Cyclic Tensile Research Articles

Influence of pre‐strain and bake hardening on the static and fatigue strength of a DP600 steel sheet

AbstractThe influence of pre‐strain on the tensile and fatigue properties of a dual phase DP600 steel was studied. The material was pre‐strained by uni‐axial tension in rolling and transverse direction. Thereafter, specimens were cut from the deformed plates in parallel or orthogonal to pre‐strain direction. It was found that pre‐strain increases yield and tensile strength. Results suggested that strain path change primarily affects the elastic‐plastic transition during early stage of reloading. Pre‐strained specimens showed an increase in high cycle regimes as a consequence of yield strength increment, irrespective of imposed pre‐straining direction. A modified stress life equation that accounts for pre‐strain was proposed and showed good agreement with experimental data. Bake hardening enhanced both tensile and high cycle fatigue resistance. Walker equation was successfully fitted to account for tensile mean stress. In low cycle fatigue, negligible influence of pre‐strain was observed due to cyclic softening and residual stress relaxation.

Flexible and recoverable ion-conductive hydrogels with cross-linked triple network for highly sensitive wearable Motion-monitoring sensors

The practical applications of hydrogel-based sensors are limited by their low mechanical strength, insufficient sensitivity and machining difficulties caused by electrically conductive fillers. It is of great practical importance for facile preparation of hydrogel sensor combined with excellent mechanical properties and conductive sensitivity. In this work, a series of ion-conductive hybrid hydrogels based on in-situ polymerized polyacrylamide with double and triple network structures have been fabricated via both chemical and physical cross-linking. The triple network structured hydrogels achieve the tensile strength of 2.105 MPa and compression strength of 57.56 MPa, which are much higher than those of hydrogels with double network due to the established energy dissipation gradient. The reversible non-covalent cross-linking in the network also leads a relatively fast recovery in tensile cycle tests and double compression tests, enabling the excellent stability of strain sensing in the durability test. The hydrogels show the tensile resistance sensitivity factor of 0.9352 at 0–120% strain and a high pressure sensitivity factor of 5.134 kPa−1 with the pressure ranged from 0 to 6 kPa. The hydrogels are also assembled as flexible sensors for human motion monitoring, and the signals of motions can be distinguished by different resistance variation values and certain curve shape, showing the potential of applications on flexible device and human-machine interface.

Effects of 3d Transition Metal Substitutions on the Phase Stability and Mechanical Properties of Ti-5.5Al-11.8[Mo]eq Alloys.

The phase stability, mechanical properties, and functional properties of Ti-5.5Al-11.8[Mo]eq alloys are focused on in this study by substituting 3d transition metal elements (V, Cr, Co, and Ni) for Mo as β-stabilizers to achieve similar β phase stability and room temperature (RT) superelasticity. The ternary alloy systems with the equivalent chemical compositions of Ti-5.5Al-17.7V, Ti-5.5Al-9.5Cr, Ti-5.5Al-7.0Co, and Ti-5.5Al-9.5Ni (mass%) alloys were selected as the target materials based on the Mo equivalent formula, which has been applied for the Ti-5.5Al-11.8Mo alloy in the literature. The fundamental mechanical properties and functionalities of the selected alloys were examined. The β phase was stabilized at RT in all alloys except for the Ti-Al-V alloy. Among all alloys, the Ti-Al-Ni alloy exhibited superelasticity in the cyclic loading-unloading tensile tests at RT. As a result, similar to the Ti-5.5Al-11.8Mo mother alloy, by utilizing the Mo equivalent formula to substitute 3d transition metal elements for Mo, a RT superelasticity was successfully imposed.

Development of finite element model based on indirect tensile test for various asphalt mixtures

