Loading Stress Level Research Articles

Effect of loading frequency on tensile fatigue behavior of ultra-high-strength engineered cementitious composites

The ultra-high-strength engineering cementitious composites demonstrates pseudo strain hardening behavior when subjected to uniaxial tension, making it a promising material for enduring repeated or fatigue loads. Extensive research has been conducted on the quasi-static, dynamic, and fatigue behavior of this composites. However, due to the challenges of conducting direct tensile testing on concrete, investigations into the tensile fatigue behavior of ECC, particularly for ultra-high-strength ECC, remain limited. The fatigue behavior of concrete can be influenced by various factors. This study focuses on the impact of loading frequency. Several series of tensile fatigue tests were conducted under different loading frequencies and stress levels. The test results revealed that fatigue life increases with higher applied loading frequencies and decreases with increasing stress levels. The analysis of the test results includes the examination of failure modes, fatigue life, deformation, and secondary strain rates. A probabilistic model of fatigue failure, considering the discreteness of the initial static strength, was proposed based on the fatigue life. This model aligned well with the experimental results, providing valuable insights into the behavior of ultra-high-strength ECC under tensile fatigue conditions.

Analysis of Equivalence between Loading Rate and Stress Level of Fatigue Characteristics of Asphalt Mixture

Analysis of Equivalence between Loading Rate and Stress Level of Fatigue Characteristics of Asphalt Mixture

Experimental study on bending fatigue performance of ambient-cured ultra-high-performance concrete thin plate with embedded steel wire mesh

Ambient-cured ultra-high-performance concrete (ACUHPC) with embedded steel wire mesh (SWM) was studied experimentally. ACUHPC-SWM thin plates having different SWM wire ratios and fatigue load stress levels were tested to clarify the effects of SWM on static and fatigue bending behavior. Subsequently, the fatigue equations for ACUHPC-SWM materials (with different failure probabilities) and reinforcement efficiency of the SWM were proposed. Finally, the reinforcement mechanism of the SWM was revealed by comparing it with ACUHPC thin plates. The results show that the inclusion of wire mesh significantly improved the cracking strength, ultimate static strength, energy dissipation performance, and fatigue ultimate strength of ACUHPC, with maximum increases of 23.63 %, 260.32 %, 940.42 %, and 196.34 %, respectively. As the wire mesh ratio increased, the improvement in fatigue ultimate strength initially increased and then decreased once the wire mesh ratio exceeded 1.884 %. The mechanism according to which SWM enhances ACUHPC involves the action of the mesh as a bridge to delay crack development and increase toughness.

Precursors of rock failure under cyclic loading and unloading: From the perspective of energy and acoustics

The precursor characteristics of rock failure under cyclic loading and unloading are crucial to the prevention of underground engineering disasters. Taking sandstone as an example, this work conducted rock failure experiments under uniaxial cyclic loading and unloading combined with active wave velocity and passive acoustic emission (AE). The damage evolution process of rock was analyzed from the perspectives of energy, AE, and wave velocity. The results show that as the cyclic load stress level increases, the rigidity of the rock continuously increases, the proportion of elastic energy may reach a peak, and there will be an extreme time difference between the wave velocity and stress. The occurrence of the peak point of elastic energy proportion is a sufficient and unnecessary condition for the precursor characteristics of rock failure. The appearance of numerous AE events with high amplitude and high count is an effective precursor characteristic of rock failure. The stress continues to increase while wave velocity decreases is a key precursor characteristic of rock failure. There is a clear linear relationship between wave velocity drop and dissipated energy, it implies that both the which can quantitatively evaluate the rock damage under cyclic loading and unloading. The variation of wave velocity exhibits obvious anisotropic characteristics. This paper is not only a beneficial supplement to the theoretical system of rock failure precursors, but also provides new ideas and methods for rock stability monitoring and failure disaster early warning in underground engineering.

