Strength Degradation Model Research Articles

Damage and failure characteristics of single fractured cyan sandstone subjected to freeze–thaw cycles under uniaxial compression

In order to explore the freeze–thaw (FT) damage characteristics of fractured rock masses, FT cycle tests and uniaxial compression tests were conducted on intact and pre-cracked cyan sandstone samples, respectively. The degradation laws of physical and mechanical properties of samples under FT cycle conditions were analyzed, and the FT damage mechanism and failure mechanism were explored accordingly. The results indicate that as the number of FT cycles (N) increases, the saturation weight change rate and porosity of the sample continue to increase, while the P-wave velocity continues to decrease. The closure stress, initiation stress, damage stress, and peak stress decrease linearly with increasing FT cycles, and increase linearly with increasing crack inclination angle (β). Moreover, as FT cycles increase, the geological strength index GSI decreases linearly, while the damage factor (D) increases linearly. A Hoek Brown strength degradation model for fractured rocks subjected to FT damage under uniaxial compression was obtained, and its high reliability was verified using test results. The FT damage of cyan sandstone is mainly dominated by the frost heave force inside the rock, and the degree of FT damage of fractured samples is much higher than that of intact ones. The concentration of frost heave force at the crack tip leading to cracking is the key to exacerbating the FT damage in fractured rock masses. However, the crack initiation mechanism is still controlled by β. Whenβ = 90°, it exhibits tensile initiation mode, and whenβ = 45°, it mainly exhibits shear initiation mode. Moreover, the fracture of the single crack sample conforms to the compression-shear fracture criterion. Taking the sample with pre-crack inclination angle of 45° as an example, the variation of KI and KII, and the relationship between them are discussed accordingly. The results of this paper are beneficial to clarify the influence of FT cycles on the mechanical properties of cyan sandstone, and can provide reference for the study of related issues.

Experimental study on the bond-slip behaviour between corroded bars and concrete

Pull-out tests were conducted to investigate the effects of corrosion of the both longitudinal bar on the bond-slip behaviour of reinforced concrete specimens. The main experimental variables include concrete strength (26.7 MPa, 37.7 MPa, 45.2 MPa) and expected corrosion rate (0%, 4%, 8%, 12%), with a total of 36 specimens fabricated. The results show that the relative bonding strength of specimens under different concrete strengths gradually decreases with increasing corrosion rate, but the higher the concrete strength is, the faster its degradation rate. Based on the experimental results of this work and the achievements of other scholars, a modified relative bonding strength degradation model is presented by accounting for the influence coefficient of concrete strength.

Analysis of static stiffness properties of column-architrave structures of ancient buildings under long term load-natural aging coupling

The column-architrave structures were widely used in the Yingxian wooden pagoda and pagoda has been in service for many years. Over time, it has been subjected to stress and the effects of natural aging. In order to analysis the ability of the structures to resist deformation under specific static excitation, a study of the static stiffness properties of column-architrave structures in ancient buildings considering long term load-natural aging coupling was carried out. Based on the Gerhards model, a strength degradation model has been developed for column elements and architrave elements, taking into account long-term loading and natural aging effects; By constructing a regular model and a realistic model, the deformation of the regular model under long term self weight load and the deformation of the realistic model under long term self weight load and aging are simulated by finite element model; The relationship between deformation values and static stiffness as a function of time is investigated through multiple points of the column-architrave structures, and the reliability of the proposed model is verified by extracting and comparing data from the automated real-time 3D monitoring system of the wooden pagoda. The results show that the numerical simulations of both models show that the middle of the architrave is more easily deformed than the mortise and tenon joint structures; The deformation resistance of the realistic model is lower than the deformation resistance of the regular model, and the natural aging effect has a non-negligible deterioration effect on the deformation of the column-architrave structures; For the column and architrave elements, they are likely to become unstable at 3330 AD and 3543 AD respectively, which means that after 1307 and 1520 years, the static stiffness of both elements tends to decrease with time in service, with the decrease in static stiffness of the architrave elements being less than that of the column elements.

Influence of Corrosion on the Bond–Slip Behaviour between Corroded Bars and Concrete

Pull-out tests were conducted to investigate the effects of corrosion of both the longitudinal bars and stirrups on the bond slip behaviour of reinforced concrete specimens. The main experimental variables include concrete strength (26.7 MPa, 37.7 MPa and 45.2 MPa) and expected corrosion loss (0%, 4%, 8% and 12%), with a total of 63 specimens fabricated. The results show that the relative bonding strength of specimens under different concrete strengths gradually decreases with increasing corrosion loss, but the higher the concrete strength is, the faster its degradation rate. The influence of stirrup corrosion on the peak slip can be ignored, but it will further aggravate the degradation of the bonding strength of the specimens. This reduction in bonding strength is linearly related to the stirrup corrosion loss. Based on the experimental results of this work and the achievements of other scholars, a modified relative bonding strength degradation model and a bond–slipbond–slip constitutive model of corroded reinforced concrete are presented by accounting for the influence coefficient of concrete strength. The results show that the constitutive model is in good agreement with the relevant experimental results.

