Tension-compression Loading Research Articles

Experimental Study on Mechanical Properties of Novel FRP Bars with Hoop Winding Layer

Due to the fact that steel reinforcement is vulnerable to corrosion, FRP bars with light weight, high strength, and excellent durability have become a good substitute for ordinary steel bars. FRP bars have high tensile strength, but their compressive strength is relatively low and often neglected, so the application of FRP bars in compression members has been restricted. This paper proposes a new pultrusion‐winding‐pultrusion method to improve the compressive ability of FRP bars. A hoop FRP layer is winded on the outer surface of the pultruded FRP core, and a longitudinal pultruded layer and ribs are also added on the outermost surface. In this paper, mechanical properties of this novel FRP bar with hoop winding layer are investigated. First, monotonic tensile and compressive tests on traditional and novel GFRP bars were conducted. Then, cyclic tension‐compression loading tests were also carried out on the two types of GFRP bars. Test results showed that the compressive ultimate bearing capacities of GFRP bars with winding layers were 10∼20 kN greater than those of the traditional GFRP bars, and the compressive ductility of the novel GFRP bars was also improved. Furthermore, the tensile stress‐strain behaviors of both GFRP bars were linear‐elastic and the added winding layer did not greatly influence the tensile properties of the GFRP bars. Moreover, for the cyclic loading test, the compressive ultimate load of GFRP bars was 80%∼90% of that under monotonic compressive test, and the tensile ultimate load was 45%∼65% of that under monotonic tensile test. Compared with the GFRP bar without winding layer, the overall stiffness of the novel GFRP bar was greater than that of the traditional one and the ultimate load of the novel GFRP bar was also greater. In addition, seeing that the residual displacement of the novel GFRP bar was greater than that of the traditional GFRP bar, winding hoop fibers on the outer surface of the core is a useful way to improve the energy dissipation capacity of the GFRP bar.

Very high cycle fatigue characterization of additively manufactured AlSi10Mg and AlSi7Mg aluminium alloys based on ultrasonic fatigue testing

AbstractThis paper investigates long fatigue life characteristics of AlSi10Mg and AlSi7Mg aluminium alloys produced using direct metal laser sintering. An in‐house built ultrasonic fatigue testing machine was used to carry out fatigue tests on specimens machined from heat‐treated AlSi10Mg and AlSi7Mg alloys subjected to constant fully reversed tension–compression loadings. The fatigue life data acquired for these two additive manufacturing (AM) aluminium alloys were compared with previously published data under heat treatment and non‐heat treatment conditions. It was found that fatigue data of tested specimens fit more closely the data of specimens without any heat treatment. Fatigue test results indicate that effects of heat treatment on high cycle fatigue (HCF) and very high cycle fatigue (VHCF) performance of AM alloys should be further investigated in the context of the microstructural changes.

Cyclic response of electrodeposited copper films. Experiments and elastic–viscoplastic mean-field modeling

The goal of the present work is to identify and model the elastic–viscoplastic behavior of electrodeposited copper films under tension–compression loadings. From the experimental point of view, as proposed in the literature, a film of copper is electrodeposited on both sides of an elastic compliant substrate. The overall specimen is next subjected to tensile loading–unloadings. As the substrate remains elastic, the elastic–plastic response of copper under cyclic loading is experimentally determined. A clear kinematic hardening behavior is captured. To model the mechanical response, a new elastic–viscoplastic self-consistent scheme for polycrystalline materials is proposed. The core of the model is the tangent additive interaction law proposed in Molinari (2002). The behavior of the single grain is rate dependent where kinematic hardening is accounted for in the model at the level of the slip system. The model parameters are optimized via an evolutionary algorithm by comparing the predictions to the experimental cyclic response. As a result, the overall response is predicted. In addition, the heterogeneity in plastic strain activity is estimated by the model during cyclic loading.

