Repeated Loading Conditions Research Articles

EFFECT OF GEOGRID REINFORCEMENT IN WEAK SUBGRADES

Geogrids can be identified as comparatively low cost alternative for pavement construction on soft subgrades. However, comprehensive mathematical models are yet to be developed to define the composite behaviour of geogrid pavements. Therefore, it is required to perform pavement model testing to understand the performance of geogrid reinforcements under different pavement and geogrid conditions. Hence, this research study aims to compare the behaviour of composite geogrid in a weaker subgrade (CBR <3%) by performing two pavement model tests under repeated loading conditions. A steel test box with length, width and height of 1m, 1m, 1.2m respectively were used to construct both models with a subgrade having 500mm thickness and 3% CBR. A 200mm thick granular layer was compacted on top of the subgrade by achieving 91% degree of compaction. One model was selected as the control section and a composite geogrid at the base subbase interface was included in the other one. Both pavement models were tested for more than 100,000 repeated load cycles using a 200mm diameter plate on top of the granular base to simulate a tyre pressure of 550kPa. The results demonstrated that the inclusion of a composite geogrid significantly reduced the rutting depth of the granular layer and achieved a Traffic Benefit Ratio (TBR) of 5 at 50mm rutting. Furthermore, a significant reduction of pressure transmission to the subgrade by the composite geogrid was observed.

Finite Element Analysis of Beam – Column Joints Reinforced with GFRP Reinforcements

Glass Fibre Reinforcement Polymer (GFRP) reinforcements are currently used as internal reinforcements for all flexural members due to their resistance to corrosion, high strength to weight ratios, the ability to handle easily and better fatigue performance under repeated loading conditions. Further, these GFRP reinforcements prove to be the better alternative to conventional reinforcements. The design methodology for flexural components has already come in the form of codal specifications. But the design code has not been specified for beam-column joints reinforced internally with GFRP reinforcements. The present study is aimed to assess the behaviour of exterior beam-column joint reinforced internally with GFRP reinforcements numerically using the ABAQUS software for different properties of materials, loading and support conditions. The mechanical properties of these reinforcements are well documented and are utilized for modelling analysis. Although plenty of literature is available for predicting the joint shear strength of beam-column joints reinforced with conventional reinforcements numerically, but no such study is carried for GFRP reinforced beam-columns joints. As an attempt, modelling of beam-column joint with steel and with GFRP rebars is carried out using ABAQUS software. The behaviour of joints under monotonically increasing static and cyclic load conditions. Interpretation of all analytical findings with results obtained from experiments. The analysis and design of beam-column joints reinforced with GFRP reinforcements are carried out by strut and tie model. Strut and Tie models are based on the models for the steel reinforced beam-column joints. The resulting strut and tie model developed for the GFRP reinforced beam-column joints predicts joint shear strength. Joint shear strength values obtained from the experiments are compared with the analytical results for both the beam-column joints reinforced with steel and GFRP reinforcements. The joint shear strength predicted by the analytical tool ABAQUS is also validated with experimental results.

A kinematic hardening model based on endochronic theory for complex stress histories

While the field of soil constitutive modeling is rich, the availability of models that capture the behavior of different soil types under complex loading is limited. This study presents the development and validation of the Multi-Mechanical Model (MMM). The MMM is an elasto-plastic kinematic hardening model, which extends an earlier endochronic model by introducing the third stress invariant, accounts for shear-volumetric coupling in granular materials, and displays ratcheting behavior during stress reversals. A series of triaxial and isotropic consolidation tests are used to calibrate the MMM to materials found in conventional unbound granular materials and subgrades, such as crushed limestone, Leighton Buzzard sand, and Vicksburg Buckshot clay. The MMM is tested against resilient modulus test data that present complex stress histories. The study shows the MMM is straightforward in its calibration, while also displaying a robust capability in capturing the complex behaviors of the different soil types under various loading conditions. While positioned for applications in other areas, the MMM’s capabilities in capturing the short-term pore pressure response of fine-grained materials, the shear-induced volume change and stress-hardening effects of granular materials, and ratcheting behavior during repeated loading conditions make it well-suited for addressing pavement problems.

