Dynamic Fracture Mechanics Research Articles

Dynamic splitting tensile test of short carbon fiber C/SiC ceramic matrix composites

In order to investigate the dynamic fracture mechanics behavior and damage morphology of C/SiC ceramic matrix composites, dynamic splitting tensile tests on the C/SiC composites with three different volume fractions of short carbon fiber (16.0%, 21.0%, 24.8%) were carried out by the split Hopkinson pressure bar, the destructive interface part of C/SiC composites was scanned by using scanning electron microscopy and the failure characteristics and toughening mechanism of C/SiC composites were analyzed. The experimental results show that the dynamic tensile strengths of the C/SiC composite specimens with the same short carbon fiber volume fraction increase with the increasing of the impact pressure in the dynamic splitting tensile failure process, the failure of the specimens is significantly correlated with impact pressure and short carbon fiber volume fraction. When the short carbon fiber volume fraction is 16.0%, the tensile strength of the C/SiC composite specimens is the lowest. After the impact, the overall destruction of the C/SiC composite specimens is related to impact pressure and short carbon fiber volume fraction.

Micro-structure response and fracture mechanisms of C/SiC composites subjected to low-velocity ballistic penetration

Dynamic response and fracture mechanisms of Carbon Fiber Reinforced Silicon Carbide Composites (C/SiC) especially during low-velocity ballistic penetration are studied both experimentally and numerically. The gas-gun facility is used to fire spherical metallic projectile for striking velocity of 150ms−1 on the target panels, and the impact phenomenon is captured through high-speed photography. A micro-structure based approach is employed to model C/SiC target in this paper. This proposed numerical technique captured the mechanical response (residual energy, expansion process and velocity of debris cloud, fracture morphology and mode) of target, with adequate accuracy. The fracture modes involve void collapse, delamination, fiber bundle splitting and breakage. The debris cloud possesses two types of constituents, classified by fragments' volume and high-energy powdering column at the front. The experimental and calculated results emphasize that the impact velocity, projectile shape and hardness have significant influence on the mechanical behavior of C/SiC composites, including fragment size, fracture surface morphology, fracture mode and mechanism.

Validation of a low blow specimen technique for R-curve determination using a drop tower

Abstract: Fracture mechanics based component design requires appropriate fracture mechanics toughness data with respect to both, loading rate as well as test temperature. Taking high-rate loading into account such as with accidental scenarios, different standards such as ASTM E 1820 or BS 7448-3 provide some information on dynamic fracture mechanics testing. Nevertheless, the designations differ so that a validation of the own material specific test method used for dynamic R-curve determination is mandatory. In order to address this for ductile cast iron materials an experimental method for the reliable determination of dynamic J-integral crack resistance curves at -40 °C following the multiple specimen approach has been established and validated. The experimental concept offers some additional valuable features. Single values of dynamic crack initiation toughness can be determined using a single specimen technique based on crack sensors. Furthermore, an experimentally independent method is provided according to which CTOD δ 5 R-curves can be established. The focus of the present paper is on the validation of the experimental low blow technique using a drop tower test system. A drop tower test system was developed and set up to perform low blow tests at temperatures down to -40 °C. The system allows for a variation of the impact mass and height and was optimized for testing of ductile cast iron at stress intensity rates from approximately 5∙10 4 to 3∙10 5 MPa√ms -1 . This range of loading rate is characteristic for instance with crash scenarios of heavy sectioned DCI casks for radioactive materials. In order to address characteristic challenges of impact tests (test duration of microseconds up to milliseconds, inertial effects, signal oscillations), an appropriate full bridge strain gage method for the measurement of force directly on the specimen as well as a non-contact measurement of load line displacement using an optical extensometer have been developed and validated. The low blow test requires either to prevent bouncing strikes of the hammer by using the stop block technique or to catch the hammer after its first strike. Both options are not part of the experimental concept and setup which have been realized here. The paper describes investigations which have been performed in order to make sure that bouncing strikes of the hammer do not cause additional crack extension in the specimen. This is necessary to ensure a unique relation between the work done and the achieved crack extension. The investigations covered the analysis of limit loads of the specimen with respect to the measured force. The measured stiffness of the specimen was assessed and signals of crack sensors were analyzed. Furthermore, an analysis of the mechanical behavior of the loading system and the specimen by optical observation was performed. Corresponding results are discussed in the paper. It had finally been proven that additional crack extension in the specimen due to bouncing strikes of the hammer is not to be expected under the given conditions of test setup, material and loading. It can be seen as a major experimental advantage that the striker does not have to be catched after the low blow test.

