Stress Wave Loading Research Articles

Effect of geometric parameters on dynamic fracture toughness of compact tensile specimens

In this paper, the dynamic fracture toughness test and numerical simulation of CT specimens of 2A12-T4 aluminum alloy were carried out by using strain gauge method and finite element method based on Hopkinson tensile bar experiment device. The influence of crack transverse shape, loading hole size and dimension on the dynamic fracture performance of CT specimens was explained. The experimental and simulation results show that for CT specimens with different internal cracks, the time to reach stress equilibrium at the same tensile stress wave loading is roughly the same, while the time of initial cracking varies slightly. The dynamic fracture toughness obtained by concave arc-shaped internal crack and straight line-shaped internal crack is basically the same, while other changes in crack shape have a large impact. For CT specimens with straight line-shaped cracks, the position of the loading hole has a small effect on the dynamic fracture of the CT specimen under the same tensile stress wave. However, when the position of the loading hole is fixed, changing the diameter of the loading hole has a significant effect on the dynamic fracture toughness of the CT specimen.

Modelling the Blast Cracking Processes of Rock Masses Using a Total Lagrange Meshless Method

The dynamic damage of surrounding rock masses has a great impact on the tunnel stability during the drilling and blasting construction processes. However, traditional FEM has some difficulties in simulating the blast damage processes of rock masses during mesh re-divisions. In our work, the smoothing kernel function in traditional SPH method has been improved, and the momentum equation considering the blasting load is derived, which realizes the modeling of the dynamic damage evolution processes under the SPH framework. Firstly, two typical examples are numerically simulated: 1) Upper slow and lower steep “Y” shaped fissure and 2) Upper short and lower long “Y” shaped fissure. The simulation results are compared with previous experimental results to verify the correctness of the SPH method. Then, the progressive blast damage processes are simulated, and the effects of different stress wave loading modes, different pre-existing fissure distances, different fissure dip angles as well as different ground stress levels on the blast cracking morphologies are investigated. Results show that: The blast cracks first appear around the blast hole, then the radial cracks propagate to the model boundary; The radial crack length increases with the increase of peak blast stress wave; The length of the horizontal radial crack decreases and the length of the vertical radial crack increases with the increase of the horizontal ground stress; The existence of pre-existing fissure prevents the further extension of the blasting cracks, and the increase of the fissure dip angle leads to the crack propagation at the fissure tips; The increase of fissure distance leads to the larger crack propagation in the around the fissure; The damage count increases with the increase of peak blast stress wave; The increase of the ratio of horizontal stress to vertical stress decreases the damage count; The damage counts increase with the increase of fissure dip angles; The damage degree of the model increases with the increase of the fissure distance.

Research on simulating coal crack development under high voltage electric pulse stress waves using zero thickness cohesive elements

High voltage electric pulse (HVEP) technology has demonstrated significant potential in enhancing coal seam permeability. However, there is a notable gap in numerical simulation studies that investigate the cracks development patterns in coal under the influence of this technology. In this study, a HVEP coal rock fracturing system was utilized to conduct a physical simulation experiment of coal sample electrical fragmentation. A finite-discrete element method (FDEM) model, incorporating zero-thickness cohesive elements, was developed to simulate the process of stress wave-induced crack propagation in coal, triggered by the plasma channel. The results indicate that over time, the stress wave typically exhibits an initial rapid rise followed by an oscillatory decline. The peak value of the stress wave decreases significantly with distance, stabilizing beyond a propagation distance of 10 mm. Furthermore, the fracturing efficacy of coal by HVEP stress waves is closely linked to specific characteristic parameters: within the range of 75–125 MPa, a positive correlation exists between the stress wave peak and the complexity of the coal crack network. Reducing the stress wave loading rate can increase the crack area to some extent, although excessively slow loading rates may lead to excessive energy dissipation during propagation, which is detrimental to crack expansion. Conversely, when the ratio of β to α ranges from 1 to 100, decelerating the attenuation rate of the stress wave aids in generating stress concentration within the crack zone, thereby facilitating crack propagation. The established numerical model aims to advance understanding of HVEP’s fracturing mechanisms and enhance its application in coalbed methane extraction.