Fatigue Crack Growth in Plastically Compressible Solids: Role of Negative Stress Ratio, Plastic Compressibility and Strain Softening

The effect of cyclic loading on fatigue crack growth in plastically compressible solids is investigated at negative stress ratio under plane strain and small scale yielding conditions. The material is characterized by a finite strain elastic viscoplastic constitutive model with hardening and hardening-softening-hardening hardness functions. Displacements corresponding to the isotropic linear elastic mode I crack field are prescribed on a remote boundary. The plastic crack growth, crack tip opening displacement (CTOD) and near crack tip stress fields are presented using finite element method. Material hardening/ softening has a major relevance on crack growth, CTOD and the evolution of stress distribution. It is revealed here that the negative stress ratio can significantly influence the loading conditions at the crack tip and thereby increase the crack growth for tension–compression loading for hardening material whereas the fatigue crack growth of plastically compressible hardening-softening-hardening material is only slightly affected by the negative stress ratio albeit it is accepted in literature that compressive loads contribute to fatigue crack growth significantly. In the present studies, the CTOD variation with applied load and the near stress distribution are also very unusual in nature.

Comparative Investigation on Aluminum Material Modeling with applications to Springback Prediction

A comparative investigation on springback prediction of aluminum stampings has been completed using an industry-type “shotgun” die as the benchmark case. 5xxx aluminum alloy at a thickness of 1.5 mm was used for the shotgun die tryout. After stamping tryout, the formed panel was scanned to capture its sprung geometry. The studied aluminum alloy was characterized by means of uniaxial tensile test and hydraulic bulge test to determine the material isotropic hardening behavior and planar anisotropy. In order to determine the material kinematic hardening behavior, the exclusive Yoshida model fitting test was conducted in tension-compression (T-C) and compression-tension (C-T) loading modes with the one-cycle and three-cycle loading schemes. The shotgun benchmark case was simulated using FEA software LS-DYNA®. By comparing the simulation results with the tryout measurements, the accuracy of springback prediction was examined to evaluate the influence of material modeling, loading mode and loading cycle in the Yoshida model fitting test. Based on the current benchmark study, the best practice in springback prediction and compensation of aluminum stampings and in conducting the Yoshida model fitting test of aluminum alloys were summarized for industrial applications.

Mesoscopic Analysis of the Mechanical Properties of Beishan Granite Based on the Equivalent Crystal Model

The internal microstructure of rock has an important influence on its macroscopic mechanical properties and fracture mechanism. In this paper, an equivalent crystal model (ECM) consisting of a bonded particle model and smooth joint model is established, which can simultaneously reflect the structure and content of mineral granules in rock. To verify the applicability and reliability of the ECM to further study the macroscopic nonlinear mechanical behaviour and fracture mechanism of rocks from a mesoscale perspective, this paper carried out numerical simulations of direct tension, uniaxial compression and triaxial compression loading using the particle flow code software, and the numerical results were compared with the test results of Beishan granite. The main research results are summarized as follows. (1) Under the condition of triaxial compression, with the increase in the confining pressure, the axial peak strain, radial peak strain, volume peak strain and post-peak residual strain of the rock all show an increasing trend. The rock exhibits an axial splitting failure mode dominated by tensile cracks and gradually changes to a shear failure mode dominated by shear cracks at an angle of approximately 45° from the loading direction. (2) Under direct tensile conditions, the failure of the rock manifests as cracks initiating from the middle of the specimen and extending approximately perpendicular to the tensile load direction, forming an approximately horizontal macroscopic crack composed of mineral granular boundaries. (3) The use of the ECM can reproduce a high ratio of the uniaxial compressive strength to the uniaxial tensile strength of the rock, and its strength characteristics show obvious nonlinear characteristics and meet the Hoek–Brown strength criterion. The rock strength and failure type obtained from the simulation are basically consistent with the test results, demonstrating the reliability of the ECM.

Influence of Crack Width in Alternating Tension–Compression Regimes on Crack-Bridging Behaviour and Degradation of PVA Microfibres Embedded in Cement-Based Matrix

The use of high-performance polymeric microfibres in enhancing the ductility of cementitious composites is widespread. A vivid example is the application of strain-hardening cement-based composites (SHCCs) in the construction industry. However, there are a few challenges which need to be addressed with respect to material design. For instance, the ductility of SHCC diminishes under alternating tension–compression loading, where the fibres lose their crack-bridging capacity due to specific damage mechanisms. The damage development and its influence on crack-bridging capacity have been studied in previous works by the authors. The paper at hand focuses on the influence of crack width on the crack-bridging capacity of polymeric microfibres in conjunction with the number of cycles in an alternating tension–compression regime with different cyclic compressive force levels. It shows that bridging capacity can be markedly influenced by crack width: an increase in crack width leads to more severe damage to the fibres and thus to lower crack-bridging capacity. Then, after analysing the specimens by means of electron microscopy, a hypothesis is presented to address the effect of crack width on damage development. Finally, a simple approach is proposed for estimating the influence of different parameters on fibre degradation.

