Fracture Initiation Behavior Research Articles

T-strain effects in kinked interfacial fracture of bonded composites

Strain-based criteria are employed to predict fracture initiation behavior in bi-material cracked components. Two versions of the maximum tangential strain criterion are developed theoretically for the case of a crack existing at the interface of two dissimilar isotropic elastic solids to predict kinking angle and the mixed-mode interfacial fracture toughness. The first version (MTSN) is developed based on the singular strain fields while the second version (EMTSN) takes into account the effects of the first non-singular strain term (T-strain) as well as the singular terms. The theoretical effect of T-strain on the calculation of the kinking angle and the mixed-mode interfacial fracture toughness is studied. Then, both MTSN and EMTSN criteria are examined by comparing theoretical predictions to the previously reported experimental results for the kinking angle and mixed-mode interfacial fracture toughness in different bonded composite specimens. It is found that considering T-strain can significantly improve the theoretical predictions of the experimental results provided by the maximum tangential strain criterion.

A study of the rupture behavior of a ship side plate laterally punched by a full-shape bulbous bow indenter

This paper deals with analytical, experimental and numerical studies with respect to the rupture behaviors (including the fracture initiation and crack propagation) of a ship side plate induced by the lateral impact loading from a full-shape bulbous bow indenter. Analytical expressions were developed to calculate the whole collapse resistance, especially the initial fracture prediction and crack propagation resistance of the side plate. Small scale tests and numerical simulations were conducted to validate the proposed analytical method. The specimens were quasi-statically punched by a combine-shaped bulbous indenter, which is specifically designed to distinguish the fracture initiation and crack propagation behaviors. The results from these three methods well correlated which confirmed the validation of the proposed analytical method. In addition, the experimental observations demonstrate that the crack propagation resistance is not related to the crack number generated in the plate. Moreover, in the crack propagation process, failure of the plate is due to the tension effect at the crack tips with high stress concentration, and the plate can still absorb energy significantly after rupture. Finally, the influence of friction effect on the collision response is discussed.

Numerical/experimental study of the monotonic and cyclic viscoelastic/viscoplastic/fracture behavior of an epoxy resin

This paper investigates the viscoelastic/viscoplastic/fracture behavior of an epoxy resin. A state-of-the-art pressure-dependent elastoplastic constitutive model (Melro et al. 2013) is expanded to include viscoelasticity, viscoplasticity and a modified damage formulation with linear softening and shrinking pressure-dependent fracture surface. A water plasticization model with a single degradation factor is proposed. A set of new quasi-static and fatigue experiments is used to calibrate the model and assess its predictive capabilities. The model correctly represents the rate dependent plasticity and fracture initiation behavior of the studied epoxy. The stiffness and strength degradations after plasticization are also accurately captured. The model is found to be less suitable in reproducing the measured loading-unloading behavior, which displayed strong nonlinearity in combination with limited permanent deformation. Nevertheless, reasonably accurate fatigue life predictions in the low-cycle regime are obtained.

Characteristics of fracture propagation in compact limestone formation by hydraulic fracturing in central Sichuan, China

Compact limestone formations are significant and serve as enrichment petroleum sources in the central Sichuan basin. However, they are difficult to stimulate by hydraulic fracturing owing to their low porosity, low permeability, and high density. The fracture geometry is neither a bi-wing fracture found in a homogeneous sandstone formation nor a complex fracture network occurring in a shale formation with well-developed discontinuities and high brittleness. In this study, a series of laboratory stimulated experiments were conducted to investigate the fracture initiation and propagation behavior in compact limestone formation. Concurrently, the effects of the horizontal stress difference, pump rate, fracturing fluid viscosity, and completion method were also studied. The results revealed that a complex fracture network was formed with difficultly in a compact limestone formation. The fractures initiated from open-hole section or perforations to propagate in four forms, resulting in three categories of the main fracture geometry: transverse fracture, longitudinal fracture, and complex fracture. Complex fracture formations occurred under a small horizontal stress difference, whereas a simple fracture was induced by a large horizontal stress difference, similar to a transverse or longitudinal fracture. Moreover, the cement strength, development degree, and distribution position of the discontinuities had a significant effect on the fracture geometry. A low fluid viscosity or low pump rate was demonstrated to be an effective method for activating and opening the discontinuities, forming complex fractures under a small horizontal stress difference rather than under a large horizontal stress difference. However, the variable pump rate fracturing with a low-viscosity fluid could promote the interaction of the induced fracture and discontinuities, leading to a large stimulated reservoir volume under the large horizontal stress difference. In addition, perforated fracturing generated a more complex fracture at a higher fracture pressure than an open-hole fracturing. Furthermore, an acid fluid should be injected into limestone formation before the fracturing operation to reduce the rock strength, resulting in a low fracture pressure of limestone formation.