Electric Potential Response Characteristics of Coal Under Stress Wave Loading

Electric Potential Response Characteristics of Coal Under Stress Wave Loading

Sucrose-mediated formation and adhesion strength of Streptococcus mutans biofilms on titanium

Biofilms consist of bacterial cells surrounded by a matrix of extracellular polymeric substance (EPS), which protects the colony from many countermeasures, including antibiotic treatments. Growth and formation of bacterial biofilms are affected by nutrients available in the environment. In the oral cavity, the presence of sucrose affects the growth of Streptococcus mutans that produce acids that erode enamel and form dental caries. Biofilm formation on dental implants commonly leads to severe infections and can restrict osseointegration necessary for the implant to be successful. This work determines the effect of sucrose concentration on biofilm EPS formation and adhesion of Streptococcus mutans, a common oral colonizer, to titanium substrates simulating common dental implants. Biofilm formation and profiles are visualized at high magnification with scanning electron microscopy (SEM). Large mounds and complex structures consisting of bacterial cells and EPS can be seen in biofilms at sucrose concentrations that are favorable for biofilm growth. The laser spallation technique is used to apply stress wave loading to the biofilm, causing the biofilm to delaminate at a critical tensile stress threshold. The critical tensile stress threshold is the adhesion strength. Because laser spallation applies the stress loading to the rear of the substrate, bulk adhesion properties of the biofilm can be determined despite the heterogenous composition and low cohesion strength of the biofilm. Statistical analysis reveals that adhesion strength of biofilms initially increase with increasing sucrose concentration and then decrease as sucrose concentration continues to increase. The adhesion strength of bacterial biofilms to the substrate in this study is compared to the adhesion of osteoblast-like cells to the same substrates published previously. When sucrose is present in the biofilm growth environment, S. mutans adhesion is higher than that of the osteoblast-like cells. Results of this study suggest sucrose-mediated S. mutans biofilms may outcompete osteoblasts in terms of adhesion during osseointegration, which could explain higher rates of peri-implant disease associated with high sugar diets. Further studies demonstrating adhesion differentials between biofilms and cells including co-cultures are needed and motivated by the present work.

Study of finite fracture of the micro contact on the friction interface induced by stress waves

Frictional interface commonly exist in nature and kinds of materials. The dynamic behavior of the frictional interface has a profound impact on the macro strength and other properties of materials. This paper mainly studies the influence of finite fracture of the micro contact the interface under stress wave loading theoretically and simulatively. Based on the micro contact fracture mechanism of friction, a two-dimensional interface friction model including a triangular micro bulge is established with linear elastic constitutive relationship and D-P failure criterion. The results show that the interaction of the incoming stress disturbance and the micro bulge will induce the fracture of the bulge, and with the decrease of the inclination angle of the micro-contact, the fracture position of the triangular micro contact gradually moves upward from the root, and a crack propagating towards the matrix occurs in the top corner of the triangle. Moreover, in the micro process of fracture, a three wave profile centered on the fracture surface: longitudinal wave, transverse wave and interface wave, will be generated A new interesting phenomenon is that at the moment of loading, a micro stress disturbance is produced synchronously from the interface and propagates to the substrate in the form of longitudinal wave. More comparative examples and analysis show that the mechanism of this disturbance is related to the overall gravity micro adjustment acting on the interface. This work connects fracture and wave, and reveals the finite fracture of micro contact of the friction interface and wave effect, which is expected to provide an effective way for earthquake prediction, so as to advance the earthquake prediction time.

