Cyclic Uniaxial Compression Tests Research Articles

Study on the evolution rules of coal deformation and failure characteristics caused by multiple mining disturbances

During coal mining operations, the coal will be deformed and damaged due to multiple mining disturbances (MMD), often resulting in disasters, like rock burst. To understand the evolution rules of coal deformation under MMD and its final fracture characteristics after impact dynamic load loading, reduce the adverse effects of mining disturbances, and improve disaster prevention and control capabilities, quasi-static uniaxial cyclic loading-unloading (L-U) and dynamic axial compression tests were conducted on large-sized coal-like samples. During the tests, three-dimensional (3D) laser scanning and acoustic emission (AE) monitoring technology were utilized to accurately capture the full-field deformation and AE response data, facilitating a systematic analysis of deformation and fracture characteristics. The results show that: (1) Under the cyclic L-U effect induced by MMD, each loading cycle causes compression deformation with partial recovery during unloading, presenting an overall “wavy” variation trend. (2) The maximum load is the most critical factor affecting the damaged coal deformation, with smaller load resulting in less overall sample deformation. (3) After the impact dynamic loading, the damaged samples suffered large-scale impact splitting failure, with the compressive-shear layer failure mainly occurred inside the holes. (4) Lower loading during cyclic L-U process correlate with reduced damage degree, and smaller debris particles with a higher fractal dimension when impact failure occurs, indicating a more severe impact failure. (5) With multiple cycles of L-U, the cracks inside the sample gradually extend and expand from around the hole to the outside. The greater the load and the number of cycles, the more serious the crack damage will be. (6) In the practical mining process, it is crucial to reinforce roadway interiors while minimizing low-loading cyclic disturbances induced by MMD. The study has obtained the deformation evolution rules and failure characteristics of coal under MMD, providing a theoretical basis for the prevention and control of corresponding engineering disasters.

Crack Evolvement of Ancient Brick Masonry Under Uniaxial Compression Fatigue Loading

ABSTRACT Cracking is an essential intuitive indicator of damage in masonry structures. Evaluating the progression of fatigue cracks in ancient brick masonry structures is vital for the preservation and safety of ancient buildings. By conducting amplitude cyclic uniaxial compression tests on ancient brick masonry specimens, using the video image correlate-3d (VIC-3D) technology to collect images of the entire process from loading to failure, the development of fatigue cracks is analyzed and concluded. A reversed S-shaped function for fatigue crack evolution was established based on the fatigue damage mechanism. The model parameters’ physical significance, value range, and their impact on the model curve were discussed. The fatigue damage evolution equation was derived from the fatigue crack evolution model. The research results show that a reversed S-shaped function for fatigue crack evolution covers various types of fatigue crack evolution laws, with strong adaptability and high accuracy. The damage evolution curve established, based on the fatigue crack evolution model, has a reversed S-shaped change rule, and was divided into three stages: the first stage (0 N f ~ 0.1 N f ) fatigue damage grows faster; the second stage (0.1 N f ~ 0.9 N f ) fatigue damage growth is stable, close to linear change; when the cycle ratio exceeds 0.9, it enters the third stage, and fatigue damage increases rapidly.

Mechanical responses of chemically corroded sandstone under cyclic disturbance: Insights from fatigue properties and macro-micro fracturing mechanism

