Tensile Stress Field Research Articles

Theoretical Rayleigh and Love Waves from Explosions in Nonspherical Cavities and from Tectonic Release in Compressive Stress Fields

Ben‐Menahem and Mikhailov (1995) developed a spherical mapping approximation to compute displacement fields due to explosions in nonspherical cavities embedded in elastic media. The explosion‐generated stress distribution was mapped onto the surface of an equivalent virtual spherical cavity. In this article, we use these techniques to analyze Rayleigh‐ and Love‐wave radiation patterns for a horizontal cylinder cavity, modeling an explosion in a tunnel for diameter‐to‐length ratios ranging from a line source (zero diameter) to that of a disk (zero length). The equivalent tectonic release is that created in a horizontal direction tensile stress field. With this orientation, the Love‐wave radiation pattern is identical to that of a vertical strike‐slip fault with a strike of 45° from the axis of symmetry. Assuming temporal step-function pressures and ignoring near-field nonlinear elasticity, shock waves, and reverberations in the air-filled cavity, we express the displacement fields for either explosion or tectonic release, in terms of a multipole expansion of spherical eigenvectors. This is done for both axisymmetric ellipsoidal and right cylindrical cavities. As in Ben‐Menahem and Mikhailov, at long wavelengths the dominant spherical harmonics are found to be, at most, second order. This is true even for extreme geometries such as the finite-length line source and the disk. For long-period surface waves, the first two terms suffice. At very long periods, the radiation field is the same as that due to the rapid formation of a spherical cavity in the presence of tensile stress, and differs by only a sign in case the initial stress is compressive instead. The two sources differ only in their respective second-order moment tensor time functions.

A silicon microwire under a three-dimensional anisotropic tensile stress

Three-dimensional tensile stress, or triaxial tensile stress, is difficult to achieve in a material. We present the investigation of an unusual three-dimensional anisotropic tensile stress field and its influence on the electronic properties of a single crystal silicon microwire. The microwire was created by laser heating an amorphous silicon wire deposited in a 1.7 μm silica glass capillary by high pressure chemical vapor deposition. Tensile strain arises due to the thermal expansion mismatch between silicon and silica. Synchrotron X-ray micro-beam Laue diffraction (μ-Laue) microscopy reveals that the three principal strain components are +0.47% (corresponding to a tensile stress of +0.7 GPa) along the fiber axis and nearly isotropic +0.02% (corresponding to a tensile stress of +0.3 GPa) in the cross-sectional plane. This effect was accompanied with a reduction of 30 meV in the band gap energy of silicon, as predicted by the density-functional theory calculations and in close agreement with energy-dependent photoconductivity measurements. While silicon has been explored under many stress states, this study explores a stress state where all three principal stress components are tensile. Given the technological importance of silicon, the influence of such an unusual stress state on its electronic properties is of fundamental interest.

Evaluation of cracks in an offshore crane runway girder

AbstractThis paper deals with the evaluation of fatigue cracks in a box type radial crane runway girder with full penetration welds between the web and flange. After 20 years of service fatigue cracks were observed which were initiated in the flange at the toe of the full penetration weld with the web. The observed cracks in the crane runway girder vary in length from a few mm to 330 mm with a summation of the lengths of all observed cracks being 750 mm, on a total length of 56000 mm, thus being only 1.3 %.The investigation consists of the fatigue analysis described in this paper and additional experimental investigations with scale tests of approximately 1:2 on equivalent I sections with a concentrated load and with a line load to investigate whether cracks stop when they have grown through the residual tensile stress field. For this analyzed crane runway girder with multi‐layered full penetration welds and with small cracks at the weld toes in the flange, based on EN 1993‐6 the damage would be 1.0 for a design class 98 which is not far from the ”class“ 92 found by Kuhlmann et al, [1] for tests with rolling wheels, however, related to weld failures from the root of not fully penetrated fillet welds in combination with crack initiations at the weld toe.

Experimental and analytical investigation of fatigue crack propagation of T‐welded joints considering the effect of boundary condition

AbstractT‐welded joints are commonly employed in ship and ocean structures. The fatigue failure of structure components subjected to cyclic loading always occurs in T‐welded joints because of the metallurgical differences, tensile residual stress fields and stress concentrations. The former researches about T‐welded joints fatigue have in common that the boundary condition needs to be taken into account as an influencing parameter to predict the crack propagation during cyclic loading. In this paper, the crack growth behaviour in T‐welded joint processed with Q345D steel (Pingxiang Iron & Steel Co., Ltd, Hukou, Jiangxi Province in China) under the fatigue loading was analysed via analytical model and verified via experiment. The results show that the influence of boundary condition should be considered in T‐welded joint structure during crack propagation in weld toe area. The correction factor concerning the effect of boundary condition and modified Paris' equation was proposed according to the experimental results and verified by the following repeated experiments.

