Biaxial Loading Research Articles

Mechanical behavior of backfilled pillar under biaxial loading

Mechanical behavior of backfilled pillar under biaxial loading

A comparative review on bubble-voided reinforced concrete slab – An innovative concept for lightweight concreting

The bubble-voided reinforced concrete (RC) slab is a relatively new concept of light-weight concreting, introduced to the industry. In this concept, the consumption of unnecessary concrete between the tension zone and the neutral axis, inside a RC slab (one-way or two-way), can be avoided by the introduction of voids at that very zone in the form of hollow plastic bubbles. The bubble-voided slab is lightweight in comparison to the solid slab, and self-weight can effectively decrease upto 30% to 40%, through the application of spherical voided-formers. Although, the bubble-voided slab’s load capacity is slightly less, compared to the equivalent solid slab. But, unlike the hollow-core slab, bubble-voided slab performs well against bi-axial loading. As, the concrete consumption is less in bubble-voided slabs, so less amount of cement needs to be manufactured, and, this way, the bubble-voided concreting method can indirectly be beneficial in reducing the Carbon-Di-Oxide (CO2) emission in the atmosphere. The bubbles can be made of plastic waste, which is an innovative way to recycle that waste plastic to use in industry, and, also reduce plastic wastage from the nature. So, it can be understood that bubble-voided concreting is very much eco-friendly. The present authors have explored through the evolution in the researches on the bubble-voided concrete slab, and, discussed about the critical findings of previous researchers. A comparative discussion is also, presented in this study on various input parameters and load vs deflection curves presented in previous studies. The comparisons showcase that the depth of slab (DS), grade of concrete, and, percentage of void, are the key factors which influence the load vs deflection behaviour of bubble-voided RC slabs. Future scopes in this field of study are identified, such as, studies on the effect of thermal loading, impact loading, and, cyclic loading on the bubble-voided RC slabs.

Numerical study on the behaviour of concrete-encased composite slender columns under biaxial loading

Slender steel–concrete composite structural members that utilise the merits of steel and concrete are becoming integral elements in modern-day construction. Encased Steel-Concrete [ESC] has encased outer concrete portion that provides higher stiffness against buckling and protects the inner core steel. The steel portion has greater bending resistance and provides ductility. Amongst the various structural elements, the ESC Column [ESCC] members are often implemented in construction. The present study comprises the behaviour of slender ESCCs subjected to biaxial loading through Finite Element [FE] parametric studies using ABAQUS. The FE model and analysis are validated against the experimental results from existing research work. In this study, a total of 54 slender ESCC- FE models is analysed to access the effect of the governing factors: slenderness ratio, compressive strength of concrete, yield strength of steel, steel contribution ratio, load eccentricity ratio and transverse reinforcement spacing It is found that the ultimate load-carrying is increased about 5.09%, 17.49% and 20.11% for increased structural steel contribution ratio, yield strength of steel, and concrete strength, respectively. Further, the load-carrying capacity (0% to 7.08%) decreases when the eccentricity (0 mm to 200 mm) and slenderness increase.

Changes in the microstructure of the human aortic adventitia under biaxial loading investigated by multi-photon microscopy.

Among the three layers of the aortic wall, the media is primarily responsible for its mechanical properties, but the adventitia prevents the aorta from overstretching and rupturing. The role of the adventitia is therefore crucial with regard to aortic wall failure, and understanding the load-induced changes in tissue microstructure is of high importance. Specifically, the focus of this study is on the changes in collagen and elastin microstructure in response to macroscopic equibiaxial loading applied to the aortic adventitia. To observe these changes, multi-photon microscopy imaging and biaxial extension tests were performed simultaneously. In particular, microscopy images were recorded at 0.02 stretch intervals. The microstructural changes of collagen fiber bundles and elastin fibers were quantified with the parameters of orientation, dispersion, diameter, and waviness. The results showed that the adventitial collagen was divided from one into two fiber families under equibiaxial loading conditions. The almost diagonal orientation of the adventitial collagen fiber bundles remained unchanged, but the dispersion was substantially reduced. No clear orientation of the adventitial elastin fibers was observed at any stretch level. The waviness of the adventitial collagen fiber bundles decreased under stretch, but the adventitial elastin fibers showed no change. These original findings highlight differences between the medial and adventitial layers and provide insight into the stretching process of the aortic wall. STATEMENT OF SIGNIFICANCE: To provide accurate and reliable material models, it is essential to understand the mechanical behavior of the material and its microstructure. Such understanding can be enhanced with tracking of the microstructural changes caused by mechanical loading of the tissue. This study provides therefore a unique dataset of structural parameters of the human aortic adventitia obtained under equibiaxial loading. The structural parameters describe orientation, dispersion, diameter, and waviness of collagen fiber bundles and elastin fibers. Eventually, the microstructural changes in the human aortic adventitia are compared with the microstructural changes in the human aortic media from a previous study. This comparison reveals the cutting-edge findings on the differences in the response to the loading between these two human aortic layers.

