Problems and feasibilities of testing the cyclic behaviour of thin sheet steels

This report deals with the testing problems of thin sheet steels subjected to cyclic load and introduces at the same time the appropriate techniques to procure the reliable and significant testing results. Practically, a proper judgement of cyclic fatigue behaviours of such conventional deep drawing steels is only possible, if the specimens were to be tested either under strain control mode in elastic-plastic range with sufficient plastic deformations or load controlled in elastic range by using the notched specimens with adequate local strain concentration. Besides the study of specific material properties, as absolutely determined by using the unnotched specimens, the influence of notch geometry can also be investigated by means of the notched specimens. For a promising performance of elastic-plastic cyclic tests on thin sheets the use of a particular specimen shape with uncritical length of high buckling resistance is most essential. Moreover, a special grip with great precision is necessary to ensure an axial and rigid alignment of specimens. In the course of a development an optimal specimen shape and its gripping device has been designed. The cyclic properties of some thin sheet steels, obtained in this way, are also illustrated.     

Evaluation of fatigue damage in materials using indentation testing and infrared thermography

Damage identification due to fatigue has been studied on 304-Stainless Steel and Al-Cu-Mg alloys 2014-T651 and 7175-T7351, using two different experimental methods: a) cyclic indentation, and b) infrared thermography. Indentation response during load controlled cyclic loading is used to characterize fatigue response of materials. The load vs. depth of penetration data obtained continuously during fatigue testing is used to obtain information on cyclic stress-strain behavior and onset of failure. Infrared thermography is used to study the heat generation during fatigue loading on specimens. The variables that affect the process are: frequency of loading, magnitude of strain (elastic-plastic), thermal properties. The temperature curve can be considered to be having three regions, initial region of rapid increase in temperature, followed by stable temperature rise and final rapid heat generation prior to failure. The slopes in the initial region and stable region are independent of prior damage history in materials in case of specimens subjected to pure elastic load reversals. In case of elastic-plastic loadings, the rate at which the temperature rises in initial region changes as a function of fatigue damage and can thus be used to estimate prior damage in materials.     

Novel Formulation to Study Thermal Postbuckling of Circular Plates with Edges Elastically Restrained against Rotation

A novel formulation is used to study the thermal postbuckling behavior of circular plates, with the edges supported to not have lateral deflection and elastically restrained against rotation. The elastic restraint is mathematically represented by an elastic rotational spring. The circular plate is subjected to a uniform edge compressive radial load, developed because of a uniform temperature rise. The formulation is on the basis of on the radial tensile load developed in the plate because of the large deflections of the plate with edges immovable in the plane normal to the edge and the linear buckling load corresponding to the uniform edge radial compressive load. The developed radial tensile load is obtained by using Berger’s approximation. The numerical results obtained from the present investigation in terms of the ratios of the postbuckling to the buckling loads for several rotational spring stiffness values compare well with those obtained by using the versatile finite-element analysis.     

Nonlinear Analytical Solution for Cable Truss

In this technical note the nonlinear closed-form static solution of the suspended biconvex and biconcave cable trusses with unmovable, movable, or elastic yielding supports subjected to vertical distributed load applied over the entire span is presented. Irvine’s linearized forms of the deflection and the cable equations are modified because the effects of the nonlinear truss behavior needed to be incorporated in them. The concrete form of the system of two nonlinear cubic cable equations is derived and presented. From a solution of a nonlinear vertical equilibrium equation for a loaded cable truss, the additional vertical deflection is determined. The transformation analytical model serves to determine the response, i.e., horizontal components of cable forces and deflection of the geometrically nonlinear truss, due to the applied loading, considering effects of elastic deformations, temperature changes, and elastic supports. The deflection of asymmetric prestressed cable trusses has been compared with Irvine’s linear solution as well as the nonlinear finite element model results.     

矫直机压弯量计算法的探讨

随着计算机设定矫直机压弯量（俗称压下量）的精确化需要,初步提出一种解析方法和数值方法。并在大挠度压弯时把弹复挠度与残留挠度分开计算,在计算残留挠度时,将其作为假想外力作用所产生的等效挠度,再将此等效挠度分为弹性挠度与弹塑性挠度使用数值积分法计算,可获得相当精确的结果。与现场经验值对比验证,可得出切实可行的结论     

Detecting Multiple Open Cracks in Elastic Beams by Static Tests

This paper concerns with the identification of multiple open cracks in a beam by measurements of the damage-induced variations in the static deflection of the beam under a prescribed load condition. Each crack is simulated by an equivalent linear spring connecting the two adjacent segments of beam. Sufficient conditions on the static measurements which allow for the unique identification of the damage are presented and discussed for nonuniform beams under some ideal boundary conditions. The inverse analysis is based on an explicit expression of the crack-induced variation in the deflection of the beam under a given load distribution and it provides exact closed-form expressions of position and severity of the cracks in terms of the measured data. The theoretical results are confirmed by a comparison with static tests carried out on a steel beam with localized damages.     

