The J Integral of a Slightly Curved Elasticity-plasticity Crack when Enduring Quasi Static Loads

The J Integral of an elastic-plastic curved crack under quasi-static loads has been mainly worked in the paper,and the J Integral has been calculated as a practical application of a second order perturbation method and theorem of surname KA where the effect of quasi-static applied stresses and normal and shear stresses on the boundaries of plasticity area are synthetically taken into consideration.A regular pattern of variations of the J Integral of an elastic-plastic curved crack with the variations of curved crack shape parameters under different quasi-static loads has been mainly carried out.Continuity and unity of a elastic-plastic fracture and linear elastic fracture has been demonstrated from the viewpoint of the mechanical parameter of the curved crack tip J integral.The elastic-plastic slightly curved crack tip J integral has been calculated approximately,an integral loop having been reasonably selected and continuity and unity of the fracture characteristic of centre penetration straight line crack and curved crack have been demonstrated.

Residual Stress and Fracture Toughness Study in A516 Gr70 Steel Joints Welded and Repaired by Arc Processes

Structural components made of steel are used in several areas and require welding for assembly. In some situations, repair of the weld bead, also performed by electric arc welding, can be used to correct, and eliminate any discontinuities. However, electric arc welding causes the presence of residual stresses in the joint, which can impair its performance and not meet specific design requirements. In this paper, welded joints made of ASTM A 516 GR 70 steel plates, with a thickness of 30.5 mm, welded by the MAG—Metal Active Gas process (20% CO2) and using a “K” groove were analysed. The joints were manufactured with seven welding passes on each side of the groove. After welding, one batch underwent repair of the bead by TIG welding (Tungsten Insert Gas) and another batch underwent two repairs by TIG welding. Were presented results of the behaviour of the residual stress profile measured by X-ray diffraction and the Vickers microhardness profile in the joints as well the fracture toughness in the conditions only welded and submitted to repairs. The results indicated that the greater number of repair passes reduced the residual compressive stress values obtained in the material manufacturing process and caused a stabilization on the Vickers hardness values. It was concluded that compressive residual stresses did not play a major role in the R-curve results. The presence of discontinuities in the welded joint caused greater influence on the behaviour of the R curve.

EFFECT OF TEMPERATURE ON THE FRACTURE MODES OF BHW-35 STEEL UNDER MODE-Ⅱ LOADING

Finite element method(FEM)has been used to analyze the stress and strain fields and the stress tri-axial levels around the tip of the crack under mode-Ⅱ loading.The results show that: under mode-Ⅱ loading,the direction of the maximum tensile stress and that of the maximum tri-axial levels(R_ )exist at an angle of-75.3°from the original crack plane:the maximum shear stress and R_ =0 exist along the original crack plane. Mode-Ⅱ loading experiment using BHW-35 steel at different temperatures show that there are two kinds of fracture mode.opening mode(or tensile mode)and sliding mode(or shear mode).A de- crease in temperature causes the fracture mode to change from shear mode to tensile mode.For BHW- 35 steel,this critical temperature is about-90C.Actually.under any kind of loading mode(mode Ⅰ,mode Ⅱ,mode Ⅲ or mixed mode),there always exist several kinds of potenital fracture modes (for example,opening mode.sliding mode.tearing mode or mixed mode).The effect of temperature under mode-Ⅱ loading is actually related to the change of the elastic-plastic properties of the material.

METHOD FOR CALCULATION AND SIMULATION ON PROPAGATION OF SURFACE CRACK ON SPHERICAL VESSELS MADE OF STEELS

To avoid leaking or bursing by crack on surface of preddure vessels made of streels,a method for analyzing the principle of propagation of a surface crack is established,and used on a spherical vessel with program calculation and simulation,the program is written in QBASIC and the reaults are provided and discussed.

Study on fracture mechanics parameters of surface crack in overmatched weldments

In this paper, the J integral of surface cracks in overmatched weldments with different matching ratio has been investigated using direct evaluation method. The influence of matching ratio of joints on the crack driving forces (J integral, crack mouth opening displacement CMOD) and strain distributions along the contour have been discussed in detail. The results indicate that the comprehensive effects of crack position and the matching ratio of welded joint dominate the distribution of strain along the contour. Moreover, the crack driving force increases with the decrease of the matching ratios in overmatched weldments.

Structural Integrity Assessment of Welded Pressure Vessel Produced of HSLA Steel

Welded joint is a critical region of a welded structure.The operational safety of welded pressure equipment mostly depends on the behaviour of loaded weldments.Safety of welded structure is dependent on the properties of welded joint as whole and of its constituents (parent metal,heat affected zone and weld metal).In this paper the behaviour of welded joint cracked constituents is considered.Structural integrity assessment procedure is applied to welded pressure vessel produced of high-strength low-alloyed steel,operating at-40°C,comparing crack driving force and material crack resistance,using path-independent contour J-integral as fracture mechanics parameter.The comparison of crack driving force,expressed by J-integral and material resistant curve,J-R curve,provide the possibility to determine the extent of the stable crack as well as the critical crack size for its final fast propagation and also to assess the structural integrity of a cracked pressure vessel.

Analysis of Mode-Ⅰ Crack Tip Shielding by Microcracking in Brittle Materials

The stress shielding effect of profuse microcracks at the tip of a macroscopic stationary mode Ⅰ crack is studied. The analysis method adopted combines the micromechanical approach with the effective elastic medium approach. The anisotropic constitutive relation of the effective elastic medium is based on the DMG damage model developed by the authors for microcrack weakened brittle materials undergoing damage in form of elastic modulus degradation as a result of stable microcrack growth. The stress and strain fields at the crack tip and the condition of path independence of J integral in the damage zone are discussed under some reasonable approximations. A modified J integral method is thereby proposed to calculate the ratio of near tip to remote stress intensity factors and compared with the conventional method of J conservation.