Essential work of fracture, crack tip opening displacement, and J-integral relationship for a ductile polymer film

Abstract

A unique set of double-edge notched tension specimens of a Polyethylene Terephthalate Glycol-modi¿ed ¿lm was tested in mode I, plane stress. The load was registered on a universal testing machine. The displacements, ligament lengths, and video frames were recorded by a Digital Image Correlation system. With these registered data, the essential work of fracture, J-integral, and crack tip opening displacement (CTOD) fracture concepts have been applied. The onset of crack initiation was through a complete yielded ligament. The analysis showed that the intrinsic speci¿c work of fracture, we, is the speci¿c energy just up to crack initiation, which is an initiation value. we has both a coincident value and the same con-ceptual meaning as Jo, the J-integral at the onset of crack initiation. The relationship between Jo and CTOD is also determined. The in¿uence on the notch quality when the specimens were sharpened by two different procedures, femtosecond laser ablation and razor blade sliding, was analysed in detail.Peer ReviewedPostprint (author's final draft

The post-yield fracture of a ductile polymer film: Notch quality, essential work of fracture, crack tip opening displacement, and J-integral

“…This does not happen with the femtolaser ablated specimens which does not have material volume accumulation at the root of the sharpened notch. The volume accumulation of plastically deformed material is small for low tough polymers and equal w e values can be obtained for the femtolaser and the razor blade sharpened specimens [18]. However, in the toughest polymers the razor blade sharpening can originate a large volume accumulation of material at the notch root, resulting higher values of w e compared with the femtolaser sharpened specimens [14].…”

“…High values of w e [22] and up to three times higher values of w e have been reported [23] for razor blade sharpened specimens when the notch tip was blunt or damaged. With razor blade sharpening is easy to generate a volume accumulation of deformed material, at the root of the sharpened notch [14,18,22]. This does not happen with the femtolaser ablated specimens which does not have material volume accumulation at the root of the sharpened notch.…”

“…(a) The ligament length is fully yielded prior to the onset of crack propagation. Full ligament yielding must show a load drop in the related load-displacement curves [13,18]. This requirement ensures that the fracture mechanism is the same irrespective of the ligament length.…”

“…Although Eq. (3) is a reasonable size criterion, it appears to be too restrictive considering the evidences encountered in amorphous copolyesters [13,18].For pure plane stress conditions, another method is based on the Hill criterion [19][20][21] but does not ensure full ligament yielding.…”

“…(c) Identical sharp notches without plastic deformation as material volume accumulation at the notch tip, and without thickness increase of the notch root. This requirement [18] guarantees the same value for the crack tip opening displacement at initiation and results in the overlapping of the heads of the nominal stress-displacement and self-similar ligament length-displacement curves for the specimens having different ligament lengths.…”