Axial Stress State Research Articles

Effects of axial static stress and confining pressure on the dynamic compressive behaviours of granite

This study aims to investigate experimentally the effects of axial static stress and confining pressure on the dynamic compressive behaviours of granite. A new index of confining pressure increase factor (CPIF) was introduced to evaluate the confining pressure-induced strength enhancement, a modified definition was proposed to determine the dynamic deformation modulus, and the roles of energy storage limit (ESL) were highlighted to explain the energy absorption and release process. The results showed that when the axial static stress was less than a threshold value that was equivalent to the rock’s elastic limit, it coupled with the confining pressure and elicited positive effects on the strength enhancement of the rock. However, once the threshold value was exceeded, the axial static stress weakened the rock while the confining pressure began to strengthen the rock in a more significant way. Moreover, the dynamic deformation modulus increased as the static stresses increased (including axial static stress and confining pressure) when the rock remained in the linear elasticity under the static loading, while it declined when the rock suffered from damage before the impact. Furthermore, the results revealed that the ESL tended to play critical roles in the process of energy absorption and release in different situations.

Study on Dynamic Characteristics of Deep Siltstone under High Static Stress and Frequent Dynamic Disturbance

The mechanical characteristics of siltstone under high static load and frequent dynamic disturbance were investigated by the improved split Hopkinson pressure bar (SHPB), which is used to simulate the failure phenomenon of the extracted ore drift owing to high stress concentration and blasting disturbance in Dongguashan copper mine at the depth of more than 900 m. The maximum dynamic strain, dynamic stress, and dynamic deformation modulus of rock of each impact were researched to provide theoretical basis for mining production. The results show that the dynamic stress‐strain curves of siltstone before the peak stress present two characteristics of linear and nonlinear variation and different degrees of resilience after the peak stress under high static stress and frequent dynamic disturbance. The strain rate rises in a linear trend with the increasing times of dynamic disturbance, but the value of the strain rate is approximately constant in the process of one impact. With the increase of the strain rate, the maximum dynamic strain rises at the same static stress, while the peak dynamic stress and dynamic deformation modulus decrease linearly. Because the inner stored elastic energy of rock determines the value of resilient strain after unloading, the elastic energy may increase with the increase of static axial stress in a certain range and then the resilient strain increases. Accumulative times of dynamic disturbance and static axial stress show the relationship that the former is obviously reduced with the increase of the later.

Осесимметричное напряжённое состояние равномерно слоистого пространства, содержащей периодическую систему внутренних дискообразных трещин

Осесимметричное напряжённое состояние равномерно слоистого пространства, содержащей периодическую систему внутренних дискообразных трещин

Axial-torsional fatigue and cyclic deformation of 304L stainless steel at room temperature

This work investigates the axial-torsional fatigue and cyclic deformation behaviour of 304L stainless steel at room temperature. Four fully reversed strain-controlled loading paths (axial, torsional, proportional axial-torsional, and 90º out-of-phase axial-torsional) and a fully-reversed shear strain-controlled with static axial stress loading were investigated. For axial, torsional, torsional with static stress and few proportional experiments, an initial cyclic softening was followed by secondary hardening related to martensitic transformation. Secondary hardening was not observed for non-proportional loading nor for some proportional experiments. The influence of the non-stabilized cyclic deformation behaviour on the fatigue life estimates of two multiaxial critical plane fatigue models (Smith–Watson–Topper and Fatemi–Socie) was investigated. Life estimates based on the stress-strain hysteresis loops corresponding to the maximum softening and to the half-life were similar for the two models.

Simulation of Fatigue Crack Growth in Weld Joint of Thin Structures Using Characteristic Tensor

It is known that fatigue life of welded structures, such as automotive and ship structures, is greatly influenced by welding residual stress. In case of real structures, the orientation of the stresses produced by static load and that produced by repeated load and that of welding residual stress are generally different from each other and they form a highly complex multi axial stress states. On the other hand, the fatigue behaviour has been mostly studied for uniaxial stress state without residual stress. The crack growth rate is related to the stress intensity factor range ΔK such as in the Paris Low [1]. To predict the fatigue behaviour of real welded structures, the three dimensional stresses produced by static load, repeated load and welding must be rationally correlated with a scalar value ΔK. The author proposes a Characteristic Tensor which characterizes the singular stress field at crack tip and its invariant parameters, such as principal values or invariants which has similar form as strain energy density, are used to compute ΔK in a consistent manner. Its potential usefulness is demonstrated through simple examples.

