Rissfortschritt in duktilen Stählen unter zyklischer Belastung: Einfluss des Herstellungsverfahrens und der Lastgeschichte

Translated title of the contribution: Crack propagation in ductile steels under cyclic Loading: influence of manufacturing process and the load history

Xiang Zhou

Research output: ThesisDoctoral Thesis

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Abstract

Due to its technical importance and complexity, numerous scientists are working on the field of fatigue of materials. Most of the estimations in practice are based on the results of laboratory tests. However, the laboratory results often significantly deviate from the real behaviour of components. One of the main reasons is that in most cases the fatigue crack emanates from flaws, for example a manufacturing defect, where certain residual stresses are present. The residual stresses are difficult to verify in laboratory tests and not considered in estimations. Another reason is the load history. A change in load amplitudes often causes a variation in the material properties, which can influence the following crack propagation behaviour drastically. The current work is focused on these two topics. All fatigue tests were carried out on deep notched 8-point bending specimens. The fatigue crack growth behaviour was determined on five similar steels with ultimate tensile strengths between 500 and 1000 MPa. The investigation of the influence of the pre-cracking process and the overload effect are analysed in one material. The crack growth behaviour was investigated by the R-curve for the threshold of the stress intensity range and the fatigue crack propagation rate. The step-wise increasing constant load method was used to obtain the R-curve. The crack length was measured by the potential drop method. The results of crack propagation in similar steels show that the flow stress or the ductility exhibits an influence on the crack propagation behaviour. The materials with higher ultimate tensile strength and lower ductility exhibit a steep rise in the R-curve for the threshold. However, the threshold remained almost unaffected. At higher range of stress intensity (ΔK), the materials with lower ductility show a faster crack propagation rate. The investigation of the effect of a pre-crack delivers the following results: To initiate a pre-crack using compressive-compressive loading, it is recommended to keep the applied ΔK as small as possible but use a sufficiently large number of cycles. Large amplitudes and an insufficient number of cycles causes a shift of the crack resistance curve to longer crack extension. In order to understand the effect of load history, the effect of overload on short cracks was studied in the current research. Overload in tension leaves a plastically deformed zone, which increases the crack resistance immediately thereafter. After the crack has grown through the plastic zone produced by an overload, this deformed area is still causing a premature contact of crack faces, which favors the formation of other crack closure mechanisms such as oxide-induced crack closure and increases thereby the R-curve for ΔKth over a long distance. On the other hand, the overload in compression leaves a plastically deformed zone, which favors the propagation of the fatigue crack. The crack growth resistance curve in this case is shifted to longer crack propagation, resulting in an R-curve for the threshold with a slow increase. However the long crack threshold is not affected.
Translated title of the contributionCrack propagation in ductile steels under cyclic Loading: influence of manufacturing process and the load history
Original languageGerman
QualificationDr.mont.
Supervisors/Advisors
  • Pippan, Reinhard, Assessor A (internal)
  • Antretter, Thomas, Supervisor (internal)
Publication statusPublished - 2015

Bibliographical note

no embargo

Keywords

  • Crack propagation
  • load history
  • overload effect
  • crack closure
  • precracking

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