An exponential strength degradation model for clay: From mathematical model to numerical application

Investigation in strain-softening behavior of clay is of great significance for offshore geotechnical engineering. Although several models could be used to characterize the degradation of undrained shear strength of clay with cumulative shear strain, it is necessary to improve accuracy of them. In this study, based on full-flow cyclic penetration test, an exponential strength degradation model with one parameter α is proposed, where α is the function of the fully remolded resistance factor and the ratio of initial extraction to initial penetration resistance. The reliability of the proposed model is verified by comparing indices (strength and sensitivity) calculated by it and by other existing strain-softening models. In addition, a large deformation finite element (LDFE) analysis based on the proposed model is conducted for pipe-clay interaction, and efficiency of the model is verified by comparing numerical results with centrifuge test results. The influence of α on the normalized penetration resistance were discussed, and demonstrate the necessity of accurately evaluating degradation of undrained shear strength of clay.

Tensile strength degradations of mineral grain interfaces (MGIs) of granite after thermo-hydro-mechanical (THM) treatment

Granite for hosting the deep geological repository (DGR) is subjected to thermo-hydro-mechanical (THM) treatment for decades of years, while the variations in the tensile strengths of mineral grain interfaces (MGIs) with THM treatment are still unknown. Served as an important parameter in determining the macro mechanical properties of granite, quantifying the tensile strength degradation of MGI with THM treatment is crucial for ensuring the long-term safety of DGR. In this study, the varied tensile strengths of MGIs in the granite after different THM treatments were first directly measured based on a self-developed single-MGI mechanical test system. Three-bending tests were conducted on six-hundred MGIs, which were submitted to various THM treatments with a temperature of 25–175 °C and a pore pressure of 0–15 MPa. The tensile strengths of the interfaces of quartz, feldspar and the interfaces between quartz and feldspar were derived based on the Dugdale–Barenblatt (D–B) model. Results show that the average tensile strength of the interfaces of quartz is 1.06 times larger than that of interfaces between quartz and feldspar, and 1.13 times larger than that of the interfaces of feldspar. Besides, the tensile strength of MGIs seems more susceptible to the pore pressure than to the temperature, and the degradation of tensile strength is the most obvious in the interfaces between quartz and feldspar, followed by the interfaces of feldspar and finally the interfaces of quartz after THM treatments. Based on the characteristics of the tensile strength distributions of different MGIs after THM treatments, the tensile strength degradation models of MGIs related to THM treatments were proposed in meso scale.

Residual Strength Modeling and Reliability Analysis of Wind Turbine Gear under Different Random Loadings

A novel method is proposed to investigate the pattern of variation in the residual strength and reliability of wind turbine gear. First, the interaction between loads and the effect of the loading sequence is considered based on the fatigue damage accumulation theory, and a residual strength degradation model with few parameters is established. Experimental data from two materials are used to verify the predictive performance of the proposed model. Secondly, the modeling and simulation of the wind turbine gear is conducted to analyze the types of fatigue failures and obtain their fatigue life curves. Due to the randomness of the load on the gear, the rain flow counting method and the Goodman method are employed. Thirdly, considering the seasonal variation of load, the decreasing trend of gear fatigue strength under multistage random load is calculated. Finally, the dynamic failure rate and reliability of gear fatigue failure under multistage random loads are analyzed. The results demonstrate that the randomness of residual strength increases with increasing service time. The seasonality of load causes fluctuations in the reliability of gear, providing a new idea for evaluating the reliability of the wind turbine gear.

