Influence of hot working parameters on microstructure evolution,tensile behavior and strain aging potential of bainitic pipeline steel

The microstructure evolution, tensile properties and aging behavior were studied in a low-carbon pipeline steel by performing a number of physical simulations in a thermo-mechanical simulator. The austenite status, before entering the finishing stage, was varied by varying parameters in the austenitization and the roughing stages. The finishing parameters and the subsequent cooling strategy were kept fixed throughout all applied simulations. It was found that starting the finish rolling stage with pancaked austenite is more effective in motivating the formation of acicular ferrite and refining martensite/austenite phase than refining the prior austenite grains. The former effect resulted in improving both of ultimate tensile strength and proof stress without significant ductility decrease. Increasing the deformation during roughing stage resulted in stimulating the transformation kinetics during subsequent processing and consequently decreasing the untransformed austenite that forms the martensite/austenite microconstituent. Furthermore, increasing the austenitization temperature is found to enhance both of strength and ductility when starting the finishing stage with pancaked austenite. On the other hand, attaining aging effect in the studied steel was only possible after creating new dislocations by pre-straining. An increase up to 63 MPa in the yield strength is recorded due to aging after 2% pre-straining.     

Effect of phase on debond strength in shape memory alloy reinforced composites

Systematic single fiber pullout tests were performed on epoxy composites embedded with nickel titanium shape memory alloy (SMA) wires. The SMA wires were tested in the austenitic or martensitic states to study and decouple the elastic moduli from martensite transformation or reorientation stresses in the analysis of debond loads. The results reveal that the SMA wires that were in the austenite phase consistently produced higher debond loads as compared to that of those wires that started in the martensite phase, likely due to differences in the Poisson’s ratio. Additionally, there appears to be a relationship between the elastic modulus and debond load where reinforcements with a higher elastic modulus displayed lower debond loads. Lastly, for SMA reinforcements that underwent a martensitic phase transformation or reorientation, the debond load was equivalent to the martensite transformation or reorientation load. The results of this work illustrate the sensitivity of SMA reinforced composites on the mechanical behavior and phase transformation characteristics of the constituent materials.     

Size effect on the optimum actuation condition for SMA-composites

Techniques of using shape memory alloy (SMA) wires as actuators inside the hybrid composites have been studied extensively for improving mechanical properties of structures. There were many experimental findings and theoretical predictions agreed the higher the actuation temperature on built-in SMA wires, the greater the improvement in mechanical response of a composite due to recovery action of wires. However, due to the limitation of interfacial shear strength, over-actuation (by means of electrical resistive heating) of SMA wire is able to induce the development of interfacial crack inside the composite structure. When the maximum interfacial shear strength is attained due to vigorous recovery action of SMA wire under relatively high temperature, interfacial debond can be initiated. In this paper, a typical SMA–matrix cylinder model is employed to study the captioned risk of SMA-composite actuation. Applying the criterion of optimum actuation condition (OAC), target actuation level can be determined to prevent structural failure due to over-actuation. Effects of geometric factors including wire embedded length and matrix-to-wire radius ratio on the interfacial shear stress distribution are evaluated prior to the study of size effect on OAC. Results indicate that the size effect become negligible when these two geometric factors are sufficiently large and as a result, the governing equation of OAC can be greatly reduced to a simple relation between externally applied stress and actuation temperature on the SMA wire. This simplified model is able to enhance the application of OAC and provide a simple and explicit solution to determine an ideal range of actuation levels for a large scale SMA-composite structure in the design stage.     

Low-cycle fatigue life of superelastic NiTi wires

This paper analyzes low-cycle fatigue life under strain control (ε_a–N_f curve) of NiTi wires in bending-rotation tests. These were carried out on stable austenite, superelastic, and stable martensite wires, with strain amplitudes from 0.6% to 12%. For strain amplitudes up to 4%, ε_a–N_f curves of superelastic wires are close to those values reported in the literature, and close to that of the stable austenite wire. For higher strain amplitudes, the fatigue life of superelastic wires increases with strain and approaches the fatigue life of stable martensite wire. This unusual behavior causes a “Z-shaped” curve. Fatigue crack characteristics were studied by scanning electron microscopy. Simple finite element models of the test were also developed. The results suggest that the abnormal shape of the superelastic wire curve is associated to the changes in fatigue properties that occurs when the superelastic material transform to martensite.     

