Modulus Degradation Research Articles

Experimental study on high-cycle flexural fatigue behavior of cement mortar for ballastless track of high-speed railway

Cement mortar (CM), an important component of concrete, is the phase where fractures and failures predominately appear. The present paper conducted static and fatigue tests regarding four maximum stress levels (0.65, 0.70, 0.75, and 0.80) on CM specimens, aiming at determining the flexural fatigue properties of CM for ballastless track concrete. The S-N curve were fitted, and the fatigue damage model were established. Results demonstrate that the fatigue strain evolution curves showed a three-stage inverted S-shaped law. The CM specimens’ fatigue modulus degradation values ranged between 3.2 and 4.5 GPa. The fatigue damage evolution curves rose as stress levels increased, indicating that higher fatigue loads would lead to larger damage in earlier stages and smaller damage in later stages.

Experimental study on fatigue damage of self-compacting concrete of CRTS III slab track

To study fatigue performances and establish the fatigue damage model of self-compacting concrete (SCC) of China Railway Track System (CRTS) III slab track under different stress levels, static and fatigue tests were conducted. In the static test, the flexural strength of SCC was obtained. In the fatigue test, fifteen specimens were divided into five groups. The maximum stress levels were 0.65, 0.70, 0.75, 0.80, and 0.85 for each group, and the minimum stress level was 0.10 for every group. The S-N curve of SCC is obtained and shows that the fatigue life of SCC decreases with the increase of the maximum stress level. The strain evolution and elastic modulus degradation can be divided into 'Fast-Slow-Fast' three stages. With the maximum stress level increases, the proportion of elastic modulus degradation increases and decreases in the first and third stages, respectively. The fatigue damage model derived from the improved elastic modulus method and the Chaboche fatigue damage model can reflect the damage evolution of SCC under different stress levels. Moreover, the proposed model can provide theoretical guidance for the further numerical simulation of SCC for ballastless tracks under the fatigue load.

Thermo-mechanical coupled behavior of laminated beams with temperature-dependent viscoelastic interlayers

The present research proposes analytical solutions of simply-supported laminated beams bonded by viscoelastic interlayers under both non-uniform temperature field and mechanical loads, in order to study its time-varying thermo-mechanical coupled behavior. The temperature has two effect mechanisms to the laminated beams, including the thermal expansion difference and the temperature-dependent property of viscoelastic interlayer. In the analytical model, the non-uniform temperature field is solved first by the transfer matrix method, based on the heat transport theory. The standard linear solid model is utilized to simulate the time-varying characteristic of the interlayer, which renders the mechanical property of the laminated beam changes over time. Then, the temperature effects on the interlayer modulus degradation are determined according to the Time-Temperature-Superposition-Principle (TTSP). By virtue of the state space method and Fourier series expansion, the general solutions of stress and displacement components are solved with undetermined coefficients. The analytical solutions are solved by means of Laplace transform and Cramer rule. The analysis results indicate that the present solutions are in accordance with the finite element solution and have superior computational efficiency. The influences of temperature, geometric and material parameters on the long-term thermo-mechanical coupled behavior of the laminated beam are studied in detail.

Experimental study on uniaxial compression of granite under real-time loading at designated low-temperature conditions

A low temperature and the degree of saturation have important influence on the mechanical properties of rock surrounding cave-type underground liquefied natural gas storage. Uniaxial compression tests were carried out on dry and saturated granite under different real-time loadings at designated low-temperature conditions (25, 0, −20, −40, −50 and −60°C). The porosity and microstructure were then observed by nuclear magnetic resonance spectroscopy and scanning electron microscopy. The results show that with the decrease in temperature, the uniaxial compressive strength (UCS) and porosity of both dry and saturated granite monotonically increase, while the elastic modulus undergoes an increase–decrease transition and finally stabilises. At the same temperature, the UCS and elastic modulus of dry granite are generally greater than those of saturated granite. The reinforcement in the UCS and elastic modulus is caused by cold shrinkage, while the degradation in the elastic modulus, particularly for saturated granite, is attributed to freezing damage below 0°C. The results of this study can provide technical support for the long-term stability analysis and evaluation of underground cavernous liquefied-natural-gas-storage reservoirs.