Engineering rock masses are likely to be subjected to coupled chemical corrosion and cyclic load disturbance, deteriorating the internal structure of rocks and further threatening the long-term stability of engineering structures. In this study, to investigate the combined effect of cyclic loading and chemical corrosion, the cyclic uniaxial compression tests are conducted on sandstone specimens chemically corroded by different corrosion times (0, 20, 40, 60 days) and pH values (1, 3, 7, 11, 13). Our results indicate that chemically corroded sandstone specimens featured by an increase in irreversible strain and hysteresis energy density compared with natural rocks. A coupling damage variable is proposed to reflect the initial damage induced by chemical corrosion and the mechanical damage induced by cyclic disturbance, and the accumulation rate of coupled chemical-fatigue damage variables significantly increases with increasing corrosion time or enhancing acidity and alkalinity. Fatigue failure modes of tested specimens transform from splitting failure to unidirectional shear failure as corrosion time increases, and chemically corroded specimens are featured by ductile failure characteristics compared to natural state. Moreover, X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) are employed to analyze the micro-fracturing mechanism of sandstone. Cement minerals dominate the reactions in acidic solutions, whereas quartz takes the lead in alkaline conditions. Due to pronounced influence of cement minerals on the macroscopic mechanical properties of sandstone, acid corrosion specimens tend to experience more severe damage. Micro-cracks inside rocks under cyclic loading have sufficient time to pass through weakly bonded particles, and chemical reactions markedly weaken inter-particle bonding degree, and thus the fatigue fracture of chemically corroded specimens predominantly exhibits intergranular (IG) fracture.

Ultralong and highly axially aligned nerve guide conduits produced by electro-centrifugal spinning

The fabrication of nerve guide conduits (NGCs) with axially aligned fibers has been realized by electrospinning; however, the reported maximum length of such NGCs is limited to 2 cm which cannot repair a long injured peripheral nerve. In this context, a modified electro-centrifugal spinning (ECS) system is developed to produce ultralong and highly axially aligned NGCs. To this end, medical-grade poly (lactic-co-glycolic acid) PLGA with L-to-G ratios of 8/2, 7/3, and 5/5 loaded with antibiotic ciprofloxacin (CIP) are used to investigate the corresponding spinning mechanisms and drug release behaviors. Specifically speaking, first, the effects of chloroform/trifluoroethanol co-solvents and drug loading on the spinnability and fiber orientation of PLGA are systematically investigated, and tubular conduits are produced with the established method. Second, uniaxial tensile tests and cyclic compressive tests for the conduits with and without drug loading are conducted; PLGA conduits with an L-to-G ratio of 8/2 are further annealed to enhance their mechanical properties. Last, the drug release behavior of the conduits in vitro is studied. As a result, highly fiber-aligned NGCs with a length of over 10 cm, combined with excellent mechanical properties and well-controlled drug release features can be efficiently produced. Therefore, this work provides a practical strategy for producing NGCs with high efficiency.

Fatigue behavior investigation of artificial rock under cyclic loading by using discrete element method

In numerous engineering projects, such as tunnel construction, underground gas storage in caverns, and the impact of earthquakes, rock materials experience cyclic loading. However, the response of rock materials to cyclic loading, which may vary in terms of waveform, frequency, and amplitude, is not uniform. Consequently, comprehending the mechanical attributes and fatigue behavior of rocks holds significant importance in engineering designs. This study focuses on subjecting a collection of cylindrical concrete specimens, categorized into three groups with distinct mechanical properties, to both monotonic and cyclic loading within a laboratory environment. Three groups of cylindrical concrete samples with different mixing designs are prepared to evaluate the effect of the mechanical strength of the samples on the fatigue response of the rock. In such a way that: The first group (G1) with Uniaxial Compressive Strength (UCS): 19 MPa, the second group (G2) with (UCS): 49 MPa, and the third group (G3) with (UCS): 71 MPa. In addition, the discrete element method is utilized to simulate uniaxial cyclic compression tests under various loading conditions, such as monotonic and cyclic loading. The resulting fracture propagation patterns and geometries are then analyzed. Numerical and experimental results show that the cyclic uniaxial compressive strength, due to the presence of damage and fatigue phenomena, has decreased relative to the monotonic tests. Also the cyclic elastic modulus values (Ec) have decreased by 53 %, 25 %, and 11 % relative to the monotonic elastic modulus (Em) for the three groups of G1, G2, and G3, respectively. The Stress-life (S-N) model is used to express the fatigue behavior of rock, and the final equation for the S-N model is expressed.

Experimental Study on the Localized Deformation and Damage Behavior of Polymer-Bonded Explosive Simulant under Cyclic Compression.

