Unstable Crack Propagation Research Articles

Mechanics of dynamic fracture in notched polycarbonate

Fracture toughness of brittle amorphous polymers (e.g. polymethyl methacrylate (PMMA)) has been reported to decrease with loading rate at moderate rates and increase abruptly thereafter to close to 5 times the static value at very high loading rates. Dynamic fracture toughness that is much higher than the static values has attractive technological possibilities. However, the reasons for the sharp increase remain unclear. Motivated by these observations, the present work focuses on the dynamic fracture behavior of polycarbonate (PC), which is also an amorphous polymer but unlike PMMA, is ductile at room temperature. Towards this end, a combined experimental and numerical approach is adopted. Dynamic fracture experiments at various loading rates are conducted on single edge notched (SEN) specimens with a notch of radius 150μm, using a Hopkinson bar setup equipped with ultra high-speed imaging (>105fps) for real-time observation of dynamic processes during fracture. Concurrently, 3D dynamic finite element simulations are performed using a well calibrated material model for PC. Experimentally, we were able to clearly capture the intricate details of the process, for both slowly and dynamically loaded samples, of damage nucleation and growth ahead of the notch tip followed by unstable crack propagation. These observations coupled with fractography and computer simulations led us to conclude that in PC, the fracture toughness remains invariant with loading rate at Jfrac=12±3kN/m for the entire range of loading rates (J̇) from static to 1×106kN/m−s. However, the damage initiation toughness is significantly higher in dynamic loading compared to static situations. In dynamic situations, damage nucleation is quickly followed by initiation of radial crazes from around the void periphery that initiate and quickly bridge the ligament between the initial damaged region and the notch. Thus for PC, two criteria for two major stages in the failure process emerge. Firstly, a mean stress based defect initiation is suggested. The value of the critical mean stress for defect initiation under dynamic loading is found to be 115±5MPa, which is significantly higher than its static value of 80MPa. The critical normal plastic stretch needed for crazes to nucleate from the nucleated defect is estimated to be about 1.78±0.2.

Effect of localized KrF excimer laser treatment on fracture behaviors of freestanding <110> and <100> single crystal silicon beams

Microscale silicon structures oriented along and orientations were laser treated with different conditions with the cross section shape and tensile strength investigated after the treatment. Finite element simulation was performed to examine the temperature distribution at different conditions during laser treatment. Using a low energy (1.2 J/cm2) and high tilt angle (65°) led to a more preserved cross section with a slight strength improvement. The strength improvement was limited due to other surfaces that were not affected by laser treatment. An improvement of 30 % in tensile strength was achieved with a higher energy (4 J/cm2) lower tilt angle (45°) treatment that was consistent for different sample orientations. The cross section of the samples treated at such condition was significantly changed however. The effect of sample orientation on fracture behaviour was studied and unstable crack propagation was observed for oriented samples that was more significant after laser treatment.

29pm1-F-1 ナノスケールの特異応力場を有するSi中のき裂の伝ぱ基準評価

In order to investigate the criterion of unstable crack propagation induced by singular stress field less than 10 nm, the fracture toughness is examined using nano-scale silicon (Si) single crystal specimens with a precrack. Critical value of stress intensity factor at fracture, K_<IC>, is evaluated to be 0.93〜1.09 MPam^<1/2> where the minimum size of singular stress field is 6.3 nm. This clearly means that the fracture mechanics concept based on the continuum mechanics keeps the validity even in the single digit nanometer scale.

A laser irradiation disc test for fracture testing of refractory fine ceramics

Abstract The critical stress intensity factor, tensile strength and crack stability were analysed for a zirconia refractory by parameter identification based on a laser irradiation test and a finite element simulation. Furthermore, the results for the specific fracture energy were determined for different assumed cohesive behaviours. The tests were carried out on notched discs that were irradiated at their centres. During the tests, temperatures are recorded by a thermo vision camera and the crack propagation by an acoustic emission recorder. Acoustic emission allowed for the determination of the onset of crack initiation (time t 1 ) and unstable crack propagation (time t 2 ). A finite element model representing the geometry of the sample was built to determine the fracture mechanical parameters from t 1 and t 2 . This method is believed to be favourable for rather brittle refractory ceramics, whereas for less brittle materials, a wedge splitting procedure according to Tschegg is considered more favourable.

