Yanyao Jiang

Direct observation of twinning–detwinning–retwinning on magnesium single crystal subjected to strain-controlled cyclic tension–compression in [0 0 0 1] direction

Cyclic deformation of magnesium single crystal under fully reversed tension–compression along the [0 0 0 1] direction at a strain amplitude of 0.5% was investigated in ambient air. In situ light microscopy of twins was conducted on the prismatic plane. Fundamental morphology evolution of twinning–detwinning–retwinning was characterized in situ. The critical resolved shear stress of twinning was found to be ∼2.4 MPa. With increasing loading cycles, the activity of twinning–detwinning–retwinning exhausted and the residual twins accumulated. During detwinning at each loading cycle, the residual twins were observed to form on the vicinities of twin boundaries which had been fully expanded at the previous tensile peak. Microcracks were found on the boundaries of residual twins and the basal slip bands.

Multiaxial Fatigue of 16MnR Steel

Uniaxial, torsion, and axial-torsion fatigue experiments were conducted on a pressure vessel steel, 16MnR, in ambient air. The uniaxial experiments were conducted using solid cylindrical specimens. Axial-torsion experiments employed thin-walled tubular specimens subjected to proportional and nonproportional loading. The true fracture stress and strain were obtained by testing solid shafts under monotonic torsion. Experimental results reveal that the material under investigation does not display significant nonproportional hardening. The material was found to display shear cracking under pure shear loading but tensile cracking under tension-compression loading. Two critical plane multiaxial fatigue criteria, namely, the Fatemi-Socie criterion and the Jiang criterion, were evaluated based on the experimental results. The Fatemi-Socie criterion combines the maximum shear strain amplitude with a consideration of the normal stress on the critical plane. The Jiang criterion makes use of the plastic strain energy on a material plane as the major contributor to the fatigue damage. Both criteria were found to correlate well with the experiments in terms of fatigue life. The predicted cracking directions by the criteria were less satisfactory when comparing with the experimentally observed cracking behavior under different loading conditions.

Fatigue of AL6XN Stainless Steel

Tension-compression, torsion, and axial-torsion fatigue experiments were conducted on the AL6XN alloy to experimentally investigate the cyclic plasticity behavior and the fatigue behavior. The material is found to display significant nonproportional hardening when the equivalent plastic strain amplitude is over 2 X 10 -4 . In addition, the material exhibits overall cyclic softening. Under tension-compression, the cracking plane is perpendicular to the axial loading direction regardless of the loading amplitude. The smooth strain-life curve under fully reversed tension-compression can be described by a three-parameter power equation. However, the shear strain-life curve under pure torsion loading displays a distinct plateau in the fatigue life range approximately from 20,000 to 60,000 loading cycles. The shear strain amplitude corresponding to the plateau is approximately 1.0%. When the shear strain amplitude is above 1.0% under pure shear, the material displays shear cracking. When the shear strain amplitude is below 1.0%, the material displays tensile cracking. A transition from shear cracking to tensile cracking is associated with the plateau in the shear strain-life curve. Three different multiaxial fatigue criteria were evaluated based on the experimental results on the material for the capability of the criteria to predict fatigue life and the cracking direction. Despite the difference in theory, all the three multiaxial criteria can reasonably correlate the experiments in terms of fatigue life. Since the cracking mode of the material subjected to pure torsion is a function of the loading magnitude, the prediction of cracking orientation becomes rather challenging.

An Experimental Study of the Crack Growth Behavior of 16MnR Pressure Vessel Steel

An experimental investigation was conducted on the crack growth behavior of a pressure vessel steel, 16MnR, in ambient air. Standard compact tension specimens were subjected to Mode I loading with several R-ratios and loading amplitudes. Three circular notch sizes ranging from very sharp notch to blunt notch were used. In addition to constant amplitude loading, experiments were conducted to study the influences of overload and loading sequence on crack growth. The results show that the R-ratio has an insignificant influence on the crack growth of the material. The size of the notch together with the R-ratio and loading amplitude has a great influence on the early crack growth from the notch. A single tensile overload during a constant amplitude loading experiment retards the crack growth significantly. Right after the application of an overload, the crack growth rate is higher than that of the stable crack growth observed in the constant amplitude loading. The crack growth rate decreases and reaches a minimum value before it gradually increases and reaches the stable crack growth curve. In high-low sequence loading with the maximum load in the second step lower than that of the first loading step, the preceding higher constant amplitude loading results in a significant crack growth retardation in the second loading step. This phenomenon is similar to the effect of a single tensile overload on the constant amplitude loading. An existing model making use of the stress intensity factor is discussed with respect to its capability to describe the observed crack growth behavior with the influence of overload and sequence loading.

