Uniaxial Loading Research Articles

Experimental and theoretical study on high-temperature creep of VT6 titanium alloy under multi-axial loading conditions

In the framework of damage mechanics, we discuss a new mathematical model that describes the kinetics of the stress–strain state and damage accumulation during material degradation by the mechanism of long-term strength under complex multiaxial stress state. An experimental and theoretical technique is proposed for determination of material parameters and scalar constitutive functions for damaged media based on specially set experiments on laboratory specimens. The results of experimental studies and numerical simulations of short-term high-temperature creep of VT6 titanium alloy under uniaxial and multiaxial loading are presented. Numerical results are compared with the data obtained through experiments. Particular attention is paid to simulating the process of unsteady creep for complex deformation modes, accompanied by rotation of main areas of stress, strain and creep strain tensors. It is shown that the developed version of the constitutive relations of the damaged media enables us to describe the processes of unsteady creep and long-term strength of structural alloys under multiaxial stress with the accuracy sufficient for engineering calculations.

Analysis of the Internal Structure of Brenna Sandstone Samples with Respect to the Differences in Measured Quasi-elastic Moduli

The Young’s modulus and Poisson’s ratio determined for the rocks under uniaxial or triaxial loading conditions represent necessary input parameters for solving many geotechnical tasks. However, these effective stiffness moduli, particularly the Poisson’s ratios, measured on different samples sometimes substantially differ, even for visually compact and macroscopically homogeneous rock. This has been observed for Brenna sandstone, especially under conditions of conventional triaxial extensions. The aim of this study was, thus, to reveal differences in rock structure that could cause such behavior. Several complementary methods were used to investigate the structure of this sandstone: X-ray computed tomography (CT), visual analysis using optical scanning and stereo-microscopy, and a new method combining water jet erosion with these visualization techniques. Analyses of this structure revealed mechanically more resistant compact thick and adjacent thin weak layers. In rock samples, these layers have different patterns and orientations. To demonstrate the influence of layer orientation on effective stiffness moduli measured under different loading conditions, FEM calculations were performed for idealized structural models. The outcomes of this numerical analysis are in qualitative agreement with the results of the loading experiments and the layered structure revealed in the samples. The ability of the proposed method using water erosion to analyze the rock structure in detail was verified. The method offers an interesting alternative to standard visual and X-ray CT techniques. The numerical calculations indicate the importance of latent layered defects in sedimentary rocks for fine measurements of stiffness moduli (especially the Poisson’s ratios) used as inputs for geotechnical applications.

Elliptic Rigid Inclusion in Soft Materials of Harmonic Type

Abstract We investigate finite plane deformations of an elliptic rigid inclusion embedded in a soft matrix that is made of a particular class of harmonic-type hyperelastic materials. The inclusion is assumed to be perfectly bonded to the matrix, which is subjected to a constant remote in-plane loading. Utilizing the Cauchy integral techniques associated with conformal mappings, we derive closed-form solutions for the full-field deformation, Piola stress, and Cauchy stress in the entire matrix. Numerical examples are presented to illustrate the current solutions in comparison with those established from linear elasticity theory. We find that in terms of the Cauchy stress around the inclusion, the maximum normal stress component always appears at the endpoints of the major axis of the inclusion, irrespective of the magnitude of the remote loading, while the maximum hoop stress component occurs not exactly at the above-mentioned endpoints when the remote loading exceeds a certain value. In particular, we identify an exact explicit formula for determining the relative rotation of the inclusion during deformation induced by a remote uniaxial loading of arbitrarily given magnitude and direction.

An experimental study of the relationship between modulus of elasticity, Poisson's ratio, and the mechanical properties of high-performance concrete

