Dynamic Fracture Initiation Toughness Research Articles

Dynamic fracture characterization of glass fiber composites reinforced with through-thickness nylon fibers

ABSTRACT A detailed experimental study is performed to investigate the mode-I dynamic fracture initiation toughness of through-thickness Nylon fibers reinforced glass/epoxy composites using a wet electrostatic flocking method. Composites of a double cantilever beam specimen configuration of two different deniers 1.5D and 3D of 0.75 mm long Nylon fibers with two different fiber densities 150 and 300 fiber/mm2 and composites without Nylon fibers (control) are fabricated. The dynamic fracture initiation toughness is investigated by using a modified split Hopkinson pressure bar setup with a novel fixture in conjunction with a high-speed video camera. In addition, rate sensitivity on dynamic fracture initiation toughness is also investigated for control and composites of 1.5D with fiber density of 300 fibers/mm2 as a function of three different rates of dynamic fracture toughness ( G ˙ = 1500, 2010, and 2,150 kJ/m2/s). Results show that for the composite of 3D–0.75 mm Nylon fiber, G IC values reach 0.78 kJ/m2 (110% increase) and 1.3 kJ/m2 (250% increase) at flock densities of 150 and 300 fibers/mm2 compared to the control sample with the lowest G IC value at 0.37 kJ/m2. The same pattern is observed for a 1.5D–0.75 mm configuration at the flock fiber density of 150 fiber/mm2 where G IC value demonstrates a 75% increase compared to control conditions. The dynamic fracture initiation toughness of flocked composites demonstrates significant rate sensitivity.

Dynamic response and fracture characteristics of thermally-treated granite under dynamic loading

In the process of deep underground resource development and utilisation, the response of the surrounding rock mass is affected by high temperatures and the dynamic loadings that can trigger strain bursts. Therefore, strain burst failure is highly related to the rock mass's dynamic fracture toughness and energy absorption capacities. Understanding the dynamic fracturing behaviour of rock mass is crucial for designing stable underground structures and controlling the hazard associated with strain bursts. In this paper, dynamic Mode I fracture toughness tests using a split Hopkinson pressure bar (SHPB) were conducted on Cracked Chevron Notched Semicircular Bend (CCNSCB) granite specimens to investigate the relationship between the strain burst mechanism and its dynamic fracture propagation. The tests were performed on thermally treated granite specimens (from 25 to 250 °C) under a wide range of impact velocities. The dynamic fracturing characteristics and crack propagation speeds were measured by a high-speed camera (HSC). The dynamic fracturing process and the effects of thermal damage on the dynamic fracture modes were identified by detailed image analysis. Energy partition characteristics in dynamic fracture of granite samples were quantified. The results revealed that dynamic fracture initiation toughness and energy partitions highly depend on loading rate and temperature. As the loading rate increased, the failure modes changed from axial splitting to pulverisation. Under the same dynamic loading, an increase in the temperature can exacerbate the fragmentation degree of granite. Under the same dynamic loading, the dissipated and released energies increased with increasing temperature. When the loading rate was high, the loading rate strengthening effect became remarkable, and the dynamic fracture toughness of granite increased under all temperatures. Finally, the dynamic energy mechanism of strain burst with the increased loading rate and heat-treatment temperature was further discussed.

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.

Determination of static and dynamic fracture initiation toughness and numerical simulation of dynamic 3-point bend experiments of Al6063-T6

The objective of this research is to find out the static and dynamic fracture initiation toughness of aluminium alloy Al6063-T6. An in-house modified Hopkinson Pressure Bar (MHPB) along with the digital image correlation (DIC) system has been used for 3-point bend experiments under dynamic loadings. However, conventional UTM has been used for quasi-static experiments. Dynamic experiments have been performed at different projectile velocities of 13.37, 14.23, 15.95, and 20.09 m/s, while quasi-static experiments have been performed at a 1 mm/min displacement rate. Dynamic force is calculated by using strain gauge measurement at two-point on the bar while the crack initiation time is calculated using combined results from strain gauge and digital image correlation. Experimental results found that the value of dynamic fracture initiation toughness (DFIT) is higher as compared to static fracture initiation toughness (SFIT). However, DFIT has continuously increased with increasing loading rates. A numerical simulation of pre notched specimen under 3-point bend dynamic loading has been performed in ABAQUS software using the J-C constitutive and fracture model. Simulation results were compared with the experimental results and found good agreement in both these results.

