Dynamic Shear Failure Research Articles

Coupled Thermo-Poro-Elastic modeling of near wellbore zone with stress dependent porous material properties

In this study, the Thermo-Poro-Elastic approach was applied to investigate the dynamic shear and tensile failure condition in the near wellbore zone. The necessary data to perform the analysis were collected from literature. To increase the simulation accuracy, the stress dependency of the material properties such as porosity, permeability, bulk density, bulk compressibility, bulk heat capacity, bulk thermal expansivity, bulk thermal conductivity, and Biot's poroelastic coefficient was also considered. The coupled study was conducted with cool, isothermal, and hot wellbore fluid to investigate the temperature influence. The obtained results were compared with the results of the uncoupled static approach. The computations take into account the changes in material properties due to the stress alteration in near wellbore zone. Results revealed that the pore pressure in the near wellbore zone was very sensitive to the wellbore fluid temperature which consequently influences the dynamics stress distribution in the studied near wellbore zone and its stability state. For the first case, when wellbore fluid temperature was lower than the formation temperature, shear strength decreased abruptly at the early time steps and then increased to a steady value, which is equal to the shear failure stress estimated by the uncoupled approach. The estimated tensile strength was high originally and descended to the value estimated by the uncoupled approach at infinity. In the isothermal and hot wellbore fluid cases, an abrupt change occurred in the shear strength; however, for hot wellbore fluid, the shear strength then decreased permanently with a slope depending on the wellbore fluid temperature. At the first step of modeling for all three cases, the initial tensile strength was the same. When formation rock was exposed to a cool wellbore fluid, tensile strength was maximum at initial stages and then decreased to the estimated value by the uncoupled approach. Rising wellbore fluid temperature causes an initial tensile instability shock (decrease in tensile strength) vanishing by time. Based on the results, average principal stress distribution in near wellbore zone intensely alters permeability distribution. The sensitivity of other parameters is less than permeability and can be neglected in further studies.

Dynamic Shear Deformation and Failure of Ti-6Al-4V and Ti-5Al-5Mo-5V-1Cr-1Fe Alloys.

To study the dynamic shear deformation and failure properties of Ti-6Al-4V (Ti-64) alloy and Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) alloy, a series of forced shear tests on flat hat shaped (FHS) specimens for the two investigated materials was performed using a split Hopkinson pressure bar setup. The evolution of shear deformation was monitored by an ultra-high-speed camera (Kirana-05M). Localized shear band is induced in the two investigated materials under forced shear tests. Our results indicate that severe strain localization (adiabatic shear) is accompanied by a loss in the load carrying capacity, i.e., by a sudden drop in loading. Three distinct stages can be identified using a digital image correlation technique for accurate shear strain measurement. The microstructural analysis reveals that the dynamic failure mechanisms for Ti-64 and Ti-55511 alloys within the shear band are of a cohesive and adhesive nature, respectively.

Dehydration-driven stress transfer triggers intermediate-depth earthquakes

Intermediate-depth earthquakes (30–300 km) have been extensively documented within subducting oceanic slabs, but their mechanics remains enigmatic. Here we decipher the mechanism of these earthquakes by performing deformation experiments on dehydrating serpentinized peridotites (synthetic antigorite-olivine aggregates, minerals representative of subduction zones lithologies) at upper mantle conditions. At a pressure of 1.1 gigapascals, dehydration of deforming samples containing only 5 vol% of antigorite suffices to trigger acoustic emissions, a laboratory-scale analogue of earthquakes. At 3.5 gigapascals, acoustic emissions are recorded from samples with up to 50 vol% of antigorite. Experimentally produced faults, observed post-mortem, are sealed by fluid-bearing micro-pseudotachylytes. Microstructural observations demonstrate that antigorite dehydration triggered dynamic shear failure of the olivine load-bearing network. These laboratory analogues of intermediate-depth earthquakes demonstrate that little dehydration is required to trigger embrittlement. We propose an alternative model to dehydration-embrittlement in which dehydration-driven stress transfer, rather than fluid overpressure, causes embrittlement.

Dynamic shear deformation and failure of Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy

To study the high strain rate shear behaviour of Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) alloy, dynamic compression tests of hat shaped specimens have been conducted using a split Hopkinson pressure bar. Localized shear deformation is induced in Ti-55511 alloy by subjecting it to high local shear strain rate under dynamic compression tests. The experimental results demonstrate that: a) for a certain range of the velocity of the striker bar, the shear stress and local shear strain rate are independent on the applied force; b) no α→β phase transformation occurs in the localized shear region, and the formation of a shear band does not mean the occurrence of phase transformation; c) the microhardness within the shear band is lower than that of the matrix area and the region close to the shear band, and the microhardness in the bottom region of the shear band is lower than that of the top region; and d) brittle shear bands without microvoids and microcracks are observed for Ti-55511 alloy.

