Dynamic Fracture Tests Research Articles

Experimental investigation of dynamic effects in a two-bar/three-point bend fracture test

A Hopkinson pressure bar has been modified to measure the dynamic fracture properties of materials at loading rates greater than approximately 10(6) MPa ms. Some fundamental dynamic effects associated with the incident stress pulse, such as stress wave propagation characteristics along the Hopkinson bar and within the cracked specimen, the specimen's dynamic response excited by the stress pulse, and the specimen contact situations with the impactor and supports, need to be understood. To better comprehend these fundamental issues, an experimental investigation of these dynamic effects with the emphasis on "loss of contact" was first performed on a two-bar/three-point dynamic bend fracture test setup using a voltage measurement circuit across the specimen/loading-pin interfaces and high-speed photographs. It was demonstrated here that the three-point bend specimen employed with the current two-bar/three-point bend test setup remains in contact with the impactor and supports throughout the first loading duration and that "loss of contact" does not occur. A further improvement using a pulse-shaping technique was employed for achieving a tailored incident pulse. The effect of pulse shaper on the rise time and duration of the incident pulse as well as the dynamic stress equilibrium in the cracked three-point bend has been investigated, for the first time here, with profound implications for significantly improved dynamic three-point bend fracture testing.

A finite element analysis for using Brazilian disk in split Hopkinson pressure bar to investigate dynamic fracture behavior of brittle polymer materials

An experimental method of using the Brazilian disk specimen in the split Hopkinson pressure bar (SHPB) apparatus is proposed to perform dynamic fracture testing of brittle materials. The average force on the specimen, which is obtained according to the one-dimensional elastic wave propagation theory, is adopted to determine the dynamic stress intensity factor (DSIF) by extending the quasi-static formula of the stress intensity factor (SIF) to impact loading condition. A finite element model for the Brazilian disk-SHPB dynamic fracture testing system is established. The numerical analysis confirms the validity of the one-dimensional experimental measuring principle. The effects of rise time of incident stress pulse and specimen geometry are discussed. It is shown that there exists a matching relation between the specimen and the pressure bar system.

Tensile and fracture behavior of polymer foams

Tensile and mode-I fracture behavior of cross-linked polyvinyl chloride (PVC) and rigid polyurethane (PUR) foams are examined. Tension tests are performed using prismatic bar specimens and mode-I fracture tests are performed using single edge notched bend (SENB) specimens under three-point bending. Test specimens are prepared from PVC foams with three densities and two different levels of cross-linking, and PUR foam with one density. Tension and quasi-static fracture tests are performed using a Zwick/Rowell test machine. Dynamic fracture tests are performed using a DYNATUP model 8210 instrumented drop-tower test set up at three different impact energy levels. Various parameters such as specimen size, loading rate, foam density, cross-linking, crack length, cell orientation (flow and rise-direction) and solid polymer material are studied. It is found that foam density and solid polymer material have a significant effect on tensile strength, modulus, and fracture toughness of polymer foams. Level of polymer cross-linking is also found to have a significant effect on fracture toughness. The presence of cracks in the rise- and flow direction as well as loading rate has minimal effect. Dynamic fracture behavior is found to be different as compared to quasi-static fracture behavior. Dynamic fracture toughness ( K d) increases with impact energy. Examination of fracture surfaces reveals that the fracture occurs in fairly brittle manner for all foam materials.

Viscoelastic Effect on the Fracture Toughness of GFRP: Experimental Approach

The dynamic fracture tests were carried out for a glass fiber reinforced plastic specimen with a crack and dynamic fracture toughness was evaluated by examination of cracking at an initial slit root. Before the crack initiated at the slit root, a whitened damage zone was created surrounding the slit tip. The damage zone consists of micro cracking in the matrix, debonding between a fiber and the matrix, and fracture of the fiber. The comparison of the dynamic fracture toughness and the static fracture toughness value shows that the former is around 12 MPa√m and apparently higher than the later, which is 7 MPa√m. To understand those experimental results and mechanics of the damage zone, a dynamic debonding test was carried out and dynamic bonding strength was estimated as around 70 MPa.

Preparation, morphology and thermal/mechanical properties of epoxy/nanoclay composite

Abstract A novel approach assisted with solvent was developed to disperse clay into epoxy matrix. The dispersion of clay was examined by means of optical microscopy (OM), wide angle X-ray scattering (WAXS) and transmission electron microscopy (TEM). Batches of cured samples containing 1–3 wt.% silane-modified clay (SMC) were prepared and their thermal/mechanical properties were studied by dynamic mechanical analysis (DMA), tensile and fracture tests. Improvements on storage modulus, Young’s modulus and fracture toughness were achieved with incorporation of SMC clay. The fracture surfaces of the nanocomposites were imaged by scanning electron microscopy (SEM) to investigate the toughening mechanisms.