The Attenuation Ability of Saturated Joints Filled with Granular Materials under High-Amplitude Stress Wave Loading

Existing experimental evidence shows that the propagation of explosive waves in the free fields of soils is remarkably affected by the degree of saturation. In the surrounding rocks of underground protective structures, the underground water is normally unavoidable, which is supposed to reduce the isolation efficiency of a passive antiblast barrier. To investigate the effect of water saturation on the stress wave attenuation ability of infilled joints, impact tests were carried out on artificial joints filled with dry and saturated granular materials using a split Hopkinson pressure bar (SHPB). The test results revealed that under the same conditions, the stress and energy transmission coefficients of the waves crossing saturated sand-filled joints were about 3.16–4.13 times and 9.75–11.4 times those of joints filled with dry sand, respectively. The dynamic stress-strain relationship of the filling layer during the impact process and the crushing index of the infill were analyzed. The results showed that the compressibility and the granular crushing index of the dry sand were much greater than that of the saturated sand, and the dynamic stress-strain relationship of the dry sand exhibited three-stage nonlinear characteristics. The experimental results quantitatively uncovered the serious adverse effect of water on the wave absorption properties and markedly diminished the potential of the filled joints as a wave elimination barrier, which should be a matter of great concern in the design of underground protective structures.

Mixed-mode dynamic fracture behavior of a rubber toughened epoxy adhesive under stress wave loading

This work seeks to measure mixed-mode fracture parameters for a dynamically growing crack in a core shell rubber toughened epoxy adhesive (EPONTM 828 with PARALOID™ EXL-2691J particles). Digital image correlation technique coupled with ultrahigh-speed photography and a hybrid finite element-based post-processing approach are used to extract fracture parameters. The dynamic loading is accomplished by using a striker to induce stress waves in an incident bar in contact with a free-standing semi-circular specimen. A companion finite element analysis is performed for investigating suitable experimental parameters and configurations for implementing the approach and determining specific experimental parameters (crack length, crack angle, and impact velocity) necessary to achieve the desired experimental conditions (mode mixity at crack initiation and strain rate). The critical fracture parameters are used to generate mixed mode fracture envelope and fracture toughness-mode mixity relationship for the material. Further investigation is carried out to evaluate various fracture criteria for predicting initial crack kink angles. The measured dynamic fracture quantities are also compared with the quasi-static counterparts to assess strain rate-dependence of the adhesive. The results reveal a decrease in critical stress intensity factors of approximately 10 % and 30 % for the mode I and mode II states, respectively, with increasing strain rate. Post-initiation, the dynamic crack propagates at a near-constant mode I stress intensity factor of ∼1.4 MPa-√m. Under dynamic conditions, the critical, effective stress intensity factors are relatively constant with respect to mode mixity. This is in contrast to quasi-static results where the effective values increased with respect to mode mixity. Experimentally measured crack kink angles agree well with those predicted by the maximum energy release rate criteria and the maximum tangential stress criteria, particularly under mode I dominant conditions.

Experimental study on crack overlaps between two blast-induced cracks in PMMA

Phenomena of crack overlaps are very common and essential in tunnel constructions by rock blasting. In this study, in conjunction with the high-speed camera, directional fracture controlled blasting experiments are performed on three sets of PMMA specimens using the optical caustics method. The lenticular fragments formed as two cracks approach each other are measured and compared, as well as other fracture parameters, such as crack velocity, mode I and II dynamic stress intensity factors and curvature of crack propagation path. It is found that both mode I stress intensity factor and crack velocity exhibit an abrupt rise upon crack interaction. Further, mode II stress intensity factors show more complicated variations in comparison with mode I, divided into three loading regimes, i.e. crack initiation, blast stress wave loading and crack overlap. And an appropriate ratio of crack offset distance/horizontal distance (h/s) between two boreholes for apparent mixed mode failure exists.

Crack propagation induced by a plane stress wave in a general anisotropic elastic material

Abstract The stress intensity factors of a semi-infinite crack propagating with constant speed in an anisotropic elastic solid under a uniform stress wave loading are considered. The crack is assumed to start propagating at some arbitrary time after an obliquely incident plane stress wave strikes the crack tip. It is shown that the stress intensity factor of the propagating crack has the form of the product of a universal matrix function of the crack speed and an equivalent stationary crack stress intensity factor of t*, which is the time that would have elapsed since the incident wavestruck the crack tip if the crack tip had been always at its instantaneous position. The present result is a generalization of that obtained by Freund for isotropic materials.

