Crack initiation stress and strain of jointed rock containing multi-cracks under uniaxial compressive loading: A particle flow code approach

The ratio of crack initiation stress to the uniaxial compressive strength(SCI,B/SUC,B) and the ratio of axial strain at the crack initiation stress to the axial strain at the uniaxial compressive strength(B,UCB,CI,A,A/SSSS) were studied by performing numerical stress analysis on blocks having multi flaws at close spacing's under uniaxial loading using PFC3 D. The following findings are obtained: SCI,B/SUC,B has an average value of about 0.5 with a variability of ± 0.1. This range agrees quite well with the values obtained by former research. For joint inclination angle, β=90°,B,UCB,CI,A,A/SSSS is found to be around 0.48 irrespective of the value of joint continuity factor, k. No particular relation is found betweenB,UCB,CI,A,A/SSSS and β; however, the average B,UCB,CI,A,A/SSSS seems to slightly decrease with increasing k. The variability ofB,UCB,CI,A,A/SSSS is found to increase with k.Based on the cases studied in this work,B,UCB,CI,A,A/SSSS ranges between 0.3 and 0.5. This range is quite close to the range of 0.4to 0.6 obtained for SCI,B/SUC,B. The highest variability of ± 0.12 forB,UCB,CI,A,A/SSSS is obtained for k=0.8. For the remaining k values the variability ofB,UCB,CI,A,A/SSSS can be expressed within ± 0.05. This finding is very similar to the finding obtained for the variability of SCI,B/SUC,B.

Effect of microvoids on microplasticity behavior of dual-phase titanium alloy under high cyclic loading(Ⅰ):Crystal plasticity analysis

A crystal plasticity finite element(CPFE)model was established and 2D simulations were carried out to study the relationship between microvoids and the microplasticity deformation behavior of the dual-phase titanium alloy under high cyclic loading.Results show that geometrically necessary dislocations(GND)tend to accumulate around the microvoids,leading to an increment of average GND density.The influence of curvature in the tip plastic zone(TPZ)on GND density is greater than that of the size of the microvoid.As the curvature in TPZ and the size of the microvoid increase,the cumulative shear strain(CSS)in the primaryα,secondaryα,andβphases increases.Shear deformation in the prismatic slip system is dominant in the primaryαphase.As the distance between the microvoids increases,the interactive influence of the microvoids on the cumulative shear strain decreases.

Dynamic response of fixed-roof oil-storage tank structure under blast loading

In order to investigate the damage and deformation mechanism of large scale steel fixed-roof oil-storage tanks under the combustible gas explosion, a series of explosion experiments of scaled models are conducted. The l： 25 scaled numerical models of oil-storage tanks with a capacity of 5 000 m3 are also set up by ANSYS/LS-DYNA software, and their damage processes under the blast impact are numerically simulated. Both the experimental results and the numerical simulations show that the blast loading curve displays a pressure jump instantaneously at the moment of contact with the experimental models, and the overpressure peaks at the stagnation area of the outer surface on the blast side. The yield range first appears at the stagnation area and then propagates to the neighboring parts, and the irregular plastic hinge circle obviously appears around the deformation area, which results in the concaved buckling of the tank inner surface. During the whole process, the inner liquid not only impacts on the structures, but also absorbs and consumes part of the blast energy.

Numerical study of RC beams under various loading rates with LS-DYNA

Having an accurate understanding of concrete behavior under effects of high strain rate loading with the aim of reducing incurred damages is of great importance. Due to complexities and high costs of experimental research, numerical studies can be an appropriate alternative for experimental methods. Therefore, in this research capability of the finite element method for predicting concrete behavior at various loading conditions is evaluated by LS-DYNA software. First, the proposed method is presented and then is validated in three stages under different conditions. Results of load-lnidspan displacement showed good agreement between experimental and finite element results. Capability of finite element method in analyses of beams under various rates of loading was also validated by low error of the results. In addition, the proposed method has reasonable ability to evaluate reinforced concrete beams under various loading rates and different conditions.