Beyond time: Enhancing corrosion resistance of geopolymer concrete and BFRP bars in seawater

To improve the durability of Basalt fiber reinforced polymer (BFRP) bars reinforced geopolymer concrete (GPC), it is important to study the time-dependent variation of the corrosion resistance ability of GPC and BFRP in a seawater environment. This paper presents an experimental investigation to study the time-dependent mechanical properties and durability of BFRP bars and geopolymer materials synthesized by granulated blast furnace slag (GGBFS), fly ash, and silica fume. The resulting GPC and Portland cement (PC) concrete were exposed to artificial seawater. The mechanical properties of GPC were evaluated by analyzing and comparing the volume expansion and strength loss rates of GPC and PC concrete in an artificial seawater environment. The corrosion resistance of geopolymer (GP) mortar and PC mortar was evaluated by studying the migration ability and pore structure in corrosive ions attack (Cl−, SO42−, Mg2+) in artificial seawater. Moreover, the time-dependent tensile strength of BFRP was comparatively investigated by immersing in different solutions (tap water, artificial seawater, and alkaline simulated seawater). In addition, the dual interface transition zones (ITZs) characteristics of BFRP reinforced GPC under artificial seawater were also investigated by SEM and BSE tests. The results showed that the volume expansion rate and strength loss rate of GPC decreased by 77.6% and 8.7%, respectively, after 360 days of seawater corrosion compared with PC concrete. This enabled the development of a time-dependent strength model of GPC in marine environments. The coefficient of ions diffusion in GP mortar is much lower than that of PC mortar, and GP mortar shows excellent resistance to ion migration. In addition, the effect of seawater corrosion on the tensile strength of BFRP bars increases with the increase of bars' diameter, and the ultimate strengths of BFRP bars with diameters of 6 mm and 8 mm were 695 MPa and 663 MPa, respectively. The tensile strength degradation model of BFRP bars in geopolymer concrete under seawater corrosion was established. After 360 days of seawater immersion, the average porosity of the ITZ between geopolymer and aggregates, and the average porosity of the ITZ between geopolymer and BFRP bars increased insignificantly compared to that of PC concrete. This research can provide a theoretical basis for the service life prediction of BFRP reinforced geopolymer concrete within marine environments.

Effect of pre-fatigue damage on the residual strength of 2198-T8 aluminum-lithium alloy sheet with holes

The 2198-T8 aluminum-lithium alloy is the advanced aircraft aluminum-lithium alloy used for airframe construction. During the service of the aircraft, the rivet joints of the fuselage skin are vulnerable to fatigue damage, which is accompanied by the formation of fatigue cracks near the rivet holes. The influences of pre-fatigue damage on the residual strength of 2198-T8 aluminum-lithium alloy with rivet holes were studied by combining the digital image correlation (DIC) technique and the micro and macro fracture morphology. The results show that with the increase of the pre-fatigue loading ratio, the residual strength and residual strain of 2198-T8 aluminum-lithium alloy gradually decrease, and when the pre-fatigue loading ratio is greater than 90 %, the attenuation trend is sharply accelerated. The relationship between residual strength and residual strain satisfies the residual strength degradation model, and the formula has a goodness of fit of 0.9878. The fatigue cracks at the hole edge propagate in both vertical and horizontal directions, and the propagation rate increases with the increase of the pre-fatigue loading ratio, and the fatigue striations gradually widen.

Hybrid physics-BP neural network-based strength degradation model of corroded reinforcements under the simulated colliery environment

This study proposes a hybrid physics-artificial neural network (ANN)-based framework for developing a physics-based strength degradation model of corroded reinforcements taken from deteriorating reinforced concrete (RC) beams respective under a single harsh colliery environment and coupled action of corrosive environment and sustained load. This proposed degradation model can well account for the impacts of random variables (e.g., the diameter of reinforcing steel bars) on the strength degradation behavior of corroded reinforcements. The main contributions of this study include the following aspects: Firstly, based on the experimental results, a BP-ANN surrogate model is established to substitute the experiment method for generating the residual yield and ultimate strengths of corroded reinforcements under different exposure conditions. In addition, a new data sample is formed with the experimental and simulated data obtained by this trained surrogate model. Applying the Bayes theorem, the physics-based strength degradation model respecting to corrosion degree and diameter of corroding reinforcements is constructed. The posterior probability distributions of unknown model parameters are developed, which is beneficial for representing the epistemic uncertainty in assessing the service reliability of deteriorating RC components and structures. Finally, a sample from experimental data is randomly selected to verify the accuracy of this physics-based strength degradation model. Analytical results indicate that the diameter of reinforcements as well as the coupled action of corrosive simulated colliery environment and sustained load have considerable influences on the strength degradation of corroded reinforcing steels. This study provides an effective approach to developing a physics-based strength degradation model of corroding reinforcements and highly improves the use efficiency of experimental results.

