Mechanoluminescent Visualization of Crack Propagation for Joint Evaluation

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 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. Prepare and apply the ML paint following the steps below. 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·s. Apply the ML paint on the surface of the DCB specimen by spraying using an air spray or spray can (Figure 3). 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). Post treatment, cure the specimen by heating the ML paint sprayed on the specimen at 80 °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. Perform quality confirmation. Confirm that the sprayed ML paint is roughly uniform on the surface. Ensure a thickness of approximately 50-100 µm using a microscope or coating thickness gauge8 (Figure 4). ​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 "roughly uniform" in step 1.4.1. 2. ML measurement for the DCB test For the experimental setup for the ML measurement, perform the following steps. 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. 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. Perform ML observation in the DCB test. Set the surroundings to ensure dark conditions. 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. 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. Start the camera recording 5 s before finishing the blue light irradiation. 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. 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). 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: (Equation 1) where 2H denotes the thickness (mm) of the DCB specimen, B denotes the width of the specimen, λ denotes the crack opening displacement (COD) compliance (mm/N), Pc denotes the load (N), and α1 denotes the slope of (a/2H) and (B/λ)1/3. 3. ML measurement for the lap-shear (LS) test 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. 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° angle to each surface. Perform the ML observation in the lap-shear (LS) test. Maintain dark conditions. Set the camera recording conditions: recording rate = 10-50 fps; exposure time = 0.02 s or 0.1 s; gain = maximum. 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. Start the camera recording 5 s before finishing the blue light irradiation. 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. 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). 4. Information for the ML measurement and data analysis Perform excitation before the ML test. 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. 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. 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. Determine the highest ML point. 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. Create an ML contour image. 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.