Tensile Loading Experiments Research Articles

Understanding the asymmetric orientations and stress states in polycrystalline NiTi SMA by in-situ synchrotron-based high-energy X-ray diffraction

The stress-induced martensite transformations (SIMTs) dramatically affect the recoverable strain and mechanical response of polycrystalline NiTi. An in-depth understanding of the propagation manner and orientation preference of SIMTs is therefore crucial. In this work, we present a unique asymmetric anisotropy of SIMTs and lattice strains induced by Lüders-type deformation in polycrystalline NiTi, achieved through a combination of in-situ synchrotron X-ray diffraction and uniaxial tensile loading and unloading experiments. Our experimental findings reveal that in polycrystalline NiTi under uniaxial deformation, the asymmetry of SIMTs is attributed to the inhomogeneous strain field caused by the Lüders-type mechanism. The asymmetrical SIMT starts with the forward Lüder band and disappears along with the backward Lüder band. The austenite with the favored orientation of ⟨110⟩A//loading direction (LD) transformed and recovered back at a higher rate compared to other orientations during both loading and unloading.

Fluid-solid coupling numerical simulation of entire rat caudal vertebrae under dynamic loading

Trabeculae bone undergoes directional growth along the applied force under physiological loading. The growth of bone structure relies on the coordinated interplay among osteocytes, osteoblasts, and osteoclasts. Under normal circumstances, bone remodeling maintains a state of equilibrium. Excessive bone formation can lead to osteosclerosis, while excessive bone resorption can result in osteoporosis and osteonecrosis. The investigation of the structural characteristics of trabeculae and the mechanotransduction between bone cells plays a vital role in the treatment of bone-related diseases. In this study, a fluid-solid coupling model of the entire vertebral bone was established based on micro-CT images obtained from rat tail vertebrae subjected to tensile loading experiments. The flow characteristics of bone marrow and the mechanical response of osteocytes in different regions under physiological loading were investigated. The results revealed a U-shaped distribution of wall fluid shear stress (FSS) along the longitudinal axis in trabecular bone, with higher FSS regions exhibiting greater mechanical stimulation on osteocytes. These findings elucidate a positive correlation between the mechanical microenvironment among osteocytes, osteoblasts, and osteoclasts, providing potential strategies for the prevention and treatment of bone diseases.

Reveling the orientation preference along with localized Lüders-type deformation in polycrystalline NiTi SMA by in-situ synchrotron-based high energy X-ray diffraction

The stress-induced martensite transformations (SIMTs) in near-equiatomic NiTi shape memory alloys (SMAs) predominantly occur through localized, inhomogeneous, and intense Lüders-type mechanisms, which significantly influence the recoverable strain and mechanical response of the material. An in-depth understanding of the propagation manner and orientation preference of SIMTs is therefore crucial. In this study, we present a unique asymmetric anisotropy of SIMTs and lattice strains induced by Lüders-type deformation in polycrystalline NiTi, achieved through a combination of in-situ synchrotron X-ray diffraction and uniaxial tensile loading experiments. Our experimental findings reveal that in polycrystalline NiTi under uniaxial tensile loading, the austenite with the favored orientation of ⟨110⟩A//loading direction (LD) is consumed faster compared to other orientations, resulting in residual austenite with an orientation of ⟨431⟩//LD within the Lüders banding area. In contrast, the high-strain residual austenite with few favored orientations transforms fairly slowly and remains well beyond the transformational plateau. Our work provides valuable new insights into the microstructural nature of the Lüders-type deformation mechanism of polycrystalline NiTi, and the enhanced understanding of these complex interactions holds promise for optimizing the performance and design of SMAs in practical applications.

Mechanically Prestressed Pneumatically Driven Bistable Soft Actuators

Abstract Bistable soft robots are gaining momentum for their fast speed. This study presents a novel asymmetric mechanically prestressed, pneumatically driven, bistable laminated soft actuator. Its two orthogonal stable shapes are created by prestretching two orthogonal elastomer matrix composites before bonding them to a thin core layer. Two fluidic layers with fluid channels are bonded on either side of the core layer to actuate and trigger the snap-through process of the actuator. An analytical model is proposed as follows: the actuator net energy is calculated based on polynomials with unknown coefficients, and the stable shapes of the actuator are computed as a result of pneumatic pressure and external loads with the Rayleigh–Ritz method. Bistable actuators are fabricated with different prestrains, and motion capture and tensile loading experiments are conducted for model validation. A gripper is fabricated with two bistable actuators and demonstrated to grasp a variety of objects. Sensitivity studies are performed to identify the actuator response as a function of a variety of design parameters.