Uniaxial cyclic compression tests were performed to investigate the compression deformation and damage of polymer-bonded explosive (PBX) simulant, particularly shear localization. The macroscopic mechanical behavior and mesoscale failure mechanisms of the PBX simulant were analyzed by optical observation and SEM scanning methods. After each cyclic compression, the specimen was scanned by X-ray computed tomography (CT), and the internal 3D deformation of the specimen was calculated using the digital volume correlation (DVC) method. The results show that the stress-strain curve of the PBX simulant exhibits five stages and coincides with the morphological changes on the surface of the specimen. The mesoscale failure mechanism is dominated by particle interface debonding and binder tearing, accompanied by a small amount of particle breakage. There are three bifurcation points (T1, T2, and T3) in the curves of the normal and shear strain components with compression strain. It was found that these bifurcation points can reflect the full progression of the specimen from inconspicuous damage to uniformly distributed damage, shear localization, and eventual macroscopic fracture. The strain invariant I1 can quantitatively and completely characterize the deformation and damage processes of the PBX simulant under cyclic compression.

Properties of Hyper-Elastic-Graded Triply Periodic Minimal Surfaces.

The mechanical behaviors of three distinct lattice structures-Diamond, Gyroid, and Schwarz-synthesized through vat polymerization, were meticulously analyzed. This study aimed to elucidate the intricacies of these structures in terms of their stress-strain responses, energy absorption, and recovery characteristics. Utilizing the described experiments and analytical approaches, it was discerned, via the described experimental and analytical procedure, that the AM lattices showcased mechanical properties and stress-strain behaviors that notably surpassed theoretical predictions, pointing to substantial disparities between conventional models and experimental outcomes. The Diamond lattice displayed superior stiffness with higher average loading and unloading moduli and heightened energy absorption and dissipation rates, followed by the Gyroid and Schwarz lattices. The Schwarz lattice showed the most consistent mechanical response, while the Diamond and Gyroid showed capabilities of reaching larger strains and stresses. All uniaxial cyclic compressive tests were performed at room temperature with no dwell times. The efficacy of hyper-elastic-graded models significantly outperformed projections offered by traditional Ashby-Gibson models, emphasizing the need for more refined models to accurately delineate the behaviors of additively manufactured lattices in advanced engineering applications.

Fracture and energy evolution of rock specimens with a circular hole under multilevel cyclic loading

Underground structures gradually face increasing multilevel cyclic loading as construction progresses. In this paper, the fracture and energy evolution of rock specimens with a circular hole under multilevel cyclic loading were investigated using an experimental-theoretical approach. A series of multilevel cyclic loading tests were conducted on two types of rock (sandstone and granite) using the MTS 815 rock mechanics test system. Digital image correlation (DIC) and acoustic emission (AE) techniques were employed to monitor the fracture behavior of the rock with deformation and acoustic emission data recorded respectively. The total energy density, elastic energy density, and dissipated energy density of specimens are obtained by theoretical calculation. The results indicate that, despite the similarity in cracking sequence and fracture evolution of rock specimens with a circular hole between multilevel cyclic loading and uniaxial compression tests, there are differences in the AE activity under these two loading conditions. Additionally, the cracks around the hole under multilevel cyclic loading display a distinct “open-close-reopen” behavior. The energy density of rock specimens with a circular hole grows nonlinearly with stress. The elastic and dissipative energy density exhibits a linear relationship with the total energy density before rock failure. The fitting value of the elastic and dissipative energy density can be regarded as the energy storage coefficient and the energy dissipation coefficient, which could be further used to quantify the energy storage and dissipation capacities of rock. Under multilevel cyclic loading, the propagation of remote fractures and their coalescence with the shear fractures is the primary resource of AE energy, with only a few supplies given by micro-fissure closure and primary fractures. The dissipation energy in the rock structure can be described by the growth of AE energy. The findings indicate that there are distinct stages of energy release during the fracture evolution of cracks around the circular hole. These stages can be predicted by changes in AE energy or macroscopic energy, revealing the evolution stages of cracks and the modes of failure.