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.

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.

Quasi-static and dynamic fracture toughness of a γ-TiAl alloy: Measurement techniques, fractography and interpretation

Intermetallic titanium aluminides (TiAl) offer an enormous potential for high temperature applications due to their high specific strength, creep and oxidation resistance. However, their use for structural components is limited by their low ductility and fracture toughness. During service, along with a high number of load cycles, the material may also be subjected to unexpected impact loads, which requires therefore knowledge about dynamic material parameters for a damage-tolerant component design. Here we present a feasibility study to determine the fracture toughness of a third generation TiAl-alloy over a wide range of loading rates. Quasi-static crack resistance curves (R-curves) were determined using the direct current potential drop (DCPD) technique. The dynamic experiments were performed with a drop tower with impact velocities ranging from 1 to 10 m/s on pre-cracked SENB specimens. To exclude the influence of inertial forces, the measurements were carried out with strain gauges applied in the elastic near-field of the crack tip. Despite a critical consideration of possible systematic errors of this measurement method, it was found that the dynamic fracture initiation toughness slightly increases up to loading rates of K̇I = 106 MPa√m/s. The fracture behaviour of the material and possible reasons for the toughness increase are discussed based on comprehensive fractographic investigations as well as crack path analyses within the microstructure.

Effects of bedding planes on the fracture characteristics of coal under dynamic loading

To investigate the influence of bedding planes on its fracture characteristics under dynamic loading, testing and numerical simulations are carried out on semi-circular bend specimens of coal. It is shown that the fracture load and dynamic initiation fracture toughness decrease as the increase of bedding-plane angle. The crack propagation direction is jointly controlled by the maximum principal stress and bedding planes. Based on the digital speckle correlation method, it is found out that, due to stress concentration, strain at the initial loading stage concentrates around crack tip. After crack initiation, there is a specific strain gradient with a candle flame-like shape on the surface of a specimen. The opening displacements at crack tip can be divided into stable and linearly increasing phases. Further, a continuum-based discrete element method is applied to virtually reproduce these fracture characteristics, which are instructive to study dynamic anisotropy in fracture of coal.

Experimental study of the quasi-static and dynamic fracture toughness of freshwater ice using notched semi-circular bend method

The dynamic fracture toughness of ice involves its fracture resistance under impact loading, which is essential for impact or blasting parameter selection and evaluation. Temperature is an important factor influencing ice mechanical properties. To examine the evolution of ice fracture characteristics at different loading rates and temperatures, notched semi-circular bend method was applied to determine the freshwater ice quasi-static and dynamic fracture toughness. A modified low-temperature split Hopkinson pressure bar system was developed to measure dynamic ice fracturing properties. A high-speed camera captured ice specimen cracking initiation and propagation processes. The experimental results indicate that the overall initiation fracture toughness (IFT) of freshwater ice under dynamic loading exceeds that under quasi-static loading. The loading rate effect on the dynamic fracture toughness and failure modes greatly exceeds the test temperature effect. The freshwater ice dynamic IFT increases approximately linearly with the loading rate and rapidly at temperatures from –5°C to –45 °C. In all tests, cracks always initiate from the notch plane. Two symmetrical curved cracks then propagate from the contact end of two support rollers to the flat contact end, ultimately leading to different sample fragmentation distributions. These results improve the understanding of the dynamic fracturing mechanism in ice engineering applications.

Numerical study for dynamic fracture toughness of AA7075-T651 under dynamic loading

Finite element simulations for three-point bend specimens of aluminium AA7075-T651 were performed by using ABAQUS software. The simulation parameters, load point displacement and crack mouth opening displacement obtained, were used to evaluate the dynamic stress intensity factor. Three-point bend specimens impacted with an incident bar at three different striker velocities on modified Hopkinson pressure bar (MHPB) used to evaluate load point displacements and crack mouth opening displacements during the experiment were simulated. Load point displacements and crack mouth opening displacements were obtained using finite element simulation with maintaining same experimental conditions and it was found a good agreement between experimental and numerical results. These results were used to evaluate dynamic initiation fracture toughness at different loading rates using dynamic stress intensity factor and the time of initiation of fracture obtained with an experimental method using MHPB conjugation with digital image correlation (DIC) method.

