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.

A New Method for Determining J_(IC) of High StrengthHigh Fracture Toughness Steels by SingleThree Point Bend Specimen

A single three point bend specimen compliance method for determining JIC of high strength high fracture toughness steels is presented and a formula for calculatinff J-integral is proposed as follows.It is simple and valid for high strength high fracture toughness steels. The values of JIC and KIC measured by this method are in good agreement with those measured by standard test method.

A new approach for flow simulation in complex hydraulic fracture morphology and its application:Fracture connection element method

Efficient flow simulation and optimization methods of hydraulic fracture morphology in unconventional reservoirs are effective ways to enhance oil/gas recovery.Based on the connection element method(CEM)and distribution of stimulated reservoir volume,the complex hydraulic fracture morphology was accurately described using heterogeneous node connection system.Then a new fracture connection element method(FCEM)for fluid flow in stimulated unconventional reservoirs with complex hydraulic fracture morphology was proposed.In the proposed FCEM,the arrangement of dense nodes in the stimulated area and sparse nodes in the unstimulated area ensures the calculation accuracy and efficiency.The key parameter,transmissibility,was also modified according to the strong heterogeneity of stimulated reservoirs.The finite difference and semi-analytical tracking were used to accurately solve the pressure and saturation distribution between nodes.The FCEM is validated by comparing with traditional numerical simulation method,and the results show that the bottom hole pressure simulated by the FCEM is consistent with the results from traditional numerical simulation method,and the matching rate is larger than 95%.The proposed FCEM was also used in the optimization of fracturing parameters by coupling the hydraulic fracture propagation method and intelligent optimization algorithm.The integrated intelligent optimization approach for multi-parameters,such as perforation number,perforation location,and displacement in hydraulic fracturing is proposed.The proposed approach was applied in a shale gas reservoir,and the result shows that the optimized perforation location and morphology distribution are related to the distribution of porosity/permeability.When the perforation location and displacement are optimized with the same fracture number,NPV increases by 70.58%,which greatly improves the economic benefits of unconventional reservoirs.This work provides a new way for flow simulation and optimization of hydraulic fracture morphology of multi-fractured horizontal wells in unconventional reservoirs.

ON J-INTEGRALS IN THE PLANE FRACTURE

In this paper, we discuss J-integrals near models I and II crack tips for the plates of linear-elastic isotropic homogeneous material and orthotropic composite material, using the theories of complex function and calculus, and obtain the result as follows: (1) The various J-integrals are transformed into standard form of line integrals with respect to coordinates: J = ∫rP(x, y)dx+Q(x, y)dy. (2) Independence of path of the various J-integrals is proved. (3) Computing formulae of J-integrals are derived.

Theory and Calculation of the J-Integral for Coupled Chemo-Mechanical Fracture Mechanics

In this paper,by introducing a chemical field,the J-integral formulation is presented for the chemo-mechanical coupled medium based on the laws of thermodynamics.A finite element implementation of the J-integral was performed to study the mode I chemo-mechanical coupled fracture problem.For derivation of the coupled J-integral,the equivalent domain integral(EDI)method was applied to obtain the mode I J-integral,with expression of the area integrals based on constitutive relationships of a linear elastic small deformation for chemo-mechanical coupling,instead of the finite deformation problem.A finite element procedure is developed to compute the mode I J-integral,and numerical simulation of the y-direction stress field is studied by a subroutine UEL(User defined element)developed in ABAQUS software.Accuracy of the numerical results obtained using the mode I J-integral was verified by comparing them to a well-established model based on linear elastic fracture mechanics(LEFM).Furthermore,a numerical example was presented to illustrate path-independence of the formulated J-integral for a chemo-mechanical coupled specimen under different boundary conditions,showing a high accuracy and reliability of the present method.The variation laws of J-integral and the y-direction stress field with external chemical,mechanical loading and time are revealed.The J-integral value increases with larger external concentration loading in the same integral domain.The extent of diffusion is much greater with larger concentration,which leads to a stronger coupling effect due to the chemical field.This work provides new insights into the fracture mechanics for the chemo-mechanical coupled medium.

RELATIONSHIP BETWEEN J-INTEGRAL AND FRACTURE SURFACE AVERAGE PROFILE

To investigate the causes that led to the formation of cracks in materials, a novel method that only considered the fracture surfaces for determining the fracture toughness parameters of J-integral for plain strain was proposed The principle of the fracture-surface topography analysis （FRASTA） was used. In FRASTA, the fracture surfaces were scanned by laser microscope and the elevation data was recorded for analysis. The relationship between J-integral and fracture surface average profile for plain strain was deduced. It was also verified that the J-integral determined by the novel method and by the compliance method matches each other well.