Long-term deformation of rock salt under creep–fatigue stress loading paths: Modeling and prediction

Rock salt, due to its water solubility, low permeability, high plasticity, and damage self-healing ability, is one of the best candidate rock types for underground energy storage. Utilizing salt caves to construct compressed air energy storage (CAES) facilities can effectively enhance the utilization of renewable energy. Due to the need for peak shaving, the surrounding rock of the salt cavern will undergo discontinuous cyclic loading with varying gas injection rates and pressures, namely, alternating creep–fatigue loading. Considering the actual peak-shaving cycle of a CAES plant, long-term creep–fatigue tests of rock salt with different loading cycles and stress levels were conducted. The results indicate that in long-term creep–fatigue tests for rock salt, the lower the loading stress rate is, the greater the deformation of the rock salt. The variation in the stress limit has a greater effect on creep than on fatigue loading and unloading. The deformation rate of rock salt is influenced by alterations in the stress state. Based on the test results, according to the Norton creep model, a new creep–fatigue constitutive model for rock salt was established by defining a state variable that characterizes the level of rock hardening and introducing unloading as well as crack factors. This model can accurately describe the impact of historical loading and unloading processes on the viscoplastic mechanical characteristics of rock salt. The rock salt creep–fatigue test results were used to verify the constitutive model. A comparison of the fitting curve of the different stress loading paths with the test curve reveals good consistency, indicating that the model comprehensively considers the effects of time, load, and state on rock salt creep–fatigue, effectively describing the viscoplastic deformation characteristics of rock salt under different stress paths. These research findings provide important guidance for ensuring the stability of salt caverns used for CAES.

Detection of Stress Distribution in Surrounding Rock of Coal Seam Roadway Based on Charge Induction Principle

Rock burst is a worldwide prevention and control problem, and the main reason for its occurrence is the concentration of stress in the surrounding rock of the coal roadway. Therefore, it is of great significance to realize the rapid and accurate detection of the stress distribution in the surrounding rock of the roadway for the prevention and control of rock burst. Based on the principle of charge induction, this paper adopts a research method combining theoretical analysis and indoor and field tests to carry out a study on the charge induction detection of stress distribution of surrounding rock in coal seam roadways using the self-developed coal rock charge induction monitor. A theoretical analysis of the charge induction intensity in relation to the stress level is carried out. Indoor tests on the law of charge induction for graded loading of large sized coal samples are carried out. Field detection tests of the charge induction law at different drilling depths on the solid coal side and the large coal pillar side of the coal seam roadway are carried out. The results show a positive correlation between the charge signal intensity and the stress magnitude. The induced charge of coal samples has a tendency to increase with the increase in graded loading stress level. The magnitude of the induced charge can reflect the stress level of the coal body. On the solid coal side, the induced charge has a tendency of increasing and then decreasing with the increase in detection depth. The final results are in good agreement with the results of the drill chip method, which better reflects the distribution of the lateral support pressure of the roadway. On the side of the large coal pillar, the induced charge has a tendency to increase, then decrease, and then increase with the increase in probing depth, which is in good agreement with the distribution of lateral support pressure formed in the elastic core area of the large coal pillar. Therefore, the charge induction technology can be used as a fast, non-contact detection means for the partitioning and stress distribution of the roadway enclosure, which can provide guidance for the target prevention and controlling rock burst and for designing roadway support.

Smith machine squats pose high risk to ACL graft integrity after the ACL reconstruction and conventional squats are a safer alternative.

The purpose of this study was to evaluate the impact of squats after the anterior cruciate ligament (ACL) reconstruction on the ACL graft, considering new data on biomechanics, posterior tibial slope (PTS)and anterolateral ligament (ALL). Utilising finite element analysis on the new 14-component knee joint model, we have evaluated stresses on the knee elements separately for the knee with a native double-bundle ACL and with a single-bundle ACL graft for the 5° and 14° PTS variants during both conventional and Smith machine horizontal squats. Replacing a native ACL with a single-bundle graft causes an overstrain on the graft compared to the intact ACL under all conditions. Stresses on the ACL, ACL graftand ALL are much higher during the Smith machine squats compared to the conventional ones. The stress on the menisci is 3.6-4.9 times higher with conventional squats. PTS at the squats' lowest point minimally affects ACL stress but impacts menisci. The single-bundle ACL reconstruction (ACLR) does not reproduce the biomechanics of the native ACL and increases stresses in most knee joint elements, according to the current study. Conventional squats are relatively safe for the ACL graft at their lowest point. Passing the half-squat position is the most dangerous point. Smith machine horizontal squats produce stress on the ACL graft several times higher than its estimated breaking load and dangerous stress levels on the ALL. During the rehabilitation following ACLR, it is advisable to prioritise the conventional squats over Smith machine squats until ligamentisation is complete. Level III.

