Wire Stress Research Articles

Stress and strain states in a pseudoelastic wire subjected to bending rotation

Bending rotation is a load case where an originally straight wire is bent into a curved shape and subsequently forced to rotate around its longitudinal axis. Continuous bending rotation results in tension–compression cycles and is used to assess the fatigue resistance of pseudoelastic shape memory wires for medical applications. It is important to discuss fatigue on the basis of the underlying cyclic stress and strain states. To our knowledge, all previous studies on pseudoelastic bending rotation estimated the strain amplitudes by a simplified mechanical model (i.e. pure bending). In this study, a bilinear material model for pseudoelasticity is used to analytically derive a more accurate mechanical description of the stress and strain distributions during pseudoelastic bending rotation. We use numerical calculations to show that the stress and strain states, and other characteristic parameters, differ significantly between pure bending and bending rotation of pseudoelastic wires.

Increase of Burst Pressure and Optimum Design for CFRP Pressure Vessels Reinforced with SMA Wire

CFRP pressure vessels are now widely used in compressed natural gas vehicles (CNGV) and have a possibility to be employed in fuel cell vehicles (FCV) for highly compressed Hydrogen. In order to increase a driving distance in CNGV and FCV, lighter vessels proofed against higher pressure are required. In the case of internal pressure, the tensile circumferential stress at the inner radius should be decreased in order to increase the burst pressure. This paper presents a method how to increase the burst pressure of CFRP vessels. For increasing the burst pressure of CFRP vessels, the shape memory alloy (SMA) wire memorized a compressive stress is wound around the outer surface of cylindrical part of the CFRP vessel. When the internal pressure acts on the CFRP vessel, its tensile circumferential stress can be reduced by being received the compressive stress of the SMA wire and the experimental results of increasing the burst pressure are shown. The experimental results are compared with numerical ones and they agree well each other. Furthermore, optimum design of CFRP pressure vessels by the change of winding method of SMA wire is numerically obtained.

Influence of Mean Stress on Contact Wire Fatigue

When a pantograph passes underneath a contact wire, the upward force causes a bending stress in the contact wire. If this stress is high and occurs frequently, there is a possibility that the contact wire will suffer a fatigue fracture. To estimate the fatigue life of a contact wire, we have been using the S-N curve prepared for a mean stress of 88.2 MPa. In addition, a contact wire is gradually worn down by running contact strips so that the mean stress governing its fatigue life gradually increases. To estimate the fatigue life of a worn contact wire correctly, therefore, the influence of the mean stress on contact wire fatigue needs to be examined. For this paper, we carried out fatigue tests on GT110 harddrawn copper contact wire at a mean stress of 154.2 MPa to obtain another S-N curve. Then, by using parameters that included the mean stress, we prepared a fatigue curve that integrates S-N curves for any mean stress. Finally, we estimated contact wire fatigue damage using this fatigue curve and proposed a new method to establish the allowable bending strain amplitude.

Influence of Strain Ratio on Bending Fatigue Life and Fatigue Crack Growth in TiNi Shape-Memory Alloy Thin Wires

The influence of strain ratio on bending fatigue properties of TiNi shape-memory alloy thin wires and the process of fatigue crack propagation were investigated. The results obtained are summarized as follows. (1) The martensitic transformation stress of a superelastic thin wire is higher than that of a shape memory wire, resulting in shorter fatigue life of the superelastic wire. The maximum bending strain of fatigue limit is the martensitic-transformation starting strain. (2) The plane-bending fatigue life curve is expressed by a power function of maximum strain max and the number of cycles to failure. The smaller the strain ratio, the shorter the fatigue life. (3) In both rotating bending and plane bending, fatigue cracks nucleate on the surface of the wire. One fatigue crack grows preferentially and the fatigue-crack propagated region of fracture surface is fan-shaped. (4) If max is larger than 1%, max during the rotating-bending fatigue test becomes a little smaller than that of the initial value. (5) The fatigue crack length can be estimated by measuring increase in electric resistance based on decrease in cross-sectional area due to fatigue crack propagation. (6) The fatigue crack length of the notched wire is expressed by a power function of the number of cycles.

Bending Fatigue Properties of TiNi Shape-Memory Alloy Thin Wires

The influence of strain ratio on bending fatigue properties of TiNi shape-memory alloy thin wires and the process of fatigue crack growth were investigated. The results obtained are summarized as follows. (1) The martensitic transformation stress of a superelastic thin wire is higher than that of a shape memory wire, resulting in short fatigue life of the superelastic wire. The maximum bending strain of fatigue limit is the martensitic-transformation starting strain. (2) The plane-bending low-cycle fatigue life curve is expressed by a power function between maximum strain emax and the number of cycles to failure. The smaller the strain ratio, the shorter the fatigue life. (3) In plane bending, fatigue crack initiates on the surface of the wire. One fatigue crack grows preferentially and the fatigue-crack propagated region is fan-shaped. (4) If emax is larger than 1%, emax becomes a little smaller than that of the initial value during the rotating-bending fatigue test. (5) The fatigue crack length of the notched wire is expressed by a power function of the number of cycles.

