Fracture Mechanics弹塑性断裂力学（PPT32）课件.ppt

LEFM and EPFM * Elastic-Plastic Fracture Mechanics Introduction When does one need to use LEFM and EPFM? What is the concept of small-scale and large-scale yielding? Contents of this Chapter The basics of the two criteria used in EPFM: COD (CTOD), and J-Integral (with H-R-R) Concept of K- and J-dominated regions, plastic zones Measurement methods of COD and J-integral Effect of Geometry Background Knowledge Theory of Plasticity (Yield criteria, Hardening rules) Concept of K, G and K-dominated regions Plastic zone size due to Irwin and Dugdal LEFM In LEFM, the crack tip stress and displacement field can be uniquely characterized by K, the stress intensity factor. It is neither the magnitude of stress or strain, but a unique parameter that describes the effect of loading at the crack tip region and the resistance of the material. K filed is valid for a small region around the crack tip. It depends on both the values of stress and crack size. We noted that when a far field stress acts on an edge crack of width “a” then for mode I, plane strain case LEFM concepts are valid if the plastic zone is much smaller than the singularity zones. Irwin estimates Dugdale strip yield model: ASTM: a,B, W-a 2.5 , i.e. of specimen dimension. LEFM cont. Singularity dominated region For =0 EPFM In EPFM, the crack tip undergoes significant plasticity as seen in the following diagram. EPFM cont. EPFM applies to elastoc-rate-independent materials, generally in the large-scale plastic deformation. Two parameters are generally used: Crack opening displacement (COD) or crack tip opening displacement (CTOD). J-integral. Both these parameters give geometry independent measure of fracture toughness. Sharp crack Blunting crack y x ds EPFM cont. Wells discovered that Kic measurements in structural steels required very large thicknesses for LEFM condition. --- Crack face moved away prior to fracture. --- Plastic deformation blunted the sharp crack. Sharp crac