Durability of concrete subjected to acidic exposure under flexural loading

The durability of concrete under combined acidic exposure and flexural loading was studied. Concrete specimens were made and subjected to up to 168 days of exposure to flexural loading (stress levels of 0, 0.35 and 0.50 of the 28 day flexural strength) and an acidic environment. Concrete properties such as neutralisation depth, mass change, compressive strength and flexural strength were measured. The compositions of different samples were determined by means of X-ray diffraction and chemical analysis and the microstructure of samples was examined using scanning electron microscopy. The results showed that, with an increase in stress level from 0 to 0.50, the performance of the concrete deteriorated gradually and the deterioration in the tension zone was greater than that in the compression zone. The concrete performance improved within 28 days but deteriorated dramatically afterwards. The acidic solution reacted with the cement paste, leading to the formation of harmful gypsum and the decomposition and dissolution of cement hydration products. An increase in stress level increased the porosity of the concrete, accelerating the rate of acid erosion, which led to a deterioration of concrete performance.

Study on low-cycle hysteresis properties of seawater corroded steel considering loading history effects

To investigate the combined effects of wave loading history and time-dependent corrosion damage on the hysteresis mechanical properties of long-term serviced marine engineering materials, this study conducted hysteresis mechanical experiments and a generalized mechanical model research on corroded steel, considering the influence of high-cycle loading history. Steel corrosion was pre-generated by accelerated electrochemical corrosion method. Developing the early-stage high-cycle fatigue loading system and later-stage low-cycle hysteresis loading system based on the dynamic characteristics of wave loads, seismic actions, and typhoon-level pulsating wind loads. Analyze the influence of the independent and combined effects of early-stage load stress levels, cycle times, and corrosion damage on the steel capacity to withstand low-cycle high-stress hysteresis loading. Based on test results and mechanical properties, an improved generalized hysteresis mechanical model was established that encompassed monotonic tension curves, skeleton curves, and well-defined loading and unloading rules. The validity and universality of the model were verified through various loading systems and hysteretic tests of corroded steel. The results indicated that the early-stage high-cycle fatigue history led to a reduction in the material's elastic range, causing the material to transition into plasticity earlier. The combined effects of early-stage loading history and corrosion damage resulted in a significant degradation of the material's hysteresis mechanical properties.

Low cycle fatigue behavior and crack initiation mechanism of Ni-based single crystal curved thin-walled blade simulator specimen with film cooling holes

A novel curved thin-walled blade simulator (CTWBS) specimens was used to investigate low cycle fatigue (LCF) behavior and crack initiation mechanism nearby film cooling holes (FCHs) in Ni-based single crystal superalloy. LCF tests were conducted under two loading stress levels at 700 °C and 1000 °C. Experimental results demonstrated that LCF lifetime reduced as the fatigue load increased at both temperatures. Crack modes were characterized on macroscopic fracture paths, and the analysis of fatigue crack early propagation revealed its microscopic mechanism. The fracture morphology and microstructure evolution also showed temperature dependence, and oxidation played a role at high temperature. The finite element calculation was based on the crystal plasticity theory considering the back stress and slip damage. The resolved shear stress (RSS) distribution law under two temperatures led to the identification of the octahedral slip system activation type respectively. In addition, the resulting fatigue damage increased in direct proportion to the slip systems activated numbers locally surrounding the FCH. The damage distribution around the FCH was in good agreement with the experimental observation. Finally, the temperature dependence of LCF crack initiation mechanism was discussed from three perspectives: crack nucleation around the FCHs, microstructure evolution and dislocation motion near crack.

Numerical and experimental investigation on tensile fatigue performance of reinforced thermoplastic pipes

With the rapid development of offshore drilling for harvesting oil and gas from the deep sea, the fatigue performance of offshore risers is increasingly demanding. This study investigated the tensile fatigue performance of reinforced thermoplastic pipes (RTPs). The theory of three-dimensional (3D) anisotropic elastic mechanics was combined with a strain potential-energy density function to construct the 3D constitutive relation of RTPs. The stiffness degradation law of RTPs was predicted by introducing a modified 3D Hashin failure criterion and the residual stiffness model. A user-defined material (UMAT) subroutine was developed to evaluate the progressive fatigue failure process of RTPs. A uniaxial tensile fatigue experiment of RTPs was designed to validate the numerical model's correctness. The analysis examined the damage failure modes of RTPs under cyclic loading conditions and revealed the stiffness degradation mechanism. Results suggested that the matrix fatigue damage of the reinforced layer of RTPs is the primary cause of the rapid stiffness degradation, which was observed to be faster with increasing cyclic load stress levels.

