Dynamic Fracture Toughness Research Articles

Experimental study of dynamic fracture behavior of Al7075-T651 under different loading rates

The dynamic fracture initiation toughness for an aluminium alloy, Al7075-T651, has been estimated experimentally at different loading rates of 6.20, 7.87, and 10.1 M P a m / μ s . In this study, dynamic 3-point bend experiments were performed with a modified Hopkinson Pressure Bar (MHPB) associated with two high-speed cameras. A 3D digital image correlation (DIC) technique was used to get the full-field deformation of the specimen, such as crack initiation time and crack mouth opening displacements. Load point displacement as well as crack mouth opening displacement methods were used to determine the stress intensity factor (SIF). It was discovered that the dynamic fracture toughness significantly increased with increasing loading rates. The fractography carried out on the surface of fractured samples revealed transgranular facets and large dimples, which illustrated an increase in the fracture toughness with loading rates. 2D and 3D profiles of fractured surface were drawn, which relate undulation with fracture toughness.

Static and dynamic fracture behavior of rock-concrete bi-material disc with different interface crack inclinations

The fracture behavior of rock-concrete bi-material structures with interface crack is of great significance to the stability of construction projects in rock engineering. Dynamic testing of cracked straight-through Brazilian disc (CSTBD) samples was performed using a split-Hopkinson pressure bar (SHPB) system. The fracture development of rock-concrete bi-material was monitored by the digital image correlation (DIC) technique. A static fracture test using the MTS322 hydraulic servo-controlled testing system was also carried out for comparison. The results show that three typical fracture patterns can be identified for CSTBD samples under both static and dynamic loading. At a smaller interface inclination angle, the disk samples fracture along the interface, while at a larger interface inclination angle, the samples suffer tensile fracture along the loading direction. The increase of strain rate leads to an increase in the number of secondary cracks. As the inclination of interface increases from 0° to 75°, the dissipated energy increases gradually. When the inclination exceeds 75°, the dissipated energy turns to weaken. The difference in peak dissipated energy between different inclination angles weakens as the strain rate increases. The dimensionless stress intensity factors (SIFs) at the two crack tips are different, and increasing the crack length-diameter ratio will enlarge the difference. The interface inclination angle has a significant effect on both the static and dynamic fracture toughness of CSTBD samples. As the inclination of the interface increases, the difference between mixed-mode static and dynamic fracture toughness also increases. The dependence of dynamic fracture toughness on inclination is significantly higher than that of static fracture toughness, and the ratio between dynamic and static fracture toughnesses increases with increasing loading rate.

Effect of cyclic freeze–thaw treatments on the dynamic fracture characteristics of granite: Laboratory testing

In this study, the dynamic fracture toughness of freeze-thawed granite was studied. Different freeze–thaw (F-T) cycles were used to treat notched semicircular bending (NSCB) granite samples. The microstructure changes of samples treated with different numbers of F-T cycles were measured by a nuclear magnetic resonance (NMR) testing system. The dynamic fracture toughness was obtained by dynamic testing with split Hopkinson pressure bar (SHPB) equipment. In addition, the roughness of the rock fracture surface was studied using fractal geometry, and the F-T damage was defined using fractal dimensions. On this basis, a dynamic fracture toughness prediction model is proposed. The influence of the F-T cycle and loading rate on fracture toughness are discussed.

Experimental characterization of dynamic fracture toughness behavior of X80 pipeline steel welded joints for different heat inputs

Experimental characterization of dynamic fracture toughness behavior of X80 pipeline steel welded joints for different heat inputs

Effects of microstructure anisotropy on dynamic fracture behaviors of a selective laser melting nickel-based superalloy

In the study, the crack initiation and propagation behaviors of Ni-based GH3536 manufactured by selective laser melting (SLM) are quantified by the absorbed impact energy and the dynamic crack resistance curves. It is found that the SLM fabricated alloy exhibited slightly lower strength but much higher dynamic fracture toughness along the building direction, compared to those along transverse directions and the hot-rolled counterpart. The microstructure and defects of the SLM GH3536 alloywere characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), electron back-scattered diffraction (EBSD) and micro-computer tomography (μ-CT). Furthermore, the correlation between microstructure anisotropy and dynamic fracture behaviors are explored by the KAM, fractography and crack trajectories analyses. It is concluded that the anisotropy related to dynamic fracture mechanism of GH3536 alloy manufactured by SLM can be attributed to the ‘weakest link’ interface due to the laser melted pool geometry, the grain boundary or dendrite, which is strongly dependent on the relative orientation difference between the building direction and the crack plane.

