Debonding initiation in a NiTi shape memory wire–epoxy matrix composite. Influence of martensitic transformation

The effect of martensitic transformation on the debonding initiation in single shape memory NiTi wire–epoxy matrix composite was studied. Three different heat treatment cycles were performed in order to obtain wires with different transformation characteristics. The composite samples were prepared using NiTi wires and steel wire for comparison. The single-wire specimens were subjected to the pull-out test. The in situ observations of the interfacial debonding and sliding behaviour during the test were carried out.     

钢丝与混凝土粘结拔出行为的试验与模拟

该文首先建立了钢丝从混凝土基材中拔出的试验方法,并试验测定了不同锚固长度的钢丝拉拔行为,获得了拉拔荷载与拔出端位移之间的定量关系。在此基础上,建立了基于细观力学分析的钢丝拔出模型。通过模型计算,获得了钢丝与混凝土之间的界面粘结参数,并计算分析了锚固长度、倾斜角度等因素对钢丝拔出行为、最大承载力的影响。试验与模型计算结果表明,钢丝在混凝土中的拔出行为可分为脱粘和拔出两个阶段。在脱粘阶段,钢丝与混凝土之间的粘结强度可视为常数;在拔出阶段,钢丝与混凝土之间的粘结强度随拔出位移首先快速减小,之后减小速率逐渐降低,并趋于恒定值。对相同养护龄期,钢丝锚固长度越长,钢丝极限拔出荷载越大,同时与最大拔出力相对应的拔出位移也逐渐增大。钢丝埋置深度和角度相同时,养护龄期越长,钢丝抗拔能力增强。钢丝拔出方向与锚固方向不一致时,钢丝拔出荷载将增大,钢丝拔出方向与埋入方向夹角越大,抗拔极限承载力越大。模型与实验结果对比表明,所建模型及其所获得的粘结参数可良好地预测不同锚固长度的钢丝从混凝土中的拔出行为。     

The debonding and pull-out of ductile wires from a brittle matrix

An experimental investigation into the debonding and pull-out of nickel wires from epoxy resin and cement paste matrices has been carried out. Above a critical embedded length both the debonding and pull-out stresses attain limiting values. A theory based on the model of a yielded zone travelling up the wire behind a debonding front was shown to describe the observed dependence of the limiting debonding stress on the yield stress, diameter and surface roughness of the wire. Pull-out behaviour subsequent to debonding was explained using this model in terms of an unyielded plug at the end of the wire. Orientation of the wire to the loading direction was found to raise the limiting debonding and pull-out stresses due to enhanced friction at the wire exit point.     

Analysis of pull-out tests on fibres embedded in brittle matrices

A graphical interpretation and analysis of pull-out tests is developed to indicate combined effects of factors such as the tensile strength of the fibre and the bond characteristics of the fibre-matrix interfaces on the mode of pull-out failure. In addition to the critical embedment length the significance of two other embedment lengths is shown. One is the minimum length at which a tensile failure of a reinforcing element is not preceded by debonding and the other one is the maximum embedment length at which a complete debonding occurs instantaneously. The graphical analysis is applied to a typical pull-out test result and quantitative values of the ultimate elastic shear bond and the frictional bond are obtained.     

Analysis of the effect of embedded fibre length on fibre debonding and pull-out from an elastic matrix

Experimental results obtained from single fibre pull-out tests on specimens with different fibre embedment lengths, consisting of a brass-coated steel wire as fibre and a cementitious mortar as matrix, are analysed using the appropriate theories reviewed in the first part of this paper. The analyses indicate that both adhesional bonding and frictional resistance to slipping along the portion of the interface over which the adhesional bond has failed contribute significantly to the total resistance to completion of fibre debonding and initiation of fibre pull-out in these specimens. Estimated values of the adhesional (maximum) interfacial bond shear strength and the frictional resistance to slipping obtained from the apparent variation of maximum pull-out load with embedded fibre length are compared and found, for theories which are similar, to be generally in agreement.     

