Evaluation of dynamic fracture toughness of a bonded bi-material interface subject to high-strain-rate shearing using digital image correlation

Composite and adhesively bonded materials are widely used in numerous applications. Notably, the strength of these materials is highly dependent on the interfacial strength, for they have dissimilar interfaces in themselves. Besides, since most products made of these materials are prevalently subjected to impact loading, as in the case of a collision with space debris, a bird strike on airplanes and car accidents, the dynamic interfacial strength of these materials becomes an important design parameter.

The dynamic fracture characteristics and parameters of bi-material interfaces have been extensively investigated experimentally based on optical techniques. Despite the good progress, the typical method used in most of those research had the disadvantage that it is difficult to analyze the results and could only be applied to transparent materials. Therefore, the application of these study results has been limited. Recently, digital image correlation (DIC) has been identified as a promising method. This technique is advantageous to the existing methods. It does not require complicated optical systems to measure the displacement and strain distributions. Moreover, it is also appliable in cases involving non-transparent materials.

Nevertheless, an efficient technique for computing the fracture toughness values of bonded bi-materials interfaces directly from the DIC results is yet to be developed. To address this issue, Assistant Professor Tomohisa Kojima, Mr. Yuta Kimura (master’s student), Associate Professor Shuichi Arikawa and Professor Mitsuo Notomi, who are researchers at Meiji University in Japan, evaluated the dynamic fracture toughness of a bonded bi-material interface subjected to high-strain-rate shearing using the DIC method. Their work is currently published in the research journal, Engineering Fracture Mechanics.

In their approach, aluminum material at the center and polymethyl methacrylate (PMMA) resin layers bonded on both sides formed the three-layered test piece. These materials exhibited dissimilar elastic wave propagation characteristics. Next, high-strain-rate shear tests were conducted with Split-Hopkinson pressure bar (SHPB) set-up. Finally, a combination of an ultrahigh-speed video camera and DIC was used to investigate the propagation of the elastic stress wave and locate the interface crack tip, then the fracture toughness was evaluated at the aluminum/PMMA bonded interface.

As a result of the DIC, it was possible to determine the process by which 1) the elastic stress wave propagated to the aluminum section, 2) the wave was transmitted to the PMMA section, and 3) the crack developed at the interface. It provided an in-depth understanding of the process involved in the crack development at the interface. The authors successfully located the interface crack tip through displacement distributions obtained using DIC. They also obtained the true interfacial stress by correcting the strain value at the interface obtained using DIC. The distribution of the true interfacial stress suggested that mode II (shearing mode) fracture appears when the crack is sufficiently shorter than the length of the bonded interface, and mode I (opening mode) and mode II fractures appear when the crack is longer in comparison.

 Eventually, the authors evaluated the fracture toughness at the bonded interface using the obtained true interfacial stress values. It was noted that the obtained fracture toughness values compared well with the literature values, indicating that the proposed method in their study is effective and reliable for evaluating the dynamic fracture toughness of bonded bi-material interfaces.

In a statement to Advances in Engineering, the authors acknowledged that their study would advance the development of composites and adhesively bonded materials with high-impact resistance.