Dynamic fracture behavior and fracture toughness analysis of rock-concrete bi-material with interface crack at different impact angles

The crack initiation and propagation behavior of interface crack under impact are of great significance for evaluating the stability of rock-concrete system and other layered structures. Dynamic test using split-Hopkinson pressure bar (SHPB) system was performed on bi-material cracked straight-through Brazilian disc (CSTBD) specimens, and the fracture processes of specimens were monitored by means of a high-speed camera. The effects of the impact angle, concrete strength and strain rate on the dynamic fracture behavior were investigated. The results show that three typical fracture types can be identified: interface fracture, combined fracture (interface/shear and tension fracture) and tensile fracture. The interface fracture appears under small impact angle, while the combined fracture occurs with the further increase of impact angle. The crack initiation time of wing cracks and the number of secondary cracks are mainly affected by the strength of concrete in the bi-material specimen. Crack initiates earlier and more secondary cracks are generated in concrete material (weak material), and the number of secondary cracks increases with increasing strain rate. The dissipated energy increases gradually as the impact angle increases from 0° to 75°, while it decreases when the impact angle exceeds 75°. As the concrete strength increases, the difference in dissipated energy for different impact angles decreases. Furthermore, the effects of impact angle, concrete strength and loading rate on the dynamic fracture toughness for different loading modes are also discussed. The results indicate that the normalized stress intensity factors (SIFs) at the two tips of the interface crack are distinct and the difference increases with increasing crack length-to-diameter ratio. The impact angle, concrete strength and loading rate influence the dynamic fracture toughness significantly.

Investigation of dynamic fracture behavior and energy dissipation of water-bearing coal under impact load

The stability of residual coal pillar in coal mine is significantly affected by the ponding in goaf and the dynamic disturbances such as blasting vibration and mine tremor activity in the production process of surrounding coal mines. In this paper, the effects of different loading pressure and water-bearing state on the dynamic fracture mechanical properties and energy dissipation law of coal samples were studied. The dynamic fracture characteristics of dry, natural and water-saturated notched semi-circular bending (NSCB) coal samples were tested under different loading pressure by using the impact loading system of split Hopkinson pressure bar (SHPB). A high-speed camera was used to record the dynamic crack propagation process. Combined with image processing method and fractal dimension calculation software, the influence of loading pressure and water-bearing state on fractal characteristics of crack propagation in coal specimens was further analyzed. The results show that at the beginning of crack initiation, water-saturated coal samples are easy to fracture compared to dry and natural coal samples. During the crack propagation, the dry and natural coal samples have a shorter crack penetration time compared to the water-saturated samples. The fracture toughness of the saturated coal sample is sensitive to the loading pressure and there is a threshold value. When the loading pressure is lower than 0.483 MPa, the dynamic fracture toughness of the dry coal sample is greater than the saturated coal sample. While the loading pressure is higher than 0.483 MPa, the dynamic fracture toughness of the saturated coal sample is greater than the dry coal sample. The fractal dimension of the crack propagation path of the coal sample changed as a “V” shape with the loading pressure. The fractal dimension of the crack propagation path of the dry coal sample is always higher than the water-saturated coal sample. The fracture energy of the coal sample meets the y=a·bx exponential growth relationship with the loading pressure. The fracture energy of water-saturated coal sample is the largest and the natural state is the least during the fracture process. The research results can provide theoretical guidance for the stability assessment of residual coal pillar as well as the safe and efficient mining of deep coal resources.

