Tensile Stress Field Research Articles

引張残留応力場から圧縮残留応力場にき裂が伝播する場合の溶接残留応力の再配分とき裂進展挙動(OS6-3 損傷評価・計測,OS6 エネルギー機器の維持管理と健全性評価)

引張残留応力場から圧縮残留応力場にき裂が伝播する場合の溶接残留応力の再配分とき裂進展挙動(OS6-3 損傷評価・計測,OS6 エネルギー機器の維持管理と健全性評価)

The effect of residual thermal stresses on the fatigue crack growth of laser-surface-annealed AISI 304 stainless steel: Part I: computer simulation

The effect of residual thermal stresses on the fatigue crack growth of the laser-surface-annealed AISI 304 stainless steel, especially the effect of stress redistribution ahead of the crack tip was extensively evaluated in the study. Based on the finite element simulation, the longitudinal residual tensile stress field has a width of roughly 20 mm on the laser-irradiated surface and was symmetric with respect to the centerline of the laser-annealed zone (LAZ). Meanwhile, residual compressive stresses distributed over a wide region away from the LAZ. After introducing a notch perpendicular to the LAZ, the distribution of longitudinal residual stresses became unsymmetrical about the centerline of LAZ. High residual compressive stresses exist within a narrow range ahead of notch tip. The improved crack growth resistance of the laser-annealed specimen might be attributed to those induced compressive stresses. As the notch tip passed through the centerline of the LAZ, the residual stress ahead of the notch tip was completely reverted into residual tensile stresses. The existence of unanimous residual tensile stresses ahead of the notch tip was maintained, even if the notch tip extended deeply into the LAZ. Additionally, the presence of the residual tensile stress ahead of the notch tip did not accelerate the fatigue crack growth rate in the compact tension specimen.

Normalized method for comparing tensile behaviors of electrorheological fluids

Tensile behaviors of electrorheological (ER) fluids over a wide applied field range have been determined. Normalized tensile stress and the normalized electric field have been proposed to compare the elongations of ER fluids under different conditions. This clearly showed the nature of the stress-strain curves under different initial electric fields and the structure effect during the elongations. Corresponding to the tensile behavior change, the exponent of tensile yield stress versus electric field was also found to vary in different initial field range.

Application of circular statistics in the study of crack distribution around cemented femoral components

Cemented stem constructs were loaded in cyclic fatigue using stair climbing loading and the resulting fatigue damage to the cement mantle was determined in terms of angular position of crack and crack length. Techniques from circular statistics were used to determine if the distribution of micro-cracks was uniform. With a designated orientation of 0°–90°–180°–270° indicating lateral–anterior–medial–posterior anatomic directions, the overall distribution of cracks was not uniform ( p<0.05) with a mean crack direction in the postero-medial (249°) quadrant of the mantle. The crack angular distribution for proximal (postero-medial; 251°) and distal (antero-medial; 112°) regions of the cement mantle was also different ( p<0.025). These findings suggest that the location of cement damage depends on anatomic position and appears to correspond with the tensile stress field in the cement mantle.

Determination of the interface strength between two polymer materials by a new curved interface tensile test: Preliminary experimental results

Recently, the authors have proposed a new experimental method for the determination of adhesion strength between two different materials. A curved interface and special arrangement of materials is used for the tensile test of bimaterial specimens to avoid singular stress fields around corners and edges. The main advantage of the test consists in the fact that the strength is determined under conditions of a uniform tensile stress field normal to the interface in the region where debonding starts. The present paper presents experimental results for two bimaterial systems - PMMA/TPE and PC/TPE (two stiff standard polymers with a thermoplastic elastomer). The expected failure behaviour was observed during the experiments, thus enabling the estimation of adhesion strength by using calculated stress concentration factors. The influence of the radius of curvature is discussed in detail.

Determination of interface strength between two polymer materials by a new curved interface tensile test

A new experimental method for the determination of adhesion strength between two different materials is proposed. A curved interface and special arrangement of materials is used for the tensile test of bimaterial specimens to avoid singular stress fields around corners and edges. Assuming that normal stresses are responsible for debonding at the interface, the adhesion strength can be determined from critical loads at debonding initiation. The stress concentrations at the interface as a function of applied load are derived by the finite-element method. The calculations are made for material constants that are typical for a polymer-polymer bimaterial; however, the method is applicable for all composites of two materials which show elastic behaviour prior to first failure. The main advantage of the test consists in the fact that the strength is determined under conditions of a uniform tensile stress field normal to the interface in the region where debonding starts.

