Multiaxial Strain Research Articles

Comparison of multiaxial low cycle fatigue behavior of CP-Ti under strain-controlled mode at different multiaxial strain ratios

Multiaxial low cycle fatigue (MLCF) tests were performed on commercial pure titanium (CP-Ti) at different multiaxial strain ratios (λ = 0.865, 1.73, 3.46, 5.19). A higher λ accelerated the initial cyclic hardening for the axial stress. For torsional stress responses, all specimens showed initial cyclic hardening characteristics. Optical microscopy (OM) observations of the fracture surfaces showed that the critical plane of CP-Ti is better aligned with the maximum principal strain plane rather than the maximum shear strain plane. The fatigue-life curves obtained from the tests were constructed using the Zamrik, ESN, SWT, FS, and KBM models. According to the modified critical plane, a KBM-P model based on the maximum principal strain plane is proposed. A unified life prediction KBM-U model incorporating the effect of λ is further proposed. Scanning electron microscopy (SEM) observations of fatigue fractures showed that fatigue cracks initiated from the outer surface of the specimen at all λ. Many small cleavage surfaces of different sizes and heights were observed in the crack propagation zone. At higher λ, these cleavage characteristics were accompanied by shear dimples.

Study of plastically deformed region underneath the ball in indentation tests and evaluation of mechanical properties of materials through finite element simulation and a hybrid algorithm

Ball indentation technique has been studied extensively in literature and it has been widely applied to determine mechanical properties of different materials because of simplicity and minimal requirement of material for the tests. Originally, the material deformation was represented in terms of a representative strain, which is a non-dimensional form of indentation diameter and a representative stress, which again is the instantaneous mean pressure multiplied by some empirical factors. It is known that the state of deformation as well as stress beneath the ball is multiaxial in nature in a ball indentation test. In this work, a new hybrid algorithm for estimation of equivalent stress and equivalent plastic strain during the process of indentation in the most stressed location beneath the ball has been developed. The algorithms uses experimental load–indentation data as well as the multiaxial stress and strain parameters obtained from 2D axisymmetric elastic-plastic finite element simulations. The stress and strain multiaxial parameters are functions of applied load, material yield stress and strain hardening exponents. The algorithm is iterative in nature and uses suitable initial guess values for material yield stress and strain hardening exponents and it converges very quickly. This method can be applied to determine the material stress–strain curve for a wide range of equivalent plastic strain, yield stress as well as plastic strain hardening exponent of the engineering materials. The data points of load vs. indentation depth as obtained directly from the ball indentation tests can be used in the new algorithm without the need for conducting unloading during the test. To illustrate the new scheme, two case studies have been presented where the results from the proposed new method have been compared with those of the existing method in literature and tensile test data to check the accuracy.

Fracture Modeling Of Cracked Pipe Under Monotonic And Cyclic Loading

ABSTRACT The present work presents experiment and numerical modelling to quantify the effect of the loading type on ductile crack growth in circumferential through-wall cracked SA508 Gr.1a pipes. In tests, three types of loading conditions are considered; monotonic, displacement-controlled and load-controlled large-amplitude cyclic loading. Experimental data are compared with numerical crack growth simulation results based on the multiaxial fracture strain energy model. Simulated results show overall good agreements with experimental data for all loading types. Based on FE results, the effects of the loading type and cyclic hardening model on ductile crack growth are discussed.

Damage Assessment of 2.5D C/SiC Ceramic Matrix Composite: Effect of Fibre Off-Axis Orientation

2.5D C/SiC ceramic matrix composite is widely used for thermal protection and hot sections of gas turbine engines. The fibre axis in these components plays a big role in its performance, under multi-axial strain conditions. In the present investigation, the effect of fibre orientations has been studied. Three-point bend specimens of this composite with 30°/− 60°, 45°/− 45° and 0°/90° fibre orientations have been prepared and tested under bending loads. Various mechanical parameters such as flexural strength and Young’s modulus have been evaluated. A considerable reduction in the flexural strength and Young’s modulus has been observed in this composite with 30°/− 60° and 45°/− 45° fibre off-axis orientations. In situ damage assessment has been conducted with the help of the acoustic emission technique. Various micro-mechanics parameters such as the matrix cracking stress, matrix cracking saturation stress and stress for fibre fracture initiation have been derived using acoustic emission data as a function of the off-axis orientation of the fibres.