This study investigated the mechanical properties of high-performance concrete (HPC) with the percentage of mineral admixtures. HPC is a good choice for multi-story buildings because it has many advantages. As HPC becomes more popular, it is important to research its properties. This study used the P.C. Aitcin method to design HPC mixes with compressive strengths of 60 to 80 MPa. Fly ash is used for M60 and M70 grade HPC and Silica Fume is used for M80 grade HPC is used as mineral admixture as a 20% replacement for cement. In this research paper, the study of properties of the different ingredients used to make HPC is carried out. The testing of three concrete mixes for mechanical properties such as compressive strength, split tensile strength, modulus of elasticity, and flexural strength, as well as other important properties is done. This study explores High-Performance Concrete (HPC) by precisely controlling the water-to-cement ratio and incorporating mineral and chemical admixtures, achieving strengths from 60 to 80 MPa. It investigates key mechanical properties, establishes empirical equations, and compares findings with codes and literature. The results showed that a lower water-to-binder ratio (w/b) led to better mechanical properties. The experimental program includes the casting of 39 cubes, 39 cylinders, 39 beams and 81 columns to test the physical properties of different concrete grades. The use of a compression testing machine with a load capacity of 3000 kN to test the HPC cubes and a loading frame with a load capacity of 2000 kN to test the columns. The average compressive strength for HPC M60 to M80 was 65.40 MPa, 73.45 MPa, and 82.64 MPa respectively. The modulus of elasticity values ranged from 40 to 50 GPa. The findings of the study showed that an increase in concrete strength correlated with a decrease in the average value of Poisson's ratio. Overall, the study suggests that HPC is a good choice for high-rise buildings because it has many advantages. As HPC becomes more popular, it is important to research its mechanical properties. This study provides valuable results about the mechanical properties of HPC in comparison with IS 456:2000. From the experimental results of tested columns, it was observed that as the load increased, the deflection decreased for both uniaxially and biaxially loaded columns, highlighting the novelty of this finding. Additionally, the columns subjected to biaxial loading displayed greater sensitivity compared to their axial and uniaxial loading.

Residual mechanical properties of high strength concrete BBR9 subjected to dynamic uniaxial compressive loading

In recent years, enormous efforts have been devoted to accurately quantifying the residual load-bearing capacity of concrete at the material level when exposed to elevated temperatures. However, relatively less attention has been given to investigating the effects of dynamic events such as terrorist attacks, earthquakes, and hurricanes, which can cause complete collapse of concrete structures and have significant public and economic consequences. Hence, in the present study, a modified Kolsky compression bar system integrated with a momentum trapping technique was implemented to develop precisely controlled pre-/post peak high strain-rate loading to introduce distinctive states of damage inside the high strength concrete HSC-BBR9 specimens and to measure the corresponding plastic strain. Thereafter, the partially damaged specimens were tested to quantify the residual mechanical properties (e.g., residual stiffness and residual strength) at different plastic strain levels. Furthermore, a high-resolution laboratory micro-CT scanner was used to visualize the 3D crack network morphology in the partially damaged specimens. The findings demonstrated a remarkable decrease in the residual capacity with increasing levels of damage-induced plastic deformation. Additionally, the results showed a fundamental difference in the influence of internal damage on residual stiffness and strength to be acknowledged in the formulations of future constitutive damage models for concrete materials.

Design strategy of porous elastomer substrate and encapsulation for inorganic stretchable electronics

ABSTRACT The emergence of stretchable electronic technology has led to the development of many industries and facilitated many unprecedented applications, owing to its ability to bear various deformations. However, conventional solid elastomer substrates and encapsulation can severely restrict the free motion and deformation of patterned interconnects, leading to potential mechanical failures and electrical breakdowns. To address this issue, we propose a design strategy of porous elastomer substrate and encapsulation to improve the stretchability of serpentine interconnects in island-bridge structures. The serpentine interconnects are fully bonded to the elastomer substrate, while segments above circular pores remain suspended, allowing for free deformation and a substantial improvement in elastic stretchability compared to the solid substrates. The pores ensure unimpeded interconnect deformations, and moderate porosity provides support while maintaining the initial planar state. Compared to conventional solid configurations, finite element analysis (FEA) demonstrates a substantial enhancement of elastic stretchability (e.g. ≈9 times without encapsulation and ≈ 7 times with encapsulation). Uniaxial cyclic loading fatigue experiments validate the enhanced elastic stretchability, indicating the mechanical stability of the porous design. With its intrinsic advantages in permeability, the proposed strategy has the potential to offer insightful inspiration and novel concepts for advancing the field of stretchable inorganic electronics.