Study on Crack Branching Condition for Brittle Crack Propagation in Steels

In brittle material like glass, it is observed that crack branches when the crack speed gets more than the material critical velocity. When the crack branches, the driving force at running crack tip gets lower, so it is possible to prevent the structure from serious failure. In this paper, using the devised under-matched welded joint to implement the higher crack velocity, the mechanism of crack branching and surface roughening is investigated by experiments and the dynamic Finite Element Method (FEM) analysis.In the result, crack surface roughening happens when the crack velocity is around 800m/s, contrary to the findings of the elastic dynamic fracture mechanics. FEM analysis was conducted to explain the phenomenon and it is possible that the high stress triaxiality can be the critical condition for crack branching.

A comparative and parametric study of dynamic cohesive and linear elastic fracture mechanics models

In cohesive fracture mechanics (CFM), fundamental nondimensional parameters are the ratios of spacetime domain geometries and loadings to corresponding intrinsic scales implied by the cohesive fracture traction–separation relations (TSRs). One of these parameters is the nondimensional load-to-strength parameter which is the ratio of the applied loads, expressed in stress form, to an intrinsic strength scale implied by a TSR. Herein the radii of stress singularity from asymptotic Linear Elastic Fracture Mechanics (LEFM) solutions are derived to normalize cohesive process zone (CPZ) sizes from CFM. By approximating these nondimensional CPZ sizes, a simple small-scale yielding (SSY) indicator is derived for dynamic fracture which in turn is shown to be proportional to the square of the load-to-strength parameter. Thus, the load-to-strength parameter serves two purposes. First, increasing this ratio is shown to correspond to more ductile response for families of cohesive fracture self-similar solutions. Second being related to SSY condition, it is used to evaluate the validity of an LEFM model. Numerical results compare characteristic differences between these groups of CFM solutions, investigate the accuracy of the proposed SSY indicator, demonstrate LEFM solutions underestimate crack length and speed even when the SSY condition is satisfied, and study the evolution of the CPZ size.

Effect of loading rate on the fracture toughness and failure mechanisms of polycrystalline diamond (PCD)

Quasi-static and dynamic fracture toughness tests are performed on four grades of PCD to evaluate the effect of rate and identify dominant failure mechanisms. The results indicate that the presence of the secondary phase cobalt has a significant negative influence on the fracture toughness of PCD grades at high rates of loading. An apparent rate insensitivity was exhibited for coarse grain specimen, where appreciable amounts of secondary phase cobalt were removed, or leached from the crack-tip region. The beneficial effect of cobalt removal on dynamic toughness was observed to be dependent on grain size, with fine grain microstructures exhibiting a similar negative rate sensitivity to corresponding non-leached grades. SEM analysis was used to evaluate the resulting fracture surfaces of both static and dynamic fractured specimens. The effect of rate on the prevailing fracture mode was analysed using a dynamic fracture mechanics approach.

A source generation model for near-field seismic impact of coal fractures in stress concentration zones

To study the near-field seismic impact of coal fractures in stress concentration zones, we established a source generation model based on finite dislocation source theory and dynamic fracture mechanics, derived an analytical expression for near-field seismic displacements caused by coal fractures in the zone and numerically computed the resultant near-field seismic displacements within the coal mass. The results show that (1) the larger difference between the vertical and horizontal normal stresses in the stress concentration zone leads to a greater fracture speed, which thereby causes a stronger seismic impact; (2) the P-wave component in the near-field seismic displacements mainly impacts on the middle of the roadway, while the SH- and SV wave components mainly affect the junctions between the roadway and both the roof and the floor, and the damage caused by the SH- and SV waves within the coal mass is more significant than that caused by the P-waves; and (3) the effective way to mitigate the seismic impact induced by coal fractures in stress concentration zones is to reduce the difference between the vertical and horizontal normal stresses as far as possible. It is hoped that this study will provide a better understanding of the seismic impacts induced by coal fractures in stress concentration zones and thus help engineers to discover ways to prevent roadway failure.

