Abstract: The Acoustic Emission (AE) monitoring technique is widely used in mechanical and material research for detection of plastic deformation, fracture initiation and crack growth. However, the influence of AE features (such as signal amplitude, frequency, rise time and duration) on the fracture parameters (such as brittle or ductile mode of propagation and fracture propagation speed) is not completely understood. In this paper, the effect of loading conditions on fracture behavior was studied using AE monitoring during tensile testing of an aluminum alloy specimen. The fracture development was observed using a high speed video camera and was analyzed using the finite element method. The hardware and software produced by Physical Acoustics Corporation (USA) was used. Variations in AE parameters were analyzed and correlated to the stress-strain curves obtained during testing. It is shown that the strain rate and the presence of a crack (modeled by a notch on the sample), affect the fracture mode (brittle or ductile) and a relative amount of the mode dependent AE signatures.
Abstract: Aluminum honeycombs are lightweight and have good energy absorption capability. They are widely used in industrial products and also as core materials in various fields of engineering such as aerospace, automotive and naval engineering because of their high specific strengths and they can undergo large plastic deformation to absorb high impact energy. In the applications of aluminum honeycombs they are not only subjected to pure compressive or indentation load but sometime also under combined compression-shear load. The mechanical response and crushing behavior under combined compression-shear loading condition is still limited in literature. In this paper, quasi-static out-of-plane combined compression-shear tests were conducted to study the deformation mechanism of different types of HEXCELL® aluminum honeycombs with different cell sizes and wall thicknesses. Three types of aluminum honeycombs were used in this study. A universal MTS machine with specially designed fixtures was employed in the quasi-static loading tests. The experiments were conducted at three different loading angles, that is, 30°, 45° and 60° and in TL and TW (T is out-of-plane direction and L, W are the two in-plane directions) plane orientation loading directions of aluminum honeycomb. The effects of different loading angle and different plane orientation are reported in this experimental study. Similarly, the effects of cell size and cell wall thickness were also analyzed.
Abstract: We perform finite element homogenization (FEH) analysis to investigate the effect of strain hardening on the monotonic and cyclic loading behavior of plate-fin structures with two pore pressures. As a typical base metal of plate-fin structures, 316 stainless steel is considered and assumed to be the viscoplastic material that obeys the Ohno-Wang kinematic hardening rule. The plate-fin structures are assumed to be periodic and subjected to uniaxial monotonic and cyclic loadings in the stacking direction. A periodic unit cell is used for FEH analysis. Results are compared with those based on three special cases derived from Hill’s macrohomogeneity equation. It is found that the mean pore pressure entirely affect the homogenized viscoplastic behavior. It is further found that the differential pore pressure causes the remarkable accumulation of ratcheting strain in the periodic unit cell, although this internal ratcheting gives no effect on macroscopic relations, resulting in providing a closed hysteresis loop for the plate-fin structures.
Abstract: The size effects observed in the torsion of thin FCC single crystal wires is modelled by the employment of mechanism-based strain gradient crystal plasticity (MSG-CP). In the formulation the total slip resistance in each active slip system is assumed to be due to a mixed population of forest obstacles arising from both statistically stored and geometrically necessary dislocations. The MSG-CP constitutive model is implemented into the Abaqus/Standard FE platform by developing the User MATerial subroutine UMAT. By implementing the formulation, the relationship between the non-dimensional torque and the surface strain of the thin copper single crystal wires of different diameters is obtained with the  direction along that of the wire axis. The simulation results of torsion reveal size effects, which is in a qualitative agreement with those reported in existing literatures. An appreciable axial elongation is also found in the torsion of single crystal wires.Key words Size effect, MSG-CP model, Torsion
Abstract: Tensile properties of ferrite lamella in pearlite under lattice strain are examined by a strain gradient crystal plasticity analysis. Tensile direction is made to be parallel to the lamella. Obtained results of macroscopic stress-strain relation of the lamella show significant increase of yield stress and strain hardening rate with the reduction of the lamella thickness and further increase of the yield stress with positive normal lattice strain parallel to the tensile direction in the ferrite layer. Whereas normal lattice strain perpendicular to the tensile direction contributes little to the tensile properties.
Abstract: Compressive residual stresses and high hardness introduced by induction quenching are often used to improve the mechanical performance of crankshafts in engines. In this paper, in order to predict the fatigue strength of the crankshafts in a diesel engine accurately, the quenching process and the stress fields of the crankshaft are simulated by finite element method (FEM) and then the residual stresses are obtained. Quenching is a high non-linear process because of the coupling of the temperature, phase transformation and stress/strain fields. The transient temperature field is firstly solved using a quasi-coupling method to simulate the relationship of the temperature and the phase transformation. Based on the results of the temperature and phase transformation fields, the thermo-elastic-plastic analysis is then carried out to calculate the stress fields including the thermal stresses and the phase transformation stresses. At last the residual stress distribution at the room temperature is obtained.
Abstract: Based on our extensive studies on the experimental, theoretical and numerical results on various tubes under axial compression/impact in the last few years, we propose a set of Key Performance Indicators (KPIs) to assess and compare the energy absorbing performance of tubular structures with various configurations, so as to guide the design of energy absorbers whilst to archive a certain degree of optimization. The KPIs have five factors: Effective stroke ratio (ESR), Non-dimensional Load-carrying capacity (NLC), Effectiveness of energy absorption (EEA), Specific energy absorption capacity (SEA), Stableness of load-carrying capacity (SLC).The paper presents a series of diagrams to compare the energy absorbing performance of various tubes in terms of the four KPIs as described above. The work is valuable to engineering designs and applications, as well as to the further studies of the topic.
Abstract: The dynamic double-notched experiments by using Split Hopkinson Pressure Bars (SHPB) and high-speed camera were performed on bulk metallic glass. In the double-notched experiment, shear crack propagating process was captured with the high temporal resolution of high-speed camera and the crack front propagating velocity was estimated to be 1137m/s. the shear strain/shear stress curve of BMG under dynamic loading was also obtained. Static in-situ SEM tensile experiments were included to study the multiple shear bands propagating behavior on a glassy ribbon. It was found that shear bands propagates progressively in an intermittent and discontinuous manner, and the choice of which shear bands to propagate and which ones to keep still among multiple shear bands is quite stochastic. This is explained qualitatively from the view point of energy.
Abstract: This paper proposes an integrated hydraulic bulge and forming limit testing method and apparatus for sheet metals. By placing a PU (Polyurethane) plate between molds and uniformly applying hydraulic pressure to sheet metals, a biaxial stress-strain relationship and forming limit diagram (FLD) displaying both left and right sides were acquired using the same apparatus. An uniaxial tension test and traditional drawing test were conducted to compare the results obtained from the proposed hydraulic bulge and forming limit testing methods, respectively. A close correlation between the results of the stress-strain relationship and FLD in both comparisons verified the feasibility and capability of this integrated hydraulic testing method and apparatus for use with sheet metals.
Abstract: The cyclic growth and recovery of warpage were observed in experiments on Si/solder/Cu layered plates subjected to cyclic thermal loading . In the present study, the experiments were analyzed using representative material models for the solder and Cu layers in finite element analysis. The warpage growth/recovery behavior observed was reproduced well in the analysis using the Armstrong-Frederick and Ohno-Wang models for the solder and Cu layers, respectively. Material ratcheting due to non-proportional cyclic loading was found to happen in the solder layer as a consequence of the CTE mismatch, while material ratcheting due to proportional cyclic loading occurred in the Cu layer as a result of the significant temperature dependence of viscoplasticity in the solder layer.