Theoretical instructions for experimenting controllable Hopkinson pressure bar

Hopkinson bar loading technique needs theoretical instruction for desired stress waves, to achieve controllable deformation under constant high strain rate loading, for it is the indispensable access while investigating material's rate-dependence and deformation mechanism under transient loading. In this work, theoretical and experimental studies are focused on the involved issues. Firstly, the theoretical deduction based on stress wave theory is performed for constant true strain rate loading, and the equation is formulated of an ideal incident strain wave for different materials. The formulated waves are further clarified by case studies of specimens with various mechanical behaviors, and they are found to present concave characteristics under large deformation. Secondly, an efficient methodology is proposed for single stress wave loading, and the target requirement is deduced theoretically. It is easily achieved experimentally by performing specific strain-controlled experiments: prior to and beyond peak stress of epoxy. Finally, some methods are proposed for achieving the formulated stress waves, and experimental verifications are also conducted on various materials.

Dynamic propagation behaviors of pure mode I crack under stress wave loading by caustics

Abstract It is always a difficult task to study the dynamic fracture of prefabricated cracks under stress wave loading. To investigate loadings of stress waves on dynamic cracks, a crack propagation testing configuration consisting of a one-point bend specimen loaded in split Hopkinson pressure bar (SHPB) was used, which loaded an unconstrained polymethyl methacrylate (PMMA) plate (120 mm × 60 mm × 5 mm) on the edge opposite to the cracked edge. A modified simple pre-cracked specimen geometry under impact stress wave loading to generate pure mode I crack at crack initiation was demonstrated, which can avoid the superposition and interference of various waves to facilitate the research. The numerical simulation was performed firstly by ABAQUS to prove the existence of the mode I field at the crack tip leading to crack propagation and indicate the stress distribution and evolution in the specimen caused by the propagation of the impact stress wave, analyze the propagation characteristics of the wave. Then dynamic caustics method in conjunction with high-speed photography was utilized in SHPB impact experiment. The propagation of shock stress wave in the specimen and its interaction with the prefabricated crack and the stress concentration at the tip of the prefabricated crack were analyzed. The corresponding stress intensity factor history is precisely determined. Finally, it is concluded that the observed distortion phenomenon at the impact point belongs to a caustic behavior under compression load, which reflects the stress concentration at the impact point. And the mode I failure occurs along the pre-crack direction. Specifically, the pre-crack shows obvious pure mode I crack propagation characteristics under symmetrical reflected tensile wave, the stress at the crack tip changes from compressive stress to tensile stress. And crack propagates under tensile stress wave reflected from its two free boundaries without crack, while the compressive stress wave can not make crack initiate and has little influence on crack propagation. Which agree with the numerical prediction.

Combined electromechanical dynamic fracture behavior of lead zirconate titanate (PZT)

AbstractCoupon specimens of poled and depoled lead zirconate titanate (PZT) are examined under combined stress wave and electric loading conditions. Mode‐I crack initiation and fracture behavior is examined using ultrahigh‐speed imaging and two‐dimensional digital image correlation. The dynamic critical stress intensity factor () is extracted using measured displacement fields ahead of the impulsively loaded crack tip, and compared between poled and depoled plates that were either under no electric field, positive 0.46 kV/mm electric field, or negative 0.46 kV/mm electric field. Poled specimens had a poling direction and applied electric field direction normal to the crack front. The addition of an electric field resulted in a crack‐enhancing effect, where the dynamic fracture toughness of poled specimens under 0.46 kV/mm was almost half that of samples with no electric field. Depoled samples experienced almost no change in dynamic fracture toughness with the addition of an electric field.

