The aim of the project B01 is to investigate the influence of a preexisting ductile damage on a material’s fatigue properties. On the basis of fatigue tests, it will be determined to what extent a current forming-induced damage reduces the service life of a component compared to a non-deformed reference state (initial state).

The scientific objective of the subproject is to qualify a short-term testing methodology to monitor the interaction between ductile damage and fatigue damage to facilitate an assessment of the material in terms of its service properties. The methodology to be developed is based on an instrumented multi-step fatigue test which has already been successfully used by the project coordinator for the evaluation of fatigue properties of metallic materials. The in-situ monitoring is accomplished by the use of an enhanced measurement setup including sensors for stress-strain hysteresis measurement in order to detect temperature changes and changes of the electrical resistance caused by deformation. A locally resolved assessment of the strain and temperature fields is carried out by means of optical measurement systems. Thus, the damaged material regions can be localized on the basis of strain concentration and local heat sources. The affected areas are examined in detail employing a multi-parameter micro-magnetic analysis to assess their microstructural composition and the local degree of damage. The damage evolution is experimentally investigated, starting from a metallographic analysis of the test objects carried out in association with the project partners in section B. In addition, test series are conducted on heat-treated test objects in order to separate the effects of work hardening from damage caused by geometrical defects such as pores and micro-cracks.

 

170721 TPB01 en

Proposed experimental method for the analysis of the cyclic damage evolution as a function of ductile pre-damage

The primarily evaluated material in TP B01 is 16MnCrS5 which is commonly used in bulk forming. A transfer of the developed strategy to the sheet metal DP800 is planned. In the second half of the first funding period, an assessment is carried out along the process chain of bulk forming in order to generate a basic understanding of the interaction of the individual forming processes with regard to the achieved material state and its intrinsic damage. The results are transferred to a process-damage-property-relationship based on established analytical approaches for the mathematical description of the material behavior under alternating stress. The identified relationship is scheduled to serve as a tool for the design of damage-controlled forming processes in the second funding period.

In the second funding period, the research will focus on the fatigue behavior of materials 16MnCrS5 and DP800 as well as on very high-cycle fatigue tests (VHCF, Nf >> 1E7). In the third funding period, the behavior of component-like test objects will be examined and a strategy for quality assurance in industrial practice will be developed based on the previously established methods.

Project leader
Prof. Dr.-Ing. Frank Walther
Department of Materials Test Engineering (WPT), TU Dortmund University

Project coordinator
Lukas Lücker M. Sc.
Department of Materials Test Engineering (WPT), TU Dortmund University