The Finite Element (FE) method allows for use of advanced material models and an effective and adequate analysis of mechanical behaviours for flexible pavements. This study aimed to validate the FE modelling of four types of asphalt mixtures considering the stress and strain responses derived from Cyclic Indirect Tensile Test (CITT). This research investigated the influence of asphalt surface layers with thicknesses of 40, 60, 80, 100, 120, 140 and 160 mm on the stress and strain at the bottom of asphalt surface and base layers. The viscoelastic behaviour of the material was described by the generalised Maxwell model using the Prony series method. The relaxation modulus was converted from the master curve constructed from the CITT results. The tensile stresses of the numerical models matched the measured values, showing a high Coefficient of Determination (R2 > 0.96) and low Mean Absolute Percentage Errors (MAPE ≈ 10%). In contrast, the fitting of tensile strain was better at reduced frequencies higher than 100 Hz representing the low temperature (high frequency) conditions. It was also found that a surface layer having a thickness up to 100 mm significantly reduced stress and strain at the bottom of both the asphalt surface and base layers. Asphalt mixtures with larger dynamic modulus showed smaller stress responses at the bottom of the asphalt layers under the same vehicle load. The tensile strain at the interface between courses highly depended on the stiffer layer. The experimental compressive stress and strain values showed a linear correlation with the tensile stress and strain evaluated in the FE model. After validating the constitutive relationships implemented in the FE approach against the experimental CITT results, the numerical model incorporating the master curve developed in this study was a very useful tool to predict the stress and strain responses of flexible road pavements.

Biomechanical Comparison of Nonlocked Minimally Invasive and Locked Open Achilles Tendon Simulated Rupture Repairs.

Open repair of Achilles tendon ruptures is associated with a risk of infection and other wound complications. Although percutaneous repairs reduce these complications, they may increase the risk of nerve injury. This study was designed to determine whether a percutaneous nonlocking repair can approach the gapping resistance offered by a standard open repair under conditions approximating typical postoperative physiotherapy. Ten pairs of cadavers Achilles tendons were transected in situ 5 cm above the insertion. One tendon from each pair was repaired using an open 4-strand Krackow locking loop, and the contralateral tendon was repaired with the Achillon system using the same suture material. Displacement transducers were attached to the medial, lateral, anterior, and posterior aspects of the tendon, spanning the repair. Each tendon underwent 1000 tensile loading cycles to 86.5 N, simulating passive ankle range-of-motion physiotherapy. Gapping was documented on the 1st, 50th, 100th, 500th, and 1000th cycles. The ultimate tensile strength of each repaired tendon was then measured by distracting until gross failure occurred. Gapping of the percutaneous repairs exceeded that of conventional open repairs on the first, 500th, and 1000th load cycles. All 10 conventionally repaired tendons withstood 1000 load cycles without gross failure, but 4 of 10 percutaneous minimally invasive repairs failed, one on the 9th load cycle and the others between the 100th and 500th cycles. On average, tendons repaired with the open technique withstood 66% greater tensile load in failure testing than those repaired with the percutaneous technique. Open Krackow Achilles tendon repairs may better withstand more aggressive postoperative physiotherapy than nonlocked percutaneous repairs. The study suggests that surgeons should consider locking suture approaches to avoid loss of repair integrity with early motion.

3D printed carbon nanotube/polyaniline/gelatin flexible NH3, stress, strain, temperature multifunctional sensor

The stability of 3D printed wearable flexible sensors is crucial for the practical application of hydrogel in wearable electronic devices and human-machine interfaces. However, conventional NH3 detectors have to face poor mechanical properties, and most hydrogel sensors have unstable humidity detection. Herein, we present a simple strategy to prepare in situ polymerized PANI conductive substances on CNTs. The PNI-CNTs were directly accessed in gelatin, and the thin-film sensors were used to 3D printed. The sensors enable monitoring NH3 detection over a wide range of 10–1200 ppm and stable humidity detection cycle. The hydrogel sensor shows excellent mechanical properties and keeps monitoring stress, strain temperature and human motion. In addition, the sensor shows high fatigue resistance after >2000 tensile cycles under fixed strain. The PANI-CNTs-gelatin sensor opens up a new horizon in human-computer interaction.

Crack mechanism and experimental verification on straightening of AZ31B magnesium alloy plate

