Dynamic Fracture Behavior Research Articles

Evaluation of modelling parameters for AA2014-T6 in simulating dynamic fracture event

The material behavior of the aluminum alloy AA2014-T6 under different strain rates and temperatures must be examined in order to simulate their responses under extreme loading conditions like dynamic fracture and impact. In the present study, several uniaxial tensile experiments, including quasi-static, dynamic, and triaxiality, were carried out at different temperatures to evaluate the material constants for the Johnson cook constitutive and failure models of AA2014-T6. It was observed that material flow stress exhibited positive strain rate sensitivity at high strain rates and negative sensitivity when temperature increased from 25 to 250 °C. From the triaxiality experiments, it was also observed that the plastic flow increased while the ductility of the material decreased with increasing stress triaxiality. The dynamic fracture behavior of a pre-cracked 3-point bend specimen of AA2014-T6 was studied at different loading rates. Applied load and load point displacement (LPD) were measured using strain gauges. In addition, crack mouth opening displacement (CMOD) and crack initiation time were calculated using the 3D digital image correlation technique. It was found that the loading rates highly influence the dynamic fracture initiation and propagation toughness of the material. A 3D finite element simulation of dynamic 3-point bend experiments was performed in ABAQUS using the J-C damage evaluation criteria, and incident stress obtained from the dynamic experiments was used as an input load to minimize the error. In accordance, with the actual experiments conducted, the material geometry, boundary conditions, and contact circumstances were considered. The authors found good agreement between both the experimental and numerical results.

Dynamic fracture mechanism and behaviour of V-notch white marble specimens with strain gauge and digital image correlation in three-point bending tests

A three-point bending test was conducted on white marble specimens with V-shaped notches. The fracture mechanism of these specimens, including crack initiation toughness and propagation speed under different opening angles, was studied, and their special fracture behaviour was analysed. In this experiment, a falling weight loading device was used to impact the V-notch white marble specimens with different opening angles at the same impact speed. To measure the crack tip initiation toughness, a strain gauge theory is formulated in this study. The calculation results are in good agreement with the digital image correlation (DIC) calculation results. The error is approximately 10%, and the results obtained by the acute angle patching method are stable. The experimental results also show that when the opening angle is large, the specimens crack, and the energy dissipated by the crack initiation is considerable. Owing to the particularity of the white marble structure, the rock fracture behaviour during the dynamic fracture process is clearly observed in the experiment. All the specimens exhibited spreading crack branches at the crack tip under stress. The foregoing occurs because the original defects at the crack tip deform and grow after sustaining stress. Some of the cracks combine to form the main crack and produce branches. In this study, the microcrack areas on the crack tip and fracture path observed in the experiment are collectively referred to as the crack growth area. The expansion of the crack growth area precedes the expansion of the main crack. The expansion of the strain concentration area is calculated by DIC; it reflects the dynamic formation of cracks on the surface of the specimen. Scanning electron microscopy shows that microcracks mainly occur under the intergranular failure mode.

Dynamic fracture behavior for inclusion-initiated cracks in graded non-homogeneous magnetic-electric-elastic material

In this paper, dynamic anti-plane problem in graded non-homogeneous magnetic-electric-elastic material with a conductive cylindrical rigid inclusion is studied. Cracks are generated from the inclusion edge under the action of SH guided waves. By applying a pair of reversed shear stresses to the crack surface, the mechanical model of the crack is achieved, and Green’s function method is used in this process. The scattering fields of displacement, electric potential and magnetic potential are constructed by the wave function expansion method. Numerical results clarified the factors that affect the dynamic stress intensity factor of the crack tips, including material parameters, inclusion parameters, wave number and incident angle. It is expected that the investigation of magnetic-electric-elastic material with inclusion-initiated crack defects can provide some references for the project designing, manufacture and application of non-homogeneous multi-fields coupling materials.

