Plasticity Effect on Crack Tips with Help of IR-Camera

Abstract

The assessment of the mechanical performance of engineering materials subjected to high cycle fatigue requires very long and costly testing procedures. Therefore, there is a strong drive from industry to find alternative ways of evaluating high cycle fatigue performances. Under deformation the structural evolution is observed at all scale levels, and its development leads to irreversible deformation and failure. These processes are accompanied by accumulation and dissipation of energy. Most fatigue crack propagation studies have been conducted from either a continuum or micro-mechanics point of view. An energy approach could represent a solution. Recent developments in IR-camera permits to take the problem from an energetic point of view. The scope of the work is to perform a crack growth test according to ASTM E647 on a Compact Specimen and correlate energy production with the growth rate. The heat is composed by a thermo- elastic coupling term and by mechanical dissipation term. Energy dissipation in metallic materials subjected to mechanical deformation is a result of the movement of dislocations, deriving from plasticity at the crack tip, and generates a monotonously increasing variation in temperature during the test. Experimental data are contaminated by some noise, either because of the data acquisition process and because of naturally occurring phenomena (such as material inhomogeneity). A first pre-processing step in analysing such datasets is denoising, that is, estimating the unknown signal of interest from the available noisy data. Two ways for denoising have been performed. The first involved the use of classical spatial filtering and then filtering in the frequency domain. The second is a novel approach which employees wavelets. A MATLAB framework has been developed to automatize the process of data processing that is quite long. In this way new functionalities and changes of parameters can easily be made. Numerical transient simulations have also been performed to try to replicate the temperature variation. Both the case with only the thermo-elastic effect (linear material model) and with plastic heat dissipation (multilinear kinematic model) have been considered. Any discrepancies with real data might tell us what model is more accurate.