Numerical Simulation of Mechanical Characteristics of Roadway Surrounding Rock under Dynamic and Static Loading

Mechanical characteristics of roadway surrounding rock under different stress wave disturbances are the key to design roadway supporting scheme. In this study, the 2802 transportation roadway in Zhangcun Coal Mine is selected as the engineering background. The distribution of stress, displacement, and plastic zone in surrounding rock under the impact of different stress waves is studied. Results show that the stress and displacement of the roof, floor, and coal walls present fluctuating change with time during the stress wave loading process. With the increase of disturbing intensity of stress wave, the resistance ability for stress wave disturbance of the roof is lower than that of the floor, while the resistance ability of two sides is the same. The volume of plastic zone in roadway surrounding rock was calculated by the self-compiled FISH code. The relationship between the plastic zone volume and the stress wave disturbing intensity in different states is explored. The cubic polynomial relationship between the volume and the disturbing intensity in the state of shear_past and tension_past is obtained. Under the simulated condition, the disturbing intensity of stress wave has the greatest influence on the increase of shear_past volume when it equals 11 MPa. While the disturbing intensity of stress wave has the greatest influence on the increase of tension_past volume, it equals 7 MPa. Meanwhile, the relation between stress wave disturbing intensity and surrounding rock stress and displacement is obtained respectively. The achievements provide a theoretical base for roadway surrounding rock support under dynamic and static loading.

Dynamic crack initiation and growth in cellulose nanopaper

Cellulose nanopaper (CNP) made of cellulose nanofibrils has gained extensive attention in recent years for its lightweight and superior mechanical properties alongside sustainable and green attributes. The mechanical characterization studies on CNP at the moment have generally been limited to tension tests. In fact, thus far there has not been any report on crack initiation and growth behavior, especially under dynamic loading conditions. In this work, crack initiation and growth in self-assembled CNP, made from filtration of CNF suspension, are studied using a full-field optical method. Dynamic crack initiation and growth behaviors and time-resolved fracture parameters are quantified using Digital Image Correlation technique. The challenge associated with dynamic loading of a thin strip of CNP has been overcome by an acrylic holder with a wide pre-cut slot bridged by edge-cracked CNP. The ultrahigh-speed digital photography is implemented to map in-plane deformations during pre- and post-crack initiation regimes including dynamic crack growth. Under stress wave loading conditions, macroscale crack growth occurs at surprisingly high-speed (600–700 m/s) in this microscopically fibrous material. The measured displacement fields from dynamic loading conditions are analyzed to extract stress intensity factors (SIF) and energy release rate (G) histories. The results show that the SIF at crack initiation is in the range of 6–7 MPa m1/2, far superior to many engineering plastics. Furthermore, the measured values increase during crack propagation under both low- and high-strain rates, demonstrating superior fracture resistance of CNP valuable for many structural applications.Graphical abstract

Energy absorption within elastic range for AZ31 magnesium alloy

Energy absorption for AZ31 magnesium Alloy was investigated with Split Hopkinson Pressure Bar using single stress wave so as to avoid multiple stress wave loading. The stress wave amplitude, which was in elastic stress range and propagated along the AZ31 magnesium bar, was reduced with increasing propagating distance, and with increasing stress wave amplitude, the stress wave amplitude reduction along the magnesium bar was increased losing more energy as compared with that of the stress wave with lower amplitude. The drastically decreased stress wave amplitude could be explained based on dislocations movements, which was similar to the established theory of damping for the explanation of the energy loss during cyclic loading. However, it was not the case for LY12 aluminum alloy: the stress wave amplitude changed slightly without drastic energy loss regardless of the variation of stress wave amplitude.

Experimental study on the bump-inducing mechanism of slicing in thick coal seams with hard roofs

