Characterization of interfacial fracture using strain fields

An experimental study was conducted to develop and introduce strain gage techniques as an effective and simple alternative experimental method to investigate interfacial fracture in isotropic-isotropic and isotropic-orthotropic bimaterials. The study looked to develop the requisite strain field equations and experimental techniques to obtain static and dynamic interfacial fracture parameters using strain gages and, in the static case, comparing to several optical experimental methods. In the first part of the study, the strain gage technique was compared to three widely used experimental techniques—photoelasticity, coherent gradient sensing (CGS), and caustics—as applied to the stationary interfacial crack problem. These experimental methods were used to obtain the complex stress intensity factor (CSIF), K, in the region of the crack tip within the interface of PSM-1-polycarboate/aluminum, PMMA/aluminum, PSM-1/Scotchply (unidirectional composite), and PMMA/Scotchply bimaterials and then compared to theory. In the second part of the study, strain field equations were developed to investigate subsonic interfacial crack propagation in isotropic-isotropic and isotropic-orthotropic bimaterials. Strain field equations were derived from the available stress field equations and critically evaluated in a parametric study. The feasibility of using strain gages was then demonstrated in experiments in which strain gage techniques were used to obtain values of the dynamic CSIF. Experiments were conducted using PSM-1/aluminum and PSM-1/Scotchply specimens. For the isotropic-isotropic experiments, strain gages were mounted first on Material-1 (PSM-1) and then on Material-2 (aluminum). For the isotropic-orthotropic experiments, strain gages were mounted on the Scotchply (Material 2) first with fiber orientation perpendicular to the interface (α = 90°) and then parallel (α = 0°). Two first-order strain gage analyses were developed to determine dynamic CSIF (a 2-gage method and a 3-gage method) and subsequently compared to a higher-order photoelastic analysis. Results showed that strain gages methods were effective at determining static and dynamic CSIF. Also, strain gage methods compared favorably to the results from the other optical techniques (as well as theoretical predictions in the static case). Thus, strain gages are feasible and effective to use to investigate interfacial crack problems in isotropic-isotropic and isotropic-orthotropic bimaterials