Study on Numerical Simulation Method of Fracture Behavior of Pipeline Girth Weld

Abstract The failure accidents in girth weld of pipelines occur frequently due to the combination of internal defects and external loads. However, the research on the fracture behavior of girth weld defects is relatively poor at present. To solve this problem, the cracking behavior and strain evolution law of the inner wall defects of the pipe girth weld are studied in combination with full-scale tests (FST). The constitutive and Gurson-Tvergaard-Needleman damage parameters of the pipe base metal zone, weld zone and heat-affected zone (HAZ) are calibrated through the small punch test (SPT) and single edge notch bending (SENB) test. On this basis, the welded pipe model with inner wall defects is established, and a numerical simulation method for dynamic fracture behavior based on damage mechanics is formed. The numerical simulation method is verified by FST data and theoretical calculation. The results show that the numerical results are consistent with the FST and theoretical calculation in the elastic stage, plastic stage, and fracture stage, and the error is within 10%. The novel numerical simulation method is provided as a means for the fracture behavior research of pipeline girth weld.

Effect of interlayer carbon nanotube films on the quasi-static and dynamic mode Ⅰ fracture behavior of laminated composites – An experimental and numerical investigation

A combined experimental and numerical study was conducted to investigate the quasi-static and dynamic mode Ⅰ fracture behavior of laminated composites toughened by carbon nanotube (CNT) films. Fracture tests under quasi-static and dynamic loading conditions were carried out by using electronic universal testing machine and electromagnetic Hopkinson bar device respectively. It’s illustrated that the CNT films can suppress the propagation of interlayer cracks effectively. Dynamic strengthening effect was also observed for CNT toughened composites during tests. For numerical simulations, the three-linear cohesive zone model (CZM) was proposed to simulate the Mode I delamination of CNT films toughened composites under quasi-static loading condition. Furthermore, rate-dependent parameters were introduced into this model by user-defined subroutine to describe the dynamic fracture behavior of toughened composites. This developed numerical model was validated by comparing with dynamic crack propagating speeds obtained from experimental tests. During the dynamic delaminating process, the average crack propagation rate of toughened composites decreased while the energy release rate increased, indicating that the interlayer CNT films can hinder the dynamic delamination effectively.

Experimental investigation on effect of loading rate on fracture behavior of coal-seam roof rock

The fracture behavior of coal-seam roof rock in coal mining is a key and controlling factor for the mode optimization of the artificial roof caving. However, the fracture mechanism of roof rock under loading is not clear. In the work, the split Hopkinson pressure bar (SHPB) experiment was carried out using semicircular bending samples from the sandstone of coal-seam roof rock in the Junger mining area of Inner Mongolia at the loading rate of 0.35–3.78 GPa·m0.5·s−1, and the dynamic fracture behavior and energy dissipation mechanism of samples under different loading rates were investigated. The result shows that the dynamic stress–strain process of the hard roof rock includes four stages: linear instantaneous compaction, linear elastic compression, failure, and fracture extension, in which the failure forms changes from brittle fracture to ductile fracture with the increase of loading rate. The mode I fracture toughness increases linearly under confining pressure. In addition, the propagation orientation of induced fractures is parallel to the loading direction, and the gravel in the samples can inhibit fracture extension, resulting in changing the fracture extension path. Further, the energy absorption efficiency of the samples during the fracture process decreases with the increase in the loading rate.

Toughening ultrastrong low-density steel by textured δ-ferrite lamellas

By both the Charpy V-notched impact and the projectile tests, we here investigated the dynamic fracture behavior of a recently developed ultrastrong lightweight steel comprising a hierarchical martensitic matrix, dispersed ultra-fine-retained austenite grains and oriented δ-ferrite lamellas, the latter being due to high Al and Si contents employed for low-density design. This steel shows a superior combination of specific ultimate tensile strength and impact toughness to other ultrastrong steels and has successfully arrested a real steel-cored bullet shot. These are attributed to the densely textured δ-ferrite lamellas that can deflect the propagating cracks until they are trapped and enclosed besides austenite-to-martensite transformation crack closure, leading to more energy consumed before failure. These results suggest a new pathway for toughening ultrastrong lightweight steels.