When plates with edge cracks in the rolling process is straightened by cyclic tensile and compressive stress, the tip of edge crack always accompanied by stress concentration, which leads to crack propagation. In this paper, damage parameters are imported into the plate straightening model based on determining the GTN damage parameters of magnesium alloy materials by inverse finite element calibration method, the influence of different straightening process schemes and prefabricated V-shaped crack geometry on crack growth is analyzed through the way of the combination of simulation and straightening experiment. The results show that the peak values of equivalent stress and equivalent strain under each straightening roll appear at the crack tip. The value of longitudinal stress and equivalent stain decrease with the distance to crack tip becomes larger. The peak value of longitudinal stress appears when the crack circumferential angle is about 100°, and the crack tip is easy to form crack propagation; when the plate passes roll 2 and roll 4, the equivalent stress and strain concentration at the crack tip are most obvious; when the reduction reaches a certain degree, the void volume fraction (VVF) reaches the VVF of the material breaking; with the increase of the entrance reduction, the number of VVF at the crack tip which reaches the material fracture increases, and the length of crack propagation increases; the stress concentration at the tip of V-shaped crack with large length–width ratio is obvious, and the VVF is more likely to reach the VVF at the time of material fracture, crack initiates and propagates easily.

Experimental study of thermomechanical behaviour of Gum Metal during cyclic tensile loadings: the quantitative contribution of IRT and DIC

ABSTRACT Thermomechanical behaviour of Gum Metal (Ti–23Nb–0.7Ta–2.0Zr–1.2O, at.%) under cyclic tension was experimentally investigated using infrared thermography and digital image correlation. The thermomechanical characteristics of particular stages of the subsequent loading-unloading cycles of Gum Metal were identified, i.e. (I) the linear, elastic loading accompanied by the temperature drop, (II) the nonlinear super-elastic loading related to the temperature growth, (III) the transient stage (at which both the superelastic-like behaviour and the plastic one are present simultaneously) and the temperature starts growing fast, (IV) the plastic deformation with a significant growth of temperature, (V) the superelastic-like unloading accompanied by a fast drop in temperature, (VI) the transient unloading with a slower decrease in temperature and (VII) the elastic unloading, with a slight increase in temperature. Thermoelastic effect in Gum Metal during both loading and unloading was analysed in each tensile cycle. Finally, the evolution of strain and temperature fields just before unloading in each cycle was discussed and a comparison of the fields at selected stages of cycles 12 and 24 was presented. The results of this work enabled us to identify the non-dissipative processes of elastic and superelastic-like deformations as well as the dissipative process of plastic deformation.

Unraveling the failure mechanism of wire arc additive manufactured low carbon steel under tensile and high cycle fatigue loading

Wire arc additive manufacturing (WAAM) is one of the common additive manufacturing processes for efficiently building large components due to its high deposition rate and higher catchment efficiency with an easy and relatively simpler arrangement. However, WAAM components often observe defects that adversely affect some mechanical properties. The present work employs a two-camera digital image correlation (DIC)-based technique for simultaneous strain measurements on two mutually orthogonal surfaces to capture the strain accumulation due to defects and subsequent failure behavior under uniaxial tensile loading of WAAM deposited ER70S-6 low carbon steel samples. The elastic cyclic loading tests are also performed at different stress amplitudes, and their fracture surfaces are analyzed with scanning electron microscopy. Due to the weak interlayer consolidation, multiple delaminations and strain accumulation sites are observed. Propagation of delamination contributed towards the failure, and dimples as well as intergranular fractures are observed on failure surfaces. Premature failure with a reduced number of cycles to failure is observed during high cycle fatigue tests due to catastrophic fast failure in the neighborhood of the internal defects. While the fatigue crack initiation and propagation are distinctly observed on fracture surfaces, transition regions from fatigue fracture and fast fracture are mostly obscure.

Critical Assessment of the Bonded Single Lap Joint Exposed to Cyclic Tensile Loading

Single shear or single lap joints are the most prevalent type of adhesive joints used in advanced engineering applications, where they are exposed to fatigue loadings in their services. Although their mechanical performances under static loading have been investigated extensively, the studies related to the fatigue performances were limited. For that purpose, single lap joints’ (SLJs) reaction to fatigue tensile loading was studied by varying the adherend thickness (3 mm to 6 mm) and fatigue load (3250 N to 1500 N). ABAQUS/Standard was used to create its advanced FE model. To represent the progressive damage in the adhesive layer, the fatigue damage model via the Paris Law, which links the rate of the crack expansion to the strain energy release rate (SERR), was integrated into the cohesive zone model with bi-linear traction–separation characteristics. The model was written in a UMAT subroutine. The developed model was validated using experimental data from the literature. The crack initiation cycle (Ni), the failure cycle (Nf), the fatigue load limit, the strain energy release rate, the crack propagation rate, and variation of stress components with their dependency to design parameters were investigated in depth. It was found that the service life of the SLJs with thicker adherends was more responsive to the amount of stress applied. When exposed to lesser loads, the SLJs’ life span changed more noticeably.