Behavior of RC beams strengthened with NSM CFRP strips under flexural repeated loading

Strengthening with near surface mounted carbon fibre reinforced polymers (NSM-CFRP) is a strengthening technique that have been used for several decades to increase the load carrying capacity of reinforced concrete members. In Iraq, many concrete buildings and bridges were subjected to a wide range of damage as a result of the last war and many other events. Accordingly, there is a progressive increase in the strengthening of concrete structures, bridges in particular, by using CFRP strengthening techniques. Near-surface mounted carbon fibre polymer has been recently proved as a powerful strengthening technique in which the CFRP strips are sufficiently protected against external environmental conditions especially the high-temperature rates in Iraq. However, this technique has not been examined yet under repeated loading conditions such as traffic loads on bridge girders. The main objective of this research was to investigate the effectiveness of NSM-CFRP strips in reinforced concrete beams under repeated loads. Different parameters such as the number of strips, groove size, and two types of bonding materials (epoxy resin and cement-based adhesive) were considered. Fifteen NSM-CFRP strengthened beams were tested under concentrated monotonic and repeated loadings. Three beams were non-strengthened as reference specimens while the remaining were strengthened with NSM-CFRP strips and divided into three groups. Each group comprises two beams tested under monotonic loads and used as control for those tested under repeated loads in the same group. The experimental results are discussed in terms of load-deflection behavior up to failure, ductility factor, cumulative energy absorption, number of cycles to failure, and the mode of failure. The test results proved that strengthening with NSM-CFRP strips increased both the flexural strength and stiffness of the tested beams. An increase in load carrying capacity was obtained in a range of (1.47 to 4.49) times that for the non-strengthened specimens. Also, the increase in total area of CFRPs showed a slight increase in flexural capacity of (1.02) times the value of the control strengthened one tested under repeated loading. Increasing the total area of CFRP strips resulted in a reduction in ductility factor reached to (0.71) while the cumulative energy absorption increased by (1.22) times the values of the strengthened reference specimens tested under repeated loading. Moreover, the replacement of epoxy resin with cement-based adhesive as a bonding material exhibited higher ductility than specimen with epoxy resin tested under monotonic and repeated loading.

An investigation on the mechanical behavior of expanded polystyrene (EPS) geofoam under different loading conditions

The mechanical properties of Expanded Polystyrene (EPS) Geofoam are important to be investigated. Loading rate dependency and repeated load effects on the EPS stress strain behaviors were studied by performing compression tests. In the present study an investigation was done to simulate the actual working conditions and hence to evaluate the mechanical behavior of EPS under these conditions. A series of loading cycles were applied in accordance with a defined procedure on EPS cubic samples with densities of 0.30 kN/m3 the elastic properties were evaluated at each loading condition (repeated load and different loading rates values). The significant effects of those loading conditions specially rate dependant stress strain behavior were observed. For the repeated load conditions the residual deformations occurs after each load application was the keyword of repeated load effects, where the relative deformation will increases as the load repeated on the same sample. For the loading rates effects when EPS foam is subjected to compressive loading, the entrapped air within the cells is compressed and viscous force is generated. Viscous forces increase with the loading rate. The detailed of this investigation will be discussed in this article.

Memory Surface Hardening Model for Granular Soils under Repeated Loading Conditions

AbstractThe prediction of the stress-strain response of granular soils under large numbers of repeated loading cycles requires subtle changes to existing models, although the basic framework of kinematic hardening/bounding surface elastoplasticity can be retained. Extending an existing model, an extra memory surface is introduced to track the stress history of the soil. The memory surface can evolve in size and position according to three rules that can be linked with physical principles of particle fabric and interaction. The memory surface changes in size and position through the experienced plastic volumetric strains, but it always encloses the current stress state and the yield surface; these simple rules permit progressive stiffening of the soil in cyclic loading, the accurate prediction of plastic strain rate accumulation during cyclic loading, and the description of slightly stiffer stress-strain response upon subsequent monotonic reloading. The implementation of the additional modeling features re...

