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Full-field characterisation of the deformation near fatigue crack tips using advanced imaging and diffraction techniques

Student thesis: Doctoral Thesis

Critical engineering components and structures are often exposed to cyclic loads and hence susceptible to fatigue fracture. The integrity of many components and structures is assessed following a damage tolerant approach, allowing fatigue cracks to grow to a sub-critical size before repairing or replacing a part. Commonly, damage tolerant approaches are based on linear elastic fracture mechanics without considering the plastic deformation known to occur at the crack front, which may have an influence on steady-state crack propagation. On the other hand, shielding effects in the crack wake, such as closure, are often included in crack growth models, although their impacts on fatigue crack growth behaviour have by large not been directly investigated. Full-field studies of a fatigue crack tip have, until very recently, been restricted to limited model materials and mostly to surfaces, whilst the 3D bulk behaviour around a fatigue crack tip has rarely been examined. This work aims to address these issues by revisiting fatigue crack growth using imaging and diffraction techniques on selected engineering materials.

Near-tip strain fields of fatigue cracks have been examined in the heat-affected zone of a welded CrNiMoV steel joint using Digital Image Correlation. A systematic error assessment was carried out on the measurements of displacements and strains; and the effects of selected key parameters on the random and systematic errors with and without loads are reported. An “optimal” subset size was found to be about 5 times the speckle or feature size as a result of the trade-off between systematic and random errors. Random errors appear to be predominant under the selected experimental conditions. A step size of ¼ of the subset size or smaller is recommended. The following experiments could hence be carried out at low error levels and with confidence about their precision and accuracy.

The fatigue crack growth behaviour was subsequently examined using in situ image correlation on the surface and in situ Energy-Dispersive X-ray Diffraction in the interior of the specimen. Normal strains ahead of the crack tip were mapped during fatigue crack growth to capture a characteristic “critical” failure strain for the onset of crack growth at selected observation points along the crack path, both on the surface and in the bulk of the specimen for the first time. Strain accumulation as well as critical strains of 1.6% on the surface and 0.6% in the bulk were found during steady state fatigue crack growth, supporting the development of a strain-based crack growth propagation model, and demonstrating that predictive models based on physical quantities instead of empirical data are possible.

The effects of crack closure on fatigue crack growth in 3D were examined using Digital Volume Correlation near the surface and in the interior of ductile nodular cast iron. The crack opening displacements behind the crack tip, the evolution of the near-tip strains ahead of the crack tip and the J-integral were measured as a function of load. No clear evidence for crack closure was found either along the crack wake, nor ahead of the crack front. Therefore, no conclusion could be drawn on the effect of closure on the deformation field around the crack front for the case studied. However, significant amounts of plasticity were measured at low stress intensities, representing up to 50% of the fracture energy at stress intensities below 12 MPa√m. This contradicts previous studies and indicates that a linear-elastic approach may not be suitable for quantifying closure effects in this material.

Although bridging has been observed in microstructural studies of nuclear graphite, a quasi-brittle material, its impact on the global crack behaviour under cyclic or sustained cyclic loadings has not been examined. 3D full-field studies have been carried out using volume correlation to obtain the crack opening displacements, the near-tip strains ahead of the crack tip and the crack driving force in a Brazilian disk specimen under mode I, mode II and mixed mode I+II loading conditions. The evolution of strains and the J-integral during cyclic loading were studied using Finite Element analysis, evoking a damage-plasticity model. The results shed some light on the lack of quantifiable effects of bridging on the global crack tip response, but also on potential interlocking effects discovered in pure shear mode. The general assumption that such microstructural events have shielding effects on crack growth in quasi-brittle materials can hence not be upheld without reservation. Further, the evolution of accumulated residual strains during dwell loading suggests the occurrence of creep effects in the material, which may contribute significantly to the improvement of current fracture models.
Original languageEnglish
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Award dateMar 2019

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