Experimental and numerical research on cyclic tensile performance of high-strength hydraulic concrete subjected to freeze–thaw damage

Hydraulic concrete structures in cold regions are vulnerable to freeze–thaw (FT) cycles, which will significantly weaken their ability to resist tensile load, thus threatening the safe operation of structures. Direct tensile properties of high-strength hydraulic concrete subjected to different FT cycles (0, 100, 200, 300 and 400 FT cycles) are investigated through experimental analysis and numerical simulation. The results show that the tensile strength, initial elastic modulus and ultimate strain of hydraulic concrete decrease linearly with the increase of FT cycles, and the cumulative rate of residual strain is independent of FT damage, but increases linearly with the increase of unloading strain. Based on FT damage caused by temperature and mechanical damage caused by external load, a cyclic tensile model considering FT effect is proposed. Moreover, the evolutions of principal strain, main crack width and internal microcrack of hydraulic concrete subjected to different FT cycles under cyclic tensile load are revealed by digital image correlation (DIC) and acoustic emission (AE) techniques. Based on experimental analysis, a DEM cyclic tensile prediction model considering loading–unloading constitutive and real aggregate is established to accurately describe the cyclic tensile response of hydraulic concrete subjected to different FT cycles.

A Transparent, and Self-Healable Strain-Sensor E-Skin Based on Polyurethane Membrane with Silver Nanowires

Electronic skin (E-skin) is increasingly utilized in modern society, yet current E-skin technology suffers from issues, such as opacity, hardness, and fragility. To address these challenges, a novel E-skin was developed using polyurethane (PU) as the matrix material and silver nanowires (AgNWs) as the sensing material. By leveraging the small degree of microphase separation and lack of crystallization in the PU, combined with the appropriate length–diameter ratio of the AgNWs, the resulting E-skin exhibited a visible light transmittance of 75%. The E-skin also showed excellent self-healing properties (83.63% efficiency in the third repair) and mechanical properties (with almost no degradation after 60 tensile cycles) due to the reversible dynamic cross-linking network within the PU. The synergistic effect of PU and AgNWs resulted in exceptional sensing performance for the E-skin, with a gauge factor of 46 (when ε = 10%). Moreover, the E-skin demonstrated signal stability during human joint motion monitoring and successfully identified different movement states, highlighting its potential for diverse applications. This research presents a simple yet effective approach for producing transparent, durable, and stable E-skin.

Evaluation of Piezoresistive and Electrical Properties of Conductive Nanocomposite Based on Castor-Oil Polyurethane Filled with MWCNT and Carbon Black.

Flexible films of a conductive polymer nanocomposite-based castor oil polyurethane (PUR), filled with different concentrations of carbon black (CB) nanoparticles or multiwall carbon nanotubes (MWCNTs), were obtained by a casting method. The piezoresistive, electrical, and dielectric properties of the PUR/MWCNT and PUR/CB composites were compared. The dc electrical conductivity of both PUR/MWCNT and PUR/CB nanocomposites exhibited strong dependences on the concentration of conducting nanofillers. Their percolation thresholds were 1.56 and 1.5 mass%, respectively. Above the threshold percolation level, the electrical conductivity value increased from 1.65 × 10-12 for the matrix PUR to 2.3 × 10-3 and 1.24 × 10-5 S/m for PUR/MWCNT and PUR/CB samples, respectively. Due to the better CB dispersion in the PUR matrix, the PUR/CB nanocomposite exhibited a lower percolation threshold value, corroborated by scanning electron microscopy images. The real part of the alternating conductivity of the nanocomposites was in accordance with Jonscher's law, indicating that conduction occurred by hopping between states in the conducting nanofillers. The piezoresistive properties were investigated under tensile cycles. The nanocomposites exhibited piezoresistive responses and, thus, could be used as piezoresistive sensors.

