Tensile Stress Distribution Research Articles

Influence of short heat pulses on properties of martensite in medium carbon steels

The process of tempering a martensitic medium carbon steel was investigated with the aim to study resulting material properties. The experimental results were used to model residual stresses caused by local heating, with the finite element method. Tempering was followed for times from 0.1s up to 1h by using laser heating and conventional salt bath furnace treatments within the temperature interval 500–700°C. In addition, the thermal expansion was evaluated using dilatometry. Experiments showed that the initial stages of martensite decomposition, associated with loss of crystal tetragonality, proceed almost instantly. An initial large decrease of hardness within the first tenth of a second of the tempering process was measured, followed by only limited further softening with increased tempering time. Thus for the current material the tempering time had limited influence on hardness, governed primarily by the peak temperature during the heating process. Finite element modelling of rapid local heating and cooling showed that the tempering behaviour and associated dilatation effects yield a peak temperature dependent residual stress field with a broad tensile stress distribution for the case of un-tempered martensite. However, for tempered martensite the residual stress field depends primarily on the heating rate and peak temperature and shows large gradients with tensile stresses in the surface and compressive below. Thereby, for both cases, residual stresses were obtained but with completely different residual stress gradients.

The Brazilian Disc Test for Rock Mechanics Applications: Review and New Insights

The development of the Brazilian disc test for determining indirect tensile strength and its applications in rock mechanics are reviewed herein. Based on the history of research on the Brazilian test by analytical, experimental, and numerical approaches, three research stages can be identified. Most of the early studies focused on the tensile stress distribution in Brazilian disc specimens, while ignoring the tensile strain distribution. The observation of different crack initiation positions in the Brazilian disc has drawn a lot of research interest from the rock mechanics community. A simple extension strain criterion was put forward by Stacey (Int J Rock Mech Min Sci Geomech Abstr 18(6):469–474, 1981) to account for extension crack initiation and propagation in rocks, although this is not widely used. In the present study, a linear elastic numerical model is constructed to study crack initiation in a 50-mm-diameter Brazilian disc using FLAC3D. The maximum tensile stress and the maximum tensile strain are both found to occur about 5 mm away from the two loading points along the compressed diameter of the disc, instead of at the center of the disc surface. Therefore, the crack initiation point of the Brazilian test for rocks may be located near the loading point when the tensile strain meets the maximum extension strain criterion, but at the surface center when the tensile stress meets the maximum tensile strength criterion.

Mesoscopic surface folding in EK-181 steel polycrystals under uniaxial tension

The work is experimental and theoretical study of folded structures formed on free surfaces of polycrystalline materials under uniaxial tension. General mechanisms by which the folded deformation relief develops are demonstrated with the example of EK-181 steel. Numerical simulation shows that the polycrystalline structure of the material can be a condition responsible for local curvature of its initially flat surface and hence for periodic distribution of normal tensile and compressive stresses.

Influence of the third monomer component on the temperature-dependent crystallite modulus and tie chain fraction evaluated for ethylene-tetrafluoroethylene copolymers

Temperature dependence of apparent crystallite modulus along the chain axis Ecapp has been measured on the basis of X-ray diffraction measurement under the assumption of homogeneous tensile stress distribution for the uniaxially-oriented ethylene-tetrafluoroethylene (ETFE) alternating copolymers with and without the third monomer component of short or long side groups. The Ecapp of the 2-components alternating ETFE copolymer was found to decrease remarkably in the phase transition temperature region between the low- and high-temperature phases at around 80 °C. For the ETFE copolymer sample with the third comonomer component of long C4F9 side groups the Ecapp was found to show the similar temperature dependence to that of the pure alternating copolymer although the Ecapp value itself was slightly lower than the latter. The ETFE copolymer with short CF3 side groups was found to show remarkably low Ecapp value in the temperature region from −50 to 50 °C since the crystal structure transferred already to the high-temperature disordered phase in this temperature region. The bulk Young’s moduli of these copolymer samples, estimated by the dynamic viscoelastic measurements, were analyzed on the basis of a complex mechanical model consisting of parallel and series sequences of Ectrue and Ea, where Ectrue and Ea were the true moduli of the crystalline and amorphous regions, respectively. The temperature dependences of the bulk moduli observed for these three samples were successfully reproduced by estimating the Ectrue as a function of temperature. The Ectrue estimated at a low-temperature, was 215 GPa for ETFE 2-components sample, which was consistent with the value 230 GPa calculated theoretically for the planar-zigzag chain conformation. The parallel crystalline component, which was assumed to be a kind of taut tie chain to support the mechanical toughness of the bulk sample, was found to be lower in population for ETFE 2-components sample and higher for the copolymer samples containing the third monomer component. This tendency is consistent with that of the actually-observed stress crack resistance or the mechanical toughness in a high-temperature region near the melting point: ETFE < ETFE-CF3 < ETFE-C4F9. In this way, the terpolymer sample has an important role to create the higher content of taut tie chains, resulting in the higher stress crack resistance.