On the mechanics of sinusoidal interfaces between dissimilar elastic–plastic solids subject to dominant mode I

We compute the effective toughness of a weak, sinusoidal cohesive interface between two dissimilar elastic–plastic solids subjected to dominant mode I loading. We use a computational model that takes into account the cohesive behavior of the interface, and the elastic–plastic behavior of two different materials. Using a dimensionless analysis, we found relationships between the normalized fracture toughness, the sinusoidal aspect ratio (amplitude to wavelength) and key dimensionless groups that are related to material behavior. Transitions between several failure mechanisms, including unstable crack propagation, intermittent periods of stable/unstable behavior, such as stick–slip, void-to-void crack growth, crack tip blunting, and stable crack propagation are found in terms of the sinusoidal geometry parameters and the assumed constitutive behavior of the two materials. We present design maps in which the sinusoidal aspect ratio arises as a natural design variable that can be manipulated to increase the resistance to crack propagation in bimaterial interfaces.

Critical Crack Length for the Initiation of Unstable Propagation in Relation to Fracture Toughness and the Evaluation of Stress and Deformation Characteristics Obtained From Tensile Testing

The paper is concerned with the relation between temperature and critical crack length for the initiation of a brittle crack unstable propagation at the yield strength level stress. For the purpose of calculation, results of dynamic fracture toughness test were applied; the experiment was supplemented with measuring stress and deformation characteristics in a determined test temperature range. For comprehensive assessment of test temperature effects, a fractographic analysis of fracture surfaces was conducted.

On-line Monitoring of Engineering Components Using Acoustic Emission Technique

Acoustic emission technique (AET) has been used for on-line monitoring of pressure vessels with artificial defects in the laboratory and for monitoring structural integrity of storage vessels in heavy water plant. The laboratory studies are helpful for getting better insight into the deformation and fracture aspects in the vessels, towards better utilisation of AET for structural integrity of components. AE results generated during hydrotesting of the storage vessels were used to confirm the structural integrity of the vessels. AE was used to monitor compression loading of plain and fly ash concrete. This showed three stages due to crack closure/microcracking, steady crack propagation and unstable crack propagation. An attempt was made to compare the types of cracking in concrete using AE parameters.

The effect of aging on impact toughness and fracture surface fractal dimension in SAF 2507 super duplex stainless steel

The relation between room temperature impact toughness and fractal behavior of the fracture surfaces of SAF 2507 super duplex stainless steel, aged between 0 and 288h at 475°C, has been studied. Fractography was performed in a scanning electron microscope and the fractal dimension was determined according to the slit island method. A monotonical decrease in fractal dimension was observed as the time of aging increases from 0 to 288h. For the entire range of aging times, a complete transition from ductile to brittle behavior was observed as the impact toughness decreases from 284J (0-24h) to 43J (288h). Ductile fracture occurred by nucleation growth and coalescence of micro-dimples, while the brittle behavior was characterized by the propagation of cleavage cracks in ferrite, controlled by plasticity of the surrounding austenite. Partially based on several previous concepts and relationships well established for brittle materials, a direct linear correlation between impact toughness and fractal dimensional increment has been developed. The concept of critical volume of material is proposed and tentatively related with the unstable crack propagation event, through both, the so called critical length (microstructurally significant distance) and the size of the largest micro-dimple and cleavage facet (structural parameter) developed in ductile and brittle fracture respectively.