Analysis of Texture and Anisotropic Elastic Properties of Additively Manufactured Ni-Base Alloys

Additive manufacturing of metallic materials generates strong crystallographic textures, leading to anisotropic elastic properties on the macroscopic scale. The impact of the processing parameters on the resulting texture requires suitable techniques for the prediction and the experimental determination of elastic properties to exploit the anisotropy in the design process. Within this study mechanical as well as microstructure based approaches are applied on a batch of specimens manufactured from IN718 by selective laser melting to assess the elastic behavior on macroscropic scale. Tensile loading experiments and the impulse excitation technique are applied for the determination of elastic properties without additional constitutive data. Furthermore, the elastic behavior is estimated from single-crystal elastic properties and texture data measured by electron backscatter diffraction and high energy X-ray diffraction. The results of the applied approaches are discussed and compared, allowing also to assess the homogeneity of the elastic properties within the batch of specimens.

Crack Propagation for Glass Fiber Reinforced Laminates Containing Flame Retardant: Based on Single-Edge Tensile Loading

Research on crack propagation for fiber reinforced composites containing flame retardant is rare. The micro-cracks propagation is a reason for delamination and debonding failure of fiber reinforced composites. To study the crack propagation of continuous glass fiber reinforced epoxy resin laminates that contained ammonium polyphosphate flame retardant (GFRP-APP), the quasi-static single-edge tensile loading (SETL) experiments for the end-notched GFRP-APP specimens were carried out by MTS universal electronic testing machine. The crack propagation of the end-notched 90� GFRP-APP specimen includes two types, both of which belong to opening type (mode I). Namely, one type is mode I multi-cracks propagation without preexisting crack, and the other is mode I fiber bridge propagation with preexisting crack. The intralaminar fracture toughness along fiber direction of GFRP-APP is approximately 8.4 N/mm, which is calculated by area method. The opening displacement-tensile force curves can be divided into three stages for 90� GFRP-APP specimen without crack, i.e., crack gestation, crack birth and crack propagation. However, the 90� GFRP-APP specimen with crack not contains the crack birth stage. Additionally, the microscopic morphology for the fracture face of pure epoxy resin and GFRP-APP, and the phase analysis for GFRP-APP were performed by scanning electron microscope (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). As a conclusion, the pores and interfaces in materials were the guiding factors of micro-crack propagation, and the ammonium polyphosphate flame retardant particle contributed extra interfaces.

The local deformation behaviour of MMCs – an experimental study

AbstractThe local deformation behaviour of powder metallurgy metal matrix composites (MMCs) with two different SiC particle sizes and aging conditions are investigated and compared to the behaviour of the un-reinforced matrix. In situ tensile loading experiments are carried out in a scanning electron microscope. Images taken at different deformation stages are analysed by a system for local deformation measurement. It is found that even the pure matrix material deforms inhomogeneously, showing a shear-band pattern which is independent of the loading stage. The MMCs with coarse reinforcements deform mainly due to shear bands which are induced by fractured particles. The MMCs with small particles exhibit a shear-band pattern which is controlled by the particle arrangement. The fracture of small particles does not induce far-reaching shear bands and, therefore, these materials have a higher strength and ductility.

Designing additively manufactured lattice structures based on deformation mechanisms

The post-yield mechanical behavior of additively manufactured lattice structures (AMLS) is governed by the interplay between intrinsic (microstructural) and extrinsic (structural topology) properties at different length scales. Herein, we introduce a novel design optimization approach that accounts for scale separation and size effects, which control deformation mechanisms, to achieve a certain targeted macroscopic mechanical response. The new topological designs are guided by finding a direct correlation between the distribution of local stresses within struts and the underlying microstructures. The local stresses are computed using a strut-level yield criterion that has been calibrated to strut-level tensile, compressive, and shear loading experiments. Therefore, the local response of the struts, including tension-compression asymmetry, build direction dependence, and size effects, are accounted for in the yield surface, enabling a more accurate representation of the local stress state. Accurate calculation of the stress state for a given microstructure and topology combination allows for optimizing the topology for the given strut-level microstructure. The interplay between the topology and microstructure is assessed by investigating the unit cell-level deformation mechanisms and quantifying their influence on the global stress-strain relationship via finite element simulations. Using these relationships, a new set of topologies is designed, built, and validated with experiments. On average, the new topologies demonstrate 40% and 72% improvement in energy absorption capacity and flow stress, respectively, compared to topologies that had been previously optimized using constitutive models, which are homogeneous throughout the unit cell. The goal of the presented article is to demonstrate that simultaneously considering the effects of topology and microstructure on the mechanical behavior of AMLS has the potential to substantially improve key performance metrics, including ultimate strength and energy dissipation. The distinguishing and novel feature of our approach is that the topological optimization is performed while accounting for the heterogeneous distribution of strut-level microstructural features and concomitant mechanical behavior, which leads to new insights relative to peak AMLS structural performance.

