Strain rate dependence of HPFRCC cylinders in monotonic tension

High-Performance Fiber-Reinforced Cementitious Composite (HPFRCC) materials exhibit strain hardening in uniaxial, monotonic tension accompanied by multiple cracking. The durability of HPFRCC materials under repeated loading makes them potentially suitable for seismic design applications. In this paper, the strain rate dependence of tensile properties of two HPFRCC materials in cylindrical specimens is reported from a larger study on strain rate effects in tension, compression and cyclic tension–compression loading. The cylindrical specimens were loaded in monotonic tension at strain rates ranging from quasi-static to 0.2 s⁻¹. To evaluate the impact of specimen geometry on tensile response, coupon specimens loaded in monotonic tension under a quasi-static strain rate were compared to corresponding cylindrical specimens made from the same batch of material. Tensile strength and ductility of the HPFRCC materials were significantly reduced with increasing strain rate. Multiple cracking, strain hardening, strain capacity, and the shape of the stress–strain response were found to be dependent on specimen geometry. SEM images taken of the fracture plane of several specimens indicated that pullout and fracture of the fibers occurred for both HPFRCC materials studied here.     

Experimental testing of reinforced concrete and reinforced ECC flexural members subjected to various cyclic deformation histories

Engineered Cementitious Composite (ECC) materials have been designed to exhibit high tensile ductility compared to traditional concrete. ECCs have also shown improved damage tolerance in compression. When reinforced with steel, ECC components have been proposed for enhanced seismic resistance in structural applications. Because of the uncertainty associated with ground motions, determining an appropriate cyclic deformation history for seismic testing of structural components is a challenge. Three reinforced ECC and three reinforced concrete beams were tested under three different cyclic loading protocols. Cracking, strain in the steel reinforcement, and hysteretic response were monitored. The reinforced ECC beams exhibited an increase in ductility and hysteretic energy dissipated over the reinforced concrete beams, particularly when subjected to a deformation history containing large initial deformation pulses. The presence and magnitude of initial pulses did not affect ductility or failure mode of the ECC beams, and is not expected to be relevant in design of reinforced ECC beams for collapse prevention.     

Cyclic modeling of FRP-confined concrete with improved ductility

Confinement by fiber reinforced polymer (FRP) wraps can significantly enhance strength and ductility of concrete columns. Behavior of FRP-confined concrete in uniaxial compression can be characterized by its bilinear stress–strain and unique dilation properties. A number of models have in recent years been developed to capture these characteristics under monotonic loading. None, however, have addressed the cyclic response of FRP-confined concrete. A total of 24 FRP-confined concrete stub specimens were tested in uniaxial compression under different levels of loading and unloading, with different fiber type, wrap thickness, and loading patterns. Based on a regression analysis of test results, a constitutive model is developed that includes cyclic rules for loading and unloading, plastic strains, and stiffness and strength degradations. The proposed model is validated by comparing analytical predictions with experimental results of an independent test series. Good agreement was shown between the analysis and experiments, confirming the ability of the model to predict the cyclic behavior of FRP-confined concrete. The model could be easily implemented in a fiber element model for flexural analysis of cyclic loaded beam-columns in conjunction with a strain gradient approach.     

Cyclic tensile response of a pre-tensioned polyurethane

In the research reported in this paper, we subject a polyurethane to uniaxial tensile loading at a quasi-static strain rate, a high strain rate and a jumping strain rate where the specimen is under quasi-static pre-tension and is further subjected to a dynamic cyclic loading using a modified Kolsky tension bar. The results obtained at the quasi-static and high strain rate clearly show that the mechanical response of this material is significantly rate sensitive. The rate-jumping experimental results show that the response of the material behavior is consistent before jumping. After jumping the stress–strain response of the material does not jump to the corresponding high-rate curve. Rather it approaches the high-rate curve asymptotically. A non-linear hyper-viscoelastic (NLHV) model, after having been calibrated by monotonic quasi-static and high-rate experimental results, was found to be capable of describing the material tensile behavior under such rate jumping conditions.     

