Cyclic-Loading Induced Lattice-Strain Asymmetry in Loading and Transverse Directions (original) (raw)
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Most of crystalline materials exhibit a hysteresis on their deformation curve when mechanically loaded in alternating directions. This Bauschinger effect is the signature of mechanisms existing at the atomic scale and controlling the materials damage and ultimately their failure. Here, three-dimensional simulations of dislocation dynamics and statistical analyses of the microstructure evolution reveal two original elementary mechanisms. An asymmetry in the dislocation network junctions arising from the stress driven curvatures and the partial reversibility of plastic avalanches give an explanation to the traction-compression asymmetry observed in FCC single-crystals. These mechanisms are then connected in a physically justified way to larger-scale representations using a dislocation density based theory. Parameter-free predictions of the Bauschinger effect and strain hardening during cyclic deformation in different materials and over a range of loading directions and different plast...
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The present investigation focuses on an observation regarding the initial elastic response of a triangular geometrically and structurally disordered lattice during medium-to-high strain rate loading. Namely: a transition from the short-time modulus of elasticity to the long-time one, which is not accompanied by the corresponding change of the stiffness tensor. It is demonstrated that the difference between the two moduli is, in the case of the homogeneous biaxial test simulations performed herein, a consequence of the geometrical and structural disorder "quenched" within the lattice. The investigation is performed on the triangular lattice with the first-neighbor central interactions under practically identical in-plane conditions over eight decades of strain rate.
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A lot of elements of the machines operate under different types of loading. Rarely the stress states are simple, but contrary they are usually becoming complex in different cases. Therefore, many material laboratories focus their efforts on the investigations under multiaxial stress states in spite of the well know fact that such kinds of tests are not easy to perform. Since the number of multiaxial experiments is still modest, the current knowledge about materials behaviour under such conditions is insufficient for reasonable modelling. Such situation takes place for example during an effective fracture predictions of engine turbine blades [1], [2], failure assessments of ball bearing [3] or damage analysis of vehicle drive shaft [4]. On the other hand, complex loadings play an essential role in modifications of technological processes [5], [6]. Taking into account an increase of the industrial devices lifetime and possible redesigning of certain technological processes, the experi...
Lattice strain evolution during cyclic loading of stainless steel
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A uniaxial tension/compression test specimen was cycled between fixed total strain limits of ±0.4% for eight successive cycles. The sample was loaded using a dedicated Instron hydraulic load frame on the ENGIN station of the PEARL beam line at the ISIS facility of the Rutherford Appleton Laboratory. The load frame was aligned to allow simultaneous monitoring of longitudinal and transverse lattice strain components. There was a strong experimental hkl-dependency of the lattice strain response in both the effective stiffness and in the lattice strain loop hysteresis. The experimental data were compared with numerical predictions obtained from a self-consistent elasto-plastic model for the simulation of polycrystal deformation. A cyclic hardening law was developed and implemented into the modelling scheme, providing theoretical predictions in good agreement with experimental observations.
On the evolution and modelling of lattice strains during the cyclic loading of TWIP steel
Acta Materialia, 2013
The evolution of lattice strains in fully annealed Fe-24Mn-3Al-2Si-1Ni-0.06C twinning-induced plasticity (TWIP) steel is investigated via in situ neutron diffraction during cyclic (tension-compression) loading between strain limits of ±1%. The pronounced Bauschinger effect observed upon load reversal is accounted for by a combination of the intergranular residual stresses and the intragranular sources of back stress, such as dislocation pile-ups at the intersection of stacking faults. The recently modified elasto-plastic self-consistent (EPSC) model which empirically accounts for both intergranular and intragranular back stresses has been successfully used to simulate the macroscopic stress-strain response and the evolution of the lattice strains. The EPSC model captures the experimentally observed tension-compression asymmetry as it accounts for the directionality of twinning as well as Schmid factor considerations. For the strain limits used in this study, the EPSC model also predicts that the lower flow stress on reverse shear loading reported in earlier Bauschinger-type experiments on TWIP steel is a geometrical or loading path effect.
An Influence of Cyclic Loading on Stress Component Reduction in the Transversal Direction
2016
The paper reports results of tests where monotonic deformation was conducted in assistance of cyclic straining in the perpendicular direction. Two variants of strain signals combinations were applied, i.e. monotonic tension with torsion cycles and monotonic torsion with tension-compression cycles. The amplitudes of cyclic strain were lower than ±1%, frequency levels were selected within the range from 0.005 to 1 Hz. Independently of the cyclic strain direction applied the material response was similar. A stress reduction with the cyclic strain amplitude increase was obtained for direction of monotonic loading.
