Simulation of microfracture process of brittle polycrystals: Microcracking and crack propagation
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Keywords:
Brittleness
Interface (matter)
Mixed mode
Mode (computer interface)
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Paris' law
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Most intelligent materials are brittle. From the engineering point of view, it is just important to evaluate the fracture toughness. KIC is a standerd value to evaluate the brittleness. However, it is difficult to prepare (form) test specimens. Vickers hardness tester is a good tool to know many mechanical propaties without fracture. In this paper, we introduce evaluate methods of fracture toughness for brittle materials by means of Vickers hardness tester. Indentation fracture (IF) method is easy to detarmine KIC to measure crack length around the indentation, load, diagonal and Young's modules. On the other hands, we have suggested a new method to evaluate brittleness by means of critical deformation energey (Ef). Ef is calcurated by critical load (Pf) and critical diagonal (df) only. This method is useful to evaluate brittleness of small or thin specimens.
Indentation
Brittleness
Vickers hardness test
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For an actual crack growth in structures subjected to the applied stress from the various directions, it is important to study about the fatigue crack propagation behavior under mixed-mode condition. In particular under the condition, crack surfaces tend to contact when the load is applied because of the compressive residual stress distributed near the crack and the zigzag crack surface morphology. In this study, using slant cracks with compressive residual stress induced in mode I fatigue crack propagation under the stress ratio of R= –1 and 0, stress intensity factors (KI)est and (KII)est were evaluated from the measured crack opening and sliding displacements. As a result, the stress intensity factor (KII)est for the crack made under the stress ratio of R= –1 with the slant angle of 45 deg. was decreased owing to the crack surface contact, while (KI)est showed relatively large values in spite of compressive residual stress.
Paris' law
Zigzag
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Residual stresses are present in engineering components as an unintended consequence of manufacturing processes, but they are also deliberately introduced to beneficial effect during surface engineering procedures. Plasma nitriding is a process of particular importance for forming tools and dies, giving significant advantages in wear, and fatigue resistance through the generation of near-surface compressive residual stresses. A precise knowledge of the level and distribution of residual stresses that exist in engineering components is necessary for an accurate design and prediction of a component's fatigue resistance. However, measurement of residual stresses is not always possible, which is especially true for forming tools. Therefore, other methods for residual stress evaluation and prediction are required. Results of this investigation show that residual stress level and depth in plasma nitrided tool steel increase with nitriding time and temperature. On the other hand, experimental data indicates that the residual stress distribution in plasma nitrided tool steels can be determined on the basis of microhardness depth distribution. A minimum in the microhardness depth profile derived over depth corresponds to the location of the compressive residual stress maximum. Furthermore, the residual stress level can be extrapolated by using reference residual stress data and genetic programming. In this way the residual stress level and distribution can be obtained even for components where measurement is not possible.
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Brittleness
Intensity
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Allowance (engineering)
Autofrettage
Fatigue limit
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Deposition
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Deep hole drilling
Hole drilling method
Bar (unit)
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In this paper, an evaluation method of fracture toughness to apply interfacial fracture mechanics was investigated in adhesively bonded double-cantilever beam (DCB) joints. Four types of adhesively bonded DCB joints with an interface crack were prepared for analyses of the stress intensity factors using boundary element method(BEM) and the fracture toughness test. From the results of BEM analysis and fracture toughness experiments, it is found that the stress intensity factor, K1 is a parameter driving the fracture of adhesively bonded joints. Also, the evaluation method of fracture toughness by separated stress intensity factors of mixed mode cracks was proposed and the influences of mode components for its fracture toughness are investigated in adhesively bonded DCB joints.
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