This research was supported by a pioneering project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) and the Research and Development Program for Promoting Innovative Clean Energy Technologies through International Collaboration (JPNP20005) commissioned by NEDO. N. T. is grateful to Shimadzu Co. for providing the auto-monitoring software for distinguishing the points of highest ML intensity in Supplementary Figure 1. N. T. is grateful to Ms. Y. Nogami and Ms. H. Kawahara for spraying the ML paint for ML testing. Additionally, N. T. is grateful to Ms. Y. Kato, Ms. M. Iseki, Ms. Y. Sugawa, Ms. C. Hirakawa, Ms. Y. Sakamoto, and Ms. S. Sano for assisting with the ML measurements and analysis in the 4D visual sensing team (AIST).

The present study was conducted using DCB specimens. DCB is a standard test specimen that is often used for studying crack growth and fracture mechanics16,17,18.
1. Preparation of the test specimen
<ol>
	<li>Perform surface pretreatment before the application of the ML paint (see Table of Materials). Wipe the test specimen surface (on which the user wants to spray the ML paint) with a solvent such as isopropyl alcohol (IPA) or ethanol for surface degreasing.</li>
	<li>Prepare and apply the ML paint following the steps below.
	<ol>
		<li>Weigh 20 g of the main epoxy reagent for the ML paint (see the Table of Materials), including SrAl2O4:Eu2+ ML material, and 3.1 g of the curing reagent, and mix them with an organic solvent such as toluene and ethyl acetate in a measuring cup to obtain a viscosity of 100 mPa&#183;s.</li>
		<li>Apply the ML paint on the surface of the DCB specimen by spraying using an air spray or spray can (Figure 3).</li>
		<li>Dry the specimen gradually overnight at room temperature. 
		NOTE: The ML paint was prepared by mixing ML and polymer resins. ML materials1,2,3,4 and polymer materials can be used as opposed to commercial ML paints. However, the protocol in this study is described using commercial ML paint or spray cans (as shown in Figure 3) to ensure good performance. Although the content rate of the ML material depends on the efficiency, 25 wt% or over 50 wt% of the ML material was selected as the content rate in the ML paint22. The viscosity described in step 1.2.1 was evaluated using a viscometer8,9 (see the Table of Materials).</li>
	</ol>
	</li>
	<li>Post treatment, cure the specimen by heating the ML paint sprayed on the specimen at 80 &#176;C for 1 h. 
	NOTE: The conditions of the post-treatment must be within the range of conditions that are appropriate for curing the resin of the ML paint and that do not affect the test piece and bonding performance.</li>
	<li>Perform quality confirmation.
	<ol>
		<li>Confirm that the sprayed ML paint is roughly uniform on the surface.</li>
		<li>Ensure a thickness of approximately 50-100 &#181;m using a microscope or coating thickness gauge8 (Figure 4). 
		&#8203;NOTE: Low thickness is appropriate for preventing load distribution in the ML epoxy reagent. The uniformity of the sprayed ML paint is necessary for utilizing ML visualization for adhesive testing because intense mechanoluminescence can be observed at the crack tip owing to the high stress concentration. Therefore, the sprayed ML paint is expressed as being &#34;roughly uniform&#34; in step 1.4.1.</li>
	</ol>
	</li>
</ol>
2. ML measurement for the DCB test
<ol>
	<li>For the experimental setup for the ML measurement, perform the following steps.
	<ol>
		<li>Mount the ML paint-sprayed specimen onto the mechanical testing machine using a special zig (see the Table of Materials) for the DCB test16,17,18, as shown in Figure 5A. 
		NOTE: The DCB test specimens must comply with international standards for DCB tests16,17,18.</li>
		<li>Place cameras (a CCD, charge-coupled device, or a CMOS, complementary metal oxide semiconductor; see the Table of Materials) in front of each surface of the test specimen such that they face the position of the crack tip to be monitored8,9,10,11,12 (Figure 5B). Check the camera conditions to ensure that it can record the afterglow (AG) during the estimated measurement time of the mechanical testing. 
		NOTE: Although a four-way camera system is not mandatory for all the directions of the specimen, the number of cameras is dependent on the face of the specimen that the user wants to focus on and record.</li>
	</ol>
	</li>
	<li>Perform ML observation in the DCB test.
	<ol>
		<li>Set the surroundings to ensure dark conditions.</li>
		<li>Set the camera recording conditions: recording rate = 1 or 2 frames per second (fps); exposure time = 0.5 s or 1 s; and gain = maximum.</li>
		<li>Irradiate the ML paint-sprayed DCB specimen with 470 nm blue light for excitation using a blue LED (see the Table of Materials) from every camera direction for 1 min.</li>
		<li>Start the camera recording 5 s before finishing the blue light irradiation.</li>
		<li>Wait in the dark condition for 1 min to ensure that the afterglow settles down. 
		NOTE: The settle-down time can be changed according to the type of ML sensing material and camera, especially in relation to the balance of the mechanoluminescence and afterglow intensities in the recorded movies.</li>
		<li>Apply a mechanical load16,17,18 using a mechanical testing machine with a loading rate of 1 mm/min to obtain the ML image (Figure 5C and Movie 1).</li>
		<li>Calculate the crack length (a) by using the information on the crack tip position, which is determined from the ML point during crack propagation in the ML paint-sprayed specimen (Movie 1), to obtain the fracture toughness, G1c (kJ/m2), value using Equation 18,9,16,17,18. 
		NOTE: <img alt="Equation 1" src="/files/ftp_upload/64118/64118eq01.jpg" style="margin: auto;" /> (Equation 1) 
		where 2H denotes the thickness (mm) of the DCB specimen, B denotes the width of the specimen, &#955; denotes the crack opening displacement (COD) compliance (mm/N), Pc denotes the load (N), and &#945;1 denotes the slope of (a/2H) and (B/&#955;)1/3.</li>
	</ol>
	</li>
</ol>
3. ML measurement for the lap-shear (LS) test
<ol>
	<li>For the experimental setup for the ML measurements, mount the ML paint-sprayed LS specimen on a mechanical testing machine19,20, as shown in Figure 6A.</li>
	<li>Place cameras (a CCD or CMOS camera) in front of each surface of the test specimen such that they face the position of the crack tip to be monitored (Figure 6A). 
	NOTE: The LS test specimens must comply with international standards for the LS test19,20. In the case of dissimilar material joints, different strain distributions will appear on each of the four surfaces of the LS specimens. Thus, a four-way camera system or at least a two-way camera system is recommended for use on each of the four surfaces, as shown in Figure 6A, to capture two surfaces with each camera at a 45&#176; angle to each surface.</li>
	<li>Perform the ML observation in the lap-shear (LS) test.
	<ol>
		<li>Maintain dark conditions.</li>
		<li>Set the camera recording conditions: recording rate = 10-50 fps; exposure time = 0.02 s or 0.1 s; gain = maximum.</li>
		<li>Irradiate the ML paint-sprayed DCB specimen with 470 nm blue light for excitation using a blue LED from every camera direction for 1 min.</li>
		<li>Start the camera recording 5 s before finishing the blue light irradiation.</li>
		<li>Wait in the dark condition for 30 s for the afterglow to settle down. 
		NOTE: The settle-down time can be changed according to the ML sensing material and camera used, especially in relation to the balance of the mechanoluminescence and afterglow intensities in the recorded movies.</li>
		<li>Apply a mechanical load19,20 using a mechanical testing machine with a loading rate of 1-5 mm/min to obtain the ML images (Figure 6B and Movie 2).</li>
	</ol>
	</li>
</ol>
4. Information for the ML measurement and data analysis
<ol>
	<li>Perform excitation before the ML test.
	<ol>
		<li>Although the ML intensity is proportional to the strain energy, the ML intensity gradually decreases according to the load cycles2,3,6,12, as shown in Figure 7A. Therefore, perform excitation before the ML test to generate reproducible ML results, as mentioned in step 2.2.3 and step 3.2.3.</li>
	</ol>
	</li>
	<li>Choose the waiting time for a high ML/AG ratio. 
	NOTE: The ML sensor shows the afterglow (AG) after excitation as a long persistent phosphor and shows the mechanoluminescence at the moment of the load application, as shown in Figure 7B.
	<ol>
		<li>Select the waiting time after excitation and the camera conditions to ensure that the ratio of ML/AG (the so-called ML index) is sufficiently high (as mentioned in step 2.2.4 and step 3.2.4) because the afterglow functions as base noise against the ML pattern (i.e., the measurement signal)2,3,4.</li>
	</ol>
	</li>
	<li>Determine the highest ML point.
	<ol>
		<li>Determine the position of the crack tip by recognizing the position with the highest ML point as the crack tip8,9. 
		NOTE: The highest ML point can be determined via visual inspection, image processing software, an auto-monitoring system, and an ML movie, as shown in Supplementary Figure 1.</li>
	</ol>
	</li>
	<li>Create an ML contour image.
	<ol>
		<li>If the ML points and patterns are difficult to distinguish, then create an ML contour image and use ML patterns by converting the ML raw images using an image processing software, such as ImageJ (see the Table of Materials), as shown in Figure 8.</li>
	</ol>
	</li>
</ol>