Investigation of Mixed-Mode I/II Fracture under Impact Loading Using Split-Hopkinson Pressure Bar

Mixed-mode fracture of construction building materials under impact loading is quite common in civil engineering. The investigation of mixed-mode crack propagation behavior is an essential work for fundamental research and engineering application. A variable angle single cleavage semi-circle (VASCSC) specimen was proposed with which the dynamic fracture test was conducted by using a Split-Hopkinson pressure bar (SHPB). Notably, the mixed-mode crack propagation velocity could be detected by the synchronized crack velocity measuring system. With experimental results, the dynamic initiation stress intensity factors KI and KII were calculated by the experimental-numerical method. Additionally, the crack path of mixed-mode I/II fracture can be predicated precisely by using numerical method. Thus, a comprehensive approach of investigation on mixed-mode I/II fracture under impact loading was illustrated in this paper. The study demonstrates that the mixed-mode I/II crack would transform from complicated mode I/II to pure mode I during crack propagation, and several velocity decelerations induced crack deflection. The dynamic initiation fracture toughness of mixed-mode crack was determined by the experimental-numerical method. The VASCSC specimen has a great potential in investigating mixed-mode fracture problems with the SHPB device.

Investigation of dynamic fracture properties of multi-crack tunnel samples under impact loads

Cracks and joints generally exist in various underground structures, and affect the overall mechanical behaviour (strength, elastic modulus, fracture, etc.) of wall rocks significantly. Studying the crack failure behaviour in cracked tunnel samples containing single or multiple cracks can improve our understanding of the dynamic fracture mechanism of cracked tunnels. To investigate the dynamic fracture parameters of cracks in surrounding rock mass, several cracked tunnel model samples having single or multiple cracks were prepared from green sandstone and polymethyl methacrylate. Four groups of impact experiments were conducted using the drop hammer impact testing equipment to determine dynamic fracture properties. Corresponding numerical simulations were performed using a modified version of the finite difference method software and a traditional version of the finite element method software. The effect of single or multiple cracks on crack propagation speed, dynamic fracture initiation time, and fracture initiation toughness under impact loads was analysed and discussed in this study. By comparing the experimental and simulation results, we concluded that multiple cracks have a significant influence on crack propagation speed because the crack propagation speeds of multi-crack samples were larger than those of single-crack samples. Furthermore, the dynamic fracture initiation time decreases with the number of cracks. The dynamic fracture initiation toughness KId and KIId of green sandstone at the crack tip of single-crack samples was 1.94 times greater than that of multi-crack samples.

A novel method to determine the mixed mode (I/III) dynamic fracture initiation toughness of materials

A novel hybrid experimental-numerical approach is proposed to determine the mixed-mode (I/III) dynamic fracture initiation toughness of engineering materials. To demonstrate the methodology, cylindrical aluminum alloy specimens with V-notch spiral cracks on the surface at spiral inclined angles of $${\beta _{sp}=0}^{\circ }, {11.25}^{\circ }, {22.5}^{\circ }, {33.75}^{\circ }$$ and $${45}^{\circ }$$ are subjected to dynamic torsion load using a torsional Hopkinson bar apparatus. The torque applied at the onset of fracture is measured through strain gages attached to the incident and transmitter bars. Stereo digital image correlation (StereoDIC) is performed to measure the full field deformation and crack mouth opening displacement (CMOD) as a function of loading time. Results are used to estimate the crack initiation time. The dynamic stress intensity factors are extracted numerically based on the dynamic interaction integral method using ABAQUS. The mode-I $$ {(K}_{Id})$$, mode-III $$(K_{IIId})$$, and mixed-Mode ($$K_{(I/III)d})$$ dynamic initiation toughness data as a function of spiral angles are presented and discussed.

