Damien Texier

Comparison of Two X-Ray Residual Stress Measurement Methods: Sin2 ψ and Cos α, Through the Determination of a Martensitic Steel X-Ray Elastic Constant

a dorian.delbergue.1@ens.etsmtl.ca, b damien.texier@etsmtl.ca, c martin.levesque@polymtl.ca, d philippe.bocher@etsmtl.ca Abstract. X-ray diffraction technique for residual stresses measurement is usually associated to the sin 2 ψ method, a method based on the interception of the diffraction cone and line detectors. To overcome this loss of information, the cos α method is an alternative method which uses a single exposure to collect the entire diffraction cone via a 2D detector. The present paper compares both sin 2 ψ and cos α methods, through the X-ray elastic constant (XEC) determination of a quenched and tempered martensitic steel. The full-cone measurement method demonstrates a smaller scatter and a better repeatability of the measurements. This latter point is of considerable interest since larger scatter in XEC may result in large variation in residual stress values, especially at high stress levels.

Microstructural statistics for fatigue crack initiation in polycrystalline nickel-base superalloys

In advanced engineering alloys where inclusions and pores are minimized during processing, the initiation of cracks due to cyclic loading shifts to intrinsic microstructural features. Criteria for the identification of crack initiation sites, defined using elastic-plastic loading parameters and twin boundary length, have been developed and applied to experimental datasets following cyclic loading. The criteria successfully quantify the incidence of experimentally observed cracks. Statistical microstructural volume elements are defined using a convergence approach for two nickel-base superalloys, IN100 and René 88DT. The material element that captures the fatigue crack-initiating features in René 88DT is smaller than IN100 due to a combination of smaller grain size and higher twin density.

Micromechanical testing of ultrathin layered material specimens at elevated temperature

Abstract The mechanical characterisation of ultrathin specimens at very high temperature is challenging in terms of specimen preparation and mechanical testing under ‘inert’ atmospheres. An experimental procedure for the local characterisation of mechanical and thermal properties at elevated temperature is presented. Various common ‘inert’ atmospheres were investigated up to 1373 K in order to identify the most efficient environmental conditions to prevent surface reactivity and degradation of specimens. A NiCoCrAlYTa-coated monocrystalline Ni-based superalloy was used to exemplify the capabilities of the technique because of its high surface reactivity at very high temperature, i.e. oxidation and sublimation. Purified overpressured argon atmosphere combined with oxygen getters was found to be particularly suitable for the study of alumina-forming alloys at elevated temperature.

Surface Roughness Effects on the Fatigue Behavior of As-Machined Inconel718

Surface finish of machined components plays a key role in their life performance. The aim of this research is to investigate the effect of different roughness parameters on high cycle fatigue behavior of Inconel718. Rotating bending fatigue tests have been performed on Inconel718 specimens with various surface roughnesses produced by turning. Height and amplitude distribution roughness parameters were investigated. Statistical analyses show that a valley material component (Mr2), as one of the amplitude distribution parameters, is the most relevant parameter for the high cycle fatigue life of machined specimens. Observations conducted at the surface of broken specimens gage length, have shown the impact of surface roughness and residual stresses on the crack propagation mode. When the roughness increases, valleys were shown to be deeper and larger, leading to a higher Mr2 value and an increase of stress concentration.