Crack closure and stress intensity factor measurements in nodular graphite cast iron using three-dimensional correlation of laboratory X-ray microtomography images

Abstract

3D tomographic images of a nodular graphite cast iron obtained using a laboratory X-ray source were used to analyze the opening of a fatigue crack during in-situ mechanical loading. Direct image analysis and digital image correlation are utilized to obtain the 3D morphology and front location of the crack as well as the displacement fields in the bulk of the specimen. From DIC results, it is possible to extract the Crack Opening Displacement (COD) map in the whole sample cross-section and to compute Stress Intensity Factors (SIF) all along the crack front even for COD values that are less than the image resolution. The comparison of COD maps with local values of SIF enabled for an estimation of the opening SIF, K op , equal to 6 MPa√m.

Influence of closure on the 3D propagation of fatigue cracks in a nodular cast iron investigated by X-ray tomography and 3D volume correlation

“…For the selected size of elements, displacement uncertainty was assessed to be about 0.04 voxel. The reader is referred to [18] for details about the methodology of uncertainty evaluation.…”

“…The difference is the residual error that directly provides a 3D image of the crack [18]. Measurement of the SIF along the crack front was carried out using a method introduced by Hamam et al [8] and adapted to 3D images [18]. The residual error is used to retrieve the crack position while the 3D displacement fields obtained with DIC are fitted in each plane orthogonal to the crack front with relevant mechanical equations in order to extract K I , K II and K III SIF values along the crack front.…”

“…the deformed image, cannot be perfectly matched to the reference image once it has been corrected for by the measured displacement field. The difference is the residual error that directly provides a 3D image of the crack [18]. Measurement of the SIF along the crack front was carried out using a method introduced by Hamam et al [8] and adapted to 3D images [18].…”

“…The material used in the present study was previously characterized using laboratory tomography during in-situ static tensile loading of a cracked specimen [18]. This analysis allowed for the quantification of crack closure through the determination of the opening stress intensity factor, K op .…”

“…Digital Volume Correlation (DVC) was also used to extract the COD maps in the crosssection of a pre-cracked specimen [17] while analysis of the measured 3D displacement field allowed for the SIFs to be computed [18]. It is to be emphasized that DVC, like the microstructural tracking method, can only be applied to materials with a suitable texture, i.e.…”