Study on mode I dynamic fracture characteristics with a mini three-point bending specimen for the split Hopkinson bar technique

In recent years, dynamic fracture experimental techniques have been developed rapidly in the field of engineering and scientific research. However, there still exist some difficulties in dynamic fracture tests, such as the limitation of loading rate, the accuracy of transmitted wave, the difficulty of crack initiation, and etc. To solve such problems, a novel mode I dynamic fracture experimental technique is proposed in this work, with a mini three-point bending(3PB) specimen and two kinds of fixtures specially designed for the split Hopkinson pressure bar (SHPB) system. In this method, the dynamic stress intensity factor curve (DSIF) is obtained by the experimental-numerical method, and the crack initiation time is determined by the strain gauge method. To verify the feasibility and reliability of the proposed method, the dynamic fracture toughness (DFT) of two kinds of ultra-high-strength steel (UHSS) is determined by this new method and then compared with the results of other methods. Meanwhile, the loading rate effect of fracture toughness, the size effect of crack initiation time and the failure mechanisms of the two materials under mode I fracture are also studied. The results show that fracture toughness depends on the energy consumed to form two types of fracture regions within the fracture initiation time. This work provides a new method for studies of the dynamic fracture behaviors of brittle solids.

Mixed-mode I/II dynamic fracture behavior in PMMA driven by stress waves

Mixed-mode I/II dynamic fracture behavior in PMMA driven by stress waves

Dynamic fracture behavior of SA508-3 steel for nuclear power equipment under medium-and low-loading rates

Abstract The heat treatment of SA508-3 steel was carried out by means of quenching and manual aging. The microstructure and mechanical properties of the prepared SA508-3 steel were analyzed and tested. The results show that the main composition of the matrix of SA508-3 steel is bainite, and the main composition of the second phase is alloy cementite containing Fe, Mn and C. The second phase is distributed at both the matrix and grain boundaries. The second phase can prevent the dislocation from moving by fixing the dislocation, and the second phase at the grain boundary can strengthen the matrix by hindering the grain boundary movement. The stress-strain curves of SA508-3 steel under different loading rates show that when the strain rate is greater than 0.5 m s−1, the fracture mode of the steel is brittle fracture, and when the strain rate is less than 0.5 m s−1, the fracture mode of the steel is ductile-brittle bonding fracture. The second phase of the crack first diffuses to the grain boundary, reducing the strength of the grain boundary. When the loading rate is high, the second phase at the grain boundary cannot diffuse in time, and the material undergoes transgranular fracture and intergranular fracture.

DyFraNet: Forecasting and backcasting dynamic fracture mechanics in space and time using a 2D-to-3D deep neural network

The dynamics of material failure is a critical phenomenon relevant to a range of scientific and engineering fields, from healthcare to structural materials. We propose a specially designed deep neural network, DyFraNet, which can predict dynamic fracture behaviors by identifying a complete history of fracture propagation—from the onset of cracking, as a crack grows through the material, modeled as a series of frames evolving over time and dependent on each other. Furthermore, the model can not only forecast future fracture processes but also backcast to elucidate past fracture histories. In this scenario, once provided with the outcome of a fracture event, the model will reveal past events that led to this state and can also predict future evolutions of the failure process. By comparing the predicted results with atomistic-level simulations and theory, we show that DyFraNet can capture dynamic fracture mechanics by accurately predicting how cracks develop over time, including measures such as the crack speed, as well as when cracks become unstable. We use Gradient-weighted Class Activation Mapping, Grad-CAM, to interpret how DyFraNet perceives the relationship between geometric conditions and fracture dynamics, and we find that DyFraNet pays special attention to the areas around crack tips that have a critical influence in the early stage of fracture propagation. In later stages, the model pays increased attention to the existing or newly formed damaged regions in the material. The proposed approach offers the potential to accelerate the exploration of dynamical processes in material design against failure and can be adapted for all kinds of dynamical problems.