The effects of texture in titanium alloys for engineering components under fatigue

The mechanical response of textured Ti 6/4 plate material is assessed through an evaluation of monotonic properties under tension and torsion loading and fatigue testing of plain section and notched specimen geometries. Significant variations in modulus, yield strength, ultimate tensile strength and ductility are demonstrated for testpieces taken from the plate materials parallel to either the transverse or longitudinal rolling direction. Cyclic performance is also shown to be sensitive to orientation with different cyclic stress–strain curves defined in the two orientations. The relationship between the principal stress axis and the dominant basal plane texture is shown to control fatigue crack initiation lives and the ultimate mode of fracture. Whilst loading parallel to the transverse direction offers the strongest monotonic and cyclic stress–strain response, fatigue tests performed on specimens orientated parallel to the longitudinal rolling direction provide the optimum cyclic life. These effects are discussed with reference to the inherent, anisotropic mechanical response of α+β titanium alloys, which results from the hexagonal crystallographic form of the α phase and the availability of preferential slip systems. It is argued that the anisotropic response could be utilised to an engineering advantage by matching critical stressing directions to the specific properties offered by the texture.     

Lateral behavior of concrete under uniaxial compressive cyclic loading

In reinforced concrete structures under seismic loading, concrete is subjected to compressive cyclic stress. Although cyclic stress–strain response has been described before, the cyclic behavior of strains in the direction orthogonal to loading has not been characterized yet. Such behavior can be of great importance for evaluating the efficiency of the confinement under cyclic loading. For this purpose an experimental program on cylindrical specimens of concrete strength from 35 to 80 MPa subjected to uniaxial cyclic compression was carried out. Stress versus longitudinal and lateral strains curves have been obtained both for the hardening and softening branches under monotonic and cyclic loading. Governing parameters of the lateral behavior are identified and correlated to describe the response of the lateral strain. Additionally, an analytical model to obtain the lateral deformations of concrete under cyclic uniaxial compression has been formulated and verified experimentally. Finally, some examples are presented in order to illustrate the applicability of the proposed model and its possible incorporation into a 3D constitutive cyclic model.     

Damage accumulation model for aluminium‐closed cell foams subjected to fully reversed cyclic loading

Although metal foams are a relatively new material, substantial knowledge has been accumulated about their mechanical properties and behaviour under monotonic loads and tension–tension and compression–compression cyclic loads. However, there are very few reports of the behaviour of metal foams under tension–compression‐reversed loading. In this paper, we examine some of the rare published data regarding the tension–compression cyclic response of metal foams, develop a statistical model of the fatigue lifetime and propose two damage accumulation models for aluminium‐closed cell foams subjected to a fully reversed cyclic loading. In developing these models a fatigue analysis and a failure criterion for the material are needed; the fatigue models considered are the Coffin–Manson and the statistical Weibull model, and the failure criterion used is the one described by Ingraham et al. (Ingraham, M.D., DeMaria, C.J., Issen, K.A. and Morrison, D.J.L. (2009). Mater. Sci. Eng. A. 504:150–156). The models developed are compared with the experimental published data by Ingraham et al. (Ingraham, M.D., DeMaria, C.J., Issen, K.A. and Morrison, D.J.L. (2009). Mater. Sci. Eng. A. 504:150–156) and a final analysis was performed to determine whether it is preferable to use the total or plastic strain amplitude for the fatigue analysis.     

双相型不锈钢S22053循环本构关系研究

为研究国产双相型不锈钢S22053在循环荷载下材料的力学性能和本构关系,该文采用S22053热轧钢板加工成母材试件,进行单调和循环加载试验,得到14种加载制度下的应力-应变曲线。基于单调加载试验曲线,分析了单调荷载下的材料性能;利用等应变幅加载试验曲线拟合了Chaboche混合强化参数,并用有限元软件ABAQUS对试验进行模拟计算,对比、验证了拟合效果。结果表明:双相型不锈钢S22053延性较好,没有明显的屈服平台和屈服点,比例极限较低;循环骨架曲线可以采用Ramberg-Osgood模型进行拟合;采用不同分量模型标定的3组混合强化参数均能较好的模拟材料的循环受力特征,其中三背应力分量模型(N2L1)拟合效果最好。研究结果可用于分析计算双相型不锈钢结构在地震作用下的受力性能。     

国产低屈服点钢材循环加载试验研究

为研究不同荷载作用下国产低屈服点钢的材料力学行为,对LY100、LY160及LY225钢材共46个试件进行了单调拉伸试验及12种不同加载制度的循环加载试验。对国产低屈服点钢的单调性能、滞回性能、破坏形式、延性特征等进行了分析,并与其他结构钢材的力学性能进行了对比。结果表明,低屈服点钢在循环荷载作用下有明显循环强化现象,塑性变形能力强,且与普通钢材相比延性及耗能能力突出。该试验结果为后续研究低屈服点钢本构模型提供基础。     