Disturbance and recovery in high speed (110) cleavage in single crystalline silicon

Stress perturbations and material defects can significantly affect the fracture initiation and propagation behaviors in brittle materials. In this work, we show that (110)[110] cleavage in silicon deflects onto (111) plane in the presence of contact stresses. The deflection is however not permanent as the crack returns to the (110) plane after a certain length of propagation, even in the case where the crack velocity is up to 78% of the Rayleigh wave speed. The recovery behavior indicates that the (110)[110] cleavage is invariably prevailing when perpendicular to the maximum stress. Following this indication, it can be concluded that the observed (110)[110]–(111) deflection in previous literature is more likely driven by the external disturbance rather than the crack velocity induced toughness evolution. We also highlight that the extra energy for the (110) recovery is minimized at the expense of a large propagation distance upon the plane switch.

Analysis of hydraulic fracture initiation and vertical propagation behavior in laminated shale formation

The extent of hydraulic fracture vertical propagation extent is significant in evaluating simulated reservoir volume for laminated shale reservoirs. Given that it is affected by the discontinuities (beddings, natural fractures, and other factors), fracture geometry is complex in the vertical plane and is different from a simple fracture in a homogeneous sandstone reservoir. However, the propagation mechanism of hydraulic fracture in the vertical plane has not been well understood. To clarify this mechanism, several groups of large-scale tri-axial tests were deployed in this study to investigate the fracture initiation and vertical propagation behavior in laminated shale formation. The influences of multiple factors on fracture vertical propagation were studied. The results showed that hydraulic fracture initiation and propagation displayed five basic patterns in the vertical plane of laminated shale formation. The ultimate fracture geometries could be classified into four categories: simple fracture, fishbone-like fracture, fishbone-like fracture with fissure opening, and multilateral fishbone-like fracture network. Furthermore, the favorable geo-stress conditions for forming the complex fracture network were as follows: vertical stress difference close to 6MPa and vertical stress difference coefficient from 0.2 to 0.5. In addition, when q·μ-value (the product of injection rate and fracturing fluid viscosity) was roughly 3×10−9, a complex fracture geometry of fishbone-like fracture with bedding opening was formed; however, extremely small or extremely large values were both harmful. Variable injection rate fracturing with low viscosity fracturing fluid of 3mPa·s was proved to be an effective treatment to improve the connectivity of induced hydraulic fracture with the discontinuities. Moreover, because of the influence of cementing strength on fracture communication effects between hydraulic fracture and the beddings, the overall propagation region generally displayed an ellipse in shape with beddings opening asymmetrically along two wings of the main hydraulic fracture.

Crack initiation behavior in single crystalline silicon

In this article, we report special fracture initiation behavior of (110) cleavage in silicon single crystal. It is found that the static energy release rate always exceeds the material toughness upon crack initiation. According to high-speed imaging measurements and fractographies, it is found that the crack has an important initial velocity, i.e. up to 3000m/s, and then reaches the steady-state propagation regime very quickly. The experimentally revealed initial crack velocity is in good agreement with estimation from the energy flux at crack initiation.

上部ベイナイト組織鋼の脆性き裂発生挙動に及ぼすミクロ組織の影響

上部ベイナイト組織鋼の脆性き裂発生挙動に及ぼすミクロ組織の影響

Finite Element Simulations of Crack Propagation in Al2O3/6061Al Composites

In this work, the local fracture initiation behaviour of an Al2O3/6061Al composite is studied numerically. The damage behaviour of the microstructure is evaluated in consideration of the path and the amount of damage as well as the stress–strain performance of the microstructure. The damage behaviour of the ductile matrix has been simulated using the damage parameter D. For the simulation of fracture of the ceramic particles, a normal stress criterion is applied. For the analysis of the damage behaviour of the transition zone between particulate and matrix, both damage models (D parameter and normal stress criteria) are applied in this region. Parameter studies of crack propagation prediction in the Al2O3/6061Al composite on the basis of an Element Elimination technique have been performed for two differently heat-treated variants resulting in different mechanical properties. In addition, residual stress effects on the damage behaviour are examined for various microstructural situations.

