with a three-dimensional finite-element simulation. The technique has been already demonstrated by using different materials, such as ceramics, metals, polymer and concerts [1–4]. The technique is promising to investigate fracture behavior of materials used in pressure vessel, piping structural components and weldments and other unconventional materials. On the other hand, there is still a demand in finding a standard method to determine the dynamic fracture toughness of engineering materials. Usually, a modified split Hopkinson pressure bar (MSHPB) apparatus is used to investigate the dynamic fracture initiation toughness of materials. The principle of modified MSHPB is similar to general SHPB experiment, except in the case of MSHPB only incident and striker bar are required [5, 6]. In MSHPB experiment, a three point bend single edge specimen will be sandwich between the incident bar and supporting frame. The striker bar, made of the same material used for the incident bar, will be projected towards the incident bar by the help of compressed air and generate a well-defined loading pulse that propagates towards the specimen. Once the compressive stress wave reach the specimen, some of the wave will propagate to the specimen and some of the wave reflected back to the incident bar. Using a strain gage bonded in the incident bar, both the incident and reflected waves can be recorded. Using one dimensional wave theory the force acted on the sample can be calculated as [5, 6] FðtÞ¼ εiðtÞþεrðtÞ ð ÞEA Where F is the force, εi and εr are the incident and reflected strain respectively, E is the Young’s modulus, and A is the cross sectional area of the bar. Now, the fracture intensity factor can be calculated using quasi-static equation, [5, 6] KIðtÞ¼ FðtÞ B ffiffiffifffiiWp F a W whereKI is the stress intensity factor, B and W are the specimen thickness and width respectively, a the initial crack length and f(a/W) is the geometric factor. The above equation can be used to calculate the fracture intensity factor if the time of fracture is sufficiently long enough to neglect the inertia effect. To satisfy this condition and to avoid the transient effect, in most cases the experiment has to be conducted at lower speed [8]. This limits the application of the method described above, in the case of only lower strain rate loading. In order to study the dynamic fracture toughness of material at medium and higher loading rate, a better approach is essential. Here we are presenting a technique that can be used to study the dynamic fracture toughness of materials, without the influence of inertia effect. 37.2 Experimental The proposed experiment is based on the spiral notch sample loaded under pure torsion as proposed by Wang [1–4] for quasi-static fracture. The cylindrical specimen that has spiral notch grove around the specimen at a 45o pitch angle will undergo pure torsion loading. Near the crack front a compression and tension stress fields will be generated on the concentric cylinder of the sample. The tension load, perpendicular to the grooved line, will be responsible to creating a mode I crack opening fields. In the case of dynamic loading, since the loading, torsion, the will be no inertia force associated, which makes the method promising. 37.2.1 Material and Specimen Geometry In this experiment a polycarbonate specimen is machined from cylindrical bar as shown in the figure. A v-notched spiral group with 45 pitch angle was machined on the surface of the specimen. The diameter of the cylinder and the gage length are 0.5 and 1 in. respectively, and the depth of the grove is 0.1 in. Since polycarbonate is relatively brittle material, the deep notch was not required to create the opening mode fracture; however it was made to be consistent with other materials under investigation. It should be mentioned that, if the material is ductile, either a deep notch or fatigue pre-crack is required. The head of the sample is made hexagonal to easily attache´ the sample with the tow bars. As can be seen in the figure, a plane-strain condition is achieved on every plane normal to the spiral groove, which makes the SNTT unique compared with conventional methods (Fig. 37.1) [1]. 308 A. Kidane and J.-A.J. Wang
RkJQdWJsaXNoZXIy MTMzNzEzMQ==