Plastic strain localization induced by microstructural gradient in laser cladding repaired structures

Laser Cladding is an additive manufacturing technology well suited for the repair of complex metallic components. The repair is a two-step process: first, one removes the worn region and then, the initial geometry is reconstructed locally. The aim of this work is to study the influence of the microstructural gradient on the strain localization in repaired structures. More precisely, we perform in-situ SEM tensile tests completed by EBSD observations of the microstructure in the interface neighborhood between the base material and the repaired region. Furthermore, we monitor the evolution of the local plastic strain distribution at the grain level until failure. This is performed by Digital Image Correlation methods and superposition of grains contours and strain maps. The observations of grain size and plasticity are compared with predictions provided by a Hall-Petch model. The study emphasizes the importance of the microstructural gradient in the vicinity of reparation interface, more precisely it reveals that this gradient induce multiaxial strains and that the strain localization phenomenon is governed mainly by a grain size effect.

A variable parameters damage model for concrete

A novel four-parameters damage constitutive model for concrete in strain space is proposed in this paper. It is based on the Heish-Ting-Chen constitutive model and is derived in the framework of the thermodynamics of irreversible process. Because tension damage is a major fracture mode in concrete used for hydraulic structures, an equivalent strain is introduced that converts the multi-axial strain state into the equivalent tension strain to determine the tension damage. Considering the change in the equations' parameters during progressive failure, a variable parameters constitutive model is established. The parameters of the damage constitutive model are obtained by the increasing section and the decreasing section of stress-strain curve and it is demonstrated that the proposed model performs better than the regular constitutive model in the decreasing section. The numerical implementations provide a good description of the failure process as well as an accurate prediction of the behaviour of concrete.

Microstructure-sensitive computational multiaxial fatigue of Al 7075-T6 and duplex Ti-6Al-4V

Multiaxial fatigue experiments are expensive and typically require specialized equipment and specimens, but they are essential to support fatigue design of components that experience multiaxial stress states. To leverage limited experimental data, we pursue parametric simulations regarding the influence of multiaxial strain state on extreme value distributions (EVDs) of fatigue crack formation driving force in realistic microstructures. Ensembles of Statistical volume elements (SVEs) for both Al 7075-T6 and Ti-6Al-4V polycrystalline microstructures are subjected to multiaxial cyclic straining to compute EVDs of Fatigue Indicator Parameters (FIPs) that serve as computable surrogate measures of the driving force for fatigue crack formation.

Lubricant-Added Conductive Composite for Direct Writing of a Stretchable Electrode.

Stretchable electrodes, which are essential components of next-generation electronic devices, should be highly conductive under multiaxial tensile strain, durable under repetitive stretching, and patternable for integrating stretchable devices. Herein, a lubricant-added stretchable conductive composite of a polydimethylsiloxane-based elastomer containing silver flakes is reported. The added lubricant minimizes changes in conductivity during stretching and maximizes elastic durability by reducing friction. The conductivity varies from 1933.3 S·cm-1 at 0% strain to 307.5 S·cm-1 at 300% uniaxial stretching and 1264.1 S·cm-1 at 50% biaxial stretching. Furthermore, the composite exhibits high durability, even after 1000 cycles of stretching at 200%, and the conductive composite paste can be applied to fine-linewidth direct writing.

Tubular Sensor with Multi-Axial Strain Sensibility and Heating Capability Based on Bio-Mimic Helical Networks

Sensors are crucial in industrial robotics and smart production line owing to the basic needs for human–machine communication and real-time monitoring. However, a lack of versatile sensing capabili...

Closed-Loop Haptic Feedback Control Using a Self-Sensing Soft Pneumatic Actuator Skin.