Multiaxial fatigue life prediction based on effective factor with shear/axial stress ratio for Ti60 titanium alloy in very high cycle regime

Fatigue failure mechanism of Ti60 titanium alloy under tension–torsion multiaxial loading in the very high cycle regime was studied first, and the results showed that the shear/axial stress ratio directly affects the failure mechanism. When the shear/axial stress ratio is relatively small, cracks tend to originate from the subsurface in single-point initiation mode, and the larger the shear/axial stress ratio, the more cracks tend to originate from the surface in multi-point initiation mode. Then, a pragmatic equivalent method for multiaxial to uniaxial load based on the law of shear/axial stress ratio on fatigue strength was proposed. Based on this method and uniaxial S-N curve, the fatigue lives of Ti60 specimens under shear/axial stress ratios of 1.37, 1.95, and 3.53 was predicted, and the experimental results showed that the prediction error is basically within a factor of 3.

Characterisation and mathematical modelling of nonlinear mechanical behaviour of 3D printed short carbon fibre reinforced composites

Short carbon fibre reinforced plastic composites (SCFRP) that can be used in 3D printing exhibit excellent properties such as high specific strength and modulus, fatigue resistance, and efficient production at a lower cost. However, in the process of 3D printing, short carbon fibres tend to be distributed along the axial direction of the printed filament when they flow in the nozzle, leading to anisotropy and dispersity at the macro level. This brings difficulties to the design and application of materials. This paper developed a novel fix point iteration method based on classical laminate theory and Euler's integral method to accurate predict the nonlinear mechanical behaviour of SCFRP and verified via experiments. Results showed that the predicted curve and experimental data were in excellent agreement with only a 5 % error for 0°/90° laminate under uniaxial tensile loading. The accuracy of strength prediction for the -45°/45° laminate was improved and stabilised with the introduction of the fix point iteration method. These findings provide a useful framework for predicting the mechanical properties of the short fibre reinforced composited prepared by 3D printing.

Understanding seawater-induced fatigue changes in glass/epoxy laminates: A SEM, EDS, and FTIR study

Composite polymeric materials have exhibited significant advantages when operating in hostile conditions such as the marine environment. However, there is a lack of information concerning three-dimensional variations induced by prolonged exposure to seawater in composite structures and their impact on fatigue performance. Thus, this study aims to investigate the effects of long-term exposure to natural seawater on both the fatigue strength and three-dimensional chemical changes of GFRP laminates. Fatigue tests were conducted under uniaxial cyclic loading at a stress ratio R = 0.1, considering three groups of specimens: a control group not immersed in natural seawater, a group immersed for 230 days in natural seawater, and a group immersed for 910 days in natural seawater. After the tests, fracture surfaces were analysed using scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS). SEM examination revealed the microstructural occurrence of fibre-matrix debonding, reducing the load transfer between the fibre and matrix, contributing to the fatigue life decrease in immersed specimens. The fatigue results revealed that the reduction in fatigue life of the specimens immersed for 230 days was approximately 66 % of that observed in the control specimens increasing to 95 % when the immersion period extended to 910 days. Moreover, the effect of immersion time on the chemical structure of various regions within the specimens was assessed using Fourier-transform infrared (FTIR) spectroscopy. The main findings suggest the existence of a concentration gradient within the laminate and that regions near permeable surfaces are more vulnerable to chemical changes compared to regions located further away, even after reaching saturation.

Specimen width affects vascular tissue integrity for in-vitro characterisation

The preparation of slender specimens for in-vitro tissue characterisation could potentially alter mechanical tissue properties. To investigate this factor, rectangular specimens were prepared from the wall of the porcine aorta for uniaxial tensile loading. Varying strip widths of 16 mm, 8 mm, and 4 mm were achieved by excising zero, one, and three cuts within the specimen along the loading direction, respectively. While specimens loaded along the vessel’s circumferential direction acquired consistent tissue properties, the width of test specimens influenced the results of axially loaded tissue; vascular wall stiffness was reduced by approximately 40% in specimens with strips 4 mm wide. In addition, the cross-loading stretch was strongly influenced by specimen strip width, and fiber sliding contributed to the softening of slender tensile specimens, an outcome from finite element analysis of test specimens. We may, therefore, conclude that cutting orthogonal to the main direction of collagen fibers introduces mechanical trauma that weakens slender tensile specimens, compromising the determination of representative mechanical vessel wall properties.

