Repeated Loading Conditions Research Articles

Aluminum boron carbide metal matrix composites for fatigue-based applications: A comprehensive review

Metal matrix composites (MMCs), known for their low density and superior stiffness, have gained significant attention in various load bearing and fatigue-prone applications. These composites offer distinct advantages over traditional monolithic metals, including enhanced mechanical and tribological properties, making them prime candidates for lightweight structural applications. Among ceramic reinforcements, boron carbide stands out due to its lower density, high elastic modulus, excellent refractoriness, and superior hardness, positioning it as an ideal reinforcement material. This review delves into the processing techniques, material properties, and microstructural characteristics of boron carbide-reinforced MMCs, with a focus on aluminum LM6 alloy as the matrix. Additionally, the paper explores the latest advancements and emerging opportunities for employing these composites in fatigue-critical applications, highlighting their potential to revolutionize industries requiring high performance and durability under repeated loading conditions.

Study of the fatigue fractography of dual phase low carbon steel used in automotive industry

Fatigue properties play a crucial role as they are vital to ensuring the durability and integrity of components subjected to repeated loading conditions over long periods. The main objective of this work is to investigate the fatigue behavior of dual-phase low-carbon steels used in automotive applications using a rotating bending fatigue machine. Heat treatments were carried out to analyze the microstructure's effect on the fatigue properties, including quenching low-carbon steel samples at 800 °C and 900 °C. Hardness and tensile tests were performed, and the microstructure was inspected to examine the constitute phases. With the assistance of a scanning electron microscope, fractographic analyses were carried out to reveal the fracture features of the samples at different lifetime ranges. The results show that various failure mechanisms occur depending on the stress levels. Additionally, the specimens quenched at 900 °C exhibited higher fatigue strength.

Cyclic response and stability of shape memory alloy cables considering training, displacement history, and prestrain effects

Superelastic shape memory alloy (SMA) cables have been considered in the development of various seismic protection technologies. The cyclic nature of seismic loads necessitates a comprehensive understanding of the mechanical behavior of SMA cables under repeated loading conditions. As SMAs undergo repeated phase transformation cycles, alterations in their properties occur due to defects generated and modified during the transformation. When utilizing superelasticity, this response can be stabilized after numerous transformation cycles. In various applications, components made of SMAs undergo a stabilization process referred to as training to enhance the predictability of the alloy’s response. Despite the widespread acceptance of training cycles to achieve stable cyclic response, a well-defined protocol is lacking. This study investigates the effects of various factors, including training strain amplitude, repeated loading, low-cycle fatigue loading, loading at high strain amplitudes, displacement loading history, and prestrain on the cyclic response of a large-diameter SMA cable. A total of 14 SMA cable specimens are tested under various tensile loading conditions. The experimental results, including stress-strain curves and various response parameters such as peak stress, residual strains, energy dissipation capacity, and equivalent viscous damping are examined in detail. The results indicate that training SMA cables at a strain amplitude near the end of phase transformation plateau leads to a highly stable cyclic response. However, it is worth noting that, in comparison to untrained cables, training induces a softening effect in the cable response.

Comparative experimental study of biomechanical features of suture materials in tendon repair

Introduction Many different suture configurations and pathomorphology of tendon repair have been described for tendon repair over the past 20 years. However, the biomechanical properties of suture material at primary flexor tendon repair have not been sufficiently explored. A cyclic loading test is performed to evaluate the performance of the different sutures under repeated loading conditions simulating dynamic conditions in postoperative rehabilitation procedures.The objective was to compare the strength of suture materials under cyclic loading on a biological model of a tendon.Material and methods Eighty porcine digital flexor tendons were examined in a pilot study. The sutured tendons were tested with a universal testing machine. Tendon repair was produced using polypropylene in group I, braided polyamide suture in group II, complex polytetrafluoroethylene thread in group III and a thread of superelastic titanium nickelide in group IV. The standard Chang protocol was used for cyclic loading.Results The percentage of intact sutures was 25 % in group I and in group II, 80 % in group III and 85 % in group IV after completing the entire load cycle. A pairwise comparison showed suture disruption being more common for group I and group II as compared to group III and group IV. Irreversible gap was more common for group 1 as compared to group IV. Neither knot ruptures nor tissue cutting were seen in the groups.Discussion The topic of biomechanical properties of suture material remains poorly understood. Although static load testing is commonly used in current experimental studies and cyclic testing is suitable for simulating postoperative conditions. The search continues for the “ideal” suture material for flexor tendon repair to prevent tears and retain tensile properties until the repair reaches strength.Conclusion The threads of polytetrafluoroethylene and nickelide-titanium showed the best biomechanical properties for tendon repair in the form of linear strength, good elasticity and low plasticity of the suture material. There were no significant differences between polypropylene and braided polyamide threads.