Axial stress-strain behavior of high-strength concrete confined by circular thin-walled steel tubes

The axial stress-strain behavior of high-strength concrete confined by circular thin-walled steel tube was experimentally investigated. Specifically, 18 column specimens with the key parameters of concrete compressive strength and diameter-to-thickness ratio of steel tube were tested by monotonic or cyclic axial compression. The failure mode, load versus axial displacement curves, strain and stress states of the steel tube, interaction behavior between the steel tube and the concrete core, and stress-stain characteristics of steel tube confined concrete were described and discussed in detail. The loading path affects little on the strength and stiffness of the confined concrete. As the concrete grade increases from C40 to C80, the specimens show higher brittleness with a ratio of yield to peak loads changing from 0.6 to 0.9. Due to the accumulation of frictional stresses between the steel tube and the concrete, the stress state of steel tube varies along the tube height. The equivalent confinement of steel tube was estimated by the average transverse stress which was based on the observed stress distribution. The stress-strain model proposed by Mander et al. was modified and the predicted stress-strain curves for steel tube confined concrete are in good agreement with test results.

Experimental characterization of dynamic behaviour of gelatin-based material using DIC

In this paper, the dynamic response of gelatin-based soft material under impact loading is investigated. The dynamic tests are principally performed by the classical SHPB (Split Hopkinson Pressure Bars) technique. However, due to the very low mechanical impedance of the specimen compared with the Hopkinson bars, the feeble impact forces are measured by highly sensitive piezoelectric polyvinylidene fluoride (PVDF) pressure sensors instead of SHPB measurement system. The PVDF pressure sensors are placed on the interfaces between the specimen and the bars. During the impact test, the non-equilibrium stress state and inhomogeneous strain fields are developed in the specimen; a digital image correlation (DIC) technique is proposed to identify the inhomogeneous displacement fields using high speed photography. A non-parametric approach based on the DIC technique is developed to deduce the transient stress fields in the longitudinal and transverse directions from the displacement fields measured by DIC. The validation of the calculated stress fields is performed by comparing them with the stress measurements from the PVDF pressure sensor at the bottom end of the specimen. Furthermore, stress-strain response is carried out using this approach throughout the specimen. It is clearly shown that the average highest strain rate varies with position in the specimen. This lead to multiple stress-strain relations determined at different strain rates by only one impact test. The significant strain rate sensitivity is observed at the tested rate range from 81/s to 269/s. Under compression loading, the axial stress state is developed as a simple compression only in the central part of the specimen due to the friction at the interfaces between the specimen and the bars. According to the calculated results based on movement of “long waves”, the region of the simple compression stress state in the central part of the specimen is localized. It is observed that the axial stress is much more important than the transverse stress in the central part and this confirms the assumption of uni-axial compression stress state in the specimen.

Pulsed magnetic field-induced changes in the meso- and nanostructure of Co49Ni21Ga30 martensite

A near single-variant martensitic state of Co[Formula: see text]Ni[Formula: see text]Ga[Formula: see text] alloy was induced by a static axial stress of 66[Formula: see text]MPa. Upon application of a magnetic field pulse, the single-variant state of stressed specimen was transformed into a hierarchic martensitic structure at micron, submicron and nanometer scales. The martensitic structures, which were induced during the magnetic field pulse, did not disappear after the field was switched off. This stability of the field-induced martensitic structure is attributed to the appearance of additional twinning systems with hardly mobile twin boundaries.