Research on the dynamic fracture toughness of reef limestone

The basic properties of reef limestone were measured to explore the dynamic fracture characteristics of reef limestone. The test results show that the physical and mechanical properties of blocky reef limestone are linearly related to the density. Hence, accurate mechanical parameters can be ascribed to standard mechanical specimens. Next, the fracture toughness of the reef limestone was measured using notched semi-circular bend (NSCB) specimens. Meanwhile, fracture energy and characteristics were studied with the help of a high-speed camera and scanning electron microscope. The findings show that the fracture toughness and energy of reef limestone are linearly related to the density with a rate effect. Moreover, the dynamic compressive strength is also linearly correrlated with the dynamic fracture toughness of reef limestone under the same loading rate; the fracture toughness of saturated reef limestone is larger than that of dry reef limestone. Besides, reef limestone has a rough fracture surface for its various crystal shapes and arrangements, with cracks propagating along the interface of varied crystals.

Research on dynamic cracking properties of cracked rock mass under the effect of thermal treatment

Deep fractured rock mass is more prone to failure with the coupling effect of suddenly dynamic disturbances loads (explosions, seismic waves, etc.) and high temperature environment conditions, these factors have a great effect on the dynamic stability of fractured rock mass. In the present study, some traditional cracked straight through Brazilian disc (CSTBD) specimens made of granite and sandstone were heated from 25 °C to 700 °C to obtain dynamic fracture properties of high temperature soft and hard rocks. A modified traditional split Hopkinson pressure bar (SHPB) was applied for dynamic fracture experiments under impact loads, high speed camera, digital image correlation (DIC) and fracture surface scanning technology were used to study the fracture morphology of the specimens, the dynamic damage mechanism of high temperature soft and hard rocks under impact loads was obtained. The test results indicate that high temperature can greatly affect the roughness of the fracture surfaces under impact loads during crack formation and propagation, and the fractal dimension increases with the increase of temperature. Observing the dynamic fracture strength and fracture toughness of the CSTBD specimens, it can be found that the strength of the granite decreases gradually with high temperature treatment, and the dynamic fracture strength of the green sandstone increases firstly and then gradually decreases. At 500 °C–600 °C, the dynamic fracture strength of the two types of rocks decreases rapidly. Dynamic critical crack tip opening displacement of the specimens with different thermal temperatures increases with temperature, but the fracture process was similar and the cracks would follow the loading direction.

Determination of dynamic mode I fracture toughness of rock at ambient high temperatures using notched semi-circular bend method

To investigate the influence of loading rate and high temperature on the dynamic fracture toughness of rock, dynamic fracture tests were carried out on notched semi-circular bend specimens under four temperature conditions based on the split Hopkinson pressure bar system. Experimental and analytical methods were applied to investigating the effect of temperature gradient on the stress waves. A high-speed camera was used to check the fracture characteristics of the specimens. The results demonstrate that the temperature gradient on the bars will not significantly distort the shape of the stress wave. The dynamic force balance is achieved even when the specimens are at a temperature of 400 °C. The dynamic fracture toughness linearly develops with the increase of loading rate within the temperature range of 25−400 °C, and high temperature has a strengthening effect on the dynamic fracture toughness.

Loading rate effect and failure mechanisms of ultra-high-strength steel under mode II fracture

40Cr and 30CrMnSiNi2A are both ultra-high-strength steel (UHSS) that is usually used in engineering structures such as aircraft landing gear, wing girder, and fastening bolts. Under dynamic loading, fracture properties of such materials are very important for structural design. A novel mode II dynamic fracture testing technique is adopted to study the mode II dynamic fracture characteristics of these two materials under high loading rates. The mode II dynamic fracture specimen is designed for the SHPB equipment, and the stress intensity factor curve at the crack tip is determined by an experimental-numerical method. The crack initiation time of the specimen is determined by the strain gage method. In the end, the mode II dynamic fracture toughness (KIId) of the two materials is obtained, and the loading rate effects are compared and analyzed in detail. The results show that within the loading rate range of this research (1.08 ∼ 7.73 TPa·m1/2/s), the KIId of the two materials has a positive correlation to the loading rate. Both of the materials exhibit a gradual transition process from tensile failure to adiabatic shearing failure mode. According to the fracture morphologies of the two materials, the failure mechanism of the two materials is analyzed for different failure modes. Failure mode transition (FMT) is specially investigated with the increase of the loading rate for both of the materials.