Maximum shear modulus and modulus degradation curves of an unsaturated tropical soil

The maximum shear modulus (G0) and the modulus degradation curve (G/G0 versus γ) are important information in the evaluation of the soil mechanical behavior, both for dynamic and static loads. Dynamic tests (resonant column and cyclic triaxial tests) are not routinely performed in geotechnical practice in Brazil, and the geotechnical literature on the dynamic behavior of unsaturated tropical soils is limited. This paper presents and discusses seismic dilatometer (SDMT), resonant column, and triaxial test with bender elements and internal instrumentation to determine G0 and the modulus degradation curve in an unsaturated tropical sandy soil profile. It was observed that G0 tends to increase non-linearly with soil suction and net stress (σ - ua). It was also observed that the in situ G0 values determined with the SDMT were higher than those from laboratory tests (bender elements and resonant column). The modulus degradation curves determined with resonant column were used to define the reference curve via SDMT for the studied site. Soil suction influence in shear modulus degradation curves determined with unsaturated triaxial compression tests with local instrumentation is also presented and discussed.

Shear stiffness of sand-fines binary mixtures: Effects of sand gradation and fines content

In this study, a comprehensive experimental programme was carried out on five clean sands of different gradations mixed with low plastic fines using an apparatus that incorporates both resonant column (RC) and bender element (BE) functions. The sand gradation is characterized by uniformity coefficient of host sand (Cus) and grain size ratio (Rd), which is defined as the ratio of the mean grain size of sand to that of fines. The effects of fines content (FC) and sand gradation on small strain shear modulus G0 are analyzed from the experimental results. Both RC and BE test results show that G0 continuously decreases in a non-linear manner as FC increases to 30% and becomes as low as nearly 50% of that of clean sand at the highest FC. The reduction rates with respect to FC are similar regardless of the testing type and the stress dependence of G0 is insensitive to FC. Meanwhile, G0 decreases as Cus or Rd increases at a given FC and void ratio or at a fixed equivalent skeleton void ratio e*. A new predictive model in terms of e* is proposed to evaluate the G0 of sand-fines mixtures to account for the effects of void ratio, confining pressure, FC, and sand gradation within ±20% deviation. Moreover, the rate of shear modulus degradation with shear strain and the damping ratio are higher for sand-fines mixtures with higher FCs.

Predicting the mechanical properties of E-glass fiber-reinforced polymer bars after exposure to elevated temperature

This paper presents the experimental investigations on the tensile, flexural, and shear performance of E-glass fiber-reinforced polymer (GFRP) bars after exposure to elevated temperatures from ambient to 500 ℃. Two types of GFRP bars with nominal diameters of 12 mm and 16 mm were directly exposed to several target temperatures for 0.5 h, and their mechanical properties were tested after cooling. The experimental results showed that the GFRP bars gradually loosened, and their surface color darkened with the increasing exposure temperature. After experiencing high temperatures, the bars’ tension, bending, and shear failure modes remained unchanged. However, their mechanical strengths showed a phased decline in three typical temperature ranges of 20 to 120 ℃, 120 to 270 ℃, and 270 to 420 ℃. In addition, no significant tensile and flexural elastic modulus degradation was observed. The measured strengths were compared with the results of several existing prediction models. A simplified model was proposed to predict the residual strength of GFRP bars after high-temperature exposure. Based on fire safety requirements, a fire-resisting reduction factor, CF, was proposed to determine the design strength of GFRP bars used in concrete slabs. For GFRP bar-reinforced concrete slabs with a cover thickness of 60 mm, the suggested CF for tensile strength and the other two strengths should be 0.65 and 0.75, respectively, to achieve structural safety after 2-h fire disasters.