In this article, we achieve a closed-loop control over haptic feedback, first time for an entirely soft platform. We prototyped a novel self-sensing soft pneumatic actuator (SPA) with soft strain sensors, called SPA-skin, which withstands large multiaxial strains and is capable of high-frequency sensing and actuation. To close-loop control the haptic feedback, the platform requires a cohesively integrated system. Our system consists of a stretchable low profile (<500 μm) SPA and an ultra-compliant thin-metal film strain sensor that create a novel bidirectional platform for tactile sensing via force-tunable vibratory feedback. With this prototype, we demonstrated control of the actuator shape in real time up to 100 Hz at output forces up to 1 N, maintained under variable mechanical loadings. We further characterized the SPA-skin platform for its static and dynamic behavior over a range of actuation amplitudes and frequencies as well as developed an analytical model of this system to predict the actuator inflation state only using the embedded sensor's resistance. Our SPA-skin is a multifunctional multilayer system that can readily be implemented as a high-speed wearable bidirectional interface for contact sensing and vibrotactile feedback.

Numerical prediction of thermal aging and cyclic loading effects on fracture toughness of cast stainless steel CF8A: Experimental and numerical study

This paper presents a FE simulation method to predict the thermal aging effect on fracture behavior of cast stainless steel CF8A under monotonic and very low-cyclic fatigue loading conditions. A multi-axial fracture strain energy damage model is determined from tensile and C(T) test data of un-aged CF8A under the monotonic loading condition. To simulate fracture behavior of aged CF8A, the multi-axial fracture strain energy of aged CF8A is obtained by multiplying the thermal aging constant determined from tensile test data by the multi-axial fracture strain energy of the unaged CF8A. The model is then extended to predict fracture behaviours of unaged and aged CF8A under fully-reversed cyclic loading. For the damage model, the model determined for the monotonic loading is applied to simulate fracture behavior under cyclic loading, based on the assumption that fracture energy is not dependent on the loading mode. It is shown that simulation results can capture the thermal aging effect on fracture behavior under monotonic and cyclic loading well.

The Biaxial Strain Dependence of &lt;italic&gt;J&lt;/italic&gt;C of a (RE)BCO Coated Conductor at 77 K in Low Fields

Recently, we designed and commissioned a “crossboard” sample holder that can apply biaxial strains in the plane of a (RE)BCO coated conductor. It allows us to measure the critical current density ${\boldsymbol{J}_C}$ for arbitrary combinations of x - and y -strain. Understanding the in-field, in-plane, biaxial strain dependence of a tape's ${\boldsymbol{J}_{\bf{C}}}({\boldsymbol{B},{\boldsymbol{\varepsilon }_{\boldsymbol{xx}}},{\boldsymbol{\varepsilon }_{\boldsymbol{yy}}}})$ is crucial for applications such as CORC or Roebel cables, as the cables are subjected to multiaxial strains during manufacturing and operation. Here, we present experimental data for ${\boldsymbol{J}_{\bf{C}}}({\boldsymbol{B},{\boldsymbol{\varepsilon }_{\boldsymbol{xx}}},{\boldsymbol{\varepsilon }_{\boldsymbol{yy}}}})$ on a SuperPower SCS4050 APC tape in magnetic fields up to 0.7 T, at 77 K. We also outline a theoretical model for the biaxial strain dependence of ${\boldsymbol{J}_{\bf{C}}}$ and use it to parameterize our data and show that the fraction of A-domains and B-domains are roughly equal $({\boldsymbol{f} = 0.49 \pm 0.03})$ and that the strain sensitivity of the critical temperature is 1.8 ± 0.1 K%−1 and −1.3 ± 0.1 K%−1 along their a - and b- axes, respectively, for all the domains in this (RE)BCO tape. For the first time, we show both parabolic and linear strain dependencies of ${\boldsymbol{J}_{\bf{C}}}$ in a single tape by changing the angle between the applied strain direction and the twin boundaries in the (RE)BCO layer.