Mechanical properties of short polypropylene fiber enhanced recycled concrete under cyclic compression

AbstractThis study investigated the mechanical properties and stress–strain relationship of polypropylene fiber recycled aggregate concrete (PFRAC) subjected to uniaxial cyclic compression. A total of 78 cylindrical specimens were designed for uniaxial cyclic compression test with coarse aggregate replacement rate, polypropylene fiber volume fraction, and loading rate as variation parameters. The failure process of the specimens under cyclic loading was observed and recorded. The stress–strain curve of PFRAC was summarized. The important mechanical properties of PFRAC were obtained, such as plastic strain, accumulated energy dissipation. The microstructure of PFRAC was analyzed by scanning electron microscopy, and the crack resistance mechanism of PFRAC under cyclic loading was revealed. The results show that the PFRAC under cyclic loading appeared oblique splitting failure. The compressive stress–strain curve envelope of PFRAC subjected to cyclic loading exhibited similarity to the monotonic counterpart. The addition of polypropylene fibers can improve the peak stress, peak strain, residual stress, post‐peak ductility, and accumulated energy dissipation of the recycled concrete. In addition, a constitutive relation formula for the PFRAC stress–strain curve under cyclic loading is proposed, and the fitting result is good.

Rheological Modeling of Bituminous Mixtures Including Polymer-Modified Binder and Fine Crumb Rubber Added through Dry Process

In recent years, both dry and wet processes to include waste rubber (WR) in bituminous mixtures have had little success in Italy due to technical and economic reasons. However, the necessity to recycle this waste material and the increasing interest in low-noise emitting pavement is encouraging researchers and stakeholders to explore new solutions. In this context, a hot-mix asphalt (HMA) manufactured with polymer-modified bitumen and fine WR added through a dry method has been recently developed. This paper deals with the rheological characterization and modeling of this innovative HMA, in comparison with two reference mixtures, with ordinary polymer-modified bitumen and with an asphalt rubber binder produced through a wet process. The complex modulus was measured through uniaxial cyclic compression tests at different temperatures and frequencies. The Huet-Sayegh rheological model was used to simulate the experimental data. The results showed that the time-temperature superposition principle is valid, despite the presence of polymers and WR. The Huet-Sayegh model allows a good fitting of the dynamic modulus and loss angle data to be achieved. The viscous response of the mixture including polymer-modified bitumen and rubber powder is higher than the other HMAs, which is mainly associated with the nature of the modifiers, the binder content in the mix and the less severe short-term aging.

Modeling of Hyper-Viscoelastic Properties of High-Damping Rubber Materials during the Cyclic Tension and Compression Process in the Vertical Direction.

With the rapid development of the economy and urbanization, the construction of the urban rail transit system has had a great impact on the work, life, and health of residents in buildings along the rail transit line. Thus, it is particularly urgent and necessary to develop base isolation technologies to control and reduce the impact of vibrations of rail transit systems on building structures. High-damping rubber isolation bearings have shown significant effectiveness in the reduction of this impact, and their isolation performance mainly depends on the mechanical and damping energy dissipation characteristics of the high-damping rubber material. This paper aims to investigate the hyper-viscoelastic properties of the high-damping rubber material used for high-damping rubber isolation bearings during the cyclic tension and compression process in the vertical direction. These properties include hyperelastic parameters, viscoelastic coefficients, and the relaxation times of the material. For this purpose, uniaxial cyclic tension and compression tests were conducted. A three-element Maxwell rheological model combining a strain energy density function was proposed for modeling the hyper-viscoelastic behaviors of the materials during the cyclic tension and compression process. Based on the obtained results, an iterative identification procedure was used to determine the constitutive parameters of the material for each loading-unloading cycle. The aforementioned parameters were further expressed as a function of the number of cycles. New insights into hyper-viscoelastic property changes in this high-damping rubber material during the cyclic tension and compression process were gained in this work. These investigations could facilitate the development of computational tools, which would regulate fundamental guidelines for the better controlling and optimization of the isolation performance of the high-damping rubber material used for high-damping rubber isolation bearings, which have a wider perspective of applications in the urban rail transit system.