<ol><li>Xu, C. -N., Watanabe, T., Akiyama, M., Zheng, X. -G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Direct+view+of+stress+distribution+in+solid+by+mechanoluminescence%5BTitle%5D)+AND+%22Applied+Physics+Letters%22%5BJournal%5D)">Direct view of stress distribution in solid by mechanoluminescence.</a> Applied Physics Letters. 74 (17), 2414-2417 (1999).</li>
<li>Xu, C. -N., Ueno, N., Terasaki, N., Yamada, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mechanoluminescence+and+Novel+Structural+Health+Diagnosis%5BTitle%5D)+AND+%22NTS+Inc%22%5BJournal%5D)">Mechanoluminescence and Novel Structural Health Diagnosis.</a> NTS Inc. , (2012).</li>
<li>Terasaki, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Innovative+first+step+toward+mechanoluminescent+ubiquitous+light+source+for+trillion+sensors%5BTitle%5D)+AND+%22Sensors+and+Materials%22%5BJournal%5D)">Innovative first step toward mechanoluminescent ubiquitous light source for trillion sensors.</a> Sensors and Materials. 28 (8), 827-836 (2016).</li>
<li>Feng, A., Smet, P. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((A+review+of+mechanoluminescence+in+inorganic+solids%3A+Compounds%2C+mechanisms%2C+models+and+applications%5BTitle%5D)+AND+%22Materials%22%5BJournal%5D)">A review of mechanoluminescence in inorganic solids: Compounds, mechanisms, models and applications.</a> Materials. 11 (4), 484(2018).</li>
<li>Terasaki, N., Xu, C. -N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mechanoluminescence+recording+device+integrated+with+photosensitive+material+and+europium-doped+SrAl2O4%5BTitle%5D)+AND+%22Japanese+Journal+of+Applied+Physics%22%5BJournal%5D)">Mechanoluminescence recording device integrated with photosensitive material and europium-doped SrAl2O4.</a> Japanese Journal of Applied Physics. 48, (2009).</li>
<li>Terasaki, N., Xu, C. -N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Historical-log+recording+system+for+crack+opening+and+growth+based+on+mechanoluminescent+flexible+sensor%5BTitle%5D)+AND+%22IEEE+Sensors+Journal%22%5BJournal%5D)">Historical-log recording system for crack opening and growth based on mechanoluminescent flexible sensor.</a> IEEE Sensors Journal. 13 (10), 3999-4004 (2013).</li>
<li>Terasaki, N., Fujio, Y., Sakata, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Visualization+of+strain+distribution+and+portent+of+destruction+in+structural+material+through+mechanoluminescence%5BTitle%5D)+AND+%2235th+International+Committee+on+Aeronautical+Fatigue+and+Structural+Integrity+%28ICAF%29+Conference+and+29th+ICAF+Symposium+%28ICAF%22%5BJournal%5D)">Visualization of strain distribution and portent of destruction in structural material through mechanoluminescence.</a> 35th International Committee on Aeronautical Fatigue and Structural Integrity (ICAF) Conference and 29th ICAF Symposium (ICAF. 75, 1961-1967 (2017).</li>
<li>Terasaki, N., Fujio, Y., Sakata, Y., Horiuchi, S., Akiyama, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Visualization+of+crack+propagation+for+assisting+double+cantilever+beam+test+through+mechanoluminescence%29%5BTitle%5D)+AND+%22The+Journal+of+Adhesion%22%5BJournal%5D)">Visualization of crack propagation for assisting double cantilever beam test through mechanoluminescence).</a> The Journal of Adhesion. 94 (11), 867-879 (2018).</li>
<li>Terasaki, N., Fujio, Y., Horiuchi, S., Akiyama, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mechanoluminescent+studies+of+failure+line+on+double+cantilever+beam+%28DCB%29+and+tapered-DCB+%28TDCB%29+test+with+similar+and+dissimilar+material+joints%5BTitle%5D)+AND+%22International+Journal+of+Adhesion+and+Adhesives%22%5BJournal%5D)">Mechanoluminescent studies of failure line on double cantilever beam (DCB) and tapered-DCB (TDCB) test with similar and dissimilar material joints.</a> International Journal of Adhesion and Adhesives. 93, 40-46 (2019).</li>
<li>Terasaki, N., Fujio, Y., Horiuchi, S., Akiyama, H., Itabashi, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mechanoluminescent+study+for+optimization+of+joint+design+on+cross+tension+test%5BTitle%5D)+AND+%22The+Journal+of+Adhesion%22%5BJournal%5D)">Mechanoluminescent study for optimization of joint design on cross tension test.</a> The Journal of Adhesion. 98 (6), 637-646 (2022).</li>
<li>Terasaki, N., Fujio, Y., Sakata, Y., Uehara, M., Tabaru, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Direct+visualization+of+stress+distribution+related+to+adhesive+through+mechanoluminescence%5BTitle%5D)+AND+%22ECS+Transactions%22%5BJournal%5D)">Direct visualization of stress distribution related to adhesive through mechanoluminescence.</a> ECS Transactions. 75 (45), 9-16 (2017).</li>
<li>Terasaki, N., Ando, N., Hyodo, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Mechanoluminescence+visual+inspection+of+micro-crack+generation+through+fatigue+process+in+flexible+electronics+film%5BTitle%5D)+AND+%22Japanese+Journal+of+Applied+Physics%22%5BJournal%5D)">Mechanoluminescence visual inspection of micro-crack generation through fatigue process in flexible electronics film.</a> Japanese Journal of Applied Physics. 61, (2022).</li>