Fracture properties of basalt-fiber-reinforced bridge concrete under dynamic fatigue loading

Basalt fibers with different lengths and contents were incorporated into concrete to improve the expansion and extension of microfractures in bridge deck concrete. A fracture performance test of basalt-fiber-reinforced bridge concrete (BFRBC) was conducted using a three-point bending loading test. A fatigue test was performed using a self-designed loading device, and load stress levels of 0.5 and 0.7 were used to investigate the deterioration trend of the fracture toughness of the BFRBC under dynamic fatigue loading. The inhibition effect of basalt fiber on concrete cracks under fatigue loading was investigated. The crack resistance mechanism of basalt fiber on concrete under fatigue loading was revealed by scanning electron microscopy using Image-Pro Plus and MATLAB software to extract relevant crack parameters. The results of the fracture performance of BFRBC showed that basalt fiber with an appropriate length and content can greatly improve the fracture performance of concrete, and 12-mm basalt fiber was adopted in concrete with a content of 0.08%. At a stress level of 0.7 after 200,000 fatigue load cycles, the deterioration of concrete without fiber content was the most significant, and the initial fracture toughness and unstable fracture toughness decreased by 22.67% and 29.28%, respectively. In addition, the crack area density, average crack width, and maximum crack length of the BFRBC decreased by 46.69%, 21.94%, and 21.35%, respectively, because of the incorporation of basalt fiber. BFRBC disperses and transmits cracks through the bridging action of fibers in its internal stresses, delaying the generation and propagation of cracks and, thus, improving the crack resistance of concrete.

Mechanical properties of Q345 structural steel after pre-fatigue loading

This study investigates the residual mechanical properties of Q345 structural steel previously damaged by pre-fatigue loading. Fatigue damage is induced by subjecting Q345 structural specimens to various loading cycles without causing fracture under three pre-fatigue loading stress levels, which is referred to herein as pre-fatigue damage. Subsequently, tensile tests are performed on these specimens to obtain the stress–strain curves and associated parameters, including yield strength, ultimate strength, elastic modulus, and ductility. Based on the test results, the effect of pre-fatigue loading on the mechanical properties of Q345 structural steel is comprehensively discussed and analyzed. Significant differences are observed in the fracture positions and morphological features of the Q345 steel structure after pre-fatigue loading. Additionally, pre-fatigue loading significantly affects the residual strength and ductility of Q345 structural steel. In particular, the residual ductility of Q345 structural steel reduced significantly after being subjected to different cyclic loadings. However, the effect of pre-fatigue loading on the elastic modulus of Q345 structural steel is almost negligible.

Mechanical properties of CFRP bar with different elevated-temperature experiences

Material properties of carbon fiber reinforced polymer (CFRP) at elevated temperatures are of great importance to promote its application in civil engineering. Previous literatures focused mainly on steady-state temperature tests, neglecting the thermal experiences that CFRP may undergo in real fire scenarios. To address this gap, it conducted four types of tensile tests on CFRP bars to simulate different elevated-temperature experiences ranging from 25 to 300 °C. This study investigated properties of CFRP under tensile tests at room temperature and steady-state tests, as well as properties at elevated temperatures with sustained load and after exposure to elevated temperatures. The tests showed that CFRP bars without sustained load remain about 85%, 75%, and 52% of their original strength at 100 °C, 150 °C and 300 °C, after 0.5 h thermal experience, respectively. In addition, the ultimate stress decreased by 11% when heating duration increased from 0.5 h to an hour and thereafter remained stable at 300 °C. Meanwhile, the CFRP bars with sustained load (stress level of 0.3) at 300 °C presented a 7% reduction of tensile strength compared to those without sustained load. For the CFRP bars exposed to 300 °C for an hour and cooled naturally to room temperature, the tensile strength and modulus decreased by 17% and 5%, respectively, but remained higher than those tested at 300 °C due to the solidification of softened resin. This investigation provides insights into the behavior at elevated temperature of CFRP.