Mechanical performance of low-carbon ultra-high performance engineered cementitious composites (UHP-ECC) with high-volume recycled concrete powder

Reusing recycled concrete powder (RCP) from construction waste for ultra-high performance engineered cementitious composites (UHP-ECC) markedly enhances its sustainability. This paper explores the mechanical properties and environmental benefit of low-carbon UHP-ECC containing high-volume RCP as the replacement of cement, ground granulated blast furnace slag (GGBS) and silica sand. The results indicate that UHP-ECC has a loose microstructure when a high percentage of cement is replaced with RCP, while replacing silica sand with RCP results in a denser microstructure. Substituting 75% of cement with RCP reduces the compressive strength and flexural strength of UHP-ECC by 24% and 17.6%, respectively. Substituting RCP for silica sand and GGBS does not significantly affect the compressive strength of UHP-ECC, albeit slightly reducing its flexural strength. Under the application of uniaxial tensile loading, the introduction of RCP improves the ductility of UHP-ECC at a 25% substitution rate, whether replacing silica sand or binder materials. However, as the substitution rate increases, replacing cement and silica sand with RCP decreases the ductility of UHP-ECC. Specifically, reductions of 10% and 7.6% in ultimate tensile strain are discerned when substituting 75% of cement and 100% of silica sand with RCP in UHP-ECC, respectively. Conversely, substituting a large volume of GGBS with RCP can further enhances the ductility of UHP-ECC, with an 15.5% increase in ultimate tensile strain when replacing all GGBS with RCP. By adjusting the RCP content to serve as an alternative to traditional binders and sand, it is possible to achieve a more sustainable UHP-ECC characterized by ideal mechanical strength and ductility, and RCP-blended UHP-ECC demonstrates favorable environmental and economic benefits.

Mesoscopic mechanical study on quasi-static compression of coral aggregate seawater concrete after high temperature exposure

Coral aggregate seawater concrete (CASC) is a pioneering concrete made by blending and curing coral reefs, coral sands, cement, mineral admixtures, chemical additives, and seawater. The investigation of its mechanical properties is important for both theoretical and engineering purposes. This paper aims to investigate the quasi-static mechanical properties of CASC after exposure to high temperatures using a combined approach of experiments and numerical simulations. The simulation uses LS-DYNA software and a three-dimensional random aggregate mesoscopic model to analyze the process of concrete damage and crack propagation. Various mechanical parameters such as peak stress, peak strain, and compressive strength after exposure to different temperatures are calculated. The simulation results show that under quasi-static uniaxial compressive loading, microcracks initiate within the mortar and interfacial transition zone (ITZ) of CASC. These cracks then propagate through the coarse aggregates, leading to aggregate failure and the disintegration of the concrete structure. The macroscopic damage traits of CASC specimens reveal concentrated damage in the central region. As the temperature increases during heat treatment, the severity of damage to CASC specimens also increases. However, as the strength grade increases, the damage condition of CASC tends to improve.

Characterising mechanical properties and failure criteria of steel slag aggregate concrete under multiaxial stress states

To advance the application of steel slag aggregate concrete (SAC) in civil engineering, this study introduces a comprehensive experimental program to explore the failure mode, strength, deformation, and failure criteria of SAC under multiaxial stress conditions. In this investigation, SAC specimens containing 50% stabilised steel slag as a coarse aggregate were examined for mechanical behaviour (e.g. stress–strain relationships) under diverse loading scenarios, including uniaxial tensile and compressive loadings, biaxial compression, and true triaxial compression loadings. Subsequently, three distinct failure criteria (i.e. the Ottosen criterion [1], William–Warnke criterion [2], and Guo criterion [3]) for SAC were formulated and authenticated based on the experimental outcomes. The results showed varied failure modes in the SAC, such as prism type, splitting tensile, sheet-like tensile, and oblique shear, which depended on the stress state of the specimen during failure. A significant elevation in the ultimate strength and deformation of SAC specimens was observed, with an increment in the first-to-third principal stress ratio (σ1/σ3). Furthermore, the failure criterion introduced by Guo was validated as adept at reproducing all geometric characteristics of the concrete failure surface and precisely forecasting SAC’s ultimate strength of the SAC. Overall, this investigation proposed a reliable failure criterion based on true triaxial test data that can not only describe the multiaxial strength characteristics of SAC but also provide design advice for the applications of SAC in engineering structures.