The Effect of Discrete Grippers on Sheet Metal Flexible-Gripper Stretch Forming

Flexible-gripper stretch forming is a new flexible manufacturing process of sheet metal. Four numerical simulation models of discrete grippers were established using finite element method, the effect of the numbers of grippers on stress distribution, thickness distribution and gripping surface shape of the spherical parts were studied. The results show that the clamped edge of the spherical parts formed by using integral grippers is straight，the tensile stress distribution is uneven and the thickness is obviously reduced. However, the stress distribution on the parts formed by using discrete grippers stretch forming is relatively uniform, with the increase in the numbers of grippers, the thickness increases, and the more gripping surface curvature of the parts is close to the curvature of die, the better forming quality of the parts is.

Formation and Growth of the Crack in Bonded Joints Under Mode I Fracture: Substrate Deflection at Crack Vicinity

Adhesive bonding is now commonly used in aircraft, cars, boats, etc. In these applications, thin panels are often bonded. In such thin structures, heterogeneous mechanical loading along the bondline edge (or potential crack front) is likely to arise due to 3D structural effects. The crack front and its vicinity is a special region, in that it is where structural properties of the adherend material meet those of the adhesive (discontinuity). To investigate the stress distribution in this region, we have observed the deflection of a flexible adherend in an asymmetric wedge bonded joint loaded in Mode I. A sensitive laser profilometry technique was used to observe the main vertical beam displacement and curvature along the length, as well as the resulting transverse, or anticlastic effect, due to Poisson's ratio. From this analysis is evaluated the heterogeneous tensile stress distribution in the adhesive in the vicinity of the crack front.

단층라멜라 돔의 시공 중 접합부 강성에 따른 좌굴특성

Single-layer latticed domes with rigid-joint have an advantage in the construction cost and the aesthetic. But, in single-layer latticed domes, the joints are hard to discriminate between pin-joint and rigid-joint, and consisted of semi-rigid joint in practical. And the erection of large roof structures requires special techniques. As one of these special techniques is the Step-Up erection method. This paper verified buckling characteristics of single-Layer lamella domes according to the Joint flexibility under construction by Step-up method. The results are follows: As erection steps increase, the buckling strength decreases. It is occurred the joint buckling by snap through on the top of dome when the joint flexibility close the rigid. And large tensile stress distribution appeared in circumferential member of bottom boundary when the step of construction is low. As the step of construction increase, large compressive stress distribution showed in the top of dome.

Effect of roller shapes on strip buckling in a continuous annealing furnace

The effect of roller shapes on strip buckling in a continuous annealing furnace was focused on. The tensile stress distribution, the transverse compressive stress, and the critical buckling stress of the strip were studied by the finite element method (FEM) when the flat roller, crown roller, single taper roller, and double taper roller were used, respectively. Simulation results show that strip buckling is most likely to occur with the crown roller, then the double taper roller, and finally the single taper roller. Also, strip buckling can not occur when the flat roller is used. Considering strip snaking, the single taper roller and double taper roller are suggested in the continuous annealing furnace. The double taper roller with a better strip snaking-prevention ability should be applied in the sections with high strip temperature, and the single taper roller with a better buckling-prevention ability should be used in the sections with low strip temperature.

Multiscale Simulation of Electro-Chemo-Mechanical Coupling Behavior of PEN Structure under SOFC Operation

Numerical simulations are performed to characterize the micro- and macro-scale aspects of the electro-chemo-mechanical coupling behavior of a positive-electrolyte-negative (PEN) structure under SOFC operation. The mathematical homogenization method is applied to evaluate the macroscopic material properties of porous electrodes by performing the corresponding microscopic analyses. In particular, the potential transfer conditions at pore surfaces and triple-phase boundaries are homogenized to be source terms in the macroscopic electron-ion coupled transport problem. A case study is conducted to identify the major source of generation and evolution of cracks running through a disc-shaped cell. In this context, we are concerned especially with the effects of the radius of the cell, the thickness of an electrolyte and the profile of temperature's in-plane distribution on the macroscopic tensile circumferential stress distributions.