<li>Terasaki, N., Xu, C. -N., Yasuda, K., Ichinose, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Fatigue+crack+detection+of+steel+truss+bridge+by+using+mechanoluminescent+sensor%5BTitle%5D)+AND+%22Proceedings+of+Sixth+International+Conference+on+Bridge+Maintenance%2C+Safety+and+Management+%28IABMAS%29%22%5BJournal%5D)">Fatigue crack detection of steel truss bridge by using mechanoluminescent sensor.</a> Proceedings of Sixth International Conference on Bridge Maintenance, Safety and Management (IABMAS). 6, 2542-2549 (2012).</li>
<li>Fujio, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Sheet+sensor+using+SrAl2O4%3A+Eu+mechanoluminescent+material+for+visualizing+inner+crack+of+high-pressure+hydrogen+vessel%29%5BTitle%5D)+AND+%22International+Journal+of+Hydrogen+Energy%22%5BJournal%5D)">Sheet sensor using SrAl2O4: Eu mechanoluminescent material for visualizing inner crack of high-pressure hydrogen vessel).</a> International Journal of Hydrogen Energy. 41 (2), 1333-1340 (2015).</li>
<li>Da Silva, L. F. M., Öchsner, A., Adams, R. <a target="_blank" href="http://scholar.google.com/scholar?hl=en&safe=off&q=author%3aDa+Silva%2C+LFM+%22Handbook+of+Adhesion+Technology.%2C+2nd+edition%22">Handbook of Adhesion Technology., 2nd edition</a>. , Springer. Cham, Switzerland. (2018).</li>
<li>International Organization for Standardization. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((ISO+22838%3A2020.+Composites+and+reinforcements+fibres+-+Determination+of+the+fracture+energy+of+bonded+plates+of+carbon+fibre+reinforced+plastics+%28CFRPs%29+and+metal+using+double+cantilever+beam+specimens%5BTitle%5D)+AND+%22International+Organization+for+Standardization%22%5BJournal%5D)">ISO 22838:2020. Composites and reinforcements fibres - Determination of the fracture energy of bonded plates of carbon fibre reinforced plastics (CFRPs) and metal using double cantilever beam specimens.</a> International Organization for Standardization. , (2020).</li>
<li>International Organization for Standardization. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((ISO+25217%3A2009.+Adhesives+-+Determination+of+the+mode+1+adhesive+fracture+energy+of+structural+adhesive+joints+using+double+cantilever+beam+and+tapered+double+cantilever+beam+specimens%5BTitle%5D)+AND+%22International+Organization+for+Standardization%22%5BJournal%5D)">ISO 25217:2009. Adhesives - Determination of the mode 1 adhesive fracture energy of structural adhesive joints using double cantilever beam and tapered double cantilever beam specimens.</a> International Organization for Standardization. , (2009).</li>
<li>International Organization for Standardization. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((ISO+15024%3A2001.+Fibre-reinforced+plastic+composites+-+Determination+of+mode+I+interlaminar+fracture+toughness%2C+GIC%2C+for+unidirectionally+reinforced+materials%5BTitle%5D)+AND+%22International+Organization+for+Standardization%22%5BJournal%5D)">ISO 15024:2001. Fibre-reinforced plastic composites - Determination of mode I interlaminar fracture toughness, GIC, for unidirectionally reinforced materials.</a> International Organization for Standardization. , (2001).</li>
<li>International Organization for Standardization. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((ISO+4587%3A2003.+Adhesives-Determination+of+tensile+lap-shear+strength+of+rigid-to-rigid+bonded+assemblies%5BTitle%5D)+AND+%22International+Organization+for+Standardization%22%5BJournal%5D)">ISO 4587:2003. Adhesives-Determination of tensile lap-shear strength of rigid-to-rigid bonded assemblies.</a> International Organization for Standardization. , (2003).</li>
<li>International Organization for Standardization. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((ISO+22841%3A2021.+Composites+and+reinforcements+fibres-Carbon+fibre+reinforced+plastics%28CFRPs%29+and+metal+assemblies-Determination+of+the+tensile+lap-shear+strength%5BTitle%5D)+AND+%22International+Organization+for+Standardization%22%5BJournal%5D)">ISO 22841:2021. Composites and reinforcements fibres-Carbon fibre reinforced plastics(CFRPs) and metal assemblies-Determination of the tensile lap-shear strength.</a> International Organization for Standardization. , (2021).</li>
<li>International Organization for Standardization. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((ISO%2FCD+8065.+Composites+and+reinforcements+fibres+-+Mechanoluminescent+visualization+method+of+crack+propagation+for+joint+evaluation%5BTitle%5D)+AND+%22International+Organization+for+Standardization%22%5BJournal%5D)">ISO/CD 8065. Composites and reinforcements fibres - Mechanoluminescent visualization method of crack propagation for joint evaluation.</a> International Organization for Standardization. , (2022).</li>
<li>Azad, A. I., Rahimi, M. R., Yun, G. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/pubmed?term=((Quantitative+full-field+strain+measurements+by+SAOED+%28SrAl2O4%3AEu2%2B%2CDy3%2B%29+mechanoluminescent+materials%5BTitle%5D)+AND+%22Smart+Material+Structure%22%5BJournal%5D)">Quantitative full-field strain measurements by SAOED (SrAl2O4:Eu2+,Dy3+) mechanoluminescent materials.</a> Smart Material Structure. 25 (9), 095032(2016).</li>
</ol>