基于内聚力模型的形状记忆合金短纤维增强树脂基复合材料的模拟分析

通过单纤维拔出实验和单轴拉伸实验, 测定了形状记忆合金(SMA)增强树脂基复合材料的界面脱粘剪切强度和单向随机分布SMA短纤维增强复合材料的拉伸强度。根据蒙特卡罗法和边界条件控制方程, 编写了适于软件调用的单向随机分布短纤维增强复合材料的APDL语言生成程序, 建立数值模拟模型。基于指数型内聚力模型, 对SMA纤维与环氧树脂基体界面分离(即界面脱粘)过程进行了有限元模拟。结果表明: 相同纤维体积分数下, 随着纤维长细比的减小, 复合材料整体弹性模量逐渐降低; 温度驱使SMA纤维弹性模量发生变化, 可以有效提高复合材料整体弹性模量。     

考虑加/卸载速率影响的Ti-Ni形状记忆合金简化本构模型

系统研究了Ti-Ni形状记忆合金丝(SMA)应力-应变曲线、特征点应力、耗能能力、等效阻尼比随材料直径、应变幅值、加载速率、加载循环次数的变化规律;针对SMA唯象Brinson本构模型无法描述SMA动态力学性能的缺点,结合前述试验结果,提出了一种可考虑加/卸载速率影响的SMA简化本构模型。应用该模型对试验用SMA丝进行模拟,所得应力-应变曲线各特征点平均误差仅为3%,结果表明:所建立的速率相关SMA简化本构模型可较为精确地描述SMA在应力诱发相变过程中的超弹性力学行为,同时可反映加/卸载速率和应变幅值等主要因素对其动力本构模型的影响;该模型结构形式简单,具有较好的工程应用前景。     

Experimental investigation on the debonding strength in shape memory alloy wire reinforced polymers

The interfacial shear strength between the shape memory alloy (SMA) wire and epoxy matrix was evaluated experimentally using a single wire pull-out test. Moreover, the effect of pre-strain in SMA wires on the interfacial behavior was studied by pre-straining the SMA wires to 2% and 4% pre-strains. Experiments were conducted in both martensite and austenite phases of SMA. Results showed that pre-straining SMA wire in the martensite phase caused enhancement in interfacial shear strength due to recovery force generation. Further, 9.7% and 33% improvements in the interfacial shear strength were achieved at 2% and 4% of SMA pre-strain, respectively. However, the enhancement of interface behavior did not occur, when the SMA wires were subjected to pre-strain in the austenite phase.     

SMA丝表面纳米SiO_2改性对SMA/环氧树脂复合材料界面粘结强度的影响

为了研究形状记忆合金(SMA)丝增强环氧树脂复合材料的界面粘结行为,首先通过单纤维拔出试验测定了SMA/环氧树脂界面的粘结强度,重点考察了埋入深度对界面极限粘结强度及其拔出行为的影响。然后,结合ABAQUS有限元分析方法,利用基于表面内聚力行为的单元对SMA丝拔出过程中应力分布随拔出时间的变化关系进行了模拟。最后,针对SMA/环氧树脂复合材料界面粘结强度较弱的缺陷,提出了利用纳米SiO2改性SMA丝表面提升材料界面粘结强度的方法,并通过拔出试验进行了验证。结果表明:随着埋入深度从1.0cm增加到1.5cm和2.0cm,最大拔出载荷显著增加,平均界面粘结强度却逐渐下降。当纤维埋入深度为2.0cm时,在0.300s时临界脱粘出现。利用在SMA表面涂覆纳米SiO2颗粒的方法可以增加纤维的表面粗糙度,进而有效提高SMA丝增强环氧树脂复合材料的临界拔出强度。研究结论为SMA丝在实际工程领域中的应用提供了理论指导。     

An enhanced FEM model for particle size dependent flow strengthening and interface damage in particle reinforced metal matrix composites

By incorporating the dislocation punched zone model, the Taylor-based nonlocal theory of plasticity, and the cohesive zone model into the axisymmetric unit cell model, an enhanced FEM model is proposed in this paper to investigate the particle size dependent flow strengthening and interface damage in the particle reinforced metal matrix composites. The dislocation punched zone around a particle in the composite matrix is defined to consider the effect of geometrically necessary dislocations developed through a mismatch in the coefficients of the thermal expansion. The Taylor-based nonlocal theory of plasticity is applied to account for the effect of plastic strain gradient which produces geometrically necessary dislocations due to the geometrical mismatch between the matrix and the particle. The cohesive zone model is used to consider the effect of interfacial debonding. Lloyd’s experimental data are used to verify this enhanced FEM model. In order to demonstrate flow strengthening mechanisms of the present model, we present the computational results of other different models and evaluate the strengthening effects of those models by comparison. Finally, the limitations of present model are pointed out for further development.     