3909 面外負荷を受ける平板への光学的手法の適用(S17-2 光学的手法,S17 実験力学における最近の展開)

Dynamic fracture behavior caused by the out-of plane loading is evaluated by the method of caustics. Out-of plane displacement is measured by the C.G.S. method under static loading and it is in good agreement with theoretical one. Diameter of caustic pattern formed by the out-of plane loaded plate is compared with that one calculated by the theoretically induced formula. Dynamic fracture testing apparatus is assembled with the ultrahigh-speed camera, and caustic pattern under dynamic loading is obtained. Moreover, dynamic fracture characteristics of PMMA plate are discussed.

Rheological and mechanical properties of PVC/CaCO 3 nanocomposites prepared by in situ polymerization

Poly(vinyl chloride) (PVC)/calcium carbonate (CaCO 3) nanocomposites were synthesized by in situ polymerization of vinyl chloride (VC) in the presence of CaCO 3 nanoparticles. Their thermal, rheological and mechanical properties were evaluated by dynamic mechanical analysis (DMA), thermogravimetry analysis (TGA), capillary rheometry, tensile and impact fracture tests. The results showed that CaCO 3 nanoparticles were uniformly distributed in the PVC matrix during in situ polymerization of VC with 5.0 wt% or less nanoparticles. The glass transition and thermal decomposition temperatures of PVC phase in PVC/CaCO 3 nanocomposites are shifted toward higher temperatures by the restriction of CaCO 3 nanoparticles on the segmental and long-range chain mobility of the PVC phase. The nanocomposites showed shear thinning and power law behaviors. The ‘ball bearing’ effect of the spherical nanoparticles decreased the apparent viscosity of the PVC/CaCO 3 nanocomposite melts, and the viscosity sensitivity on shear rate of the PVC/CaCO 3 nanocomposite is higher than that of pristine PVC. Moreover, CaCO 3 nanoparticles stiffen and toughen PVC simultaneously, and optimal properties were achieved at 5 wt% of CaCO 3 nanoparticles in Young's modulus, tensile yield strength, elongation at break and Charpy notched impact energy. Detailed examinations of micro-failure micromechanisms of impact and tensile specimens showed that the CaCO 3 nanoparticles acted as stress raisers leading to debonding/voiding and deformation of the matrix material around the nanoparticles. These mechanisms also lead to impact toughening of the nanocomposites.

VISCOELASTIC EFFECT ON THE DYNAMIC FRACTURE TOUGHNESS OF GFRP

The dynamic fracture tests were carried out for a glass fiber reinforced plastic specimen with a crack and dynamic fracture toughness was evaluated by examination of cracking at an initial slit root. Before the crack initiated at the slit root, a whitened damage zone was created surrounding the slit tip. The damage zone is consisted of micro cracking in the matrix, debonding between a fiber and the matrix, and fracture of the fiber. The comparison of the dynamic fracture toughness and the static fracture toughness values showed that the former is around 12 Mpa√m and apparently higher than the later, which is 7 MPa√m. Analysis using ANSYS shows that in the viscoelastic material the stress ahead the crack tip is lower than in the elastic material.

Determination of dynamic fracture-initiation toughness using three-point bending tests in a modified Hopkinson pressure bar

We present a procedure for measuring the dynamic fracture-initiation toughness of materials. The method is based on three-point bending tests at high loading rates, performed in an experimental device which is a modification of the classical split Hopkinson pressure bar. Coupled with the loading device, a high-speed photography system was used to measure the crack mouth opening displacement (CMOD) directly on the specimen. The stress intensity factor was calculated by three different simplified methods and the time to fracture was obtained from an appropriate specimen instrumentation. To evaluate the results derived from the simplified methods, a two-dimensional full-numerical analysis of the dynamic bending fracture test was made. The model includes the specimen, the input bar, the impacting projectile and the supporting device and takes into account the possible loss of contact during the experiment between the input bar and the specimen and between the specimen and its supports. From the tests and numerical results, it can be concluded that the CMOD procedure, together with the knowledge of the time to fracture determined using crack gages, seems to be the best method for measuring dynamic fracture-initiation toughness.

Dynamic Loading Fracture Tests of Ferritic Steel Using the Direct Current Potential Drop Method

Abstract To apply the leak-before-break concept to nuclear piping, the dynamic strain aging of low alloy steel materials has to be considered. For this goal, J-R tests are needed over a range of temperatures and loading rates, including rapid dynamic loading conditions. In dynamic J-R tests, the unloading compliance method cannot be applied and usually the direct current potential drop (DCPD) method is used. But, the DCPD method is known to have a problem in defining the crack initiation point due to a potential peak that arises in the early part of loading of ferromagnetic materials. In this study, the characteristics of measured DC potential peaks were investigated for SA106 Gr. C piping steels, and the definition of crack initiation point was determined by backtracking from the physically-measured final crack length. It is proposed that this technique could be applied as an improved DCPD method applicable for the dynamic loading J-R test.

