On determination of apparent fracture toughness and fracture process zone

Suvanit Chitsiriphanit, Purdue University

Abstract

The existence of crack-like flaw cannot be precluded in any engineering structure. The strength of material approach of failure predicts that the material fails when the stress exceeds some critical value. When a cracked plate is subjected to a small load, the plate does not fail, although the stress field near the crack tip becomes very high. In the fracture mechanics approach, instead of comparing the maximum stress value with a critical stress value, the material failure is predicted by the stress intensity factor ( KI) with some critical value (Kc). This value is called the critical stress intensity factor or the fracture toughness of the material for mode I depending on the problem geometry and the loading condition. Note that, this critical value (Kc) is commonly believed a material property. The structure will fail when the stress intensity factor (KI) exceeds the fracture toughness. Under certain circumstances, the second parameter in the near tip stress filed is necessary to be included in the K field to characterize the fracture toughness of brittle material. The main objective of this research is to study fracture mechanisms and investigate the degree of K-dominance zone of brittle material at multi length scale. The first part of this research is to determine the two-parameter model and also introduce alternative approach (namely, effective crack tip approach) to predict fracture load. The results showed that fracture load based on Kc- constant could be over or under predicted dependent on the degree of K-dominance zone. The second part of this study is to investigate Liner Elastic Fracture Mechanics (LEFM) of brittle materials at atomistic scale using molecular dynamics simulation. The evidences of this research showed that K-dominance zone also exists at nanoscale and the multiscale nature occurs in brittle solid. Furthermore, two-parameter model was proposed to predict apparent fracture toughness and strain energy release rate respectively. In the final part, we focused on fracture process zone (FPZ). The failure mechanisms of nano-structured brittle material were examined via the results from MD simulation. The stress intensity factor can still be used to characterize the occurrence of crack growth if fracture process zone (FPZ) length is contained well within the region of dominance of the singular elastic field which corresponds to the concept of the correlation of fracture process zone length and K-dominance zone size. If it is not valid, second parameter will need to be included for characterizing brittle materials.

Degree

Ph.D.

Advisors

Sun, Purdue University.

Subject Area

Aerospace engineering

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