Study of the martensitic transformation in NiTi–epoxy smart composite and its effect on the overall behavior

In this work, the effect of wire phase transformation on the overall thermo-mechanical behavior of NiTi–epoxy composites has been investigated. The shape memory wire received in as drawn condition was subjected to three heat treatments which results to different transformation characteristics. Composite specimens were manufactured by casting followed by curing and post curing process. The mechanical behavior of samples has been determined using standard tensile test. The effect of wire volume fraction and test temperature was investigated as well.It is found that the martensitic transformation occurring in the wire affects the mechanical behavior of the composite specimens. In this way, using the wire with higher transformation stress improves the composite tensile strength. This is achieved either by increasing the test temperature or by using the wires heat treated at lower temperatures. From the experimental results, the martensitic transformation can change the debonding mode. It seems that on the constraint of matrix, the transformation occurs simultaneously at several points in wires that result in regular debonded/undebonded pattern.     

Pull-out of short rods and fibres

An experimental study has been carried out of the mechanics of pull-out of short nylon rods embedded in a rubber block, considered as a model of fibres embedded in an elastic resin. Two stages of pull-out were observed. First, rubber became detached from the base of the rod creating a large internal cavity. This process was apparently initiated by internal rupture of rubber under the rod, and occurred when the local triaxial tension reached a value similar to the tensile (Young's) modulus of the rubber. After cavitation, a cylindrical debond propagated up the rod, starting at the embedded end and ending in complete pull-out. Pull-out forces were found to be consistent with a simple fracture mechanics treatment based on the elastic compliance of partially debonded specimens, both measured and calculated by finite element analysis. The fracture energy for debonding is deduced to be about 300 J m^–2. The effects of varying rod diameter and depth of embedment are shown to be in satisfactory agreement with theory.     

Improved adhesion between nickel–titanium SMA and polymer matrix via acid treatment and nano-silica particles coating

Shape memory alloy (SMA) composites are the desirable candidate for smart materials that used in intelligent structures. However, the overall mechanical performance of SMA composites depends immensely on the quality of the interaction between SMA and polymer matrix. Therefore, it is necessary to find out an approach to enhance the interfacial property of this composite. In this paper, we modified nickel–titanium SMA wire with nano-silica particles before and after acid treatment. The modification effect on the interfacial strength between SMA and epoxy resin was evaluated. Contact angle analysis, scanning electron microscopy (SEM) observation, and single fiber pull-out test were carried out. The bonding characteristics between modified wire and liquid/cured resin were investigated. We then embedded SMA wire into woven glass fabric/epoxy composite laminates, and manufactured this hybrid composites via vacuum assisted resin transfer molding processing. Three-point-bending test of the hybrid composites was performed to validate the modification effect. Fiber pull-out experiment demonstrates that the interfacial shear strength increases by 6.48% by nano-silica particles coating, while it increases by 52.21% after 8 h acid treatment and nano-silica particles coating simultaneously. For hybrid composites, flexural strength of the two specimens increases by 19.8 and 48.2%, respectively. In SEM observation, we observed large debonding region in unmodified composites, while interfacial adhesion between modified wire and epoxy keeps strong after flexural damage.     

Prediction of interfacial fracture between concrete and fiber reinforced polymer (FRP) by using cohesive zone modeling

Interfacial debonding between concrete and fiber reinforced polymer (FRP) is investigated through integrating experiments and computations. An experimental program is designed to evaluate interfacial fracture parameters of mode-I through cutting and bonding specimens with an FRP sheet. The evaluated fracture parameters, i.e. the fracture energy and the bonding strength, are confirmed by predicting FRP debonding failure with the cohesive zone modeling approach. In the cohesive zone model, a traction-separation relation for FRP debonding is proposed with a shape index while providing various initial descending slopes. Computational results of the cohesive zone model agree well with three-point bending test results for both FRP debonding and plain concrete fracture. Furthermore, both experimental and computational results demonstrate that the fracture energy and the cohesive strength are essential fracture parameters for the prediction of FRP debonding behavior.     

Finite element modeling of cyclic thermal response of shape memory alloy wires with variable material properties

This paper considers the influence of variable thermal conductivity and electrical resistivity on the cyclic thermal response of a shape memory alloy (SMA) wire. The specific boundary value problem of an SMA wire under zero-stress conditions is considered, wherein the wire, in an initially martensitic state, is heated by electrical current and cooled by convection. An isothermal boundary condition is considered at the ends of the wire. The heating is continued until the transformation from martensite to austenite has essentially taken place. A Galerkin finite element method (FEM)-based numerical approach is used to solve the boundary value problem. The two key parameters of interest are the maximum possible transformation achievable (i.e. extent of actuation) and the time period for one cycle. It is seen that for short wires, the assumption of constant thermal conductivity underestimates the actuation and overestimates the cycle time significantly. The assumption of a constant electrical resistivity does not affect the actuation but significantly overestimates the cycle time. For long wires, the thermal conductivity effect is diminished whereas the electrical resistivity effect is very similar to that for the short wires.     