ON J-INTEGRALS IN THE PLANE FRACTURE OF COMPOSITE MATERIALS

By using a complex function method in this paper, the complex form of J-integral of mixed mode crack tip for unidirectional plate of linear-elastic orthotropic composites is obtained first by substituting crack tip stresses and displacements into general formula of J-integral. And then, the path-independence of this J-integral is proved. Finally, the computing formula of this J-integral is derived. As special examples, the complex forms, path-independence and computing formulae of J-integrals of mode I and mode II crack tips for unidirectional plate of linear-elastic orthotropic composites are given.

Analytic Expression of Meso-Criterion on Dynamic Fracture

This study presents a meso-criterion of dynamic fracture, on the basis of stress in integral form In such way the difficulty due to the singularity of stress distribution at the crack tip is overcome. A micro-parameter, the atom radius, is introduced into the criterion.Meanwhile a characteristic time concept is taken into account for describing the inertia effect of material. The criterion reveals The criterion reveals the effects of loading rate, defect and sample geometry,material constants including the micro-structure parameter.

ANTI-PLANE FRACTURE ANALYSIS OF FUNCTIONALLY GRADIENT MATERIAL INFINITE STRIP WITH FINITE WIDTH

The special case of a crack under mode Ⅲ conditions was treated, lying parallel to the edges of an infinite strip with finite width and with the shear modulus varying exponentially perpendicular to the edges. By using Fourier transforms the problem was formulated in terms of a singular integral equation. It was numerically solved by representing the unknown dislocation density by a truncated series of Chebyshev polynomials leading to a linear system of equations. The stress intensity factor （SIF） results were discussed with respect to the influences of different geometric parameters and the strength of the non-homogeneity. It was indicated that the SIF increases with the increase of the crack length and decreases with the increase of the rigidity of the material in the vicinity of crack. The SIF of narrow strip is very sensitive to the change of the non-homogeneity parameter and its variation is complicated. With the increase of the non-homogeneity parameter, the stress intensity factor may increase, decrease or keep constant, which is mainly determined by the strip width and the relative crack location. If the crack is located at the midline of the strip or if the strip is wide, the stress intensity factor is not sensitive to the material non-homogeneity parameter.

A Pipeline Fracture Model of Hydrogen-induced Cracking

The relationship between hydrogen concentration and crack tip stress, strain field, hydrogen diffusion and internal pressure respectively in the crack tip process zone was investigated, and the length of the crack tip process zone of hydrogen-induced cracking （HIC） was determined. Based on the mechanism of fracture of micro-crack nucleation, a dislocation model was presented for the fracture criteria of HIC. The influence factors of pipeline tube fracture ductile KISCC in the presence of hydrogen was analyzed, and the critical pressure bearing capability of a pipeline with hydrogeninduced cracking and the critical J-integrity （JISCC） were calculated, which is very important for pipeline safety.

Numerical methods for solving singular integral equations obtained by fracture mechanical analysis of cracked wedge

The numerical solutions to the singular integral equations obtained by the fracture mechanical analyses of a cracked wedge under three different conditions are considered. The three considered conditions are:(i) a radial crack on a wedge with a nonfinite radius under the traction-traction boundary condition,(ii) a radial crack on a wedge with a finite radius under the traction-traction boundary condition, and(iii) a radial crack on a finite radius wedge under the traction-displacement boundary condition. According to the boundary conditions, the extracted singular integral equations have different forms. Numerical methods are used to solve the obtained coupled singular integral equations, where the Gauss-Legendre and the Gauss-Chebyshev polynomials are used to approximate the responses of the singular integral equations. The results are presented in figures and compared with those obtained by the analytical response. The results show that the obtained Gauss-Chebyshev polynomial response is closer to the analytical response.

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.

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.

Thermal stress and fracture temperature prediction for flexible pavement

Analytical solutions of thermal stresses in multilayered elastic system whose materials characteristics are dependent on temperature are derived by a transfer matrix and integral transformation method.The resulting formulation is used to calculate thermal stresses in the low temperature cracking problem of asphalt pavement.Numerical simulations and analyses are performed using different structural combinations and material characteristics of base course.And fracture temperatures are predicted for a given flexible pavement constructed with three types of asphalt mixtures based on the calculated results and experimental data.This approach serves as a better model for real pavement structure as it takes into account the relationships between the material characteristics and temperature in the pavement system.

A PRELIMILARY STUDY ON FRACTURE GEOCHEMICAL CHARACTERISTICS IN THE MAYANG Cu DEPOSIT, HUNAN

The Mayang Cu deposit, western Hunan is a middle scale sandstone type Cu deposit predominant of native copper. From an angle of geological integrative field, fracture geochemical features are studided and discussed in this paper. In fracture structure, variant regularity and the ratios of element couples participating in metallogensis that their geochemical natures are similar or strongly correlative, reveal changing laws of metallogenic temperature and pressure fields and migrating direction of metallogenic elements. The theoretical study tallies much better with productive practice. They are meaningful to conduct prospecting and exployation.

