Macroscopic Failure Mechanism Research Articles

Study on Static Strain Aging Kinetics of High-Carbon Steel Wires and Its Impact on High-Strength Steel Cords

Under quasi-static loading, an irregular failure mode of high-strength thin carbon steel cords were observed after low-temperature thermal aging. Character and kinetics of damage in such wire ropes highly depend on the plastic elongation of the steel wires, which is significantly modified by the strain aging effect. In this paper, the static strain aging effect on heavily drawn high-carbon steel wires and their cords is experimentally studied in the 80–200 °C temperature range. The kinetics of the aging process is studied in detail. Experimental data are fit by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetic model. The temperature dependence of the static strain aging process is given by means of the Arrhenius equation. The associated JMAK exponents, the apparent activation energy and the pre-exponential constant are determined. Quantitative analysis of the affected strength and strain parameters is given, and based on this, the macroscopic failure mechanism is fundamentally explained.

Dynamic tensile properties of ROP/OCC natural hybrid fibers reinforced composites

The dynamic tensile properties of natural hybrid fibers reinforced composites with components of recycled-office-paper (ROP) fibers and old corrugated cardboard (OCC) fibers at different proportions are investigated by using an aluminum split Hopkinson tension bar (SHTB) at the strain rates ranging from 300/s to 1500/s. The influence on strain–stress relationship and strength by changing its component proportions are tested under quasi-static and dynamic loading. It is observed that the strain-rate sensitivities of the composites are varied by changing component proportions. The type of composites with 100% OCC fibers has a negative response to strain-rate effect. Nonetheless, that of 100% ROP fibers is not sensitive to the strain-rate effect. The type of composites with 75% ROP/25% OCC fibers is subject to strain-rate effect when the stress rates are less than 1000/s, but it has a negative response to the strain-rate effect when the strain rates are greater than 1000/s. The failure modes and their corresponding microscopic and macroscopic failure mechanism are also discussed. The composites with different component proportions are failed by combining with fibers break, fibers pull-out, fibers curvature shape changing and scattered, which are fibers failure and fiber–matrix bonding failure.

Using Acousto-Optic Coupling Technology to Investigate the Fracture Evolution of Rock-Like Materials during Inclined Shear Tests

This study used synchronized acoustic and optical nondestructive AE and ESPI technology with an inclined shear test to investigate the micro- and macroscopic failure mechanisms of cement mortar specimens under shear force. Crack opening displacement feedback was used to control stable damage to obtain complete loading curves of specimens sheared at different shear angles (β). AE technology was used to investigate the time and space distributions of the occurrence of microseismic sources produced by shearing within rocks. Synchronized comparisons were made using the interference images caused by the exterior of the rock material changing shape because of the shearing detected by electronic speckle pattern interferometry (ESPI). This was then used to examine three characteristics of the fracture evolution of rock-like material: localization, initiation of cracks, and crack propagation. The results indicate that shearing stiffness and shear strength are reduced following increases in the shear angle. In addition, the fracture propagation angle tends to change direction and its value also increases. Using AE methods to observe the evolution of microcracks in materials, the time and position of their localization were determined. These results were then compared with the crack initiation and propagation positions detected using ESPI. It is found that the fracture trends observed using these two methods were largely identical.

Influence of the Lode parameter and the stress triaxiality on the failure of elasto-plastic porous materials

This work makes use of a recently developed “second-order” homogenization model to investigate failure in porous elasto-plastic solids under general triaxial loading conditions. The model incorporates dependence on the porosity and average pore shape, whose evolution is sensitive to the stress triaxiality and Lode parameter L. For positive triaxiality (with overall tensile hydrostatic stress), two different macroscopic failure mechanisms are possible, depending on the level of the triaxiality. At high triaxiality, void growth induces softening of the material, which overtakes the intrinsic strain hardening of the matrix phase, leading to a maximum in the effective stress–strain relation for the porous material, followed by loss of ellipticity by means of dilatant shear localization bands. In this regime, the ductility decreases with increasing triaxiality and is weakly dependent on the Lode parameter, in agreement with earlier theoretical analyses and experimental observations. At low triaxiality, however, a new mechanism comes into play consisting in the abrupt collapse of the voids along a compressive direction (with small, but finite porosity), which can dramatically soften the response of the porous material, leading to a sudden drop in its load-carrying capacity, and to loss of ellipticity of its incremental constitutive relation through localization of deformation. This low-triaxiality failure mechanism leads to a reduction in the ductility of the material as the triaxiality decreases to zero, and is highly dependent on the value of the Lode parameter. Thus, while no void collapse is observed at low triaxiality for axisymmetric tension (L=-1), the ductility of the material drops sharply with decreasing values of the Lode parameter, and is smallest for biaxial tension with axisymmetric compression (L=+1). In addition, the model predicts a sharp transition from the low-triaxiality regime, with increasing ductility, to the high-triaxiality regime, with decreasing ductility, as the failure mechanism switches from void collapse to void growth, and is in qualitative agreement with recent experimental work.