Physical and Mechanical Behaviors of Compacted Soils under Hydraulic Loading of Wetting–Drying Cycles

Exposed geo−infrastructures filled with compacted soils experience cyclic wetting–drying effects due to environment and underground water fluctuations. Soil physical and mechanical behaviors are prone to deterioration to a great extent, e.g., swelling, collapse, or even slope failure, resulting in huge losses to human life, safety, and engineering construction. In this paper, hydraulic loading tests of wetting–drying cycles were carried out on compacted fine soil via a one−dimensional pressure plate apparatus equipped with bender elements. The influences of wetting–drying paths on the soil characteristics of moisture content, void ratio and shear modulus were obtained and analyzed. Results showed that cyclic wetting–drying effects weakened the soil’s water retention capacity. It was observed that it was harder for pore water to approach saturation at a lower matric suction level and to be expelled at a higher matric suction level. Typical swelling and shrinkage deformations occurred during the hydraulic loading processes, and volume expansion was generated after the drying–wetting cycles at a given value of matric suction, which deteriorated the densely compacted soils to a relatively looser state. Then, a unified soil–water characteristic surface was proposed to describe the unique relationships of moisture content, void ratio, and matric suction. Moreover, the small−strain shear modulus of the soil, in terms of shear wave velocity, was reduced by 32.2–35.5% and 13.8–25.8% at the same degree of saturation during the first and second wetting paths, respectively. Therefore, the volume expansion and modulus degradation resulting from the wetting–drying cycles should attract particular attention to avoid further distresses in the practical engineering.

Multiple Sulfur Corrosion Simulation on the Oil–Paper Insulation System Under Synergistic Electric and Thermal Fields

Sulfur corrosion in transformers results in insulation deterioration failures. Therefore, in this study, multiple sulfur corrosion was simulated on the oil–paper insulation system. The mechanical properties and hydrogen bond of the insulating paper as well as the mean square displacement and shear viscosity of the insulating oil were investigated under different combinations of electric and thermal fields. The electric field could promote the degradation of the elastic modulus of cellulose. The influence of the AC electric field on the modulus is not as obvious as that of the DC electric field. Temperature has a significant effect on the modulus parameters. With the increase in the temperature, the elastic modulus decreases considerably. The radial distribution function of the model was also investigated. Cellulose can easily form intramolecular hydrogen bonds under AC electric fields. Furthermore, intermolecular hydrogen bonds are easily formed under a DC electric field. The shear viscosity of insulating oil decreases with the increase in the temperature. The presence of the electric field could increase shear viscosity. The results of the study can provide some reference for the microscopic mechanism of multiple corrosive sulfides and investigate effective protection techniques against sulfur corrosion.

Resonant Column Calibration and Dynamic Torsional Shear Testing Using Stepped Frequency Sweeps

Abstract This article introduces a new apparatus calibration procedure along with testing protocols, data acquisition, and reduction procedures for a fixed-free resonant column test in which a fixed amplitude sinusoidal torque is applied to the specimen at discrete frequencies over a wide bandwidth that encompasses the resonant frequency. Recently developed resonant column theory, consistent with ASTM D4015-21, Standard Test Methods for Modulus and Damping of Soils by Fixed-Base Resonant Column Devices, is shown to hold for all frequencies in this bandwidth and enables determination of shear modulus, damping, and shear strain amplitude at each frequency. For off-resonant frequencies, shear strain amplitudes are much lower than those at resonance; hence, under a constant applied torque, the method measures values over a range of shear strains. These procedures are facilitated using a spectrum analyzer. The new and more robust approach for apparatus calibration relies on conducting stepped frequency sweeps (SFS) on calibration rods and allows concurrent determination of all apparatus calibration factors, which apply to the full frequency bandwidth for testing soils. Test results on sand are used to illustrate the developed procedures and recommendations are provided for determining shear modulus and damping over the full shear strain range of the apparatus. If the fixed amplitude applied torque is low, the test provides shear modulus and damping at very low strains. For sufficiently high values of the applied torque, a single SFS allows semicontinuous characterization of the shear modulus degradation and shear damping increase behavior. The article provides recommendations for selecting frequency bandwidth, frequency step values, and spectrum analyzer settings. For testing with large input torques, SFS should be from high frequency to low frequency to more accurately capture the response function for strain softening behavior. The total time for SFS is on the order of 10 min, and the number of applied loading cycles can be controlled.