The influence of out-of-plane motion on the deformation measurement of planar biaxial tests on biological soft tissue

Planar biaxial testing is a popular technique to characterize a wide range of biological soft tissues. Nevertheless, a lot of variability exists between experimental setups. One particularly important variation relates to the difference in strain measuring techniques to characterize the deformation during the test: digital image correlation using one camera (measuring the out-of-plane motion as false strain), or two cameras (providing a stereo view). This study aimed to compare the two methods and provide practical guidelines for their use. A planar biaxial experiment was performed on ovine arterial samples on which both methods produced similar results in terms of strain measurements and material properties. The two-dimensional approach using a single camera is an appropriate and less time-consuming alternative to measure the deformation of soft biological tissue during planar biaxial loading conditions for similar setups.

Effect of horizontal stress on the mesoscopic deformation and failure mechanism of layered surrounding rock masses in tunnels

The deformation and failure behaviors of layered surrounding rocks under high in-situ stress are important indicators for tunnel stability evaluation. The attitude of bedding planes is too complex, which impedes the understanding of the rock deformation mechanism under different horizontal stress, especially from the microscopic perspective. In this study, high-precision and easy-to-prepare 3D printing materials are used to simulate layered rock masses. The biaxial loading test system is adopted to explore the creep properties of layered rocks during tunnel excavation in the case of different horizontal stress. The mesoscopic deformation field and failure evolution mechanism of the layered surrounding rocks are revealed using digital image correlation technology. The test results are verified with the numerical simulation method. The shear displacement and plastic strain zone of surrounding rocks in the case of different layer dip angles are studied, and the plastic failure region is given. This paper provides significant experimental and theoretical guidance for the excavation and support of layered soft rock tunnels.

Experimental and numerical study of resistance of multi-cell L-shaped concrete-filled steel tubular stub columns under biaxial eccentric compressive loading

This study aims to understand the cross-sectional resistance and develop practical resistance equations for multi-cell L-shaped concrete-filled steel tubular (ML-CFST) stub columns under biaxial eccentric compressive loading based on experimental testing and numerical analysis. Eighteen specimens were firstly tested to investigate experimentally their failure modes, nonlinear structural behavior, and the influence of different loading angles (θ) and eccentricities on the load-carrying capacity. Secondly, detailed nonlinear finite element models were developed and validated against test results, and further used for parametric analysis to explore untested scenarios. This facilitated the construction of a failure surface, which was interpreted as a family of Mx-My interaction curves under a variety of axial compression ratios (n), where Mx and My are the bending moments about the minor and major axes, respectively. Results showed that ML-CFST specimens exhibited favorable structural performance, and n and θ had a considerable influence on the normalized Mx-My interaction curves, compared to other geometric and material strength parameters (i.e., depth-to-width ratio of section limb, steel-to-concrete area ratio, steel yield strength and concrete compressive strength) studied. To facilitate engineering use, a dimensionless interaction equation for normalized Mx-My was developed in good agreement with experimental results.

Mechanical properties and dynamic multifractal characteristics of shale under anisotropic stress using AE technology

The mechanical properties and failure mechanism of shale under anisotropic stress is the key to evaluate the stability of wellbore. Meanwhile, the fracture instability process of shale needs to be characterized by the reasonable monitoring technology. This paper performs a series of uniaxial, biaxial and true triaxial loading tests on shale specimens. A monitoring technology, Acoustic emission (AE), is adopted to record the fracture process of shale. The effects of anisotropic stress on the mechanical behaviors of shale are analyzed. The evolution characteristics of AE event rate are applied to describe the fracture process of shale. The transition from AE quite period to AE active period effectively characterizes the macro-cracking process within shale specimens. The multifractal theory is adopted to analyze the dynamic multifractal characteristics and to quantitatively characterize the fracture instability process of shale. The distribution difference of multifractal parameters can be served as an early warning information for the stable and unstable macro-cracking within shale. The anisotropic stress evidently affects the failure mechanism of shale. The results of this study will be important to ensure the production efficiency of shale gas and the stability of drilling operation.