Damage analysis for structural integrity and durability of composite materials

Composite structures for mechanical and aerospace applications are designed to retain structural integrity and remain durable for the intended service life. Since the early 1970s important advances have been made in characterizing and modelling the underlying mechanical behaviour and developing tools and methodologies for predicting fracture and fatigue of composite materials. This paper presents a review of the concepts and analyses related to this area, and illustrates these by a few examples. The topics discussed are composite material strength in tension, compression and shear, damage and its progression in monotonic and cyclic loading, fatigue life prediction and damage induced changes in visco‐elastic response.     

Response of boron carbide subjected to high-velocity impact

This article presents an evaluation of the response of boron carbide (B₄C) subjected to impact loading under three different conditions. Condition A is produced by plate-impact experiments where the loading condition is uniaxial strain and the stresses and pressures are high. Under plate-impact loading the material fails at the Hugoniot Elastic Limit (HEL) and the failed material undergoes high confining pressures and relatively small inelastic strains. Condition B is produced by projectile impact onto thick targets where the stresses and pressures are dependent on impact velocity, but they are generally lower than those from plate impact. Under thick-target impact/penetration most of the material fails under compression, the inelastic strains are large and the material appears to exhibit more ductility than under condition A. Lastly, condition C is produced by projectile impact and perforation of thin targets where the stresses and pressures are a combination of compression and tension. Under thin-target perforation the material fails in both tension and compression. The Johnson–Holmquist–Beissel (JHB) constitutive model is used to evaluate the material behavior for each of the three conditions, but it is not possible to accurately reproduce the experimental results of the three conditions with a single set of constants. Instead, three different sets of constants are required to accurately model the three impact conditions. These three models/constants are used to provide insight into the complex response of B₄C, and to identify possible mechanisms that are not included in the JHB model.     

FRP-confined concrete under axial cyclic compression

One important application of fiber reinforced polymer (FRP) composites in construction is as FRP jackets to confine concrete in the seismic retrofit of reinforced concrete (RC) structures, as FRP confinement can enhance both the compressive strength and ultimate strain of concrete. For the safe and economic design of FRP jackets, the stress–strain behavior of FRP-confined concrete under cyclic compression needs to be properly understood and modeled. This paper presents the results of an experimental study on the behavior of FRP-confined concrete under cyclic compression. Test results obtained from CFRP-wrapped concrete cylinders are presented and examined, which allows a number of significant conclusions to be drawn, including the existence of an envelope curve and the cumulative effect of loading cycles. The results are also compared with two existing stress–strain models for FRP-confined concrete, one for monotonic loading and another one for cyclic loading. The monotonic stress–strain model of Lam and Teng is shown to be able to provide accurate predictions of the envelope curve, but the only existing cyclic stress–strain model is shown to require improvement.     

双轴受压状态下的高延性纤维增强水泥基复合材料本构模型

基于Darwin和Pecknold考虑混凝土双轴力学行为的方法,建立一个同时考虑双轴受压状态下非线性力学行为和抗压强度变化的高延性纤维增强水泥基复合材料(ECC)二维正交各向异性本构模型。在因双轴加载而产生的正交各向异性的2个方向上引入等效单轴应变,建立非线性应力-等效单轴应变关系以考虑ECC的双轴非线性行为,并采用一条双轴强度包络线确定2个方向上的抗压强度。推导模型的显式数值算法,编写包含该算法的用户自定义材料子程序UMAT,并嵌于有限元计算程序ABAQUS v6.14中。通过对两组不同配合比的ECC试件在不同应力比下的双轴受压加载试验进行数值分析验证本模型的有效性。数值计算得到的主压应力方向上的应力-应变曲线及预测的抗压强度与试验结果吻合较好,表明该文提出的本构模型能够有效地预测ECC在双轴受压状态下的非线性力学行为和破坏强度。     