The combined effect of temperature and seawater on the compression properties of carbon fiber vinyl ester composites for sandwich structures

For naval applications, composite sandwich structures are of significant interest, and are often manufactured using thin (2 to 4 mm) composite facings made from carbon or glass fiber reinforcement, attached to a thick (25 to 50 mm) section of PVC cellular foam or balsa wood-based core materials using a suitable polymeric resin. In the present study, we focus on the hygrothermal effect on the fiber-dominated compression properties of carbon fiber reinforced vinyl ester resin based polymeric composite (CF/VE), used as “skin” for a polymeric composite sandwich material. Hygrothermal conditioning is achieved by saturating samples in simulated seawater at 40°C. Compression properties are evaluated for coupons extracted along warp and fill undergoing- no conditioning, conditioning till saturation (up to 6 months), and long-term conditioning (2 years). Sea-water saturation yields in up to 12% drop in compression strength with a further 3–4% drop resulting from long-term conditioning. No statistically significant modulus degradation is noticed due to short or long-term hygrothermal exposure. The failure mechanism of the warp extracted coupon, which fails in a splitting failure mode originating due to the delamination between the 0/90 interface, or the fill extracted coupon, which fails due to the instability caused by tow micro-buckling, remains unchanged due to combined exposure (short or long-term) of seawater and temperature. The loss in strength is attributed to the degradation of the fiber-matrix interface, which is validated via conducting single fiber push-in tests with a nominal diameter of 7 micron for conditioned and unconditioned coupons.

Static Unified Inelastic Model: pre- and post-yield dislocation-mediated deformation

Modelling dislocation glide over the initial part of a stress–strain curve of metals received little attention up to now. However, dislocation glide is essential to ones understanding of the fundamental relationship between inelastic deformation and the evolution of the dislocation network structure. Therefore, we present a model of dislocation-driven deformation under static loading conditions.We reproduce repeated cyclic uniaxial tensile tests on Interstitial-Free and Low-Alloy steels. The elastic mechanical behaviour is described by isotropic linear elasticity, pre-yield anelastic mechanical behaviour by a dislocation bow-out model with dissipation, and the post-yield evolution of dislocation network structure by a statistical storage model. We hypothesise that when the local anelastic compliance is lower than the global plastic compliance, deformation is mechanically recoverable, and vice versa. This hypothesis is corroborated with the classical Taylor relation. We report the relation between stable and unstable dislocation glide using this prototypical modelling framework.We find four structural variables, that are based on dislocation physics, to describe the stress–strain curve: total dislocation density, average dislocation segment length, dislocation junction formation rate, and average dislocation junction length. Firstly, we quantify the dislocation network evolution during uniaxial monotonic loading, and verify work-hardening by dislocation junction formation and a Taylor-type equation for flow. Finally, we present a semi-empirical relation for the evolution of the dislocation network structure. Which allows us to: refine the physical interpretation of the Taylor relationship, and rationalise experimental observations on apparent modulus degradation by thermomechanical processing. Both these findings circumvent the limitations of current, physics-based hardening models.

A viscoelastic–viscoplastic model for adhesives subjected to reversed cyclic load

This study aims to describe a toughened adhesive’s ratcheting–recovery behavior under reversed cyclic load using a viscoelastic–viscoplastic model. As most adhesives are based on synthetic polymers, their tensile and compression response can be different. A series of load–Sunload tests were conducted on bulk adhesives and bonded joints involving tension/compression–shear loads to characterize the initial yield surface. The effect of hydrostatic stress was studied by considering the instantaneous response and yield strength under tensile and compression loads. Given the observed modulus degradation and extensive permanent strain during reversed cyclic tests, time-dependent damage factors were considered for both viscoelastic and viscoplastic responses. The model was implemented in a finite element (FE) code and used to model the shear response to reversed cyclic load with varying frequency. Good agreement between the model and experiment was obtained. The consideration of both hydrostatic stress and damage was required to describe the observed adhesive reversed cyclic response.