Analytical layerwise solution of nonlinear thermal instability of SMA hybrid composite beam under nonuniform temperature condition

Abstract

Thermal buckling and post-buckling behavior of composite beam reinforced with shape memory alloy (SMA) wires under nonuniform temperature distribution is explored. Thermo-mechanical behavior of SMA wires is formulated by using the one-dimensional Brinson SMA model. Considering von Karman strain–displacement relation, corresponding nonlinear governing equations are obtained and solved analytically. Heat conduction equation is employed and through-the-thickness temperature distribution is obtained by discretization scheme of layerwise method. Influence of SMA-wire positioning across the thickness, temperature distribution, SMA wire pre-straining level and volume fraction of SMAs upon the thermal buckling and post buckling of reinforced beam are examined and discussed in detail.     

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.     

Vibration characteristics of laminated composite plates with embedded shape memory alloys

Shape memory alloy (SMA) is commercially available for a variety of actuator and damping materials. Recently, SMA wires have also become commercially available for the design of smart composite structures because SMA wires with a small diameter can be easily produced. In this work, two types of SMA-based composites are presented for investigating the vibration characteristics. First, laminated composite plates containing unidirectional fine SMA wires are fabricated. By measuring the vibration mode of a clamped cantilever, the influence of both SMA arrangement and temperature on the vibration characteristics is made clear. Next, laminated composite plates with embedded woven SMA layer are fabricated. The stiffness tuning capability is evaluated by impact vibration tests with different temperatures. It is found that the stiffness tuning capability may be improved by increasing the volume fraction of SMAs and by controlling accurately the internal stress according to the phase transformation temperature of SMAs from martensite to austenite. The theoretical prediction on the natural frequency considering the SMAs behavior and laminated structures is proposed and their results agree reasonably with experimental ones.     

Finite element simulation of low velocity impact on shape memory alloy composite plates

Delamination of composite materials due to low velocity impacts is one of the major failure types of aerospace composite structures. The low velocity impact may not immediately induce any visible damage on the surface of structures whilst the stiffness and compressive strength of the structures can decrease dramatically.

Shape memory alloy (SMA) materials possess unique mechanical and thermal properties compared with conventional materials. Many studies have shown that shape memory alloy wires can absorb a lot of the energy during the impact due to their superelastic and hysteretic behaviour. The superelastic effect is due to reversible stress induced transformation from austenite to martensite. If a stress is applied to the alloy in the austenitic state, large deformation strains can be obtained and stress induced martensite is formed. Upon removal of the stress, the martensite reverts to its austenitic parent phase and the SMA undergoes a large hysteresis loop and a large recoverable strain is obtained. This large strain energy absorption capability can be used to improve the impact tolerance of composites. By embedding superelastic shape memory alloys into a composite structure, impact damage can be reduced quite significantly.

This article investigates the impact damage behaviour of carbon fiber/epoxy composite plates embedded with superelastic shape memory alloys wires. The results show that for low velocity impact, embedding SMA wires into composites increase the damage resistance of the composites when compared to conventional composites structures.     

Mechanical Properties and Interlaminar Fracture Toughness of Glass‐Fiber‐Reinforced Epoxy Composites Embedded with Shape Memory Alloy Wires

The effects of the content and position of shape memory alloy (SMA) wires on the mechanical properties and interlaminar fracture toughness of glass‐fiber‐reinforced epoxy (GF/epoxy) composite laminates are investigated. For this purpose, varying numbers of SMA wires are embedded in GF/epoxy composite laminates in different stacking sequences. The specimens are prepared by vacuum‐assisted resin infusion (VARI) processing and are subjected to static tensile and three‐point‐bending tests. The results show that specimens with two SMA wires in the stacking sequence of [GF₂/SMA/GF₁/SMA/GF₂] and four SMA wires in the stacking sequence of [GF₄/SMA/GF₂/SMA/GF₄] exhibit optimal performance. The flexural strength of the optimal four‐SMA‐wire composite is lower than that of the pure GF/epoxy composite by 5.76% on average, and the flexural modulus is improved by 5.19%. Mode‐I and II interlaminar fracture toughness tests using the SMA/GF/epoxy composite laminates in the stacking sequence of [GF₄/SMA/GF₂/SMA/GF₄] are conducted to evaluate the mechanism responsible for decreasing the mechanical properties. Scanning electron microscopy (SEM) observations reveal that the main damage modes are matrix delamination, interfacial debonding, and fiber pullout.

     

Effect of pre-strain on interfacial friction damping in carbon nanotube polymer composites

This paper investigates the effect of mechanical pre-strain on interfacial friction damping in nanotube polymer composites. Oxidized single-walled carbon nanotubes were dispersed in a polycarbonate matrix using a solution mixing technique. To characterize the damping response, the material storage and loss modulus was measured by application of dynamic (sinusoidal) load to the nanocomposite in the uniaxial direction. A static pre-strain (in 0.35-0.85% range) was then superimposed on the dynamic strain to quantify its effect on the material response. The results indicate that application of pre-strain facilitates the activation of interfacial slip at the nanotube-polymer interfaces at relatively low dynamic strain amplitudes. This is because pre-strain raises the interfacial shear stress for the nanotube inclusions allowing the critical stress for tube-matrix interfacial slip to be reached at lower strain amplitudes. In this way pre-strain significantly improves the effectiveness of the nanotube-matrix sliding energy dissipation mechanism for damping enhancement in composite structures.     

