X‐ray fractography of a diesel engine crankshaft

Abstract

A B S T R A C T X-ray (XR) diffraction is a well-known experimental technique used for measuring residual stresses in metallic materials. If we apply the XRD technique to the fracture surface of a broken part, it becomes a fractographical technique, that is to say that it is possible to relate the results of the measurement to the loading condition that lead a component to fail. However, in the past this technique was mainly used to analyse standard specimens and not mechanical components and there are few experimental investigations concerning the possibility of using this technique to investigate the cause of fatigue failures. In this paper, after a brief introduction to the technique, XRD fractography is applied to a diesel engine crankshaft that failed under known fatigue loading. It was possible to determine the load that lead the crankshaft to fail and to evidence some original aspects about the application of this technique to real machine parts. Comparison with finite element results served to confirm that XRD can be used as complementary tool to scanning electron microscope (SEM) observation or as a substitute to SEM observation, in the case of damaged fracture surface. N O M E N C L A T U R Ed = interatomic spacing of a family of crystallographic planes E = elastic modulus FWHM = full width at half maximum of the diffraction peak K max = maximum value of the stress intensity factor R = stress ratio of a fatigue cycle r pc = cyclic plastic zone size r pm = monotonic plastic zone size UTS = ultimate tensile strength α = coefficient of relationship between the plastic zone size and the stress intensity factor K = stress intensity factor range ε = strain ψ = angle between the normal of the sample and the normal of the diffracting plane θ = Braggs' angle of the material σ ys = monotonic yield stress σ ys = cyclic yield stress I N T R O D U C T I O NX-ray (XR) diffraction is an experimental technique used in many scientific and industrial fields to measure different properties of materials. It can be applied to crysCorrespondence: M. Guagliano. talline and poly-crystalline materials and consists of measuring the angle of diffraction of an incident XR beam on a surface. In mechanical engineering, XR diffraction is mostly used to measure residual stresses in materials and machine parts: 1,2 the main advantages of this technique with respect to others is that it is non-destructive if used to obtain residual stresses on the surface and that