An Investigation of Hydraulic Fracturing Initiation and Near-Wellbore Propagation from Perforated Boreholes in Tight Formations

In this study, hydraulic fracturing tests were conducted on 10 and 15 cm synthetically manufactured cubic tight mortar samples. The use of cube samples allowed application of three independent stresses to mimic real far field stress conditions. A true triaxial stress cell was used for this purpose. The lab test parameters were scaled to simulate the operations at field scale. The hole and perforations were made into the sample after casting and curing were completed. Various scenarios of vertical and horizontal wells and in situ stress regimes were modeled. These factors are believed to play a significant role in fracture initiation and near-wellbore propagation behavior; however, they are not independent parameters, hence should be analyzed simultaneously. In addition to experimental studies, analytical solutions were developed to simulate the mechanism of fracture initiation in perforated boreholes in tight formations. Good agreements were observed between the experimental and analytical results. The results of this study showed that a lower initiation pressure is observed when the minimum stress component is perpendicular to the axis of the perforations. It was also seen that, even when the cement sheath behind the casing fails, the orientation of the perforations may affect the initiation of the induced fracture noticeably. Furthermore, it was found that stress anisotropy influences the fracturing mechanism in a perforated borehole, and affects the geometry of the initiated near-wellbore fracture.

On fracture initiation angle near bi-material notches – Effects of first non-singular stress term

The effect of first non-singular stress term of elastic stress field near bi-material notches (I-stress) on the fracture initiation angle is investigated. A modified maximum tangential stress (MMTS) criterion is suggested for predicting the fracture initiation angles which takes into account the effect of I-stress as well as the singular terms. It is shown that the I-stress can play an important role in fracture initiation behavior adjacent the interface corners and neglecting this term may introduce significant errors in predicting the fracture initiation angle. Then, to evaluate the proposed criterion, a finite element (FE) simulation is done on a typical test specimen and the presented criterion is applied for predicting the fracture initiation direction. It was observed that the MMTS predictions are very close to the results obtained from the FE analysis (which takes into account all stress terms) showing the reasonable accuracy of the MMTS criterion for bi-material notch problems.

Mechanical/thermal stress intensification for mode I crack tip: Fracture initiation behavior of steel, aluminum and titanium alloys

A line crack in an infinite plate is subjected to the combined mechanical and thermal loading. The problem is solved in the framework of thermoelastic theory by integrating with the complex function method. An analytical solution is obtained under the boundary condition that the constant temperature is retained on the crack surfaces while the remote uniform heat flow is applied. In this case, the mode II thermal stress intensity factor vanishes. Thermal loading only induces the mode I stress intensity factor. The heat flow along the vertical direction to the crack line has no influence on the thermal stress intensity factor. The problem is a typical mode I crack problem. Three materials, say 38Cr2Mo2VA steel, LY12 aluminum and TC6 titanium alloys, are studied. The applied failure stresses are calculated under the combination of mechanical and thermal loading by using the strain energy density factor theory. The fracture behaviors of three materials are compared. Their fracture resistance is quite different. It is found that the thermal loading would enhance the crack propagation. If the direction of heat flow is changed to an opposite direction, the thermal effect would impede the crack growth. That is the thermal effect has the positive or negative effect which depends on the direction of heat flow.

The initiation and propagation of thermal shock cracks in graphite

Thermal shock resistance of a commercial grade of graphite was studied using an arc-discharge test. The thermal shock fracture initiation and crack propagation behaviour of the graphite disks at different input power levels were determined and analysed using fracture mechanics. The temperature gradient was measured experimentally and the profiles were force fitted with an even fourth-order polynomial. The thermal stresses were calculated from the force fits. A radial notch was introduced to the disk specimens to enable calculation of the thermal stress intensity factors. The crack mouth opening displacement was monitored using a special displacement transducer. The thermal stress and stress intensity factors were found to increase with increasing input current (and hence increasing thermal gradient). The thermal shock fracture toughness determined using the arc-discharge technique was found to increase from 0.8 to 1.4 MPa m 1/2 at temperatures from 220 to 420 °C. The longer the notch length, the shorter the time to crack, the smaller the crack mouth opening displacement jump and the shorter the unstable crack growth.

Study on Fracture Properties of High-Density Polyethylene (HDPE) Pipe

Butt-fusion welding is the main technology to join high-density polyethylene (HDPE) plastic pipes, which are widely used in transport the water, gas and corrosive liquid. Investigation shows that one of the failure modes of HDPE pipe is the crack slowly grows across the thick direction and leads to failure at last, so that it is very important to study the resistance to crack initiation of HDPE pipe and its butt-fusion welded joint. In this study, the elastic-plastic fracture mechanics parameter, crack opening displacement (COD) is used to describe the fracture initiation behaviors for the HDPE materials and its butt-fusion welded joints. The resistance to initiation fracture of HDPE pipe materials and butt-fusion welded joints were investigated at different temperature by using multiple specimen resistance curve method and silicon-rubber replica method. The results show that saturation initial crack COD- δis of HDPE pipe materials and butt-fusion welded joints decreases with the decreasing temperature. The δis of butt-fusion welded joints is lower than that of HDPE pipe materials. Investigation also proved that the silicon-rubber replica method is more suitable for HDPE engineering material than the multiple specimen method. At the same time the statistic distribution of the δis of HDPE butt-fusion welded joint was conducted. The results show that the value of the δis has the statistic variance inherently. The optimum fitting distribution of COD is Weibull distribution with three parameters.