Dynamic fracture toughness of cellular materials with different microstructures

Ultra-lightweight cellular materials have been widely utilized owing to their excellent specific stiffness, strength and fracture resistance ability. However, current understanding of the fracture resistance of cellular materials is mostly limited to quasi-static loading conditions, and their resistance to dynamic fracture is comparatively less understood. In this study, we investigate the dynamic fracture behaviors of the triangular, kagome, and hexagonal honeycomb cellular materials using numerical methods. Both single-edge notched bend and single-edge notched tension specimens under dynamic loading are considered, and the J values of dynamic fracture toughness of the three configurations are determined. Simulation results demonstrate that the fracture resistance of the triangular honeycomb increases with increasing loading velocity, whereas the fracture toughness of the two other structures exhibits slight variation with respect to the loading rates. After examining the fracture response by changing the strain-rate-dependence of the constituent material, we concluded that the microstructure has a more significant effect on the rate-dependence of fracture toughness than the constituent material. A mechanism based on crack deviation is proposed to account for the distinct rate-dependence of fracture toughness in specimens with different microstructures.

Study on the effect of loading conditions on the fracture behavior of pipeline with girth weld

There is increasing attention on the fracture of pipelines with girth weld, as a result of the accidents caused by additional loads occurring occasionally. However, there is little research on the dynamic cracking behavior of girth welds under different load forms. To further understand the fracture process of girth weld, the crack initiation and dynamic fracture behavior of welded pipe under pure bending load are firstly analyzed by numerical simulation method based on Gurson-Tvergaard-Needleman (GTN) model. It is found that the bearing capacity of the pipeline is reduced due to the existence of the weld. Subsequently, the distribution of the axial stress and strain and the fracture morphology are analyzed, which indicates that the softening of HAZ donated to the brittle fracture of the pipe. The opening angle and displacement are studied respectively, from which can be obtained that WOD is linearly related to the deflection change, and the fracture resistance of the pipe girth weld can be characterized by the slope. Finally, the bearing capacity and fracture toughness of the pipe in different loading conditions are analyzed. The results show that the bearing capacity of pipeline girth weld is greatly reduced due to shearing.

Dynamic response of the TiB2–SiC/B4C composite ceramic under dynamic loading

AbstractTiB2–SiC/B4C composite ceramics have application as lightweight armor ceramic material, and therefore, it is particularly important to research their dynamic mechanical properties and fracture behavior under high strain rates. In the present research, the dynamic compression strength and Hugoniot elastic limit (HEL) of these composite ceramics were determined by split Hopkinson pressure bar technology and plate impact equipment, respectively. The experimental investigation results illustrated that the composite ceramic exhibited outstanding dynamic compression strength (2750 MPa) and displayed a remarkable strain‐rate strengthening effect. The HEL was calculated as 18.49 GPa, and the commonly used P–μ form of the Hugoniot curve was derived from the calculated data of the shock velocity and particle velocity. The dynamic fracture morphologies of the TiB2–SiC/B4C composite ceramic and monolithic B4C ceramic (correlation data) at macro and micro scales were observed and analyzed in detail. The dynamic fracture mechanisms were summarized in‐depth into the following three points. First, the strengthening mechanism offered by the in situ second phases could significantly improve the comprehensive property. Second, the mixed‐inter/transgranular fracture mode was conducive to energy absorption. Third, the anchoring effect and thermal expansion coefficient mismatch contributed to the energy dissipation and fracture toughness improvement. This work provides the foundation for the application and development of lightweight armor ceramic materials.