Selective Laser Sintered food: A unit cell approach to design mechanical properties

Selective Laser Sintering is a 3D printing technique that is suitable for tailoring food structures on both macroscopic and microscopic length scales. A unit cell approach was employed to design properties using a spatial distribution of printing parameters within a single product. Support structures were printed and a successive checkerboard strategy was used to effectively reduce warpage effects that occur because of the product size. Cyclic uniaxial compression tests were performed to mechanically characterize the products. Damage evolution was controlled by the process parameters and cracks propagated through the interface between separate unit cells, which was weakened through the printing strategy. Printing with higher area energy density decreased the ductility and increased the stiffness of the geometry. • A unit cell approach was utilized for Selective Laser Sintering of food. • Unit cells consisting of two phases were printed with varying laser parameters. • Existing strategies for reducing warpage were adopted for food printing. • Stiffness and fracture were tailored by controlling the process parameters.

Compaction Behavior and Damage Constitutive Model for Porous Cement Mortar under Uniaxial Cyclic Loads.

The void compression stage causes porous cement mortar to present special mechanical properties. In order to study the compaction behavior and the damage evolution of the porous material, cement mortar specimens with an average porosity of 26.8% were created and cyclic uniaxial compression tests were carried out. The irreversible strain accumulated in the tests was obtained by cyclic loading and unloading. As the secant modulus of the porous cement mortar increases with stress in the pre-peak deformation stage, its damage variable is defined according to the accumulated irreversible strain instead of modulus degradation. The strain-based damage indicator fitted with the damage evolution law is characterized by linear accumulation at the beginning and has an acceleration rate of about 0.3 in the pre-peak deformation stage, and the damage value converges to 1 at failure. Based on the Weibull distribution, a constitutive damage model of porous cement mortar is improved by considering both the damage evolution during the plastic deformation stage and the mechanical behavior in the compaction stage. The theoretical envelope curves obtained by the constitutive model are in good agreement with the experimental envelope curves of cyclic uniaxial compression in the compaction and pre-peak stages, and the average absolute error is about 0.54 MPa in the entire pre-peak stage, so the proposed damage constitutive model can characterize the damage-induced mechanical properties of porous cement mortar in the compaction and pre-peak stages.

AN EXPERIMENTAL STUDY ON THE PERMANENT STRAIN OF DENSE-GRADED ASPHALT AND POROUS ASPHALT USING UNIAXIAL CYCLIC COMPRESSION TEST WITH CONFINEMENT

This study investigated the permanent strain of dense-graded asphalt and porous asphalt material using uniaxial cyclic compression test with confinement. Based on the test results, the comparison of the permanent strain was carried out. The results showed that the permanent strain resistance of porous asphalt material was better than that of the dense-graded asphalt material. In addition, the use of higher performance-graded asphalt binder improved remarkably the permanent strain resistance of porous asphalt material. The analysis of the two prediction models showed that the power model performed better in prediction of the permanent strain. In future, more studies need to be conducted to investigate the effect of other factors such as moisture, freeze/thaw cycle on the mechanical performance of the porous asphalt material.

Experimental assessment on the fatigue mechanical properties and fracturing mechanism of sandstone exposed to freeze-thaw treatment and cyclic uniaxial compression