Cohesive Model-Based Approach for Fatigue Life Prediction of Reinforced-Concrete Structures Strengthened with NSM FRP

This paper presents a fatigue-life prediction model of reinforced concrete beams strengthened with near-surface mounted (NSM) fiber-reinforced polymer (FRP) composite rods under flexure. This model is based on the fracture-mechanics approach using a cohesive model, and accounts for different bridging properties in steel and FRP reinforcement in strengthened concrete structures. It predicts the number of fatigue cycles to ultimate failure in the strengthened members, in which the failure is defined by unstable propagation of an effective crack in concrete. Several sets of experimental results in the literature are selected to verify the efficiency and precision of this model, in which the writers compare typical stress versus fatigue-life curves. This model is capable of predicting the fatigue life of NSM-strengthened concrete beams to a good precision and relates the fatigue life to the structural stress in an explicit manner.

Finite element analysis of the microdroplet test method using cohesive zone model of the fiber/matrix interface

This paper presents a finite element (FE) modeling methodology to simulate and assess the importance of various factors affecting the failure mechanisms in the microdroplet test used to measure the interface shear strength (IFSS) of S-glass fiber epoxy matrix. These factors include the effect of processing-induced residual thermal stresses, progressive debonding with interfacial friction, unstable crack propagation and frictional fiber sliding on mixed-mode loading at the interface. Cohesive zone modeling approach is employed to simulate the crack propagation and interfacial failure. Mode II dominated traction separation laws are determined through numerical simulations of microdroplet test results. The importance of including the effect of residual stresses and interfacial friction in determining the cohesive traction–separation laws is illustrated. The sensitivity of the results for mode I traction–separation parameters is studied. The developed holistic modeling methodology allows for accurate determination of traction–separation behavior of the interface. This modeling effort also attempts to identify improvements to the microdroplet test method which is widely used to assess the degree of fiber/matrix adhesion.

Crack Initiation and Crack Propagation in Heterogeneous Sulfate-Rich Clay Rocks

Brittle fracture processes were hypothesized by several researches to cause a damage zone around an underground excavation in sulfate-rich clay rock when the stress exceeds the crack initiation threshold, and may promote swelling by crystal growth in newly formed fractures. In this study, laboratory experiments such as unconfined and confined compression tests with acoustic emission monitoring, and microstructural and mineralogical analyses are used to explain brittle fracture processes in sulfate-rich clay rock from the Gipskeuper formation in Switzerland. This rock type typically shows a heterogeneous rock fabric consisting of distinct clayey layers and stiff heterogeneities such as anhydrite layers, veins or nodules. The study showed that at low deviatoric stress, the failure behavior is dominated by the strength of the clayey matrix where microcracks are initiated. With increasing deviatoric stress or strain, growing microcracks eventually are arrested at anhydrite veins, and cracks develop either aligned with the interface between clayey layers and anhydrite veins, or penetrate anhydrite veins. These cracks often link micro-fractured regions in the specimen. This study also suggest that fracture localization in sulfate-rich clay rocks, which typically show a heterogeneous rock fabric, does not take place in the pre-peak range and renders unstable crack propagation less likely. Sulfate-rich clay rocks typically contain anhydrite veins at various scales. At the scale of a tunnel, anhydrite layers or veins may arrest growing fractures and prevent the disintegration of the rock mass. The rock mass may be damaged when the threshold stress for microcrack initiation is exceeded, thus promoting swelling by crystal growth in extension fractures, but the self-supporting capacity of the rock mass may be maintained rendering the possibility for rapidly propagating instability less likely.

Modeling damage mechanics of railway tracks to evolve control strategies for derailment prevention

Purpose – With an eye to prevent derailment of high-speed trains, vis-à-vis unwarranted loss of lives and property, this paper aims to develop a formalism of designing a suitable control system with embedded decision support system. Design/methodology/approach – A model of rolling contact fatigue (RCF) crack propagation in railway tracks is designed, simulating the alarming stress intensity factor around the advancing fatigue cracks. COMSOL multi-physics software is employed to design the RCF crack monitoring system with acoustic emission (AE) count signals, describing the damage threshold of railway tracks. Findings – Simulation experiment on stress intensity factor for cracks in real life rail sections has enabled to describe the maximum working stress; it has been noticed that the threshold value of stress intensity factor (∼ 41 MPa m1/2) for the onset of unstable crack propagation is reached at a fatigue crack length of 11.5 mm. It is further noticed that the observed AE count at a particular instant of time in a specific location of railway track is a true indication of the vulnerability of rail failures. Originality/value – The proposed model, a completely new of its kind, bears a high socio-technological value as it entails the design of an intelligent control system to prevent train accidents.