Hot workability and densification behavior of sintered powder metallurgy Al-B4C preforms during upsetting

The workability and the densification behavior of a porous Al–B4C preforms have been carried out in the present investigation. Hot upsetting tests have been carried out on Al–B4C powder metallurgy (P/M) preforms having an initial preform density of 0.9 and having different B4C compositions of 2%, 4% and 6%. The samples were compressed between two flat dies in a hydraulic press of 50 ton capacity under varying deformation temperatures such as 200°C, 300°C, 400°C and 500°C under the tri–axial stress state condition. The workability and densification behavior of Al–B4C preforms were analyzed till the initiation of cracks on the outer surface of the preform. The experimental results were analyzed for the various deformation parameters such as axial strain, relative density, formability stress index and different stress ratio parameter under the tri–axial stress state condition. The formability and densification behavior were discussed with the axial strain (εz) during the hot upsetting process. The relationships between the various stress ratio parameters (σq/σeff, σm/σeff) and formability stress index (βσ) as a function of the relative density under the tri–axial stress state condition were established.

A unified multiscale ductility exhaustion based approach to predict uniaxial, multiaxial creep rupture and crack growth

Numerical and analytical methods for predicting uniaxial damage have largely depended on the constituent components of the stress/strain measured data which have inherent scatter. Models developed for this purpose have also attempted, with some degree of success, to address the fundamental issues of failure mechanisms within a multiaxial stress state context. This paper presents a new analytical/empirical/a postpriori unifying approach to predict creep damage and rupture under uniaxial/multiaxial and crack growth conditions by deriving a multiscale based constraint criterion. Essentially, the model links the global constraint due to geometry in a globally isotropic material with a microstructural constraint arising from creep diffusional processes occurring in a sub-grain locally anisotropic microstructure. Furthermore, it is shown that the model is consistent with the established NSW crack growth model (Nikbin et al., 1984, 1986; Tan et al., 2001) which is routinely used to determine the plane stress/strain bounds for cracking rates in fracture mechanics geometries and cracked components. The concept assumes that at very short times an initial upper shelf material tensile strength and global plasticity and power law creep control creep damage failure and sub grain multiaxial axial stress state dependent failure strain dominates the long term diffusion/dislocation controlled creep response. It is established that the material yield strength in the short term and a measure of creep failure strain at the creep secondary/tertiary transition region described at the limits by the Monkman-Grant failure strain (Monkman and Grant, 1963), are the important variables in both the uniaxial and multiaxial failure processes. For verification creep constitutive properties from long term data from uniaxial and multiaxial and crack growth tests on Grade P91/92 martensitic steels from various databases (EPRI, private communications; NIMS data base), are used to establish the procedure.

Non-destructive Assessment of the Axial Stress State in Iron and Steel Truss Structures by Dynamic Measurements

This paper is concerned with the inverse identification of the stress state in axially loaded slender members of iron and steel truss structures using measured dynamic data. A methodology is proposed based on the finite element model updating coupled with nature-inspired optimization techniques, in particular the particle swarm optimization. The numerical model of truss structures is calibrated using natural frequencies and mode shapes from vibration tests, as well as additional information of the axial forces in selected truss members based on the experimentally identified modal parameters. The results of the identification are the axial forces or corresponding stresses in truss structures and the joint rigidity in relation to pinned and rigid conditions. Attention is given to several examined aspects, including the effects of the axial tensile and compressive forces on the dynamic responses of trusses, mode pairing criteria, as well as modeling assumptions of joints and the use of a joint rigidity parameter. Considering the pairing of modes, it is performed by adapting an enhanced modal assurance criterion that allows the selection of desired clusters of degrees-of-freedom. Thus, information extracted from the measurements related to specific modes is utilized in a more beneficial way. For modeling of joints, the numerical model of a truss structure includes rotational springs of variable stiffness to represent semi-rigid connections. Moreover, a fixity factor is introduced for practical estimation of the joint flexibility. The effectiveness of the proposed methodology is demonstrated by case studies involving simulated and laboratory experimental data.

