Papers by Keyword: Dynamic Fracture

Abstract: Results of experimental investigation of concrete and rocks under high rate loads of microsecond duration (2.5 µs) are presented. Tested samples are rectangular plates (150/150/15 mm) with a notch (70 mm long and 3 mm thick) containing a pre-crack (about 1 mm long). Dynamic loading was created utilizing electric wire explosion technique. An electric wire was placed between the faces of the notch at a distance of 20 mm from the tip of the pre-crack. After the wire explosion the trajectory of the extended crack and the fracture surface were studied. Measured dependence of the crack extension on the loading pulse energy gives the possibility of determining the threshold (minimal) amplitude of the load required to initiate fracture in the studied samples.

Abstract: The mechanical properties of concrete materials vary with the loading rate underdynamic conditions, which can influence the dynamic fracture behavior of structures. The ratedependency is reported as due to the microscopic mechanisms, such as a material inertia effectand the Stefan effect. In this study, the rigid-body-spring network (RBSN) is employed forthe fracture analysis, and the visco-plastic damage model is implemented to represent the rateeffect in this macroscopic simulation framework. The parameters in the Perzyna type visco-plastic formulation are adjusted through the direct tensile test with various loading rates asa preliminary calibration. As the loading rate increases, the strength increase is presented interms of the dynamic increase factor (DIF), and compared with the experimental and empiricalresults. Next, the flexural beam test is conducted for plain and reinforced concrete beams underslow and impact rates of loading. At the failure stage, different crack patterns are observeddepending on the loading rate. The impact loading induces the failure to be more localizedon the compressive zone of the beam, which is due to rather the rate dependent materialfeatures. In structural aspects, the reinforcement exerts stronger effects on reducing crack widthand improving ductility at the slow loading rate. The ductility is also evaluated through thecomparison of load-deformation curves until the final rupture of the beams. This study canprovide understandings of the structural rate dependent behavior and the reinforcing effectunder dynamic loadings.

Abstract: The dynamic fracture behavior of PVB laminated glass during impact has been studied by both theoretic and finite element approaches. To make the analysis of cracking on PVB laminated glass more accurate and direct, high-speed photography method is introduced in this paper. Different crack patterns as well as their sequences of appearance are visualized. Finally, typical crack is measured in order to obtain important fracture characteristics such as crack velocity and acceleration.

Abstract: Pedestrian-vehicle accident without road marks has long been a headache to accident investigators. This paper suggested a new method with the application of fracture mechanics to estimate impact speed in pedestrian-vehicle. Firstly, a windshield crack propagation model based on the crack initiation model put forward by Freund [1] is established. In the model, crack bluntness coefficient is an unknown parameter, depending on various factors, so speed domain is then divided into five intervals and sample real-world accident cases are employed to the calibrate crack bluntness coefficient in different speed intervals. Further, fourth-order Runge Kutta’s method is used to solve the differential equation. Five additional real-world accident cases are then employed to verify the accuracy of the model. Results show good agreement between the model results and the real impact speeds. Finally, the advantages and limitations of this method are discussed.

Abstract: The purpose of this paper is to discuss the nonlocal effect on dynamic crack propagation velocity. Some experimental phenomena in dynamic fracture and simulative results using molecular & atom dynamics were analyzed and discussed in this paper. The authors found that there were still some disagreements on the dynamic crack propagation velocity. Based on these researches, we introduced nonlocal field theories into the estimation of dynamic crack propagation velocity. The dynamic crack propagation velocity is affected not only by the crack instability, but by characteristic length of material. A nonlocal characteristic length parameter M is defined through a double pile-up dislocation model. According to the Mott’s research method for crack velocity in dynamic fracture and the nonlocal field theories, an approximate theoretical dynamic propagation velocity is obtained. And we conclude that the velocity is related to the combined activity of the nonlocal characteristic length parameter M, the velocity of longitudinal wave, constant k, crack length and Poisson’s ratio.

Abstract: The phenomena occurring during rapid crack propagation in brittle single crystals were studied by cleaving silicon specimens on the low energy cleavage planes under tensile and bending. The experiments revealed new phenomena not previously reported, and new crack path instabilities in particular. The well defined boundary conditions of the tested specimens and crack velocity measurements enabled rationalization of the observed phenomena and the velocity-surface instabilities relationship in particular. In contrast to amorphous materials, the observed instabilities are generated at relatively low velocity, while at high velocity the crack path remains stable. No evidences for mirror, mist, and hackle instabilities, typical in amorphous materials, were found.

Abstract: The damage of thin steel plate subjected to contact explosions was a very complex nonlinear process. The initial crevasse of plate subjected to contact explosions had influence upon the whole damage process, so the critical damage study was significant on the anti-explosion of ship structure. In virtue of the wave solution of the plate plastic dynamic response and the dynamic fracture theory, the critical blasting charge was derived theoretically when onset of initial circumferential crack namely critical damage happened. And the expression of critical deformation was also obtained. Lastly the results calculated using proposed method was compared with numerical simulation and the experiment in the published literature, which were almost coincided. It could be seen that the present method could perfectly solve the critical damage of thin plate under contact explosions theoretically, and provide references for defensive engineering.

Abstract: A recoverable plate impact testing technology has been used for studying the growth mechanisms of mode II crack. The results show that interactions of microcracks ahead of a crack tip cause the crack growth unsteadily. Failure mode transitions of materials were observed. Based on the observations, a discontinuous crack growth model was established. Analysis shows that the shear crack grows unsteady as the growth speed is between the Rayleigh wave speed cR and the shear wave speed cs; however, when the growth speed approaches 2cs, the crack grows steadily. The transient microcrack growth makes the main crack speed to jump from subsonic to intersonic and the steady growth of all the sub-cracks leads the main crack to grow stably at an intersonic speed.

Abstract: The dynamic fracture experiments were conducted on the heat treated magnesium alloys; AZ31B-O, AZ31B-200 °C, and AZ31B-430 °C. Cross shaped specimens with the crack on their center were used for the experiments. Dynamic fracture behavior near a crack tip under equal and unequal biaxial stress was observed by the caustics method. From the observation, the stress intensity factor and the fracture toughness value were calculated. As a result, the effect of heat treatment was found. However, no clear relation such as correlation between dynamic stress intensity factor and heat treatment temperature was deduced.

Abstract: The Mode I and Mode II dynamic fracture toughness (DFT) values, KId and KIId for high strength steels 40Cr and 30CrMnSiNi2A were evaluated using a combined experimental-numerical method. The tests were performed on three point bend (3PB) and shear specimens with Hopkinson pressure bar. The time of crack initiation was determined by a strain gauge. With the assistance of 3-D transient finite element analysis, the temporal evolution of the dynamic stress intensity factor under different loading rates was obtained, and the DFT was determined by the fracture initiation time. In Mode I tests, a brittle transgranular fracture was found in 40Cr, while evidence for ductile fracture was observed on the failure surface of 30CrMnSiNi2A. In Mode II tests, both tensile cracks and adiabatic shear bands were found for the two steels. The effect of ligament size on fracture toughness was discussed. The correlations of KId and KIId with loading rate were also investigated. The micromechanisms of loading rate effect on the DFT were discussed for both modes.