Experimental study and analysis for the long-term behavior of the steel–concrete composite girder bridge

Due to the influence of the meteorological factors, the long-term behaviors of steel–concrete composite girders of the real bridge is different from that of indoor laboratory models. In order to address this issue, a long-term field test of 4 years was performed on the simply-supported steel–concrete composite girder bridge to study the temperature, shrinkage and creep effects. The temperature field models (TFM) and mechanical behavior models (MBM) were built in sequence and verified by the test results of the concrete deck and steel girders of 2018 and 2019 since the temperature and strain changed greatly during this period. The temperature calculated by the TFM agreed well with the test results. Although the stress of composite girders obtained by the test is different from that of the MBM, the difference in the whole service stage is small. The test results of the steel cross beam, the steel–concrete interface, the wet joint and the concrete continuous layer were also discussed, and the former three were all under compressive state, while the concrete continuous layer is under tensile state in some time period with the maximum tensile stress 3.12 MPa. Then, the verified models were used to separate the temperature strain from the long-term tested strain, and the remaining tested strain is the shrinkage and creep strain. In summer and winter, the shrinkage and creep stress calculated by the age-adjusted effective modulus method (AEMM) was different from the tested values due to the influence of air humidity, initial load age, stress level and inherent error of the prediction model. Much attention should be paid to the influence of the meteorological factors on the long-term behaviors of steel–concrete composite girder bridges.

Long-Term Prestress Loss Calculation Considering the Interaction of Concrete Shrinkage, Concrete Creep, and Stress Relaxation

In order to accurately calculate the long-term prestress losses of prestressed tendons, a time-varying model of long-term prestress loss considering the interaction between concrete shrinkage, creep, and the stress relaxation of prestressed tendons was constructed. Then, a method for calculating the long-term prestress losses of concrete structures was developed. A long-term prestress loss test of a prestressed concrete T-beam in a long-term field test environment was carried out. The measured values of long-term prestress losses are compared with the calculated results of JTG 3362-2018, AASHTO LRFD-2007, and the time-varying law model. The results show that the long-term effective tension of the T-beam decreases gradually with the increase in the load holding time. At the beginning of loading, the tensile force changes rapidly and then gradually slows down. The later the tensile age or the higher the initial loading stress level, the smaller the long-term prestress losses of the prestressed tendons. The long-term prestress loss values calculated by JTG 3362-2018, AASHTO LRFD-2007, and the time-varying law model increase with the increase in the load holding time. In the early stage of loading, the rate of change slows down and tends to be stable. The calculated results of JTG 3362-2018 and AASHTO LRFD-2007 are significantly different from the measured values. However, the calculated results of the time-varying law model are in good agreement with the measured values. The average coefficients of variation of the long-term prestress loss calculated by JTG 3362-2018, AASHTO LRFD-2007, and the time-varying law model are 17%, 10%, and 5%, respectively. The time-varying law model of the long-term prestress losses of prestressed tendons is accurate, and the long-term prestress loss of prestressed reinforcement can be predicted effectively.

Development of Artificial-Neural-Network-Based Permanent Deformation Prediction Model of Unbound Granular Materials Subjected to Moving Wheel Loading.

The majority of existing regression models for unbound granular materials (UGMs) consider only the effects of the number of loading cycles and stress levels on the permanent deformation characteristics and are thus unable to account for the complexity of plastic deformation accumulation behavior influenced by other factors, such as dry density, moisture content and gradation. In this study, research efforts were made to develop artificial-neural-network (ANN)-based prediction models for the permanent deformation of UGMs. A series of laboratory repeated load triaxial tests were conducted on UGM specimens with varying gradations to simulate realistic stress paths exerted by moving wheel loads and study permanent deformation characteristics. On the basis of the laboratory testing database, the ANN prediction models were established. Parametric sensitivity analyses were then performed to evaluate and rank the relative importance of each factor on permanent deformation behavior. The results indicated that the developed ANN prediction model is more accurate and reliable as compared to previously published regression models. The two major factors influencing the magnitude of accumulated plastic deformation of UGMs are the shear stress ratio (SSR) and the number of loading cycles, of which the calculated influence coefficients are 38% and 41%, respectively, while the degree of influence of gradation is twice that of the confining pressure.