An Experimental Study of Excavation Damaged Zone in Anisotropic Rock

When a tunnel or an underground structure is excavated in rock mass, rock disturbed or damaged zone (EDZ) is formed around the excavation due to the stress concentration resulting from stress redistribution. Recent studies on the rock EDZ revealed it’s important to structural stability around underground opening. In this study, the fracture and damage mechanisms of rock induced by the accumulation of microcracks were investigated by AE tests. The results of the experiments showed that tensile failure was the major microscopic failure mechanism of rock in excavation damaged and disturbed zone. The expression of the damage magnitudes in each AE source leads to accurate prediction of macroscopic failure mechanisms. In addition, the orientation of the macroscopic failure plane could be estimated by the orientational distribution of microcracks.

Artificial midline-fascia of the human abdominal wall for testing suture strength

To reduce testing of human abdominal wall closure-modalities in test animals, a fibre reinforced rubber with identical mechanical properties compared to the human midline fascia (linea alba: LA) was developed. The microscopic structure of the human LA, stress-strain behaviour, maximum tensile force and macroscopic failure mechanism in tensile tests with human LA were defined as indicators for the required properties of the fibre reinforced rubber. A composite consisting of latex rubber and cotton fibres was developed that shows mechanical properties comparable to the human abdominal wall. The results of the tensile tests on sutured artificial LA were highly similar to those performed on sutured human LA. The material presented in this study is proposed as a substitute for human and animal tissues presently used to test suture techniques. A protocol for an approach to develop artificial fibrous soft tissue like fascie and tendon was drawn up.

Load-rate dependency of ultimate tensile strength in ceramic matrix composites at elevated temperatures

Ultimate tensile strength of three continuous fiber-reinforced ceramic matrix composites, including SiC f/CAS-II (1D), SiC f/MAS-5 (2D) and SiC f/SiC (2D), was determined as a function of load (test) rate in air at 1100–1200 °C. All three composites exhibited a significant dependency of ultimate tensile strength on test rate. The application of the preload technique as well as the prediction of life from one loading configuration (constant stress-rate) to another (constant stress) suggested that the overall phenomenological macroscopic failure mechanism of the composites would be a power-law type of slow crack growth or damage evolution/accumulation. It was further found that constant stress-rate testing could be used as an alternative to life prediction test methodology for ceramic matrix composites, at least for a short range of lifetimes and when ultimate tensile strength is used as a failure criterion.

Estimation of cracking and damage mechanisms in rock under triaxial compression by moment tensor analysis of acoustic emission

In highly stressed conditions, the excavation damaged zone induced by stress redistribution and disturbance must be evaluated after tunnel excavation. Therefore, the deformation and fracture characteristics of rock must be investigated. In this study, the fracture and damage mechanisms of rock induced by the accumulation of microcracks were investigated by moment tensor analysis, as well as by the moving point regression technique, both of which were applied to acoustic emission (AE) and strain data obtained from triaxial compression tests. Damage thresholds before the peak strength of rock under triaxial compression were determined by the moving point regression technique using acoustic emission data. The results showed that damage thresholds, except the crack closure stress, increased linearly with confining pressure. The results of the moment tensor analysis showed that shear failure was a major microscopic failure mechanism of rock under triaxial compression. In addition, shear failure became more dominant as the confining pressure increased. In this analysis, the expression of the damage magnitudes in each AE source as relative crack volumes leads to accurate prediction of macroscopic failure mechanisms, as well as major failure planes in the rock. In addition, the orientation of the macroscopic failure plane could be estimated by the orientational distribution of microcracks.

Strength prediction of bolted joints in graphite/epoxy composite laminates

A parametric finite element analysis was conducted to investigate the effect of failure criteria and material property degradation rules on the tensile behaviour and strength of bolted joints in graphite/epoxy composite laminates. The analysis was based on a three-dimensional progressive damage model (PDM) developed earlier by the authors. The PDM comprises the components of stress analysis, failure analysis and material property degradation. The predicted load–displacement curves and failure loads of a single-lap single-bolt joint were compared with experimental data for different joint geometries and laminate stacking sequences. The stiffness of the joint was predicted with satisfactory accuracy for all configurations. The predicted failure load was significantly influenced by the combination of failure criteria and degradation rules used. A combination of failure criteria and material property degradation rules that leads to accurate strength prediction is proposed. For all the analyses performed, the macroscopic failure mechanism of the joint and the damage progression were also predicted.