A B S T R A C T Compressive residual stresses are often deliberately induced at holes and notches to improve the fatigue performance of parts with these features. A modification to the FASTRAN II deterministic life prediction model was developed to account for the presence of compressive residual stresses. They were accounted for by replacing the opening stress as the closure mechanism with a closure mechanism consisting of the compressive residual stress and a reduced opening stress. An experimental program was developed to investigate the effects of residual stresses on fatigue lives. Compressive residual stresses were induced in the notches of 2024-T3 aluminium single-edge notch tension [SE(T)] specimens using tensile overloads. The residual stress distributions were depicted by finite element analyses. Four sets of specimens, each subjected to a different overload level, were tested under constant amplitude loading. Because of the variability observed in experimental fatigue lives, a probabilistic approach was used to predict the distribution of fatigue lives of the SE(T) specimens with compressive residual stresses. The variability of the experimental fatigue lives for the SE(T) specimens was predicted using the residual stresses, measured particle distributions for the material, a modified version of FASTRAN II, and conditional probability. The predicted fatigue life distributions for the four residual stress levels examined agreed with the experimental data with average errors between 5 and 15%.Keywords compressive residual stress, fatigue life prediction, crack growth modelling.
N O M E N C L A T U R Ea=half-length of the surface crack c=crack length in the width direction CDF=cumulative distribution function da/dN=crack growth rate E=elastic modulus f (a)=boundary correction factor Hz=cycles per second K I =mode I stress intensity factor K t =stress concentration factor, based on gross cross-section area N=cycles R=stress ratio defined as S min /S max R eff =stress ratio defined as (S min +S res /K t )/(S max +S res /K t ) S max =maximum applied stress S min =minimum applied stress S o =crack opening stress S or =modified opening stress S res =local compressive residual stress at notch root S tot =initial stress state of the notch surface defined as S tot =S max +S res /K t SE(T)=single-edge tension DK=stress intensity factor range DK eff =effective stress intensity factor range
The high cycle fatigue (HCF) resistance of Ti-6Al-4V for gas turbine engine pplications is studied when the material is first subjected to low cycle fatigue (LCF). The high cycle fatigue (HCF) threshold is determined after small LCF surface cracks are formed in notch tension specimens. LCF loading at two stress ratios, R = 0.1 and R -1.0, is used to initiate the LCF cracks, which are detected using direct current potential difference (DCPD). The surface crack sizes are measured under load using a static loading fixture and a scanning electron microscope (SEM). In addition to the SEM surface measurements, heat tinting is used to mark the crack profiles before HCF testing so that fractography can be used after failure to measure the 2D crack geometry. The LCF surface-cracked specimens are tested at room temperature in lab air at 600 Hz using a step-loading procedure at two stress ratios, R = 0.1 and R = 0.5. The LCF loading history is found to affect the HCF threshold compared to what is predicted from long crack threshold values obtained from other crack geometries. Variations in HCF crack growth thresholds obtained on specimens with LCF crack sizes from 25 to 175 μm are attributed to overload and underload effects from the LCF precracking.
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