High Strain Rate Compressive Behaviour of Selective Laser Melted Ti-6Al-4V

Ti-6Al-4V alloy is one of the most important engineering alloys, combining attractive properties with inherent workability. The aim of this study is to investigate the effect of strain rate on the compressive mechanical properties of Ti6Al4V alloy manufactured by a selective laser melting process. The mechanical tests were performed by means of a compression split Hopkinson pressure bar apparatus under high strain rate ranging from 1400 s-1 to 4500 s-1. The true stress-strain curves obtained from static and dynamic compressive tests show strain rate sensitivity from quasi-static (peak strength 1300MPa) to high strain rate (peak 1500 MPa). Within the high strain rate range tested, the strain rate sensitivity is not remarkable. The fractographic analysis shows a relatively smooth and smeared fractured surface along with a dimple like structure. The observation of elongated dimples confirms the operation of a dynamic shear failure mechanism for the additively manufactured Ti-6Al-4V parts.

Experimental study on the prediction of rockburst hazards induced by dynamic structural plane shearing in deeply buried hard rock tunnels

To investigate methods for predicting bursts induced by the shear failure of structural planes in the deeply buried hard rock tunnels, shear tests were performed under various normal stresses on completely occlusive granite joints that were created via tension splitting. In these experiments, stress drops resulting from shear off of asperities on the surface of granite rigid structural plane, which can trigger a fault-slip rockburst, were reproduced, and the acoustic emissions (AEs) were monitored during shearing. The AE characteristics were analysed, and a method based on the AE b-value was developed to predict stress drops and the fault-slip rockbursts induced by the stress drops. The b-value continuously decreased before the stress drop and dropped to approximately 0.8 or less at the point of the violent post-peak shear stress drop. Moreover, the b-value tended to decrease to a relative minimum during the stick-slip period when the energy increased sharply if the energy rate was greater than 104, and a lower b-value was associated with a higher rockburst probability and intensity. The b-value generally decreased as the normal stress increased, which increased the risk of rockburst induced by the dynamic shear failure of granite joints. Moreover, the static and dynamic shear failure of the joints was easily distinguished when the b-value was used as a predictor.

Improved Method Used to Investigate the Dynamic Shear Failure of AISI 1045 Steel Cylinder Under Blast Loading

This study describes an improved method used to determine the dynamic shear failure of materials under blast loading conditions. The proposed method is a convenient and effective and generates material failure along the controlled fracture trajectories under uniform uniaxial stress conditions at a strain rate of [Formula: see text][Formula: see text]s[Formula: see text] to [Formula: see text][Formula: see text]s[Formula: see text]. The method was developed by performing experiments on AISI 1045 steel with heat treatment. Specimens were filled with composition B explosives and detonated, and fragments were recovered. Most of the macroscopic shear cracks originated at the notch tips and connected adjacent internal and external notches. The phenomena of multipath and bifurcate shear fracture trajectories were also observed. Fractography analysis of the fragments provided data on damage and failure mechanism and revealed that dynamic shear fracture was characterized by microvoids and microcracks. The adiabatic shear band of AISI 1045 steel was detected on the fracture surface boundary. This finding indicates that adiabatic shear localization is the underlying failure mechanism of the studied material.

Influence of loading positions on dynamic shear failure behaviors of RC beams

Influence of loading positions on dynamic shear failure behaviors of RC beams

Study on Dynamic Shear Resistance of RC Beams

The aim of this study was to investigate the dynamic shear failure behavior of RC beams under rapid loading through an experimental study and also to set up a strut-and-tie model with loading rate effect to predict the dynamic shear resistance of RC beams. Thus, rapid loading test with 24 RC beams with a shear span-to-deep ratio of 1.9 was performed, in which shear reinforcement ratio and loading rate were variable. All of the RC beams exhibited shear compression failure. Although the shear resistance increases with increasing loading rate, the influence of loading rate on the shear resistance clearly depends on shear reinforcement ratio. The strut-and-tie model with loading rate effect was finally developed, in which the thickness of the compression strut was formulated to be increased with an increase in loading rate. The developed strut-and-tie model was good agreement with the experimental results.