Characterizing the Wave Environment in the Fatigue Analysis of Flexible Risers

As the offshore industry moves towards deeper water developments and continues to embrace harsh environments, unbonded flexible pipes are increasingly being utilized as a cost effective riser solution. Furthermore, with the advent of issues such as nonpristine annuli environments, the fatigue performance of these flexible risers is becoming a critical issue. This paper presents an overview of the comparisons between deterministic and stochastic global fatigue analysis techniques. Methods used to perform both deterministic and stochastic analyses are outlined, from performing the global analyses to using local models to generate armor wire stresses and subsequent fatigue damage. The paper identifies the key issues in the analysis performed and presents key results and conclusions with regard to the characterization of the wave environment in the global fatigue analysis of flexible risers.

Shape Memory Alloy Wire Reinforced Composites for Structural Damage Repairs

Shape memory alloys (SMA) have demonstrated their potential use in various smart structural applications. SMA undergo a reversible phase transformation from martensite to austenite as temperature increases. This transformation leads to shape recovery and to the associated recovery strains. SMA can also be used to enhance the capacity of a damaged structure, especially adhesively bonded joints. One approach is to use SMA-reinforced patches for enhancing adhesively bonded joints. To this end a design strategy, by which one can integrate the properties of SMA reinforcement to improve the interlaminar stresses of the bonded joint must be developed. Therefore, an analytical solution for evaluation of the stresses in a SMA wire reinforced composite patch used for repairing cracks in banded joints was created. The variables considered in the model are the fraction of SMA wires, the associated phase transformation strain, patch thickness, and adhesive layer's thickness and mechanical properties. A finite element analysis (FEA) was also conducted to verify the integrity of the results obtained through the proposed solution, and good agreement was obtained.

Calculation of the rolling pressure distribution and force in wire flat rolling process

In this study a slab analysis is developed to predict the rolling pressure distribution and rolling force in the wire flat rolling process. In the slab analysis the variations of width of contact area between the rolls and wire during rolling is considered. Two differential equations are derived and solved using a modified Eulerian numerical method. The effects of friction coefficient, reduction in height and yield stress of wire on the rolling pressure distribution and rolling force are assessed. It is found that in the flat rolling of wire, there exists a maximum in the pressure distribution similar to that of the strip rolling process. Also, the effects of friction coefficient, reduction in height and yield stress on the neutral angle are investigated. It is observed that by increasing the reduction in height and friction coefficient, the neutral angle is increased and the yield stress has no effect on the neutral angle. A good agreement is found between the calculated and measured rolling force.

Allowance for Internal Microstresses and Texture in Evaluation of Residual Stresses in Cold-Drawn Steel Wires with Micromechanical Models

Under consideration is an allowance for internal grain-scaled stresses in the evaluation of residual macroscopic stress in highly textured polycrystals. The grain-scaled stresses have been evaluated with two micromechanical models: (a) a simplified model developed for a very strong <110> fiber texture and suggesting the corresponding peculiarity of slip pattern in constitutive crystals, and (b) a full-constraint model allowing for accommodation nature of the multi-slip pattern in these crystals. Both models satisfactorily predict the position and relative intensity of the nonlinearities of d{hkl}-sin2y curve. The former model, by treating magnitude of grain-scaled stresses as a fitting parameter, provides better correspondence to experimental data. The latter model, free of fitting parameters, overestimates the grain-scaled stresses, and, consequently, the oscillations of d{hkl}-sin2y curve.

A Temperature-based Controller for a Shape Memory Alloy Actuator

This paper presents a robust nonlinear control that uses a state variable estimator for control of a single degree of freedom rotary manipulator actuated by shape memory alloy (SMA) wire. A model for SMA actuated manipulator is presented. The model includes nonlinear dynamics of the manipulator, a constitutive model of the shape memory alloy, and the electrical and heat transfer behavior of SMA wire. The current experimental setup allows for the measurement of only one state variable which is the angular position of the arm. Due to measurement difficulties, the other three state variables, arm angular velocity and SMA wire stress and temperature, cannot be directly measured. A model-based state estimator that works with noisy measurements is presented based on the extended Kalman filter (EKF). This estimator estimates the state vector at each time step and corrects its estimation based on the angular position measurements. The estimator is then used in a nonlinear and robust control algorithm based on variable structure control (VSC). The VSC algorithm is a control gain switching technique based on the arm angular position (and velocity) feedback and EKF estimated SMA wire stress and temperature. Using simulation it is shown that the state vector estimates help reduce or avoid the undesirable and inefficient overshoot problem in SMA one-way actuation control.