Investigations into the static and dynamic fracture initiation and propagation toughness of AA2014-T6 incorporating temperatures effects

The fracture toughness of a material is an important parameter to quantify the reliability and safety of an engineering structure. In this article, the effect of loading rate and temperature on static and dynamic initiation and propagation toughness fracture behavior of AA2014-T6 is studied. Pre-crack three-point bend specimens are used in determining the static and dynamic fracture initiation and propagation toughness. Static three-point bend experiments are performed using a conventional UTM combined with the heating chamber at different displacement rates (1 mm/min, 10 mm/min, and 100 mm/min) and a wide temperature ranging from −150 °C to 200 °C. Similarly, the dynamic experiments are carried out on modified Hopkinson pressure bar (MHPB) under different loading rate and temperature. ASTM E399 was utilized in evaluating the static fracture initiation toughness, while the dynamic fracture initiation toughness is measured with the help of load point displacement (LPD) and crack initiation time. The 3D digital image correlation (DIC) in combination with ultra-high-speed imaging is used to calculate the crack initiation time and crack mouth opening displacement (CMOD). The energy conservation-based method was utilized in calculating the propagation fracture toughness in both static and dynamic loading conditions. Results show that the propagation toughness increases with temperature while the static fracture initiation toughness declines linearly. In addition, both initiation and propagation fracture toughness increases with the loading rates. Also the dynamic fracture investigation reveals that the positive rate sensitivity for dynamic propagation toughness increases with the increase in temperature. Results also show that the value of dynamic fracture initiation toughness is maximum at room temperature as the yield strength and plastic zone size are optimum in room temperature environments.

Mesh‐size independent rigid‐body spring network for simulation of the dynamic fracture behavior of concrete

AbstractIn this study, a numerical approach is developed for simulating the rate‐dependent behaviors of concrete without mesh sensitivity. The rheological units (e.g., dashpot) are integrated with the six directional springs in the rigid‐body spring network (RBSN) elements to reflect the rate‐dependent behavior of concrete. Previously, the viscoplastic damage model was associated with the elemental degrees of freedom. However, in the present approach, a viscoelastic constitutive law is newly defined for the normal direction as a function of the strain rate, from which the internal forces can be updated from the regularized elemental stresses. Such improvements are validated through the numerical simulations of the direct tensile test and spalling test using the modified split‐Hopkinson pressure bar (SHPB). The simulated results show consistent structural responses without mesh dependence on the strain rate. In addition, the influences of the softening curve shapes on the crack localizations are examined. It is shown that the rheological parameters can be optimized and determined for consistent applications through virtual experiments. The proposed numerical approach enables the mesh construction procedure without concerning the mesh‐size sensitivity due to the strain rate, and various applications are expected for the simulations of detailed numerical models of concrete materials and structures under high loading rates.

Exploring the dynamic fracture performance of epoxy/cement based piezoelectric composites with complex interfaces

Dynamic intensity factors (IFs) are the key parameters in comprehending and forecasting dynamic fracture behavior of piezoelectric composites. A domain-independent interaction integral (DII-integral) is developed to extract the dynamic stress intensity factors (SIFs) and electric displacement intensity factor (EDIF). The DII-integral is theoretically proved that there is no need to consider the electromechanical material derivatives and complicated material interfaces have no effect on the application of the method. Evaluating the results with the published data yields a good agreement and good domain-independence of the proposed I-integral is checked for multi-interface piezoelectric composites. The numerical simulations show that the amplitudes of the dynamic IFs decrease as the size of piezoelectric particle increases and crack damage is prevented as the number of piezoelectric particles increases. The peak values of the dynamic IFs are not monotonically related to the distance between the particles due to the superposition of elastic waves. The dynamic IFs increase when the volume percentage of piezoelectric fiber rises, while the opposite change is observed in laminated composites. Generally, the dynamic IFs of fiber-reinforced composites are smaller (larger) than that of laminated composites when the volume fraction of piezoelectric ceramics is higher (lower) than 40 percent.