The authors have nothing to disclose.

In terms of the ML behavior observed from the side view, intense mechanoluminescence originating from the strain concentration was recorded at the tip of the initial crack (Figure 5C). Subsequently, movement of the ML point was observed along the adhesive layer at the crack propagation time, reflecting the crack tip. In previous studies, microscopic observations showed that the highest ML point was only 0-20 &#956;m ahead of the crack tip and could be adopted as the reference for the crack tip position8. In the conventional method, the crack tip is identified via visual inspection, but this leads to a significant amount of human error due to the small size of the crack tip, even when using a magnifying glass. Specifically, patience is required to mark the position of the crack tip during the DCB test, which, in turn, requires several minutes, particularly for structural adhesive joints16,17,18. Therefore, ML visualization in the DCB test is important for identifying the position of the crack tip automatically and with higher precision. Previously, the position and shape of the ML line on the top view were shown to synchronize with the crack failure front line in the adhesive layer9. Therefore, ML sensing in the top view of the adherend was utilized as an indicator of the inner cracks from the outside surface of the adherend.
However, the limitations of this method include the dark test environment and the decrease in ML and AG intensity during the DCB test over several minutes, as shown in Figure 7B. This leads to an unclear ML point and AG pattern, which reflect the crack tip and specimen geometry, respectively. To overcome this limitation, infrared light, such as light at a wavelength of 850 nm that does not influence SrAl2O4:Eu2+ ML material, was used to irradiate the DCB specimen during the DCB test to clarify the condition of the specimen9. Alternatively, blue light at 470 nm used to illuminate the specimen for 1 s every 5 min or 10 min to recover the ML and AG intensities even during the DCB testing2,9, as explained in Figure 7A.
ML contour images and movies during the LS test were recorded using a four-way camera system (Figure 6C). In this case, the adherends were sand-blasted aluminum (A5052), and the adhesive was a two-component epoxy adhesive. The tensile shear strength (TSS) value was 23 MPa, which was calculated using the load value (N) at rupture under tensile load and the adhesive bonded area (mm2). Furthermore, the TSS value can be considered as an indicator of the strength of a structural adhesive joint18. Although the TSS value is usually used as an index of adhesive strength, the background physical properties, such as mechanical behavior, which are crucial for improving the joint design, were not investigated.
The ML images clearly provided information on the mechanical behavior during the destruction process of the single-lap adhesive joint (Figure 6C). In brief, intense mechanoluminescence was first observed at the edge of the adhesively bonded and lapped area, which shows the strain concentration at the early stage of the LS test. Second, the ML points moved from both adhesive edges to the center along the adhesive layer to appear together in the left and right views of the ML images. This indicates shear strain and crack propagation along the adhesive layer, which denotes cohesive failure (CF) in this case.
Additionally, the ML lines in the front and back views indicated the occurrence of crack propagation, which is the same phenomenon as in the DCB test. Finally, after the two ML points combined at the center, intense mechanoluminescence was observed at the center point in the adhesive layer. This indicated the strain concentration in the adhesive layer and the subsequent generation of a transverse crack across the adhesive layer, similar to in a previous work11. This information is useful to determine the location of the stress/strain concentration. Hence, it implies that improvement in stress dispersion is required to achieve a strong and reliable joint design.
Unlike the DCB test, the LS test causes the high-speed rupture of adhesive joints. The LS test generates a high strain rate in the adhesive layer, which is followed by highly intense mechanoluminescence that saturates in the recorded ML image, accumulates many events in one image, and produces an unclear ML image. In these cases, a smart choice of recording rate can be used for troubleshooting (e.g., selecting a high recording rate, such as 25 fps, which fits the speed of the event in the LS test)11.

Mechanoluminescent (ML) sensing materials are functional ceramic powders that emit intense light repeatedly under mechanical stimuli. This phenomenon is observed even within regions of elastic deformation1,2,3,4. When dispersed onto the surface of a structure, individual ML particles function as sensitive mechanical sensors, and the two-dimensional (2D) ML pattern reflects the dynamic strain distribution. The ML emission pattern presents a mechanical simulation of the strain distribution2,3,4,5,6,7,8,9,10,11,12 (Figure 1A).
As shown in Figure 1B, ML sensors have been applied to visualize two-dimensional (2D) and three-dimensional (3D) dynamic mechanical behaviors in elastic, plastic, and destruction processes using coupon test specimens comprising recent advanced lightweight structural materials (e.g., high-tensile strength steel5,6, aluminum, carbon fiber-reinforced plastic [CFRP]7), the adhesive joint for damage tolerance design8,9,10,11, and product components (e.g., gear and flexible electronics file for foldable phones12, and complicated adhesive and/or welding joints used for validating computer-aided engineering [CAE] results in laboratory-level testing2,8,9,10,11). Additionally, ML sensors have been successfully used in practical applications, such as the structural health monitoring (SHM) of buildings and bridges for detecting crack propagation or the probability of a strain concentration leading to structural degradation2,6,13, the monitoring of the inner crack propagation in interlaminar layers7,9, the prediction of the lifespan of high-pressure hydrogen vessels9, impact tests of mobility for visualizing the impact wave propagation or excitation in vibration mode14, and visual sensing of sports tools to determine the appropriate physical settings to increase the chances of winning. In the protocol, ML visualization was selected for monitoring crack propagation and the subsequent changes in mechanical behavior during adhesive joint evaluation testing.
There are several reasons for selecting this theme. The first reason is the significant increase in the importance of adhesive joints in recent years. Recently, due to the need for significant CO2 reduction and energy saving, various types of lightweight materials have been developed and applied in the mobility and transportation industries, such as for automobiles, aircraft, and trains. As part of this trend, adhesive technology has gained importance as a key technology for freely joining different lightweight materials (dissimilar material joints) in a multi-material strategy15. Furthermore, the ML visualization method for determining adhesive strength, especially in dissimilar materials, has been suggested by various international standards16,17,18,19,20. The evaluation of adhesive strength is essentially destructive testing, and the obtained adhesive strength can be mainly classified into two types: (1) fracture toughness energy (Gc), which is determined using the position of crack propagation during the load application, and (2) adhesive strength, which is determined using the load at the rupture of the adhesive joint. Although the double cantilever beam (DCB) test and single lap-shear (LS) test are representative evaluation methods of fracture toughness and adhesive strength, respectively, and represent the most frequently used adhesive testing methods worldwide15,16,17,18,19,20, the crack tip is too small to distinguish the stress/strain distribution. Hence, the fracture toughness energy (Gc) value is highly scattered. As a result of recommendations from researchers examining adhesives and other individuals in the industry, mechanoluminescent (ML) visualization has been investigated for monitoring crack propagation and the subsequent changes in mechanical behavior during adhesive joint evaluation testing8,9,10,11,21. The second reason for selecting this theme in this protocol is that stress/strain is highly concentrated at the crack tip, which generates intense mechanoluminescence at the ML point during crack propagation, and this is potentially the most user-friendly methodology among various&#160;ML testing applications. Additionally, this method can be utilized without advanced experience in sample preparation and highly efficient ML materials.
Therefore, in this study, the protocol of ML visualization is explained for monitoring crack propagation and the subsequent changes in the mechanical behavior during adhesive joint evaluation testing, as shown in Figure 2.