Development and experimental consideration of SMA/CFRP actuator for vibration control

One of the significant advantages of shape memory alloy (SMA) is its large recovery force generated by phase transition at specific temperature when initial deformation had been applied. Because of manufacturing and processing difficulties, SMA materials have been often used by the way of wires embedded in a variety of matrices when they are applied as actuators. Its great advantage, i.e. large generated force and displacement, raises, on the other hand, a problem at the SMA/matrix interface. The purpose of this paper is a proposition of a safety SMA actuator form with which no debonding occurs even when the maximum shear stress is generated by recovery force in SMA wires. Superior interfacial strength was obtained by the way of embedding SMA wires, memorized to sinusoidal curve shape, in the supported parts of the actuator. It provides holding stress over the fracture stress of SMA wires. An actuator form with eight SMA wires semi-embedded in two supporting parts made by CFRP was, thus, proposed. Besides, there exist again some difficulties to use SMA materials as active actuators, for example, its slow response speed. Particular driving system with cooling system is constructed in consideration with characteristics of SMA materials. The first and the second mode vibration suppressions of 2 m beam were carried out to evaluate the efficiency of the proposed actuator, mounted on one side closed to the clamping edge, as an active actuator, and the system. Control effect on beam structure is, then, demonstrated experimentally even for around 5 Hz.

Monitoring based maintenance utilizing actual stress sensory technology

In recent years, many infrastructures have been deteriorating. In order to maintainsustainability of those infrastructures which have significant influence on social lifelines,economical and rational maintenance management should be carried out to evaluate the lifecycle cost (LCC). The development of structural health monitoring systems, such asderiving evaluation techniques for the field structural condition of existing structures andidentification techniques for the significant engineering properties of new structures, can beconsidered as the first step in resolving the above problem. New innovative evaluationmethods need to be devised to identify the deterioration of infrastructures, e.g. steeltendons, cables in cable-stayed bridges and strands embedded in pre- or post-tensionedconcrete structures. One of the possible solutions that show ‘AtoE’ characteristics, i.e.,(a)ccuracy, (b)enefit, (c)ompendiousness, (d)urability and (e)ase of operation,elasto-magnetic (EM) actual stress sensory technology utilizing the sensitivity ofincremental magnetic permeability to stress change, has been developed. Numerousverification tests on various steel materials have been conducted. By comparingwith load cell, strain gage and other sensory technology measurement results,the actual stresses of steel tendons in a pre-stressed concrete structure at thefollowing stages have been thoroughly investigated: (i) pre-stress change due toset-loss (anchorage slippage) at the tendon fixation stage; (ii) pre-stress changedue to the tendon relaxation stage; (iii) concrete creep and shrinkage at the longterm pre-stressing stage; (iv) pre-stress change in the cyclic fatigue loading stage;and (v) pre-stress change due to the re-pre-stress setting stage. As the result ofthis testing, it is confirmed that EM sensory technology enables one to measureactual stress in steel wire, strands and steel bars precisely without destroying thepolyethylene covering sheath and enables one to provide adequate accuracy andreliability for monitoring actual stresses of those steel tendons during the life cycle ofinfrastructures. An example of a field application at a cable-stayed bridge is described.

Backstepping Control of a Shape Memory Alloy Actuated Robotic Arm

This paper investigates a nonlinear controller designed to stabilize a single-degree-of-freedom rotary shape memory alloy (SMA) actuated robotic arm. To this end, a bias-type robotic arm was built using 150 pm Flexinol SMA wire. This robot is designed to lift and position lightweight objects. Upon complete phase transformation, the SMA wire actuates the robot to rotate up to 1350. A linear spring is used to extend the wire to its original length because the SMA wire can only apply force in one direction. To measure the angular position of the robotic arm, an optical rotary encoder was used. To stabilize the robot, a model-based controller was developed. The controller incorporates the SMA actuated robot model with nonlinear control techniques. The model consists of three parts: the dynamics/kinematics of the arm, the thermoruechanical behavior of SMA wire, and the heat transfer model of the wire. The model-based backstepping controller determines the applied voltage to the SMA wire for positioning the arm at the desired angle by first calculating the wire's stress to stabilize the arm. The voltage to the SMA wire is then calculated based on the desired stress and the SMA's thermomechanical and heat transfer models. A series of simulations were performed to investigate stabilizing performance of the controller. Moreover, other issues such as robustness of the control design was evaluated. The results show that the control algorithms is able to globally and asymptotically stabilize the robot. The results further indicate that the sliding mode control has better robustness properties.