Fracture of U- and V-notched Al6061-T6 plates: The first examination of the Fictitious Material Concept under mixed mode I/III loading

In this study, for the first time, the load-carrying capacity (LCC) of test specimens containing U-shaped and V-shaped notches made of Al6061-T6 aluminum alloy under combination of out-of-plate shear and tensile loading (mixed mode I/III) is studied. U-notched specimens are made with different notch tip radii and V-notched specimens are made with different notch tip radii and notch opening angles. At first, the specimens are subjected to different mixed mode I/III loading conditions using a universal tension-compression loading device and their LCCs are measured experimentally. Then, considering the ductile behavior of the material under investigation, it is attempted to use the Equivalent Material Concept (EMC) in theoretical analysis, since it does not require complex and time-consuming elastic-plastic analyses. Initial evidence for the combination of the EMC with the brittle fracture criteria of point stress (PS) and mean stress (MS) indicates that both criteria show large errors in predicting the LCC of the test samples. The cause is the large failure strain of the Al6061-T6 alloy. Therefore, a newly developed concept similar to the EMC, called the Fictitious Material Concept (FMC), is applied. Based on the FMC, instead of equalizing the elasticity modulus of the ductile material and the fictitious material, the failure strains of those materials at the ultimate points are assumed to be equal. It is shown that combining the FMC with the PS and MS criteria leads to acceptable predictions of the LCC of the tested specimens.

Initial yield behaviour of closed-cell aluminium foams in biaxial loading

This study investigates the initial yield characteristics of closed-cell aluminium foam (Alporas) in multiaxial stress states using experimental and numerical approaches. The mechanical responses of foams with two relative densities (11.7% and 17.2%) are presented in uniaxial compression, uniaxial tension, biaxial compression, biaxial tension and combined tension-compression loading conditions. Cubic and dog-bone specimens are fabricated for uniaxial compression and tension tests respectively. A Modified Maltese Cross Specimen (MMCS) geometry is custom designed for testing closed-cell foams in biaxial stress states. The experimental investigations reveal a consistently stiffer response of closed-cell foams in tensile dominated stress states. This is attributed to the stretching mode of deformation observed in the cell walls in tensile loading conditions, in contrast to the bending/buckling dominant deformation in compressive loading conditions. To complement the experimental data, a wide range of multiaxial stress states is simulated using irregular 3D Voronoi models. An initial yield surface normalized with respect to the uniaxial compressive yield strengths is constructed in the mean-effective stress space using both experimental and numerical data. The normalized initial yield surfaces appear considerably shifted towards the positive mean stress axis. An yield criterion for closed-cell aluminium foams that adequately captures this asymmetric nature of the yield surface about the effective stress axis is proposed, which requires only uniaxial compressive yield strength as a parameter. An empirical relation function estimating the uniaxial compressive yield strength based on the density of the foam material is proposed to facilitate direct application of the yield criterion in early stages of the design.

On the microstructure evolution during low cycle fatigue deformation of wrought ATI 718Plus alloy

In the present work, strain-controlled low cycle fatigue (LCF) tests were performed at 704 °C using 0.25 Hz in cyclic frequency with tension-compression loading on the forged alloy 718Plus to investigate the fatigue performances determined by structures, and the corresponding microstructure evolutions and deformation mechanisms were analyzed through techniques of scanning electron microscope (SEM) and transmission electron microscope (TEM). The fatigue specimens from different locations (core and rim) showed similar response on cycle stress, but smaller grain size contributed to superior fatigue performance. Fatigue deformation induced the generation of η precipitates in grain interior and promoted the growth of grains and η precipitates, but brought insignificant change on the morphology of spherical γ′ phase and slightly increase in particle size and γ′/γ lattice misfit, indicating its excellent mechanical stability at 704 °C. A transition area was observed between the η precipitates and γ matrix, which was determined to be γ phase via high resolution transmission electron microscopy (HRTEM) and elements mapping techniques. Twinning and dislocation planar slip bands were confirmed to be the primary fatigue deformation modes of alloy 718Plus. Dislocations piled up and formed networks at the edges of η precipitates during the fatigue process, inducing the initiation and propagation of micro-cracks. As the cycle accumulated, the η and γ′ precipitates were sheared by gliding dislocations, resulting in weakened resistance to deformation and fatigue softening.