<table><tbody><tr><td>Aluminum plate</td><td>Engineering Test Service Co.,Ltd.</td><td>A5052&amp;nbsp;</td><td>A5052 is defined name as quality of aluminum in standards.</td></tr><tr><td>Blue LED</td><td>MORITEX Co.</td><td>MBRL-CB13015</td><td></td></tr><tr><td>Camera</td><td>Baumer</td><td>TXG04 or VLU-12</td><td>CCD or CMOS</td></tr><tr><td>Coating thickness gauge&amp;nbsp;</td><td>KETT</td><td>LZ-373</td><td></td></tr><tr><td>Epoxy adhesive</td><td>Nagase ChemteX Co.</td><td>Denatite2202</td><td>structual adehsive</td></tr><tr><td>ImageJ</td><td>National Institutes of Health</td><td>Image J 1.53K</td><td>Image processing software</td></tr><tr><td>Mechanical testing machine&amp;nbsp;</td><td>SHIMADZU Co.</td><td>EZ Test EZ-LX</td><td></td></tr><tr><td>Mechanoluminescnet (ML) paint</td><td>Sakai Chemical Industry Co. Ltd.</td><td>ML-F2ET3</td><td>The ML paint in 1.1&amp;nbsp; is 2 components epoxy paint , and consisting of epoxy main reagent and curing reagent as described in 1.2.1.&amp;nbsp; SrAl&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;:Eu&lt;sup&gt;2+&lt;/sup&gt; ML ceramic perticle is including in main epoxy reagent.</td></tr><tr><td>Microscope</td><td>keyence</td><td>VHX-6000</td><td></td></tr><tr><td>Stainless steel&amp;nbsp; plate</td><td>Engineering Test Service Co.,Ltd.</td><td>SUS631</td><td>A631 is defined name as quality of stainless steel in standards.</td></tr><tr><td>Viscometer</td><td>Sekonic. Co.</td><td>Viscomate VM-10A</td><td></td></tr></tbody></table>

mechanoluminescent visualization of crack propagation for joint evaluation

In this study, methods for the mechanoluminescent (ML) visualization of crack propagation and mechanical behavior to evaluate adhesive joints are demonstrated and explained. The first step involved sample preparation; an air spray was used to apply ML paint to the surface of the adhesive joint specimens. The performance of the ML sensor was described to examine the measurement conditions. The results of ML sensing during a double cantilever beam (DCB) test and a lap-shear (LS) test are demonstrated as these are the most frequently and widely used methods for evaluating adhesives. Originally, it was difficult to directly quantify the crack tip and strain/stress distribution and concentration because the crack tip was too small, and the effects of the strain could not be observed. The mechanoluminescence, crack propagation, and mechanical behavior during mechanical testing can be visualized via the ML pattern during the adhesive evaluation. This allows for the recognition of the precise position of the crack tips and other mechanical behaviors related to structural failure.

ML images and movies during the DCB and LS test were collected using two-way and four-way cameras, respectively.
Figure 5C shows the ML images and movies in the side view, which can be used to recognize the crack tip. Furthermore, the top view is shown to reflect the failure front at the crack propagation time during the DCB test. In this case, the adherends were sand-blasted aluminum (A5052, see the Table of Materials), the adhesive was composed of two components of epoxy adhesive, and the geometry complied with international standards. Regarding the ML behaviors in the side view, intense mechanoluminescence was observed at the position of the initial crack owing to the strain concentration at this point. Subsequently, movement of the ML point, which reflects the crack tip, was observed on the adhesive layer at the crack propagation time. Using ML images in the DCB test, the position of the crack tip during crack propagation was defined and used to calculate the crack propagation length (a) and the associated fracture toughness, G1c, value, as explained in step 2.2.7.
Figure 6B shows the ML contour images and movies during the LS test. The images and movies were recorded using a four-way camera system. In this case, the adherends were sand-blasted aluminum (A5052), and the adhesive was a two-component epoxy adhesive. Figure 6B clearly provides information on the mechanical behavior during the destruction process of the single-lap adhesive joint. In brief, intense mechanoluminescence was first observed at the edges of the adhesively bonded and lapped areas. Second, the ML points moved from the adhesive edges to the center along the adhesive layer to appear together in the left and right views of the ML image. Finally, after combining the two ML points at the center, intense mechanoluminescence was observed at the center point in the adhesive layer. ML images in the LS test can be used to understand the mechanical behavior of adhesive joints during the destruction process, which is difficult to simulate.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/64118/64118fig01.jpg" /> 
Figure 1: Properties of the ML sensor. (A) Mechanoluminescence under tensile load for a stainless steel plate with a hole and numerical analysis (simulation) of the Mises strain distribution. (B) Examples of ML visual sensing to visualize the dynamic 2D/3D mechanical behavior of products, structural materials, and 3D printing materials under the application of mechanical load, vibration, and impact. The arrows with an &#34;F&#34; indicate the direction of the force under mechanical load. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64118/64118fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/64118/64118fig02.jpg" /> 
Figure 2: ML visual sensing for various internationally standardized adhesive evaluation tests. These standards describe the methods to obtain various indexes of adhesive strength, such as fracture toughness energy (Gc), tensile shear strength (TSS), peel strength, and cross tension strength (CTS). The arrows indicate the direction of the force under mechanical load. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64118/64118fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/64118/64118fig03.jpg" /> 
Figure 3: Applying ML sensor paint. (A) Examples of ML paint and spray cans and (B) a photograph of spraying. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64118/64118fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/64118/64118fig04.jpg" /> 
Figure 4: Illustration of ML paint-sprayed specimens. (A) A DCB specimen and (B) an LS specimen. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64118/64118fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/64118/64118fig05.jpg" /> 
Figure 5: ML measurement during the DCB test. (A) Photograph of the experimental setup and (B) illustration of the camera positions. (C) ML measurement during the DCB test. CAM 1 and CAM 2 denote the CCD camera described in step 2.1.2. The arrows indicate the direction of the force under mechanical load. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64118/64118fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/64118/64118fig06.jpg" /> 
Figure 6: ML measurement during the LS test. (A) The experimental setup and (B) ML measurement during the LS test using a four-way camera system. The arrows indicate the direction of the force under mechanical load. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64118/64118fig06large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/64118/64118fig07.jpg" /> 
Figure 7: Basic properties of the employed ML sensor. (A) The ML intensity across load cycles and (B) the relationship between the ML and AG intensities and waiting time after excitation using a blue LED. The inset illustrates the definition of the ML and AG intensities in the time-luminance curve. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64118/64118fig07large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/64118/64118fig08.jpg" /> 
Figure 8: Comparison of ML expression in the ML images. (A) The raw image in 12-bit grayscale and (B) the contour image. The arrows with &#34;F&#34; indicate the direction of the force under mechanical load. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64118/64118fig08large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Movie 1: ML movie during the DCB test. Recording rate: 1 fps. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64118/Fig. S1 ML movie during DCB test.mp4" target="_blank">Please click here to download this Movie.</a>
Movie 2: ML movie during the LS test. Recording rate: 25 fps. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64118/Fig. S2 ML movie during LS test.mp4" target="_blank">Please click here to download this Movie.</a>
Supplementary Figure 1: Methods to distinguish the position of the point of highest ML intensity. (A) Visual inspection, (B) image processing software, and (C) auto monitoring system. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/64118/Supplementary Figure 1.tif" target="_blank">Please click here to download this File.</a>