WITHDRAWN: Analysis of dynamic fracture mechanics parameters of one-point bend specimens based on modified Timoshenko’s beam theory

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STUDY ON MICROSTRUCTURE AND DYNAMIC FRACTURE BEHAVIOR OF Q460 STEEL WELDING JOINTS

Effects of welding heat input on the microstructure and dynamic fracture toughness (JId) of the CO2 shielded arc welded joints of Q460 high strength low alloy steel were investigated. The mechanism of effects on the dynamic facture behavior of the welded joint was also discussed. The results showed that there existed the allotriomorphic ferrite at the columnar interface in the fusion zone of welded joint under the condition of low heat input. The morphological characteristics of columnar crystal in the fusion zone gradually decreased and the allotriomorphic ferrite disappeared as the heat input increased. The fusion zone was mainly composed of acicular ferrite, and its average size increased with increasing heat input. The welded joint exhibited the optimal dynamic fracture toughness under the condition of medium heat input while it showed the lowest value under low heat input within the temperature range of -70 °C to room temperature. When the temperature decreased from room temperature to -70 °C, the dynamic fracture mechanism of Q460 welded joint changed from ductile fracture to brittle cleavage fracture. Under the condition of low heat input, the allotriomorphic ferrite characterized by the planar growth at the columnar interface in the fusion zone of welded joint can lead to the rapid intergranular crack propagation at low temperature. The fine acicular ferrite in the fusion zone of the welding joint obtained at medium heat input which *国家自然科学基金项目51371044和51571052以及中央高校基本科研业务费专项资金项目L1502027资助 收到初稿日期: 2015-12-01,收到修改稿日期: 2016-01-22 作者简介:冯祥利,男, 1972年生,博士生 DOI: 10.11900/0412.1961.2015.00617 第787-796页 pp.787-796

Highly Stretchable and Conductive Core–Sheath Chemical Vapor Deposition Graphene Fibers and Their Applications in Safe Strain Sensors

Highly stretchable and conductive core–sheath nanofibers are significant for flexible and wearable microelectronics. Core–sheath fibers were massively fabricated from ultralong chemical vapor deposition (CVD)-grown graphene bundles. They exhibited superior conductivity and excellent mechanical properties that exceeded those of the reduced graphene oxide fibers. The intrinsic dynamic fracture procedure and mechanism of the core–sheath nanofibers were investigated. Furthermore, safe strain sensors based on as-prepared core–sheath CVD graphene fibers have been demonstrated as a proof-of-concept application. The performance of strain sensors has been greatly improved by using CVD graphene fibers.

The Construction of Dimensionless Nonlinear Dynamic Model and the Research on Chaotic Features of Vibration Cutting Based on Dynamic Fracture Mechanics

In this paper, the author constructs a dimensionless nonlinear dynamic model of Vibration cutting system described by nonlinear differential equations. The model is established on the relationship between cutting force and cutting speed and the relationship of cutting depth to the cutting resistance and the time. It is the nonlinear dynamic friction model of vibration cutting considering tool-chip friction. Based on the model and the method of chaotic identification, Lyapunov Exponent, the further research on the vibration cutting produce behavior and its classification are conducted.

Effect of inhomogeneous distribution of non-metallic inclusions on crack path deflection in G42CrMo4 steel at different loading rates

An inhomogeneous distribution of non-metallic inclusions can result from the steel casting process. The aim of the present study was to investigate the damaging effect of an inhomogeneous distribution of nonmetallic inclusions on the crack extension behavior. To this end, the fracture toughness behavior in terms of quasi-static J-?a curves was determined at room temperature. Additionally, dynamic fracture mechanics tests in an instrumented Charpy impact-testing machine were performed. The fracture surface of fracture mechanics specimens was analyzed by means of scanning electron microscopy. It was shown that an inhomogeneous distribution significantly affected the path and, therefore, the plane of crack growth. Especially clusters of non-metallic inclusions with a size of up to 200 ?m exhibited a very low crack growth resistance. Due to the damaging effect of the clusters, the growing crack was strongly deflected towards the cluster. Furthermore, crack tip blunting was completely inhibited when inclusions were located at the fatigue precrack tip. Due to the large size of the non-metallic inclusion clusters, the height difference introduced by crack path deflection was significantly larger than the stretch zone height due to the crack tip blunting. However, the crack path deflection introduced by a cluster was not associated with a toughness increasing mechanism. The dynamic loading ( 1 0.5 5 s MPam 10 ? ? K? ) did not result in a transition from ductile fracture to brittle fracture. However, the crack growth resistance decreased with increased loading rate. This was attributed to the higher portion of relatively flat regions where the dimples were less distinct.

