Measuring material plastic response to cyclic loading in modern rail steels from a minimal number of twin-disc tests

Advances in rail materials from conventional rail steels to those with higher yield points and the potential of additively manufactured laser clad coatings to improve the durability of railway track components presents a new challenge in characterisation. Many of these new and novel materials have either limited test samples available or are more resistant to strain and therefore present challenges in characterisation. The method reported here uses twin disc tests to simulate cyclic loading experienced by rail steel in service. A sample from a single test condition is analysed, measuring the shear yield stress and the accumulated shear strain at multiple depths below the contact surface, from which a Shear Yield Stress – Plastic Shear Strain (SYS-PSS) relationship is extracted. Knowledge of the stress history of a rail sample is not required to apply the method and minimal samples are required, providing a technique which can be used on rail steel samples removed from service.

Coupling numerical model of hydraulic fracturing seepage in soft coal based on elastoplastic damage

In order to guide the field application of hydraulic fracturing of soft coal in coal mine, based on the elastic-plastic damage theory, the coupling numerical model of soft coal hydraulic fracturing seepage was studied. The porosity strain relationship equation, permeability strain relationship equation, the relationship between permeability and volume plastic tensile strain and volume plastic shear strain of coal and rock mass are derived, and the plastic correction equation and softening parameters are defined. The stress coupling equation and yield criterion are programmed and embedded into the finite difference software FLAC3D for numerical solution. The numerical simulation shows that the numerical calculation model of soft coal hydraulic fracturing conforms to the actual law, and the field fracturing radius investigation experiment is consistent with the numerical simulation results.

Bearing Capacities of Shallow Skirted Foundations After the Action of Multi-Directional Cyclic Displacements Considering Soil Degradation

Shallow skirted foundations have been applied widely in ocean engineering. Under the action of external excitation, the shallow skirted foundations on soft soil undergo cyclic displacements during service state. Under the action of cyclic displacements, the foundations drive the surrounding soft soil to produce a continuous accumulation of absolute plastic shear strain, which weakens the shear strength of the soft soil around the foundations. Therefore, the bearing capacities of shallow skirted foundations reduce after the action of cyclic displacements considering soil degradation. In order to study the evolutions of bearing capacities of shallow skirted foundations after the action of multi-directional cyclic displacements considering soil degradation, the elastoplastic finite element models of shallow skirted foundations with different embedment ratio are established. Cyclic displacements are applied along different displacement loading paths, and the evolutions of soil shear strength and bearing capacities of shallow skirted foundations after the action of cyclic displacements are analyzed. The results show that the soil softening zone gradually expands from the stress concentration zone of the soft soil to the surrounds with increasing number of loading cycles. Due to the enlargement and weakening of the soil softening zone, the failure envelopes of shallow skirted foundations gradually shrink, but the shrinkage trend gradually converges with increasing number of loading cycles. The shapes of the failure envelopes are relatively less affected by the cyclic number of displacements. The size of the failure envelopes is greatly affected by the loading paths while the shape of the failure envelopes is relatively less affected.

Stability analysis of strain-softening slopes based on the gravity increase method

With traditional slope stability analysis methods, it is difficult to accurately describe the progressive failure process and dynamic variation law of slope stability. The strain-softening constitutive model was therefore used to simulate the progressive failure process of a strain-softening slope based on the gravity increase method (GIM), with the displacement interface employed to determine the sliding surface. A sensitive analysis of the characteristic parameters within the softening stage was then conducted. The results are as follows: There are similar space-time evolution laws of residual strength factor and shear strain increment, with failure starting from the slope toe and extending gradually. The sliding surface of strain-softening slopes is located between that of the slope with peak strength and the sliding surface of the slope with residual strength. The stability coefficient shows an exponential growth trend with the increase of residual cohesion and residual plastic shear strain threshold, with a positive linear correlation between the residual friction angle and stability factor. The residual friction angle is the most sensitive factor in slope stability, followed by the residualcohesion, with the residual plastic shear strain threshold being the least sensitive.

A New Unified Solution for Circular Tunnel Based on a Four-Stage Constitutive Model considering the Intermediate Principal Stress