Quantitative phase analysis by X-ray diffraction of martensite and austenite in strongly oriented orthodontic stainless steel wires

A method is described for measuring the volume fractions and textures of martensite and austenite in strongly textured stainless steel orthodontic wires using a conventional X-ray diffractometer. These wires display a classic fibre texture with the 111 of the FCC austenite phase and the 110 of the BCC martensite phase aligned parallel to the wire axis. The samples analysed consisted of wire cross-sections bundled together and chemically polished in an epoxy disc. In this form the dominant lines in the XRD patterns are the austenite (111) and the martensite (110). On the basis of X-ray diffraction results from these two lines only, procedures are described for, (a) correcting the X-ray intensity data for both the finite size and irregular cross-sectional shape of the specimens in relation to the X-ray beam footprint, (b) separately measuring the texture of the austenite and martensite phases and, (c) correcting the 111 and 110 integrated intensities for texture. These procedures are illustrated using X-ray data from four different orthodontic wires. The factors limiting the accuracy of the phase analysis are discussed.     

Investigation of the evolution of retained austenite in Fe–13%Cr–4%Ni martensitic stainless steel during intercritical tempering

The microstructure and amount of retained austenite (the austenite remained at room temperature) evolved in Fe–13%Cr–4%Ni martensitic stainless steel during intercritical tempering at 620 °C have been investigated. The amount of retained austenite showed a parabolic trend with increase in tempering time, which can be attributed to the gradual decrease in the thermal stability of the reversed austenite (the austenite formed at high temperature). The influences of chemical composition, morphology of reversed austenite, and mechanical constraints originating from tempered martensite matrix on the thermal stability have been discussed. The precipitation and growth of M₂₃C₆ in reversed austenite dilute the carbon concentration in reversed austenite. The spheroidization of lathy reversed austenite during tempering decreases the interfacial energy barrier to the phase transformation of reversed austenite to martensite. Furthermore, the decrease in the strength of martensite matrix lowers the strain energy associated with the transformation of reversed austenite to martensite. All these factors during tempering weaken the thermal stability of reversed austenite and facilitate the phase transformation of reversed austenite to martensite during the cooling step of intercritical tempering.     

Strain Measurement in a Shape Memory Alloy with Strain Gauges

Abstract:  The recovery of shape due to temperature-induced martensite to austenite phase transformation in shape memory alloys (SMA), such as nickel titanium alloys, has been investigated and utilised in applications for many years. One of the problems in incorporating SMA in host materials, such as composites, is the poor bonding characteristics of the SMA. A closely related problem in experiments is the measurement of strains in SMA specimens; as thin wires have been used in many of the experimental investigations, strains have been deduced from length measurements. In the current work, electrical resistance strain gauges have been bonded on SMA strips. Tensile strains up to 8% during tensile loading as well as compressive strains during strain recovery due to subsequent heating have been measured. Strains determined by other methods, such as extensometer and length measurements, are also reported and compared. The major contribution of this work is the successful bonding of strain gauges on SMA and the measurement of large strains (up to 8%).     

Vibration characteristics of SMA composite beams with different boundary conditions

Recently, the development of shape memory alloy (SMA) actuators, in the forms of wire, thin film and stent have been found increasingly in the fields of materials science and smart structures and engineering. The increase in attraction for using these materials is due to their many unique materials, mechanical, thermal and thermal-mechanical properties, which in turn, evolve their subsequent shape memory, pseudo-elasticity and super-elasticity properties. In this paper, a common type of SMA actuator, Nitinol wires, were embedded into advanced composite structures to modulate the structural dynamic responses, in terms of natural frequency and damping ratio by using its shape memory and pseudo-elastic properties. A simple theoretical model is introduced to estimate the natural frequency of the structures before and after actuating the embedded SMA wires. The damping ratios of different SMA composite beams were measured through experimental approaches. The natural frequencies changed slightly at a temperature above the austenite finish temperature of composite beams with embedded non-prestrained SMA wires. However, the increase of the natural frequencies of the beams with embedded prestrained SMA wires were found in both the theoretical prediction and experimental measurements. The damping ratios of SMA composite beams increased with increasing the temperature of the embedded wires with and without being pre-strained. Compressive and local failures of the beams with high wire content are a possible explanation.     