Experimental Study on the Mechanical Properties of Reinforced Pervious Concrete

Pervious concrete (PC) has gained popularity as an environmentally friendly solution for mitigating the urban heat island effect and promoting sustainable construction. However, its lower compressive strength, attributed to its higher porosity required for permeability, poses challenges for withstanding heavy vehicle loads on pavements. Our study aims to improve the flexural strength of regular PC by adding advanced reinforcing materials like steel wire mesh or glass fiber mesh. This results in reinforced pervious concrete, referred to as RPC, which offers enhanced strength and durability. The primary objective of our research is to investigate the mechanical behavior of RPC, with a specific emphasis on essential design parameters such as PC elastic modulus, modulus of rupture, and stress–strain characteristics under both single and repeated loading conditions. Our findings reveal that the influence of repeated loading on the compressive strength and elastic modulus of PC pavement is negligible, as there are no significant differences observed between the two loading protocols. Notably, our statistical analysis indicates that the PC strength (fc′) averages around 15 MPa. Moreover, empirical formulas for the elastic modulus (Ec = 3072fc′) and modulus of rupture (fr = 0.86fc′) are derived from our research. Furthermore, our study establishes that the stress–strain behavior of PC closely aligns with the general concrete model proposed by a previous scholar, providing valuable insights into the material’s structural performance. These findings contribute to a better understanding of RPC’s mechanical properties and offer potential solutions for improving its suitability for heavier vehicular loads.

Quantifying the Residual Stiffness of Concrete Beams with Polymeric Reinforcement under Repeated Loads.

Current technology development ensures a variety of advanced materials and options for reinforcing concrete structures. However, the absence of a uniform testing methodology complicates the quantification and comparative analysis of the mechanical performance of the composite systems. The repeated mechanical loads further complicate the issue. This research extends the recently developed residual stiffness assessment concept to the repeated loading case. It provides an engineer with a simplified testing layout and analytical model to quantify the residual flexural stiffness of standardized laboratory specimens subjected to repeated cycling loads. This model explicitly relates the particular moment and curvature values, requiring neither iterative calculations nor the load history. Thus, this feature allows residual stiffness quantification under repeated loading conditions, including complete reloading of the beam samples imitating the structural strengthening procedure; the proposed technique is equally efficient in quantifying the residual stiffness of the beam samples with any combinations of fiber-reinforced polymer (FRP) reinforcements, i.e., embedded bars, near-surface-mounted strips, and externally bonded sheets. This study employs 12 flexural elements with various reinforcement and loading layouts to illustrate the proposed methodology's efficiency in quantifying the residual strength of the tension concrete, which estimates the efficiency of the reinforcement system. The explicit quantifying of the residual resistance of the FRP reinforcement systems under repeated load cycles describes the essential novelty of this work.

