Determination of Calibration Function for Fatigue-Crack Propagation by Measurement Surface Deformation

摘要

Components and structures exposed to elastic dynamic loading respond with elastic strains on the surface of the material. Mechanical response could be monitored by deformations on the surface. The measurements and monitoring of these parameters could be performed with electronic devices for on-line measurements, controlled by computerized systems. Fatigue-induced flaw growth was monitored on a four-point specimen, loaded by cyclic dynamic bend forces. The flaw growth was monitored by strain gauges with a standard resistance of 120 Ω. After performance of fractal-graphical measurements, a flaw-growth analysis was performed to determine the shape, propagation, and cross sections of the crack. To determine the stress-intensity factor, a numerical model was developed based on measured crack shapes, material properties, and cyclic loading data of the actual tested specimen. The investigation results showed that the derived calibration curve could be used to predict surface deformations as a result of crack propagation and growth, but not crack initiation. With the determination of surface deformation, one could follow the crack transition from the semi-elliptical surface crack to the through-thickness crack. The stress-intensity factor has been determined numerically by using the finite-element method for five different fatigue-crack fronts. Results show that fatigue crack on the surface of specimens propagated under an almost constant stress-intensity factor value. Consequently, in our case, the fatigue-crack growth rate was constant during transition from a surface semi-elliptical crack to a through-thickness crack front. The aim of this paper is to describe methodology and results based on experimental and numerical modeling during crack propagation and potential use of this technique for online monitoring purposes.