Dislocation-based entropy as a criterion for fracture

Abstract

Griffith’s 1920 fracture criteria is well known in the engineering community. Griffith produces a simple formula for the fracture stress of a material using the first law of thermodynamics by relating the decreased elastic energy of an applied load to the increased surface energy of a crack. However, evidence in fatigue research, most notably the concept of fracture fatigue entropy (FFE), suggests that the fracture mechanism is more intimately related to irreversibility and can be treated via the second law of thermodynamics. In this paper, Griffith’s model is approached from the perspective of the second law of thermodynamics, producing a new equation for fracture stress centered on entropy. Fracture tests are performed on 304 stainless steel (SS) and 110 copper specimens. Experimental crack lengths at fracture determined via digital image correlation (DIC) are compared to predicted crack lengths at fracture determined via the entropic fracture stress relation to assess accuracy. Predicted crack lengths at fracture are determined to be accurate to within 2.88% for 304 SS specimens and 7.62% for copper specimens, both of which are comparable to the corresponding accuracy of Griffith criteria predictions. Further, it is shown that the developed entropy model is capable of predicting the entropy-based debonding strength of a material, matching experimental debonding strength values evaluated via DIC to within 25%.