Seismicity during lateral dike propagation: Insights from new data in the recent Manda Hararo–Dabbahu rifting episode (Afar, Ethiopia)

Seismicity released during lateral dike intrusions in the Manda Hararo–Dabbahu Rift (Afar, Ethiopia) provides indirect insight into the distribution and evolution of tensile stress along this magma‐assisted divergent plate boundary. In this paper, 5 dike intrusions among the 14 that form the 2005–present rifting episode are analyzed with local and regional seismic data. During dike intrusions, seismicity migrates over distances of 10–15 km at velocities of 0.5–3.0 km/h away from a single reservoir in the center of the rift segment, confirming the analogy with a slow spreading mid‐ocean ridge segment. Comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression. Dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward, suggesting an asymmetry of tension in the brittle‐elastic lithosphere. Seismicity during dike injections is concentrated near the propagating crack front. In contrast, faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions. The seismic energy released during dike intrusions in the Manda Hararo Rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion. The low level of seismic energy accompanying dike intrusions, despite their significant volumes, is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary.

The analysis of diagonal crack propagation in concrete beams

This paper deals with flexural concrete members reinforced longitudinally but without transverse reinforcement. The conducted experimental investigations have shown that beams without web reinforcement may fail without attaining their full flexural capacity and then shear governs their failure. In the paper, there are presented recent results of the author’s own experiments, which aimed at disclosing some aspects of the propagation of cracks in longitudinally reinforced concrete beams without stirrups. The experimental program has been designed to investigate the influence of the shear span-to-depth ratio on diagonal crack propagation and load carrying capacity of tested beams. The obtained test results were compared with numerical results made on the basis of Finite Element Method. In the performed numerical simulation the same longitudinally reinforced concrete beams as in the test were considered. During the numerical analysis, the special attention was paid on a tensile stress distribution and on changes of diagonal crack propagation in dependence of the shear span-to-depth ratio in calculated beams.

Thermal effects in high-power Nd:YAG disk-type solid state laser

An investigation of thermal effects in a high-power Nd:YAG disk-type solid state laser pumped with different pump beam transverse profiles is carried out by numerical simulation based on the finite element method (FEM). Impact of the heat sink on the thermal effects is included in the simulation. The distribution of first principle stress, thermally induced birefringence, including the distribution and variation of the birefringence loss, are studied. The characteristics of the phase variation are analyzed with consideration of the temperature gradient, deformation, strain and thermal stress. Thermal lensing is explored as a function of pump power and of the radius pumped with different pump beam transverse profiles. The non-parabolic part of optical phase distortion is simulated. Furthermore, the characteristics of the bi-focus of the disk laser are also studied. Experiments on the maximum tensile stress distribution and depolarization loss are carried out. The presented calculations are in qualitative agreement with the experimental observations.

Discussion on the computational algorithms of stress inside nano-scale solid material

Abstract This paper describes the definitions and differences of the computational methods of virial stress and engineering stress in nano-scale problems. According to simulation results of tensile testing of a single-crystalline nano-scale copper bar, the average stresses in a cross-section of the copper bar, calculated with different methods, are compared. The values and distributions of local tensile stresses reveal further insights into these two methods. The advantages and disadvantages of these two methods are also discussed in detail.

Effect of pulsed current on temperature distribution, weld bead profiles and characteristics of gas tungsten arc welded aluminum alloy joints

Temperature distribution and weld bead profiles of constant current and pulsed current gas tungsten arc welded aluminium alloy joints were compared. The effects of pulsed current welding on tensile properties, hardness profiles, microstructural features and residual stress distribution of aluminium alloy joints were reported. The use of pulsed current technique is found to improve the tensile properties of the weld compared with continuous current welding due to grain refinement occurring in the fusion zone.

Channeled local structural transformations in polycrystal surface layers in alternate cyclic bending

The paper presents experimental data on the stage evolution of channeled local structural transformations in the direction of maximum tangential stress in polycrystal surface layers under cyclic loading. At the first loading stage, islands of surface layers are extruded; at the second loading stage, they coagulate into folded structures. The geometric parameters of the folded structures correlate with the stacking fault energy of the material. The obtained results support the concept of physical mesomechanics about a chessboard distribution of tensile and compressive normal stresses at the “weakened surface layer - crystalline substrate” interface.

Diverse Embedment Model for Steel Fiber-Reinforced Concrete in Tension: Model Verification

Results obtained from the Diverse Embedment Model (DEM) for analysis of steel fiber-reinforced concrete, described in an accompanying paper, are compared with experimental results produced by several independent researchers. Variation of the fiber orientation factor, which accounts for the effects of finite member dimensions on fiber orientation and embedment, is also theoretically investigated and compared with experimental data. Verification studies show that the proposed model provides accurate predictions of the tensile stress and crack width relationship of uniaxial tension specimens containing straight or end-hooked steel fibers. In addition, the proposed model provides accurate calculations of the distribution of tensile stress provided by fibers. The proposed model is also shown to be useful in modeling aspects of the tensile behavior, such as crack spacing and tension stiffening, of fiber-reinforced concrete (FRC) members reinforced with ordinary steel reinforcing bars.