Influence of Heterogeneity of Mechanical Properties on Hydraulic Fracturing in Permeable Rocks

Numerical simulations of circular holes under internal hydraulic pressure are carried out to investigate the hydraulic fracture initiation, propagation and breakdown behavior in rocks. The hydraulic pressure increases at a constant rate. The heterogeneity of the rocks is taken into account in the study by varying the homogeneity index. In addition, the permeability is varied with the states of stress and fracture. The simulations are conducted by using a finite element code, F-RFPA2D, which couples the flow, stress and damage analyses. The simulation results suggest that the fracture initiation and propagation, the roughness of the fracture path and the breakdown pressure are influenced considerably by the heterogeneity of rocks. The hole diameter elongation and the stress field evolution around the fracture tip during the fracture propagation can also provide useful information for the interpretation of the hydraulic fracturing behaviour.

An investigation of fracture initiation and resistance-curve behavior in concrete

This paper examines the effects of mix strength on the fracture initiation and resistance-curve behavior of concrete. The fracture initiation toughness and the resistance-curve behavior are shown to increase with increasing mix strength. However, the extent of stable crack growth is shown to decrease with increasing mix strength. The measured resistance-curve behavior is associated with the beneficial effects of crack-tip shielding due to ligament bridging mechanisms. The observed small- and large-scale bridging phenomena are then modeled using fracture mechanics models. The studies show that the measured resistance-curve behavior is predicted largely by considering the beneficial effects of ligament bridging. Finally, the implications of the results are then discussed for the design of durable concrete structures.

The effect of prestrain on ductile fracture initiation behavior of TMCP steel

The effect of prestrain on ductile fracture initiation behavior of TMCP steel

Micromechanical modeling of fracture initiation in 7050 aluminum

Mechanical testing and finite element calculations have been carried out to characterize the fracture initiation behavior of the high-strength aluminum alloy 7050-T7451. Results show that fracture initiation is well predicted for two specimen types of differing constraint using the stress-modified, critical plastic strain micromechanical model. The relation between stress triaxiality and critical plastic strain was found from a series of notched tensile specimens. Data from these tests are interpreted using both companion finite element modeling and common, semi-empirical relations, and these two approaches are compared. Multiple, interrupted tests of standard, highly constrained single edge notched bend specimens are used to obtain the J– R curve in 7050 for small amounts of tearing to experimentally identify initiation. Companion modeling and the stress-modified, critical plastic strain relation are used to find the length scale for fracture, l ∗ , needed for initiation predictions. The calibrated stress-modified, critical plastic strain relation and length scale are then used to predict fracture initiation of a low-constraint specimen. The prediction is within 5% of the experimental measurements. Finally, various aspects of the procedure followed in the present work are compared to previous efforts using similar approaches.

Ductile and brittle fracture initiation behavior for compressively prestrained steel

The authors have found that a ductile crack initiation for compressively prestrained steel can be evaluated quantitatively via a limit curve of ductile crack initiation, which is presented by stress triaxiality-critical equivalent plastic strain relationship. In this research, validity of the limit curve approach, with which ductile crack initiation for compressively prestrained steel is predicted, was examined and stress-based quantitative evaluation of brittle fracture initiation for the compressively prestrained steel was investigated through experiments and numerical analysis. In this investigation, validity of the limit curve approach was confirmed and it is found that brittle fracture occurs with the critical cleavage fracture stress depending on compressive prestrain.

Thermal shock behaviour of industrial carbon anodes

Thermal shock resistance of industrial carbon anodes was studied from both the direct thermal shock test and indirect measurements of thermal shock resistance parameters derived from thermal and mechanical properties. The direct thermal shock test was conducted using the so-called arc discharge technique. In conjunction with finite element and fracture mechanics analysis, the thermal shock fracture initiation and crack propagation behaviour of carbon anodes were characterised in the thermal shock tests. The thermal shock test results were correlated with the thermal shock resistance parameters derived from thermal and mechanical properties. It was found that increased extent of non-linear stress–strain behaviour, which can be described by a lower brittleness function, χ, and a higher γWOF/γi ratio, leads to significantly higher thermal shock crack propagation resistance.