The axial tensile performance of DSCW-RC joints with lap-spliced bars

Joints formed between double-skin concrete shear walls (DSCWs) and reinforced concrete (RC) foundations commonly utilize lap-spliced bars to transfer forces. However, the behavior and design of these critical connections have not been thoroughly investigated. This study experimentally analyzes a steel-reinforced DSCW-to-RC foundation anchorage joint under monotonic uniaxial tension loading. Seven 1:3 scale specimens were tested to examine the effects of varying lap-spliced bar diameter, steel plate thickness, and stud bolt spacing. Crack propagation, load-displacement response, and reinforcement strain were measured. The results characterize two potential failure modes: ductile steel yielding and brittle stud shearing. Steel strains developed linearly up to yielding, demonstrating uniform transmission of tensile loads. Increased plate thickness and decreased stud spacing enhanced capacity. To mitigate sudden shear failures, the number and strength of studs must exceed the expected tensile demand. New design recommendations are proposed, including minimum stud bolt shear strength and the use of confining tie bars. This study provides an improved understanding of the behavior of DSCW-to-RC foundation lap-spliced connections, enabling resistance to axial demands while avoiding brittle failure modes. The experimental data will inform future analytical models and design provisions for these critical structural joints.

Effect of thickness on fatigue behavior of L-PBF fabricated Ti-6Al-4V alloy using a novel specimen geometry

One of the most promising advantages with additive manufacturing is the ability to fabricate complex geometries. These geometries can have significant impacts on thermal histories, microstructures, defects, and fatigue behavior. This study aims to investigate the effects of thickness variation on the fatigue life of AM fabricated components subjected to uniaxial loading through the introduction of a novel specimen geometry. The geometry is designed with a constant cross-sectional area and varying thicknesses by changing the diameter, ensuring constant stress throughout the gage section. Results found minimal relationship between the thickness and fatigue behavior for the as-built surface condition.

An origami-inspired energy absorber

The design of effective and compact energy absorption systems is key to the survivability and durability of many man-made structures and machines. To this end, this work presents the design, assessment, and implementation of a novel origami-inspired energy absorber that is based on the Kresling origami pattern. The absorber consists of a Kresling origami column positioned between the loading point and an energy dissipation module. By exploiting its unique inherent translation-to-rotation coupling feature, the primary function of the Kresling column is to transmit uniaxial incident loads (shock or impact) into localized rotational energy that can then be dissipated in a viscous fluid chamber. The proposed system has several unique advantages over traditional designs including the ability to (i) dissipate energy associated with both torsional and uniaxial loads, (ii) control the rotational velocity profile to maximize energy dissipation, and (iii) customize the restoring-force behavior of the Kresling column to different applications. Furthermore, the proposed design is more compact since it can realize the same stroke distance of the traditional translational design while being considerably shorter. Through extensive computational modeling, parametric studies, and experimental testing, it is demonstrated that the proposed design can be optimized to absorb all the imparted energy; and out of the absorbed energy, around 40% can be dissipated in the viscous fluid, while the rest is either dissipated by the viscoelasticity of the origami column or stored in it as potential energy.

Experimental Study on Energy Evolution and Acoustic Emission Characteristics of Fractured Sandstone under Cyclic Loading and Unloading

