Test Method Microstructure-Oriented Fatigue Assessment of Construction Materials and Joints Using Short- Time Load Increase Procedure*

Physical measurement methods have been employed during the fatigue tests to determine the influence of joining, environment and manufacturing processes on the development of fatigue damage taking the applied loadings into account. The fatigue damage development determined in stepwise and continuous load increase tests serves as a basis for determination of suitable loading parameters in single step tests. The corrosion behaviour of the corrosion-susceptible light metals (aluminium, magnesium) and composites of metals and carbon fiber-reinforced plastics (CFRP) Lightweight materials are distinguished by good mechanical properties combined with low density and faces challenges in many areas of new joining technologies. In addition to the systematic microstructureoriented characterization of the fatigue behaviour, development of application-oriented measurement and analysis methods for component design and operational condition-monitoring with the aim of remaining lifetime assessment is another key area. On the basis of a precise characterization, evaluation and prediction of fatigue mechanisms taking into account the expected load and the corresponding fatigue life, component design can be optimized resulting in a reduction of safety factors. Besides promising part design and application-oriented material selection, the understanding of the fatigue performance of the employed materials and joints is of critical importance for a reliable and economic operation of cyclically loaded components. The fatigue behaviour is very closely related to the microstructure and characterized by the complexity of the interacting operations. Alternating stresses cause micro-plastic deformation in the material and characteristic dislocation structures by cyclic hardening and/or softening mechanisms. These dislocations can be the starting points of a fatigue crack. The fatigue-related property changes lead to crack initiation followed by crack propagation, and ultimately to failure. Stress-strain-hysteresis, change in deformation-induced temperature and change in DC-based electrical resistance measurements are applied for the detailed characterization of structural-mechanical processes in construction materials and joints under multiple-step and single-step fatigue loading. Results concerning the influence of joining technologies on austenitic steel AISI304, carbon-fiber reinforced polymers (CFRP) and beech wood materials, of environmental media on magnesium alloys Mg-4Al-2Ba-2Ca (DieMag422) and Mg-10Gd-1Nd, and of manufacturing processes on titanium alloy Ti-6Al-4V and wood-plastic composites (WPC) are discussed. The load-and cycle-dependent change in microstructure was investigated by light and electron microscopy and correlated with fatigue properties, to reach a preferably precise description of process structure property relationship in a qualitative and quantitative manner. The time-efficient load increase procedure applied for evaluation of joining, environmental and manufacturing influence on fatigue performance is suitable for production-accompanied usage