Dynamic fracture behaviour of AA7475-T7351 alloy at different strain rates and temperatures

The fracture behaviour of the aluminium alloy AA7475-T7351 at different loading rates and temperatures (-150 °C to 200 °C) is investigated here. The three-point bend experiments are conducted at Zwick/Roell-50 UTM (crosshead speed up to 100 mm/min), MTS-250 machine (crosshead speed upto 1000 mm/min) and modified Hopkinson pressure bar (striker velocity 11–19 m/s) to understand the initiation and propagation fracture toughness of AA7475-T7351 aluminium alloy along the rolling (L-T) and transverse (T-L) directions. The digital image correlation (DIC) technique is used to obtain the load point displacement (LPD) and crack mouth opening displacement (CMOD) and the same is used to calculate the stress intensity factor (SIF). The fracture surface is studied using a digital stereo microscope (DVM6) and scanning electron microscope (SEM). The fracture toughness of AA7475-T7351 is found to depend on the loading rates and temperatures and found to vary between 50 and 66 MPm1/2 along the L-T direction, whereas the corresponding value along the T-L direction is comparatively less.

Dynamic shear fracture behavior of rocks: insights from three-dimensional digital image correlation technique

The influence of the loading rate on the mode I fracture property of rocks has been extensively investigated through experiments. However, the rate-effect of the dynamic mode II fracture has rarely been reported. This study employed the modified split Hopkinson pressure bar (SHPB) system to conduct a series of dynamic mode II fracture tests under a wide range of loading rates using the modified short-core-in-compression (SCC) method. The complete dynamic fracture processes of rock were observed using the three-dimensional high-speed digital image correlation (3D HS DIC) technique. The results reveal that the dynamic mode II fracture toughness increases with the loading rate at a power relationship. By virtue of the evolution of the surface principal strain and displacement, the dynamic fracture and damage process and the deformation characteristic of the SCC specimen were analyzed in detail. The dynamic fracture processes are divided into the elastic stage, plastic deformation stage, unstable crack growth stage, and post fracture stage. The crack propagation path of the SCC specimen was obtained under impact loading. It was found that new cracks first initiate from two notch tips and then propagate toward the central area in the expected shear fracture band (ESFB); the path is nearly independent of the loading rate. Additionally, the macro–micro fracture surface morphology of the SCC specimen was measured using a 3D optical scanner as well as scanning electron microscope (SEM) respectively. It was found that the roughness and curvature in the fracture surface created by shearing generally decrease as the loading rate increases, in accordance with fractal theory. The SEM result indicates intergranular fracture occurs more frequently on the shear fracture surface under static loading, while transgranular fracture is more common on the shear fracture surface at high loading rates.

Effect of reflected stress wave on dynamic crack propagation and arrest behavior of sandstone specimens under impact loading

The interaction of stress waves generated by blasting or excavation disturbance with cracks or joints significantly affects the mechanical behavior of rock masses with some pre-existing cracks. To better understand the dynamic damage evolution mechanism of fractured rock mass, the influence of stress waves on cracks in rock mass must be investigated. Thus, to study the effect of reflected stress wave on dynamic fracture behavior of fissured rock mass, a large-size arc-shape boundary single cleavage triangle (ABSCT) configuration sandstone specimen was proposed in the present study. A series of impact tests were performed on the specimen with 0°, 65°, 95°, and 125° ABSCT using the drop plate impact testing system to determine dynamic fracture properties. The test was numerically simulated using the modified finite difference software and the traditional finite element software. The effect of the reflected stress wave caused by the arc-shape boundary of the ABSCT specimen on the crack propagating speed, crack fracture toughness, and compressive stress along the crack trajectory under dynamic loads was analyzed and discussed. The results show that the ABSCT specimen has a crack arresting effect on a propagating crack. The average crack propagation speed of three specimens with arc-shape boundaries is almost 0.63 times that of the flat-bottom specimen, indicating that the arc boundary has an obvious effect on the crack propagating velocity. The average crack propagating speed decreases with the increase in arc boundary radian. The compressive stress on the crack path of three arc boundary specimens is found to be greater than that of the flat-bottom specimen. Furthermore, the crack arrest toughness is greater than the average crack propagating toughness at the arc-shape boundary specimen.