Papers by Keyword: Brittle Fracture

Abstract: The article deals with the fracture mechanism of thermomechanically processed low-carbon steel tested at low temperatures The low temperature impact toughness of the low-carbon Fe360 steel was defined in the initial (as delivered) state and after its combined treatment: quenching and equal channel angular pressing (ECAP) as well as quenching, equal channel angular pressing, and annealing. It was stated that the combination of quenching and the equal channel angular pressing provided higher strength and led to increased resistance to brittle fracture. Post-deformation annealing, due to high ductility of the material, allows for higher values of impact toughness and power intensity of fracture.

Abstract: This paper presents a new energy approach based on the concept of a volume stain-energy-density factor useful to deal with the general problem of brittle fracture for sharp V-notches under mixed-mode I and II. The basic assumption is that the fracture occurs when the volume strain energy density factor arrive at its critical value in the direction of crack initiation where the deviator strain energy density factor is minimum. The method is validated taking into account experiment data already reported in the literature and the analytical predictions are in good agreement with experiment data. It indicates that the accuracy of the new approach is undoubtedly very satisfactory and can be applied in engineering.

Abstract: The coupled Finite Fracture Mechanics (FFM) criterion is applied to investigate brittle fracture in rounded V-notched samples under mode I loading. The approach is based on the contemporaneous fulfilment of a stress requirement and the energy balance, the latter being implemented on the basis of a recently proposed analytical expression for the stress intensity factor. Results are presented in terms of the critical crack advance and the apparent generalized fracture toughness, i.e. the unknowns related to the system of two equations describing the FFM criterion. A validation of the theory is performed by employing varying root radius notched, as-quenched, AISI 4340 steel specimens fracture results.

Abstract: The research work deals with the evaluation of limit states of machine needles failure where significant operational wear was anticipated. It focuses on the description of limit states, their causes and subsequent brittle fracture as a result of degradation. The experiment dealt with an evaluation of prepared microstructures followed by the fractography observation. Due to the fact that chemical composition of the material was not originally determined distribution of the chemical elements by linear and area method was performed within the experimental part. The work confirmed incorrect selection of the base material that although coated was typical by rapid degradation and failure which resulted in economic losses.

Abstract: B₄C/Al composites of different boron carbide contenting were prepared by the liquid mixing method and hot-rolled to 3mm. Microstructure and mechanical properties of the rolled composite were analyzed after the stress relieving heat treatment. Results showed that: the distribution of boron carbide particles was uniform, and only a few particles appeared fractured. With Ti addition, a reaction product layer was fabricated surrounding B₄C particles, which acted as a diffusion barrier to separate B₄C particles from liquid aluminum to protect them. With the mass fraction of the B₄C particles increasing, it can improve the tensile strength but degrade the plasticity of the composites. The fracture surface of composites showed both ductile and brittle fracture characteristics and when the mass fraction of the B₄C particles increased the ductile fracture feature decreased.

Abstract: In order to optimize the well control equipments of nitrogen drilling, the wellhead equipments have been inspected and analyzed systematically, and the failure analysis has been performed for the fracture at the through hole of two valve plates of flat valve at the side outlet of the multi-function four-way valve in this paper. The fracture reason of the valve plates is explored from macroscopic morphology analysis, chemical composition analysis, mechanical property test, SEM analysis and macroscopic analysis respectively. SEM scanning shows that more precipitates have been separated out nickel-based solid solution in Ni60 welded alloy layer. The spectrum analysis shows that these precipitate phases are rich chromium evolution phases. The valve plate cracks are originated in the welded alloy layer. The final fracture of valve plate is a cleavage fracture and the mechanism is brittle fracture.

Abstract: A new method to recycle radial tyre is studied, using Ultra-High Pressure Water jet to cut the tread rubber. The dynamic constitutive model of radial tyre material is builded, fitting model parameters with least square method. And the stress distribution law and dissociation characteristics of rubber block under impact are studied by finite element analysis.Through the SEM observation of cross section, the relationship is preliminarily discussed between stress distribution and micro-morphology of fracture interface and dissociation pathways. The split Hopkinson pressure bar apparatus is used to obtain the dynamic response law of material in a wide range of Strain Rate. The experimental results show that viscoelasticity of the rubber material is not fully reflected from the perspective of the stress wave propagation. The rubber material has a brittle transition and eventually forming fine powder. And the experimentally measured material crushing threshold is found agreed well with the numerical simulation results. It provides important mechanical foundation for further research on the mechanical force desulfurization of material.

Abstract: —Abstract: The use of high-strength rock bolt in the underground often occured brittle fracture in the threaded section, this is closely related to bolt material impact toughness. Tested the impact absorbed energy of four manufacturers of the yield strength of 500MPa rock bolt and the impact absorbed energy value ranging from 19J to 165J. Tested bending test of the bolt rod and threaded section that has a different impact absorbed energy, the results showed that: the impact toughness affects the cold bending of the rock bolt thread segment, when the impact absorbed energy value is less than 30J rock bolt thread segment cold bending poor; impact toughness has no obvious affect on the cold bending of rod.

Abstract: Charpy impact toughness values show large statistical scatter, particularly in the ductile-to-brittle transition temperature (DBTT) range. Although the statistical distribution of Charpy absorbed energy has not been clarified, critical values of the stress intensity factor, J-integral and crack-tip opening displacement (CTOD) at brittle fracture generally show the Weibull distribution with two or three parameters. This study proposes a brittle fracture model, based on the weakest link theory, for evaluating the scatter in Charpy absorbed energy KV. The numerical results show that the amplitude of the opening stress fields ahead of the V-notch at varying amounts of KV are uniquely characterized as the square of the applied load. With these numerical results, the Weibull shape parameter of the statistical distribution of KV is almost equal to 2. The proposed statistical model is verified through experimental results. It is found that the statistical distribution of KV is characterized by a two-parameter Weibull distribution with the shape parameter of 2 under the condition of pure brittle fracture.

Abstract: In this work, ECAP technique has been applied to 5052 Al-Mg alloys where the alloys find wide application in building and construction, automotive, cryogenic and marine applications. The alloy samples extruded by ECAP with four different deformation routes for 4 passes each were tensile tested and the observation of the fracture surfaces were carried out by using digital camera and Scanning Electron Microscope (SEM). The experimental results showed that the initial pre-annealed sample showed cup and cone shape of fracture, whereas the fractured ECAPed samples of all deformation routes exhibited shear feature in which the fracture surface made an angle with pressing direction of ECAP. The change of size and shape of dimples fracture from larger and irregular shape dimples in initial sample to much smaller and grains-look like dimples in ECAPed sample indicating the grain refinement occurred after ECAP. Among the ECAP deformation routes applied, route B_C resulted in smallest dimples size, i.e. about below 2 mm in average. The post annealing subjected to ECAP samples recovered almost completely the fracture feature and fracture morphology to that of the initial one, which was achieved after annealing at 300°C.