A synchrotron X-ray diffraction study of non-proportional strain-path effects
Common alloys used in sheet form can display a significant ductility benefit when they are subjected to certain multiaxial strain paths. This effect has been studied here for a polycrystalline ferritic steel using a combination of Nakajima bulge testing, X-ray diffraction during biaxial testing of cruciform samples and crystal plasticity finite element (CPFE) modelling. Greatest gains in strain to failure were found when subjecting sheets to uniaxial loading followed by balanced biaxial deformation, resulting in a total deformation close to plane-strain. A combined strain of approximately double that of proportional loading was achieved. The evolution of macrostrain, microstrain and texture during non-proportional loading were evaluated by in-situ high energy synchrotron diffraction. The results have demonstrated that the inhomogeneous strain accumulation from non-proportional deformation is strongly dependent on texture and the applied strain-ratio of the first deformation pass. Experimental diffraction evidence is supported by results produced by a novel method of CPFE-derived diffraction simulation. Using constitutive laws selected on the basis of good agreement with measured lattice strain development, the CPFE model demonstrated the capability to replicate ductility gains measured experimentally.
Material responses to the laboratory simulation of complex cyclic loadings
Journal of KONES, 2008
The paper presents investigations identifying an influence of complex cyclic loading controlled by the strain signals on the selected mechanical properties of the 2024 aluminium alloy. Investigations of proportional and non-proportional loading paths in the form of square and circle were carried out at room temperature using thin-walled tubular specimens enabling realisation of complex stress states due to acting of axial force and twisting moment. The hardening effect of the material due to cyclic loading was observed on the basis of: stress responses into the strain controlled loading programme; hysteresis loop variations and amounts of stress amplitude. In the case of the non-proportional cyclic loading an additional hardening effect was identified. Second-order effects, which allow better understanding of the influence of proportional and non-proportional loading paths on mechanical behaviour of engineering materials, were identified. In the case of the circular loading path a delay of the maximum stress signals with respect to strain ones is demonstrated. For the square loading path a softening effect was observed. It is reflected by the rapid stress drop on this direction which is perpendicular to that where the stress level begins to turn back. The yield surface approach is also applied in order to assess the mechanical properties variations due to cyclic loading of the material.
Stress asymmetry in cyclic deformation of b.c.c. metals
Acta Metallurgica et Materialia, 1992
Asymmetric slip in b.c.c, metals is related to both the symmetry about ~(111) screw dislocations and elastic anisotropy. The elastic anisotropy differs quite substantially from one b.c.c, metal to the next and this anisotropy has been observed in measurements of asymmetric stresses in plastic deformation experiments. In this paper, data from cyclic deformation of b.c.c, metals, with special emphasis on tungsten which is elastically isotropic, are used to demonstrate the role of screw dislocation geometry and elastic anisotropy on stress asymmetry. The interaction of dislocation geometry and thermal activation of screw dislocation motion is discussed. Rrsumr-Le glissement asymrtrique dans les mrtaux cc est li6 fi la fois ~. la symrtrie autour des dislocations vis ~ (111) et ~. l'anisotropie 61astique. L'anisotropie 61astique varie trrs substantiellement d'un m&al cc ~i rautre et cette anisotropie est observre dans les mesures de contraintes asymrtriques dans des exprriences de drformation plastique. Dans cet article, les donnres provenant de la drformation cyclique de mrtaux cc, avec une attention particulirre pour le tungstrne qui est 61astiquement isotrope, sont utilisres pour drmontrer le r61e de la gromrtrie des dislocations vis et de l'anisotropie 61astique sur l'asym&rie des contraintes. L'interaction de la gromrtrie des dislocations et de l'activation thermique du mouvement des dislocations vis est discutre. Zusammenfassung-Die Asymmetric der Gleitung von krz. Metallen h~ingt sowohl mit der Symmetrie um I2(lll)-Schraubenversetzungen als auch mit der elastischen Anisotropie zusammen. Die elastische Anisotropie unterscheidet sich yon einem zum anderen krz. Metall betr~chtlich; diese Anisotropie wird bei Messungen der asymmetrischen Spannungen in Experimenten der plastischen Verformung beobachtet. In dieser Arbeit wird mit Daten zur zyklischen Verformung von krz. Metallen mit Schwerpunkt Wolfram, welches elastisch isotrop ist, die Rolle der Schraubenversetzungsgeometrie und der elastischen Anisotropie bei der Spannungsasymmetrie dargestellt. Die Wechselwirkung der Versetzungsgeometrie und der thermischen Aktivierung der Bewegung der Schraubenversetzungen wird diskutiert.