The multi-axial contour method is a recent development of the contour method of stress measurement. It permits measurement of the 3D residual stress distribution in a body, based on the assumption ...that the residual stresses are due to an inelastic misfit strain (eigenstrain) that does not change when a sample containing residual stresses is sectioned. The eigenstrain is derived from measured displacements due to residual stress relaxation when the specimen is sectioned. By carrying out multiple cuts, the full residual stress tensor in a continuously processed body can be determined, where the specimen has an initial length-to-width aspect ratio of 2:1. In the present study, first a finite element simulation of the technique was carried out to verify the method numerically. The method was then used to determine the residual stresses in a VPPA-welded sample, and the results were validated by neutron diffraction measurements.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A finite-element model of the interaction of an elastic–plastic asperity junction based on cylindrical or spherical asperities is used to predict sliding friction coefficients. The modelling differs ...from previous work by permitting greater asperity overlaps, enforcing an interface adhesional shear strength, and allowing material failure. The results of the modelling were also used to predict friction coefficients for a stochastic rough surface. The asperities were based on the titanium alloy Ti-6Al-4V, and the magnitudes of the predicted friction coefficients were generally representative of experimental measurements of sliding friction. The results suggest that friction arises from both plasticity and tangential interface adhesion.
► Model of sliding friction based on elastic–plastic asperities using finite-element analysis. ► Higher asperity overlaps, tangential adhesion, and asperity failure allow realistic friction predictions. ► Model based on Ti6/4: tests reveal friction coefficients within the range of model predictions. ► Explanation of sliding friction based on plastic deformation and tangential interface adhesion. ► Some level of tangential interface adhesion seems necessary to generate realistic friction.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SBCE, SBJE, UPUK
The present paper describes an experimental technique to accurately measure the tangential contact stiffness between two rough contacting surfaces manufactured from the titanium alloy Ti–6Al–4V. The ...digital image correlation method is employed to measure the local displacement field. The effect of normal contact pressure, nominal contact area and fretting wear on tangential contact stiffness is investigated. The experiments indicate that the tangential contact stiffness is approximately proportional to the nominal contact area and the normal pressure raised to the power of 0.64. Multiple experiments with the same parameters show good repeatability given the number of variables involved.
► Digital image correlation for accurate measurement of tangential contact stiffness. ► We use flat pads with rounded corners clamped against the flat surface of a specimen. ► Effect of contact pressure, contact area and wear on stiffness is investigated. ► Using the same experimental parameters, the repeatability of results is investigated.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SBCE, SBJE, UPUK
Ground motion effect on a roller-compacted concrete (RCC) dams in the earthquake zone should be taken into account for the most critical conditions. This study presents three-dimensional earthquake ...response of a RCC dam considering geometrical non-linearity. Besides, material and connection non-linearity are also taken into consideration in the time-history analyses. Bilinear and multilinear kinematic hardening material models are utilized in the materially non-linear analyses for concrete and foundation rock respectively. The contraction joints inside the dam blocks and dam-foundation-reservoir interaction are modeled by the contact elements. The hydrostatic and hydrodynamic pressures of the reservoir water are modeled with the fluid finite elements based on the Lagrangian approach. The gravity and hydrostatic pressure effects are employed as initial condition before the strong ground motion. In the earthquake analyses, viscous dampers are defined in the finite element model to represent infinite boundary conditions. According to numerical solutions, horizontal displacements increase under hydrodynamic pressure. Besides, those also increase in the materially non-linear analyses of the dam. In addition, while the principle stress components by the hydrodynamic pressure effect the reservoir water, those decrease in the materially non-linear time-history analyses.
•Study of cracks in single crystal and bi-crystal hcp Ti of varying crystallographic orientation.•Assessment of stress distribution and stress intensity factors in elastically anisotropic ...media.•Quantitative study of plastic strain distribution in bi-crystal of varying orientation and morphology.•Furthering understanding of cold-dwell facet fatigue.
The Sih, Paris and Irwin analytical solution for cracks in anisotropic elastic media has been developed for an hcp Ti single crystal and shown to lead to crack tip normal stresses which are independent of crystal orientation but other stress components which are dependent. Detailed finite element studies confirm that the stress intensity remains independent of crystal orientation but ceases to do so in an edge-cracked bi-crystal.