The coupling problem of freeze-thaw (F-T) treatment and cyclic loading is extremely prominent for rock engineering in Tibetan Plateau, which has triggered many serious geotechnical engineering disasters. It is thus essential to systematically characterize the fatigue mechanism of rocks exposed to F-T treatment and cyclic loading. In this study, a series of cyclic uniaxial compression tests are conducted on sandstone subjected to 0, 10, 20, 30, 40 and 50 artificial F-T cycles to investigate the influence of F-T treatment on the fatigue mechanical properties of sandstone, regarding the fatigue deformation characteristics, energy evolution and coupled F-T-fatigue damage. Our testing results indicate that sandstone specimens exposed to higher F-T cycles are featured by larger irreversible strain and faster damage accumulation, leading to higher fatigue damage variable and lower fatigue life. By virtue of the digital image correlation (DIC) technique and scanning electron microscope (SEM), the progressive fatigue fracture process and the microscopic morphologies of fracture surfaces of the F-T treated sandstone specimens are revealed. During the cyclic loading process, the strain field in specimens uniformly distributes initially, and then it localizes continually with increasing cyclic number, triggering the eventual fatigue fracture. With increasing F-T cycles, the fatigue fracture of sandstone is more ductile and progressive. Moreover, the hidden linkage between microscopic fracture mechanism and macroscopic mechanical properties is further analyzed based on the test results. In addition, the microscopic fracture mechanism of F-T treated sandstone under cyclic loading and monotonic loading is also compared and discussed: the primary micro-mechanism causing the fatigue fracture of sandstone is the inter-granular (IG) fracture, while the micro-mechanism triggering the monotonic failure is the trans-granular (TG) fracture.

Investigation of the Reusability of a Polyurethane-Bound Noise-Absorbing Pavement in Terms of Reclaimed Asphalt Pavement.

A key aspect of sustainable pavement construction is the use of environmentally-friendly designed pavement materials. These materials are characterized by the fact that they are renewable raw materials, require a low amount of energy during production and in the best case, are made from a high proportion of recyclable materials in order to reduce waste. A number of recent studies have demonstrated the recyclability of waste materials that can be very well utilized in road construction. This study describes the recycling of a new and innovative topcoat system that already contains recycled materials. However, the focus is on guaranteeing the mechanical performance of the innovative absorption layer where different portions of used material are added. Therefore, low-temperature behaviour, durability, fatigue and noise absorption are investigated in detail and it is concluded that their function is preserved. In order to investigate these characteristics, the impedance measuring tube, the uniaxial cyclic compression test (UCCT), the three point bending test (3PB), the uniaxial tension stress test (UTST) and the thermal stress restrained specimen test (TSRST) are used. However, the examined absorption material can be reused to build innovative roads.

Enhanced damping capacity of nanolaminated graphene (reduced graphene oxide)/Al-Mg-Si composite

Cyclic uniaxial compression tests were carried out on micro-pillars fabricated from reduced graphene oxide (RGO)-reinforced Al-Mg-Si alloy that went through peak-ageing treatment. The composite pillars were found to have a significantly better damping capacity and improved resistance to plastic deformation as compared to the unreinforced matrix. These observations were further corroborated by the much higher damping coefficient of the composite pillars (η = 0.3081 ± 0.0803), almost doubled that for the alloy matrix alone (0.1560 ± 0.0457). Site-specific TEM analysis revealed that the enhanced damping property of the composite came from a combined effect of dislocation impedance at the RGO/Al interface, the dislocation looping and cutting at the precipitates, and the formation of nano-twins in the alloy matrix upon deformation.

Formation mechanism of the distinctive granular fracture surface in subsurface fracture of Ti6Al4V alloy

The formation mechanism of the granular regions often observed in subsurface fractures of Ti6Al4V alloy was experimentally investigated. To simulate and simplify the phenomenon of crack opening and closure, which is strongly related to fatigue crack propagation and strength properties, uniaxial cyclic compression tests were conducted in air and vacuum using test pieces with rounded surfaces under contact conditions. The results showed ring-like patterns on the parts of the rounded surfaces subjected to cyclic contact regardless of the ambient environmental conditions. However, in the case of vacuum only, fine surface asperities were observed, which were quite similar to those in the subsurface fractures. Microstructural refinement was confirmed by cross-sectional observation of the concave and convex parts. In the refined microstructure, voids and cracks were observed, which were formed during the refinement and adhesion processes. Microstructural refinement and metallic adhesion under compression loading in vacuum are key phenomena for the formation of fine surface asperities. Based on the experimental results, a new model for the formation mechanism of the granular region was proposed. This model can explain the characteristics of the granular region and the properties of subsurface fractures, which occur at lower stress and have longer lifetimes than surface fractures.