超弹性SMA复合阻尼器的计算模型及参数分析

介绍了超弹性SMA复合阻尼器的构造及工作原理,并根据它的受力特性,建立了该阻尼器的力学计算模型,针对模型中的关键参数:阻尼器内滑条槽深与接触面摩擦系数、SMA丝的预应变及缠绕量、环境温度及位移幅值等对阻尼器力学性能的影响规律进行了分析。分析结果表明:阻尼器的初始刚度、输出力和滞回面积随内滑条槽深、SMA丝缠绕量、接触面的摩擦系数的增加而增加;在不同的位移幅值下,阻尼器的荷载-位移曲线形状分别呈三角形、四边形和五边形等多边形形式,表现为变刚度的特性;阻尼器的屈服位移和输出力随环境温度的升高而线性增加;当对SMA丝施加一定的初始预应变,SMA复合阻尼器表现为摩擦型阻尼器。最后,将模型结果与实验结果进行了对比,两者基本吻合。     

Freeze-thaw degradation of FRP-concrete interface: Impact on cohesive fracture response

Results from an experimental investigation into the influence of freeze-thaw action on the FRP-concrete interface fracture properties are presented. The FRP-concrete bond behavior is investigated using a direct shear test. The cohesive stress transfer between FRP and concrete during debonding is determined from spatially continuous measurements of surface strains obtained at different stages of the debonding load response. The non-linear material law for the interface shear fracture, which provides a relation between the interface shear stress as a function of relative slip between the FRP and concrete, is established for specimens subjected to different levels of damage associated with freezing and thawing action. The influence of freeze-thaw action on the cohesive stress transfer during crack propagation, and on the cohesive interface fracture parameters is evaluated using a statistical hypothesis testing method. A larger percentage decrease in the interface fracture energy due to freeze-thaw cycles compared to the corresponding decrease in the ultimate nominal stress at debonding was noted. A decrease in the length of the cohesive stress transfer zone and the maximum interface cohesive stress were also observed with freeze-thaw cycling.     

A parametric study on residual stresses and loads in drawing process with idle rolls

A comprehensive study of drawing process with flat idle rolls of round wires is presented through 3D mechanical finite element simulations. An elastic–plastic model is used for the wire material and contact behavior is simulated by a sliding–sticking friction model. The results of numerical simulations are compared with measurements on wires produced with a laboratory equipment. The comparison of drawing load and some geometrical characteristics of experimental samples with numerical model predictions allowed to establish a good correspondence of model with experimental findings, thus validating the numerical model. Residual stress of flat roll drawn wires, pressure distribution on the forming rolls and drawing load are studied. The effects of main process parameters such as initial workpiece diameter, forming rolls diameter and percentage of deformation are investigated. The results present a helpful insight into the process parameters effect in wire drawing with flat idle rolls thus furnishing the basic guidelines for process design and optimization.     

基于超弹性特性的SMA复合材料层合板低速冲击损伤数值模拟方法

基于ABAQUS有限元软件结合VC++6.0程序设计,建立了含不同铺层角度、不同排列密度形状记忆合金(SMA)纤维的复合材料层合板有限元模型。将基于Brinson本构模型的SMA分段线性超弹性模型以及判断复合材料层内失效的三维HASHIN失效准则编译至ABAQUS/VUMAT子程序,使用界面单元模拟复合材料层间区域,建立了SMA复合材料层合板的低速冲击损伤及冲击后剩余强度数值模拟方法。对比了不含SMA纤维层合板、含SMA纤维层合板、含普通金属丝层合板在不同冲击能量下的损伤响应。进一步分析了SMA纤维体积分数和直径变化对冲击响应的影响。冲击后剩余压缩强度模拟结果表明:冲击能量为16J时,含体积分数25%、直径0.5mm的SMA纤维层合板的冲击后剩余压缩强度相比不含SMA纤维层合板提高5.78%、相比含普通金属丝层合板提高4.69%。随着SMA纤维体积分数提高,层合板的抗低速冲击能力增强,当体积分数一定时,较细的(0.3mm)SMA纤维比粗的(0.6mm)SMA纤维对层合板的抗低速冲击能力增强效果更好。