Residual stresses and fatigue performance

The effect of shot peening (SP) and deep rolling (DR) on fretting fatigue life of Aliminum7075-T6 is investigated in this work. The results show that fretting fatigue reduces the normal fatigue life by 67%. For low cycle fatigue, SP while being superior to DR, increases the fretting fatigue life by 300% for the specimens tested in this work. For high cycle fatigue, however, the effect of DR on fretting fatigue resistance is more profound than SP such that an increase of about 700% is observed for DR. Numerical simulation of SP and DR is also performed in this work. The results indicate that DR gives rise to a larger compressive residual stress than SP under the conditions considered in the simulations. This agrees quantitatively with the experimental results obtained from high cycle fatigue tests.

The effects of deep rolling and shot peening on fretting fatigue resistance of Aluminum-7075-T6

The effects of laser peening and shot peening on fretting fatigue in Ti–6Al–4V coupons

Metal forming is not only shaping the form of a product, it is also influencing its mechanical and physical properties over its entire volume. Advanced analysis methods recently enable accurate prediction of these properties and allow for setting these properties deterministically during the forming process. Effective measurement methods ensure the setting of these predicted properties. Several real examples demonstrate the impressive achievements and indicate the necessity of a paradigm change in designing products by including manufacturing-induced effects in the initial dimensioning. This paradigm change will lead to lightweight components and serve environmentally benign designs.

Metal forming beyond shaping: Predicting and setting product properties

Surface Engineering to Improve the Durability and Lubricity of Ti-6Al-4V Alloy

X-ray diffraction residual stress measurement has been utilized as nondestructive tool for the characterization of fretting fatigue damage in shot peened samples of Ti–6Al–4V. Prior to fretting fatigue damage, compressive residual stresses were found to be uniform over the entire face of the sample and independent of the measurement direction. After fretting fatigue, inside and in the vicinity of the fretting damage zone large relaxation of compressive residual stress was observed. An anisotropic residual stress distribution has been observed in the fretting fatigue damaged region. Residual stress measurements in interrupted fretting fatigue experiments showed that the relaxation of residual stress increases as the number of fretting fatigue cycles increase. The results are discussed in the light of their importance in establishing X-ray diffraction residual stress measurement technique as a nondestructive tool to characterize fretting fatigue damage.

Effects of fretting fatigue on the residual stress of shot peened Ti–6Al–4V samples

The 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.

Effects of shot‐peening intensity on fretting fatigue crack‐initiation behaviour of Ti–6Al–4V

The technique of measuring small changes in acoustic wave velocity due to external or internal stress has been used for quantitative determination of residual stress in materials during the last decade. Application of similar methodology with focused acoustic waves leads to residual stress measurement with spatial resolution of a few millimeters to a few microns. The high spatial resolution residual stress measurement required development of new methodologies in both the design of acoustic lenses and the instrumentation for acoustic wave velocity determination. This paper presents two new methodologies developed for the measurement of residual stress with spatial resolution of a few millimeters. The design of new type of acoustic lens for achieving higher spatial resolution in residual stress measurement is introduced. Development of instrumentation for high precision local surface wave velocity measurement will be presented. Residual stresses measured around a crack tip in a sample of Ti–6A1–4V using a focused beam will be compared with X-ray diffraction measurements performed on the same region of the sample. Results of residual stress measurements along a direction perpendicular to the electron beam weld in a sample of Ti–6A1–4V, determined using focused acoustic waves and X-ray diffraction technique, are also presented. The spatial resolution and penetration depth of X-rays and focused acoustic beams with reference to residual stress measurements are discussed.

Residual stress measurement with focused acoustic waves and direct comparison with X-ray diffraction stress measurements

Local variation in surface skimming longitudinal wave (SSLW) velocity has been measured using a scanning acoustic microscope. A very narrow width electrical impulse has been used to excite the transducer of the acoustic lens. This permits the separation of the SSLW signal from the direct reflected signal in the time domain. A simple method of measuring the time delay between the directly reflected signal and the SSLW signal at two defocuses has been utilized for the local measurement of SSLW velocity. The variation in the SSLW velocity measured over an area of the sample is scaled and presented as an image. The method has been implemented to image the variation of the SSLW velocity around a crack tip in a sample of Ti-6Al-4V. Since the SSLW velocity is known to change linearly with the stress, the SSLW velocity image is considered as a representation of the image of stress around the crack tip. Local stress variation in the same region of the crack tip is directly measured using x-ray diffraction. The SSLW velocity image is compared with the x-ray diffraction stress image. The contrast in the two images, spatial resolution, and the penetration depth into the sample of acoustic waves and x rays are discussed.

Local surface skimming longitudinal wave velocity and residual stress mapping

Several nondestructive evaluation (NDE) techniques are in use to inspect heat damage in organic matrix composites Majority of the techniques detect major damage like cracking, delamination and blistering An NDE technique sensitive to early stages of heat damage (“incipient” damage) is still elusive This paper presents, a novel non‐contact NDE method, based on detection and measurement of heat developed during the propagation of an acoustic wave through the material The methodology was used to characterize the heat damage in carbon fiber epoxy composite The results of temperature changes in heat damaged regions in the composite and its correlation with exposure time are presented

Shamachary Sathish

Papers

Effects of fretting fatigue on the residual stress of shot peened Ti–6Al–4V samples

Effects of shot‐peening intensity on fretting fatigue crack‐initiation behaviour of Ti–6Al–4V

Residual stress measurement with focused acoustic waves and direct comparison with X-ray diffraction stress measurements

Local surface skimming longitudinal wave velocity and residual stress mapping

Thermo‐Elastic Characterization of Heat Damage in Carbon Fiber Epoxy Composites