It is estimated that approximately 40% of demand for steel comes from failed products that need to be replaced. Although steel in general can be recycled, a startlingly large amount is prematurely recycled before the actual deterioration of its structural integrity. This leads inevitably to potentially avoidable premature recycling, which consumes enormous amounts of energy and effort and the high carbon footprint involved cannot be gainsaid as well. This is especially important in case of quenched and tempered steels which are employed in load bearing parts with high hardness and fatigue strength as well as impact strength requirements. Conventionally used methods, such as ultrasound and X-rays facilitate the detection of macroscopic cracks, but they can only provide insufficient information regarding the potential for remaining fatigue life in steel components already in use.

Within the scope of a current research project, which is currently being carried out in cooperation between the Steel Institute (IEHK) of RWTH Aachen University and the Department of Materials Science and Materials Testing (WWHK) of the University of Applied Sciences Kaiserslautern, the re-use potential of steels and steel structures is being considered using the example of a quenched and tempered SAE 4140 steel, in order to be able to use them directly in a second application after their initial use, without having to re-melt the material.

It is important to note that the development of fatigue damage in the High-Cycle-Fatigue (HCF) regime occurs predominantly in the area of the surface of the component. To detect this in the fatigue tests and to characterize the cyclic deformation behavior of the specimens under cyclic loading, the specimens are instrumented in the fatigue tests with various optical, thermographic, resistometric and micromagnetic measurement techniques.

To provide an initial database in terms of S-N curves, the fatigue tests are carried out and evaluated according to the StressLife and SteBLife methods. Based on these data, pre-damage states are defined into which the remaining specimens are transferred, to then further load them in the Very-High-Cycle-Fatigue (VHCF) regime using high-frequency testing techniques. To quantify the pre-damage in the material, supplementary analytical methods such as micro-hardness measurements and light electron microscopy as well as scanning electron microscopy are used.

The overall aim of this research project is that the information gained can be used in an overarching model for the residual life assessment of materials and structures already in use.