Dynamic Stress Intensity Factor Of Crack Research Articles

The effect of a pre‐existing crack on a running crack in brittle material under dynamic loads

AbstractBrittle material usually contains plenty of cracks or micro‐cracks, which raises a question that how a pre‐existing crack affects a running crack as they are approaching. In order to investigate such issue, impact experiments were conducted by using double crack semi‐circle (DCSC) specimens. Polymethyl methacrylate (PMMA) was selected to make the DCSC specimens, and a modified SHPB system was used to perform the impact tests. The finite difference code AUTODYN was employed to simulate the crack propagation behavior and propagation path, and the simulated crack paths agree well with the impact test results. Meanwhile the stresses around the crack tips were analyzed and the crack propagation direction was investigated. To calculate the crack dynamic stress intensity factors (DSIFs), finite element code ABAQUS was employed. The results show that the spacing D between the vertical crack tip and the inclined crack center affects the stress distribution and fracture behavior of the vertical crack largely. As the vertical crack approaches the inclined crack, the crack speed is slow down, and tensile stress appears at the inclined crack tip. As the spacing D is small than 50 mm, the vertical crack connects with the middle area of the inclined crack. As the spacing D is 50 mm, the vertical crack connects with either the middle area or the upper tip of the inclined crack. As the spacing D is larger than 50 mm, the vertical crack coalesces with the upper crack tip.

Study of the fracture behavior of mode I and mixed mode I/II cracks in tunnel under impact loads

Blast excavation may induce radial cracks around a tunnel, and these cracks may initiate and propagate under nearby mining activities. In order to study the dynamic fracture behavior of mode I and mixed mode I/II cracks in a tunnel under impact loads, a new large sample, i.e. a tunnel with a radial crack (TWSRC) was proposed. Impact tests by using the TWSRC samples and a drop-weight impact testing device were also implemented. The advantage by using the TWSRC samples is that the corresponding results could be directly employed to tunnel engineering practice, and could lead tunnel designers to improve tunnel dynamic stability and to prevent tunnel hazards. The pre-crack was designed in a tunnel roof and was parallel to the symmetrical axis of the tunnel. The ratio α of the distance between the crack and the symmetrical axis to the radius of tunnel roof was in the range between 0 and 1. The crack initiation time and the crack propagation characteristic were measured by strain gauges and crack propagation gauges. A finite difference commercial code AUTODYN was used in the simulation of crack propagation paths and the fracture mechanism was analyzed. A finite element commercial code ABAQUS was employed to calculate crack dynamic stress intensity factors (DSIFs), and the experimental-numerical method was used to measure crack initiation toughness. The results showed that for mode I cracks, the cracks propagate along the pre- exiting crack direction, whereas the mixed mode I/II cracks, they propagate with a certain angle to form a wing crack at the early stage, and finally they propagate along the major principal stress direction. For mixed mode I/II cracks, the critical mode I SIF increases slightly with the distance to the symmetrical axis, whereas the critical mode II SIF increases largely as compared to the mode I SIF.

Study of mode I crack dynamic propagation behaviour and rock dynamic fracture toughness by using SCT specimens

AbstractTo study crack dynamic propagation behaviour and rock dynamic fracture toughness, a single cleavage triangle (SCT) specimen was proposed in this paper. By using these specimens and a drop‐weight test system, impact experiments were conducted, and the crack propagation velocity and the fracture time were measured by using crack propagation gauges. To examine the effectiveness of the SCT specimen and to predict the test results, finite difference numerical models were established by using AUTODYN code, and the simulation results showed that the crack propagation path agrees with the test results, and crack arrest phenomena could happen. Meanwhile, by using these numerical models, the crack dynamic propagation mechanism was investigated. Finite element code ABAQUS was applied in the calculation of crack dynamic stress intensity factors (SIFs) based on specimen dimension and the loading curves measured, and the curves of crack dynamic SIFs versus time were obtained. The fracture toughness (including initiation toughness and propagation toughness) was determined according to the fracture time and crack speeds measured by crack propagation gauges. The results show that the SCT specimen is applicable to the study of crack dynamic propagation behaviour and fracture toughness, and in the process of crack propagation, the propagation toughness decreases with crack propagation velocity, and the crack arrest phenomena could happen. The critical SIF of an arrest crack (or arrest toughness) was higher than the crack propagation toughness but was lower than the initiation toughness.

Finite Geometrically Similar Element Method for Dynamic Problem of a Cracked Specimen

A group of geometrically similar elements is automatically generated layer by layer around the tip of crack. By taking advantage of the same stiffness and similar mass of similarly shaped elements, the combined stiffness matrix and combined inertia matrix in area can be expressed by the displacement row matrix of outer polygon. The global stiffness matrix will be obtained if the combined stiffness matrix and combined inertia matrix of area are assembled to those of another elements according to corresponding nodes. The small set of generalized coordinates can be obtained through solving the equation, and then the dynamic stress intensity factor of crack will be obtained. The three points bending with single crack and shearing model with double cracks in explosive loading were calculated with finite geometrically similar element method.