Cyclic plasticity of aerospace metals: I. Modelling of aluminium 7075-T6 for structural fatigue analysis, II. Experimental characterisation and modelling of additively manufactured Ti-6Al-4V

摘要

Aircraft structures, as is the case for many engineering structures, contain discontinuities such as holes and notches, which harbour the potential for fatigue crack initiation. At the root of such discontinuities, localised plasticity can occur even though the rest of the structure is experiencing elastic deformation. In the cyclic regime, a number of different cyclic transient effects can occur depending on the control mechanism (stress, or strain), which include progressive relaxation of mean stress, accumulation of strain with cycles (strain ratcheting), plastic shakedown, and cyclic hardening/softening. Strain-life methodologies have routinely been used in the aeronautical industry for many years for fatigue analysis of aircraft. However, the current strain-life fatigue methods apply a simplified method of cyclic plasticity calculation, namely the Masing plasticity model. Consequently, cyclic transient effects on fatigue life are not considered in the strain-life fatigue predictions. In addition, additive manufacturing (AM) presents a new challenge. AM is a promising new technology that can significantly alter how future aircraft structures are to be manufactured. One of the ground-breaking aspects of this evolving technology is the ability to manufacture material with microstructure composed of a wide range of crystallographic phases, which is only possible in traditional manufacture methods through post thermos-mechanical process. The technology begins to enhance the potential of tailoring the fabrication of components to contain specific microstructure to achieve the required mechanical properties for particular loading conditions, which has a significant effect on the future design process. This thesis enhances the current understanding of the application of elastoplastic models to strain-life fatigue predictions, through an experimental and computational investigation of 7075-T6, and a parameter optimisation scheme. Furthermore, it begins to develop the cyclic elastoplastic and low cycle fatigue behaviour of the titanium alloy Ti-6Al-4V manufactured using the selective laser melting (SLM) additive manufacturing process. The main findings of this research are the following: Cyclic elastoplastic behaviour of AA 7075-T6 An in depth understanding of macroscopic behaviour of AA7075-T6 was successfully linked to the extensive micro-mechanism analysis of the material which occurred of the past 40 years. The first asymmetric stress-controlled analysis of the material was also conducted, where a plastic shakedown of the strain ratcheting occurred. An initial cyclic softening was also noticed for first time in AA7075-T6, which occurred during the low peak stress tests. A closer inspection of the evolution of the effective stress and backstress in asymmetric strain-controlled results showed that a difference in the micro-mechanism occurring in tension and compression contributed to the slowing down of the relaxation rate, which was dependent on the applied strain amplitude. Elastoplastic constitutive model development and improvements to strain-life fatigue predictions Experimentally noticed plastic shakedown in AA7075-T6, can be successfully modelled through the modification of kinematic hardening rules to contain a linear backstress rule. A parameter optimisation workflow which could be applied to determine elastoplastic constitutive model parameters to be used to improve strain-life fatigue calculations was determined. The results of an extensive investigation into the potential improvements offered by the application of elastoplastic constitutive models to strain-life fatigue found that the Multicomponent Armstrong-Frederick (MAF) model improved the strain-life fatigue prediction accuracy. This was based on both statistical and deterministic methods of comparing the predicted and experimental fatigue lives calculated from the application of P-3C spectra. However, all the applied elastoplastic constitutive models improved the fatigue predictions compared to the traditional Masing model. Cyclic elastoplastic investigation of SLM Ti-6Al-4V Comparison with symmetric strain-controlled results obtained from mill-annealed Ti-6Al-4V coupons showed that the micro-mechanisms associated with an SLM ’ martensite resulted in quite different mean stress relaxation behaviour, where the mean stresses in the SLM martensite microstructure were relaxed faster than the mill-annealed microstructure. Additionally, through an experimental program which used test coupons fabricated at different build orientations, the mechanical anisotropy of SLM Ti-6Al-4V was observed in both monotonic and cyclic results, where the diagonally manufactured coupon had the largest yield stress in both, while the diagonal and horizontal coupons were more ductile than the vertical coupons. Elastoplastic constitutive modelling of SLM Ti-6Al-4V Different uniaxial elastoplastic features of SLM vertically fabricated martensite Ti-6Al-4V were shown to be successfully captured through the application of phenomenological elastoplastic constitutive models. The simulation results gathered for each of the tested kinematic hardening models demonstrated very good agreement with symmetric strain-controlled hysteresis loop development corresponding to the experimentally gathered results at 1%, 1.5%, 2%, and 2.5%. Furthermore, good simulations results were achieved for mean stress relaxation and strain ratcheting.