The Investigation of the Fracture Behavior of a Chinese 9% Cr Steel Welded Joint under Creep-Fatigue Interactive Loading

To understand the premature-fracture mechanisms of long-term service damage of an advanced alloy’s (Chinese P92 steel) welded joint, the creep-fatigue (CF) experiments with holding times of 30, 120, 300, 600 and 900 s were individually performed at 923 K. The cyclic softening, inelastic-strain amplitudes and stress-relaxation behaviors were compared between welded and base-metal (BM) specimens. From the results, the failure stage of the welded specimens occupies 45% of the lifetime fraction, while it only takes up 20% of the lifetime fraction in BM specimens with short holding times (30 and 120 s). Furthermore, only two softening stages were observed for both kinds of CF specimens with long holding times. The absence of a third softening stage in longer-held specimens indicates that the processes of macroscopic-crack initiation, propagation and rupture were accelerated. Based on the observation of the fracture surfaces, the fracture mechanism shifted from fatigue-dominated damage to creep-fatigue interaction when the holding period was increased.

Exploring the Effect of Asperity Order on Mechanical Character of Joint Specimen from the Perspective of Damage

The joint morphology is multiscale. The effect of each asperity order on the mechanical properties of joints is different. The shear mechanical properties of joint specimens are related to its surface damage characteristics. At present, there are still few studies on the effect of roughness on the shearing mechanical properties of joint from the perspective of damage of each asperity order. In this paper, the standard roughness profile was chosen as initial morphology. The standard roughness profile was decomposed into waviness and unevenness by the method combine the ensemble empirical mode decomposition (EEMD) and the cut-off criterion. Then, the joint specimen which contains waviness and unevenness and the specimen which only contains waviness were prepared by the 3D engraving technology. The 40 sets of joint specimens with different asperity order were subjected to direct shear tests under different normal stresses. Based on the 3D scanning technology and ICP iterative method, the damaged area and the damage volume were calculated. Based on the damage volume data and the acoustic emission (AE) data, the effect of asperity order to the joint mechanical behaviour was studied. The results indicate that (1) under low normal stress, the unevenness plays a control role in the failure mode of the joint specimen. Under low normal stress, the joint surface containing only waviness exhibits slip failure, and the joint surface with unevenness exhibits shear failure. With the increase of the normal stress, the failure mode of the specimen containing only waviness changes from slip failure to shear failure; (2) the unevenness controls the damage degree of the joint specimen. The damaged area, damage volume, AE energy rate, and accumulative AE energy of the joint specimen with unevenness are larger than those of the specimen with only waviness, and this difference increases with the normal stress increase; (3) the difference between the joint specimen with unevenness and specimen with only waviness mainly exists in the prepeak nonlinear stage and the postpeak softening stage. The characteristic parameters of acoustic emission generated in the postpeak softening stage of the joint specimen with unevenness are greater than those of the specimen with only waviness. This phenomenon can be used to explain the stress drop difference at the postpeak softening stage; (4) the AE b value can be used to evaluate the damage of joint specimens. Analysing the damage difference of each asperity order under different normal stresses is of great significance to the analysis of the influence of the morphology of the joint surface on the mechanical properties of the joint.

On Durability of Structurally Inhomogeneous Materials

Numerical methods used to calculate strength are based on energy approaches and minimization of functionals of one type or another. Yet the model of a material is limited to stable processes of deformation. As a result, a considerable number of deformation properties related to realization of the softening stage in materials of structural elements remains unaccounted for. To describe fracture as a new phenomenon in the behavior of structures, one needs to apply newer experimental and calculational approaches. The article cites results of modelling and experimental notions on the stage of softening in materials and its role in determining their durability. It is proposed to define the durability of a structurally inhomogeneous material as its capacity of equilibrium deformation beyond its ultimate strength under specified loading conditions. That reflects nonlocality of criteria for the failure of the material, their dependence both on its own properties and the geometry of a structural element. Complete stress-strain diagrams for structural materials of various classes and examples on how the softening stage is realized in structural materials are given.