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Mechanoluminescent Visualization of Crack Propagation for Joint Evaluation

In this study, a protocol is presented that describes the use of mechanoluminescent (ML) visualization for monitoring crack propagation and mechanical behavior during adhesive joint evaluation testing.

In this study, methods for the mechanoluminescent (ML) visualization of crack propagation and mechanical behavior to evaluate adhesive joints are ...

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2.1K Views.  National Institute of Advanced Industrial Science and Technology (AIST). Om gebruik te maken van mechanoluminescente detectieresultaatfouten, toont dit protocol aan dat de voorbereiding van de mechanoluminescente sensor, de soorten meetomgeving en de opnameomstandigheden die moeten worden aangenomen voor herhaalbaarheid. Het voordeel van deze methode is directe visualisatie van de scheurpunt en sterkte, spanningsverdeling en concentratie in mechanische stimuli die oorspronkelijk moeilijk direct te kwantificeren zijn. Dit protocol richt zich op het visualiseren van...

Video: Mechanoluminescent Visualization of Crack Propagation for Joint Evaluation

A Method for Studying the Temperature Dependence of Dynamic Fracture and Fragmentation

The dynamic fracture of a body is a late-stage phenomenon typically studied under simplified conditions, in which a sample is deformed under uniform stress and strain rate. This can be produced by evenly loading the inner surface of a cylinder. Due to the axial symmetry, as the cylinder expands the wall is placed into a tensile hoop stress that is uniform around the circumference. While there are various techniques to generate this expansion such as explosives, electromagnetic drive, and existing gas gun techniques they are all limited in the fact that the sample cylinder must be at room temperature. We present a new method using a gas gun that facilitates experiments on cylinders from 150 K to 800 K with a consistent, repeatable loading. These highly diagnosed experiments are used to examine the effect of temperature on the fracture mechanisms responsible for failure, and their resulting influence on fragmentation statistics. The experimental geometry employs a steel ogive located inside the target cylinder, with the tip located about halfway in. A single stage light gas gun is then used to launch a polycarbonate projectile into the cylinder at 1,000 m/sec-1. The projectile impacts and flows around the rigid ogive, driving the sample cylinder from the inside. The use of a non-deforming ogive insert allows us to install temperature control hardware inside the rear of the cylinder. Liquid nitrogen (LN2) is used for cooling and a resistive high current load for heating. Multiple channels of upshifted photon Doppler velocimetry (PDV) track the expansion velocity along the cylinder enabling direct comparison to computer simulations, while High speed imaging is used to measure the strain to failure. The recovered cylinder fragments are also subject to optical and electron microscopy to ascertain the failure mechanism.

Fracture and fragmentation are late stage phenomena in dynamic loading scenarios and are typically studied using explosives. We present a technique for driving expansion using a gas gun which uniquely enables control of both loading rate and sample temperature.

The dynamic fracture of a body is a late-stage phenomenon typically studied under simplified conditions, in which a sample is deformed under uniform ...

In situ Compressive Loading and Correlative Noninvasive Imaging of the Bone-periodontal Ligament-tooth Fibrous Joint

This study demonstrates a novel biomechanics testing protocol. The advantage of this protocol includes the use of an in situ loading device coupled to a high resolution X-ray microscope, thus enabling visualization of internal structural elements under simulated physiological loads and wet conditions. Experimental specimens will include intact bone-periodontal ligament (PDL)-tooth fibrous joints. Results will illustrate three important features of the protocol as they can be applied to organ level biomechanics: 1) reactionary force vs. displacement: tooth displacement within the alveolar socket and its reactionary response to loading, 2) three-dimensional (3D) spatial configuration and morphometrics: geometric relationship of the tooth with the alveolar socket, and 3) changes in readouts 1 and 2 due to a change in loading axis, i.e. from concentric to eccentric loads. Efficacy of the proposed protocol will be evaluated by coupling mechanical testing readouts to 3D morphometrics and overall biomechanics of the joint. In addition, this technique will emphasize on the need to equilibrate experimental conditions, specifically reactionary loads prior to acquiring tomograms of fibrous joints. It should be noted that the proposed protocol is limited to testing specimens under ex vivo conditions, and that use of contrast agents to visualize soft tissue mechanical response could lead to erroneous conclusions about tissue and organ-level biomechanics.

In situ Compressive Loading and Correlative Noninvasive Imaging of the Bone-periodontal Ligament-tooth Fibrous Joint

In this study, the use of an in situ loading device coupled with micro-X-ray computed tomography for fibrous joint biomechanics will be discussed. Experimental readouts identifiable with an overall change in joint biomechanics will include: 1) reactionary force vs. displacement, i.e. tooth displacement within the alveolar socket and its reactionary response to loading, 2) three-dimensional (3D) spatial configuration and morphometrics, i.e.&#160;geometric relationship of the tooth with the alveolar socket, and 3) changes in readouts 1 and 2 due to a change in loading axis, i.e. concentric or eccentric loads.

This study demonstrates a novel biomechanics testing protocol. The advantage of this protocol includes the use of an in situ loading device coupled to a ...