Landau–Khalatnikov simulations for the effects of external stresses on the ferroelectric properties of Pb(Zr,Ti)O 3 thin films

The effects of external stresses on ferroelectric properties of Pb(Zr,Ti)O 3 thin film capacitors were studied with a phenomenological Landau–Khalatnikov equation. In this simulation, Landau free energy density functional ( G) considering the interaction between ferroelectric polarization ( P) and external stresses (σ) was used; G ̂ =a 1P 2+a 11P 4+a 111P 6−Q′ 12(σ 1 2+σ 2 2)P 2−EP. As reported experimentally, polarization and coercive field increased with compressive stress. With tensile stress, however, polarization and coercive field decreased. The simulated external stress effects on ferroelectric properties are in good agreement with experimental results of quadratic dependence.

Linear elastic fracture mechanics (LEFM) analysis of the effect of residual stress on fatigue crack propagation rate

In this research, both residual and applied stresses are converted to stress intensity factors independently and combined using the superposition principle. The fatigue crack propagation rates are predicted. Experiments using two different loading modes, constant applied stress intensity factor (SIF) range, and constant applied load modes are done for samples with and without initial tensile residual stresses. The samples with initial tensile residual stresses exhibit accelerations of the crack propagation rates. The results show that the weight function method combined with the three-component model provides a good prediction of fatigue crack propagation rates in tensile residual stress fields.

CFK-Zugelemente verankern

A new anchoring system for tension elements made of composites is presented in this contribution. The anchoring system reduces the chances for premature failure at the ends of the tension elements which might be caused by stress concentration. The system provides tensile force initiation at the end of the tension element, in order to procedure a uniform tensile stress field. In this respect, the results of the comparative tensile test on CFRP (Carbon Fiber-Reinforced Plastic) laminates as well as finite-element simulations are addressed. Differently designed anchoring systems were tried out by tensile tests on CFRP laminates and compared to the standard anchoring system by experimental and theoretical investigations. Some of the new anchoring systems reveal improvements in comparison to the standard clamping device. The new anchoring system might be utilized to improve the tensile testing procedure for CFRP laminates, the improvement of force initiation at adhesively bonded external CFRP reinforcements as well as for the pre-loading of tension elements for cable-stayed bridges.

HRTEM investigation of the multiplicity of [formula omitted] symmetric tilt grain boundary in Cu

Σ=9 [0 1 ̄ 1]/(1 2 2) symmetric tilt grain boundary in Cu is investigated using high resolution transmission electron microscopy (HRTEM). Modified mirror and glide-mirror symmetric structural units (SUs) are found to coexist in the grain boundary. Partial dislocations with Burgers vectors close to half a displacement shift complete (DSC) lattice are introduced to connect the glide-mirror symmetric structural unit (SU) with the modified mirror symmetric one in the grain boundary. The Burgers vectors are analyzed to be close to a/18 [1 2 2] L // a/18 [ 1 2 2 ] R and a/36 [ 4 ̄ 1 1] L // a/36 [ 4 ̄ 1 1] R . The volume expansion per unit area of the grain boundary, δV/ A, in the glide-mirror symmetric region is determined as −0.00238 nm which is a little smaller than that in rigid-body model, while the δV/ A in the modified mirror symmetric region is computed as 0.03221 nm which is larger than that in the same model. The two values of δV/ A are corresponding to compressive and tensile stress field, respectively. One possible mirror and glide-mirror symmetric structural model is proposed according to atomic relaxations.

Laser induced decohesion of coatings: probing by laser ultrasonics

The aim of the present study is to investigate a conventional laser-ultrasonics technique for the determination of intrinsic properties of oxide coatings and their adhesion strength on a metallic substrate. The good agreement between experiments and computations in an epicenter configuration allows determining the longitudinal wave velocity as well as the Young's modulus of the oxide coatings versus the porosity. For a critical value of the laser energy, a breakdown at the coating–substrate interface is generated by the laser irradiation. The critical tensile stress field developed at the coating/substrate interface, which leads to the interfacial fracture, can be easily calculated. The value of the practical adhesion which is defined is found to be in accordance with those obtained by classic contact techniques (tensile adhesion test, indentation, bending test). Finally, this work demonstrates that this quantitative, contactless test fits well to simultaneously characterise the oxide coatings and evaluate the coating–substrate adhesion.