Analysis of the Elastic-Plastic Theoretical Model of the Pull-Out Interface between Geosynthetics and Tailings

Aiming at the strain-hardening and strain-softening phenomena between geosynthetics and tailings during pull-out tests, bilinear and trilinear shear stress-displacement softening models were proposed. The pull-out process of the hardening reinforcement was divided into the elastic stage, elastic-hardening transition stage, and pure hardening stage. The pull-out process of the softened reinforcement was divided into the elastic stage, elastic-softening transition stage, pure softening stage, softening-residual transition stage, and pure residual stage. The expressions of the interface tension, shear stress, and displacement at the different stages under a pull-out load were derived through the interface basic control equation. At the same time, the evolution law of the interface shear stress at different pull-out stages was analysed, and the predicted results of the two elastic-plastic models were compared with the experimental results. The results show that the predicted results are in good agreement with the experimental data, which verifies the validity of the proposed two elastic-plastic models for the progressive failure analysis of reinforcement at the pull-out interface. During the process of pull-out, the transition stage is not obvious. When the reinforcement is in the elastic stage, the nonlinearity and maximum value of the interface shear stress increase with an increase in the elastic shear stiffness, while the tensile stiffness shows the opposite trend. When the reinforcement is in the hardening or softening stage, the larger the hardening (softening) shear stiffness is, the larger the change range of shear stress is and the more obvious the hardening (softening) characteristics of the reinforcement are. The results comprehensively reflect the progressive failure of reinforcement-tailing interfaces with different strain types and provide theoretical support for the study of the interface characteristics of geosynthetic-reinforced tailings.

Meso-Scale Simulation of Concrete Based on Fracture and Interaction Behavior

Based on the cohesive zone model, a meso-scale model is developed for numerical studies of three-phase concrete under tension and compression. The model is characterized by adopting mixed-mode fracture and interaction behavior to describe fracture, friction and collision in tension and compression processes. The simulation results match satisfactorily with the experimental results in both mechanical characteristics and failure mode. Whole deformation and crack propagation process analyses are conducted to reveal damage evolution of concrete. The analyses also set a foundation for the following parametric studies in which mode II fracture energy, material parameter, frictional angle and aggregates’ mechanical characteristics are considered as variables. It shows that the mixed-mode fracture accounts for a considerable proportion, even in tension failure. Under compression, the frictional stress can constrain crack propagation at the beginning of the damage and reestablish loading path during the softening stage. Aggregates’ mechanical characteristics mainly affect concrete’s performance in the mid-and-late softening stage.

Experimental Study of Strain-Softening Stage in Materials

Physical theory of reliability is based on research into degradation processes of various origins which take place in a material of a stressed construction. Experimental evaluation of parameters carried out for such processes is a practically important problem by itself. One of the approaches to solving this problem is related to the studies into the stage of material softening due to deformation. This paper analyzes the issues of experimental validation of material softening properties in terms of a phenomenological approach to the problem of structural fracture. Results of deformation analysis for the “machine – model specimen” system, using catastrophe theory are used to form requirements for carrying out experiments which investigate the softening stage of materials. The success of such experiments – which should include recording a branch descending to zero on a computer diagram – is possible when small specimen, made from structurally heterogeneous materials, are strained in a sufficiently rigid testing machine. Thus, the conditions for manifestation of the softening stage connect properties of the material with properties of the load-applying system. Therefore, the material's limiting state – preceding the fractured – also depends on the conditions of loading, and the criteria of that fracture would be nonlocal. In consideration of the results of diagrams plotted from various bases for deformation measurement, a necessity of utilizing local material characteristics for calculation purposes is discussed. As an example of using the complete diagrams for determining kinetics of material degradation from external load, the results of specimen testing, which follows a cyclic training, are cited.