On the Validation of Williams' Stress Function for Dynamic Fracture Mechanics

The Finite Block Method (FBM) for computing the Stress Intensity Factors (SIFs) and the T-stress under transient dynamic load is presented. In order to capture the stress intensity factor and the T-stress, the Williams' series of stress function is introduced in the circular core for statics generally. In the Laplace domain, the Deng's series of stress and displacement is too complicated to be used easily like Williams' series. However, the numerical solutions show that Williams' solution of series is still valid with smaller core size. Comparisons have been made with the solutions given by the finite element method (ABAQUS).

Dynamic failure and fracture mechanism in alumina ceramics: Experimental observations and finite element modelling

The application of ceramics in composite armour requires the investigation on the failure and fracture mechanisms of the ceramics under dynamic loading. In this study, the split-Hopkinson pressure bar technique with high speed photography was used to directly observe the dynamic macro-cracking and fragmentation process in alumina and to characterise the stress and strain histories. An experimentally validated 3D finite element model of the full scale test was developed to predict the stress distribution and damage evolution in alumina that were related to the experimental observations. It was found that the damage (cracking) evolution is determined by the multiaxial stress state, in particular the substantial localised tensile stress concentration and the stress direction. The cracking process leads to the longitudinal axial splitting failure on the surface and the internal diagonal damage bands. The dynamic failure process in alumina is dominated by both the intergranular and transgranular fracture mechanisms.

Stress and Fracture Analyses of Solar Silicon Wafers During Suction Process and Handling

Solar wafer/cell breakage depends on the combination of the stresses generated in the handling and the presence of structural defects such as cracks. Suction process is a common loading during silicon wafer handling. This paper presents a systematic static and dynamic analysis of the suction process. Optimum suction pad diameter and locations are obtained by minimizing the stress distribution under both static and dynamic loading, and the effect of the impact time on the crack driving force is also investigated in this optimum situation. The results show that the four pads configuration with diameter of 20 mm placed in a rhombus shape with 18 and 38 mm diagonal lengths yields lowest maximum principle stress among the cases analyzed. In the dynamic fracture analyses, the maximum J integral appears at 800 and 1400 μs for continued holding and unloading cases after reaching the maximum load, respectively. The J integral for the unloading cases are always smaller than the holding cases. It has been found that when the impact time is longer than 3 s and 5600 μs the dynamic fracture mechanics analysis of the suction impact process can be replaced by a static fracture mechanics analysis for the holding and unloading cases, respectively.

Dynamic fracture and local failure mechanisms in heterogeneous RDX-Estane energetic aggregates

Local failure initiation mechanisms, such as the nucleation and propagation of multiple cracks, have been investigated in energetic aggregates with a viscoelastic estane binder and crystalline RDX grains that have been subjected to dynamic thermo-mechanical loading conditions. A dislocation density-based crystalline plasticity, finite viscoelasticity, dynamic fracture nucleation and propagation, and non-linear finite-element formulations were used to study crack nucleation and propagation due to dynamic, tensile mechanical strain-rate loading conditions in RDX-Estane energetic aggregates. The interrelated effects of grain boundary (GB) misorientations, porosity, grain morphology, dislocation densities, polymer binder relaxation, and crystal-binder interactions were coupled with adiabatic plasticity heating, thermal decomposition, and viscous dissipation heating to fundamentally understand and predict aggregate behavior and local failure initiation mechanisms. The predictions indicate that local failure occurs when cracks nucleate at the peripheries of internal porosity and subsequently propagate toward the viscoelastic estane binder where crack arrest occurs at the interface, which results in large inelastic deformations and temperature accumulations at the interfaces.