Bioengineering

Full-field Strain Measurements for Microstructurally Small Fatigue Crack Propagation Using Digital Image Correlation Method

A novel measurement approach is used to reveal the cumulative deformation field at a sub-grain level and to study the influence of microstructure on the growth of microstructurally small fatigue cracks. The proposed strain field analysis methodology is based on the use of a unique pattering technique with a characteristic speckle size of approximately 10 &#181;m. The developed methodology is applied to study the small fatigue crack behavior in body centered cubic (bcc) Fe-Cr ferritic stainless steel with a relatively large grain size allowing a high spatial measurement accuracy at the sub-grain level. This methodology allows the measurement of small fatigue crack growth retardation events and associated intermittent shear strain localization zones ahead of the crack tip. In addition, this can be correlated with the grain orientation and size. Thus, the developed methodology can provide a deeper fundamental understanding of the small fatigue crack growth behavior, required for the development of robust theoretical models for the small fatigue crack&#160;propagation in polycrystalline materials.

Microstructurally small fatigue crack growth behavior is investigated using a novel methodological approach combining crack growth rate measurement and strain-field analysis to reveal the cumulative deformation field at sub-grain level.

A novel measurement approach is used to reveal the cumulative deformation field at a sub-grain level and to study the influence of microstructure on the ...

Crack Monitoring in Resonance Fatigue Testing of Welded Specimens Using Digital Image Correlation

A procedure using digital image correlation (DIC) to detect cracks on welded specimens during fatigue tests on resonance testing machines is presented. It is intended as a practical and reproducible procedure to identify macroscopic cracks at an early stage and monitor crack propagation during fatigue tests. It consists of strain field measurements at the weld using DIC. Images are taken at fixed load cycle intervals. Cracks become visible in the computed strain field as elevated strains. This way, the whole width of a small-scale specimen can be monitored to detect where and when a crack initiates. Subsequently, it is possible to monitor the development of the crack length. Because the resulting images are saved, the results are verifiable and comparable. The procedure is limited to cracks initiating at the surface and is intended for fatigue tests under laboratory conditions. By visualizing the crack, the presented procedure allows direct observation of macrocracks from their formation until rupture of the specimen.

Digital image correlation is used in fatigue tests on a resonance testing machine to detect macroscopic cracks and monitor crack propagation in welded specimens. Cracks on the specimen surface become visible as increased strains.

A procedure using digital image correlation (DIC) to detect cracks on welded specimens during fatigue tests on resonance testing machines is presented. It ...

A Method to Estimate Cadaveric Femur Cortical Strains During Fracture Testing Using Digital Image Correlation

This protocol describes the method using digital image correlation to estimate cortical strain from high speed video images of the cadaveric femoral surface obtained from mechanical testing. This optical method requires a texture of many contrasting fiduciary marks on a solid white background for accurate tracking of surface deformation as loading is applied to the specimen. Immediately prior to testing, the surface of interest in the camera view is painted with a water-based white primer and allowed to dry for several minutes. Then, a black paint is speckled carefully over the white background with special consideration for the even size and shape of the droplets. Illumination is carefully designed and set such that there is optimal contrast of these marks while minimizing reflections through the use of filters. Images were obtained through high speed video capture at up to 12,000 frames/s. The key images prior to and including the fracture event are extracted and deformations are estimated between successive frames in carefully sized interrogation windows over a specified region of interest. These deformations are then used to compute surface strain temporally during the fracture test. The strain data is very useful for identifying fracture initiation within the femur, and for eventual validation of proximal femur fracture strength models derived from Quantitative Computed Tomography-based Finite Element Analysis (QCT/FEA).

In this protocol, the femur surface strains are estimated during fracture testing using the digital image correlation technique. The novelty of the method involves application of a high-contrast stochastic speckle pattern on the femur surface, carefully specified illumination, high speed video capture, and digital image correlation analysis for strain calculations.

This protocol describes the method using digital image correlation to estimate cortical strain from high speed video images of the cadaveric femoral ...

A Virtual Simulation Experiment of Mechanics: Material Deformation and Failure Based on Scanning Electron Microscopy

This work presents a set of comprehensive virtual experiments to detect material deformation and failure. The most commonly used pieces of equipment in mechanics and material disciplines, such as a metallographic cutting machine and a high-temperature universal creep testing machine, are integrated into a web-based system to provide different experimental services to users in an immersive and interactive learning environment. The protocol in this work is divided into five subsections, namely, the preparation of the materials, molding the specimen, specimen characterization, specimen loading, nanoindenter installation, and SEM in situ experiments, and this protocol aims to provide an opportunity for users regarding the recognition of different equipment and the corresponding operations, as well as the enhancement of laboratory awareness, etc., using a virtual simulation approach. To provide clear guidance for the experiment, the system highlights the equipment/specimen to be used in the next step and marks the pathway that leads to the equipment with a conspicuous arrow. To mimic the hands-on experiment as closely as possible, we designed and developed a three-dimensional laboratory room, equipment, operations, and experimental procedures. Moreover, the virtual system also considers interactive exercises and registration before using chemicals during the experiment. Incorrect operations are also allowed, resulting in a warning message informing the user. The system can provide interactive and visualized experiments to users at different levels.

This work presents a three-dimensional virtual simulation experiment for material deformation and failure that provides visualized experimental processes. Through a set of experiments, users can become familiar with the equipment and learn the operations in an immersive and interactive learning environment.

This work presents a set of comprehensive virtual experiments to detect material deformation and failure. The most commonly used pieces of equipment in ...

Performing Microscope-Mounted Y-Shaped Cutting Tests

Y-shaped cutting has recently been shown to be a promising method by which to understand the threshold length scale and failure energy of a material, as well as its failure response in the presence of excess deformation energy. The experimental apparatus used in these studies was vertically oriented and required cumbersome steps to adjust the angle between the Y-shaped legs. The vertical orientation prohibits visualization in standard optical microscopes. This protocol presents a Y-shaped cutting apparatus that mounts horizontally over an existing inverted microscope stage, can be adjusted in three dimensions (X-Y-Z) to fall within the objective&#39;s field of view, and allows easy modification of the angle between the legs. The latter two features are new for this experimental technique. The presented apparatus measures the cutting force within 1 mN accuracy. When testing polydimethylsiloxane (PDMS), the reference material for this technique, a cutting energy of 132.96 J/m2 was measured (32&#176; leg angle, 75 g preload) and found to fall within the error of previous measurements taken with a vertical setup (132.9 J/m2 &#177; 3.4 J/m2). The approach applies to soft synthetic materials, tissues, or bio-membranes and may provide new insights into their behavior during failure. The list of parts, CAD files, and detailed instructions in this work provide a roadmap for the easy implementation of this powerful technique.

