Papers by Author: Jian Lu

Abstract: The crystal orientation, surface morphology, surface roughness and scratch properties of Au/NiCr/Ta multi-layered metallic films was examined by X-ray diffraction (XRD), atomic force microscopy (AFM) and a scratch test method, respectively. It was clarified that the surface morphology and surface roughness depend on the substrate temperature. The surface roughness decreases from 4.259nm to 2.935nm when substrate temperature changed from 100°C to 180°C, and then increases when substrate temperature above 180°C. The XRD revealed that there are only Au diffraction peaks with highly textured having a Au-(111) or a mixture of Au-(111) and Au-(200) orientation. The micro-scratch test reveals that both modes can be used for conventionally critical load determination, but the friction mode can additionally reflect the changes at different metallic film layers, the critical characteristic load was not sensitive to substrate temperature.

Abstract: The objective of this paper is to study the influence of residual stresses due to fabrication conditions on the thermomechanical behavior of carbon/epoxy laminate structures (cross ply). These studied laminates have undergone various cycles of thermal aging. The addition of a post-cure cycle after the end of the initial cycle makes it possible to reduce the residual stresses level. The incremental hole-drilling method is used to measure the residual strain in the laminates. These measured strains and the numerical calibration coefficients obtained by the finite element method allow to calculating the residual stress distribution in composite depth. The obtained results show that heat treatments of composite structures do not lead to an important reduction the initial residual stress due the fabrication conditions.

Abstract: A novel surface treatment has been developed in the present work to enhance the tribological properties of 316L Stainless Steel. This Technique involves the formation of a nanocrystalline layer ascribable to a grain refinement mechanism induced by repeated impact loadings supported by the surface. The resultant system has a layered structure, comprising nanometric grains (less than 100 nm) at the top and a strain hardened transition layer in the subsurface. Such a microstructural feature has the potential to significantlty enhance the surface hardness and to create a high compressive residual stress state. The tribological properties of the stainless steel are thus improved in terms of lower friction coefficient and increased wear resistance. Detailed studies on the response of the nanocrystalline surface layer to annealing at temperatures between 400°C and 600°C showed that an annealing at high temperature can offer much better tribological enhancement than low temperature annealings due to enhanced martensitic transformation.

Abstract: The aim of this paper is to determine the incremental residual stress profile in depth of a complex shape. The authors have adapted the hole drilling method in a crankshaft and more precisely in the filet area. A new set of calibration coefficients have been determined using the finite elements method in order to analyse residual stresses in the structure.

Abstract: By means of surface mechanical attrition treatment (SMAT), nanostructured (NS) surface layers were fabricated on a pure iron plate and a low carbon steel plate. Cr diffusion behaviors in the NS Fe phase and the SMAT low carbon steel were investigated. Experimental results showed the activation energy of Cr diffusion in the NS Fe is comparable to that of the GB diffusion, but the pre-exponential factor is much higher. A much thicker Cr-diffusion surface layer was obtained in the SMAT low carbon steel plate than in the coarse-grained one after the same chromizing treatment. The much enhanced diffusivities of Cr in the SMAT samples can be attributed to numerous GBs and triple junctions with a high excess stored energy in the NS surface layer.

Abstract: Improved mechanical behavior of surface nanostructured metallic materials produced by means of a surface mechanical attrition treatment (S.M.A.T) is investigated experimentally. Based on microscopic observations and residual stress measurements, factors leading to the high strength and yielding are discussed. The effects due to treatment, as compressive residual stresses, are in that way studied for a better understanding of their influence on the global mechanical response of the nanostructured material. In regards of this, a simple way to increase the ductility of such a nanostructured material is also presented.

Abstract: Residual stresses play an important role on the mechanical behavior of composite laminate. The development of new methods to determine the residual stresses gradient within the laminates is necessary. This article presents the adaptation of the compliance method in the case of composite laminates carbon/epoxy [02/902]s. The incremental drilling of a constant width groove allows for each increment to measure the strains (using strain gages) and displacements (using an optical device) of particularly points of the structure surface. These experimental data are compared with results given by a finite elements simulation. This comparison allows to raise the residual stresses in the composite laminate.

Abstract: Shot peening is an effective method of improving fatigue performance of machine parts in the industry by producing a thin surface layer of compressive residual stresses that prevents crack initiation and retards crack growth during service. Nondestructive evaluation of the prevailing compressive residual stresses in the shallow subsurface layer is realized by the critically refracted longitudinal (Lcr) waves. This paper presents experimental data obtained on SMAT (surface mechanical attrition treatment) steel alloy S355 sample. Comparative travel-time shows that there are statistically significant differences in treated and untreated specimen. With knowledge of the acoustoelastic constants which are obtained by a test calibration, the experimental data indicates that compressive residual stresses are distributed near subsurface (hundreds of micron). These stress results show that the Lcr technique is efficient for evaluation of residual stresses after the surface treatment.

Abstract: The aim of this work is to use ultrasonic shot peening, a mechanical surface treatment derived from conventional shot peening, in order to increase the fatigue life of TiAl alloys. The goal of this treatment is to generate compressive superficial residual stresses which are aimed to enhance fatigue crack initiation and growth resistance. For this purpose, different ultrasonic shot peening tests have been carried out on Ti-48Al-2Cr-2Nb samples in order to optimise treatment conditions. The first results reveal that it is possible to generate very high stress levels (»1000 MPa) beneath the surface, far much higher than the tensile yield stress of the material which is in the range 350-600 MPa. Such a phenomenon was also observed in ultrasonic shot peened iron or stainless steel and seems to be associated to the creation of a new homogeneous and nanometric structure below the surface of the alloy [1, 2]. In the light of these encouraging results, the shot peening treatment was optimised in terms of residual stresses profile and surface quality. The influence of different parameters of shot peening like the treatment time, the shot diameter as well as the specimen-sonotrode distance were studied. S-N curves were realised on polished specimens as well as on shot peened samples in order to study the effect of the treatment on the fatigue life of this intermetallic alloy.

Abstract: Residual stresses distribution after shot peening and its relaxation during uni-axial loading were investigated experimentally and theoretically on stainless steel of AISI304. An analytical model was proposed based upon continuum plasticity theory and elasto-plasticity finite element (FE) analysis, in which both relaxation of shot peening induced residual stress and variation of yield strength in the surface layer were taken into consideration. The results show that the value of yield strength in surface layer is altered after shot peening, and the relaxation of residual stress takes place at the point where the combination of applied strain and residual strain exceeds the true value of yield strength. A fundamental agreement was proved between measurements and predictions in simple tension and compression.