结构钢材循环荷载下的本构模型研究

在抗震分析中,钢材在循环荷载下的一维本构模型是结构抗震设计时进行结构弹塑性地震响应分析的基础.为了更为准确地模拟结构的地震反应,并能够在实际工程中应用,该文提出了结构钢材Q235B、Q345B 在循环荷载下的单轴简化本构模型,其中包括:单调加载曲线、循环骨架曲线以及滞回准则,通过建立数学模型对钢材循环荷载作用下的反应进行描述.根据提出的模型并基于大型通用有限元软件ABAQUS 提供的用户子程序接口UMAT,开发了适用于结构分析的钢材单轴循环本构模型.通过与多种加载制度下钢材反应的试验数据进行对比,进而证明该文所提出模型的正确性以及适用性,保证其用于钢结构体系在地震作用下弹塑性时程分析时的精度和可行性.分析结果表明:钢材在循环荷载下的反应与在单调荷载下的反应有很大的差别,循环荷载下的骨架曲线对于准确的数值模拟起到重要作用;钢材在循环荷载下的滞回准则也与现通用有限元软件中的本构模型有较大区别,这对于抗震计算设计的准确性有一定的影响.     

循环荷载下奥氏体型和双相型不锈钢材料本构关系研究

为研究循环荷载下不锈钢材料的本构关系,对奥氏体型S30408不锈钢和双相型S220503不锈钢材料进行了单调拉伸和大应变超低周循环加载试验。采用三种常用的单调拉伸本构模型对所得应力-应变曲线进行拟合,得到相应单调荷载下材料本构参数;采用Ramberg-Osgood本构模型对循环骨架曲线进行拟合,得到材料循环强化参数;利用Chaboche塑性本构模型,标定了两种材料的循环本构参数。结果表明:在单调拉伸荷载下,G-R-O本构模型更适用于拟合不锈钢材料的单调拉伸本构;在循环荷载下,不锈钢材料滞回曲线饱满,且随着应变增大,两种材料在加载后期均表现出了明显的循环强化现象;Ramberg-Osgood本构模型对骨架曲线拟合较好,有限元计算结果和试验滞回曲线吻合度高;表明该文标定出的强化参数、循环本构参数可用于结构体系地震响应分析之中,为准确分析不锈钢结构在地震作用下的受力性能提供参考。     

HRB600钢筋屈曲受力性能试验研究

HRB600高强钢筋已纳入我国钢筋产品国家标准,并商业化生产,但关于其非弹性屈曲受力性能、考虑屈曲的低周疲劳损伤模型的研究成果很少。对于HRB600高强钢筋的原状试件,该文完成了10个单调受压屈曲试验、16个考虑屈曲的循环拉压试验,测量了屈曲钢筋的平均应力-平均应变曲线和跨中横向位移,分析了非弹性屈曲对HRB600钢筋受压强度退化的影响,以及屈曲方向对试件强度退化、疲劳损伤的影响,提出了适用于HRB600高强钢筋、可考虑屈曲效应和不同加载方式影响的修正低周疲劳损伤模型。研究结果表明:单调受压屈曲时HRB600钢筋屈曲方向稳定,随长径比增大钢筋屈曲后强度退化加快;循环加载时HRB600钢筋屈曲方向不稳定,导致局部塑性应变集中和疲劳损伤对钢筋屈曲后强度退化的不利影响减轻;传统低周疲劳损伤模型明显低估了HRB600钢筋屈曲后的疲劳损伤,修正疲劳损伤模型能合理地考虑屈曲对HRB600钢筋低周疲劳损伤和断裂的影响。     

A notch strain calculation of a notched specimen under axial-torsion loadings

In this paper, a notch analysis model is presented for the numerical prediction of multiaxial strains of a notched 1070 steel specimen under combined axial and torsion loadings. The proposed model is based on the notion of a structural yield surface and uses a small-strain cyclic plasticity model to describe stress–strain relations. A notch load–strain curve is calculated with Neuber’s rule and incremental nonlinear finite element analysis. The presented model is applied to simulate the notch root deformations of a circumferentially notched specimen under cyclic tension–compression–torsion loading histories. The model predictions are evaluated with strain measurements at the notch root of the specimen in a comprehensive set of cyclic tests. The computed strain loops were in accord with experimental data and matched qualitatively with measured shear–axial strain histories irrespective of loading path of the test. In proportional balanced torsion-axial loading, the nonlinear shear strain–axial strain loops were calculated properly. The modeling errors were determined to be a function of the loading path shape, and compared to shear strains, axial strain predictions were more accurate.     