The incorporation of crystallographic slip demonstrates that single-crystal crack tip stresses largely remain independent of crystal orientation but that the plastic zone size and shape depends greatly upon it. Significant differences result in both the magnitude and extent of the plasticity at the crack tip with crystallographic orientation which can be quite different to that predicted using Mises plasticity. For an edge crack terminating in a bi-crystal, the slip fields which result depend upon both crystal mis-orientation and morphology.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
Abstract Due to attractive mechanical properties, metastable β titanium alloys have become very popular in many industries including aerospace, marine, biomedical, and many more. It is often the ...complex interplay among the different deformation mechanisms that produces many of the sought-after properties, such as enhanced ductility, super-elasticity, and shape memory effects. Stress induced martensitic transformation is an important deformation mechanism for these alloys. Understanding of it and the influence it has on the microstructural evolution of materials is of great importance. To this end we have developed a crystal plasticity based constitutive model which accounts for both martensitic phase transformation and slip based plasticity simultaneously in metastable β titanium alloys. We present a new formulation for the evolution of martensite transformation, based on physical principles and crystal plasticity theory. To understand and demonstrate this feature of the model, a parametric assessment of the newly developed constitutive model is conducted. This is followed by first of its kind analyses of stress induced martensitic transformation in metastable β titanium alloys. We firstly present validations against uniaxial loading experiments for different metastable β titanium alloys exhibiting stress induced martensite transformation. As part of this, single crystal simulations in metastable β titanium alloys are used for the first time to investigate the interaction of individual transformation systems during unconstrained transformation. This study shows good agreement between the experimental and simulated responses during all stages of deformation in which elastic, transformation and finally the slip stage are exhibited. Relatively ‘strong’ and ‘weak’ orientations for transformation are observed, consistent with experimental studies. The work done here demonstrates the ability of this crystal plasticity finite element method to capture physical mechanisms while bringing new insight about the interaction of different deformation mechanisms in metastable β titanium alloys.
Background
Laser powder bed fusion (L-PBF) additive manufacturing (AM) is used for building metallic parts layer-by-layer and often generates non-uniform thermal gradients between layers during ...fabrication, resulting in the development of residual stresses when parts are cooled down.
Objective
The impact of modulated laser used during the L-PBF process on residual stresses in Inconel 718 (IN718) material was investigated. The impact of build directions on residual stress is also determined.
Methods
The contour method is employed to measure the full-field residual stress component on the cross-section of samples. A complementary residual stress measurement method, incremental hole drilling, was employed for obtaining in-plane residual stress components.
Results
The results show that the residual stress distribution is sensitive to the build direction, with a higher magnitude of residual stress in the direction of build than that in the transverse direction. Multiple measurements with the same manufacturing parameters show good repeatability.
Conclusion
Residual stresses in the as-built parts are significant and hence a further consideration regarding relieving residual stresses is required when post-thermal treatments are developed.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Syntactic foams with hollow glass microspheres embedded in an epoxy matrix are used in marine, aerospace and ground transportation vehicle applications. This work presents an in situ experimental ...study of failure mechanisms in syntactic foam based on X-ray microcomputed tomography with uniaxial compression. Under different levels of compressive strain, the material was scanned using X-ray microcomputed tomography to obtain three-dimensional (3D) images of its internal microstructure. Experiments with the same parameters were carried out to investigate repeatability. The microscopic observations have suggested that damage nucleation occurs at the weakest microspheres. When applied strain increases, shear collapse bands (SCBs) develop local to the fragmented microspheres due to stress concentration and bending deformation around SCBs occurs. After significant strain, the thickness of the SCBs increases owing to the accumulation of the broken microspheres. The relationship between the volume fraction of microspheres and applied bulk strain has been characterised.
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DOBA, EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, IZUM, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, SIK, UILJ, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The paper describes multiscale experimental techniques required for determining the mechanical response of a ferritic–pearlitic steel (14MoV6). Accordingly, five different sets of specimens ranging ...from macro to micro size scales are utilized and whose experimental results have been compared. Digital image correlation is employed to measure deformations on the surface of the test specimens under tensile loading conditions. The influence of the surface conditions on the residual stress distribution was investigated by means of X-ray synchrotron diffraction. In addition, the effects of specimen size, surface conditions and the number of grains in the cross-section of the specimens on mechanical properties are examined. It is observed that key material parameters including the yield stress, tensile strength and elongation to failure are dependent on specimen size. In addition, the results demonstrate that the number of grains in the cross-section of the specimens significantly influence the material response during uniaxial tensile testing. On the other hand, the surface treatment of the micro tensile test specimens bring about reducing differences in mechanical properties between standard and miniature specimens.
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IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, SBCE, SBJE, UPUK
► The friction variation which is commonly observed during the gross-slip phase of fretting cycles is a result of wear-scar interactions. ► These wear-scar interactions were found to occur
primarily ...at local ‘peaks and troughs’ distributed throughout the nominal contact region. ► 3D profilometry and cross/auto correlation methods were used to identify these local wear-scar features and determine their size. ► A simple model has been put forward to explain how interaction of wear-scar features can produce the friction variation which is commonly observed. ► A new test rig, designed to adapt in-line fretting test machines for torsional fretting has been designed, and shown to work effectively.
A commonly observed phenomenon where the friction force increases during the gross slip phase of individual fretting cycles is investigated with the aim of identifying the physical origins of this type of frictional behaviour. Measurements of sliding friction from linear and torsional fretting tests, using the aerospace nickel alloy Udimet 720, and subsequent analysis of the post-test worn surfaces were used to investigate the phenomenon. It was found that this friction variation is due to wear–scar interaction effects. These interactions were primarily found to occur at sites distributed throughout the nominal contact area via the interference of local interlocking peaks and troughs on the worn surfaces. Cross-correlation and auto-correlation analysis of the worn surfaces was used to identify, and to show the approximate size of, these local features. Many of the features were found to be similar in size to the applied fretting stroke, but on average, the features were somewhat larger. A simple one degree-of-freedom model of the interaction of an idealised surface peak with a corresponding surface groove was developed to show how these interactions produce the type of friction variation which is commonly observed during the sliding phase.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
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