Combined mode I-mode III crack growth toughness of high density polyethylene

Abstract

As fracture mechanics has developed as a discipline, many parameters have been developed to characterize the instability condition. However, a majority of this work has been confined to the investigation of mode I fracture. Thus, we have standardized methods for experimentally determining K~o, J~, and J-resistance curves for mode I crack propagation. However, cracks in real materials can be subjected not just to tensile stresses but to complex stress states so that the development of suitable parameters to characterize mixed-mode crack initiation and propagation is important in the evolution of suitable design criteria. Further, observations indicate that initially flat cracks in some tough materials tend to reorient themselves to oblique planes during growth. For these materials, crack propagation can be said to occur under combined mode conditions. There has been a limited amount of experimental work done on mixed mode fracture. The observations on combined mode I - mode III fracture have been very scarce and there is no general agreement among authors on the effect of the addition of a mode III component to pure mode I loading. There has been some theoretical analyses of mixed mode I - mode II fracture under elastic plastic conditions. Shih [1] has analyzed the problem for small scale yielding under plane strain conditions. The difficulty is that under mixed-mode conditions in the elastic-plastic range, the fields arising from the various modes are nonlinear and not directly superimposable unlike under linear elastic conditions. The relationship between pure mode I and mode III fracture toughness values is also unclear. In view of all these factors there have been attempts made recently to delineate materials into different classes based on their shear susceptibility [2]. The material chosen for study was commercial grade high density polyethylene used in packaging applications. The average thickness of the film was 3 mm. The combined mode I - mode III fracture toughness tests were performed using modified compact tension specimens [3]. In these modified compact tension specimens the crack inclination angle 0 is defined as the angle between the plane of the crack and the loading direction. The J-integral was chosen as the most appropriate parameter to analyze the fracture of the polyethylene since polyethylene is known to exhibit large crack tip plastic zones. Manoharan et al. [3] have pointed out the modifications necessary to the ASTM E-813-87 standard in the case of mixed-mode fracture. The J-integral tests were conducted using the multiple specimen method in general accordance with ASTM standards with suitable modification [3] for mixed-mode fracture.