ABSTRACT The bump-inducing mechanism of slicing in thick coal seams with hard roofs is still not completely understood. In this study, a similar material simulation experiment was designed to research the overlying strata behavior and the stress distribution characteristics in mining the lower slice of a thick coal seam with a hard roof. The results showed that the lower part of the main roof could form a relatively stable semi-arc rock-gangue structure during the mining of the lower slice, which could make the voussoir-beam structure develop into the higher strata above. The shock wave induced by the sudden rupture of the voussoir-beam could lead to the further collapse of the semi-arch rock-gangue. The stress monitoring revealed that the stress variation was not sharp in the top slice, whereas the situation of the lower slice was very different There were many intermittent peak points in the stress variation rate curve that had a certain time interval between each point. For the fracture, the rotary process of overlying strata in the bottom slice was more severe compared with the top slice, so the coal damage gradually accumulated under stress wave loading, and when the coal stress exceeded the limit load, rock burst occurred in the roadways and coal faces. In addition, control measures including the top-coal caving mining method, pressure relief technology and support technology were proposed. The microseismic monitoring indicated that the total number of tremors with energy of 10 kJ to 100 kJ was 354 in 2014, the total number of tremors with energies of more than 100 kJ was 7, and the maximum energy of a tremor was 3530 kJ when the 8937 coal face advanced 210 m from the setup entry. The findings from this study will serve to strengthen the engineers’ confidence in thick coal seams with serious coal bumps.

Quasi-static and dynamic fracture behavior of lead zirconate titanate: A study of poling and loading rate

In this study, coupon specimens of both poled and de-poled lead zirconate titanate (PZT) are examined under quasi-static and stress-wave loading conditions. Mode-I crack initiation and fracture behavior is examined using ultra high-speed imaging in conjunction with 2D digital image correlation on symmetrically impacted specimens. Measured displacement fields ahead of the dynamically loaded crack tip are used to extract the critical stress intensity factor (KICd) at initiation and are compared with quasi-static SENT fracture toughness tests. Results demonstrate that under static conditions, the difference between poled and de-poled specimens is not statistically significant, however under dynamic conditions, poled samples exhibited a 70% greater fracture toughness compared to the de-poled specimens. To verify the increase was not due to thermal degradation of material mechanical properties during the de-poling process, re-poled specimens were also prepared and examined under dynamic fracture loading conditions. In general, impulsively loaded samples exhibited greater fracture toughness compared to their quasi-statically loaded counterparts, regardless of the poling. Specifically, the critical stress intensity factor increased seven to tenfold compared to static fracture toughness values, with poled samples exhibiting the greatest toughness of all samples explored. In terms of rate, the increase in fracture toughness under dynamic conditions likely comes from microcracking and potentially phase transformations at the crack tip. In terms of poling, the increase in fracture toughness is likely due to a bulk manifestation of ferroelastic toughening, where dynamic domain reorientation provides crack tip shielding.

A novel methodology for large strain under intermediate strain rate loading

Hopkinson bar has already worked to measure mechanical properties of materials under high strain rates, but it fails to conduct intermediate strain rates loading with considerable deformation. Thereby, a methodology is proposed for achieving enough large strains under intermediate strain rates. The experimental system based on it is then constructed: a striker bar with same length as incident bar is selected to generate continuous stress wave loading, and a long polymethylmethacrylate bar is preferred as the transmitted bar to guide and record transmitted signals. Thus, loading stress wave is achievable with a nearly constant amplitude and an infinite loading duration. The involved data interpretations are deduced, and verified both theoretically and experimentally. Its measurement capacity is confirmed by loading a thermoplastic polyurethane, and maximum strains up to 12% and 65% under strain rates of 40/s and 270/s, respectively. Finally, the involved influences and factor sensitivities are analyzed for enough experimental accuracy and extensive measurements.

Influence of Loading Peak and Loading Rate on Twin Fibers Pullout Test of Concrete Matrix

Numerical analysis software based on linear finnite element method and statistical damage theory concrete material has been adopted in this study. Dynamic numerical model of twin fibers pullout test of concrete matrix have been created by considering the meso-heterogeneity of concrete material. The whole process from micro-cracks initiation, propagation to crack penetration has been simulated. The influence of stress wave loading peak and loading rate on twin fibers pull-out test of concrete matrix under dynamic load has, therefore, been scrutinized. Results show that loading rate has effect on failure mode and crack propagation along interface of the twin fibers pull-out specimens. With increasing loading rates, the rate of interface crack propagation and damage area increase either.