Analytical method for determining the elastoplastic interface of a circular hole subjected to biaxial tension-compression loads

Based on the Tresca yield criterion, an analytical method is proposed to determine the elastoplastic interface around a circular hole in an infinite plate subjected to biaxial tension-compression loads at infinity. Comparing with the plastic region formed under the action of biaxial tension-tension or biaxial compression- compression loads, the characteristic of the plastic regions is that they cannot completely surround the hole and there may be two or four nonintersecting plastic regions around the hole. In both cases, the conformal mapping is employed to map the elastic region in the physical plane onto the outer region of a unit circle in the image plane, which transforms this kind of plane elastoplastic problem into a problem of mapping. According to the stress continuity conditions on the elastoplastic interface and the stress boundary conditions on the elastic part of the circular hole, a set of nonlinear equations containing the mapping function coefficients is established. The problem can be further transformed into an optimization problem determined by the differential-evolution algorithm. The correctness of the presented method is verified by numerical method. The influence of loads and material constant on the size and shape of the plastic regions is discussed in detail.

Equal-channel angular pressing influencing the mean stress sensitivity in the high cycle fatigue regime of the 6082 aluminum alloy

The high cycle fatigue (HCF) behavior is investigated for a 6082 aluminum alloy in an as-extruded peak-aged condition and after single-pass equal-channel angular pressing (ECAP). The focus of this study is on the effect, that ECAP processing exerts on the mean stress sensitivity of the fatigue strength. The fatigue strength at 107 cycles is compared for alternating tension-compression (R = −1), tension (R = 0) and tension-tension loading (R = 0.1) and related to the microstructural features examined by electron backscatter diffraction (EBSD) measurements and the mechanical properties determined by quasi-static tensile and compression tests.For the as-extruded peak-aged alloy, the fatigue strength depends on the stress ratio. An increase in mean stress due to a higher applied stress ratio leads to decreased fatigue strength. Single-pass ECAP enhances the fatigue strength for all stress ratios when compared to the as-extruded peak-aged alloy. Furthermore, the mean stress effect is diminished and the fatigue strength is almost the same for the tested stress ratios. The mean stress insensitive behavior of the ECAP processed condition is most likely a result of an increased sensitivity to compression loading, which is caused by the ECAP induced shear-deformation and expresses further in a minor yield strength under compression loading when compared to the as-extruded peak-aged alloy.

A novel technique for determining the dynamic Bauschinger effect by electromagnetic Hopkinson bar

Dynamic mechanical behavior of metallic materials under complicated loading conditions has attracted much attention. However, it is hard to obtain the dynamic Bauschinger effect of metallic materials due to the limitation of loading equipment. In order to investigate the relationship between the Bauschinger effect and strain rate effect of metallic materials, this paper proposes an asynchronous loading technique based on electromagnetic split Hopkinson bar system, which could provide an effective way to study the Bauschinger effect of metallic materials under high strain rate loading. We first introduce the main characteristics of the asynchronous loading device, that is, the specimen can be loaded by one cycle of continuous dynamic tension-compression loading pulse in which the two separate stress waves are created by electromagnetic pulse generators and prove to maintain their consistency. The propagation of stress waves was analyzed to ensure the continuity of the loading process. Then the dynamic loading process and the methods of data processing and stress wave separation are presented. Stress equilibrium was also analyzed in order to demonstrate the reliability of the equipment. Finally, the Bauschinger effect of 6061 aluminum alloy at 5% pre-strain during the process of dynamic compression to dynamic tension loading was studied using this method, and the corresponding quasi-static tests were also conducted for comparison. It was found that the material shows less strain-rate sensitivity under axial compression loading, while its Bauschinger stress parameter increases from 0.07 in quasi-static loading to 0.17 in dynamic loading. The results indicate that the Bauschinger effect of 6061 aluminum alloys depends on the strain rate and can be significantly enhanced under dynamic loading. This conclusion presents a challenge to the traditional conception that aluminum alloys are insensitive to strain rate.

