Elasto-plastic Fracture Mechanics Approach Research Articles

Elastoplastic Fracture Mechanics Approach to the Crack Growth Rate Computation of Modified Pipe Steels

Elastoplastic Fracture Mechanics Approach to the Crack Growth Rate Computation of Modified Pipe Steels

Assessment of Pipe Defects Using a Constraint-Modified Failure Assessment Diagram

In this study, the failure assessment diagram (FAD) is constraint modified using the assumption that the constraint T stress is proportional to the non-dimensional loading. The constraint-modified FAD has been used to compute safety factors associated with spherical and elliptic defects and a long notch. The results were compared to the classical approach, indicating the possibility of a reduction of conservatism. In addition, the use of domain constraint-modified FAD indicates that in some cases, increases in fracture toughness due to the loss of constraint lead to the use of limit analysis rather than an elasto-plastic fracture mechanics approach.

Effect of Pre-Strain on Low Cycle Fatigue Strength of Austenitic Stainless Steel SUS316NG

Low cycle fatigue tests were performed for austenitic stainless steel, SUS316NG. The effect of pre-strain history on the crack growth was investigated in detail using cellulose acetate replicas. In strain range, Δe = 2.5 % and 8 % constant tests, cracks initiated from slip bands near grain boundary, while intergranular crack initiations were prevailing when Δe = 16 % constant test. The cracks grew with several coalescences until the final fracture. Pre-strains were applied at Δe = 8 or 16 % followed by fatigue loading at Δe = 2.5 %.These tests revealed that contribution of cyclic strain hardening and softening were remarkable depending on the pre-strain history. Crack growth characteristics were well evaluated by J integral range, ΔJ, which is an elasto-plastic fracture mechanics parameter. Crack growth characteristic under Δe-constant tests as well as the pre-strained tests coincided with that measured by fracture mechanics experiment. Fatigue crack growth rates were predicted by elasto-plastic fracture mechanics approach in conjunction with cyclic stress-strain response.

Fatigue Analysis of Nuclear Welded Structures Based on Structural Stress and Elasto-Plastic Fracture Mechanics Approach

In the paper, the validity of the modified mesh-insensitive SS (structural stress) procedure to apply to the welded joints with local thickness variation is identified via the comparison of SCFs (stress concentration factors) calculated for various FE (finite element) models. FCI (fatigue crac kinitiation) cycles are determined by using the SS/EPFM (elasto-plastic fracture mechanics) approach and the various fatigue crack growth models. Fatigue test is performed to identify the validity of the fatigue analysis results. Finally, as a result of comparison among test and various analysis results, it is found that the SS/FM (fracture mechanics) approach agrees well with the fatigue test results over all cycle regions and the SS/EPFM approach is more reliable than the SS/LEFM (linear elastic fracture mechanics) approach.

The Use of Crack-Tip Opening Displacement for Testing of the Hydrogen Embrittlement of High-Strength Steels

Stress-corrosion cracks are, as a rule, brittle and often encountered under stresses much lower than the yield strength. For this reason, the methods of linear elastic fracture mechanics (LEFM) can be used for the investigation of stress-corrosion cracking (SCC). However, in some cases, these methods are inapplicable, and it is necessary to use the methods of so-called elastoplastic fracture mechanics (EPFM). In the EPFM approach, the J-integral is the most commonly used parameter for correlating crack initiation and propagation but the crack-tip opening displacement (CTOD) and crack-tip opening angle (CTOA) prove to be promising alternatives, especially for thin-sheet materials. Since both these parameters are connected with the crack geometry and, hence, reflect the level of strain at the crack tip, they appear to be useful correlation parameters for the cases of SCC, where the level of strain in the vicinity of the crack tip and, in particular, the strain rate, are the determining variables of the process. The hydrogen embrittlement of a higher-strength structural steel and welded joints of a C-Mn steel is assessed by using the CTOA and CTOD methodologies. In constant-extension-rate tests (CERT), fatigue precracked specimens were loaded with various low strain rates and electrolytically charged with hydrogen. It was discovered that hydrogen embrittlement significantly affects the crack-growth resistance curves (R-curves) thus generated and the opening angle for which the crack propagates into the material.

Toughening of polypropylene by different elastomeric systems

Rubber toughening of a series of blends constituted by a polypropylene (PP) matrix added with talc, and modified by ethylene–propylene (EPR) and/or ethylene–butene rubber (EBR), with different molecular weights, was investigated. The fracture toughness was measured by an elasto-plastic fracture mechanics approach, applying the methodology of the essential work of fracture, and by conventional testing. It was found that: (i) EBR has a higher toughening efficiency than EPR; and (ii) the fracture toughness increases by increasing the molecular weight of the dispersed elastomeric phase. Measurements of volume change and temperature increase in specimens stretched during tensile tests indicated that the presence of EBR in the blends reduces cavitation and crazing, and induces a large amount of shear yielding. Some difference in the microcavitation mechanisms induced by each of the two elastomers was shown by electron microscopy analysis. An explanation of this behaviour on the basis of structural changes within the matrix or at the rubber–matrix interface was sought. Dynamic thermomechanical analysis indicated that EBR has a stronger interaction than EPR with the PP matrix.

AN ELASTO-PLASTIC FRACTURE MECHANICS APPROACH TO FATIGUE CRACK PROPAGATION

The elasto-plastic fracture mechanics approach to fatigue crack propagation gives the following relationship ; Δa/Δn = c'·w*m L or Δa/Δn = c·wmL, where, Δa/Δn ; propagation rate of fatigue crack, w*L, wL ; cyclic plastic zone length and monotonic plastic zone length at crack tip, respectively (w*L∝wL is assumed), C, C', m ; material constants, which are related to the ductility, elastic modulus and yield strength. In addition to above facts, authors insist that the plastic constraint around the crack tip is important in fatigue crack propagation, for the following reason ; the larger the plastic constraint, the faster the propagation rate of fatigue crack, because the crack propagates in plane strain state without ductility ; and the less the plastic constraint in plane stress state, the slower the rate. The transition phenomena exists between both states, depending on plastic zone width and the thickness of the plate. Above mentioned facts can be proved experimentally.