OPTIMIZATION OF THE TEMPERATURE CONTROL SCHEME FOR ROLLER COMPACTED CONCRETE DAMS BASED ON FINITE ELEMENT AND SENSITIVITY ANALYSIS METHODS

Achieving an effective combination of various temperature control measures is critical for temperature control and crack prevention of concrete dams. This paper presents a procedure for optimizing the temperature control scheme of roller compacted concrete (RCC) dams that couples the finite element method (FEM) with a sensitivity analysis method. In this study, seven temperature control schemes are defined according to variations in three temperature control measures: concrete placement temperature, water-pipe cooling time, and thermal insulation layer thickness. FEM is employed to simulate the equivalent temperature field and temperature stress field obtained under each of the seven designed temperature control schemes for a typical overflow dam monolith based on the actual characteristics of a RCC dam located in southwestern China. A sensitivity analysis is subsequently conducted to investigate the degree of influence each of the three temperature control measures has on the temperature field and temperature tensile stress field of the dam. Results show that the placement temperature has a substantial influence on the maximum temperature and tensile stress of the dam, and that the placement temperature cannot exceed 15 °C. The water-pipe cooling time and thermal insulation layer thickness have little influence on the maximum temperature, but both demonstrate a substantial influence on the maximum tensile stress of the dam. The thermal insulation thickness is significant for reducing the probability of cracking as a result of high thermal stress, and the maximum tensile stress can be controlled under the specification limit with a thermal insulation layer thickness of 10 cm. Finally, an optimized temperature control scheme for crack prevention is obtained based on the analysis results.

Constrained Sintering in Fabrication of Solid Oxide Fuel Cells.

Solid oxide fuel cells (SOFCs) are inevitably affected by the tensile stress field imposed by the rigid substrate during constrained sintering, which strongly affects microstructural evolution and flaw generation in the fabrication process and subsequent operation. In the case of sintering a composite cathode, one component acts as a continuous matrix phase while the other acts as a dispersed phase depending upon the initial composition and packing structure. The clustering of dispersed particles in the matrix has significant effects on the final microstructure, and strong rigidity of the clusters covering the entire cathode volume is desirable to obtain stable pore structure. The local constraints developed around the dispersed particles and their clusters effectively suppress generation of major process flaws, and microstructural features such as triple phase boundary and porosity could be readily controlled by adjusting the content and size of the dispersed particles. However, in the fabrication of the dense electrolyte layer via the chemical solution deposition route using slow-sintering nanoparticles dispersed in a sol matrix, the rigidity of the cluster should be minimized for the fine matrix to continuously densify, and special care should be taken in selecting the size of the dispersed particles to optimize the thermodynamic stability criteria of the grain size and film thickness. The principles of constrained sintering presented in this paper could be used as basic guidelines for realizing the ideal microstructure of SOFCs.

Polarization and valley switching in monolayer group-IV monochalcogenides

Group-IV monochalcogenides are a family of two-dimensional puckered materials with an orthorhombic structure that is comprised of polar layers. In this article, we use first principles calculations to show the multistability of monolayer SnS and GeSe, two prototype materials where the direction of the puckering can be switched by application of tensile stress or electric field. Furthermore, the two inequivalent valleys in momentum space, which are dictated by the puckering orientation, can be excited selectively using linearly polarized light, and this provides an additional tool to identify the polarization direction. Our findings suggest that SnS and GeSe monolayers may have observable ferroelectricity and multistability, with potential applications in information storage.

Ion‐Exchanged Lithium Aluminosilicate Glass: Strength and Dynamic Fatigue

Sodium for lithium and potassium for lithium ion‐exchanges of a lithium aluminosilicate glass were conducted and the resulting strength and dynamic fatigue characteristics were studied. Four‐point bend mechanical tests revealed that greater strengthening can be achieved by the potassium for lithium ion‐exchange, compared to the sodium for lithium ion‐exchange, and that the dynamic fatigue tendency is strongly suppressed by both exchanges. This suppression of dynamic fatigue characteristics of ion‐exchange strengthened glass was explained by the ability of the surface compressive layer to delay the onset of slow crack growth. Bulk stresses continue to increase in magnitude while the crack is arrested in the surface compressive stress region. Upon offsetting the surface compressive stress, the crack rapidly propagates into a high‐magnitude tensile stress field until the fracture toughness is reached, resulting in minimal crack growth prior to material failure. A slow crack growth model utilizing a fracture mechanics weight function was developed to simulate the experiments. Dynamic fatigue characteristics of the as‐received glass, without ion‐exchange treatment, were also measured and simulated for comparison.