Effect of microwave irradiation on dynamic mode-Ι fracture parameters of Barre granite

Microwave irradiation has been considered as one of the most promising approaches to assisting mechanical rock breakage in rock engineering. Understanding the influence of microwave-induced damage on rock dynamic fracture properties is thus desirable. Using the notched semi-circular bend (NSCB) method, dynamic fracture tests were carried out on rock specimens irradiated by microwaves with different durations. Prior to the dynamic tests, X-ray micro-CT scanning and P-wave velocity measurement were employed to quantify the microwave-induced damage. Both the CT value and the P-wave velocity decrease with the increase of the irradiation duration. During the dynamic tests with loads exerted by a split Hopkinson pressure bar (SHPB) system, a laser gap gauge (LGG) technique was used to monitor the crack surface opening distance (CSOD). The results show that the dynamic initiation fracture toughness increases linearly with the loading rate, and the microwave irradiation has a significant weakening effect. Moreover, the propagation fracture toughness and fracture energy are also sensitive to the irradiation duration and the loading rate, which both increasing with the loading rate while decreasing with the increase of the irradiation duration. In addition, the dynamic fracture velocity increases with the loading rate, whereas the limiting fracture velocity and the fracture arrest toughness are significantly reduced by the microwave irradiation.

Investigation on Fracture Properties of Single-Flawed Tunnel Model Under Medium-to-Low-Speed Impacts

Dynamic fractures occur frequently in geophysical processes and engineering applications. It is thus essential to study crack and failure behaviors, such as crack time-to-initiation, crack growth rate and arrest period under dynamic loading. In this study, impact experiments were implemented by utilizing the single-flawed tunnel specimens under drop-hammer impacts. Four brittle materials, i.e., green sandstone, red sandstone, black sandstone and polymethyl methacrylate, were selected to make single-flawed tunnel specimens. Strain gauges and crack extension gauges were employed to measure the crack extension parameters. The properties of crack growth rate, crack time-to-initiation and arrest period of these four brittle materials were discussed and analyzed. The corresponding numerical simulation was performed by using the commercial software AUTODYN. The numerical results of crack growth rate and crack time-to-initiation agreed with the impact test results. The commercial software ABAQUS was applied to compute the dynamic stress intensity factors. The results show that both the dynamic initiation fracture toughness and the crack growth rate increase with the elastic moduli of these four types of brittle materials under the same loading conditions, whereas the crack time-to-initiation decreases with the increase in elastic moduli of the brittle materials under the same loading conditions.

Mode-I dynamic fracture initiation toughness using torsion load

An experimental and numerical approach is proposed to determine the dynamic fracture initiation toughness of materials from a cylindrical specimen with spiral surface crack subjected to dynamic torsional load using a torsional Hopkinson bar apparatus. The torsion load creates predominantly tensile stress perpendicular to the spiral crack of the specimen, resulting in nominally Mode I conditions. The torque applied to the specimen is measured by strain gages attached to the bar and the time at which the crack propagation initiated is measured using stereo imaging and stereo digital image correlation. Using the measured torque and the time of fracture as input, a commercial FE package, ABAQUS, is utilized to analyze the spiral crack and numerically extract the dynamic fracture parameters. A 3D format of the dynamic interaction integral method is utilized to calculate the three components of the applied dynamic stress intensity factors. The result demonstrates that the spiral crack-torsional loading configuration indeed generates nominally Mode I conditions and can be used to measure the dynamic fracture initiation toughness. To demonstrate the proposed method, three aluminum alloys; Al 7050-T6, Al 2024-T3, and Al 6061-T6, were experimentally studied. The results are consistent, repeatable and in good agreement with literature data.

A Novel Approach to Increase Dynamic Fracture Toughness of Additively Manufactured Polymer

