Dynamic Fracture Toughness Of Material Research Articles

Propagating Behavior of Stress Pulses in Large Diameter Hopkinson Tube

A large diameter Hopkinson tube has been developed to measure the dynamic fracture toughness of materials. The stress wave propagation behavior in the Hopkinson tube under different impact conditions were studied both experimentally and numerically. Both the experimental and simulated results indicated that the characteristics of the incident stress pulse critically depended on the impact alignment conditions. When the striker tube impacts the incident tube in an axial, normal alignment condition, the incident pulse shows an obvious dispersive effect. While under an oblique impact condition, a smooth incident pulse with a long rise time is achieved. Therefore, an oblique impact pulse shaping technique is proposed in this study.

Three point bending Hopkinson bar for fracture toughness measurement: A new approach for interface analysis

The arrangement of the three‐point‐bending (3PB) Hopkinson bar has been adopted as a convenient means for measuring the dynamic fracture toughness of materials. Some researchers have used this method to evaluate that property which has become an important dynamic fracture mechanics parameter. In this work, an insight of this arrangement is presented through a finite element method (FEM) analysis. A number of important aspects are approached. Among them, the history of the force conveyed to the specimen and the evolution of the dynamic stress intensity factor (SIF) during tests. A new approach, allowing estimating a more accurate effect on the specimen and also an eventual loss of contact with the incident and transmitter bars is described and explained: instead of obtaining the force, the velocity on the incident bar tip is extracted to feed the finite element model.

Influences of vibration on surface formation in rotary ultrasonic machining of glass BK7

Abstract This paper presented a fundamental investigation of the surface formation mechanisms involved in rotary ultrasonic machining (RUM) of glass BK7 process. Comparative observations of the scratches, generated in the scratching tests with and without ultrasonic, were performed using optical microscopy, white-light interferometer, and scanning electron microscopy (SEM). Giving consideration to the scratch morphologies and the abrasive process kinematics, the mechanisms of surface formation provoked by the ultrasonic superposition were investigated. Additionally, the formal machining tests with and without ultrasonic were also conducted to validate these surface formation mechanisms. As a result, a nondimensional parameter K was proposed to quantitatively describe the ultrasonic effects of the abrasives as well as to correlate these effects with the machining conditions. Due to the periodic variation in the effective work angle of the abrasive, the material accumulated slightly at the RUM groove entrance, whereas serious material accumulation appeared at the exit. The stress imbalance on the specimen surface induced by the dramatic fluctuation of the abrasive inertia load caused plenty of tortuous cracks in 0.2 μm-sized length emerge on the RUM grooves generated in the ductile material removal stage. A novel theoretical model of the surface formation mechanisms involved in formal RUM process was established by incorporating the ultrasonic effects, such as the lower dynamic fracture toughness of material, cyclical variation in the effective work angle of the abrasive, and the larger abrasive inertia force. Experimental results obtained in formal machining tests revealed that superimposing an ultrasonic vibration could distinctly reduce the cutting force of the diamond tool without seriously worsening the surface quality of the specimens.

Dynamic tensile deformation and fracture of a highly particle-filled composite using SHPB and high-speed DIC method

In this work, various tensile tests, including Brazilian disc test (BDT), flattened Brazilian disc (FBD) test and semi- circular bending (SCB) test, were carried out on a highly particle-filled composite by using a split Hopkinson pressure bar (SHPB). With the consideration of low strength and low wave impedance of the materials, a quartz crystal transducer was embedded in SHPB to measure the loading forces. A high-speed camera was used to capture the deformation and fracture process of materials. Digital image correlation (DIC) method was used to process these digital images to obtain the dynamic deformation information. Based on the measured strain fields, the crack growth path was determined and the failure mechanism of samples was analyzed. Combining SHPB and DIC method, the indirect tensile stress strain plots of disc samples were obtained, and the dynamic fracture toughness of materials was measured using both FBD and SCB tests. The results show that the tensile failure strength and fracture toughness increases with the increase of strain rates, exhibiting strain rate dependence. The high-speed DIC method combined with SHPB is effective to study the dynamic tensile behaviour of brittle materials with low strengths.