Finite Element Simulation of Fiber Reinforced Composites Under Bi-Axial Loading

Advances in testing of composite materials in recent years have mostly resulted in the ability to actively characterize the uniaxial response of composites while investigations of multiaxial load cases are rare due to various challenges. One of these challenges is the lack of a suitable specimen geometry for multiaxial testing without unwanted failure modes. Ideally, such geometry should be developed and assessed virtually by means of simulation before costly and time-consuming manufacturing and testing. Therefore, reliable, and efficient simulation methods are required that incorporate the evolution of damage in FRP composites. This study investigates various geometric features virtually of cruciform specimens to test fiber-reinforced composites subjected to in-plane biaxial tensile loadings. The goal is to achieve uniform failure in the gauge region of the specimen due to biaxial stress states and to reduce any premature failure outside the gauge region. Efficient finite element simulation in the commercial software LS-DYNA is used to identify the optimal geometric features.

Link between cracking mechanisms of trilayer films on flexible substrates and electro-mechanical reliability under biaxial loading

The propagation of cracks from a top layer in trilayer systems (Cr/Cu/Mo) on a polyimide substrate is studied experimentally by in situ synchrotron x-ray diffraction under equi-biaxial loading. The results show that depending on the thickness of the ductile Cu middle layer (100 or 500 nm), the propagation can be a direct vertical path through all layers or a more complex path. These effects are analyzed by monitoring the individual stresses of each layer along with electrical resistance and resulting crack patterns. Cracks starting from the upper Cr layer propagate instantaneously through the whole system for a 100 nm Cu layer but are strongly deflected in a 500 nm Cu layer, thus delaying the global fracture of the system measured by the increase of electrical resistance. Mechanisms are proposed and allowed to anticipate the electro-mechanical performances of stretchable systems constructed of several layers.

High Expansion Auxetic Skin Graft Simulants for Severe Burn Injury Mitigation

Burn injuries are commonly treated with split-thickness skin grafting. However, low expansions offered by spilt-thickness skin grafting inhibit the treatment of large and severe burn injuries when limited donor skin is available. To overcome this gap, in this work, it was attempted to study the expansion potential of skin grafts with novel auxetic incisions with rotating rectangle (RR), honeycomb (HC), alternating slit (AS), H-shaped (HS), Y-shaped (YS), and I-shaped (IS) unit cells, through development of skin graft simulants. Clinically relevant biaxial load testing was conducted to estimate the stress–strain response, void area, and meshing ratio. Moreover, hyperelastic constitutive models were employed to characterize the non-linear biomechanical behavior of the skin graft simulants. The maximum void area increase was observed in the HS skin graft simulant, indicating low skin cover. Overall, the IS auxetic skin graft design exhibited meshing ratio higher than traditional grafts (>3:1), low void area and stresses, which can be beneficial for large skin cover and burn wound healing. With further optimization and clinical tests, the auxetic skin graft designs may find a place with the graft manufacturers for fabrication of grafts with better surgical outcomes for severe burn injuries.

Physics of molecular deformation mechanism in 6H-SiC

Even though there have been several studies in literature of 6H SiC, a proper physics based understanding of the molecular deformation mechanisms of the material under different loading conditions is still lacking. Experimentally, the brittle nature of the material leads to difficulties associated with in-situ determination of molecular deformation mechanisms of the material under an applied load; whereas, the complex material structure along with the bonding environment prevents proper computational identification of different types of inelasticity mechanisms within the material. Molecular dynamics study (on successful verification of the interatomic potential with experimental results) of pristine single crystals of 6H SiC have been used to probe the physics of molecular deformation mechanisms of the material along with its inherent orientational anisotropy. The study elucidates the experimentally observed mechanisms of defect nucleation and evolution through a detailed analysis of radial distribution functions, x-ray diffraction as well as phonon vibrational studies of the single crystal. Studies have been presented at room temperature, initial high temperature and different types of confinement effects of the material (including hydrostatic and different biaxial loading cases). The confinement resulted in an increase in stress and stiffness whereas increase in initial temperature resulted in a decrease compared to uniaxial stress loading conditions at room temperature.