Synchrotron X-ray diffraction studies of the phase-specific deformation in additively manufactured Ni–CrC composites

Cold spray is a promising solid-state additive manufacturing process for the fabrication of metal matrix composites in the forms of coatings, and more recently, bulk materials, particularly when deposition of two phases is needed with minimal alterations of the chemistry and phase composition. While prior studies have reported improved mechanical performance in cold sprayed composites, a fundamental understanding of the response and contribution of individual constituent phases to the collective mechanical properties of the composite is yet to be developed. In this work, we use in-situ synchrotron X-ray diffraction to study the strain partitioning between the soft and hard phases in cold sprayed Ni–CrC bulk composites. We conduct stepwise uniaxial tensile loading experiments and use high energy X-ray diffraction at multiple load steps to measure phase-specific strains and stresses upon loading. We find neither plasticity in the Ni phase nor a significant load transfer or redistribution between Ni and CrC phases during the loading process and attribute them to a premature failure governed by the presence of defects.

Deformation behavior of TiO2 films deposited on NiTi shape memory alloy after tensile and water-bath heating tests

TiO2 films with thicknesses of 100 and 50 nm were deposited on NiTi shape memory alloys (SMA-NiTi) using high-power impulse magnetron sputtering. Uniaxial tensile loading, unloading, and water-bath heating experiments were conducted. X-ray diffractometry, scanning electron microscopy, and profilometry were used to characterize the phase structures of the NiTi substrates and surface morphologies of the films before and after stretching and after stretching with water-bath heating. Under uniaxial tension, the NiTi substrate experienced a B2 → B19′ martensitic phase transformation. Surface relief appeared on the film and residual deformation persisted in the NiTi after stress-relief. After heating, the NiTi substrate underwent a reverse B19′ → B2 martensitic phase transformation where the surface relief on the film disappeared and the residual deformation was eliminated. Neither cracking nor spalling was observed on the 50-nm film after tensile deformation at a maximum strain of 10%, and the shape was recovered via heating. The 100-nm film withstood a tensile deformation at a maximum strain of 10%, but slight cracking and spalling occurred during shape recovery. This is the first report on the dynamic changes in the surface morphology of inorganic films deposited on SMA-NiTi alloys. The results provide a theoretical and experimental basis for analyzing the reliability of film-modified NiTi alloy devices.

Experimental study on dynamic nanoindentation on structural weld zone

In this study, dynamic spherical indentation and finite element analysis were used to assess the strain rate sensitivity behavior of the SS400 structural steel weld zone. The influences of the loading rate on both yield strength and hardness of the base metal, heat-affected zone, and weld metal are studied using dynamic spherical nanoindentation. The strain rate sensitivity (SRS) of three microstructural phases in the weld zone was also determined using the hardness model, as a result, the SRS behavior in the weld zone was investigated. The relationship of the SRS to the minimum yield stress of the investigated weld zone was constructed and compared with the general trend reported for several types of structural steel in the literature. To verify the SRS behavior in the weld zone, the tensile loading experiments on the weld specimen and finite element (FE) simulation of the tensile process considering the SRS behaviors of base metal, heat affected zone, and weld metal were conducted. The comparison of the engineering stress-engineering strain curves obtained from the tensile experiment and FE analysis is then constructed. Thus, the SRS behavior of the SS400 structural steel weld zone was validated through experimental verification. The present study provided a basic methodology to dynamic nanoindentation on the weld zone, and the experimental results of this study can be used in the practical designs and to understand the strain rate sensitivity behavior of microstructural phases in the weld zone.

An atomistic study of damage and localized anisotropic mechanical property evolution in thermoset polymers

All-atom molecular dynamics (MD) simulations are used to explore damage initiation and the local anisotropic evolution of mechanical properties in thermoset polymers under uniaxial tension with an emphasis on changes in stiffness through a series of tensile loading, unloading, and reloading experiments. By comparing the results for classical and bond order potentials, bond breakage is found to play a minimal role in damage initiation due to the highly localized plastic deformation mechanisms. The void fraction is quantified, revealing sudden initiation points and consistent growth rates, which are correlated to averaged changes in stiffness, and the state of deformation. The efficacy of incorporating MD damage data into multiscale models is evaluated considering the sensitivity of results to topological variation. In addition, the results are compared for various reference frames, demonstrating the need for fully Eulerian stress–strain pairs to isolate material behavior from the geometric changes associated with large molecular strains.