New insights into the effects of solidification process on the thermodynamic parameters and mechanical properties of CuAlBeNbNi shape memory alloy

In this work, the influence of the processes of unidirectional solidification and squeeze casting in microstructure, thermodynamic parameters and mechanical properties of CuAlBeNbNi shape memory alloy (SMA) was investigated. The results show that the unidirectional solidification process causes an increase in phase transformation temperatures and ductility, improves fracture toughness and reduces stress-induced martensite, residual stress and dissipated energy after load-unload tensile test of the CuAlBeNbNi alloy. The squeeze casting process, on the other hand, causes an increase in the density of grain boundaries, increasing the stress-induced martensite and the elastic energy of the direct martensitic transformation, while decreasing the impact toughness, increasing the residual deformation, the residual stress and the dissipated energy after the tensile cycle tests. These results indicate that the processes of unidirectional solidification and squeeze casting offer an alternative method to modify the thermodynamic parameters, mechanical properties, and microstructural properties of the CuAlBeNbNi SMA while keeping the composition constant. This study aims to fill the knowledge gap regarding the impact of solidification processes on the properties of Cu-based SMAs, specifically investigating CuAlBeNbNi SMAs.

Influence of fatigue on superconducting REBCO tapes under repeated cyclic tensile, bending and twisting loads: A simulation-based investigation

The effect of repeated cyclic loading on the superconducting REBCO tape under tensile, bending, and torsional loadings is investigated. The Fatigue Usage Factor (FUS) is calculated for each cyclic loading condition to identify degradation using the Matake criterion. The thicknesses of the tapes' constituent layers are also varied to identify the relative influence of each layer on overall fatigue performance. When the tape was subjected to fatigue-type loading, the thickness of the copper and Hastelloy layers was found to play a significant role. This holds true under tensile, bending, and twisting loading conditions. However, out of the three loading conditions, bending exhibits the greatest variation in FUS value with changes in Hastelloy and copper layer thickness. The maximum value of the fatigue usage factor is observed in the edges of the Hastelloy layer in all three loading cases. It has been found that the properties of the layer material, as well as the adjacent layer, play an important role in determining the fatigue strength of the tape.

Additively manufactured Ti55511 alloy: Microstructure and residual stress effect on mechanical properties

Stress relief (SR) is an effective heat treatment process that potentially changes microstructure and enhances the mechanical properties of additively manufactured parts. In this study, the influence of SR on the microstructure and mechanical properties of Ti-5Al-5Mo-5V-1Cr-1Fe alloy (Ti-55511), a near-β alloy fabricated by laser powder bed fusion (LPBF), was investigated. LPBF-built Ti-55511 samples were characterized and tested under tensile and low cycle fatigue loading in the as-built (AB) and SR conditions. The obtained results show that the LPBF sample was composed of columnar grains in a single bcc β-phase in the AB condition. In contrast, the SR condition resulted in a bi-modal microstructure comprising of α + β phases. This led to significant grain refinement of the β phase and reduced the residual stress (RS) up to 91 % compared to the AB condition. At the same time, the SR treatment increased the yield strength, ultimate tensile strength, and fatigue life at the expense of ductility. The improvement in mechanical properties is attributed to the grain refinement and phase transformation resulting in complex dislocation interactions during deformation.

Piezoresistive response of carbon nanotube and carbon fiber/epoxy composites under cyclic loading

Maintaining the mechanical properties and long-term operational safety is considered the main challenge for composite materials under cyclic loading. This work presents mechanical performance of tri-component composites based on multiwall carbon nanotubes (MWCNTs), carbon fibres (CFs) and epoxy resin, and their electrical conductance property for applications like strain gauges. As a result of incorporation of MWCNTs into the epoxy resin in the composite’s morphology, their electrical, mechanical and piezoresistive performance can indicate the self-sensing of carbon fiber reinforced epoxy resin matrix (CF/Epoxy matrix) composites; and thus its influence has been systematically examined. The inclusion of multiwall carbon nanotubes increased the resin bonding to the surface of the CF’s leading to an increased electrical conductivity and mechanical performances. The piezoresistive performance was significantly influenced by the amount of MWCNTs added to the resin, where the Gauge Factor (GF) with respect to the MWCNTs concentration under cyclic tensile and cyclic bending were in the range of 0.6∼1.5 and 2.5∼5.5 respectively. Moreover, the piezoresistive behaviour of the composite samples showed reasonable sensitivity, stability, and reversibility under cyclic mechanical loading, and the samples withstood more than 500 cycles of load without detectable loss in performance. The exceptional mechanical, electrical and piezoresistive performance and easy manufacturing process of the tri-component composites make them attractive for applications such as self-monitoring structural components.