Sensitivity enhancement through geometry modification of 3D printed conductive PLA-based strain sensors

PurposeThe purpose of this study is to increasing the gauge factor, reducing the hysteresis error and improving the stability over cyclic deformations of a conductive polylactic acid (CPLA)-based 3D-printed strain sensor by modifying the sensing element geometry.Design/methodology/approachFive different configurations, namely, linear, serpentine, square, triangular and trapezoidal, of CPLA sensing elements are printed on the thermoplastic polyurethane substrate material individually. The resistance change ratio of the printed sensors, when loaded to a predefined percentage of the maximum strain values over multiple cycles, is recorded. Finally, the thickness of substrate and CPLA and the included angle of the triangular strain sensor are evaluated for their influences on the sensitivity.FindingsThe triangular configuration yields the least hysteresis error with high accuracy over repeated loading conditions, because of its uniform stress distribution, whereas the conventional linear configuration produces the maximum sensitivity with low accuracy. The thickness of the substrate and sensing element has more influence over the included angle, in enhancing the sensitivity of the triangular configuration. The sensitivity of the triangular configuration exceeds the linear configuration when printed at ideal sensor dimensional values.Research limitations/implicationsThe 3D printing parameters are kept constant for all the configurations; rather it can be varied for improving the performance of the sensor. Furthermore, the influences of stretching rate and nozzle temperature of the sensing material are not considered in this work.Originality/valueThe sensitivity and accuracy of CPLA-based strain sensor are evaluated for modification in its geometry, and the performance metrics are enhanced using the regression modelling.

Numerical and experimental investigation of longitudinal rail creep at turnouts on steep ramps under repeated loads considering realistic braking loads of vehicles

Turnouts on steep ramps are subject to both temperature loads and frequent vehicle braking loads, which may lead to an accumulation of excessive residual longitudinal rail displacement, thereby leading to turnout service failure and threatening vehicle operation safety. In this paper, the following research work was carried out to study the longitudinal rail creep at turnouts on steep ramps: Experiments were conducted to investigate the longitudinal resistance characteristics of fasteners under repeated loading, and based on the experiment results, an MBS-FE simulation model was established for the longitudinal rail creep at turnouts on steep ramps, with real vehicle braking loads and fastener resistance degradation taken into consideration in the model. The change law for longitudinal displacement of turnout rails caused by temperature load and frequent vehicle braking load was studied based on the model. A full-scale experiment was then conducted on the longitudinal rail creep at turnouts on steep ramps under repeated loading conditions in order to verify the results of the simulation. A comprehensive and in-depth study of the longitudinal rail creep at turnouts on steep ramps impacted by repeated loading was conducted through the combination of theoretical analysis and experiment. The following conclusions were drawn: The longitudinal resistance of fasteners displays typical nonlinear characteristics and degrades under repeated loading, these characteristics lead to an obvious accumulation of residual longitudinal displacement of turnout rails on steep ramps under repeated loading, and the increment of residual longitudinal displacement of turnout rails reaches a maximum after the first loading–unloading cycle, and with the increase of the number of load action, the accumulated longitudinal residual displacement of turnout rails increases rapidly in the early stage, and then the growth gradually slows down and eventually stabilizes. The simulation results were found to be very similar to the full-scale experiment results, verifying the accuracy of the analytical model established in this paper.

Amelioration of flexible pavement subbases using fly ash and natural fibres

The use of fly ash as a stabilizing element has already been demonstrated to be successful. Moreover, the CBR values of fly ash do not meet IRC criteria. This study intends to analyze the addition of coir fibre, a natural and environmentally sustainable resource, as a reinforcing material. Fly ash and coconut coir are waste materials that can be locally sourced, making them cost-effective and environmentally friendly materials. The article discusses the benefits of fly ash and coconut coir in flexible pavement subbases, highlighting their potential as a sustainable solution for road construction and maintenance. The laboratory experiments conducted on fly ash subbases reinforced with various percentages of coconut coir ranging from 0.1, 0.2, and 0.3 prove that the addition of natural fibres can enhance the subbase's strength properties, with a maximum improvement observed at a 0.2% reinforcement level. Further, the cyclic plate load tests conducted on the treated fly ash subbase at optimum conditions provide valuable insights into the subbase's performance under repeated loading conditions. The results suggest that the natural fibres performed satisfactorily in improving the subbase's load-carrying capacity and reducing rebound deformations, indicating that this approach can be a viable alternative to conventional pavement subbase materials.

