Papers by Keyword: Impact Fracture

Abstract: The impact resistance improvement is important for window glass to protect people from injury. Although it has been proved that the impact resistance of a glass plate can be improved easily by fitting a thin polymeric film, its mechanism has not been clarified yet. The purpose of this study is to clarify the reinforcing mechanism of the impact resistance of a glass plate by fitting a polymeric film. To clarify it, a numerical simulation model was built using ANSYS Autodyn to simulate the dynamic fracture of a glass plate fitted with a polymeric film. The simulation model and results were examined by comparing them to the experimental result in the previous study. The Johnson-Holmquist (JH2) damage model was used for the constitutive law of the glass plate. A polymeric film with 0.2 mm thickness (3% with the glass plate) was modeled at the non-impact surface of the glass plate. The nodes of the glass plate at the interface with the film connected the nodes of the film by perfect bonding. By comparing the simulation results to the experiment, it was indicated the importance of modeling the remaining fragments of the glass plate and the adhesive layer of the film in simulating the dynamic fracture of the glass plate fitted with polymeric film.

Abstract: The yield behavior of amorphous metals including the metallic glass shows intrinsic dependence on the hydrostatic stress, so that yield criterion models such as Mohr-Coulomb and Drucker-Prager are often used. Both the models can explain the asymmetry in the yield stress under uniaxial compression and tension conditions, while the asymmetry in the angle of fracture surface is not able to be determined based on any of those models. The free volume model is able to provide that foundation. Shibutani et al. proposed a new constitutive model for amorphous metals that was derived from some free volume models and the flow rule using the Drucker-Prager yield function as a plastic potential, and investigated the yield behavior and the formation of localized shear band under some temperature conditions using the implicit static FEM code. The formation of shear bands is an unstable phenomenon that is greatly affected by the initial imperfection. In this model, on the other hand, the temperature or the strain rate also affects the yield behavior considerably. In this study, the impact fracture of metallic glass was investigated by implementing the constitutive model proposed by Shibutani et al. into the explicit dynamic FEM code DYNA3D, with laying emphasis on reproducing asymmetry in the angle of fracture surface and the examination of effects of strain rate and temperature change.

Abstract: Combine with the characteristics of bainitic rail U20Mn2SiCrNiMo, the flash butt welding process that suitable bainitic rail is optimized for improving bainitic joints’ performance basing on the mature technology of pearlitic rail flash butt welding, through approaches of lower heat input appropriate and matched upset forging length parameters, et.. In the post welding treatment, controlling the heating process strictly and using slow cooling measures are done to further optimize the properties of the welded joints. Then the performance tests are done on the bainitic rail joints welded with optimized welding and post welding process. The study results approve that the qualified bainitic rail joints can be obtained through appropriate flash butt welding process. Tensile strength and impact energy of bainitic joints are superior to pearlitic rail joints, but the elongation is reverse. The impact fracture of bainitic rail flash welded joints appear more defects such as hot cracks and silicate inclusions than pearlitic joints. Compared with parent metal, both types of the rail joints appear higher strength, but lower elongation and impact energy. The decreased percentage of bainitic rail joints compared with their parents is larger than pearlitic joints.

Abstract: As one of the quenched and tempered steels, 40Cr steel is often used to manufacture the shaft in heavy-duty trucks. To adjust the working environment of the heavy-duty machines, 40Cr steel should have high impact property. In this study, different tempering cooling mediums were treated as critical influence on improving impact property of the steel. After heated under 520°C tempering temperature, the samples of 40Cr were cooled in air, oil and water, respectively. The experiments were carried out through charpy impact, metallographical and SEM analysis. The results has shown that 40Cr steel of high impact property and fine microstructure can be obtained when water is used for the tempering cooling medium.

Abstract: Processes of carbon steels fracture under their repetitive dynamic loading according to the compression scheme have been studied. It has been ascertained that the main cause of fracture of cyclically compressed specimens is crack initiation and propagation on exposure to tensile internal stresses forming at the crack tip during the external load removal stages. The crack growth rate is maximal at the initial stage of its advancement near the notch. When the crack propagates depthward the specimen, its rate decelerates and weakly depends on the current specimen cross section. A model of the steels fatigue fracture process under repetitive dynamic loading has been proposed. It has been revealed that medium-and high-carbon steels quenched and tempered at 300 °С have long life duration.

Abstract: From the impact experiment for the nonlinear plastic behavior with the dynamically loaded adhesively-bonded TDCB specimens, there is the range of load from 1 to 2 kN or pin displacement from 4 to 10 mm for the most part in case of the impact rates 1, 2.5, 5, 7.5 and 12.5m/s. There is also the range of energy from 5 to 10 J or energy release rate(fracture energy) from 3000 to 6000 J/m2 for the most part in case of all impact rates. The fracture energy on automotive adhesive joints can be estimated by using the fracture toughness, GIC, experimental results under high rates of loading in this study. The key fracture mechanics parameter, namely the fracture energy, GIC, was ascertained as a function of test rate and may be used to assess and model the joint performance.

Abstract: Charpy V-Notch impact tests of N1, N2 and N3 steels from 77K to 293K are possessed in this paper. With increasing the nitrogen concentration, the ductile to brittle transition temperature (DBTT) increases. The toughness of the tested steels decreases rapidly with decreasing the temperature. The change of fracture patterns of high nitrogen austenitic stainless steels is dimple → shallow dimple → mixture of quasi-cleavage facet and dimple → cleavage facet. Fracture facets with river patterns, with tear ridges, along annealing twin boundary and cross the annealing twin plane are observed in this investigation. Critical dislocation density of crack tips ρc=[6π(τp)2/(KIc)2]2 can affect ductile to brittle transition (DBT) behavior at cryogenic temperature. Deformation twinning is also frequently observed at cryogenic temperature. Crack forms along the coherent twin boundary between one twin and the matrix.

Abstract: Evidence regarding a fracture event is absolutely and definitively recorded by Nature during the fracture process. That record is in the form of the general macrocrack pattern and the surface topological features of the newly formed fracture surface. In reality, it is the only perfect record of what actually occurred during a fracture. Whenever a conflict or controversy arises regarding a fracture, it is the moral and scientific responsibility of the fractographer to analyze and interpret the record of the fracture as it was created by Nature. It is further necessary for the fractographer to then inform and educate the members of the legal community (lawyer, judge and jury) as to exactly what happened during the failure. This educational process is necessary so that the legal community can collectively understand the history of the fracture and arrive at a just and fair decision regarding responsibility and potential liability for the failure. This paper describes the overall process from the beginning of the fracture examination of the failed artifact to the final appearance in court leading to a decision by the judge or a jury. Both the technical and the human factors are addressed with varying degrees of detail.

Abstract: The impact fracture behavior of Zr-based bulk metallic glass was investigated by an instrumented impact tester using subsize Charpy specimens. Influences of loading rate and notch shape on the fracture behavior of amorphous Zr-Al-Ni-Cu alloy were examined. As a result, the maximum load and absorbed fracture energy under impact loading were lower than those under quasi-static loading. A large part of the absorbed fracture energy in the Zr-based BMG was consumed in the process for crack initiation and not for crack propagation. In addition, fractographic characteristics of BMGs were investigated. Fractured surfaces under impact loading are smoother than those under quasi-static loading. The absorbed fracture energy appeared differently depending on the appearance of the shear bands developed. It can be found that the fracture energy and fracture toughness of Zr-based BMG are closely related with the extent of shear bands developed during fracture.