Dynamic Crack Growth Normal to an Interface in Bi-Layered Materials: An Experimental Study Using Digital Gradient Sensing Technique

The dynamic fracture behavior of layered architectures is experimentally studied. Specifically, crack penetration, trapping, and branching at an interface are examined. A newly introduced optical technique called Digital Gradient Sensing (DGS) that quantifies elasto-optic effects due to a non-uniform state of stress is extended to perform full-field measurements during the fracture event using ultrahigh-speed photography. By exploiting the richness of two simultaneously measured orthogonal stress gradient fields, a modified approach for extracting stress intensity factors (SIFs) is implemented for propagating crack-tips under mixed-mode conditions. The method is first calibrated using a quasi-static experiment complemented by finite element simulations before implementing it for studying dynamic mixed-mode fracture mechanics of layered configurations. The layered systems considered consist of two PMMA sheets bonded using an acrylic adhesive with the interface oriented normally to the initial crack propagation direction. Interfaces are characterized as ‘strong’ and ‘weak’ by their crack initiation toughness. The dynamic fracture of monolithic PMMA sheet is also studied in the same configuration for comparison. The crack growth and fracture parameter histories of propagating cracks are evaluated. The interface is shown to drastically perturb crack growth behavior resulting in higher dissipation of fracture energy by exciting crack trapping, branching, and mixed-mode growth mechanisms.

Dynamic compressive property and failure behavior of extruded Mg-Gd-Y alloy under high temperatures and high strain rates

For the purpose of investigating the dynamic deformational behavior and failure mechanisms of magnesium under high strain rates, the Split Hopkinson Pressure Bar (SHPB) was used for investigating dynamic mechanical properties of extruded Mg-Gd-Y Magnesium alloy at ambient temperature (300 K), 200 °C (473 K) and 300 °C (573 K) temperature. The samples after compression were analyzed by scanning electron microscope (SEM) and metallographic microscope. Dynamic mechanical properties, crack performance and plastic deformation mechanism of extruded Mg-Gd-Y Magnesium alloy along the extrusion direction (ED) were discussed. The results show that, extruded Mg-Gd-Y Magnesium alloy has the largest dynamic compressive strength which is 535 MPa at ambient temperature (300 K) and strain rate of 2826 s−1. When temperature increases, dynamic compressive strength decreases, while ductility increases. The dynamic compression fracture mechanism of extruded Mg-Gd-Y Magnesium alloy is multi-crack propagation and intergranular quasi-cleavage fracture at both ambient temperature and high temperature. The dynamic compressive deformation mechanism of extruded Mg-Gd-Y Magnesium alloy is a combination of twinning, slipping and dynamic recrystallization at both ambient temperature and high temperature.

Three point bending Hopkinson bar for fracture toughness measurement: A new approach for interface analysis

The arrangement of the three‐point‐bending (3PB) Hopkinson bar has been adopted as a convenient means for measuring the dynamic fracture toughness of materials. Some researchers have used this method to evaluate that property which has become an important dynamic fracture mechanics parameter. In this work, an insight of this arrangement is presented through a finite element method (FEM) analysis. A number of important aspects are approached. Among them, the history of the force conveyed to the specimen and the evolution of the dynamic stress intensity factor (SIF) during tests. A new approach, allowing estimating a more accurate effect on the specimen and also an eventual loss of contact with the incident and transmitter bars is described and explained: instead of obtaining the force, the velocity on the incident bar tip is extracted to feed the finite element model.

Adaptive image-based method for integrated multi-scale modeling of damage evolution in heterogeneous concrete

This paper presents a new adaptive image-based method for integrated multi-scale modeling and computational approach to simulate the trans-scale process of dynamic fracture in concrete structures from evolving meso-damage to structural failure. Two numerical examples are presented to demonstrate the accuracy and effectiveness of the approach. The results show that, the approach can be used to reveal the dynamic fracture mechanisms of concrete structures by considering the trans-scale coupling process from meso-damage to local failure in vulnerable area and eventually to structural failure; and it can obtain satisfactory results with sufficient precision and lower cost, especially for large scale computations.