Mechanical and Biomimetic Characteristics of Bulk-Fill Resin Dental Composites Following Exposure in a Simulated Acidic Oral Environment

During the last 10 years, various companies have marketed different "bulk-fill" resin dental composites for the restoration of posterior stress-bearing teeth; however, the impact of acidic conditions on these relatively newer materials has not been thoroughly investigated. Therefore, an attempt was made to evaluate the effect of acidic beverages on the mechanical biomimetic characteristics of four bulk-fill and one conventional nanohybrid resin-based dental composites (RBCs). The specimens of each RBC were stored in two acidic beverages namely 'Orange Juice' and 'Coca-Cola', whereas 'dry' and 'distilled water' storage of specimens served as controls. After 1 week of storage, flexural and surface hardness properties of specimens were determined using a universal testing machine and Vickers hardness tester, respectively. In general, the 'Coca-Cola' beverage caused the greatest degradation of flexural strength, flexural modulus, and surface hardness characteristics in all RBCs in contrast to the 'dry', 'distilled water' controls and 'Orange Juice' storage conditions. However, the overall mechanical biomimetic performance of nanohybrid RBCs was relatively better than all other bulk-fill RBCs and may, therefore, be considered a suitable candidate for the restoration of posterior stress-bearing permanent dentition.

High-throughput 3D printing of continuous carbon fiber–reinforced Polyamide 6/Maleic Anhydride-modified Polypropylene/polypropylene composite by a multifilament feeder

A multifilament feeder was developed to achieve the high-throughput 3D printing of continuous carbon fiber–reinforced thermoplastic composites. Four filaments were supplied to the feeder and simultaneously printed by controlling the feeding speeds to quadruple the manufacturing volume. A water-resistant filament was also developed by the polymer blending of polyamide 6 (PA6) and polypropylene (PP), which provided good adhesion to the carbon fibers and water resistance, respectively. The polymer structure in the continuous carbon fiber–reinforced thermoplastic composites was characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The multilayer polymer structure was observed, which enhanced the water-resistant property. The bending test results revealed that the blending minimized the degradation of the flexural modulus of the 3D-printed unidirectional carbon fiber–reinforced thermoplastic composite under wet conditions.

Numerical study of the effect of fatigue on impact performance of RC beams

Some existing concrete structures not only face threats of possible explosions and impact, but also suffer from long-term fatigue loads induced by traffic or vibration of machines. Researchers have conducted extensive investigations on concrete structures against highly dynamic load, e.g., blast or impact. However, the fatigue damage and performance deterioration of flexural members due to long-term cyclic loads throughout the service life have not been considered in the analysis of structures against highly dynamic loads. Studies have proven that fatigue damage resulted in degradation of concrete strength and modulus of elasticity (MoE), as well as a reduction in the strength of steel bars. The deteriorated materials caused a reduction in the residual capacity and stiffness of reinforced concrete (RC) structures under quasi-static loads. Therefore, ignoring the effect of fatigue may result in an overestimation of the performance of existing structures against blast or impact loads. This study numerically investigates the effect of fatigue on the impact performance of RC beams. The grid section method (GSM) is proposed to simulate RC beams with fatigue damage. The GSM is applied to divide the RC beam into compressive and tensile zones and assign deteriorated strength and MoE of concrete to corresponding zones according to the stress level. Numerical models of the RC beam with fatigue damage are established using LS-DYNA and validated against experimental results. The effect of different factors, including fatigue cycles, reinforcement ratio, concrete strength, and impact energy, on the impact behavior of RC beams is examined. Based on the results, the correction method is proposed to take into consideration of the fatigue damage in the estimation of RC beams against highly dynamic load.