Deciphering the role of solar-induced thermal stresses in rock weathering

A dearth of direct field observations limits our understanding of individual mechanical weathering processes and how they interact. In particular, the specific contributions of solar-induced thermal stresses to mechanical weathering are poorly characterized. Here, we present an 11 mo data set of cracking, using acoustic emissions (AEs), combined with measurements of rock temperature, strain and other environmental conditions, all recorded continuously for a granite boulder resting on the ground in open sun. We also present stresses derived from a numerical model of the temperature and stress fields in the boulder, idealized as a uniform elastic sphere experiencing simple solar temperature forcing. The thermal model is validated using this study’s data. Most observed cracking coincides with the timing of calculated maximum, insolation-driven, tensile thermal stresses. We also observe that most cracking occurs when storms, or other weather events, strongly perturb the rock surface temperature field at these times. We hypothesize that these weather-actuated thermal perturbations result in a complex thermal stress distribution that is superimposed on the background stresses arising from simple diurnal forcing; these additive stresses ultimately trigger measurable cracking. Measured locations of observed cracking and surface strain support this hypothesis in that they generally match model-predicted locations of maximum solar-induced tensile stresses. Also, recorded rock surface strain scales with diurnal temperature cycling and records progressive, cumulative extension (dilation), consistent with ongoing, thermal stress-driven subcritical crack growth in the boulder. Our results therefore suggest that (1) insolation-related thermal stresses by themselves are of sufficient magnitude to facilitate incremental subcritical crack growth that can subsequently be exploited by other chemical and physical processes and (2) simple insolation can impart an elevated tensile stress field that makes rock more susceptible to cracking triggered by added stress from other weathering mechanisms. Our observed cracking activity does not correlate simply with environmental conditions, including temperature extremes or the often-cited 2 °C/min thermal shock threshold. We propose that this lack of correlation is due to both the ever-varying ambient stress levels in any rock at Earth’s surface, as well as to the fact that ongoing subcritical crack growth itself will influence a rock’s stress field and strength. Because similar thermal cycling is universally experienced by subaerially exposed rock, this study elucidates specific mechanisms by which solar-induced thermal stresses may influence virtually all weathering processes.

Periodic variation of stress field in the Koyna–Warna reservoir triggered seismic zone inferred from focal mechanism studies

The Koyna–Warna region in western India is globally recognized as the premier site of reservoir triggered seismicity (RTS) associated with the Koyna and Warna reservoirs. The region is characterized by continuous seismic activity observed since several decades, including the world's largest triggered earthquake of M6.3 which occurred in Koyna in 1967. While the role of reservoirs in triggering earthquakes has been widely discussed, the actual tectonic mechanism controlling earthquake genesis in this region is hardly understood. The Koyna–Warna region is exclusively governed by earthquakes of strike–slip and normal fault mechanism distinct from the thrust faulting seen in other active zones in the Indian region. In the present study, a comprehensive catalog of 50 focal mechanism solutions of earthquakes that occurred during the last 45years in the Koyna–Warna region is developed, both from previous literature and from moment tensor inversion studies by the authors using broadband data from a local seismic network operating since 2005. The seismicity and fault plane data have enabled precise delineation of trends of the major causative faults, which are further accentuated using the double-difference technique. Stress inversion of the focal mechanism data has provided the best fitting principal compressive and tensile stress field of the region, which in conjunction with the deciphered fault zones provides a feasible model of seismogenesis in this region. Based on the observed temporal variation of faulting mechanism a model of alternating cycles of predominantly strike–slip and normal faulting is proposed, which is attributed to a periodic peaking and relaxation respectively of the horizontal compressive stress field in this region due to the Indian plate collision with Eurasia.