Mechanism of diapirism and episodic fluid injections in the Yinggehai Basin

The diapirism in the Yinggehai Basin is a combined result of strong overpressure caused by rapid sedimentation of fine-grain sediments and the tensile stress field resulting from right-lateral slip of the boundary-fault. The diapirism showed multiple-stage, episodic nature, and caused intermittent counter-direction onlaps and changes in the thickness of strata. The shallow gas reservoirs in the diapir structural zone displayed obvious inter-reservoir compositional heterogeneities, and their filling history could be divided into 4 stages, with gases injected during different stages having different hydrocarbon gas, CO 2 and N 2 contents and different stable isotope compositions. The multiple-episode, intermittent activities of the diapirs, multiple-stage, non-continuous injections of fluids, and the transient thermal effect of fluid flow as well as the strong migration fractionation reflected episodic fluid injection and natural gas accumulation.

Residual stress in weldments

Abstract Undesirable residual stresses can be introduced into engineering components during welding. These stresses can affect the load carrying capacity and resistance to fracture of components. In order to quantify their effect it is necessary to know their magnitude and distribution. Several techniques are available for measuring the residual stresses in welds. However, only neutron diffraction is capable of determining these stresses non-destructively within the interior of components. In this paper, neutron diffraction is used to obtain the residual stress profiles generated in a selection of weldments manufactured by different processes. The processes considered include linear friction, inertia, electron beam and manual metal arc welding. It is found that these welding techniques typically produce a triaxial tensile residual stress field in the weld metal which decays rapidly through the heat affected zone to produce balancing compression in the parent material.

Constitutive modeling of ratcheting of metal particulate-reinforced ceramic matrix composites

Brittle solids fail in compression by a process of progressive microfracture. If subjected to cyclic loading beyond the elastic range with non-zero mean stress, usually there is a cycle by cycle accumulation of inelastic strain related to microfracturing in the direction of mean stress similarly like an accumulation of plastic strain in steels or other metals. This phenomenon is called cyclic creep or ratcheting. In cyclic plasticity there are several models capable to describe ratcheting, among others Armstrong–Frederick model (AF), Chaboche, Ohno/Wang and/or Jiang model. AF model, however, in most situations, overpredicts ratcheting. Within the context of brittle composites reinforced by ductile inclusions one can apply these models for describing the cyclic plasticity of inclusions while matrix remains macroscopically elastic. Using a suitable composite approach a response of representative volume element of composite may be modeled. But even with the AF model we do not get any ratcheting which contradicts to experimental observations. This is due to a strong constraint exerted by matrix on inclusions. It is proposed that the weakening constraint power of the matrix caused by microfracture damage around inclusions can reconcile the model predictions with experiment. It can be easily shown that even under compression loading a local tensile stress field develops around a softer inclusion. The treatment of local stresses makes possible to introduce a microfracture damage by an approximative, self-consistent method. The microfracture damage is closely coupled with the plasticity of inclusions and provides additional inelastic strains. The effect of lateral pressures under multiaxial loading can be straightforwardly included. Numerical results are presented and compared with experimental data.

Analogy between the effects of a mechanical and chemical perturbation on the conductivity of passive films

In order to study the passivity breakdown of a type 316L stainless steel, the electrochemical impedance and surface stresses were measured under straining conditions. The Mott–Schottky analysis, performed at high frequency, appears as very useful method to study, below the apparent yield strength, the effects of a mechanical stress on the capacitance values. The results obtained in sodium chloride media indicate that the semiconducting properties of passive films formed in a tensile stress field reflect a higher vacancies concentration than those formed in a stress-free state, suggesting that the passive film conductivity is increased. It was also shown that the Mott–Schottky plots drop in the presence of tensile surface stresses, indicating that the passive film composition may be different from that formed in the presence of compressive surface stresses. A comparison between the effects of mechanical stresses and those of chloride is proposed.

Role of flaw statistics in contact fracture of brittle coatings

A flaw statistics analysis is here developed to account for systematic differences between experimentally observed and theoretically predicted critical loads for the initiation of contact-induced radial cracks in brittle coatings on compliant substrates. Specific attention is drawn to deviations in critical load ( P R) data from ideal quadratic dependence on coating thickness ( d), i.e. P R∝ d 2, especially at low d values. It is postulated that these deviations are attributable to the existence of distributions in flaw size and location, in relation to the bell-shaped tensile stress fields responsible for initiation of the radial cracks at the coating lower surface. A statistics-based expression is derived for the mean values of P R in terms of flaw density and size distribution. Data from model bilayers consisting of glass plates of different thicknesses d bonded to polycarbonate substrates are used as an illustrative case study. Controlled pre-abrasion flaws are introduced into the lower glass surfaces before joining into the bilayer configuration, to enable a priori characterization of distribution parameters by image analysis. Finite element modelling is used to determine the tensile stress distribution at the coating lower surface. The predicted statistics-based P R( d) function is shown to fit the data within uncertainty bounds. Implications concerning the continued usefulness of the ideal, P R∝ d 2 relation for designing ceramic coatings for failure resistance are considered.