Influence of fly ash and curing on cracking behavior of concrete by acoustic emission technique

This study aims to investigate the crack growth behavior of plain and fly ash concretes with different curing periods during uniaxial compression testing using acoustic emission (AE) technique. Compressive strength of plain and fly ash concrete increase with curing periods and it becomes almost same order for 56days curing. AE results have shown three distinct stages of AE activity in both concretes. The AE generated from the three stages has been attributed to crack closure/microcracking, steady crack propagation and unstable crack propagation. Fly ash concrete with proper curing produces evident filling effect for the pores and the occurrence of pozzolonic reaction leads to reduction of microcracking and corresponding decrease in amplitude of AE activity in the initial loading of fly ash concrete. The results showed that fly ash concrete specimen can withstand higher stress and strain without unstable crack propagation. An attempt has been made to compare types of cracking in both plain and fly ash concrete using AE parameters and it has been correlated with SEM images and final cracking pattern.

Development of Cu/Insulation Layer Interface Crack Extension Simulation with Crystal Plasticity

A novel scheme for the evaluation of interface adhesion energy was examined by a detailed numerical simulation of interface crack extension. The effects of crystal orientation on the Cu/SiN interface adhesion strength of LSI was evaluated using the finite element method. Crack extension simulation was conducted with a model of the actual specimen used for the interface fracture test. The characteristics of elastic–plastic deformation, which changes significantly depending on crystal orientation, were taken into account in the model. With this scheme, the effect of orientation of single crystals on the maximum load Pmax was investigated under the condition of a constant bonding energy of the interface at the beginning of unstable crack propagation during the fracture test. The values of Pmax obtained with a number of different crystal orientations ranged over 179–311 µN. The result indicates that the crack propagates more easily in the case that slip deformation of Cu near the interface starts with a low stress, as in the case of the (111) surface. It implies that the apparent interface adhesion strength represented by the load required to debond the interface strongly depends on Cu crystal orientation, because the amount of energy used for plastic deformation of the Cu crystal changes with crystal orientation near the interface.

Prognostic anomalies of induced seismicity in the region of the Koyna-Warna water reservoirs, West India

In 2000, the region of the Koyna-Warna water reservoirs in West India was hit by two strong earthquakes, which occurred six months apart and had magnitudes M > 5. The Koyna-Warna seismic zone is a typical region of induced seismicity with a pronounced correlation between seismicity and water level variations in the reservoirs. This indicates that the stress level in the region is close to critical; thus, insignificant variations in stress caused by the variations in the water level may trigger a strong earthquake. In order to study the preparatory processes in the sources of the induced earthquakes, in this paper we analyze the seismic catalogue for the Koyna-Warna region before a pair of strong earthquakes of 2000. The induced seismicity is found to exhibit prognostic variations, which are typical of preparation of tectonic earthquakes and indicative of the formation of metastable source zones of future earthquakes. Based on the obtained results, we suggest that initiation of failure in these metastable zones within the region of induced seismicity could have been caused by the external impacts associated with water level variations in the reservoirs and by the internal processes of avalanche unstable crack propagation.

Wind blade joints based on non-crimp 3D orthogonal woven Pi shaped preforms

Current industrial designs for joining composite wind blade elements allow unstable crack propagation in the thick adhesive layer within the joint. In this work, a novel integral 3D woven Pi-joint element is developed and implemented in an I-beam joint construction that corresponds to a wind blade’s shear web and spar cap. In a series of iterative design–analysis–fabrication–testing cycles, 2.9m long I-beams that used the current practice joint and the novel proposed joint were designed, studied theoretically with the use of a 3D Mosaic analysis tool, then manufactured and loaded to failure in a cantilever flexure loading set up. Experimental data showed significant advantage in the load-bearing capacity of the joint structure with the use of novel Pi-joint element. Excellent agreement was observed between the theoretical predictions and experimental data.