Fatigue properties of parts printed by PolyJet material jetting

Purpose – This paper aims to seek to fill a gap in the literature by characterizing the fatigue life and microstructure of a printed elastomer material, the TangoBlackPlus material. Design/methodology/approach – Because the TangoBlackPlus material is marketed as “rubber-like”, the printed elastomer specimens were tested according to the ASTM D4482-11 “Test Method for Rubber Property Extension Cycling Fatigue”. The microstructure of the printed material and multi-material interface was examined by slicing specimens and examining them under an optical microscope. Findings – Findings are developed to show the relationship between elongation and expected fatigue life. Findings also indicate that the smoother, non-support encased “glossy” surface finish option for PolyJet parts improve the fatigue life of components and that there are a number of microscopic voids in the TangoBlackPlus material that seem to be concentrated at layer and print head boundaries. Research limitations/implications – This paper provides a glimpse into the fatigue properties and microstructure of printed elastomeric parts, a previously unstudied area. This work is limited in that it only looks at specimens created in a single orientation, on a single machine, with a single material. More work is needed to understand the general fatigue properties of printed elastomers and the factors that influence fatigue life in these materials. Practical implications – The authors provide several design guidelines based on the findings and previous work that can be used to increase the fatigue life of printed elastomer components. Originality/value – As additive manufacturing (AM) technology moves from a prototyping tool to a tool used to create end use products, it is important to examine the expected lifespan of AM components. This work adds to the understanding of the expected product lifecycle of printed elastomer components that will likely be expected to withstand large repeated loading conditions.

입도 및 함수비 조건에 따른 철도 노반 재료의 영구침하거동 요소시험평가

Since allowable settlement of concrete slab track is about 30mm, a lot of attention must be paid to the settlement of the earthwork (reinforced trackbed, upper subgrade, under subgrade) under the concrete track. To this end, more experimental data should be accumulated through tests for these materials. In this study, we evaluate the long-term settlement of reinforced trackbed and subgrade materials using factors such as repeated loading conditions, water content, and grain size distributions in a large triaxial test and a large oedometer test. In cases in which the performance of the reinforced trackbed layer meets the design criteria, the settlement caused by train load was considerably small. But, when the water content increases in the subgrade, unexpectedly large settlement might occur for certain grain size distributions of the subgrade materials.

철강산업 Slitting용 접압이송 벨트의 내구성실험

This study aimed to investigate a durability test of a textile composite belt for use in steel slitting. The belt was developed by combining scrim fabric and a nonwoven sheet composite with resin impregnation and a silicon finish. Because resin impregnation is necessary for a textile composite slitting belt, this study focussed primarily on examining two different types of resin influencing the belt durability, namely, thermosetting resin PVB and thermoplastic resin PET. The accelerated test conditions were obtained through a performance comparison with reference belt (50 hr with 500N load). The results indicate that thermosetting PVB is more suitable for the overall durability of a belt under repeated load conditions with friction when compared to thermoplastic PET, although PVB tends to increase the brittleness in comparison when compared to PET.

Using stress relief grooves to reduce stress concentration on axle drive shaft

The axle drive shaft has important roles such as transferring power and changing the steering angle between the axle and the wheel in a power train system. It is used in most heavy construction machinery, where a high degree of reliability is required in the power train system. However, for axle drive shafts with a long span axle, failures are common at the snap ring cut that is machined on the drive shaft when there is significant fatigue damage under repeated loading conditions. Stress relief grooves have been applied at the snap ring cut to reduce the stress concentration and improve the fatigue life of an axle drive shaft. Although several studies have described how the stress concentration can be reduced by the stress relief grooves, details of the geometries of the stress relief grooves have been subject to debate and even controversy. We investigated the effects of the stress relief grooves on the stress concentration, and estimated the fatigue life of the drive shaft by using finite element analysis, taking into account the geometric parameters such as size and location of the stress relief grooves. As a result, the stress relief grooves presented by non-dimensional geometric parameters for an r/h = 1.2 and a d/b = 2.0 enabled a 22.3% reduction of the stress concentration, and a maximum improvement in the fatigue life that was approximately 3.3 times that of drive shaft with no stress relief grooves applied. These can be an index for selecting optimal geometric shapes of the stress relief grooves.

