Fracture and Damage Mechanics V

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Authors: Peter Horst
Abstract: Widespread Fatigue Damage (WFD) is a technical expression used in the certification of aircraft. The paper presents a set of issues which may influence the probability that a structural item exhibits WFD. The paper mainly follows the line of a stochastic approach to these subjects, and includes the influence of realistic parameters - like manufacturing - and tries to assess what may be the impact of new technologies.
Authors: G.K. Sfantos, Ferri M.H.Aliabadi
Abstract: In this paper, intergranular microfracture evolution in polycrystalline brittle materials is simulated using a cohesive grain boundary integral formulation. A linear cohesive law is used for modelling multiple microcracking initiation and propagation under mixed mode failure conditions, encountering the stochastic e=ects of the grain location, morphology and orientation. Furthermore, in cases where crack surfaces come into contact, slide or separate, fully frictional contact analysis is performed.
Authors: Robert Moskovic, M.R. Wootton, Peter E.J. Flewitt
Abstract: As part of the requirement to demonstrate safe operation of the steel reactor pressure vessels for the UK Magnox fleet, surveillance samples were installed at the time of construction. These were representative of the steels and weld metals used for the manufacture of the vessels. The history of sample removal and mechanical property testing is briefly reviewed. The factors leading to changes in the fracture resistance and tensile properties will be discussed. The link is described between the mechanisms leading to changes in mechanical properties and the statistically based approach adopted to predict values of fracture toughness for the neutron dose and temperatures at key vessel locations. Predictions are compared with test results obtained from samples removed from a decommissioned reactor and also surveillance and accelerated radiation samples removed towards the end of reactor life.
Authors: Wen Yan Liang, Yong Jun Wang, Zhen Qing Wang, Hong Qing Lv
Abstract: Under the assumption that the artificial viscosity coefficient at the propagating crack tip is in inverse proportion to power law of the plastic strain rate, a rate-sensitive constitutive relationship is derived for perfect elastic-plastic material. With the adoption of the rate-sensitive constitutive relationship, it is asymptotically investigated the propagating tip fields of plane strain mode I. And the quasi-static equations are obtained separately governing the stress and strain fields at the crack-tip by means of Airy stress function. Numerical calculations of governing equations are carried out by double parameters shooting, with selections of appropriate values of each characteristic parameter by combinations of boundary, and the fully continuous stress-strain fields are obtained at the crack-tip. The analytical and computational results indicate that viscosity effect is an important factor in crack-tip fields.
Authors: Yong Yang, Li Qiang Tang, Yong Dong Li
Abstract: In the petroleum engineering, the nonlinear bulk strain or bulk strain rate must be considered in rock deformation, adopting the viscous-elastic theory, a constitutive equation of the pressure-sensitive dilatant materials was established. The governing eqations were deducted in crack-tip field of quasi-static growing crack in pressure-sensitive dilatant materials. And for the mechanical model of a rigid-viscoelastic material interfacial crack, through numerical calculate, the asymptotic solution of the continuous separate variable formal of stress, strain and displacement. The significance of the material parameter in the crack-tip field is discussed. The study of the paper will provide theoretical references for preventing the glide of the mud rock and reducing the arises of the failure of casings of well.
Authors: Yong Wang, Li Qiang Tang, Yong Yang
Abstract: In this paper, the conception of natural fracture stress in rocks is given according to mesomechanics analysis. The quantitative analysis about wellbore collapse and stratum fracture is made and the density range of drilling fluid is determined. The result is the same as the conclusion which was given by the traditional Mohr-Coulomb criterion in petroleum engineering. The new method of combining mesomechanics with fracture mechanics is used to study the wellbore stability in petroleum engineering, which can reveal the nature of the rock deformation mechanism and provide theoretical reference to design of the drilling engineering.
Authors: Tian Shu Song, Hong Liang Li, Jung Qiang Dong
Abstract: In this paper, the dynamic interaction is investigated theoretically between a crack and a circular cavity in an infinite piezoelectric medium under time-harmonic incident anti-plane shearing. The formulations are based on the method of complex variable and Green’s function. The resulting dynamic stress intensity factors at the crack’s tip and dynamic stress concentration factors at the cavity’s edge are obtained with crack-division technique. Numerical results are plotted to show how the frequencies of incident wave, the piezoelectric characteristic parameters of the material and the geometry of the crack and the circular cavity influence upon the dynamic stress intensity factors and dynamic stress concentration factors.
Authors: Qing Fen Li, Li Li, Hui Min Zhou
Abstract: The non-equilibrium grain-boundary segregation (NGS) isotherms and its kinetics serve to provide a more complete understanding of inter-granular segregation behavior in relation to mechanical properties, not only for the engineering steels but also for a wide range of structural alloys. The NGS of phosphorus and temper embrittlement dynamics on the same heat treatment condition at the same isothermal holding time in two Cr-Mo steels, 12Cr1MoV and 2.25Cr1Mo, was experimentally studied. The fracture behaviour was also observed by tensile tests in situ in a scanning electron microscope (SEM). Results show that both the concentration of phosphorus atoms in grain boundaries and the degree of embrittlement reaches a maximum at the critical time. It can be satisfactorily elucidated by the temper embrittlement mechanism of NGS caused by cooling from solution temperature to isothermal holding temperature.
Authors: Li Li, Qing Fen Li, Hui Min Zhou
Abstract: For most of the experiments and the practical problems about the non-equilibrium grain-boundary segregation (NGS), segregation will always occur during continuous cooling. The effective-time-method assumed that any continuous cooling curve for a sample can be replaced by a corresponding stepped curve. In this paper, the non-equilibrium grain-boundary segregation of phosphorus in step cooling process in an industrial steel12Cr1MoVis studied based on the effective-time-method and compared with that in isothermal holding process. Results show that the calculated result is in good accordance with the experimental observation. It provides a direct support for the effective-time-method of NGS. The step cooling process can speed-up the embrittlement of the steel. Based on the step cooling technique, a control method of grain-boundary brittleness is proposed.
Authors: Yu Pu Ma, Xin Zhi Lin, Qing Fen Li, Zhen Li
Abstract: When stress is high, delaminate damage can be induced by transverse cracks. A complete parabolic shear-lag damage model containing delamination induced by transverse cracks is therefore proposed and applied to predict the stiffness reduction by transverse cracking in cross-ply laminated composite materials. The predictions of the complete parabolic shear-lag analysis model, the incomplete parabolic shear-lag analysis model, and the complete parabolic shear-lag damage model containing delamination proposed in this paper have been compared. Results show that the young’s modulus reduction values obtained by our analysis model are better agreement with the experimental ones than other models.

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