Evaluating Flexural Performance of Repaired Reinforced Concrete Beams under Static and Repeated Loading Using Non-Destructive Methods.

Deterioration of concrete structures is one of the major issues faced by the construction industry. Repair and rehabilitation are necessary to extend the service life of such structures. This study aims to investigate the effect of repair material type, length of repaired region, and loading regime on the structural characteristics of the repaired reinforced concrete (RC) beams. To achieve this goal, a total of 30 repaired and non-repaired RC beams were prepared and tested under static and repeated loading conditions. Three types of sounding-based non-destructive test (NDT) methods are employed to determine the material deterioration and sub-surface delamination after repeated loading. Results showed that under static loading conditions, full-length repaired beams had better performance than 1/3-span repaired beams. Beams repaired with cementitious repair mortar containing modified binder and cementitious repair concrete in full length had a yield strength that was 14% and 9%, respectively, higher than that of beams repaired in 1/3 span. All RC beams with full-span repair outperformed the intact beams. After repeated loading, beams repaired with cementitious repair with modified binder over full length showed a 14% improvement in yield strength compared to control samples. It was found that repair materials that had a high compressive and flexural strength are beneficial. The resonant frequency drops correlate well with the yield strength results. The formulas proposed by Canadian Standards Association (CSA) 23.3 can effectively predict the moment resistance of both intact (control) and repaired RC beams. The ratio of experimental moment resistance values to its predictions ranges from 0.91 to 1.04.

Theoretical comparison of the thermo-mechanical fatigue characteristics of a tension rod and coil used as dewar supports

Support systems for dewars serve the dual purpose of holding the vessels together and providing a thermal insulation between the inner and the outer vessels. To decrease the heat inleak, the thermal length of the supports may be increased. The length of the support should be increased such that the support fits in the annular space between the vessels and gives the required mechanical strength. Coiled supports satisfy both the criterion. The supports experience thermal and mechanical fatigue due to the repeated filling and utilization of the cargo. This cyclic loading condition can cause mechanical fatigue failure and ductile to brittle transition of the material. While designing cryogenic dewars for long life, it is pertinent to ensure the longevity of the individual components. The life of a support depends on the number of cycles needed to cause the failure of the support. A model has been developed to predict the fatigue behaviour of support systems. Repeated loading condition was imposed to determine the life. The present work compares the fatigue characteristics of a coiled support with that of a tension rod under the same operating conditions.

Linear viscoelastic response of semi-circular asphalt sample based on digital image correlation and XFEM

Asphalt mixtures behave as linear viscoelastic materials when subjected to loading conditions that are similar to pavement operating conditions. Thus, this study proposed a novel method for extracting the linear viscoelastic properties of asphalt mixture at 25 °C using digital image correlation and repeated-load semi-circular bending (SCB) test. Digital image correlation was utilised successfully for extracting creep compliance using semi-circular samples under repeated loading conditions. A fractional time-dependent viscoelastic element converted the creep compliance data into relaxation modulus. The extracted relaxation modulus data were verified by simulating the viscoelastic response of control and modified asphalt mixtures using the extended finite element model (XFEM). The findings presented in this paper showed that digital image correlation has a high potential for bridging the gap between experimental and simulation work. The strain map provided by the prediction model was compatible with the DIC strain.

Experimental hysteretic behavior of high strength concrete columns confined by butt-welded closed composite stirrups

Six specimens of high strength concrete columns with butt-welded closed composite stirrups were tested under cyclic lateral loads, to study the structural performance (deformation) and hysteretic characteristics of the columns. The influences of the axial compression ratio and the volume-stirrup ratio were considered. According to the regression analysis of the test results, it was determined that the unloading stiffness and strength degradation rate under repeated loading conditions are mainly dependent on the two parameters of the “displacement ductility factor” and “axial compression ratio”. According to the test results, the skeleton curves were determined by the section layer and the statistical regression analysis methods. By considering the influence of axial compression ratio and the volume-stirrup ratio to the hysteretic characteristics of high strength concrete columns confined by butt-welded closed composite stirrups, the shear force-lateral displacement restoring force model was established. The results show that the axial compression ratio has a great influence on the strength degradation of the columns. As the axial compression ratio increases, the strength degradation of the columns occurs faster. On the other hand, the volume-stirrup ratio has a reverse effect on the strength degradation of the columns, with the increase of the volume-stirrup, the strength degradation of the column is observed gradually and slowly. The stiffness degradation of the columns increases as the axial compression ratio increases while the volume-stirrup ratio decreases.