Study of Strain-Softening Stage in Structurally Inhomogeneous Materials

The paper considers – in terms of modeling and experimental testing – the softening stage manifested in structurally inhomogeneous materials under quasi-static and cyclic load. The method which is used to build the theoretical basis is typical for mechanics. It starts with defining a new material property – strain-softening phase. The stage is realized in the form of a descending branch of the curve on computer diagrams recorded during simple tension or torsion tests on non-reusable specimens in a reasonably rigid testing machine. The type of such a diagram is determined by the structural inhomogeneity of a material, which is defined by distribution law for strength and plasticity properties of structural elements. Complete stress-strain curves or diagrams (CSSD) with a branch descending to zero have been plotted for materials of various classes. Degradation of parameters of a descending branch on a computer diagram recorded for trained specimens provides a new angle on the subject of relation between static and cyclic material properties. The experiments and modeling which were carried out provide basic understanding of softening in terms of deformation.

Fatigue-induced dual stiffness behavior of filled styrene–butadiene rubber

Payne and Mullins effects are widely observed in reinforced rubber materials. The mechanisms by which these two effects work are not fully understood. Several models have been proposed, including molecular slippage model, bond rupture model, and filler rupture model. In this study, two different compounds of styrene–butadiene rubber were prepared using carbon black and silica as reinforcement fillers, respectively, and subjected to cyclic fatigue process. Tensile, creep, and relaxation tests were performed on fatigued samples to assess the residual stress–strain behavior by comparing with the results from similar tests using pristine (no fatigue) samples. When the tensile stiffness behavior of fatigued specimens was evaluated, we noted that the stiffness versus strain behavior which exhibited a monotonic decreasing–increasing behavior with the pristine specimens changed to what we call “dual-stiffness” condition, where the specimens went through a first (low) turning point as with the pristine samples, but then dropped off of a peak to go through a second softening stage, similar to the first softening stage of the pristine material. We believe that such spiking (dual) stiffness behavior characterized by a “Peak” point represents a combination of both Payne and the Mullins effects active during fatigue loading. We conclude that molecular slippage and bond rupture are the main factors affecting the physical properties of carbon black-filled compounds, while breakage and recombination of the filler are the key mechanisms affecting the silica-filled compounds during the fatigue process.

An Analytical Method for Studying the Stress-Strain Relations of Bulk Metallic Glass Matrix Composites under Tension

In this contribution, an analytical model was formulated to predict the tensile stress-strain relations of bulk metallic glass matrix composites (BMGCs) based on Weng's theoretical frame for dual-ductile composites. For in-situ BMGCs, BMG matrix also exhibits the elastic-plastic deform response as well as the dendrite phases during the stretching. The shear bands are regarded as Mode-I cracks, and whereby the strain-softening stage in the stress-strain curves can be well reflected. Furthermore, multi-particle representative volume element based FEM modelling was employed to clearly explain the failure mechanisms in BMGCs as a necessary complement. The predictions are in reasonable agreement with the experimental results. The presented analytical method will shed some light on optimizing the microstructures, and is of convenience in the engineering applications.

Analysis of temperature and strain rate dependencies of softening regime for tempered martensitic steel

This article aims to analyse the influence of temperature and strain rate on the mechanical behaviour of the high-chromium martensitic steel X20CrMoV12-1. The analysis is based on two series of high-temperature uniaxial tensile tests. In a first series, the tensile tests are conducted until rupture, and temperature as well as strain rate are varied systematically. The corresponding stress–strain curves show an extended softening stage. In order to examine softening, it is crucial to distinguish between microstructural changes and strain localisation due to necking. For this reason, a tensile test at low strain rate is performed several times, while the test is terminated at different strain levels in order to examine the onset of necking. Based on the test results and surface measurements of the deformed specimens, the strain level at which necking starts is determined, and possible interactions between softening and necking are discussed. The tensile tests have been conducted in order to calibrate a mechanical model which supplies reliable predictions on the material behaviour under different loading scenarios at elevated temperatures. For this reason, a framework based on microstructural processes is presented in the second part of the article. The model applies a binary mixture approach in conjunction with an iso-strain concept. Furthermore, backstress and softening variables are introduced to consider hardening and softening effects. This procedure results in a system of three differential equations describing the mechanical behaviour.