Study on dynamic fracture properties of sandstone under the effect of high-temperature using large-scale sample

High temperatures are a crucial factor in the drilling of rock and the extraction of geothermal resources, to investigate the change rules of the dynamic fracture mechanism and physical parameters of sandstone under high temperature environment, this study conducted a series of physical parameter measurements on the sandstone treated in a range of temperatures (room temperature to 700 °C) using large single cleavage semicircle compression (LSCSC) sample. According to the experimental results, it can be observed that 400 °C is the threshold for the variation of the physical parameters of sandstone. The mass, P-wave velocity, S-wave velocity, elastic modulus and Poisson’ ratio have a significant downward trend with the temperature. The main mineral components of sandstone undergo thermochemical reactions and the variation of quartz is a fundamental cause for the deterioration of sandstone. The dynamic loading test was conducted using a drop hammer impact device and the fractured sample, the dynamic fracture test results indicate that the dynamic crack initiation time (CIT) of the samples is advanced with the temperature, but the crack speed and dynamic fracture toughness decreased with the temperature. Further discussions about SEM images and fractal box dimension of fracture surfaces can be found that with the temperature increases, the proportion of intergranular fractures of sandstone gradually increases, the effect of dynamic loads changes from destroying the grain structure of sandstone to orderly coalescing the micro-cracks in the sandstone.

Experimental data-driven uncertainty quantification for the dynamic fracture toughness of particulate polymer composites

This paper presents an experimental investigation supported by data-driven approaches concerning the influence of critical stochastic effects on the dynamic fracture toughness of glass-filled epoxy composites using a computationally efficient framework of uncertainty quantification. Three different shapes of glass particles are considered including rod, spherical and flaky shapes with coupled stochastic variations in aspect ratio, dynamic elastic modulus and volume fraction. An artificial neural network based surrogate assisted Monte Carlo simulation is carried out here in conjunction with advanced experimental techniques like digital image correlation and scanning electron microscopy to quantify the uncertainty and sensitivity associated with the dynamic fracture toughness of composites in terms of stress intensity factor under dynamic impact. The study reveals that the pre-crack initiation time regime shows the most prominent effect of uncertainty. Additionally, rod shape and the aspect ratio are the most sensitive filler type and input parameter respectively for characterizing dynamic fracture toughness. Here the quantitative results based on large-scale data-driven approaches convincingly demonstrate using a computational mapping between the stochastic input and output parameter spaces that the effect of uncertainty gets pronounced significantly while propagating from the compound source level to the impact responses. Such outcomes based on experimental data essentially bring us to the realization that quantification of uncertainty is of utmost importance for developing a reliable and practically relevant inclusive analysis and design framework for the dynamic fracture of particulate composites. With limited literature available on the determination of fracture toughness considering inertial effects, the present work demonstrates a novel and insightful experimental approach for uncertainty quantification and sensitivity analysis of dynamic fracture toughness of particulate polymer composites based on surrogate modeling.

Evaluation of mode II fracture toughness under high loading rate conditions for composite bonded joints

Composite bonded joints are widely used in the aircraft components, which are frequently subjected to impact loading. The present paper intends to evaluate the mode II fracture behavior of composite bonded joints under high loading rate conditions. Dynamic loading was applied on the end-notched flexure (ENF) specimen at 10 m/s and 15 m/s via an electromagnetic split Hopkinson pressure bar system. Then the dynamic fracture toughness was evaluated using an experimental-numerical hybrid method according to the virtual crack-closure technique and compared to the fracture toughness under quasi-static loading rate (2 × 10−5 m/s and 2 × 10−4 m/s). Results indicate that the mode II fracture toughness of adhesively bonded composite joints shows a positive rate-dependent trend. The mode II fracture toughness can increase about 31.5% when the loading rate increases to 15 m/s. At last, fracture surface inspections using scanning electron microscopy (SEM) were conducted to reveal the physical mark of such an effect.