A combined experimental and numerical approach to investigate the failure behaviors of 3D woven composites under biaxial tensile loading

The failure behaviors of 3D woven composites (3DWC) under biaxial tensile loadings are investigated through a combined experimental and numerical approach in this paper. Appropriate biaxial cruciform specimen of 3DWC is first designed according to the experimental requirements. The biaxial tensile experiments are conducted on a servo-hydraulic biaxial testing facility, and then the full-field strain distributions and crack morphologies of cruciform specimens are analyzed in detail. For the multiscale geometric reconstruction of 3DWC, a full-thickness mesoscopic RVC is first extracted from the periodic woven architecture, and then a novel full-scale geometric modeling method is developed to reconstruct the biaxial cruciform specimens of 3DWC. Particularly, in conjunction with the micromechanics of failure theory, a multiscale damage model is proposed to simulate the biaxial failure behaviors of 3DWC. With the stress amplification factor, a transfer relationship between mesoscopic and microscopic stresses is established, and then the damage states of matrix and fibers within yarns can be separately determined. By comparing the experimental and predicted results, the realistic damage mechanism of 3DWC under biaxial tensile loading can be elucidated.

An open-source biaxial tensile tester with automated pre-tensioning for mechanical studies of canvas paintings

The mechanical aspects of canvas painting conservation and the study of the effects of conservation treatments benefit greatly from quantifying the mechanical characteristics of the materials. However, this is seldom possible as only few labs have the necessary equipment. This paper presents the development of a biaxial tester to be used for samples of canvas paintings which exhibit orthotropic behavior under biaxial loads. The machine was built as the first step of ongoing Ph.D. research on the mechanics of canvas paintings. An effort was made to create a system that is easy to assemble, with parts that are easy to source and with an overall cost well below the commercial units available. The control software includes the function of automated pre-tensioning to improve the accuracy of the measurement. Our broader purpose here is to make an easy-to-replicate machine available to help conservators and conservation scientists perform tensile tests to make informed choices in materials science.

Numerical investigation on the ultimate strength behaviour and assessment of continuous hull plate under combined biaxial cyclic loads and lateral pressure

The ultimate strength of continuous hull plate under combined biaxial cyclic loads and lateral pressure is investigated in the present paper by using nonlinear finite element method. Geometric nonlinearity due to large deflection and material nonlinearity induced by kinematic hardening and isotropic hardening are both accounted for. A parametric study is designed and completed to examine the longitudinal ultimate compressive strength behaviours in the cycles on the basis of a large number of load-shortening curves. Effects of series of parameters especially the plate slenderness ratio, transverse cyclic compression, lateral pressure and cycle number are analyzed with details. It is found that the strength characteristic and collapse mode are highly affected by the coupling influence of the mentioned factors as well material behaviours. A dimensionless unified formulation as a function of the decisive factors is empirically proposed to accurately assess the ultimate strength of continuous hull plate in various cycles.

Probability embedded failure prediction of unidirectional composites under biaxial loadings combining machine learning and micromechanical modelling

This study presents a data-driven, probability embedded approach for the failure prediction of IM7/8552 unidirectional carbon fibre reinforced polymer (CFRP) composite materials under biaxial stress states based on micromechanical modelling and artificial neural networks (ANNs). High-fidelity 3D representative volume element (RVE) finite element models were used for the generation of failure points. Fibre failure and the friction between fibres and matrix after fibre/matrix debonding were taken into consideration and implemented as VUMAT subroutines, respectively. Uncertainty quantification was conducted based on a coupled experimental–numerical approach and failure probabilities were inserted into the failure points to generate the database for the training of ANNs. A total of 15 biaxial stress combinations were considered for the generation of datasets. Two strategies were considered for the construction of form-free failure criteria based on the ANNs for regression and classification problems. It is found that for the regression problems, an ANN model with 2 hidden layers and 64 neurons can achieve a mean square error (MSE) of 0.027% and a mean absolute error (MAE) of 0.78%. For the classification problems, an ANN model with 3 hidden layers and 32 neurons, presents an excellent performance in the prediction with a probability of 98.1%. A good agreement was observed between the failure strength of composites under transverse and in-plane shear predicted by these ANNs and failure envelopes theoretically predicted by Tsai–Wu and Hashin failure criteria.