Effect of fibre waviness on the compressive fatigue behavior of woven carbon/epoxy laminates

In the present work, two woven carbon/epoxy laminates with different areal weight were tested under tension-compression (R = −1) and compression-compression (R = -∞) cyclic loadings. Their performance was evaluated in terms life to final separation and stiffness degradation during the fatigue life. In addition, microscopic observations were carried out at regular intervals during the tests to identify the damage evolution at the micro-scale. The bundle waviness was found to have a large influence on the damage mechanisms and the fatigue life of the laminates both for tension-compression and compression-compression loadings, while the stiffness loss remained small until final failure in all cases.

Molecular dynamics study of fatigue mechanical properties and microstructural evolution of Ni-based single crystal superalloys under cyclic loading

The fatigue performance and deformation mechanism of Ni-based single crystal superalloys under cyclic tension–compression loading are studied by molecular dynamics simulations. The effects of temperature and strain rate on the cyclic deformation of superalloys are discussed. The results show that there are three different cyclic deformation mechanisms at different temperature ranges. In the low temperature range, dislocations and stacking faults shearing γ' phase is the main deformation mechanism. In the high temperature range, Orowan bypassing and climbing mechanism dominates the deformation. While in the medium temperature range, the deformation mechanism gradually transforms from the dislocation and stacking faults shearing the γ' phase to the Orowan bypassing and climbing. The simulation results also reveal that the superalloys exhibit a cyclic saturation stage after the initial cyclic hardening. The cyclic saturation stage is a dynamic equilibrium of dislocation proliferation and annihilation, indicating that the superalloys have excellent fatigue mechanical properties. Moreover, the dislocation density and the proportion of stacking faults increase with increasing strain rate, resulting in faster cyclic stability and higher stress amplitude for the superalloys. These results provide important information for understanding the fatigue mechanisms of superalloys from an atomistic perspective.

Density-driven damage mechanics (D3-M) model for concrete I: mechanical damage

ABSTRACT Damage in concrete has been modelled using various approaches such as fracture mechanics, continuum damage mechanics and failure envelope theories. This study proposes a new approach to model the initiation of damage in concrete that addresses some limitations associated with the existing approaches. The proposed approach defines damage in terms of changes in the density of the material at the microscopic level, where such changes are induced by mechanical loading. The suggested approach is used to simulate the response of 2D concrete bodies to uni-axial tension and uni-axial compression. The simulation results indicate that the proposed model, by means of a single constitutive function, is able to correctly predict failure patterns and aptly capture the damage mechanisms under both uni-axial tension and uni-axial compression loadings using only the information related to the microstructure, the density field and the stiffness field. As a continuation, in Part II, the ability of the D3-M approach to model fully coupled chemo-mechanical damage in concrete using a single constitutive equation will be demonstrated.

Numerical study of bilinear isotropic & kinematic elastic–plastic response under cyclic loading

The study of elastic–plastic response of metals and alloys subjected to cyclic loads has several applications with phenomenon such as Baushinger effect, ratcheting, elastic and plastic shakedown having been reported in numerous studies. With the use of finite element analysis (FEA) being increasingly popular for study of mechanical components exhibiting elastic–plastic behaviour, the validation of results from plasticity analysis is an onerous task. Hence, this paper attempts to study the low-cycle elastic–plastic response during a uniaxial tension–compression loading regime using FEA. The stress–strain relationships due to controlled loading with bilinear isotropic (BISO) and kinematic hardening (BKIN) behaviour are studied. It has been observed that while kinematic hardening has a consistent stress–strain hysteresis loop with no accumulation of plastic strain during cyclic loading, isotropic hardening has shown accumulation of stress at constant strain cycles, but no accumulation of total strain at constant stress cycles. When maintaining constant total strain on the specimen for isotropic hardening, it has been observed that while the elastic strain increases, the plastic strain decreases with the number of cycles. FE simulations with different strain and stress factors were performed and the variations of peak stresses and strains were documented.