Evolution of thermal and mechanical properties of Nitinol wire as a function of ageing treatment conditions

As-received and cold-worked 55Ni–45Ti wt% Nitinol wire samples were subjected to various ageing treatments. Experiments show that the properties of as-received Nitinol, including microstructure, critical temperatures in thermally induced phase transformation, and critical stresses/strains in mechanically induced phase transformation, change significantly after ageing process at 773 K for different durations ranging from 10 min to 5 h. BSE and EBSD/EDS analysis shows that the predominant microstructural evolution is a large increase in Ni4Ti3 precipitate volume fraction, while grain growth is negligible. DSC results show that as-received material exhibits little to no phase transformation during heating-cooling cycling at 5K/min, while various ageing durations lead to the emergence of phase transformation that occurs at different critical temperatures. Tensile loading experiments conducted using a test rig equipped with a heater revealed the evolution of mechanically induced phase transformation behaviour. Mathematical models are proposed to predict the dependence of critical temperatures and critical stresses/strains on the ageing treatment conditions and alloy composition.

Application of In Situ TEM to Investigate Irradiation Creep in Nanocrystalline Zirconium

This work characterizes the irradiation creep response of nanocrystalline zirconium by nanoscale quantitative tensile loading and ion irradiation experiments performed simultaneously in situ inside a transmission electron microscope. Microfabricated devices consisting of a freestanding Zr tensile specimen 100 nm in thickness on an elastic Si test frame were produced, and subsequent ex situ ion irradiations were performed on devices with 1.4 MeV Zr ions to a nominal damage level of 0.26 displacement per atom. Subsequent in situ creep experiments performed with and without 1.4 MeV Zr ion irradiation at different applied tensile loads revealed that creep rates were enhanced by the simultaneous radiation damage. This coupled in situ nanomechanical and irradiation methodology enables rapid quantification of both the irradiation creep compliance and associated microstructural evolution.

Experimental and numerical study on mechanical properties of cement paste pipes subjected to uniaxial tensile loading

The aim of this paper is to investigate the mechanical properties of cement paste specimens by both experimental and numerical methods. Firstly, the specimens subjected to uniaxial tensile loading were studied experimentally. Afterwards, numerical investigation was carried out based on the experimental observations. Two types of specimens were used, which were unnotched and single notched specimens. The uniaxial tensile experiments of the unnotched specimens provided the Young’s modulus and tensile strength of the specimens. The complete stress-strain responses of the specimens were derived from the uniaxial tensile experiments on the single notched specimens. The crack initiation and propagation were discussed.The uniaxial tensile loading experiments were simulated by a 3D lattice model. The local mechanical properties of lattice elements were determined through simulations. The tensile simulations of the unnotched specimen provided the Young’s modulus and tensile strength for the local lattice elements. Then, the softening behavior of lattice elements was obtained from tensile simulations of the single notched specimen. The experimental and simulated stress-strain responses and cracking process were compared with each other. It was found that the simulated results matched quite well with the experiments with the set of local mechanical properties that was determined. This set was used in a further study for the simulations on external sulfate attack.

Influence of thickness reduction in base plate on elastic stress state of patch plate joint by combination of welding and bonding

ABSTRACT For investigating the influence of thickness reduction in base plate on elastic stress state of patch plate joint by combination of welding and bonding, a series of experiment and analysis were conducted. A static tensile loading experiment was carried out. The thickness reductions of 2.5 mm were applied to both top and bottom surfaces of a base plate with a thickness of 12 mm. The thickness-reduced parts of the base plate were covered by the patch plates with a thickness of 6 mm. The joining methods of patch plates were only fillet welding, only bonding and the combination of welding and bonding. The load-carrying effect at the centre of the joint was improved by the combination of welding and bonding compared to the case with only welding. However, the stress reduction effect around the welded part could not be clearly confirmed from the experimental result. The 2D elastic analysis was performed for simulating the experiment and examining the stress reduction effect around the welded part in detail. Furthermore, the influence of the degree of thickness reduction in base plate on the stress state around the welded part was investigated. The results indicated that the stress reduction effect by the combination of welding and bonding was larger around the weld root rather than the weld toe.