Fatigue constrained topology optimization

We present a contribution to a relatively unexplored application of topology optimization: structural topology optimization with fatigue constraints. A probability based high-cycle fatigue analysis is combined with principal stress calculations in order to find the topology with minimum mass that can withstand prescribed variable-amplitude loading conditions for a specific life time. This allows us to generate optimal conceptual designs of structural components where fatigue life is the dimensioning factor. We describe the fatigue analysis and present ideas that make it possible to separate the fatigue analysis from the topology optimization. The number of constraints is kept low as they are applied to stress clusters, which are created such that they give adequate representations of the local stresses. Optimized designs constrained by fatigue and static stresses are shown and a comparison is also made between stress constraints based on the von Mises criterion and the highest tensile principal stresses. The paper is written with focus on structural parts in the avionic industry, but the method applies to any load carrying structure, made of linear elastic isotropic material, subjected to repeated loading conditions.

Utilization of Ripe Coconut Fiber in Stone Matrix Asphalt Mixes

Stone Matrix Asphalt (SMA) is a gap graded mix; characterized by higher proportion of coarse aggregate, lower proportion of middle size aggregate and higher proportion of mineral filler. In the present laboratory study, commonly available one conventional VG 30 bitumen and another modified binder, namely CRMB 60 have been used along with a non-conventional natural fiber, namely coconut fiber which is abundantly available in India to provide improved engineering properties and at the same time preventing the usual draining of binder in SMA. The role of a particular binder and fiber with respect to their concentrations in the mix is studied for various engineering properties. Marshall procedure has been followed to determine the optimum binder and optimum fiber contents and also to study the relative advantages of fiber addition in the SMA mixtures. Thereafter, the engineering properties under both static as well as repeated load conditions and moisture susceptibility characteristics have been studied. It is observed that only a marginal 0.3% coconut fiber addition brings significant improvement in the engineering properties of SMA mixes.

THE FRICTIONAL CONTACT ANALYSIS BETWEEN A TACTILE SENSOR AND ATRIAL TISSUE IN VISCOELASTICITY

This paper discusses the effect of friction in tool–tissue interaction of surgical simulation. The focus of this research is on mitral valve repair by robot-assisted surgery (RAS) method with tactile feedback. Heart tissue is considered to be touched by a tactile sensor with considering the friction effect. A regularized Coulomb law analogous to elastoplastic theory is used to represent the friction. Since the heart tissue is subjected to repeated loading condition, a generalized standard solid chain is chosen for modeling the heart tissue. To implement the generalized standard solid chain in finite element analysis (FEA), a new splitting strategy is presented. The splitting strategy avoids the complicated conversion procedure from creep function to relaxation function by dividing the total problem into several subproblems. The penalty method is used in the derivation of continuum model for the interaction between the tactile sensor and the tissue. By performing linearization for the continuum model, the discrete model for the FEA is obtained. Numerical results are obtained by using the constitutive parameters of tissue and friction coefficient, which were found experimentally.

Residual stresses and shakedown in cohesive-frictional half-space under moving surface loads

‘Shakedown’ is used to refer to a state of structures under repeated loading conditions at which the material behaviour becomes purely elastic after some initial plastic deformation. Residual stresses developed in a structure due to the initially occurred plastic deformation play an important role in helping the structure to reach the shakedown state. A better understanding of residual stresses in cohesive-frictional half-space under moving surface loads is urgently needed if shakedown theory is applied to solve pavement or railway problems. This paper is focused on residual stresses in cohesive frictional materials by using finite element analysis to investigate the development of residual stresses in a cohesive-frictional half-space under repeated moving surface loads. These numerical results illustrate how residual stresses affect the behaviour of cohesive frictional materials under repeated loading conditions.