Modified nonlinear cumulative damage model considering residual strength degradation and gear reliability analysis

AbstractThe fatigue damage of gear is caused by the coexistence and mutual influence of residual strength degradation and cumulative damage. The residual strength decreases continuously with the change of the loads and the increase of stress cycles. Most of the traditional cumulative damage models ignore the effect of residual strength degradation on cumulative damage. The exponential parameter in the traditional Corten‐Dolan model was defined as exponentials function related to the strength degradation coefficient, and the strength degradation coefficient was deduced by analyzing the residual strength degradation model, and the modified Corten‐Dolan model considering the residual strength degradation was established. Based on the stress‐strength interference model, a gear reliability model considering residual strength degradation was established. The effectiveness of the strength degradation model and the accuracy of the modified Corten‐Dolan model are verified by the analysis of the test data, which provides a new idea for the reliability analysis of gear. The accuracy of the gear reliability model proposed in this paper was verified by Monte‐Carlo (M‐C) simulation, which has practical significance for engineering application.

A strength degradation model of PBL shear connectors under fatigue load

Perfobond rib shear connectors (PBL shear connectors) have been widely used in steel and reinforced concrete composite bridge. To investigate the degeneration law of the bearing capacity of PBL shear connectors after a number of fatigue loading times, both static test on one specimen and fatigue test on six specimens were performed. The failure mode, ultimate bearing capacity and load-slippage curves were emphatically focused in static test, while residual performances, including the residual strength, residual slip and shear stiffness of PBL shear connectors after a certain number of fatigue loading were also studied. Furthermore, influence of number of cycles on the residual strength and residual slip of PBL shear connector specimen within the lifetime was analyzed. Test results showed that residual strength of PBL connector decreased nonlinearly with increasing loading times, and residual slip development showed a three phrase trend of “fast, slow and then fast”, but the shear stiffness was basically constant during whole loading process. Based on the residual strength theory and elastic foundation theory, residual static bearing capacity of PBL shear connectors after a number of fatigue loading times was proposed. By comparison, it was found that the residual shear bearing capacity determined by proposed model agreed well with the experimental results, which would have a great significance to guarantee the safety of a structure and prevent secondary damages to a composite bridge during the service period.

Tensile Behavior of Basalt-Fiber-Grid-Reinforced Mortar before and after Exposure to Elevated Temperature

This paper presents an experimental study on the tensile behavior of basalt-fiber-grid-reinforced mortar (BGRM) before and after exposure to an elevated temperature of 300 °C considering the effect of fiber grid type, short polyvinyl alcohol fiber (PVA), and high-temperature exposure time. The experimental results show that the mortar plates reinforced with woven textile T25 and fiber-reinforced polymer (FRP) grid G50 exhibited more pronounced strain-hardening behavior. The highest peak stress was obtained for the T25-reinforced plate, which was 85% and 32% higher than that of the T5- and G50-reinforced plates, respectively. Meanwhile, the bridging effect of PVA fibers in mortar can improve the tensile properties. As the high-temperature exposure time increased, the cracking and peak stress of BGRM decreased significantly. Especially for the T5-reinforced plate after exposure to elevated temperature for 2 h, the cracking and peak stress decreased by 60.5% and 38%, respectively. The positive effect of short PVA fibers on the tensile properties of the BGRM became obsolete owing to the melting of short fibers at high temperature. Furthermore, an exponential strength degradation model related to high-temperature exposure time was proposed.

Physics-Based Shear-Strength Degradation Model of Stud Connector with the Fatigue Cumulative Damage

In the whole lifetime of structures, fatigue damage accumulation will exist in the shear connector of steel–concrete composite beams. It is essential to determine the residual mechanical properties of shear connectors under long-term fatigue loads, e.g., the vehicle load on bridges. In this regard, a shear-strength degradation model is proposed for shear connectors. The Bayes theorem is used to develop posterior estimates of the unknown parameters in the degradation model based on the collected pushout test data of pre-damaged stud connectors caused by high-cycle fatigue loads. In addition, according to the proposed shear-strength degradation model, the service reliability assessment is performed with a composite bridge beam. The results indicate that (1) There is a large diversion in the traditional strength degradation model under the action of fatigue cumulative damage. More importantly, this proposed physics-based degradation model can effectively reduce uncertainty. (2) The effects of steel type and test specimen size can be well considered in the proposed shear-strength degradation model, which is beneficial for improving the reliability of risk assessment for fatigued bridges.