A semi-analytical method to predict net-tension failure of mechanically fastened joints in composite laminates

A Finite Fracture Mechanics model is proposed to predict the net-tension strength of composite mechanically fastened joints. A modification of coupled stress–energy criteria is suggested to take properly into account the effect of the R-curve in determining the crack length corresponding to the unstable crack propagation. The proposed model works for quasi-isotropic laminates and it requires as input the longitudinal strength and the fracture toughness of the laminate. The predictions obtained show good agreement when compared with the experimental results.

Essential work of fracture: An approach to study the fracture behavior of acrylic bone cements modified with comonomers containing amine groups

The fracture behavior of acrylic bone cements modified with comonomers containing amine groups was studied using the essential work of fracture approach. The cements were prepared with either 2-(diethylamino)ethyl-acrylate (DEAEA), 2-(dimethylamino)ethyl-methacrylate (DMAEM) or 2-(diethylamino)ethyl-methacrylate (DEAEM) as comonomer in the liquid phase. Double-Edge-Notched Tensión (DENT) specimens were tested in a universal testing machine at 5 mm/min. The results showed that the essential work (we) and nonspecific value of fracture (βwp) of bone cements modified with all percentages of comonomer were notably increased compared with unmodified bone cement. From Scanning Electron Microscopy (SEM) micrographs, ductile behavior was observed for modified bone cements, i.e. the crack propagation is stable, whereas the unmodified cement exhibited brittle behavior indicating unstable crack propagation. The use of the essential work of fracture approach is suggested to determine the fracture behavior of cements that do not exhibit a linear stress-strain relationship.

Control of unstable crack propagation through bio-inspired interface modification

Selective toughening can be used to improve specific interfaces in a fibre reinforced polymer (FRP) component which may otherwise act as crack initiation sites. Interfacial tougheners (such as interleaves and powder treatments) have typically been deployed in a simplistic manner. However, by discretely and judiciously introducing toughening agents, taking inspiration from nature, a step change in toughness characteristics can be demonstrated. Cracks propagating through these toughened regions give rise to interesting fracture phenomena. The work presented herein shows that by incorporating regions of variable toughness, unstable crack propagation can be avoided and graceful degradation to failure demonstrated. Using both Mode I double cantilever beam testing and finite element modelling this study shows how selective toughening can be used without unstable crack growth.

Dynamic crack nucleation, propagation, and interactions with crystalline secondary phases in aluminum alloys subjected to large deformations

The major objective of this work was to model within a continuum framework the dynamic nucleation and evolution of failure surfaces in aluminum alloys with complex microstructures, using a recently developed compatibility-based fracture criterion for large deformations. Computational analyses were conducted to understand how Mn-bearing dispersoids, Ω and θ′ precipitates affect dynamic fracture processes in an Al–Cu–Mg–Ag alloys (2139-Al). High strain-rate simulations were based on a rate-dependent dislocation-density-based crystalline plasticity formulation and a nonlinear explicit dynamic finite-element approach. Results indicate that the fracture criterion elucidated how dispersoids and precipitates have a dominant role in dynamic crack blunting, branching and arrest. Rationally orientated precipitates result in overall dynamic microstructural strengthening and enhanced uniformity of deformation. These precipitates, however, accelerated unstable crack propagation, and this is amplified in the presence of a pre-crack. In contrast, dispersoids decreased microstructural toughness and ductility, but greatly improved dynamic damage tolerance, especially in the presence of a pre-crack. It can also be predicted that low angle boundaries can change the propagation direction of ductile cracks, and contribute to damage tolerance without crack initiation. Collectively, rationally oriented precipitates and dispersoids can significantly improve the combined dynamic strength, toughness and damage tolerance of crystalline aluminum alloys.