Y-shaped cutting measures fracture-relevant length scales and energies in soft materials. Previous apparatuses were designed for benchtop measurements. This protocol describes the fabrication and use of an apparatus that orients the setup horizontally and provides the fine positioning capabilities necessary for in situ viewing, plus failure quantification, via an optical microscope.

Y-shaped cutting has recently been shown to be a promising method by which to understand the threshold length scale and failure energy of a material, as ...

Imaging of the Microstructural Failure Mechanism in the Human Hip

Imaging the bone microstructure under progressively increasing loads allows for observing the microstructural failure behavior of bone. Here, we describe a protocol for obtaining a sequence of three-dimensional microstructural images of the entire proximal femur under progressively increasing deformation, causing clinically relevant fractures of the femoral neck. The protocol is demonstrated using four femora from female donors aged 66-80 years at the lower end of bone mineral density in the population (T-score range = &#8722;2.09 to &#8722;4.75). A radio-transparent compressive stage was designed for loading the specimens replicating a one-leg stance, while recording the applied load during&#160;micro-computed tomography (micro-CT) imaging. The field of view was 146 mm wide and 132 mm high, and the isotropic pixel size was 0.03 mm. The force increment was based on finite-element predictions of the fracture load. The compressive stage was used to apply the displacement to the specimen and enact the prescribed force increments. Sub-capital fractures due to opening and shear of the femoral neck occurred after four to five load increments. The micro-CT&#160;images and the reaction force measurements were processed to study the bone strain and energy absorption capacity. Instability of the cortex appeared at the early loading steps. The subchondral bone in the femoral head displayed large deformations reaching 16% before&#160;fracture, and a progressive increase in the support capacity up to&#160;fracture. The deformation energy linearly increased with the displacement up to&#160;fracture, while the stiffness decreased to near-zero values immediately before&#160;fracture. Three-fourths of the fracture energy was taken by the specimen during the final 25% force increment. In conclusion, the protocol developed revealed a remarkable energy absorption capacity, or damage tolerance, and a synergic interaction between the cortical and trabecular bone at an advanced donor age.

The protocol enables the measurement of the deformation of the bone microstructure in the entire proximal human femur and its toughness by combining large-volume micro-CT scanning, a custom-made compressive stage, and advanced image processing tools.

Imaging the bone microstructure under progressively increasing loads allows for observing the microstructural failure behavior of bone. Here, we describe ...

A Method to Study the Correlation Between Local Collagen Structure and Mechanical Properties of Atherosclerotic Plaque Fibrous Tissue

The rupture of atherosclerotic plaques in coronary and carotid arteries is the primary cause of fatal cardiovascular events. However, the rupture mechanics of the heterogeneous, highly collagenous plaque tissue, and how this is related to the tissue's fibrous structure, are not known yet. Existing pipelines to study plaque mechanics are limited to obtaining only gross mechanical characteristics of the plaque tissue, based on the assumption of structural homogeneity of the tissue. However, fibrous plaque tissue is structurally heterogeneous, arguably mainly due to local variation in the collagen fiber architecture.
The mechano-imaging pipeline described here has been developed to study the heterogeneous structural and mechanical plaque properties. In this pipeline, the tissue's local collagen architecture is characterized using multiphoton microscopy (MPM) with second-harmonic generation (SHG), and the tissue's failure behavior is characterized under uniaxial tensile testing conditions using digital image correlation (DIC) analysis. This experimental pipeline enables correlation of the local predominant angle and dispersion of collagen fiber orientation, the rupture behavior, and the strain fingerprints of the fibrous plaque tissue. The obtained knowledge is key to better understand, predict, and prevent atherosclerotic plaque rupture events.

We have developed a mechano-imaging pipeline to study the heterogeneous structural and mechanical atherosclerotic plaque properties. This pipeline enables correlation of the local predominant angle and dispersion of collagen fiber orientation, the rupture behavior, and the strain fingerprints of the fibrous plaque tissue.

The rupture of atherosclerotic plaques in coronary and carotid arteries is the primary cause of fatal cardiovascular events. However, the rupture ...

Quasistatic Load-to-Fracture Test of CAD/CAM Occlusal Veneers on Dentin Analogs

Quasistatic Mechanical Testing for Computer-Aided Design and Manufacturing Occlusal Veneers Cemented to Milled Dentin Analog Material

Under current minimally invasive treatment regimes, minor tooth preparation and thinner biomimetic ceramic restoration are used to preserve the restored tooth&#39;s vitality, aesthetics, and function. New computer-aided design and computer-aided manufacturing (CAD/CAM) ceramic-like material are&#160;now available. To guarantee longevity, a dental clinician must know these&#160;newly launched product&#39;s mechanical strength compared to the relatively brittle glass-matrix ceramic. Furthermore, a tooth substitute has been promoted for laboratory investigation, especially after the pandemic, and more evidentiary support is required for its application.
This study developed a laboratory protocol for a monotonic load-to-fracture test to determine the fracture strength of 1 mm-thick CAD/CAM occlusal veneers. Master dies were milled from high-pressure fiberglass laminate, which has&#160;similar elastic modulus and bond strength as hydrated dentin. They were mounted into polyvinyl chloride (PVC) end caps with cold-curing epoxy resin. Occlusal veneers, also called tabletop restorations, were milled from lithium disilicate (LD) and resin nanoceramic blocks (RNC) and cemented to prepared master dies using dual-cured adhesive resin cement. They were&#160;allowed to cure fully by storing in distilled water for 48 h at 37 &#176;C.
All samples were then placed in a universal testing machine and loaded via a non-fixed 5.5 mm stainless-steel ball that allows lateral movement as would occur against the antagonist&#160;teeth. Compression was applied at a 1 mm/min rate, and the load-displacement graph was generated. The average maximum load-bearing capacity of restorations in the RNC group (3,212.80 &#177; 558.67 N) was significantly higher than in the LD group (2727.10 &#177; 472.41 N) (p &#60; 0.05). No debonding was found during the test.&#160;Both CAD/CAM materials may have a similar flaw distribution. Hertzian cone crack was found at the loading site, whereas radial cracks propagating from the cementation surface were found close to the margin in both groups.

A quasistatic load-to-fracture test with a non-fixed stainless steel ball was developed to determine the fracture strength of minimally invasive posterior computer-aided design and manufacturing restorations cemented to dentin analog materials. This test models the typical loading regime responsible for the fracture of dental restorations.

Under current minimally invasive treatment regimes, minor tooth preparation and thinner biomimetic ceramic restoration are used to preserve the restored ...

Mechanoluminescent Visualization of Crack Propagation for Joint Evaluation

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