Computational Analysis of 3D Lattice Structures for Skin in Real-Scale Camber Morphing Aircraft

Conventional or fixed wings require a certain thickness of skin material selection that guarantees structurally reliable strength under expected aerodynamic loadings. However, skin structures of morphing wings need to be flexible as well as stiff enough to deal with multi-axial structural stresses from changed geometry and the coupled aerodynamic loadings. Many works in the design of skin structures for morphing wings take the approach either of only geometric compliance or a simplified model that does not fully represent 3D real-scale wing models. Thus, the main theme of this study is (1) to numerically identify the multi-axial stress, strain, and deformation of skin in a camber morphing wing aircraft under both structure and aerodynamic loadings, and then (2) to show the effectiveness of a direct approach that uses 3D lattice structures for skin. Various lattice structures and their direct 3D wing models have been numerically analyzed for advanced skin design.

A Coarse Model for the Multiaxial Elastic-Plastic Response of Ductile Porous Materials

We propose a modeling strategy to predict the mechanical response of porous solids to imposed multiaxial strain histories. A coarse representation of the microstructure of a porous material is obtained by subdividing a volume element into cubic cells by a regular tessellation; some of these cells are modeled as a plastically incompressible elastic-plastic solid, representing the parent material, while the remaining cells, representing the pores, are treated as a weak and soft compressible solid displaying densification behavior at large compressive strains. The evolution of homogenized deviatoric and hydrostatic stress is explored for different porosities by finite element simulations. The predictions are found in good agreement with previously published numerical studies in which the microstructural geometry was explicitly modeled.

Buckled carbon nanotube network thin-film fabricated using chemically swelled elastomer substrates

We report on the fabrication of buckled carbon nanotube thin-film networks (CNTN) that increases in conductivity with applied tactile pressure. When tactile pressure was applied, the buckled nanotubes collapsed and increased in interconnected density and as a result increased the thin-film conductivity. Unlike conventional methods using mechanically expanded elastomers, we utilize chemically swollen elastomers as the expanded substrate to transfer the CNTN. As the chemical evaporates, it compresses the CNTN causing the thin-film to buckle. The CNTN compression can be controlled by using organic solvents with differing elastomer absorption rates. Our method requires no mechanical instruments and shows in-plane multi-axial uniform strain for the entire substrate surface. Since the buckling was controlled chemically, the buckled CNTN can be produced reliably, furthering the possibility of its application as the active sensing material for highly sensitive tactile pressure sensors.

Turning Maneuver Effect on Near-Surface Airfield Pavement Responses

Airport pavement structures experience heavy aircraft tire loading through a localized contact area. Distributed three-dimensionally and non-uniformly, tire-pavement contact stresses directly influence the near-surface behavior of flexible airfield pavements. The resulting high shear stress levels induced by aircraft tire loading may lead to instability through shoving or slippage cracking. As the tire turns during taxiing, the risk of near-surface damage is exacerbated. In this study, numerical modeling of an inverted pavement system and a conventional flexible pavement structure loaded with a single tire from the A-380 landing gear was developed. The analysis matrix included two tire-inflation pressures, two speeds, and rolling conditions that varied from free-rolling to two turning maneuvers. Two analysis approaches were performed: 1) use of traditional critical point strains, and 2) domain analysis, which characterizes bulk pavement behavior using multiaxial stresses and strains. The critical strains, which are used as inputs for airfield pavement design, changed negligibly under varying tire turning conditions despite the asymmetric contact stress distribution. On the other hand, domain analysis not only captured the asymmetric pavement behavior, but also identified that altering the tire movement from a free-rolling condition to turning could induce a significant increase in the potential damage.