Strain Rate Dependences of Dynamic Fracture Toughness and Fracture Energy of Rocks

Strain Rate Dependences of Dynamic Fracture Toughness and Fracture Energy of Rocks

Influence of Block Copolymer Concentration and Resin Crosslink Density on the Properties of UV‐Curable Methacrylate Resin Systems

AbstractAdditive manufacturing is on the verge of replacing established processes in dentistry, as it offers the possibility of manufacturing individual parts simply and cost‐effectively. Due to its suitability for a wide variety of materials and, above all, its high precision, the focus is currently on stereolithographic processes. Intrinsic brittleness of the used multifunctional acrylic monomers remains however one of the major challenges. One promising concept is the use of block copolymers (BCPs) guaranteeing minor effects on 3D‐printing processing and UV‐curing due to initially at least partial solubility, and hence low viscosity impact. A polycaprolactone‐polysiloxane (PCL‐PDMS‐PCL) triblock copolymer is synthesized via ring‐opening polymerization of caprolactone and used in radical UV‐cured methacrylic resin systems. Small angle X‐ray scattering measurements reveal the self‐assembly of the BCPs to objects of around 20 nm prior to curing. Subsequently, thermo‐mechanical characterization is carried out by dynamic mechanical analysis, flexural testing, and fracture toughness measurements (KIC). Transmission electron microscopy and scanning electron microscopy micrographs show a homogenous distribution of the BCPs and effective toughening via cavitation and shear yielding. The influence of the crosslink density on the toughness and the high effectiveness of block copolymers for improving fracture toughness is clearly shown.

Study of the coupling effect of elliptical cavities and cracks on tunnel stability under dynamic loads

This study to explore the coupling effect of elliptical cavities and cracks on the dynamic stability and failure modes of tunnels. A series of dynamic numerical simulations and model tests were conducted by using AUTODYN code and a modified drop hammer impact test device. A cracked tunnel model with an elliptical cavity was proposed in this study. The crack coalescence behaviour, crack initiation time and stress field around the elliptical cavity and the crack tips were calculated by using numerical simulation, and the dynamic initiation fracture toughness (DIFT) was also determined according to experimental and numerical results. According to these results, we conclude that the long and short axes of the elliptical cavity have a great influence on the crack coalescence behaviour and the stress field, and the long axis of the elliptical cavity has a greater influence than the short axis. When the inclination angle θ of the elliptical cavity is 90°, the stability of the crack is lowest, and when the inclination angle θ of the elliptical cavity is 45°, the effect of the elliptical cavity on the crack initiation behaviour is largest. DIFT was calculated and indicated that when the inclination angle of the elliptical cavity shows its smallest value, 90°, the cracked tunnel stability is lowest.

Investigation of dynamic fracture toughness on zirconia ceramic grinding performance with different grain sizes

Investigation of dynamic fracture toughness on zirconia ceramic grinding performance with different grain sizes

Determination of fibre tension fracture toughness of composite laminates at high loading rate

The dynamic fracture toughness of composite laminates in fibre tension failure mode is characterized here with compact tension (CT) sample, at loading rate of 25 m/s using bi-directional electromagnetic Hopkinson bars. Digital image correlation (DIC) with high-speed imaging is employed for obtaining the full-field strain fields. A dynamic J-integral method is proposed to analyse the dynamic fracture toughness, the kinetic energy and inertia effects involved in dynamic loading and crack propagation process has been analysed and calibrated. The fracture toughness of fibre tension failure at high loading rate is found to be about 25% lower than that in quasi-static load regime, and the R-curve effect under high loading rate is not significant than that in quasi-static load regime. It is suggested from X-ray CT and SEM images that the fibre pull-out length and size of failure process zone is reduced with increase in loading rate.

Dynamic intralaminar fracture toughness characterisation of unidirectional carbon fibre-reinforced polymer composites using a high-speed servo-hydraulic test set-up

This paper presents an experimental investigation on the intralaminar fibre-dominated fracture of unidirectional (UD) IM7/8552 carbon-epoxy composites, loaded under quasi-static (QS) and intermediate strain rates (IR). Dynamic tensile testing is carried out using a previously proposed test methodology involving a high-speed servo-hydraulic test machine, equipped with a slack adapter, and Digital Image Correlation (DIC), with the aim of providing an alternative means to Split-Hopkinson pressure bar testing for the characterisation of similar composite materials at intermediate-to-high strain rates. Three sizes of Double-Edge Notched Tension (DENT) specimens are tested at QS and IR to obtain the mode I intralaminar fracture toughness. The results of the fracture toughness tests compare well with literature data, supporting the proposed methodology. This study is a first step towards standardising a more easily accessible test method for characterising the mechanical properties of fibre-reinforced polymer (FRP) composites over a broad range of dynamic strain rates.