An experimental method for determining dynamic fracture toughness

The boundary and loading conditions in many dynamic fracture test methods are frequently not well defined and, therefore, introduce a degree of uncertainty in the modeling of the experiment to extract the dynamic fracture resistance for a rapidly propagating crack. A new dynamic fracture test method is presented that overcomes many of these difficulties. In this test, a precracked, three-point bend specimen is loaded by a transmitter bar that is impacted by a striker bar fired from a gas gun. Different levels of energy can be imparted to the specimen by varying the speed and length of the striker to induce different crack growth rates in the material. The specimen is instrumented with a crack ladder gage, crack-opening displacement gage and strain gages to develop requisite data to determine toughness. Typical data for AISI 4340 steel specimen are presented. A simple quasi-dynamic analysis model for deducing the fracture toughness for a running crack from these data is presented, and the results are compared with independent measurements.

Finite‐deformation irreversible cohesive elements for three‐dimensional crack‐propagation analysis

We develop a three-dimensional finite-deformation cohesive element and a class of irreversible cohesive laws which enable the accurate and efficient tracking of dynamically growing cracks. The cohesive element governs the separation of the crack flanks in accordance with an irreversible cohesive law, eventually leading to the formation of free surfaces, and is compatible with a conventional finite element discretization of the bulk material. The versatility and predictive ability of the method is demonstrated through the simulation of a drop-weight dynamic fracture test similar to those reported by Zehnder and Rosakis. The ability of the method to approximate the experimentally observed crack-tip trajectory is particularly noteworthy. © 1999 John Wiley & Sons, Ltd.

Finite-deformation irreversible cohesive elements for three-dimensional crack-propagation analysis

SUMMARY We develop a three-dimensional nite-deformation cohesive element and a class of irreversible cohesive laws which enable the accurate and ecient tracking of dynamically growing cracks. The cohesive element governs the separation of the crack anks in accordance with an irreversible cohesive law, eventually leading to the formation of free surfaces, and is compatible with a conventional nite element discretization of the bulk material. The versatility and predictive ability of the method is demonstrated through the simulation of a drop-weight dynamic fracture test similar to those reported by Zehnder and Rosakis. 1 The ability of the method to approximate the experimentally observed crack-tip trajectory is particularly noteworthy. Copyright ? 1999 John Wiley & Sons, Ltd.

The effects of γ-irradiation on subcritical crack growth in alumina

A constant load test rig is described which allowed the time to fracture to be measured for ceramic bars loaded in 4-point bending while exposed to 1.5 Gy/s of 60Co γ-rays. Two grades of alumina, 97.5% and 99.5%, were used to compare subcritical crack growth (SCCG) under constant load with and without exposure to γ-radiation. Dynamic fracture tests on 30 samples of each material were used to determine the Weibull modulus from which the critical failure stress values for the γ-irradiated and non-irradiated samples fractured under constant load could be determined and used to compare failure times. The time to failure for a given ratio of applied stress to critical stress was found to increase by a factor ≈9 for γ-irradiated 97.5% alumina, but the same dose rate reduced the time to failure of the 99.5% alumina by ≈2. Measurement of the length of cracks extending from the tensile surface of test samples showed a much higher proportion of short cracks in a γ-irradiated, 97.5% alumina sample compared to a non-irradiated sample which had predominantly longer cracks. The crack size distribution in γ-irradiated 99.5% alumina showed a significant increase in the number of large cracks leading to a shorter time to failure. It was concluded that ionising radiation inhibits crack growth in aluminas with silica-rich grain boundary phases.

Analysis of load oscillations in instrumented impact testing

This work deals with the dynamic fracture tests of the 3-point bending notched specimen. The apparent frequency of the load oscillations is investigated for different materials (steel, aluminium, PMMA and Al2O3). It is shown that the contact stiffness between the striker and the specimen plays a predominant role in the vibrating aspect of the impact. The 2 degrees of freedom (dof), intensively used in the literature, can predict the oscillations qualitatively, but cannot realistically model all the details of the vibrations of the specimen. A numerical model with several dof is considered by means of a modal analysis and analyzed through the experimental program.