Viscoelastic Solutions and Investigation for Creep Behavior of Composite Pipes under Sustained Compression

Composite pipes, which are widely used for transporting fluids, have a high strength, good impermeability and strong resistance to external pressure. Because the pipe bears a sustained load, and its constituent materials usually possess time-dependent properties, the creep phenomenon unavoidably occurs in the composite pipes in the long run. The aim of this study is to propose analytical viscoelastic solutions, which are then applied to a composite pipe structure to explore the creep behavior of composite pipes under sustained compression. The pipe layers and the bonding interlayer both exhibit viscoelastic properties, which are the novelty of this study. The governing equations for the viscoelastic composite pipe are built on the basis of exact elasticity theory combined with the viscoelastic theory. General solutions are derived by means of a Fourier series expansion in which the coefficients are further determined by a Laplace transform. The research results indicate that the present solution has a higher computational efficiency than the finite element solution, because of the latter involving the time discretization method. In addition, for the viscoelastic pipe, if the modulus degradation of the neighboring laminar layers is proportional, the stresses can keep constant with time, as in a purely elastic material.

A Quantitative Representation of Damage and Failure Response of Three-Dimensional Textile SiC/SiC Ceramics Matrix Composites Subjected to Flexural Loading

Abstract In the present work, the microstructure deformation and synergetic damage evolution of a three-dimensional textile SiC/SiC ceramic-matrix composite under flexural loading are investigated by in situ digital image correlation at ambient temperatures. The correlations between microstructure evolution and macro-mechanical degradation of 3D textile composites under flexural loading are established based on the experimental results. In addition, by establishing continuum damage mechanics and a thermodynamic framework with synergetic effects of microstructures, a flexural loading-induced damage evolution model is developed to reveal the relationship between the energy release rate and elastic modulus degradation. The proposed model can be used to predict the flexural stress–strain curves of 3D textile SiC/SiC composites to further improve the design and assessment of new textile architectures with specific mechanical properties.

Fatigue properties of a structural rotor blade adhesive under axial and torsional loading

AbstractAxial and torsional fatigue tests at different stress ratios were performed on a structural adhesive designed for wind turbine rotor blades. By employing previously optimized specimens, fatigue properties were recorded without influences of manufacturing‐induced defects such as pores. The Stüssi S–N model was an excellent fit to the data and was combined with a Haibach extension line to account for uncertainties in the gigacycle fatigue regime. A comparison of the results with hand‐mixed specimens revealed significant and load level‐dependent differences, indicating that manufacturing safety factors should be applied to the slope of the S–N curve. The experiments were accompanied by stiffness degradation measurements, which enabled an analysis of Young's and shear modulus degradation interactions. The degradation was modeled using power law fits, which incorporated load level‐dependent fitting parameters to allow for a full description of the stiffness reduction and a prediction of the residual fatigue life of run‐out specimens.

Experimental and numerical investigations on the micro-damage behaviour of glass fibre-reinforced plastics

The formation and influence of micro-damage in glass fibre-reinforced plastics (GFRP) is carried out by numerical and experimental investigations. A quasi-static material characterisation is performed and the stiffness and strength of unidirectional, undamaged material in longitudinal and transverse direction as well as in the 23-plane are determined. Since the desired micro-damage could not be generated in unidirectional laminates due to abrupt failure, a [0∘/90∘]s layup has been identified to be most suitable to generate micro-damage in a reproducible way in the 90° layers, which is done using a tension–tension fatigue test program. During these experiments, micro-damage is detected optically and by means of ultrasonic birefringence, where the latter method shows a reduction in shear modulus due to fibre–matrix debonding prior to final inter-fibre failure. The shear modulus degradation is indirectly measured by the change in the velocity of sound. Since this method is limited to measurements in the 13- and 23-plane, all experiments are accompanied with microstructural simulations with detailed statistical volume elements (SVE), to study the influence of micro-damage under various loading conditions and directions. For loading in fibre direction, no influence on the mechanical properties is observed for the investigated fibre–matrix debonding. However, this is different for transverse loading, where a transverse shear stiffness loss of around 8% results in a decrease of transversal stiffness of around 18%, if the load direction is perpendicular to the damage plane. A similar behaviour is observed for the strength, which is reduced by a factor of 2 for the same configuration.