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2012
Conference Paper
Titel
In-situ characterization of damage evolution in welded aluminum alloy joints during cyclic deformation in the VHCF regime by means of nonlinear ultrosonics and thermography
Abstract
The ultrasonic fatigue testing system (UFTS) with working frequencies of 18-21 kHz is nowadays an established method for studying the cyclic behavior of materials at very high numbers of cycles (Nf > 107). Latest research activities in the Very High Cycle Fatigue (VHCF) area focus on damage mechanisms of various materials with and without defects. Defects are mainly analyzed at a microstructural level, such as non-metallic inclusions in high strength steels or pores in cast alloys. In this respect the significance of crack initiation in quasi defect-free materials and early crack propagation in defect-afflicted materials is discussed. However, a study of complex structures with macroscopic defects deriving from manufacturing processes were not yet part of the ongoing research activities and will therefore be discussed in this paper using the example of welded aluminum alloy joints. In the study presented welded samples were tested by means of UFTS. Crack initiation a nd crack growth were characterized by means of nonlinear ultrasonics and infrared thermography. The nonlinear ultrasonics technique allows for a registration of the formation of higher harmonics at frequencies exceeding the fundamental frequency ( 20 kHz) due to nonlinear elastic material behavior and/or so-called contact acoustic nonlinearity [1]. With the help of a thermal imaging camera the temperature gradient of the sample surface and thus the local damage evolution in the sample can be evaluated during fatigue tests. The results of both experimental methods applied - the nonlinear ultrasonics and the thermography - are compared and discussed with regard to the influence of macroscopic and microscopic notch effects in the welded joints and their effect on the life contribution of crack initiation and crack growth phase within overall fatigue life in the VHCF regime.