Stress-Strain Behavior of Nylon-Carbon Composite Subjected to High Strain Rate Impact Loading

The aim of this study is to investigate the dynamic stress-strain relation for the hybrid composite （nylon ＋carbon）. Three groups of specimens are used with different number of carbon layers. The specimens were subjected to high velocity impact with different strain rates. SHPB （split Hopkinson pressure bar） is used in this investigation. The results show that, the stress-strain relation various with the strain rate. The maximum stress and strain are proportion directly with the strain rate. Also, the results revealed that, as the number of carbon layer increased, the maximum strain decreased.

带压开孔管道极限弯矩的试验研究

设计了2台带压开孔管道结构,采用与带压开孔中相同的焊接工艺制造模型。对模型支管在横向和纵向弯矩作用下结构的极限弯矩进行了试验研究。结果表明:支管和主管相贯区受拉侧关键点的极限弯矩值最小,此处为结构的危险区域;在结构尺寸和材料相同的情况下,承受纵向弯矩模型的极限弯矩大于承受横向弯矩的模型,二者的比值约为1.7;此外,对通过不同方法得到的极限弯矩值进行了比较,表明可以由载荷位移法代替载荷应变法确定结构的极限弯矩。

风化后红砂岩单轴压缩变形及声发射特性试验

在单轴压缩条件下,对四组(A组进行了温湿循环试验(100℃内),B组常温下干湿循环试验,C组温度循环试验,D组105℃干湿循环弹性波测试)风化后红砂岩和原状样红砂岩的声发射特性进行了对比研究,分析了不同条件下红砂岩破坏的声发射机理。结果表明,不同条件下红砂岩的变形与声发射活动存在较好的对应性,红砂岩的破坏形式都为X状共轭斜面剪切破坏。在弹性阶段,风化较为严重的A组、D组红砂岩声发射数量明显多于原状样和B组、C组。从声发射能量的多少可以判断风化较为严重的A组、D组红砂岩内部裂隙和节理相比B组、C组多。以最大相关系数为标准,不同风化程度的红砂岩载荷-声发射累计数关系都是3次多项式关系。

Finite Element Modeling and Parametric Analysis of Pavement Dynamic Responses under Moving Vehicle Load

This paper intends to develop finite element models that can simulate vehicle load moving on pavement system and reflect the pavement response of vehicle and pavement interaction.We conduct parametric analysis considering the influences of asphalt concrete layer modulus and thickness,base layer modulus and thickness,and subgrade modulus on pavement surface displacement,frequency,and strain response.The analysis findings are fruitful.Both the displacement basin width and maximum value of dynamic surface displacements are larger than those of static surface displacements.The frequency is positively correlated with the pavement structure moduli,and negatively correlated with the pavement structure thicknesses.The shape of dynamic and static tensile strain is similar along the depth of the pavement structure.The maximum value of dynamic tensile strain is larger than that of static tensile strain.The frequency of entire pavement structure holds more significant influence than the surface displacement and strain do.The subgrade modulus has a significant effect on surface displacement,frequency and strain.

Damage constitutive model of different age concretes under impact load

In order to investigate the mechanical properties and stress-strain curves of concrete at different ages under impact load,the impact compression tests of concrete at age of 1, 3, 7, 14 and 28 d were conducted with a large diameter split Hopkinson pressure bar, respectively. Based on statistical damage theory and Weibull distribution, combining the analysis of the change laws of stressstrain curves and viscosity coefficient of concrete with age, a damage constitutive model that can reflect the variation in dynamic mechanical properties with age was proposed. The stress-strain curves calculated from the proposed model are in good agreement with those from experimental data directly.

Hopkinson Tensile Tests of Flat Specimens

The SHTBT （split Hopkinson tensile bar test） is used for the determination of mechanical material properties at high strain rates, in this case within 10^2-10^3 s^-1. The duration of impact loading is within the range of 40-100 μS. The principle of SHTBT is based on the measurement and evaluation of impact loading that is common in high strain rate processes. Strain gauges are usually used to record impact loading. In this case, cylindrical capacitance detectors were used. Plastic deformation of metallic materials and alloys is a complex process that depends on a number of factors with the effect of strain rate being of particular importance. Dynamical tests are used for determination of mechanical properties; therefore, SHTBT is often used for these experiments. The present paper deals with possibilities of testing device for flat bars adapted at Hopkinson test. The aim of this work is concentrated on strain rate influence on 1.4301 austenitic steel, too.