Effect of freeze–thaw/dry–wet environment on mechanical properties of epoxy resin adhesive

Epoxy resin adhesive (ERA) is an important bond material for carbon fiber reinforced polymer (CFRP) strengthened bridge structures, and its mechanical properties often degrade when used in harsh environments. Fifty-two ERA specimens were prepared and tested to investigate the influences of freeze–thaw (F–T), chlorine dry–wet (D–W), and F–T/D–W on mechanic properties of the ERA. Static tensile tests were conducted after the ERAs were exposed to these environments for 30, 60, 90, and 120 cycles. The experimental results showed that failure of all specimens demonstrated characteristics of brittle failure. Compared with the control group, the tensile strength increased around 40.7, 22.8, and 11.3%, and the elastic modulus increased around 39.2, 34.6, and 38.3% after 120 cycles of the F–T, D–W, and F–T/D–W exposure, respectively. Meanwhile, the stress–strain relationship of the environmental specimens exhibited nonlinear characteristics. However, the stress-strain relationship of the control group without any environmental treatment showed linear. By contrast, the deformation capacity of the environmental specimens was weaker than those of the control group, except for the specimens under the 120 D–W cycles. In addition, variation for the strain energy and tensile strength retention rate of the environmental specimens demonstrated similar trends with the tensile strength, which increased first and then decreased from 30, 60, 90, to 120 cycles. Finally, a strength degradation model of the ERA was proposed to predict the tensile strength under the F–T, D–W, and F–T/D–W exposure.

Sensitivity Analysis Contact Reliability of VH-CATT Cylindrical Gear and Its Reliability with Material Strength Degradation

VH-CATT cylindrical gear is a new transmission mechanism. In order to study the influence of design parameters and material strength degradation of the VH-CATT cylindrical gear on its transmission reliability. The Kriging surrogate model based on improved quantum particle swarm optimization (QPSO) algorithm was used to establish the limit state equation of contact stress for the VH-CATT cylindrical gear based on the stress-strength interference theory. The state equation of contact stress of the VH-CATT cylindrical gear was obtained with higher precision. Also, the strength degradation model with gamma distribution was established by using the P-S-N curve to estimate the parameters of the strength degradation model with gamma distribution. The reliability sensitivity of the VH-CATT cylindrical gear was analyzed by improved first order second moment method (AFOSM) based on the limit state equation of contact stress of the VH-CATT cylindrical gear, the reliability of gear considering strength degradation was studied by using the improved first order second moment method (AFOSM). The analysis results show that the failure rate decreases with the increase of pressure Angle, tooth width, modulus and tooth radius. The failure rate increases with increasing torque. Strength degradation has a certain influence on the reliability of gear and opinion. And the reliability of the gear is lower than that of the opinion, Strength degradation has a greater influence on the failure probability of gear, at the same time, the reliability of the gear pair decreases with the strength degradation. After 15000 hours of operation, the reliability of the gear pair is only 0.63155.

Structural assessment of underground utility services pit using Bayesian inference

ABSTRACT Ageing infrastructure is becoming an increasing challenge as a result of deterioration and greater loading demands. Modern cities were built on top of complex underground infrastructure networks many of which are still in-service beyond their design life. The safety assessment of underground structures is of utmost importance to avoid catastrophic failures and develop cost-effective renewal and rehabilitation strategies. However, the lack of design documentation and absence of data on the level of structural deterioration make determination of current structural capacity a challenge. This paper presents a probabilistic based assessment framework for underground utility service pits using Bayesian updating technique, which is used to refine the probabilistic distribution of material properties from the prior distribution constructed using published data. A case study of an underground pit located in Central Melbourne is provided. Extensive experimental testing was conducted to characterise the material properties and a full-scale masonry wall was tested to understand the failure mode due to earth pressure and traffic load. The test data was used in strength prediction models to achieve a more accurate estimate for wall capacity. Further, the strength degradation models were integrated to develop the time-dependent material models, which were eventually used to compute reliability index.

Shear Strength Degradation Model for Performance-Based Design of Short Coupling Beams

Shear Strength Degradation Model for Performance-Based Design of Short Coupling Beams

A model of strength degradation for glass fiber reinforced polymer composite laminates under fatigue loading

In order to overcome the shortcoming of traditional strength modeling of glass fiber reinforced polymer (GFRP) composite laminates that require a large amount of strength test data, a strength degradation model on the account of residual stiffness degradation data is proposed. First, fatigue damage growth can be described by the gradual reduction of the stiffness and strength, and damage expressed by the two degradation methods are equivalent. Second, according this assumption, the connection between the two damage indices is established, and then a four parameters strength degradation model is obtained. Finally, the proposed model is validated by the applying experimental data of GFRP laminates, and the precision of proposed model is contrasted with other four models. Verification results indicate that if the residual stiffness degradation data is known, the residual strength degradation law can be predicted by a small number of residual strength tests and the presented model has better applicability and higher fitting accuracy.

Hybrid Bayesian-Bp Neural Network-Based Strength Degradation Model of Corroded Reinforcements Under the Colliery Environment

Hybrid Bayesian-Bp Neural Network-Based Strength Degradation Model of Corroded Reinforcements Under the Colliery Environment