Discrete shear projection method for mechanical buckling analysis of FGM sandwich plates

This paper deals with the mechanical buckling analysis of functionally graded material (FGM) sandwich plates using quadrilateral element Q4γs based on the discrete shear projection method (DSPM) to avoid shear locking. The element has four-node and five degrees of freedom per node. It allows modelling two types of FGM sandwich plates, i.e. type A with a homogeneous core and FGM face sheets, and type B with homogeneous top and bottom layers and an FGM core. Different thickness schemes have been considered leading to symmetrical or unsymmetrical geometrical stacking. The paper focuses on the buckling in simply supported square plates under uniaxial or biaxial loads. The linear classical buckling theory (LCBT) is retained to evaluate the critical loads. The results obtained by the Q4γs element are compared with those reported by Zenkour (2005) and using another in-house finite element based on DKMQ and DKQ elements. All results are in good agreement when the LCBT is used to define critical loads, but the paper includes selected non-linear analysis based on the general-purpose finite element software ABAQUS. The results show the limit of validity of the LCBT for unsymmetrical stacking in the sandwich of type A and all sandwich of type B.

Investigation on the behavior factor of leaning masonry wall structures

In force-based seismic design, the behavior factor q (or response modification factor R) is devised to prevent overdesign by reducing the design force. Different values of the behavior factor q are available for various structure types. However, masonry fence walls are not categorized in the current seismic design codes. These walls are of poor quality due to the absence of proper quality assurance and maintenance, and most of those structures are inclined. In this study, the behavior factor q of leaning masonry fence walls is experimentally estimated. We fabricate three masonry wall structures that are inclined with three different wall angles of 1.5 $$^\circ$$ , 3.0 $$^\circ$$ , and 6.0 $$^\circ$$ , respectively. Monotonically increasing biaxial lateral loads are applied to the walls using a shaking table. The value q of each leaning wall is calculated by analyzing the dynamic load–displacement relationship. A relationship between the factor q and wall angle is discussed, and a new empirical formula representing q as a function of a wall angle is proposed. The proposed empirical formula would be useful in seismic design for severely leaning masonry walls.

An Improved Approach for Forming Limit Prediction of Strongly Anisotropic Thin-Walled Tube Combining M–K Model and Ductile Fracture Criterion

Abstract Accurate prediction of the forming limit at necking of strongly anisotropic thin-walled tubes is an urgent requirement in tube hydroforming. M–K model is one of the most widely used prediction models. However, the failure criterion in M–K model is based on the ratio of major true strain increments in the groove to uniform regions, which sometimes results in overestimation of the limit strains. To address this issue, a combined model of M–K model and ductile fracture criterion (DFC) for tube was proposed in this paper, in which the failure condition of the groove region is determined by the DFC. The characteristics of the M–K + DFC model were analyzed by combining with the DF2012 phenomenological DFC. To validate the M–K + DF2012 model, the limit strains at necking of an AA6061 tube with strong in-plane anisotropy were tested through tube-controllable biaxial loading experiments. The results show that the forming limit curve (FLC) predicted by the M–K + DFC model does not exceed the FLC calculated by the M–K model and the fracture forming limit curve (FFLC) determined by the corresponding DFC. The M–K + DF2012 model provides a reasonable forming limit prediction of the AA6061 tube, and the shortcoming of the M–K model mentioned earlier is overcome. Meanwhile, a reasonable constitutive model considering the in-plane anisotropy is very important in the forming limit prediction of a strongly anisotropic thin-walled tube.

Multiaxial Plastic Deformation of Zircaloy-4 Nuclear Fuel Cladding Tubes

The following work is motivated by the desire to devise an internal pressure test that can mimic a displacement-controlled loading scenario and demonstrate how to apply the multiaxial stress and strain data from the test to develop an elastic/plastic constitutive model for a thin-walled tubular component. This is achieved by conducting simultaneous measurements of tangential and axial strain during the pressure test and integrating these strain measures into a feedback loop with the pressure controller. It is shown how data from such a test can be used to develop a large mechanical property data set relevant to biaxial loading conditions. The data obtained have high confidence evidenced by their low variability and alignment with other literature studies. Additionally, data from these internal pressure tests combined with full-tube axial tensile tests allow for the derivation of the Hill anisotropic yield function. The developed Hill yield function is validated by comparing the plastic strain ratios from the full-tube tension tests and by comparing the predicted yield stress in the tangential direction with measured values from ring tension tests in a previous study.