Experimental assessment of coir geotextile to improve the strength of weak subgrade at different load conditions

The stabilization of weak subgrade soil with natural geotextile fiber is a cost-effective to improve the stability of the low-volume roads (LVRs). The coir geotextile is naturally available eco-friendly and biodegradable material having the high strength and durability compared to the other natural material. The present study, the test were conducted to determine the effectiveness of coir fiber and coir geotextile mats under the static and dynamic loading condition by using the CBR and WTT respectively. The inclusion of the coir geotextile fiber to the BC soil subgrade improved the load bearing capacity in soaked and un-soaked condition. The study was also conducted with two types of the coir mats under the repeated loading condition with help of the fabricated mould. It has two layer flexible pavement system like subgrade and sub-base are prepared according to CBR Standard. From the results, it was concluded that the provided coir geotextile mats helps in the slow settlement, reduce the permanent deformation/rut improved the performance of the roads. The placement of coir geotextile in both the test (CBR and WTT) showed the significant improved in reinforced section improved the service life of the flexible pavement structure with lower cost of maintenance.

Evaluation of Rutting in Conventional and Rubberized Asphalt Mixes Using Numerical Modeling Under Repeated Loads

This research aimed to predict the permanent deformation (rutting) in conventional and rubberized asphalt mixes under repeated load conditions using the Finite Element Method (FEM). A three-dimensional (3D) model was developed to simulate the Wheel Track Testing (WTT) loading. The study was conducted using the Abaqus/Standard finite element software. The pavement slab was simulated using a nonlinear creep (time-hardening) model at 40°C. The responses of the viscoplastic model under the influence of the trapezoidal amplitude of moving wheel loadings were determined for different speeds and numbers of cycles. The results indicated that a wheel speed increase from 0.5Km/h to 1.0Km/h decreased the rut depth by about 22% and 24% in conventional and rubberized asphalt mixes, respectively. Moreover, increasing the number of cycles from 7,500 (15,000 passes) to 15,000 (30,000 passes) under constant speed increased the rut depth by about 25% and 30% in conventional and rubberized asphalt mixes, respectively. Furthermore, the addition of Crumb Rubber (CR) to the asphalt reduced its rut depth by 55% compared to conventional asphalt.

High‐efficiency algorithms for simulating metal failure effects under multiaxial repeated loadings

AbstractHigh‐efficiency algorithms are proposed for numerically integrating elasto‐plastic rate equations for the purpose of simulating metal failure effects under repeated loading conditions. Results are obtained with a newly established elasto‐plasticity model which can automatically simulate both low and high cycle fatigue failure effects, with reference to neither damage‐like variables nor ad hoc failure criteria. Novelties are incorporated in a few respects. First, multiaxial loading processes with arbitrarily changing principal stresses and axes can be treated in a broad sense beyond usually treated proportional loading cases. Second, by means of the commonly used ‐invariant of the deviatoric stress, multiaxial loading cases can be directly treated just as in the uniaxial loading case. Third, high cost in time consumption can be considerably reduced in evaluating both low and high cycle failure effects. Numerical examples are presented for simulating uniaxial and multiaxial fatigue failure effects and simulation results are in good agreement with experimental data. Results show that the computation speed with the new algorithms exceeds 600 times faster than usual integration algorithms.

