A B S T R A C T Although cold expansion has been shown to have a positive effect on fatigue life, little work to date has been performed to evaluate the evolution of the strain field surrounding the crack tip on the mandrel entry and exit faces in cold-expanded coupons. Full-field strain measurements using both digital image correlation (DIC) and thermoelastic stress analysis (TSA) were used to compare the strains during crack growth around holes that had and had not been cold-expanded by 4%. Simple rectangular fatigue coupons were machined from 2024-T3 aluminium sheet, half had simple open, unexpanded holes and half had cold-expanded open holes. A DIC system was used to monitor the (mandrel) entry face of the coupon while a TSA system was used to monitor the (mandrel) exit face of the coupon during cyclic tensile loading (R-ratio = 0.1). These two systems provide the first published measurements of the simultaneous mandrel entry and exit surface strains surrounding a crack initiating from a cold-expanded hole. The cracks originating from the unexpanded hole exhibit a strain field around the crack tip which is characteristic of a mode I fatigue crack while those emanating from the cold-expanded holes show a significantly lower distribution of strains around the crack tip. Crack opening displacements evaluated from the strain data obtained from DIC, are substantially reduced by the residual compressive stresses from cold expansion, while the overall fatigue data show the life improvement achieved at various applied stress levels.

The aim of this study was to quantify strain partitioning in HSLA-65 joints, welded with three types of weld filler: 6011 undermatched weld filler, 7018 matched weld filler, and 9016 overmatched weld filler. Strain measurements were made using a three-dimensional digital image correlation system while specimens were tested on a displacement controlled servo-hydraulic test frame. Crosshead displacement rates ranged from 0.056 mm/s to almost 56 mm/s. Coupons were cut from flux-core arc welded HSLA-65 plates to characterize the base metal, weld filler materials, and mismatched welds. Constitutive material properties could be extracted reliably for all coupons at the lowest displacement rate and showed that, as expected, all weld fillers had similar elastic modulus values but different yield strengths. A comparison of the peak strains at 85 per cent of maximum elongation showed that overall the failure strain was inversely related to the crosshead displacement rate. Analysis of the mismatched coupons found that the failure location was a function of weld filler and was independent of strain rate. Higher magnification imaging of the weld nugget showed that strain partitioning occurred within the weld zone, with slightly lower strains in the cap pass as opposed to the root pass.