Strain ratcheting limit stresses as a function of microstructure of WC-Co hardmetals under uniaxial cyclic loads under a stress ratio of R = −∞ at elevated temperatures

The aim of the current work was the experimental determination of limit stresses in the uniaxial cyclic compression test that lead to advancing ratcheting in different WC-Co hardmetal grades at elevated temperature. At stresses below the limit stress the plastic strain per cycle reduces and plastic strain accumulation stops after a characteristic number of load cycles. Special attention was paid to the microstructural influence on the onset of advancing ratcheting and the associated damage development at the microstructure level. WC-Co hardmetals are used in various areas such as forming and forging tools, where they are exposed to high temperatures and pressure loads. Their good high-temperature properties allow them to be used under these conditions, but these properties are influenced by the microstructure. Investigations of the mechanical properties of hardmetals had been carried out under monotonously increasing loads and cyclic tests at room temperature and elevated temperatures. In these tests, the effect of different stress ratios R = σmin/σmax on the fatigue behaviour of hardmetals were studied, However, no studies are known for WC-Co hardmetals and their limit stresses in relation to strain ratcheting under cyclic compressive loading at elevated temperature. Hence, in the current work the influence of increasing stresses as a function of microstructure and their effect on the evolution of the strain of six different WC-Co hardmetal grades are discussed. For this purpose, the materials were investigated by uniaxial cyclic compression tests at a stress ratio of R = −∞ at 700 °C and 800 °C in vacuum. The investigated hardmetal grades differ on the one hand in their WC grain size, which varies between 0.4 μm and 2.0 μm, and on the other hand in their Co-content, which varies between 6 wt% and 12 wt%. The residual strain value εres at zero applied stress was observed to stabilize with increasing number of load cycles at low applied stress ranges. Strain ratcheting occurred above a critical stress range, referred to as the limit stress for strain ratcheting. Strain ratcheting is the accumulation of plastic strain with increasing number of load cycles in which no strain stabilization occurs. Further, for all investigated hardmetal grades, the limit stresses were observed to decrease with increasing temperature. In the following, the microstructure of one hardmetal grade was analysed after loading below and above the limit stress by scanning electron microscopy and electron backscatter diffraction (EBSD). The influence of strain stabilization and ratcheting was analysed with regard to damage development and deformation behaviour of the WC and Co-phases. Strain ratcheting was observed to result in the formation of cavities and nanopores at phase boundary triple points and WC/Co interfaces. Additionally, the EBSD data showed that the fcc Co phase was transformed into hcp Co. Therefore, it is assumed that on the one hand, a certain strain value needs to be exceeded for strain ratcheting to occur and, on the other hand, that besides dislocation movement, microdefect formation and phase transformation significantly contribute to the increase in strain.

A Strain Rate-Dependent Damage Evolution Model for Concrete Based on Experimental Results

There are various definitions of damage variables from the existing damage models. The calculated damage value by the current methods still could not well correspond to the actual damage value. Therefore, it is necessary to establish a damage evolution model corresponding to the actual damage evolution. In this paper, a strain rate-sensitive isotropic damage model for plain concrete is proposed to describe its nonlinear behavior. Cyclic uniaxial compression tests were conducted on concrete samples at three strain rates of 10−3s−1, 10−4s−1, and 10−5s−1, respectively, and ultrasonic wave measurements were made at specified strain values during the loading progress. A damage variable was defined using the secant and initial moduli, and concrete damage evolution was then studied using the experimental results of the cyclic uniaxial compression tests conducted at the different strain rates. A viscoelastic stress-strain relationship, which considered the proposed damage evolution model, was presented according to the principles of irreversible thermodynamics. The model results agreed well with the experiment and indicated that the proposed damage evolution model can accurately characterize the development of macroscopic mechanical weakening of concrete. A damage-coupled viscoelastic constitutive relationship of concrete was recommended. It was concluded that the model could not only characterize the stress-strain response of materials under one-dimensional compressive load but also truly reflect the degradation law of the macromechanical properties of materials. The proposed damage model will advance the understanding of the failure process of concrete materials.