Numerical simulation of mesomechanical properties of limestone containing dissolved hole and persistent joint

In order to reveal the complex mechanical properties and mesoscopic failure behavior of limestone rock mass containing both dissolved hole defect and persistent joint, numerical models are developed using the two-dimensional particle flow code (PFC2D) to carry out the uniaxial compressive strength (UCS) tests. The stress–strain, contact force chain, vertical stress field, and crack evolution characteristics of limestone samples containing circular holes and different dip angles persistent joints were analyzed. The results show that the UCS and elastic modulus of the rock samples can be reduced by the circular hole and the persistent joint, and the persistent joints with steep dip angles have a more obvious control effect on the mechanical properties of the rock samples. With the increase in loading stress level, the contact force chain in the hole-joint combination model gradually deflects slightly along the joint dip angle from the left and right sides of the hole and gradually spreads outward. Under uniaxial compression, the initiation cracks in persistent joints and rock blocks are primarily tensile cracks. The initiation positions first appear in the persistent joint, and the initiation positions in the rock block change from the upper and lower vertices of the circular hole to the junction below the joint with the increase of the joint dip angle. The crack initiation stress in the persistent joint is smaller than that in the corresponding rock block, and the control effect of the joint dip angle on the crack initiation stress of the whole rock sample is reduced.

Self-Healing Properties of Asphalt Concrete with Calcium Alginate Capsules Containing Different Healing Agents.

Calcium alginate capsules encapsulating rejuvenator are a promising self-healing technology for asphalt pavement, but the effects of different healing agents on the self-healing performance of asphalt concrete has not been considered. In view of this, this paper aimed at exploring the effects of calcium alginate capsules containing different healing agents on the self-healing properties of asphalt concrete. Three types of capsules with sunflower oil, waste cooking oil and commercial rejuvenator were fabricated via the orifice-coagulation bath method and the interior structure, mechanical strength, thermal stability and oil content of the prepared capsules were characterized. The healing levels of asphalt mixtures with different capsules under different loading cycles and stress levels were evaluated. Furthermore, the saturates, aromatics, resins and asphaltenes (SARA) fractions and rheological property of extracted asphalt binder within test beams with different capsules after different loading conditions were assessed. The results indicated that all the three types of capsules meet the mechanical and thermal requirement of mixing and compaction of asphalt mixtures. The healing levels of test beams containing vegetable oil capsules were higher than that of waste cooking oil capsules and industrial rejuvenator capsules. The strength recovery ratio and fracture energy recovery ratio of test beams with vegetable oil capsules reached 82.8% and 96.6%, respectively, after 20,000 cycles of compressive loading at 1.4 MPa. The fracture energy recovery ratio of the waste cooking oil capsules also reached as high as 90%, indicating that waste cooking oil can be used as the healing agent of calcium alginate capsules to improve the self-healing property of asphalt mixture. This work provides a significant guide for the selection of healing agent for self-healing capsules in the future.

A New Method for Characterizing the Fatigue Performance of High-Modulus Asphalt Mixtures

Abstract The current fatigue model of high-modulus asphalt mixture (HMAM) is usually built without considering the effect of loading frequency, which leads to the imprecision in the selection of fatigue parameters for the design of high-modulus asphalt pavement structures. The purposes of this study are to solve the problem of uncertainty in the characterization of fatigue performance of HMAM at various loading frequencies and to optimize the structural resistance design of high-modulus asphalt pavements. In this study, the four-point bending beam was first used to carry out the strength test on the HMAM at ten loading rates. The effects of different loading frequencies (1, 5, 10, 20, and 40 Hz) and stress levels on the four-point bending fatigue life of HMAM were then explored. The test results show that the fatigue curve derived from the nominal stress ratio deviates significantly from the theoretical strength failure point (1,1) for Nf = 1 by extending toward both ends; however, derived from the rate-dependent stress ratio (considering the effect of loading rate), it passes through the point (1,1), which reveals the correlation between the strength failure and the fatigue damage. In addition, a new method for calculating structural strength coefficients is proposed by developing a unified fatigue model for HMAM under different loading frequencies. The results of this study could predict the fatigue life of HMAM at different loading frequencies more precisely.