Modeling Out-of-Plane Behavior of URM Walls Retrofitted with Fiber Composites

Although masonry is one of the oldest construction materials, its behavior has not been investigated as extensively as other construction materials. Out-of-plane failures are common in unreinforced masonry (URM) buildings constructed in seismic regions. Seven half-scale brick masonry walls were constructed, externally strengthened with vertical glass-fabric composite strips, and subjected to static cyclic out-of-plane loading. The flexural behavior of the tested specimens is characterized by three main stages corresponding to the first visible bed-joint crack, the first delamination, and the ultimate load. The main parameters being investigated in this study are the amount of composite, the height-to-thickness ratio h∕t, the tensile strain in composites, and the mode of failure. Based on the trends observed in the experimental phase, it was concluded that the behavior of the walls is best predicted with a linear elastic approach. It was also concluded that the ultimate strength method overestimates the flexural capacity and the ultimate deflection of the wall. Preliminary design recommendations are also proposed for tensile strain in the composite, maximum deflection, and maximum reinforcement ratio.     

Resistance of covalent crystals to microindentation

Conclusions A study was made of the resistance of various covalent crystals to microindentation, involving comparison of reduced and unreduced hardness values. A parameter p-unit indenter load necessary for the elastic-plastic penetration of an indenter to unit depth — has been introduced.A simple method is proposed for the determination of the elastic and plastic components of the displacement of an indenter during its penetration. By analogy with p, another two parameters, p_p and p_e — indenter loads necessary for the plastic and elastic penetration of an indenter to unit depth, respectively — have been introduced. It is shown that the ratio of the elastic to the plastic strain component varies from material to material, and is determined by the HV/E (or _s/E) ratio.The relative contributions from the elastic and plastic components to the total strain caused by the penetration of an indenter change with load, resulting in marked deviations from the law of similarity and in increases in hardness at small indenter loads.The elastic resistance of a material to microindentation, as characterized by the parameter p_e, enables the elastic modulus of the material to be estimated, and a method of doing this is described. On the other hand, the plastic resistance of a material to microindentation, in particular the parameter p_p determines the plastic deformation resistance of the material more accurately than does its hardness. At small indenter penetration depths the contribution from plastic deformation decreases compared with that from elastic deformation, resulting in a marked increase in hardness.Translated from Poroshkovaya Metallurgiya, No. 8(176), pp. 72–80, August, 1977.The authors are most grateful to V. P. Alekhin, G. P. Upit, and S. I. Bulychev for helpful discussions relating to the results of this investigation.     

Numerical Simulations of Dynamic Instability of Elastic-Plastic Beams

This paper numerically investigates the dynamic instability of different elastic-plastic beams subjected to transverse pulse load. Two types of beam dynamic instability, i.e., symmetrical and asymmetrical instabilities, are studied. The unstable responses of the elastic-plastic beams are illustrated by investigating the modal participation factors of the lowest nine vibration modes, which are determined by inverse derivation of the numerically simulated beam deflection response. The critical pulse loads for the beam symmetrical and asymmetrical instabilities are obtained with respect to different load durations. Characteristic diagrams for beams with different boundary conditions and subjected to different type pulse loads are given. The present study conforms that not only axial compressive load induces instability of slender members, but also transverse pulse load results in instability of elastic-plastic beams.     

Anisotropy of elastic aftereffect in FCC metals

The deformation of rolled copper and aluminum sheets has been studied at a constant load in the elastic range. Anisotropy in their elastic aftereffect has been detected. The time-dependent part of the deformation is found to be best described by a power function of time with a fractional exponent. A mathematical model is proposed to describe the elastic aftereffect of a metal using the fractal concepts of deformation. A strictly exponential time dependence is shown to transform into an anomalous dependence when a continuous relaxation-period distribution transforms into a fractal relaxation-period distribution during elastic aftereffect in fcc metals. The exponent of the power dependence of the strain on the time determines the fractal dimension of relaxation during the elastic aftereffect.     