Determination of Forming Limit Diagrams for Thin Foil Materials Based on Scaled Nakajima Test

For a better process understanding of micro deep drawing processes and reliable prediction of component failure in FE simulations, it requires the most accurate knowledge of actual material behaviour. However, it is not sufficient to describe material failure for a multi axial stress state in deep drawing using a mechanical parameter as the elongation from tensile test. A forming limit diagram and a forming limit curve are more suited to describe the limit of formability under deep drawing stress state conditions. Methods like hydraulic or pneumatic bulge tests are available to determine forming limit curves even for thin metal foil materials. Nevertheless, using these methods only positive minor strains can be achieved. Especially for a deep drawing process negative minor strains and the left side of a forming limit diagram are more important. Therefore, in this study, experiments based on scaled Nakazima tests were performed to determine complete forming limit diagrams for different foil materials with a thickness range of 20 µm to 25 µm. Scaling the test setup improves the handling of thin specimens. Results with a higher local resolution and the specimens’ size is much closer to the actual size of a micro deep drawn component. Using this testing method forming limit diagrams for the materials Al99.5, E-Cu58, stainless austenitic nickel-chromium steel X5CrNi18-10 (1.4301 / AISI 304), all produced by rolling, and an Al-Zr-foil, produced by a PVD sputtering process, were determined for the micro range.

Damage Assessment in a SiC-fiber reinforced Ceramic Matrix Composite

Assessment of the fracture behavior of a SiC-fbre-reinforced barium osumilite (BMAS) ceramic matrix composite tested under static and cyclic tension conditions is reported herein. Notched specimens were used in order to limit material damage within a predefined gauge length. Imposition of successive unloading/reloading loops was found to result in an increase by 20% in material strength as compared to pure tension; the observed increase is attributed to energy dissipation from large-scale interfacial debonding phenomena that dominated the post-elastic tensile behaviour of the composite. Cyclic loading also helped establish the axial residual stress state of the fibres in the composite of tensile nature via a well-defined common intersection point of unloading-reloading cycles. A translation vector approach in the stress-strain plane was successful in establishing the residual stress-free properties of the composite and in reconciling the scatter noted in elastic properties of specimens with respect to theoretical expectations.

Large-scale interfacial damage and residual stresses in a glass–ceramic matrix composite

The current work is concerned with the micro-mechanics of fracture of a SiC-fiber-reinforced barium osumilite (BMAS) ceramic matrix composite tested under both monotonic and cyclic tension. The double-edge notch (DEN) specimen configuration was employed in order to confine material damage within a predefined gage length. The imposition of successive loops of unloading to complete load relaxation and subsequent reloading were found to result in an increase by 20% in material strength as compared to pure tension; the finding is attributed to energy dissipation from large-scale interfacial debonding phenomena that dominated the post-elastic mechanical behavior of the composite. Cyclic loading also helped establish the axial residual stress state of the fibers in the composite, of tensile nature, via a well-defined common intersection point of unloading–reloading cycles. An approach consisting of the application of a translation vector in the stress–strain plane was successfully used to derive the residual stress-free properties of the composite and reconcile the scatter noted in elastic properties of specimens with respect to theoretical expectations.

Material characterization of Inconel 718 from free bulging test at high temperature

Macroscopic superplastic behavior of metallic or non metallic materials is usually represented by the strain-rate sensitivity, and it can be determined by tensile tests in uniaxial stress state and bulging tests in multi axial stress state, which is the actual hot forming process. And macroscopic behavior of Non-SPF grade materials could be described in a similar way as that of superplastic materials, including strain hardening, cavity and so on. In this study, the material characterization of non-SPF grade Inconel 718 has been carried out to determine the material parameters for flow stress throughout free bulging test under constant temperature. The measured height of bulged plate during the test was used for estimation of strain-rate sensitivity, strain-hardening index and cavity volume fraction with the help of numerical analysis. The bulged height obtained from the simulation showed good agreement with the experimental findings. The effects of strain-hardening and cavity volume fraction factor for flow stress were also compared.

Numerical simulation on rock failure under combined static and dynamic loading during SHPB tests

The failure process of rock subjected to combined static and dynamic loading constitutes the mechanism of many engineering applications such as rockburst prediction, rock fragmentation, as well as rock drilling and blasting. In this study, the basic principle for simulating the damage and failure process of rock under combined static and dynamic loading is introduced, and it is implemented into RFPA-Dynamics (Rock Failure Process Analysis for Dynamics), a finite element package to analyze the failure process of rock-like media subjected to dynamic loading. The extended RFPA-Dynamics code is firstly validated by simulating the stress distribution and damage evolution in rock specimens during conventional SHPB (Split Hopkinson Pressure Bar) tests. Then, it is utilized to simulate the failure process of rock under combined static and dynamic SHPB tests, and the effects of axial static stress and dynamic stress on the damage and failure process of rock are examined. The strength increase factor (SIF) under combined static and dynamic loading, depending on the rock heterogeneity, static stress and strain rate, is predicted using numerical simulations, and the mechanisms associated with the increase of dynamic strength of rock subjected to the combined static and dynamic loading are clarified. The rock strength predicted with numerical simulations, which is closely related to the rock heterogeneity, is in qualitative agreement with that observed in laboratory SHPB tests, although the predicted rock strength under combined static and dynamic loading is still lower than those from experiments.