Tensile Straightening of Superfine Wire and Residual Stress Measurement Using Focused Ion Beam

Tensile Straightening of Superfine Wire and Residual Stress Measurement Using Focused Ion Beam

PLF-12: Tensile Straightening of Superfine Wire and Residual Stress Measurement Using Focused Ion Beam(PLF-II: PLASTIC FORMING AND ADVANCED PRODUCTS)

PLF-12: Tensile Straightening of Superfine Wire and Residual Stress Measurement Using Focused Ion Beam(PLF-II: PLASTIC FORMING AND ADVANCED PRODUCTS)

Influence of residual stresses in the stress relaxation of cold drawn wires

This study demonstrates the influence of residual stresses in steel wires on the stress relaxation losses. Standard stress relaxation tests were performed on four types of wire, all with the same mechanical properties but with different residual stresses. Surface residual stresses were measured by X-ray diffraction. The experimental results show that stress relaxation losses decrease as the value of surface tensile residual stresses decrease. The role, sometimes controversial, of initial pre-stretching and heat treatments on stress relaxation losses can also be understood in the light of the residual stresses induced during cold-drawing.

Bending actuation in polyurethanes with a symmetrically graded distribution of one-way shape memory alloy wires

Structures are being actuated by embedding shape memory alloy (SMA) wires into compliant materials, such as polyurethane. To achieve bending actuation, these wires are placed in opposing wire configurations, where multiple wires are often employed to enhance the amplitude of the bending actuation response. In this investigation, a procedure has been developed for fabricating polyurethanes with a symmetrically graded distribution of SMA wires. The effects of grading the distribution of one-way SMA wires have been characterized using full-field displacement deformation measurements obtained with the digital image correlation (DIC) technique. These measurements have been used in a one-dimensional (1D) model of bending actuation to determine the “equivalent two-way shape memory effect (SME)” of the graded wire distribution. To utilize the 1D actuation model, the constitutive properties of the polyurethane structure predicted by rule-of-mixture formulations were reduced to account for the differences in strain between the SMA wires and the polyurethane matrix. The graded wire distribution was also found to significantly stiffen the polyurethane structure. The level of equivalent two-way SME therefore became limited by the maximum recovery stress of the SMA wires, with a maximum level that was approximately 75% less than previously measured levels in an opposing wire configuration. However, the bending actuation behavior was more symmetric, and the actuated bending deflections were similar to those observed when using more compliant materials. It was also predicted that the symmetrically graded wire distribution would exhibit a better balance between actuation amplitude and uniformity, which combined with the more symmetric actuation behavior makes the graded wire distribution potentially more desirable for achieving higher actuation frequencies with distributed actuation concepts in new applications, such as miniaturized double diaphragm pumping devices.

Strength Anisotropy and Residual Stress in Drawn Pearlite Steel Wire

Strength anisotropy is found by tensile and compressive tests for a drawn pearlite steel wire, where the compressive tests are performed for prepared specimens along directions of 0, 45 and 90 degrees with respect to the drawing direction. The influence of annealing on such anisotropic strength is also examined. To make clear the origin of the strength anisotropy, texture and residual strain are measured using neutron diffraction. It is revealed that residual stresses in the ferrite and cementite phases are the cause of strength anisotropy.

Residual Stresses in Wires: Influence of Wire Length

Residual stresses are one of the causes of failures in structural components. These stresses may arise in the fabrication process from many causes. They cannot be easily accounted for because they are both difficult to predict and to measure. X-ray diffraction (XRD) is nowadays a widespread technique for measuring surface residual stresses in crystalline materials. Very small specimens are often used for this purpose due to geometrical restrictions of either the diffractometer sample holder or the component to be inspected. However, the cutting process itself may affect the residual stress state in these specimens, so measured stresses could be misleading. In this work, the influence of specimen length on residual stresses was investigated in cold-drawn ferritic and pearlitic steel wires by XRD measurements and finite element simulations. In the ferritic wires, numerical simulations coincide with experimentally measured stresses. However, in the pearlitic wires the effect of the stresses in cementite (which could not be measured by XRD) has to be taken into account to explain the observed behavior. The results obtained have shown that in both materials the cutting process affects residual stresses, so it is recommended that specimens larger than five times the wire diameter be used.

Polymer Wire Chemical Sensor Using a Microfabricated Tuning Fork

We have measured the mechanical properties of a polymer wire bridged across the two prongs of a microfabricated tuning fork. The polymer wire is formed by stretching so that it has a built-in mechanical stress. Upon exposure to organic vapors, the stress in the polymer wire diminishes, which is sensitively detected from the changes in the resonance of the tuning fork. We have also shown that by trimming the polymer wire from micrometers down to nanometers the sensitivity and response improve considerably because of an increased surface-to-volume ratio and sharper resonance of the polymer wire-tuning fork system.