Dynamic vs. quasi-static shear failure of high strength metallic alloys: Experimental issues

Ductile fracture of metals by void nucleation, growth and coalescence under positive stress triaxiality is well admitted. This is not the case when metals are submitted to negative stress triaxiality. The present work aims at contributing to a better understanding of the competition between micro-mechanisms at the origin of failure of metals when submitted to shear-pressure loading at low and high strain rates. With this aim in view, experiments were carried out on Ti–6Al–4V shear-compression samples involving a stress triaxiality range comprised between −0.2 and −0.5. Results show that the material failure is the consequence of a void growth induced process. At high strain rate, due to the localization of the deformation within adiabatic shear bands, the failure of the material occurs earlier, leading to maximum shear strain smaller at high strain rate than at low strain rate. Impact tests were also carried out on Kalthoff and Winkler type double notched plates. They showed that the interaction between tension and shear waves leads to a complex Mode I–Mode II crack propagation.

Modeling combined tension-shear failure of ductile materials

Failure of ductile materials is usually expressed in terms of effective plastic strain. Ductile materials can fail by two different failure modes, shear failure and tensile failure. Under dynamic loading shear failure has to do with shear localization and formation of adiabatic shear bands. In these bands plastic strain rate is very high, dissipative heating is extensive, and shear strength is lost. Shear localization starts at a certain value of effective plastic strain, when thermal softening overcomes strain hardening. Shear failure is therefore represented in terms of effective plastic strain. On the other hand, tensile failure comes about by void growth under tension. For voids in a tension field there is a threshold state of the remote field for which voids grow spontaneously (cavitation), and the material there fails. Cavitation depends on the remote field stress components and on the flow stress. In this way failure in tension is related to shear strength and to failure in shear. Here we first evaluate the cavitation threshold for different remote field situations, using 2D numerical simulations with a hydro code. We then use the results to compute examples of rate dependent tension-shear failure of a ductile material.

Influence of Loading Rate on Shear Capacity of Reinforced Concrete Beams

This study aims to investigate the dynamic shear failure behavior of reinforced concrete (RC) beams under rapid loading through an experimental study and set up a strut-and-tie model with loading rate effects to predict the dynamic shear capacity of RC beams. Thus, rapid loading test for 48 RC beams was performed, in which shear span-to-depth ratio, shear reinforcement ratio and loading rate were variable. The RC beams exhibited diagonal tension failure, shear compression failure and flexural failure depending on the shear span-to-depth ratio and the shear reinforcement ratio. The influence of loading rate on the maximum resistance is more significant for the RC beams failed in shear than for those failed in flexure. The strut-and-tie model with loading rate effects was developed, in which the thickness of the compression strut was formulated to be increased with an increase in loading rate. The developed strut-and-tie model was in good agreement with the experimental results.

The respective influence of microstructural and thermal softening on adiabatic shear localization

It has recently been shown that dynamic shear failure of crystalline solids can be initiated by local microstructural changes (dynamic recrystallization, DRX), instead of the commonly assumed thermal softening mechanism. We systematically investigate the respective contribution of thermal and microstructural softening to the initiation of dynamic shear localization, by means of a fully coupled numerical model incorporating the two softening mechanisms in an adjustable manner. Our results indicate that, for those materials that exhibit early DRX, (e.g. Ti6Al4V), the role of thermal softening is negligible, whereas for materials with late (e.g. pure Ti) or no DRX, thermal softening effects become dominant. The strength of the thermomechanical coupling term (thermal softening) is found to determine the local temperatures, with the strongest effect being achieved in the absence of coupling, together with the formation of thermal “hot spots”. Thermal softening is found to regulate the evolution of the local temperature, in the sense that the softened material both stores and dissipates smaller increments of strain energy. The results of this study allow for a general classification of the material proneness to dynamic shear localization as a function of its thermo-physical characteristics.

Three-dimensional consolidation of a porous unsaturated seabed under rubble mound breakwater

The consolidation status of a seabed under marine structures and hydrostatic pressure is the basis for the evaluation of the liquefaction and dynamic shear failure of seabed foundation under ocean wave loading. However, only a few investigations have been conducted for the seabed consolidation under hydrostatic pressure and marine structures. Furthermore, most previous numerical models for the Biot's consolidation theory are limited to two-dimensional cases. In this study, based on Biot's dynamic poro-elastic theory (“u−p” approximation), a three-dimensional FEM seabed model is adopted to investigate the consolidation of seabed under a rubble mound breakwater and hydrostatic pressure. Numerical results show that the rubble mound breakwater significantly affect the stress/displacement fields in the seabed foundation. Based on the parametric study, it can be concluded: (1) Young's modulus of a seabed significantly affects the settlement of breakwater; and (2) the magnitude of the shear stress concentrates in the zones beneath the toes of breakwater.