Steady State Dynamic Performance of the Slab with EPS-Blocks

In this paper, novel filled materials EPS-blocks of cast-in-situ slabs are presented. And EPS-blocks with light weight which are inexpensive are validated to be sufficient strong as filled materials to bring an innovation for the development of cast-in-situ slabs. Based on modal analysis, it is found that the decreased weight has a greater influence to the mode of the slab than the weakened stiffness. According to harmonic response analysis, tensile stress distributions of the slab with EPS-blocks are depicted in figures and explained. Typically, orifice stress distributions are expounded and stress variations of nodes in different places are illustrated. From these studies, it is concluded that the steady state dynamic performance of the slab with EPS-blocks is good.

Ductility of thin copper films on rough polymer substrates

Electronic components in modern flexible electronics are connected by interconnects, which typically have the form of metal films resting on polymer substrates. This paper firstly studies experimentally the ductility of Cu films deposited on polyimide substrate with roughened surface (due to sandblasting) and finds that, upon tensile loading along the direction of film surface, the density of surface cracks in the film decreases with increasing surface roughness. The method of finite elements is subsequently employed to study the distribution of tensile stresses in the film and their influence on film cracking (initiation and propagation). It is demonstrated that a rough (curved) interface can reduce the tensile stresses along the film surface so as to restrain channel cracking of the film. Finally, the cohesive zone model is used to study the initiation and spreading of damage in the film and interfacial debonding of the curved interface. Both the interfacial damage and interface crack length are reduced as a result of interface roughening.

Minimizing Stress Concentrations in the Femoral Heads of Hip Joint Prostheses: Effect of Borehole Shapes

This paper presents a stress analysis of the ceramic femoral heads of hip joint prostheses with different borehole shapes to evaluate their mechanical reliability in terms of stress concentration. Under the ideal loading conditions used for ceramic rupture tests specified by the ISO 7206-5 standard, a finite element (FE) modeling is performed to determine the tensile and hoop stress distributions in the ceramic femoral heads. Two borehole shapes that are currently used in the manufacturing industry for hip joint prostheses, namely the flat bottom and keyhole, were first studied. Two new borehole shapes, dome arc and dome ellipse, were then introduced by the authors in the paper to minimize the stress concentration. It was found that while the currently used borehole shapes lead to high tensile notch stresses at their critical corners causing possible fracture failure of ceramic heads, the authors’ borehole designs can improve the mechanical reliability significantly. In addition, the effects of taper-bore contact length and their interface friction are investigated and discussed.

Comparative investigations of surface instabilities (“sharkskin”) of a linear and a long-chain branched polyethylene

An experimental study of the physical origin and the mechanisms of the sharkskin instability is presented. Extrusion flows through a slit die are studied for two materials: a linear low density polyethylene (LLDPE) which exhibits sharkskin instability for flow rates larger than an onset value and a low density polyethylene (LDPE) which does not show any instability over a broad range of flow rates. By combining laser-Doppler velocimetry (LDV) with rheological measurements in both uniaxial extension and shear, the distributions of tensile and shear stresses in extrusion flows are measured for both materials. The experimentally measured flow fields appear to be qualitatively similar for both the unstable (LLDPE) and stable case (LDPE): around the die exit the flow accelerates near the boundaries and decelerates around the flow axis. The fields of the axial gradients of the axial velocity component are, however, quite different in the two cases. In the unstable case there exists a strongly non-uniform transversal distribution of velocity gradients near the die exit. This non-uniformity of the distribution of gradients is significantly smaller in the stable case. Significant differences in the extensional rheological properties of the two materials are found as well. Due to its branched structure, the LDPE is able to sustain higher tensile stresses prior to failure. Measurements of the distributions of tensile stresses around the die exit reveal a stress boundary layer and a stress imbalance between the boundaries and the bulk. The magnitude of the stress imbalance exceeds the melt strength in the experiments with the LLDPE which causes the failure of the material in the superficial layers and results in the emergence of the sharkskin instability. In the experiments with the LDPE, the magnitude of the stress imbalance remains smaller than the melt strength which explains the lack of an instability. The measured shear stresses around the die exit are significantly smaller than the tensile stresses, suggesting that the shear component of the flow plays no significant role in the emergence of the sharkskin instability.