Flexural Behaviour of Hybrid Concrete Beam-Column Connections Under Static and Repeated Loads

The present study includes an experimental investigations for the behavior and the load carrying capacity of hybrid beam-column connections subjected to static and repeated loading condition. The goals were to evaluate the effect of using slurry infiltrated fiber concrete (SIFCON). Experimental program consists of testing six beam-column connections, two of them casted with normal concrete and the other using SIFCON in crtical section in addation to normal concrete. Also, the program testing three of connection subjected to static load and the similar other subjected to repeated loads. Results show an improve in flexural behavior for specimens with SIFCON as compared with normal concrete under static and condition, on the other hand, the reduction in flexural strength when was exposed to repeated loads in comparison with that under static loads reach 2.6% at hybrid connection. While, the reduction was increased to 5.1% for the specimen without SIFCON.

A Unified Approach Toward Simulating Constant and Varying Amplitude Fatigue Failure Effects of Metals with Fast and Efficient Algorithms

A new finite strain elastoplastic $$J_{\mathrm {2}}$$ -flow model is established with an explicit formulation of work-hardening and softening effects up to eventual failure, in which both a new flow rule free of yielding and an asymptotically vanishing stress limit are incorporated. The novelties of this new model are as follows: (i) Fatigue failure effects under repeated loading conditions with either constant or varying amplitudes are automatically characterized as inherent response features; (ii) neither additional damage-like variables nor failure criteria need to be involved; and (iii) both high- and low-cycle fatigue effects may be simultaneously treated. A fast and efficient algorithm of high accuracy is proposed for directly simulating high- and medium–high-cycle fatigue failure effects under repeated loading conditions. Toward this goal, a direct and explicit relationship between the fatigue life and the stress amplitude is obtained by means of explicit and direct procedures of integrating the coupled elastoplastic rate equations for any given number of loading–unloading cycles with varying stress amplitudes. Numerical examples suggest that the new algorithm is much more fast and efficient than usual tedious and very time-consuming integration procedures.

Combined effect of both surface finish and sub‐surface porosity on component strength under repeated load conditions

AbstractHigh duty engineering component life is usually demonstrated through extensive testing and statistical analysis applied to empirical curve‐fit equations. Because of this, the extent of the testing required is huge and costly: it must consider the load cycle range and test to high numbers of cycles. Additive Manufacturing (AM) for high duty components has brought to the fore the question of the effect of porosity and surface roughness on fatigue life, and how the true life of a critical component can be assessed conservatively. The authors propose the first step toward the development of a fatigue model based on well‐established engineering physics principles, by creating computational specimens with modeled surface roughness and porosity, and subjected to cyclic loading using Finite Element Analysis. They show that the combination of roughness features and sub‐surface pores leads to an equivalent plastic strain distribution pattern that suggests an emergent physical process that has not been reported before, and which indicates that the component strength and life reduction arising from surface roughness can be made significantly worse by the presence of porosity. The development of such phenomenological understanding should lead to improved life prediction techniques, more cost effective test procedures, and the development of better AM methods.

Mechanistic-empirical permanent deformation models: Laboratory testing, modelling and ranking

Abstract Geomaterials exhibit elastoplastic behaviour during dynamic and repeated loading conditions. These loads are induced by the passage of a train or vehicle which then generates recoverable (resilient) deformation and/or permanent (plastic) deformation. Modelling this behaviour is still a challenge for geotechnical engineers as it implies the understanding of the complex deformation mechanism and application of advanced constitutive models. This paper reviews on the major causes of permanent deformation and the factors that influence the long-term performance of materials. It will also present the fundamental concepts of permanent deformation as well as the models and approaches used to characterise this behaviour, including: elastoplastic models, shakedown theory and mechanistic-empirical permanent deformation models. This paper will focus on the mechanistic-empirical approach and highlight the evolution of the models, and the main similarities and differences between them. A comparison between several empirical models as well as the materials used to develop the models is also discussed. These materials are compared by considering the reference conditions on the type of material and its physical state. This approach allows for an understanding of which properties can influence the performance of railway subgrade and pavement structures, as well as the main variables used to characterise this particular behaviour. An innovative ranking of geomaterials that relate to the expected permanent deformation and classification (UIC and ASTM) of soil is also discussed because it can be used as an important tool for the design process.