The Influence Of Age On The Recovery From Worksite Resistance Exercise In Firefighters

Worksite resistance training may reduce injuries and improve performance in firefighters, however aging may prolong the recovery process. PURPOSE: This study examined the influence of age on recovery following an acute bout of worksite resistance training. METHODS: Nineteen young and 19 older career firefighters (FFs) completed an acute bout of resistance exercise in addition to pre- and post-testing 24, 48, and 72 hours post-exercise. A work-related fatigue (WRF) survey was completed to assess daily fluctuations in work demands. Ultrasonography was used to assess cross-sectional area (CSA) and echo intensity (EI) of the vastus lateralis, in addition to muscle thickness (MT) and EI of the biceps brachii. To determine maximal jump height and associated velocity, participants completed 3-4 countermovement jumps while standing on a jump mat with a linear transducer attached at the waist. Upper-body peak force (PF) was measured during an isometric upright row task, using a calibrated tension-compression load cell. Lower body PF was examined with the participants seated in a custom-built, calibrated isometric dynamometer and their right knee flexed at 60 degrees. Following 3 submaximal warm-up contractions, participants performed 3 maximal voluntary contractions for each strength assessment lasting 3-4 s. The FFs completed the circuit-style resistance exercise bout following pre-testing, which included 3 sets of 8-10 repetitions at 80% of their predicted 1-repetition maximum of the deadlift, shoulder press, lunge, and upright row. Linear mixed models, controlling for WRF, were used to analyze all primary outcomes, with subject as the random effect and group and time as fixed effects. Alpha level was set a priori at 0.05. RESULTS: There was a significant group by time interaction effect for WRF (P=0.002) and was controlled for in subsequent analyses. There were no other significant group by time interactions (P>0.171). Collapsed across time, young FFs showed greater lower body PF (P=0.006), jump performance (P<0.024), and lower VL EI (P=0.008) values. Across time points, upper-body PF (P=0.023) and jump performance (P<0.029) decreased as muscle size increased (P<0.006) for both groups. CONCLUSION: These results indicate that age may not influence the recovery from a bout of worksite resistance exercise in FFs.

Damage evolution of tuff under cyclic tension–compression loading based on 3D digital image correlation

We have investigated the damage evolution and crack propagation of tuff rocks under axially cyclic tension–compression (T–C) loading with different amplitudes. The field of axial strains in the rock surface was obtained by three-dimensional digital image correlation (3D-DIC) method. The apparent strains were used to analyze the damage evolution along with the nucleation and propagation of microcracks until rock failure. The experimental results demonstrated that a local damage zone (LDZ) was formed when the loading amplitude exceeded a certain value, and the rock sample was prone to tensile failure under T–C cyclic loading. Based on arrangement of the virtual extensometers on the field of axial strains, the axial stress–strain curves, maximum axial strain, secant moduli, and dissipated energy evolution were obtained as a function of cycle number. It was observed that the variations in these mechanical parameters and dissipated energy significantly increased in the T–C cycle test with the increase in the loading amplitude. Particularly, due to obvious accumulation of residual compressive strain during the first cycle, the maximum tension stress increased, secant modulus decreased, and the dissipated energy evidently increased under tension during the second cycle. Furthermore, the mechanical parameters and dissipated energy evolution as a function of cycle number inside and outside the LDZ under 80% amplitude confirmed that the deformation mainly occurred inside the LDZ and the energy mainly dissipated for the nucleation and propagation of tensile microcracks vertical to the loading direction inside the LDZ during rock failure process.

On the comprehensive stability analysis of axially loaded bistable and tristable metastructures

Approaches to systematic analysis of essentially nonlinear structures with multistable responses, controlled buckling and snapping behavior have received much attention recently in the context of mechanical metamaterials design. A snapping bistable element is generally a highly efficient damper, performing well even at very low forcing frequencies. In this paper, we transfer basic tools of the metamaterials analysis to macroscopic systems relevant to civil and mechanical engineering applications. Such systems are comprised of only several bistable elements. Followed by analysis of a single snapping bistable axial (two-force) element, we consider a combination of two elements with antisymmetric properties and demonstrate a robust tristable performance of the resultant structure in low-frequency or quasistatic tension-compression loading cycles. The tristability provides an overall response that is symmetric for tension and compression, which makes it interesting for applications in machinery and large-scale seismic structures.