Tension-compression-tension tertiary twins in coarse-grained polycrystalline pure magnesium at room temperature

Using electron backscatter diffraction, the microstructural features of tension–compression–tension (T–C–T) tertiary twins are studied in coarse-grained pure polycrystalline magnesium subjected to monotonic compression along the extrusion direction in ambient air. T–C–T tertiary twins are developed due to the formation of a compression–tension double twin inside a primary tension twin. All the observed T–C–T twin variants are of TiCjTj type. TiCi+1Ti+1 (or TiCi−1Ti−1) variants are observed more frequently than TiCi+2Ti+2 (or TiCi−2Ti−2) variants. The number of tertiary twin lamellae increases with the applied compressive strain.

Twin-Slip Interaction at Low Stress Stage Deformation in an AZ31 Mg Alloy

Extruded magnesium alloys with strong basal texture present tension and compression asymmetry. Dislocation slip dominates plastic deformation during tension along the extrusion direction (ED), whereas twinning is the main contributor to plastic strain when compressed along the ED. In this work, an extruded AZ31 Mg alloy was prestrained by tension along the ED to 5 and 10% of total strain, followed by compression, in order to investigate twin-slip interaction. The results show that the yield stress in compression only slightly increases with increasing prestrain. Notably, the hardening rate at the low stress stage during compression remains almost unchanged, compared to specimens without prestrain. Our results suggest that the contribution of twin-slip interaction to hardening is negligible in deformation of Mg alloys.

In situ observation of cross-grain twin pair formation in pure magnesium

ABSTRACT Cross-grain twin pairing was studied utilising in situ optical microscopy and ex situ electron backscatter diffraction in extruded pure magnesium subjected to tension in the direction perpendicular to the extrusion direction. A twin chain spanning over seven grains was rapidly developed through twin propagation and conjoining. Formation of cross-grain twin pair by twin thickening was in/ex situ observed for the first time. It was confirmed that variant selection of the cross-grain twins that carry high Schmid factors, formed either by twin propagation or by twin thickening, was dominated by the strain compatibility between the neighbouring grains.

Influence of Partial Slip Conditions on Rolling Contact Fatigue of 1070 Steel

Partial slip rolling contact was analyzed in this paper by the finite element method with the application of a robust cyclic plasticity model. The repeated rolling contact process was carried out by translating the normal pressure and the tangential traction across the contact surface step by step. The normal pressure and the tangential traction were applied to the nodes through the time-dependant amplitude functions as the concentrated nodal forces. With the detailed stress-strain responses output from the FE analysis, a general multiaxial fatigue criterion was used to predict fatigue initiation life and initiation position. The influences of partial slip conditions on the residual stresses, residual shear strain and the initiation lives are obtained.

Two Models for Predicting Fatigue Crack Growth

In this investigation two fatigue crack growth models based on the different physical assumptions were systematically analyzed. One model makes use of the size of the stable damage distribution zone near crack as the major contributor to the fatigue crack growth. This model is based on the concept that a material point failures and a new crack will form when the fatigue damage value of the material point reach the critical damage. The other model supposes that fatigue growth can be described as a process with sequentially breaking small volume elements behind the crack tip. The fatigue crack growth can be regarded as successive crack re-initiation over a critical distance. The fatigue crack growth rate can be determined as the ratio of the critical distance to the average life within critical distance. Both models use macro parameter to describe the microscopic mechanism. An elastic-plastic finite element analysis (FEA) was used to obtain the detailed stress-strain history of the notched component with a detailed consideration of the cyclic plasticity of the material using a robust cyclic plasticity model. The fatigue damage distribution and the average damage within the critical distance near the crack tip can be obtained by combining the fatigue damage parameter with the stress strain distribution from the finite element analysis. These two models were evaluated using the experimental results obtained from the crack grow experiments on compact specimens made from 16MnR. The predicting results using these two models correlate well with the experimental data. The results show that two models can well describe the notch effect on the fatigue crack growth.

Fatigue and Cyclic Plasticity Properties of a Super-Austenitic Stainless Steel

Tension-compression, torsion and axial-torsion experiments were conducted on AL-6XN® alloy. The main goal was to investigate experimentally, in detail, the cyclic plasticity behavior as well as fatigue life of AL-6XN® steel. Details of cyclic stress-strain response were collected during the experiments, which can serve as a baseline for development of cyclic plasticity model for this material. Microscopic observations of cracking behavior conducted in the present study allow connecting the fracture mechanism with fatigue life prediction. It was observed, that fatigue life of this material is a function of the fracture mode (mixed or tensile). The mixed cracking was observed in the specimens tested under higher applied strain levels, while the tensile cracking was revealed in the tests under lower strain amplitudes. Strain-life curves of the specimens failed in mixed mode and of those failed in tensile mode run parallel to each other, but the specimens that exhibit mixed failure mode show lower fatigue life as compared to the tensile mode specimens. Transition between mixed and tensile cracking orientations was studied in detail. The results of the experimental work presented in this study can serve for design of fatigue models for this material in the future.Copyright