Evaluation of Static Axial Stress in Round Bar by Eigen Mode Deflection

Evaluation of Static Axial Stress in Round Bar by Eigen Mode Deflection

Shakedown of Thick Cylinders With Radial Openings Under Thermomechanical Loadings

Shakedown and ratcheting behaviors of thick-walled cylindrical vessels with radial openings subjected to cyclic thermomechanical loadings are investigated by the inelastic finite element analysis. Different shakedown and ratcheting responses are described in the modified Bree diagram considering the influences of various opening radius ratios ri/Ri, thickness ratios Ro/Ri as well as different axial stress states. Then, two simplified shakedown assessment methods of perforated cylinders are discussed. The results indicate that elastic/plasticity shakedown boundary reduces significantly owing to the presence of a radial opening while varies slightly with the opening radius, which can be defined as 1.2 approximately in the modified Bree diagram for conservative elastic/plasticity shakedown evaluation. Moreover, shakedown/ratcheting boundary decreases significantly with increasing the opening radius or decreasing the axial stress. Finally, comparing with the calculated results, the two simplified methods are verified to be accurate and intuitive to estimate the shakedown behaviors of perforated cylinders under various thickness and opening radius conditions.

Weight optimization and fatigue design of a welded bogie beam structure in a construction equipment

A welded bogie beam in an articulated hauler has been designed, analyzed, produced and fatigue tested in order to demonstrate the features of the research project LOST (Lightweight Optimized welded STructures). The results from LOST, e.g. a new weld class system, effect of using high strength steel and post treatment, were applied. The general goal was achieved by 20% weight reduction compared with existing component and at the same time a decrease of the production cost. This was accomplished mainly due to less material usage and cutting cost, but also due to a more efficient design, which decreased the production time by 30 min. Reduced weight also causes less environmental impact and lower fuel costs during the operation of the hauler. The demonstrator was produced with welds in as-welded condition where TIG treatment and weld preparation were needed in a couple of welds. A major conclusion was that the inspection of the weld quality after production needs to be improved so that the demands are secured and later failures in service can be avoided. Another conclusion was made during the fatigue test due to a multi axial stress state, which will affect the design in future projects. The requirements on the static and fatigue strengths are fulfilled in the new bogie beam design. However, some fatigue improvement effects have not been incorporated in the analysis of the new design, e.g. compressive residual stresses.

Optimal Matching of Processing Parameters of Horizontal V-Shaped Anvils for Heavy Axial Forgings Manufacturing

The forging method with horizontal V-shaped anvils (HVA) is effective in the control of inner stress states, metal tissue, etc. FEM numerical simulation is conducted for the HVA forging method, under various processing parameters. The results show that during HVA forging, the transversal and axial stress states in the middle part of forgings are influenced by not only the ratio of the anvil width to billet height (anvil-width ratio )but also the ratio of the blank width to billet height (blank-width ratio) and the reduction ratio, while the end parts are influenced little. Among those processing parameters, the reduction ratio plays an important role. When the reduction ratio is 5% or 10%, it is possible to obtain axial and transversal tensile stress. When the reduction ratio is larger than 15%, it is easy to obtain two-way compressive stress. Therefore, in order to control the two-way tensile stress, big reduction ratio should be adopted. Small blank-width ratios result in the forgings cracks because of transversal tensile stress in common flat-anvil stretching method. On the contrary, even if the blank-width ratio is small, transversal compressive stress can be generated in forgings for the HVA forging method, only if the matching of the anvil-width ratio and the reduction ratio is suitable. The feature of the HVA forging method benefits the preventing forgings from inner cracks. The results provide the theoretical foundation for the application of the HVA forging method.