The use of the split Hopkinson pressure bar techniques in high strain rate materials testing

The use of the split Hopkinson pressure bar (SHPB) techniques for materials testing under a high rate of loading is described, including various modifications of the original compression testing set-up for tension testing, torsion testing and dynamic fracture testing. Details of measurement techniques and instrumentation are included together with the calibration of the air gun and strain wave measuring system. The SHPB techniques provide a relatively cheap and simple method for high strain rate materials testing with an acceptable level of accuracy when sufficient care is taken for the proper lubrication of the interfaces and the correct length—radius ratio of the specimen is chosen. The limitations of the method are discussed and possible improvements using FFT (fast Fourier transform) techniques to take into account dispersion effects are considered.

Determination of temperature field around a rapidly moving crack-tip in an elastic-plastic solid

The problem of local heating and temperature rise induced by dynamic crack growth in elastic-plastic solids is studied numerically. Heat generation caused by plastic work dissipation is estimated from crack-tip stress and deformation fields obtained separately by two of the authors. The temperature field in an Eulerian description is shown to be governed by a convection-dominated flow equation with a singular source term that is distributed over an irregular crack-tip region, the active plastic zone. The peak value and spatial distribution of the temperature increase are determined using two independent computer codes, which are developed by the authors based on an integral representation of the temperature field and on an upwind finite element formulation. The accuracy and reliability of the numerical methods and their solutions are studied carefully against exact, closed-form solutions for several specially designed boundary value problems. These methods are used to simulate dynamic fracture tests on AISI 4340 steel specimens, and the predicted temperature contours and maximum values are found to be in good agreement with those measured and estimated experimentally.

Fractographic aspects of blunting at high loading rate

The results of an experimental study are presented to illustrate the effect of high loading rate on the initial propagation of the crack tip (blunting) of a rate-sensitive steel. Dynamic fracture tests were performed on precracked three-point bend specimens at 15, 29 and 43 m/s impact velocities. Fracture mechanisms in the initial fracture zone (blunted zone) were examined using scanning electron microscopy. All impact velocities resulted in blunting by ductile fracture mechanisms. The blunted zone consisted of a “flat” region containing considerable local stretching, and an “inclined” region containing a combination of intergranular and transgranular separation by localized stretching and microvoid coalescence. The extent of each region was increased with increased impact velocity. The propagation mechanisms in the inclined region depended on the impact velocity. Large void coalescence dominated the inclined region at low impact velocity (smooth blunting), whereas stretch and shear fracture between small voids dominated at high impact velocity (sharp blunting). Subsequent to blunting, the main fracture continued on the flat surface by microvoid coalescence or quasi-cleavage crack growth mechanisms depending on the impact velocity.

ＨＴＴＲ炉床部構造物の耐震強度，（ＶＩ）応力集中を有する黒鉛構造物の衝撃強度特性

In order to clarify the dynamic strength characteristics of a graphite component under stress concentration condition, the static and dynamic fracture tests were performed using scale-model specimens of the component. The strain rate applied to the specimens was around 10-5l/s in the static test and 1 to 5l/s in the dynamic test. As a result of the experiment, it was found that the ratio of the dynamic strength to the static one depends on the stress gradient in the specimen; the specimen with higher stress gradient shows a higher value of the strength. Moreover, it was also found that there is a critical stress gradient at which the strength ratio reaches almost a constant value even if the stress gradient increases in the specimens.

高分子系複合材料 くさび挿入試験によるＣＦＲＰ積層材のモードＩ層間破壊じん性評価

A new estimating method for mode I interlaminar fracture toughness of CFRP laminates was proposed with the aim of applying to dynamic fracture test. In this method, a wedge indentor was used to cause mode I interlaminar fracture in a precracked coupon specimen. That is, compressive load, which is convenient to dynamic test, can be used in this method, while tensile load, which is not appropriate to dynamic test, is necessary for the conventional double cantilever beam test. Finite element analyses and quasi-static experiments were carried out to validate an estimating formula for the mode I energy release rate, which is based on the beam theory and the compliance method. As the result, this method is found to be able to estimate the mode I interlaminar fracture toughness of CFRP laminates and to obtain the same stress field as the one formed in the double cantilever beam specimen. But in this method one has to measure accurately the crack length at the fracture initiation, which is difficult under high rate loading. Three types of CFRP laminates were tested in order to investigate the effect of the toughness of matrix resin. They are: T300/2500 (Toray) which has standard epoxy matrix resin, IM600/133 (Q-C133, Toho rayon) which has toughened epoxy matrix resin, and HTA/PEEK (Toho rayon) which has thermoplastic matrix resin of high fracture toughness. The experimental results show that the toughness of the matrix resin scarcely affects the estimation of the mode I interlaminar fracture toughness and that the fracture toughness estimated by the present method is comparable to that estimated by the double cantilever beam test.