Methodology for exciting dynamic shear and moment failure in composite sandwich beams

The increasing use of composites in high performance structural applications subjected to dynamic loading drives the need for understanding basic material failure properties and structural response as a function of loading rate. Balsa core sandwich beams having woven carbon/vinylester composite facesheets were investigated in both shear and bending failure at quasi-static and dynamic loading speeds producing strain rates ranging from 10−5 to 100 s−1. Short sandwich beam tests show weak rate dependency, with a slight decreasing trend in the balsa core shear failure strain for increased strain rate. Wide scatter in core shear failure strain was observed due to the highly variable nature of the balsa material, as well as its heterogeneous tiled construction within the sandwich panel. Excitation of facesheet compressive failure in sandwich beams with weak core of 50.8 mm thickness required the use of 2.43 m long sandwich beams subject to high speed loading (10 m/s) in order to achieve 100 s−1 strain rate. Facesheet compressive failure strain in the long beam specimens under dynamic loading was measured to be 10% higher than for quasi-static loading. Comparison of long sandwich beam bending results to shorter solid laminate bending strength measurements show a different trend in rate dependency, and thus it is recommended that full-depth sandwich beams (and panels) be used to characterize the failure of these types of sandwich structures.

Influence of Loading Rate on Shear Failure Resistance of RC Beams

The aim of this study was to investigate the dynamic shear failure behavior of RC beams under rapid loading through an experimental study. Thus, rapid loading test for 48 RC beams was performed, in which shear span-to-depth ratio, shear reinforcement ratio and loading rate were variable. The RC beams exhibited diagonal tension failure, shear compression failure and flexural failure depending mainly on the shear span-to-depth ratio and the shear reinforcement ratio. The influence of loading rate on the maximum resistance is more significant for the RC beams failed in shear than for those failed in flexure.

Dynamic Shear and Axial-Load Failure of Reinforced Concrete Columns

Reinforced concrete columns with light transverse reinforcement may be susceptible to shear and subsequent axial-load failures during strong earthquake ground shaking. An experimental program examined the behavior of two half-scale, one-story frames with axial loads representative of those expected for the lower story of a multistory building. The frames were subjected to unidirectional simulated earthquake motion applied at the base. Shear failures of an interior column led to axial-load failure and redistribution of internal forces to adjacent framing components. Recorded data illustrated the details of the shear and axial failures and the redistribution to adjacent components. This paper discusses the global response of the frames and the shear and axial-load response of the shear-critical interior column.

A simple model for dynamic shear failure of stainless steel

We propose a simple model to reproduce ductile failure by shear localization in simulations of perforations tests. The model incorporates a positive feedback process of shear strain localization which results in a catastrophic decrease of flow stress. We use the model for perforation tests with 304L stainless steel. It succeeds in reproducing perforation thresholds as well as qualitative features of the perforation process, including shear band formation in some of the projectiles.

Dynamic shear band propagation and failure in AISI 4340 steel

Failure of metals and their alloys at high strain-rates is usually preceded by formation of adiabatic shear bands, along which cracks can easily nucleate and rapidly propagate causing fracture. In this study, adiabatic shearing and its effect on fracture behavior of AISI 4340 steel at high compressive strain-rate of about 1.9 × 10 3 s −1 was investigated. Cracking was initiated by formation of micropores during adiabatic heating and thermal softening along the shear bands. As adiabatic shearing proceeded, the pores coalesced to form clusters of pores. Microcracks that eventually grow and cause shear failure along the adiabatic shear bands are initiated from these clusters of pores.

Dynamic Fiber Debonding and Frictional Push-out in Model Composite Systems: Experimental Observations

In the present work a modified Split Hopkinson Pressure Bar (SHPB) system is adopted to perform dynamic fiber push-out experiments on model single fiber composite systems. A tapered punch and a support connect a monofilament composite with the incident and transmitted bars of the SHPB. The tapered punch is used to apply compressive loading to a single fiber (either steel or aluminum) embedded in a surrounding matrix material (EPON 862). The SHPB allows real time measurement of relative fiber/matrix displacement and push-out force, as the debonding and push-out event progresses. Using this technique we have studied the effect of loading rate, material mismatch, fiber length, and surface roughness on the push-out event. It was seen that maximum push-out force increases with increasing loading rate. In addition dynamic interfacial strength and toughness is highly dependent on fiber surface roughness. Results from a finite element analysis incorporating a cohesive failure model were used to extract interface strength and toughness values. It was found that the particular aluminum/EPON interface used is characterized by a dynamic shear failure strength of 48±8 MPa, a mode II fracture toughness of 160±40 N/m, and a friction coefficient of 0.2 at a sliding rate of 6 m/s. For the rates tested here these quantities were found to be approximately constant.