A Discussion on Dynamic Characteristics of Undisturbed Loess by Dynamic Triaxial Test

Based on the dynamic triaxial test system and using the fitted wave of the Wenchuan earthquake and 1 Hz constant amplitude sinusoid,the paper compares the results of tests on undisturbed loess samples under different loads and vibration modes but under same saturated conditions.Results of the comparative experiment show:The stress-strain curves have a similar trend under random seismic loading and constant amplitude sinusoidal loading,but the random seismic loading is more sensitive to failure strength of the undisturbed loess samples under the same stress.

Experimental investigation and numerical modeling for elastoplastic notch-root stress/strain analysis under monotonic loadings

Experimental investigation and numerical modeling on elasto-plastic notch-root stress/strain distributions under monotonic loadings of both the Ni-based directionally solidified(DS)superalloy and Titanium alloy were carried out simultaneously.For measuring inhomogeneous deformation fields at notch roots,an optical-numerical full-field surface deformation measurement system was developed based on the digital image correlation(DIC)method.The obtained strain distributions were then verified with reasonable accuracy by finite element simulation,where an anisotropic elastic-viscoplastic constitutive model was developed for DS superalloy and a simple isotropic stress-strain relationship was adopted for Titanium alloy.Meanwhile,factors affecting elasto-plastic notch-root stress/strain distributions were systematically investigated numerically,where the emphasis was placed on temperature,loading stress rate,sample shape,anisotropy and notch features.The results show that stress/strain behavior at notch root is significantly affected by the mentioned factors,which are concretely embodied in the distribution of tensile stress/strain,equivalent stress and accumulative equivalent plastic strain.

Composite failure prediction of π-joint structures under bending

In this article, the composite π-joint is investigated under bending loads. The ＂L＂ preform is the critical component regard- ing composite π-joint failure. The study is presented in the failure detection of a carbon fiber composite π-joint structure under bending loads using fiber Bragg grating （FBG） sensor. Firstly, based on the general f＇mite element method （FEM） software, the 3-D finite element （FE） model of composite π-joint is established, and the failure process and every lamina failure load of composite π-joint are investigated by maximum stress criteria. Then, strain distributions along the length of FBG are extracted, and the reflection spectra of FBG are calculated according to the strain distribution. Finally, to verify the numerical results, a test scheme is performed and the experimental spectra of FBG are recorded. The experimental results indicate that the failure sequence and the corresponding critical loads of failure are consistent with the numerical predictions, and the computational error of failure load is less than 6.4%. Furthermore, it also verifies the feasibility of the damage detection system.

2D and 3D discrete numerical modelling of soil arching

In this study,2D and 3D soil arching phenomena associated with piled embankments were evaluated by performing a series of discrete numerical analyses using the particle flow code(PFC3D)software.After validating the micro-parameters with experimental results,we compared the stress-displacement distribution,force chain evolution,maximum vertical displacement of particles,and deformation characteristics induced by 2D and 3D arching effects.Additional analyses were carried out to understand the influence of the fill height,pile clear spacing,friction coefficient,and porosity on soil arching with respect to the stress concentration ratio(SCR)and settlement along the elevation at various sections.The numerical results indicated that a plane soil arch in a 2D embankment overestimates the degree of load transfer and underestimates the settlement at the crest and within the embankment along the elevation in a 3D embankment.A lower equal settlement plane can be found in a 2D embankment.Furthermore,an increase of fill height and friction angle,and a decrease of pile clear spacing and porosity can help to improve the degree of reduction in load transfer and settlement in both 2D and 3D embankments.However,for partially mobilized soil arching in the 3D condition,the increase of fill height reduces the settlement of soils mainly in the portion above the square subsoil area,but has less influence over the portion above the rectangular subsoil area.