An experimental study is performed to investigate the dynamic fracture of additive manufactured Acrylonitrile Butadiene Styrene (ABS). A single edge notched bending (SENB) specimen with three orientations, namely horizontal builds with 45°/−45° (H45), 0°/90° (H90) raster orientations, and vertical builds with layers perpendicular to the pre-crack (V0) are considered for this study. In addition, a novel toughening mechanism is explored by changing the surface topology to deflect the crack paths. A modified split Hopkinson pressure bar with a copper pulse shaper (to increase the raising time of incident loading pulse) is used to conduct a three-point bend impact experiment to characterize the dynamic fracture initiation toughness and crack dynamics of 3D printed specimens. Real-time crack initiation and propagation is captured by using a high-speed video camera. Using the load history diagram, accurate fracture initiation load is found to determine dynamic fracture initiation toughness. Fracture initiation toughness is increased by 138% for a V0 specimen configuration compared to H90. Three different sized circular patterns (with diameters of 1, 1.75 and 2.5 mm) and a square pattern (with a length of 1.53 mm) are considered to observe the effect of surface topology on the dynamic fracture initiation toughness. Introducing surface pattern to the specimen increases the fracture toughness by 58% as compared to specimens without surface pattern. Surface pattern also exhibits two steps of crack growth and decreases the initial crack propagation velocity significantly for all three orientations. Additionally, higher fracture initiation toughness is achieved with the increase in the size of the pattern and the change of the pattern shape.

Experimental determination of the dynamic fracture-initiation toughness of high-strength metals

In the case of materials that may be subjected to dynamic loads or extreme conditions, it is crucial to be aware of the evolution of their fracture behaviour with the strain rate. This study describes the detailed design and careful development of a novel experimental technique that allowed measuring the dynamic fracture-initiation toughness for a wide range of loading rates based on a strong theoretical background. For such a purpose, two high-strength metallic alloys were studied: (i) a very mild strain rate dependent alloy, the AA7017-T73, and (ii) an armour steel with well-established rate effects the Mars 240. The former was selected in order to validate the newly established experimental technique. The measured dynamic fracture-initiation toughness, as expected, gave almost identical results, therefore validating the technique. Once the technique was established, the dynamic fracture-initiation toughness of the Mars 240 steel at different loading rates was obtained. The results showed an increase on the dynamic fracture-initiation toughness with increasing loading rates.

The influence of impacting orientations on the failure modes of cracked tunnel

Underground tunnels are often subjected to dynamic loadings, and the dynamic loadings may come from various directions. To study the influence of loading orientations on the failure modes of cracked tunnels, laboratory impact experiments were conducted by a drop weight testing apparatus with synchronized measuring system. Green sandstone was selected as the experimental material to fabricate the tunnel samples, a pre-crack emanating from tunnel roof was designed to simulate the natural crack around a tunnel. In the process of dynamic loading tests, strain gauges were employed to determine crack time-to-fracture and crack velocity. Numerical simulation was carried out by the finite element software ABAQUS and the finite difference software AUTODYN. Crack initiation fracture toughness (IFT) was evaluated by experimental-numerical technique and the failure behavior of tunnel model under different orientation impacts was presented in this paper. The studying results show that as impacts come from tunnel roof, tunnel failure mainly occurs near the crack tip and the tunnel bottom (or the tunnel foot); as impact come from tunnel sidewall, the tunnel failure only occurs at the two sides of arch shoulder. The dynamic initiation fracture toughness of the pre-crack in the tunnel models is greatly related to the impact loading orientations.

Tensile, Quasistatic and Dynamic Fracture Properties of Nano-Al₂O₃-Modified Epoxy Resin.

Epoxy resin, modified with different particle sizes (50 nm, 100 nm, 200 nm) and contents (1 wt %, 3 wt %, 5 wt %, 7 wt %) was manufactured. The mechanical behaviors of tensile, quasistatic fracture and dynamic fracture under SHPB (split Hopkinson pressure bar) loading were investigated. The dynamic fracture behaviors of the composites were evaluated by 2D-DIC (digital image correlation) and the strain gauge technique, and the fracture surface was examined by SEM (scanning electron microscope). According to the results, the tensile modulus and strength significantly increased for epoxy resin modified with 5 wt % Al2O3 of 50 nm. The quasistatic fracture toughness of modified epoxy resin increased with the particle content. However, the fracture toughness of epoxy resin modified with high content fillers decreased for particle agglomeration that existed in epoxy resin. The crack propagation velocity can be decreased for epoxy resin modified with particles under dynamic loading. The dynamic initiation fracture toughness of modified epoxy resin increases with both particle size and content, but when the fillers have a high content, the particle size effects are weak. For the composite under dynamic loading conditions, the toughening mechanism is also affected by particle size.