Concrete Beams Strengthened with Prestressed Near Surface Mounted CFRP

Retrofitting concrete structures with fiber reinforced polymer (FRP) has today grown to be a widely used method throughout most parts of the world. The main reason for this is that it is possible to obtain a good strengthening effect with a relatively small work effort. It is also possible to carry out strengthening work without changing the appearance or dimensions of the structure. Nevertheless, when strengthening a structure with external FRP, it is often not possible to make full use of the FRP. The reason for this depends mainly on the fact that a strain distribution exists over the section due to dead load or other loads that cannot be removed during strengthening. This implies that steel yielding in the reinforcement may already be occurring in the service limit state or that compressive failure in the concrete is occurring. By prestressing, a higher utilization of the FRP material is made possible. It is extremely important to ensure that, if external prestressing is used, the force is properly transferred to the structure. Most of the research conducted with prestressing carbon fiber reinforced polymer (CFRP) for strengthening has been on surface bonded laminates. However, this paper presents research on prestressed CFRP quadratic rods bonded in sawed grooves in the concrete cover. This method has proven to be an advantageous means of bonding CFRP to concrete, and in comparison to surface bonded laminates, the shear and normal stress between the CFRP and the concrete are more efficiently transferred to the structure. In the presented test, no mechanical device has been used to maintain the prestress during testing, which means that the adhesive must transfer all shear stresses to the concrete. Fifteen beams with a length of 4?m have been tested. The tests show that the prestressed beams exhibited a higher first-crack load as well as a higher steel-yielding load as compared to nonprestressed strengthened beams. The ultimate load at failure was also higher, as compared to nonprestressed beams, but in relation not as large as for the cracking and yielding. In addition, the beams strengthened with prestressed FRP had a smaller midpoint deflection. All strengthened beams failed due to fiber rupture of the FRP.     

复杂应力状态225Cr 1Mo钢的高温低周疲劳裂纹扩展规律

 用带预裂纹的缺口试件研究225Cr 1Mo 钢高温低周疲劳裂纹扩展规律，通过疲劳试验观察裂纹扩展寿命，应用ANSYS计算裂纹尖端应力应变分量和当量弹、塑性应变范围，利用当量J积分范围表征225Cr 1Mo 钢在复杂应力状态下的低周疲劳裂纹扩展速率。结果表明：疲劳裂纹扩展速率da/dN与当量J积分范围ΔJf的关系不受试件缺口型式和加载应变范围的影响，用当量J积分来评价225Cr 1Mo 钢的高温低周疲劳裂纹扩展规律是有效的。     

PCC Airfield Pavement Response during Thaw-Weakening Periods

A field study was performed at two regional airports in Wisconsin during spring thaw to determine its effects on portland cement concrete (PCC) airport pavements. This study was part of a research program to model the performance of airfield pavements for the Federal Aviation Administration. Subsurface temperature and falling weight deflection measurements of the pavement structures were taken at both airports and used to calculate the frost penetration depths, the changes in bearing capacity, and the joint and load transfer efficiencies. This paper summarizes the findings of this study and includes several relationships between various engineering properties of the subsurface layers below the PCC layer, along with a procedure for evaluating pavement performance using falling weight deflection data for PCC pavements during spring thaw.     

超高强度弹簧钢冷变形性能及其仿真模拟

摘要：针对60Si2CrVAT超高强度弹簧钢实际冷卷成形工况，采用准静态拉伸试验和不同微观表征手段研究了经Q&T (Quenching&Tempering)和Q-I-Q-T(Quenching-Isthothermal Quenching-Tempering)工艺热处理后试验钢的组织形貌及冷变形前后力学性能的差异，并利用Deform-3D有限元数值模拟软件分析了2种工艺参数下的弹簧钢在冷卷成形过程中的应力、应变等场量参数的变化特征，预测了其冷卷成形过程中的断裂损伤概率。结果表明，Q-I-Q-T工艺复相组织弹簧钢的塑性更好，冷变形后的断面收缩率和伸长率比Q&T工艺马氏体中温回火组织弹簧钢分别高出了65%和66%。模拟结果显示，不同组织状态下的超高强度弹簧钢在卷制过程中的等效应力和等效应变分布规律近似，但Q-I-Q-T工艺复相组织弹簧钢在卷制过程中产生的等效应力和等效应变值更小，产生断裂的概率更低。     

Modeling Permanent Deformation of Unbound Granular Materials under Repeated Loads

Finite-element analysis on a pavement structure under traffic loads has been a viable option for researchers and designers in highway pavement design and analysis. Most of the constitutive drivers used were nonlinear elastic models defined by empirical resilient modulus equations. Few isotropic/kinematic hardening elastoplastic models were used but applying thousands of repeated load cycles became computationally expensive. In this paper, a cyclic plasticity model based on fuzzy plasticity theory is presented to model the long-term behavior of unbound granular materials under repeated loads. The discussion focuses on the model parameters that control long-term behavior such as elastic shakedown. The performance of the constitutive model is presented by comparing modeled and measured permanent strain at various numbers of load cycles. Calculated resilient modulus from the complete stress-strain curve is also discussed.     