Based on the triaxial test, the elasto-perfectly plastic strain-softening damage model (EPSDM) is proposed as a new four-stage constitutive model. Compared with traditional models, such as the elasto-brittle-plastic model (EBM), elasto-strain-softening model (ESM), elasto-perfectly plastic model (EPM), and elasto-peak plastic-brittle plastic model (EPBM), this model incorporates both the plastic bearing capacity and strain-softening characteristics of rock mass. Moreover, a new closed-form solution of the circular tunnel is presented for the stress and displacement distribution, and a plastic shear strain increment is introduced to define the critical condition where the strain-softening zone begins to occur. The new analysis solution obtained in this paper is a series of results rather than one specific solution; hence, it is suitable for a wide range of rock masses and engineering structures. The numerical simulation has been used to verify the correctness of the EPSDM. The parametric studies are also conducted to investigate the effects of supporting resistance, residual cohesion, dilation angle, strain-softening coefficient, plastic shear strain increment, and yield parameter on the result. It is shown that when the supporting resistance is fully released, both the post-peak failure radii and surface displacement could be summarized as EBM > EPBM > ESM > EPSDM > EPM; the dilation angle in the damage zone had the highest influence on the surface displacement, whereas the dilation angle in the perfectly plastic zone had the lowest influence; the strain-softening coefficient had the most significant effect on the damage zone radii; the EPSDM is recommended as the optimum model for support design and stability evaluation of the circular tunnel excavated in the perfectly plastic strain-softening rock mass.

Sliding wear mechanism of ductile materials – Effect of sliding direction reversal

The work presented in this paper tries to shed more light on the mechanism by which ductile surfaces fail and leave the contact surface during loaded pure sliding contact. An extensive experimental program was designed aimed at exploring the role of plastic shear strain accumulation in surface failure. Reversing the direction of strain during testing was the main variable which was facilitated by reversing the sliding direction. Changes in structure deformation morphology and accumulated plastic strain were analyzed. The effect of different sliding direction reversal regimes during testing, compared to unidirectional sliding to the same sliding distance, was thoroughly investigated. Results came to support that plastic strain accumulation is responsible for contact surface failure and, as a result, material loss from the ductile surface during sliding. It was evident, under the test conditions used, that reversing the sliding direction at different predefined sliding distances has resulted in delaying surface failure, resulting in lower wear loss compared to that found under unidirectional sliding. Multiple strain direction reversals resulted in higher beneficial effect in delaying failure. Furthermore, the earlier the sliding reversal is carried out, the better its effect of delaying failure. Findings have been explained in terms of plastic strain accumulation that leads to failure of the surface layer after reaching a certain strain to failure limit.

Wear of Carbon Steel (0.65%C) in Rolling-Sliding Contact with Creep Ratio

In rolling-sliding contact, wear will occur when the accumulated plastic shear strain of the material at the surface exceeds its critical shear strain for failure. During rolling-sliding contact, the difference in relative velocities of two contacting components can cause slip in the contact (known as creep). The higher creep ratio may increase the severity of wear. In this work, the wear rate of the material and the behaviour of material just below the contact surface in rolling-sliding contact with various creep ratios were investigated. The carbon steel (about 0.65% C) was chosen as the test material and wear test was conducted using disc-to-disc contact testing machine with the maximum contact pressure of 1000 MPa and with various creep ratio of 1%, 5% and 7%. The results show the higher creep ratio causes the material to accumulate critical shear strain more quickly, resulting in the increase of wear (i.e., from about 0.0047μm/cycle for creep ratio of 1% up to about 0.0077μm/cycle for creep ratio of 7%). .

Study on Shear Strength of Expansive Soil Considered Softening by Triaxial Tests

Swell-shrinking, crack development and over-consolidation are the characteristics of expansive soil, and it is an over-consolidation soil undergone dry-wet cycles. So there may be some faults to analyze expansive soil slope stability adopting the traditional strength criterion and calculation methods. In this paper, triaxial tests were carried out to obtain the relations between the parameters of shear strength and generalized plastic shear strain. The soil shear strength increases and then decreases along with the increasing of shear deformation due to soil over-consolidation. Moreover, the residual strength is achieved finally. The functions are applied in simulating the relations between the parameters and generalized plastic shear strain in order to analyze expansive soil slope stability.

An Analytical Elasto-Plastic Analysis for Stability of Axisymmetric Wellbore

In this paper, a general shear strain hardening and softening Drucker-Prager model based on the existing triaxial compression test results has been introduced to model the rock behaviour which captures well the mechanical characteristic of typical soft rock formations. This elastoplastic model is then adopted to develop a rigorous analytical solution for the drained wellbore drilling problem subjected to in-plane isotropic stress field, and to simulate the borehole collapse failure. It is found that the wellbore boundary value problem can be reduced to solving a system of first order ordinary differential equations in the plastic zone, with the radial, tangential, and vertical stresses as well as the volumetric strain and plastic shear strain being the five basic unknowns. The illustration numerical example shows the distributions of stress components and volumetric strain around the borehole, in addition to the evolving plastic deviatoic strain and stress path for a rock point at the borehole surface due to the wellbore drilling. The critical mud pressures necessary to prevent borehole collapse, predicted by the elastoplastic analysis based on different wellbore instability criteria, are compared with the value corresponding to the elastic theory, which are found to be considerably lower.