To investigate the dynamic response of fractured rock under cyclic loading and unloading, a WHY-300/10 microcomputer-controlled electro-hydraulic servo universal testing machine was used to conduct uniaxial cyclic loading and unloading tests. Simultaneously, acoustic emission (AE) and a CCD high-speed camera were employed to monitor the fracturing characteristics of sandstone. The mechanical properties, energy evolution, AE characteristics, and deformation of 45° sandstone were analyzed. The results indicate that as the load cycle level increases, both the elastic modulus and deformation modulus exhibit a “parabolic” increase, with a rapid rise initially and a slower rate of increase later. The damping ratio generally shows a decreasing trend but tends to rise near the peak load. The total energy, elastic energy, dissipated energy, damping energy, and damage energy all follow exponential function increases with the load level. The b-value fluctuates significantly during the stable crack propagation phase, unstable crack propagation phase, and peak phase. When the FR (Felicity ratio > 1), the rock is relatively stable; when the FR (Felicity ratio < 1), the rock gradually extends towards an unstable state. The Felicity ratio can be used as a predictive tool for the precursors of rock failure. Shear fractures dominate during the compaction and peak phases, while tensile fractures dominate during the crack propagation phase, ultimately leading to a failure characterized by tensile fracture. High-speed camera observations revealed that deformation first occurs at the tips of the prefabricated cracks and gradually spreads and deflects toward the ends of the sandstone. This study provides theoretical support for exploring the mechanical behavior and mechanisms of fractured rock under cyclic loading and unloading, and it has significant practical implications.

Experimental Study on the Mechanical Properties of Rock–Concrete Composite Specimens under Cyclic Loading

The foundations of bridges and other tall buildings are often subjected to cyclic loads. Therefore, it is essential to investigate the mechanical properties of rock–concrete composite foundations under cyclic loads. In this paper, uniaxial cyclic loading and unloading tests were conducted on rock–concrete composite specimens using the TFD-2000 microcomputer servo-controlled rock triaxial testing machine. The stress–strain curves, elastic modulus variation, and energy dissipation were analyzed. The results showed that the stress–strain curves of composite specimens under uniaxial cyclic loading and unloading conditions formed hysteresis loops. The hysteresis loop exhibited a sparse–dense–sparse pattern under the upper stress of 27.44 MPa, which was 90% of the uniaxial strength. The elastic modulus, as well as the dissipated energy, decreased rapidly in the first few cycles and then gradually decreased at a constant rate, with the upper stress increasing to 27.44 MPa. Both the elastic modulus and the dissipated energy exhibited an accelerated stage before specimen failure. The primary failure mode of the composite specimen was split failure from concrete to sandstone. A damage variable was derived to better reflect the laws governing the damage evolution of the composite under cyclic loads.

A Data-Driven Constitutive Model for 3D Lattice-Structured Material Utilising an Artificial Neural Network

A new data-driven continuum model based on an artificial neural network is developed in this study for a new three-dimensional lattice-structured material design. The model has the capability to capture and predict the nonlinear elastic behaviour of the specific lattice-structured material in the three-dimensional continuum description after being trained through the appropriate dataset. The essential data as the input ingredients of the data-driven model are provided through a hybrid method including experimental and unit-cell level finite element simulations under comprehensive loading scenarios including uniaxial, biaxial, volumetric, and pure shear loading. Furthermore, the lattice-structured samples are also fabricated using SLA additive manufacturing technology and the experimental measurements are performed and used for validation of the model. This then illustrates that the current model/methodology is a robust and powerful numerical tool to conduct the homogenization in complex simulation cases and could be used to accelerate the analysis and optimization during the design process of new lattice-structured materials. The model could also easily be used for other engineered materials by updating the dataset and re-training the ANN model with new data.

Compression-Shear Specimen Stress-State Response and Distribution Characteristics with Wide Stress Triaxiality.

Conventional methods for studying the plastic behavior of materials involve uniaxial tension and uniaxial compression. However, in the metal rolling process, the deformation zone undergoes a complex loading of multidirectional compression and shear. Characterizing the corresponding plastic evolution process online poses challenges, and the existing specimen structures struggle to accurately replicate the deformation-induced loading characteristics. In this study, we aimed to design a compression-shear composite loading specimen that closely mimics the actual processing conditions. The goal was to investigate how the specimen structure influences the stress-strain response in the deformation zone. Using commercial finite element software, a compression-shear composite loading specimen was meticulously designed. Five 304 stainless steel specimens underwent uniaxial compressive loading, with variation angles between the preset notch angle (PNA) of the specimen and compression direction. We employed digital image correlation methods to capture the impact of the PNA on the strain field during compression. Additionally, we aimed to elucidate the plastic response resulting from the stress state of the specimen, particularly in relation to specimen fracture and microstructural evolution.