A Universal Interfacial Strategy Enabling Ultra‐Robust Gel Hybrids for Extreme Epidermal Bio‐Monitoring

AbstractA seamless and tough interface to integrate incompatible/immiscible soft materials is highly desired for flexible/wearable electronics and many soft devices with multi‐layer structures. Here, a surfactant‐mediated interfacial chemistry is introduced to achieve seamless and tough interfaces in soft multi‐layer structures, with an ultra‐high interfacial toughness up to ≈1300 J m−2 for the architectural gel hybrid (AGH). The reversible noncovalent interfacial interactions efficiently dissipate energy at the interface, thereby providing excellent durability. The interfacial toughness only decreases by ≈6.9% after 10 000 tensile cycles. This strategy can be universally applied to hybrid systems with various interfaces between an interior hydrogel (PAA, PVA, PAAm, and gelatin) and an exterior hydrophobic soft matter (ionogel, lipogel and elastomer). The AGH‐based mechano‐sensor presents high robustness and stability in a wide range of conditions, including open air, underwater, and various solvents and temperatures. Epidermal bio‐monitoring, tactile trajectory, and facial expression recognition are demonstrated using the AGH sensors in various environments. A rich set of electrophysiological signals of high quality are acquired.

Biomimetic vascular tissue engineering by decellularized scaffold and concurrent cyclic tensile and shear stresses

Decellularization by chemical approaches has harmful effects on extracellular matrix (ECM) proteins, and damages lots of functional peptides and biomolecules present in the ultrastructure. In this study, we employed a combination of chemical and physical decellularization methods to overcome these disadvantages. The induced osmotic pressure by hypertonic/hypotonic solutions dissociated and removed most of cellular membranes significantly without any detergent or chemical agent. In total, 0.025% trypsin solution was found adequate to remove the remaining debrides, and ultimately 1% Triton X-100 was utilized for final cleansing. In addition, conducting all the decellularization processes at 4 °C yielded an ECM with least damages in the ultrastructure which could be inferred by close mechanical strength and swelling ratio to the native vessel, and high quality and quantity of cell attachment, migration and proliferation which were examined by optical microscopy and scanning electron microscopy (SEM) of the histology samples. Moreover, the obtained biological scaffold (BS) had no cytotoxicity according to the MTT assay, and this scaffold is storable at −20 °C. Employing bioreactor for concurrent cyclic tensile and shear stresses improved the cell migration into pores of the BS and made the cells and the scaffold compact in analogous to native tissue. As opening angle test showed by decellularizing of the blood vessel, the residual stress dropped significantly which revealed the role of cells in the amount of induced stress in the structure. However, intact and healthy ECM explicitly recovered upon recellularization and beat the initial residual stress of the native tissue. The tensile test of the blood vessels in longitudinal and radial directions revealed orthotropic behavior which can be explained by collagen fibers direction in the ECM. Furthermore, by the three regions of the stress–strain curve can be elucidated the roles of cells, elastin and collagen fibers in mechanical behavior of the vascular tissues.Graphical

SiOC-Based Strain Gauge with Ultrahigh Piezoresistivity at High Temperatures

The nonlinear response of strain gauges at high temperatures has restricted their applications despite their high-precision and real-time measurement capability. This work addresses this limitation by utilizing the easy preparative access and versatility of silicon oxycarbide-based (SiOC) thin films as strain gauges offering outstanding high-temperature robustness and giant piezoresistive response. The sensitivity of the strain gauge is assessed with continuous cyclic loads, tensile and compressive, resulting in gauge factors of ca. 2000–5000. The linearity of the response is preserved up to 700 °C with a shift in the electrical response occurring at temperatures beyond 500 °C, switching the SiOC film from semiconducting to conducting behavior. This change causes a drop in the gauge factor of the SiOC-based thin films; nevertheless, it is still significantly higher than that of metallic and Si-based commercial strain gauges. Notably, the studied thin films can regulate the effect of temperature enabling them to be a highly sensitive device with good reversibility and replicability in high-temperature environments. Furthermore, the electrical shift at 460 °C broadens the application of the SiOC film as a current-limiting device and temperature sensor.