MAPPING COMPRESSIVE AND TENSILE STRAINS OF THE PATELLAR TENDON IN OSGOOD-SCHLATTER DISEASE USING ULTRASOUND ELASTOGRAPHY

Background Osgood-Schlatter Disease (OSD) impacts millions of young athletes every year, yet its biomechanical etiology remains unclear. It’s been postulated that increased growth velocity in children elevates tension within the patellar tendon, placing distal strain on the tibial tubercle, causing anterior knee pain, but this process has not been previously elucidated. The purpose of this study was to evaluate axial tensile and transverse compressive strains in adolescent patellar tendons during exercise to identify differences between those with and without OSD using a cumulative strain, ultrasound elastography measurement method [REF: Chimenti et al 2016]. Methods Axial tensile and transverse compressive strains of the patellar tendon in eight adolescents with OSD and ten adolescents without OSD were quantified from ultrasound elastography images. Multiaxial strains of the patellar tendon were assessed as the participant performed two different exercises: 1) participant seated with leg fully extended, participant bends knee, followed by a return to initial extended position; (2) participant standing performs a squat and then returns to initial standing position. Participants’ disease severity was clinically assessed using the VISA-P and Cincinnati knee scale. Results Mean age was 13.1 years (+/- 2.7), 39% were male (63% of OSD group). There was no statistically significant difference in patellar tendon tensile strains during the seated knee flexion and squat positions between groups (p = 0.99, p = 0.83 respectively). The OSD group had significantly less tendon compression compared to the non-OSD group during both the knee flexion (p &lt;0.01), and squat exercises (p &lt;0.05). Conclusions/Significance Adolescents with OSD had lower transverse compressive strains in the patellar tendon as compared to adolescents without OSD. In contrast, tensile strains were not increased in children with OSD. These findings appear to contradict the tension hypothesis postulating that OSD is caused by increased patellar tendon axial tension associated with rapid bone growth, Instead, impingement from the tibia leading to transverse tendon compression may lead to an adaptive stiffing the patellar tendon, However, further research is needed to confirm this hypothesis. While OSD is a common cause of knee pain in growing athletes that frequently leads to loss in participation, its etiology is unclear, making development of effective treatment approaches difficult. This study highlights differences in multiaxial patellar tendon strains in adolescents with OSD which may help direct future research on management strategies.

Application of machine learning to predict the multiaxial strain-sensing response of CNT-polymer composites

We present predictive multiscale models of the multiaxial strain-sensing response of conductive CNT-polymer composites. Detailed physically-based finite element (FE) models at the micron scale are used to produce training data for an artificial neural network; the latter is then used, at macroscopic scale, to predict the electro-mechanical response of components of arbitrary shape subject to a non-uniform, multiaxial strain field, allowing savings in computational time of six orders of magnitude. We apply this methodology to explore the application of CNT-polymer composites to the construction of different types of sensors and to damage detection.

Optofluidic and strain measurements induced by polarization-resolved nanosecond pulses in gold-based nanofluids

The influence of an optical fringe patterns on the viscoelastic properties exhibited by Au nanofluids samples was evaluated. A sensitive interferometric technique for analyzing optofluidic effects in different samples was implemented. The assistance of the plasmonic characteristics in Au nanostructures allows measuring particular mechano-optical effects at 532 nm wavelength by a Fabry-Perot interferometer to explore multiaxial strains. A representative volume of the sample was studied to determine the stability and maximum viscoelastic properties exhibited by the nanostructures. A vectorial two-wave configuration allows controlling the maximum strain induced in the sample. The oscillating nature of the colloid was examined by using interferometric optical signals reflected from a remnant drop pending at the end of an optical fiber. Nanosecond pulses were used to induce inelastic optofluidic effects. The mechanical parameters were approximated by a nonlinear second order system activated by a Dirac delta functions.

Fracture toughness prediction of thermally-aged welded stainless steels using multi-axial fracture strain FE damage model

Thermal ageing constant “C” concept was applied with multi-axial fracture strain FE damage model to predict the fracture toughness of aged gas tungsten arc weld (GTAW) type 316L stainless steel (SS). Although the model was developed for cast stainless steel (CASS), it showed good agreement with GTAW SS. Then, a study was conducted to find the relationship between the fracture toughness of aged GTAW SS and ductility-related variables such as reduction of area (R.A), multi-axial fracture strain or thermal ageing constant “C”. And it was found out that the thermal ageing constant “C” has a linear relation with the fracture toughness of aged GTAW SS.