• Charkaluk, Eric
• Limodin, Nathalie
• Bartali, Ahmed El
• Wang, Long

Abstract

The economical constraints joined to industrial and environmental requirements in terms of weight and costs reduction involve improving the mechanical properties of structural materials, in order to reduce the safety coefficients used in structure calculations. This involves refining the constitutive laws, and thus knowing and understanding the strain and damage physical mechanisms at the different scales. Many studies aimed at understanding these strain and damage micro-mechanisms at the microstructural scale in LCF. Some rely on surface techniques (AFM, EBSD, DIC) and others on bulk techniques (TEM, XRD). The present work proposes to analyse the strain field measured by digital image correlation on the microstructural scale at the surface of a specimen strained in LCF. The complete methodology is first illustrated on duplex or/and 316L stainless steels, which are used in industrial domains where severe environmental conditions and high mechanical loadings are brought together (offshore, chemical, petrochemical, paper industries…). Surface observations have been performed on a fatigued standard specimen in order to follow development and localisation of cyclic plastic strain, to identify microcrack initiation sites, and to follow the micropropagation at the surface. These in situ observations combined with an EBSD analysis, performed before testing, allow identifying activated slip systems in each phase. The morphology of the slip markings observed at the surface was characterised by roughness measurements using an interferometric profilometer. Kinematical field calculations allow revealing strain heterogeneities at the grain scale, evaluating cumulated plastic strains and their scattering in each phase/grain, and revealing the probable crack initiation sites before occurrence. Recently, these experimental surface techniques were also extended to study the influence of the casting microstructure on the mechanical properties of aluminium alloy automotive parts produced by the lost foam casting process. Indeed, the casting microstructure, which consists in hard eutectic phases and large pores and microshrinkages, has a major influence on the fatigue properties. To study this influence, full field measurements at the microstructure scale are coupled to the characterization of the fatigue specimens with X-ray microtomography prior to the fatigue test. The aim of this coupling is to link surface observations of plastic localization to the inner microstructure.

Topics

• impedance spectroscopy
• pore
• morphology
• surface
• polymer
• grain
• stainless steel
• phase
• x-ray diffraction
• atomic force microscopy
• aluminium
• crack
• fatigue
• aluminium alloy
• transmission electron microscopy
• casting
• electron backscatter diffraction