Elastic-Plastic Seismic Behavior of Long Span Cable-Stayed Bridges

This paper investigates the elastic-plastic seismic behavior of long span cable-stayed steel bridges through the plane finite-element model. Both geometric and material nonlinearities are involved in the analysis. The geometric nonlinearities come from the stay cable sag effect, axial force-bending moment interaction, and large displacements. Material nonlinearity arises when the stiffening steel girder yields. The example bridge is a cable-stayed bridge with a central span length of 605 m. The seismic response analyses have been conducted from the deformed equilibrium configuration due to dead loads. Three strong earthquake records of the Great Hanshin earthquake of 1995 in Japan are used in the analysis. These earthquake records are input in the bridge longitudinal direction, vertical direction, and combined longitudinal and vertical directions. To evaluate the residual elastic-plastic seismic response, a new kind of seismic damage index called the maximum equivalent plastic strain ratio is proposed. The results show that the elastic-plastic effect tends to reduce the seismic response of long span cable-stayed steel bridges. The elastic and elastic-plastic seismic response behavior depends highly on the characteristics of input earthquake records. The earthquake record with the largest peak ground acceleration value does not necessarily induce the greatest elastic-plastic seismic damage.     

Study of effect of load on springback in sheet metal bending

Springback remains a major concern in sheet metal bending in fabricating any final product within the permissible tolerance. Apart from the geometrical and material parameters, springback is significantly affected by the forming load also and the present study is focused on it. Sheet metal bending process involves large rotation and strain as well as large springback due to elastic recovery of the material. Therefore, a large deformation algorithm based Finite Element software was used to model a typical sheet metal bending process employed in manufacturing cylindrical structures. A Total-Elastic-Incremental-Plastic (TEIP) algorithm has been incorporated in an in-house software to handle large deformation and the elastic recovery during the unloading process. In addition, experiments have been performed on aluminum, brass, copper and mild steel sheets and substantiated with the FEM analysis.     

Simulation of Strip Rolling Using Elastoplastic Contact BEM With Friction

With rollers as elastic bodies and workpieces as elastoplastic bodies,the rolling problem can be viewed as a friction elastic-plastic contact problem.With fewer assumptions in the simulation of strip-rolling process,a boundary element method（BEM）for two-dimensional elastoplastic finite strain and finite deformation analysis of contact problems with friction was presented.All the equations for contact problems,which include multi-nonlinearities,were obtained.Incremental and iterative procedures were used to find contact pressure and friction stress.Moreover,initial strain rate algorithm and work-hardening material behavior can be assumed in the plastic analysis.Several examples were presented,and the results of contact pressure and friction stress were in excellent agreement with those of analysis.     

CSP连轧过程金属变形的热力耦合模拟分析

借助Marc商用软件,采用弹塑性大变形热力耦合有限元法(FEM),对包钢生产的1 500 mm×68mm薄板坯CSP(紧凑式带材生产)轧制第一道次的热轧过程进行了模拟。分析了变形区内轧材等效应力场、应变场及应变速率的分布和变化规律。结果表明在轧件变形区内,等效应力沿轧制方向逐渐增大,在中性面附近达到最大值(95.20 MPa),后又逐渐减少;等效应变亦沿轧制方向逐渐增大,在轧件出口处达到最大值(0.70);在轧件入口端表面附近等效应变速率有最大值,为20.74 s^-1。模拟计算的轧制力为22 203 kN,现场测得的轧制力为22 239 kN,预测误差为0.16%。     

Noncontact Photogrammetric Measurement of Vertical Bridge Deflection

This paper reports on the results from a study of vertical deflection measurement of bridges using digital close-range terrestrial photogrammetry (DCRTP). The study consisted of a laboratory and two field exercises. In the laboratory exercise, photogrammetric measurements of a 11.6 m (38 ft) steel beam loaded at midspan were made and compared with dial gauge readings and elastic beam theory. In the first field exercise, the initial camber and dead load deflection of 31.1 m (102 ft) prestressed concrete bridge girders were measured photogrammetrically and compared with level rod and total station readings. A comparison of the photogrammetric measurements with the dead load deflection diagram is also made. In the second field exercise, the vertical deflection of a 14.9 m (49 ft) noncomposite steel girder bridge loaded with two dump trucks was measured. Photogrammetric results are compared with deflections estimated using elastic finite-element analysis, level rod readings, and curvature-based deflection measurements. The paper is concluded with a discussion of work in progress to further improve the accuracy of DCRTP in the field.