3D numerical analysis of crack propagation of heterogeneous notched rock under uniaxial tension

Macroscopic notches play an important role in evaluating the fracture process zone (FPZ) and the strengths of a heterogeneous rock mass. Crack initiation, propagation and coalescence for unnotched, single-notched and double-notched rock specimens are numerically simulated in a 3-D numerical model (RFPA3D). A feature of the code RFPA3D is that it can numerically simulate the evolution of cracks in three-dimensional space, as well as the heterogeneity of the rock mass. For the unnotched case, special attention is given to the complete stress–strain curve and the corresponding AE events for the failure process of rock specimen. By comparing with published experimental results, the simulation results from RFPA3D are found to be satisfactory. For the single-notched case, the effect of the length and the depth of the single notch and the thickness of the specimen on the failure mode and peak stress are evaluated. The 3D FPZ is very different from that in two dimensions. For the double-notched case, the effects of the separation distance and overlap distance of the double notches, as well as influence of the homogeneity index (m) are also investigated. As the overlap distance increases, the direction of the principal tensile stress at each notch-end changes from a perpendicular direction (tensile stress field) to a nearly parallel direction (compressive stress field), which affects the evolution of the cracks from the two notches.

Effects of laser shock processing on high cycle fatigue crack growth rate and fracture toughness of aluminium alloy 6082-T651

The effects of laser shock processing without protective coating on high-cycle fatigue crack growth and fracture toughness were investigated. Laser shock peening treatment was performed on compact tension specimens from both sides perpendicular to the crack growth direction, followed by subsequent grinding. Fatigue crack growth tests were performed at frequencies between 116 and 146Hz, at R=0.1 and a constant stress intensity range during the fatigue crack initiation phase and K-decreasing test. A lower number of cycles was required to initiate a fatigue precrack, and faster fatigue crack growth was found in tensile residual stress field of LSP-treated specimens. The crack growth threshold decreased by 60% after LSP treatment. The fracture toughness decreased by 28–33% after LSP treatment. The fatigue-to-ductile transition boundary on fractographic surfaces show linear fatigue crack fronts in non-treated specimens and curves after LSP treatment.

Macroscopic tensile plasticity by scalarizating stress distribution in bulk metallic glass.

The macroscopic tensile plasticity of bulk metallic glasses (BMGs) is highly desirable for various engineering applications. However, upon yielding, plastic deformation of BMGs is highly localized into narrow shear bands and then leads to the “work softening” behaviors and subsequently catastrophic fracture, which is the major obstacle for their structural applications. Here we report that macroscopic tensile plasticity in BMG can be obtained by designing surface pore distribution using laser surface texturing. The surface pore array by design creates a complex stress field compared to the uniaxial tensile stress field of conventional glassy specimens, and the stress field scalarization induces the unusual tensile plasticity. By systematically analyzing fracture behaviors and finite element simulation, we show that the stress field scalarization can resist the main shear band propagation and promote the formation of larger plastic zones near the pores, which undertake the homogeneous tensile plasticity. These results might give enlightenment for understanding the deformation mechanism and for further improvement of the mechanical performance of metallic glasses.

Experimental Analysis of Residual Stress in Friction Stir Processed Cast AlSi9Mg Aluminium Alloy

The relationship between friction stir processing (FSP) parameters and longitudinal residual stress profiles in modified cast aluminium alloy AlSi9Mg is presented. The influence of tool geometry, rotational speed and the number of processing passes were analysed. To experimentally measure residual stress, the trepanation method was adopted. The results indicated that an increase in the rotational speed caused an increase in the residual stress. Also, the Triflute tool promoted a higher level of residual stress than a conventional FSP tool. The region around the FSP bead was characterised by tensile residual stress fields that were balanced by compressive stresses in the parent material. A higher residual stress is observed on the advancing side than on the retreating side. An increase in the number of processing passes increased the level of residual stress in the modified material.

Moisture absorption, desorption, and moisture-induced electrical performances of high-temperature vulcanized silicone rubber

This study focuses on the moisture absorption, moisture desorption, and moisture-induced electrical performances of high-temperature vulcanized (HTV) silicone rubber (SR). The influences of some parameters, such as filler (fumed silica and aluminum hydroxide) characteristics, relative humidity, temperature, tensile stress, and AC electric field, on moisture absorption/desorption are investigated. Results show that the tensile stress accelerates the moisture uptake and increases the saturated moisture absorption rate of HTV SR, and that the AC electric field has no significant influence on the moisture absorption of HTV SR. The dependencies of the dielectric constant, loss tangent, volume resistivity, AC breakdown strength, depolarization current, and polarization charge of HTV SR on the moisture absorption rate are examined. The depolarization current and the polarization charge as a whole increase with the increased moisture absorption rate of HTV SR.