静的破砕剤を用いた円筒岩石の破壊実験

The hollow rock cylinders of andesite were tested under uniform inner pressure due to expansive demolition agent in the center hole without outer pressure. The pressure distribution in the center hole and the stress distribution on the outer surface of the specimen were observed during from the beginning of failure to the whole fracture under tensile stress field. At the same time, the measurement of acoustic emission and the visualization of failure within the specimen by the X-ray CT scanner were performed. Based on these results, failure mechanism of rocks under tensile stress field was discussed.

Size effect in tensile strength caused by stress fluctuations

The randomness of microstructure heterogeneous materials leads to creation of microscopic random stress fields within the bulk of the material under loading. Although in average the microscopic stresses coincide with the macroscopic (e.g., externally applied) stress, the local differences (stress fluctuations) can be high, the magnitude increasing with the volume of the heterogeneous material. In the case of uniform macroscopic loading, Gaussian stress fluctuations lead to a size effect in which the tensile strength reduces as square root of logarithm of the sample size. In practice, however, the macroscopic tensile stress fields are usually nonuniform. In this case, failure is determined by the maximum value of the macroscopic stress with the scale effect controlled by the minimum degree of the macroscopic stress decrease from its maximum. Therefore, a second model is proposed which accounts for a linear stress variation. Comparison of both models with the experimental data on macroscopic strength and stress variations in dog-bone shaped samples (scale range of 1:32), shows that the model based on the assumption of uniform macroscopic stresses can only explain part of the experimental data with unrealistic values of the fitting parameters. The model which takes into account the linear part of the macroscopic stress distribution offers reasonably good accuracy. This serves as another indication that macroscopic stress nonuniformity plays a crucial role in the mechanism of size effect.

On a non-linear finite element method for piezoelectric structures made of hysteretic ferroelectric ceramics

A simple macroscopic constitutive law for ferroelectric and ferroelastic hysteresis effects of piezoceramics is presented. This constitutive model has been implemented in the public domain finite element code PSU of Stuttgart University (Germany). The fully coupled electro-mechanical boundary value problem for our hysteresis model is solved by incrementation of the loading history. In order to verify the capabilities of our tool, a multilayer-like actuator geometry is analyzed. Concerning the residual stresses present after poling, a tensile stress field perpendicular to the direction of the electrodes can be found in the passive region of the actuator where so-called poling cracks are known to occur. It is the key feature of our finite element tool that it allows the consideration, in structural mechanical analyses, of such phenomena as they are induced by the remanent hysteresis properties of piezoceramics.

Finite element analysis of piezoceramic components taking into account ferroelectric hysteresis behavior

A simplifying macroscopic constitutive law for ferroelectric and ferroelastic hysteresis effects of piezoceramics is presented. After summarizing the uniaxial formulation motivated elsewhere (Kamlah, M., Tsakmakis, C., 1999. Int. J. Solids Struct. 36, 669–695; Kamlah, M., Böhle, U., Munz, D., Tsakmakis, Ch., 1997. Smart Structures and Materials 1997: Mathematics and Control in Smart Structures, Proceedings of SPIE, vol. 3039, 144–155), it is generalized to a three-dimensional tensorial formulation. The model has been implemented in the public domain finite element code PSU of Stuttgart University. The finite element analysis is carried out in a two-step scheme: First the purely dielectric boundary value problem is solved for the history of the electric potential. Second, prescribing this electric potential, the electro-mechanical stress analysis for the mechanical boundary conditions yields the electro-mechanical fields as, for instance, the mechanical stress field. In order to verify the capabilities of our tool, a multilayer-like actuator geometry is analyzed. It is shown that the remanent polarization remaining after poling gives rise to a non-vanishing distribution of the electric potential even it is reduced to zero at the electrodes. Concerning the residual stresses present after poling, a tensile stress field perpendicular to the direction of the electrodes can be found in the passive region of the actuator where so-called poling cracks are known to occur. It is concluded that our finite element tool is suitable for studying the influence of geometry and material parameters on the stresses in critical regions of piezoceramic devices.