Characterization of fatigue crack growth with the configurational force concept

Walter Ochensberger

Research output: ThesisDoctoral Thesis

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Abstract

This thesis deals with the characterization of crack propagation in cyclically loaded elastic–plastic materials in a new way, namely, by using the concept of configurational forces. Crack extension under cyclic loads, i.e. fatigue crack growth, is the most common failure mechanism in engineering. In order to assess the lifetime of cyclically loaded components it is necessary to predict the growth rate of fatigue cracks. Cracks under low-cycle fatigue conditions and short fatigue cracks are of great practical importance, but cannot be treated with the conventional stress intensity range DeltaK concept, since linear elastic fracture mechanics is not valid. An engineering approach is to apply the experimental cyclic J-integral DeltaJ^exp in such cases. However, the conventional J-integral is based on deformation theory of plasticity, which is not applicable for cyclic loading and crack extension due to the non-proportional loading conditions. Therefore, severe doubts arise whether DeltaJ^exp is appropriate to characterize the growth rate of fatigue cracks. The concept of configurational forces provides an elegant solution to this problem, since it enables the derivation of a J-integral for real elastic–plastic materials with incremental theory of plasticity. This new type of J-integral, J^ep, keeps, in contrast to the classical J-integral, the physical meaning of a true thermodynamic driving force term in elastic–plastic materials and is applicable even under strongly non-proportional loading conditions. However, J^ep is, in general, path dependent. The aim of the current thesis is to find out, how J^ep can be used for the evaluation of the driving force of a fatigue crack in elastic–plastic materials. A cyclic J-integral term DeltaJ^ep is determined from the variations of J^ep during whole load cycles. The path dependence of J^ep is investigated by analyzing the distribution of configurational forces in two-dimensional fracture mechanics specimens with long cracks under cyclic Mode I loading. Hereby the configurational forces and the values of J^ep are computed by a post-processing procedure after a conventional finite element stress and strain analysis. Stationary and growing cracks are considered. Different load ratios, between pure tension and tension-compression loading are investigated. Loading parameters are varied in order to study the influences of contained and uncontained plasticity on the properties of DeltaJ^ep. The results provide a new, physically appropriate basis for the application of the J-integral concept for characterizing fatigue crack growth in the regime of non-linear fracture mechanics. It is shown that the experimental cyclic J-integral DeltaJ^exp, which has been strongly challenged up to now, is correct for a stationary fatigue crack. It is not strictly correct, if the fatigue crack propagates. In addition, it is demonstrated that the new parameter DeltaJ^ep is also able to accurately reflect crack growth retardation following a single overload. Moreover, in combination with a configurational force analysis, new insights are obtained into the most important mechanism for the overload effect, which is still a contentious issue among fatigue experts.
Translated title of the contributionCharakterisierung von Ermüdungsrisswachstum mittels Konzept der konfigurellen Kräfte
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Fischer, Franz-Dieter, Assessor B (internal)
  • Kolednik, Otmar, Assessor A (internal)
Publication statusPublished - 2015

Bibliographical note

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Keywords

  • Configurational force concept
  • Crack driving force
  • Cyclic J-integral
  • Low-cycle fatigue
  • Fatigue crack growth
  • Overload effect

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