Effect of silicon carbide particulate on cyclic plastic strain response characteristics and fracture of aluminium alloy composites

The cyclic stress-response characteristics of powder-metallurgy-processed high-purity aluminium alloy 2124 discontinuously reinforced with varying volume fractions of silicon carbide particulates were studied over a range of plastic strains. The specimens were cycled using tension/compression loading under total strain control. The composite material, in the heat-treated condition, displayed cyclic hardening at all cyclic strain amplitudes and for different volume fractions of the ceramic reinforcement in the aluminium alloy matrix. The degree of hardening was observed to be greater at the higher cyclic strain amplitudes than at corresponding lower strain amplitudes. Micromechanisms controlling the hardening response during cyclic straining are highlighted and rationale for the observed hardening behaviour is attributed to concurrent and competing influences of an increase in dislocation-dislocation interaction, dislocation multiplication and dislocation-particle interactions, and is a mechanical effect. The kinetics of the cyclic fracture process of the composite alloy is discussed in light of composite microstructural effects, plastic strain amplitude and concomitant response stress.     

ECC材料力学性能与本构关系研究进展

混凝土在国内外应用广泛,但普通混凝土材料存在抗拉强度低、韧性差和脆性特征明显等缺点。自20世纪90年代采用性能驱动设计方法(PDDA)成功配制工程水泥基复合材料(ECC)后,仅在几年时间里,ECC材料受到了研究者的广泛关注。相比普通混凝土,采用PDDA得到的ECC的外掺纤维与基体界面有良好的粘结作用,这使得ECC材料具有应变硬化和多缝开展等重要特征。由于ECC优异的力学性能,使用其替代混凝土便成为解决混凝土脆性、裂缝开展等相关问题的一种有效的新途径。 然而, ECC的制备极不容易,由于基体胶凝材料产地不同或者纤维种类不同,某一地区配制成功的配合比大多无法适用于其他地区。因此,根据当地情况进行ECC材料的配合比设计仍然是各国学者青睐的课题。一方面,欲使用ECC代替混凝土用于建筑结构等,就必定要深入研究ECC材料层面的基本力学性能。建立ECC的本构模型对ECC构件甚至结构层面的研究都十分重要,但相关研究较少。另一方面,由于ECC材料良好的裂缝控制能力,国内外学者也致力于使用ECC材料进行结构加固修复的研究。 各国学者先后成功配制极限拉应变大于3%的ECC,这为进一步广泛开展ECC的研究和应用创造了很好的条件,用于配制ECC的纤维种类也更加丰富。在ECC拉伸、压缩、弯曲和剪切等大量的材料试验研究基础上,近几年,一些科研团队开始尝试用ECC部分甚至完全代替混凝土来浇筑梁、柱等构件,然后进行ECC构件层次的力学性能和耐久性等研究;另有部分研究人员也致力于建立ECC的本构模型,开展数值模拟分析。此外,由于ECC优异的力学性能,也有学者提出可以采用3D打印技术来建造无筋ECC建筑。 本文从ECC材料层面的单轴单调拉压力学性能、单轴循环拉压滞回性能、多轴力学性能及破坏准则、ECC与普通钢筋和纤维增强塑料(FRP)筋两种筋材的粘结性能方面进行综述,相应介绍了几种本构模型并简要对其进行评价,以期为用ECC代替混凝土进行建筑结构设计、选取本构模型进行数值模拟分析、编制规范和技术规程等提供参考。最后,对今后进一步开展ECC力学性能研究、建立本构模型提出展望。     

Post-forming monotonic and cyclic behavior in a HSLA steel sheet after large deformations by in-plane compression

The effects of large prestrains (18–40%), produced by in-plane compression, on the asymmetry and the anisotropy of the stress response and on the fatigue life are investigated under fully reversed axial strain for a 345 MPa yield strength V–N high strength low alloy steel sheet. After prestraining, the hysteresis loops are asymmetric and the stress response is anisotropic, i.e., the response differs in directions parallel and perpendicular to that of the compressive prestrain. To understand the cyclic flow stress asymmetry, monotonic tension and compression tests were conducted in these two directions after prestraining. It is shown that the loop asymmetry is related to the Bauschinger effect after prestraining. Two cyclic stress strain curves, one corresponding to the tension side of the hysteresis loops and the other to the compression side, are defined to accurately describe the post-prestraining behavior. The amount of strengthening gained by prestraining is partially retained after cycling. Prestraining increases the fatigue life at low strain amplitudes but decreases it at high strain amplitudes.