A View on Annealing Behavior of Cu-Filled Through-Silicon Vias (TSV)

In this article, based on analyzing the effect of self-annealing of copper film both empirically and numerically, a new hypothesis has been proposed to better explain the mechanism of copper protrusion evolution with annealing temperature. Electroplated copper in TSV goes through an intense self-annealing process after deposition at room temperature. The effect of self-annealing, which has been all ignored to date, is critical to the annealing behavior of Cu-filled TSV. During self-annealing, a tensile stress field is generated in TSV due to the occurrence of the initial copper protrusion, and this tensile stress can strongly suppress the copper protrusion during annealing treatment. Atomic Force Microscope (AFM) has been used for the measurements of copper protrusion. A finite element model based on the hot deformation and self-annealing of copper film has been proposed to confirm the hypothesis.

FATIGUE LIFE ESTIMATION OF REAR FUSELAGE STRUCTURE OF AN AIRCRAFT

Integrity of the airframe structure is achieved through rigorous design calculations, stress analysis and structural testing. Finite element method (FEM) is widely used for stress analysis of structural components. Each component in the airframe becomes critical based on the load distribution, which in-turn depends on the attitude of the aircraft during flight. Fuselage and wing are the two major components in the airframe structure. The current study includes a portion of the fuselage structure. Empennage is the rear portion of the aircraft, which consists of rear fuselage, Horizontal tail and vertical tail. The air loads acting on the HT also get transferred to rear portion of the fuselage. First step in ensuring the safety of the structure is the identification of critical locations for crack initiation. This can be achieved through detailed stress analysis of the airframe In this project one of the major stress concentration areas in the fuselage is considered for the analysis. Rear fuselage portion with a cargo door cutout region will be analysed. The structure considered for the stress analysis consists of skin, bulkheads and longerons, which are connected to each other through rivets. Aerodynamic load acting on the aircraft components is a distributed load. Depending on the mass distribution of the fuselage structure the inertia forces will vary along the length of the fuselage. The inertia force distribution makes the fuselage to bend about wing axis. During upward bending, bottom portion of the fuselage will experience tensile stress. A cutout region in the tensile stress field will experience high stress due to concentration effect. These high stress regions will be probable fatigue crack initiation locations in the current work, fatigue damage calculation will be carried out to estimate the fatigue life of the structure under the fluctuating loads experienced during flight. Miner’s rule will be adopted for fatigue damage calculation.

Suppression of boron-oxygen defects in Czochralski silicon by carbon co-doping

We have investigated the influence of carbon co-doping on the formation of boron-oxygen defects in Czochralski silicon. It is found that carbon can effectively suppress the formation of boron-oxygen defects. Based on our experiments and first-principle theoretical calculations, it is believed that this effect is attributed to the formation of more energetically favorable carbon-oxygen complexes. Moreover, the diffusion of oxygen dimers in carbon co-doped silicon also becomes more difficult. All these phenomena should be associated with the tensile stress field induced by carbon doping in silicon.

Preferential Cu precipitation at extended defects in bcc Fe: An atomistic study

As a starting point to understand Cu precipitation in RPV alloys, molecular dynamics and Metropolis Monte-Carlo simulations are carried out to study the effect of lattice defects on Cu precipitation by taking Fe–Cu system as a model alloy. Molecular dynamics simulations show that owing to the high heat of mixing and positive size mismatch, Cu is attracted by vacancy type defects such as vacancies and voids, and tensile stress fields. In accordance, preferential precipitation of Cu is observed in Metropolis Monte-Carlo simulations at dislocations, prismatic loops and voids. The interaction of Cu with a stress field, e.g., that associated with a dislocation or a prismatic loop, is dominated by elastic effect and can be well described by the linear-elasticity theory. For prismatic loops, the attraction to Cu is found to be size-dependent with opposite trends displayed by vacancy and interstitial loops. The size-dependences can be explained by considering the stress fields produced by these loops. The current results will be useful for understanding the effect of neutron irradiation on Cu precipitation in reactor-pressure-vessel steels.

Experimental Investigation of Fatigue Crack Propagation in Residual Stress Fields

This work is on the interaction of residual stresses and fatigue cracks in welded steel plates. A longitudinal dummy weld was used to create high tensile residual stresses in axial loading direction. The fatigue crack was trained to start at an artificial notch and propagated through a field of high tensile residual stresses. The fatigue crack propagation as well as the re-distribution of residual stresses were studied in detail by combined experimental methods. The mean stress was varied as well as the residual stress state of the samples used. Samples were tested as-welded as well as stress-relieved at R = (σmin/σmax) = 0, R = -1 and R = -3. The materials used were a typical fine grained construction steel S355J2+N as well as a high strength quenched and tempered steel S960QL.It is shown how residual stress are degraded and in which way moderate residual stresses affect the fatigue strength of welded steel plates. The combined effect of residual stresses and load mean stresses is discussed.