digital image correlation method

  fracture toughness (KIC) is one of the most important parameters in fracture mechanics, which represents the ability of a material containing a pre-existing defect to resist tensile failure. In this paper, the crack length effect on the mode I fracture toughness of an isotropic homogeneous material was investigated. For this purpose, several disc shaped PPMA samples were loaded in pure tension by performing pseudo-compact tension (pCT) tests. Digital image correlation (DIC) method was utilized to assess and monitor the distribution of the deformation field during the tests. DIC results were also used to compare the effect of crack length on the deformation field variation in samples. Very good agreement was found between the KIC values estimated in this study and those reported in the past for the similar material; indicating that the pCT method is convenient for the assessment of KIC. The experimental results also show that the initial crack length has a tangible impact on KIC, although the magnitude of its influence is closely related to material structure and type. According to the tests results, an increase in the initial crack length leads to increase the ultimate displacement at failure point, decrease the maximum load, and the amount of absorbed energy until the moment of failure, and finally decrease the mode I fracture toughness of the material. According to the comparisson of the results, in the optimum range of sample diameter, the initial crack length is suggested between 11.5 to 15.5 mm for PMMA.

In recent years, shape memory alloys, specially NiTi, as a group of smart materials, have received more attentions in industrial applications. Martensitic phase transformation in shape memory alloys is the most important factor in their unique behavior. In this paper, the formation of stress induced martensite phase in the crack tip of superelastic NiTi (50.8% Ni) samples was investigated by using the DIC method. In particular, SEC specimens were subjected to fatigue mechanical loading, then the crack length and also displacement fields at the crack tip of specimens were measured by DIC technique. Control of the crack length was performed using a high magnification camera during the fatigue test. In the following, stress intensity factors were calculated according to ASTM standard E647-15. Obtained results from the fracture analysis show that fatigue threshold values are decreased with increasing the load ratio (R). In the present paper, for a load ratio of R=0.05, during the crack propagation, the fatigue threshold value is 〖∆K〗_th=17 MPa√m, while stress intensity factor is estimated about 35 MPa√m before the final failure. Also, as a new method in observation of the phase transformation, DIC pictures indicated the formation of stress induced martensite at the specimens crack tip.