STOCHASTIC ELASTO-PLASTIC FRACTURE ANALYSIS OF ALUMINUM FOAMS

Model I quasi-static nonlinear fracture of aluminum foams is analyzed by considering the effect of microscopic heterogeneity. Firstly, a continuum constitutive model is adopted to account for the plastic compressibility of the metallic foams. The yield strain modeled by a two- parameter Weibull-type function is adopted in the constitutive model. Then, a modified cohesive zone model is established to characterize the fracture behavior of aluminum foams with a cohesive zone ahead of the initial crack. The tensile traction versus local crack opening displacement relation is employed to describe the softening characteristics of the material. And a Weibull statistical model for peak bridging stress within the fracture process zone is used for considering microscopic heterogeneity of aluminum foams. Lastly, the influence of stochastic parameters on the curve of stress-strain is given. Numerical examples are given to illustrate the numerical model presented in this paper and the effects of Weibull parameters and material properties on J-integral are discussed.

Energy dissipation and contour integral characterizing fracture behavior of incremental plasticity

J ep -integral is derived for characterizing the frac- ture behavior of elastic-plastic materials. The J ep -integral differs from Rice’s J-integral in that the free energy density rather than the stress working density is employed to define energy-momentum tensor. The J ep -integral is proved to be path-dependent regardless of incremental plasticity and deformation plasticity. The J epintegral possesses clearly clear physical meaning： （1） the value J ep tip evaluated on the infinitely small contour surrounding the crack tip represents the crack tip energy dissipation; （2） when the global steadystate crack growth condition is approached, the value of J ep farss calculated along the boundary contour equals to the sum of crack tip dissipation and bulk dissipation of plastic zone. The theoretical results are verified by simulating mode I crack problems.

T应力作用下的J积分理论研究

T应力是表征裂尖应力场所必需的物理量之一,在其作用下J积分的性质有待研究。推导了J积分在考虑T应力情况下的表达式,采用数值模拟方法在Ⅰ型、Ⅱ型和Ⅰ+Ⅱ复合型加载工况下对该表达式进行验证,给出一种基于J积分的裂尖应力场参数计算方法。理论推导证明,J1积分的路径守恒性不受T应力影响,但J2积分的数值与积分路径起止点到裂尖距离r的平方根之间满足线性关系,J2-r曲线的系数与T应力相关。有限元仿真结果表明,试样裂纹长度对J积分的影响较为显著,网格尺寸对J积分计算结果的影响很小;相同载荷工况下,不同尺寸试样的J积分数值不同,但J2-r曲线的变化趋势与理论推导的表达式相符;在Ⅱ型和Ⅰ+Ⅱ复合型载荷工况下,T应力的求解精度较高,而在Ⅰ型载荷工况下,J2≈0,受舍入误差等因素影响,求解精度降低。

基于多试样法的PVC塑料J-R阻力曲线的简便测量方法

获得PVC材料的J-R阻力曲线以及断裂韧度J_(IC)的难点在于裂纹扩展量Δa的测量,传统染色处理以及三维测量等方法过程复杂且测试成本高。由于塑料的易切割性,可以简化Δa的测量过程。因此,基于多试样法,提出根据规范的测点要求对加载后的试件沿着裂纹纵向切割成若干片、再通过工业电子显微镜测量每个切割片上的裂纹长度,从而获得Δa的二维测量方法。通过GB 21143—2014和ASTM D6068-10两个标准,应用该方法均能获得相对一致的J-R阻力曲线和J_(IC)值,测试结果有效且准确。其中根据GB 21143—2014获得PVC的J_(IC)值为8.03 kJ/m^(2),J-R阻力曲线拟合方程的系数为9.93,指数为0.15;根据ASTM D6068-10获得PVC的J_(IC)为8.31 kJ/m^(2),J-R阻力曲线拟合方程的系数为12.7,指数为0.3;表明ASTM D6068-10获得PVC的断裂韧度J_(IC)值和拟合方程的系数、指数均大于GB 21143—2014。由于PVC材料与金属材料性能的差异,得出ASTM D6068-10更适用于测试PVC材料的J-R阻力曲线和J_(IC)值的结论。该方法将传统的裂纹染色处理和三维测量等复杂操作简化为切片和二维测量,简化测量过程,降低了测量成本,可普及应用于塑料断裂韧性的测量。

业财一体化财务管理体系的构建和应用——基于J集团的案例实践

为应对复杂的经济环境和激烈的市场竞争,促进企业高质量发展,越来越多公司基于传统的财务标准化,借助信息化、智能化等手段,向数字化的现代财务管理模式转变,推动业财深度融合,助力管理者做出科学有效的决策,以降低风险,提高效益。文章以拟上市科技服务企业J集团为例,通过梳理其过去财务管理方面存在的问题,围绕构建业财一体化财务管理体系的建设思路,详细分析了J集团通过制度建设、信息系统建设及团队建设三方面的构建与应用的过程,并对该案例实践进行经验总结,以供同业者参考。