John D. Dougherty

FATIGUE CRACK PROPAGATION AND CLOSURE BEHAVIOR OF MODIFIED 1070 STEEL : FINITE ELEMENT STUDY

Abstract A new finite element modeling methodology for simulating plasticity-induced crack closure is developed for the general purpose, commercially available, finite element code called ABAQUS. This new method utilizes substructuring techniques to maximize computational efficiency. This is achieved without sacrificing model accuracy in depicting the structural behavior of the specimen. Model refinement studies were conducted to establish guidelines for model parameters such as mesh density and element aspect ratio for the elements in the crack plane region. These techniques and guidelines were used to model a compact tension specimen and simulate a series of four tests conducted during the experimental portion of this study. Comparison of stationary crack and propagating finite element models revealed that plasticity-induced crack closure produces a significant amount of crack tip shielding. Such behavior effectively reduces the strain range and mean strain experienced at the crack tip. Direct comparisons between finite element model and experimental results were performed in terms of crack closure level, crack mouth opening displacement and surface strain gage data. These comparisons revealed excellent agreement. As a result of the study, use of an experimentally verified finite element technique in conjunction with fatigue crack growth experiments is suggested as a potential means of developing future crack growth rate models in terms of the traditional fatigue parameters of strain range and mean strain.

Fatigue crack propagation and closure behavior of modified 1070 steel: Experimental results

Abstract A combined experimental and finite element study of fatigue crack propagation and crack closure behavior, in a modified 1070 steel, has been conducted. In this paper, the experimental aspects of this study are presented and discussed. A comparison of crack closure measurement techniques using crack mouth opening displacement, back-face strain gage and a new surface strain gage method was performed. For two thicknesses of compact tension specimens, a series of constant maximum stress intensity and constant load ratio, constant load and constant load ratio, constant maximum stress intensity and increasing load ratio, single tensile overload, and conventional fatigue crack propagation tests were conducted. Implications of the influence of specimen thickness, crack length and test conditions on closure and crack growth behavior are detailed. Electron microscopy observations of the fatigue fracture surfaces were performed to assess the relative importance of crack path meandering, oxide-induced ind surface roughness-induced crack closure mechanisms. In the domain of stable crack growth, closure is dictated by the mechanism of plasticity-induced crack closure; while near-threshold behavior was found to be dominated by the conjoint and mutually interactive mechanisms of oxide-induced and surface roughness-induced crack closure. Test results reveal a significant influence of specimen thickness on closure and overall growth rate behavior. The salient advantages of the new surface strain gage method are elucidated and the conformance of experimentally measured and calculated growth rates is highlighted.

Cyclic stress response, strain resistance and fracture behavior of modified 1070 steel

A combined experimental and finite element study of fatigue crack closure in modified 1070 steel has been conducted. In this paper, the material property evaluations required for this study are presented. The monotonic and cyclic stress-strain properties, cyclic stress response, cyclic strain resistance, low cycle fatigue life and fracture behavior are examined. The low cycle fatigue tests were conducted using tension-compression cycling, under total strain amplitude control, over a wide range of strain levels. The material was found to possess medium strength and high ductility; while displaying a strain level dependent combination of cyclic strain softening and hardening behavior. The observed softening behavior is attributed to the rearrangement of dislocations produced by processing, formation of slip bands on the specimen surface and the formation of microcracks. The observed hardening behavior is ascribed to contributions from synergistic influences of dislocation multiplication, dislocation-dislocation interactions and dislocation-microstructural feature interactions. The material followed the strain-life relationships attributed to Basquin and Coffin-Manson. The fracture surfaces of the fatigue specimens showed distinct regions of crack initiation, microscopic-macroscopic crack growth and sudden fracture. The low-cycle fatigue characteristics and fracture behavior are discussed in the light of competing and mutually interactive influences of cyclic strain amplitude, concomitant response stress, intrinsic microstructural effects and dislocation-microstructure interactions during cyclic straining.

A combined experimental and finite element study of crack closure effects in modified 1070 steel

The significance and even the existence of crack closure is being questioned by several researchers. The objective of this study was to determine if crack closure occurs and to quantify its significance. An approach combining experimental measurement techniques with finite element analysis techniques was utilized. For two values of compact tension specimen thickness, a series of tests were conducted to determine the effect of maximum stress intensity, load ratio, constraint, and single tensile overload on the crack closure and fatigue crack growth behavior of a modified 1070 steel. Test results indicated that constraint has a significant influence on crack closure and crack growth rate behavior. Thin specimens exhibited consistently lower crack growth rates and higher crack closure levels than the thick specimens, except for tests conducted at a high load ratio, where crack closure did not occur. The thin specimens also exhibited a more significant overload effect. A new finite element modeling technique, which uses substructuring techniques to model the load cycling and crack propagation of an entire compact tension specimen, was developed. Comparison of stationary crack and propagating crack finite element models revealed that plasticity-induced crack closure produces a significant amount of crack tip shielding, which effectively reduces the strain range and mean strain experienced at the crack tip.