A simulation study on the thermal buckling behavior of laminated composite shells with embedded shape memory alloy (SMA) wires

In this paper, numerical simulation analyses of the thermal buckling behavior of laminated composite shells with embedded shape memory alloy (SMA) wires were performed to investigate the effect of embedded SMA wires on the characteristics of thermal buckling. In order to simulate the thermomechanical behavior of SMA wires, the constitutive equation of the SMA wires was formulated in the form of an ABAQUS user subroutine. The computational program was verified by showing the response of the pseudoelasticity and shape memory effect (SME) at various temperatures and stress levels. Modeling of the laminated composite shells with embedded SMA wires and thermal buckling analyses were performed with the use of the ABAQUS code linked with the subroutine of the formulated SMA constitutive equations. The thermal buckling analyses of the composite shells with embedded SMA wires show that the critical buckling temperature can be increased and the thermal buckling deformation can be decreased by using the activation force of embedded SMA wire actuators.     

预应变对TC4ELI钛合金变形行为的影响

对TC4ELI钛合金分别进行正向拉伸-反向压缩和正向压缩-反向拉伸试验,研究预应变对其后续反向变形行为的影响。结果表明:两种加载方式下,TC4ELI钛合金反向屈服强度均随着预应变的增大先降低后趋于稳定,表现出明显的包申格效应。预拉伸试验下的饱和预应变约为2%,预压缩试验下的饱和预应变约为4%。预压缩后强度降低更加明显。晶体取向差异和α相的高度不对称密排六方结构导致材料在加载过程中出现非均匀塑性变形,产生残余应力,这是诱发TC4ELI钛合金产生包申格效应的主要机制。      

A New Hysteretic Behavior in the Electrical Resistivity of Flexinol Shape Memory Alloys Versus Temperature

The electrical resistivity (ER) of Flexinol nickel-titanium shape memory alloys (SMA) has been measured in the range from −15 to 105°C. The investigated Flexinol wires have two diameters, 150 and 375 μm. The experimental results show new temperatures of phase transformation (TTR) evidencing the unexpected presence of the R-phase. The transformations from austenite to martensite, from austenite to R-phase, and vice versa are simultaneous. In the range [20 to 110°C] the hysteresis is almost negligible, whereas in the range [−15 to 105°C] the accommodation process of the hysteresis is observed.     

Bond behavior of superelastic shape memory alloys to carbon fiber reinforced polymer composites

In this research pull-out specimens were tested to investigate the bond behavior of superelastic NiTi (Nitinol) SMA wires to carbon fiber reinforced polymers (CFRP). A total of 45 pull-out specimens were tested monotonically up to failure. The test parameters considered include the wire diameter and embedment length. A digital image correlation (DIC) system was used to identify the onset and propagation of debonding. Based on the experimental observations two debonding mechanisms were observed: complete debonding after the onset of martensitic transformation of SMA wire, and complete debonding before the onset of wire transformation. The former mechanism predominated, while the latter mechanism governed for larger diameter wires with shorter embedment lengths. A 3-D non-linear finite element model (FEM) was developed to predict the pull-out behavior. A cohesive zone model (CZM) was used to model the interface. A parametric study was conducted using the FEM to quantify the parameters of the cohesive zone model. The results demonstrate that the proposed modeling approach can be used to characterize the bond behavior of superelastic SMA wires embedded in FRP composites.     

Effects of pre-strain on uniaxial ratcheting and fatigue failure of Z2CN18.10 austenitic stainless steel

Effects of both tensile and compressive pre-strain on cyclic deformation of Z2CN18.10 austenitic stainless steel under stress cycling with mean stress are studied. As compared to as-received material, ratcheting strain of subsequent stress cycling decreases with increasing tensile pre-strain (TP) level. Lower level of compressive pre-strain (CP) is found to accelerate ratcheting strain accumulation while higher level of CP retards the accumulation. Tensile pre-straining is beneficial to ratcheting–fatigue life while compressive pre-straining is detrimental. A modified fatigue model to address the effect of pre-straining is proposed to predict the fatigue lives of the stress cycling tests with mean stress.     

用K2ZrF6溶液处理碳化硅涂层碳纤维制造碳/铝复合丝

在碳纤维表面用CVD法涂覆碳化硅涂层,接着用K₂ZrF₆溶液处理,干燥后通过700℃的熔融铝制得碳/铝复合丝。复合丝的强度约为复合准则强度的70%。当碳纤维的体积分数为54%时,复合丝的强度达到1200MPa。用扫描电镜观察复合丝的断口发现界面结合状态良好。用X射线衍射分析了碳化硅-K₂ZrF₆一铝在高温下的反应情况,表明K₂ZrF₆的良好的改善润湿的作用是由于它与铝和碳化硅之间发生了强烈的化学反应。