A B S T R A C TThe effects of shot-peening intensity on fretting fatigue crack-initiation behaviour of titanium alloy, Ti-6Al-4V, were investigated. Three intensities, 4A, 7A and 10A with 100% surface coverage, were employed. The contact geometry involved a cylinder-on-flat configuration. Residual stress and improvement in fretting fatigue life were directly related to shot-peening intensity. The magnitude of compensatory tensile stress and its location away from the contact surface increased with increasing intensity. The relaxation of residual stress occurred during fretting fatigue which increased with increasing the number of cycles. An analysis using a critical plane-based fatigue crack-initiation model showed that stress relaxation during the fretting fatigue affects life and location of crack initiation. Greater relaxation of the residual stress caused greater reduction of fatigue life and shifted the location of crack initiation from inside towards the contact surface. Modified shear stress range (MSSR) parameter was able to predict fretting fatigue crack-initiation location, which agreed with the experimental counterparts. Also, the computed parameter showed an appropriate trend with the experimental observations of the measured fretting fatigue life based on the shot-peening intensity. a = half-width of a contact zone m = coefficient N = number of cycles N f = fatigue life s 11 = normalized longitudinal normal stress s 22 = normalized transverse normal stress s 12 = normalized shear stress S 11 = longitudinal normal stresses S 22 = transverse normal stress S 12 = shear stress S 12 max = shear stresses due to the maximum applied axial force S 12 min = shear stresses due to the minimum applied axial force S b max = maximum applied force of fatigue cycle S b min = minimum applied forces of fatigue cycle Correspondence: S. Mall.

No abstract

ABSTRACT--The x-ray diffraction technique has been used to measure surface residual stress in Ti-6AI-4V samples subjected to shot peening (SP), laser shock peening (LSP) and low plasticity burnishing (LPB). The magnitude, spatial and directional dependence and uniformity of the surface residual stresses have been investigated. The results show that residual stresses due to SP are uniform and independent of direction. LSP has been observed to produce non-uniform residual stress varying from one region to another, and also within a single laser shock. In the case of LPB, residual stresses have uniform spatial distribution but have been observed to be direction-dependent. Various components of the residual stress tensor in the LPB sample have been determined following the Dolle-Hauk method. The results of the residual stress due to three surface treatments are compared, and possible reasons for spatial and directional dependence are discussed.KEY WORDS--X-ray diffraction, residual stress, shot peening, laser shock peening, low plasticity burnishing

No abstract

No abstract

The evolution of fretting fatigue damage was investigated in shot-peened Ti-6Al-4V samples, by measuring the changes in the surface residual stress, using the X-ray diffraction technique. The surface residual stress was found to relax as the number of fretting fatigue cycles increased. The relaxation behavior of the residual stress with the increasing number of fretting fatigue cycles was observed to occur in three stages. In the first 20 pct of the fretting fatigue life, a drastic relaxation was observed. In the second part (between 20 and 70 pct), a gradually increasing behavior was observed. During the last 20 to 30 pct of the fretting fatigue life, a dramatic relaxation of the residual stress was found to occur. A complete relaxation of the residual stress occurred in the fracture region. A scanning electron microscope observation of the microstructure of the damaged region was used to examine the mechanisms leading to the relaxation of the residual stress. The development of delaminations at the early stages of the accumulation of the fretting fatigue damage was observed to be the main cause of the initial relaxation. The generation of microcracks from the voids left behind by the delaminations is responsible for the additional relaxation of the residual stress. The coalescence of the microcracks generated from different delaminated regions produced yet more relaxation of residual stress and, ultimately, the final fracture of the specimen.

No abstract

No abstract