Interlayer fracture energy of 3D-printed PLA material

The application of 3D printing in additive manufacturing processes is expanding due to its versatility and the progress in the process control. In conjunction with its fast-growing applications, the performance and properties of the material after 3D printing need thorough assessments for critical designs in medical, aerospace, and automotive industries. Although increasing the deposition height in 3D printing decreases the time of manufacturing, the properties of the products must satisfy design specifications for the product strength and surface condition. The objective of the current study is to explore the effect of deposition height on tensile fracture energy of the layered structure of a biodegradable and bioactive thermoplastic polymer, known as polylactide (PLA). The results obtained from uniaxial tensile loading experiments show that the energy required for interlayer fracture is dependent on the deposition height. Based on the experimental results, it is evident that two factors conversely affect the fracture energy: tensile residual stress and the interlayer contact area. Also, the results show that the surface roughness has no significant influence on fracture energy.

Fatigue failure characterization of multi-axial warp-knitted composites under cyclic uniaxial tensile loading

In this work, the fatigue failure mechanisms of quadri- and tri-axial warp-knitted fabric composites (QWFC and TWFC) were investigated. A series of quasi-static state tensile tests and cyclic loading experiments were carried out, and the morphology of fatigue failure was visualized by optical and scanning electron microscopes (SEM). The results showed that while the failures of QWFC and TWFC were similar with X-shape morphology, QWFC had longer fatigue life than TWFC. The SEM images illustrated that the matrix of QWFC was not uniform, and contained cracks with scaly appearance, in addition to small amount of fiber curls. Moreover, the fracture surface was flat and layered. In contrast, the matrix of TWFC had propagation of cracks along ±45° direction; its fracture surface had sphere ‘budding yeast-like’ appearance. The wrap-knitted fabric contained increased numbers of layers in the composites, thereby had longer fatigue life.

Cohesive finite element modeling of the delamination of HTPB binder and HMX crystals under tensile loading

Accurately modeling the mechanical behavior of the polymer binders and the degradation of interfaces between binder and crystal is important to science-based understanding of the macro-scale response of polymer bonded explosives. The paper presents a description of relatively a simple bi-crystal HMX-HTPB specimen and associated tensile loading experiment including computed tomography imaging, the pertinent constitutive theory, and details of numerical simulations used to infer the behavior of the material during the delamination process. Within this work, mechanical testing and direct numerical simulation of this relatively simple bi-crystal system enabled reasonable isolation of binder-crystal interface delamination, in which the effects of the complicated thermomechanical response of explosive crystals were minimized. Cohesive finite element modeling of the degradation and delamination of the interface between a modified HTPB binder and HMX crystals was used to reproduce observed results from tensile loading experiments on bi-crystal specimens. Several comparisons are made with experimental measurements in order to identify appropriate constitutive behavior of the binder and appropriate parameters for the cohesive traction-separation behavior of the crystal-binder interface.This research demonstrates the utility of directly modeling the delamination between binder and crystal within crystal-binder-crystal tensile specimen towards characterizing the behavior of these interfaces in a manner amenable to larger scale simulation of polycrystalline PBX materials. One critical aspect of this approach is micro computed tomography imaging conducted during the experiments, which enabled comparison of delamination patterns between the direct numerical simulation and actual specimen. In addition to optimizing the cohesive interface parameters, one important finding from this investigation is that understanding and representing the strain-hardening plasticity of HTPB binder is important within the context of using a cohesive traction-separation model for the delamination of a crystal-binder system.

Fracture behavior of diamondlike carbon films deposited on polymer substrates

The authors investigated the fracture behavior of diamondlike carbon films deposited on polyethylene terephthalate substrates using two types of film synthesized under different sputtering-gas pressure conditions. Specifically, the influence of the hardness, indentation Young's modulus, and scratch toughness on the multiple cracking behavior of the films was explored. Based on microscratch and nanoindentation experiments, it was found that the sputtering-gas pressure conditions played an important role in the chemical composition and concomitant changes in the mechanical properties of the diamondlike carbon films. In particular, higher gas pressure conditions resulted in a lower hardness, indentation Young's modulus, and scratch toughness, which are presumably due to the presence of a large number of C–O bonds in the diamondlike carbon structure. In situ observations during tensile-loading experiments in conjunction with multiple crack analyses showed that the films synthesized under higher gas pressure conditions exhibited a high level of crack resistance to externally applied tensile strain. The estimated crack onset strain, i.e., the strain at which the first crack was generated, was approximately 2%, which is about 1.3 times higher than that for the diamondlike carbon films synthesized under lower gas pressure conditions.