Unstable behavior of the dynamic fracture toughness

It is shown that the dynamic fracture toughness of materials substantially depends on the history of loading; this determines instability in the behaviour of this quantity under shock-wave loading. It is found that the minimal value of the dynamic fracture toughness can amount to 50% of the corresponding static value. In engineering practice, this quantity can be regarded as the initial critical parameter, which must be taken into account while choosing materials working under pulsed loading.

On the dynamic fracture toughness and crack tip strain behavior of nuclear pressure vessel steel: Application of electromagnetic force

This paper is concerned with the application of the electromagnetic force to the determination of the dynamic fracture toughness of materials. Taken is an edge-cracked specimen which carries a transient electric current I and is simply supported in a steady magnetic field B. As a result of their interaction, the dynamic electromagnetic force occurs in the whole body of the specimen, which is then deformed to fracture in the opening mode of cracking. Using the electric potential and the J - R curve methods to determine the dynamic crack initiation point in the experiment, together with the finite element method to calculate the extended J-integral with the effects of the electromagnetic force and inertia, the dynamic fracture toughness values of nuclear pressure vessel steel A508 class 3 are evaluated over a wide temperature range from lower to upper shelves. The strain distribution near the crack tip in the dynamic process of fracture is also obtained by applying a computer picture processing.

Dynamic fracture mechanics with electromagnetic force and its application to fracture toughness testing

This study is concerned with the application of the electromagnetic force to the determination of the dynamic fracture toughness of materials. Taken is an edge-cracked specimen which carries a transient electric current I and is simply supported in a uniform and steady magnetic field B. As a result of their interaction, the dynamic electromagnetic force occurs in the whole body of the specimen, which is then deformed to fracture in the opening mode of cracking. For the evaluation of dynamic fracture toughness, the extended J integral with the effects of the electromagnetic force and inertia is calculated using the dynamic finite-element method. To determine the dynamic crack-initiation point in the experiment, the electric potential method is used in the case of brittle fracture, and the electric potential and the J-R curve methods in the case of ductile fracture, respectively. Using these techniques, the dynamic fracture toughness values of nuclear pressure vessel steel A508 class 3 are evaluated over a wide temperature range.

スプリットホプキンソン棒法による動的破壊じん性の測定

The main purpose of this study was to establish an experimental method determining the dynamic fracture toughness of material at high loading rate. The experimental procedure used was based on the split Hopkinson bar technique that gives a load-displacement curve for each dynamic test. The dynamic stress intensity factor was evaluated from the dynamic strain measured by a small strain gage at the vicinity of the crack. Dynamic and static tests were conducted on 2017-T4 and 7075-T651 Al alloys using specimens of the same shape under loading rates of 1.0×106MPa√m/s and 0.2MPa√m/s, respectively. It was clarified that the fracture toughness of these materials was considerably reduced with increasing loading rate. This result was consistent with the fractographical observation using SEM.

Notch constraint effects on the dynamic fracture toughness of an unaged beta titanium alloy

The influence of notch included angle and root radius on the apparent dynamic fracture toughness of an unaged metastle beta titanium alloy, Ti3Al8V6Cr4Zr4Mo, has been examined. The apparent fracture toughness, K Id ( p), increases with both notch radius, ϱ, and included angle, ω. These results have been compared with the theoretical predictions of Tetelman et al. and Smith. The comparisons show that neither theory accurately describes the effect of varying notch constraint on the apparent dynamic fracture toughness. Although preliminary considerations indicate that qualitative descriptions of notch acuity effects may be given by recent finite element analysis of the stress and strain distributions below a notch root, there is at present no quantitative basis for determining the true dynamic fracture toughness of materials from the results of blunt notch experiments.