Simulating Ballast Shear Strength from Large-Scale Triaxial Tests

The railroad ballast layer consists of discrete aggregate particles, and the discrete element method (DEM) is the most widely adopted numerical method to simulate the particulate nature of ballast materials and their particle interactions. Large-scale triaxial tests performed in the laboratory under controlled monotonic and repeated loading conditions are commonly considered the best means to measure macroscopic mechanical properties of ballast materials, such as strength, modulus, and deformation characteristics, directly related to load-carrying and drainage functions of the ballast layer in the field. A DEM modeling approach is described for railroad ballast with realistic particle shapes developed from image analysis to simulate large-scale triaxial compression tests on a limestone ballast material. The ballast DEM model captures the strength behavior from both the traditional slow and the rapid shear loading rate types of monotonic triaxial compression tests. The results of the experimental study indicated that the shearing rate had insignificant influence on the results of the triaxial compression tests. The results also showed that the incremental displacement approach captured the measured shearing response, yet could save significant computational resources and time. This study shows that the DEM simulation approach combined with image analysis has the potential to be a quantitative tool to predict ballast performance.

Comparing finite element and constitutive modelling techniques for predicting rutting of asphalt pavements

This paper focuses on a comprehensive evaluation of the effects of different finite element (FE) modelling techniques and material constitutive models on predicting rutting in asphalt pavements under repeated loading conditions. Different simplified 2D and more realistic 3D loading techniques are simulated and compared for predicting asphalt rutting. This study also evaluates and compares the rutting performance predictions using different material constitutive behaviours such as viscoelastic–viscoplastic, elasto-viscoplastic and coupled viscoelastic, viscoplastic and viscodamage behaviours. The simulations show that the assumption of the equivalency between a pulse loading and an equivalent loading, which are commonly used as simplified loading assumptions for predicting rutting, is reasonable for viscoelastic–viscoplastic and elasto-viscoplastic constitutive behaviours. However, these loading assumptions and material constitutive models overestimate rutting as damage grows. Results show that the 2D plane strain FE simulations significantly overestimate rutting as compared with the rutting performance predictions from more realistic 3D FE simulations.

Combined use of geocell reinforcement and rubber–soil mixtures to improve performance of buried pipes

Service trench provision and maintenance of buried pipes represent major cost items in the utilities industry. Using recycled material in order to optimize the design of the buried pipe system can lead to significant cost reductions, but only if performance is not degraded. The main purpose of the paper is to investigate the mitigation of strain in buried flexible service pipes and of the settlement of backfill over such pipes by the use of geocell reinforcement (as 3D-inclusion reinforcement) with rubber–soil mixtures under repeated loading conditions. Two rubber sizes (namely chipped and shredded rubbers), three different percentages of rubber content in the mixture, two positions for soil–rubber mixture inside the trench, four levels of repeated loading and the addition of geocell reinforcement over the pipe are the variables considered. Soil surface settlement, vertical diametral strain of the pipe (as an indication of pipe wall deflection) and stress distribution in the trench, especially on pipe’s crown, are assessed and evaluated. Both cumulative and resilient strains are considered. Using a material with high resilience, like the rubber–soil mixture, could lead to some critical issues that should be considered. These include the larger settlement of the soil surface, transfer of a larger pressure onto the pipe and, consequentially, greater pipe wall strain. For the chipped rubber and soil mixture, the pipe has the highest strains under the cyclic loading irrespective of the amount of rubber in the soil. However, the shredded rubber and soil mixture, dependent on the amount of rubber content, is able to reduce the soil settlement and plastic pipe’s diametral strain, attenuating the pipe’s accumulating strains and, finally, protecting the buried pipe from fatigue under repeated loadings. This benefit is enhanced by the combined action of geocell reinforcement over rubber-modified soil. According to the results, the minimum soil surface settlement and vertical diametral strain are provided by 5% of shredded rubber–soil mixture placed over the pipe with a geocell, giving values of, respectively, 0.30 and 0.53 times those obtained in the unreinforced and untreated soil.

복공식 지하 압축공기에너지 저장공동의 내압구조에 대한 반복하중의 역학적 영향평가

In a material, micro-cracks can be progressively occurred, propagated and finally lead to failure when it is subjected to cyclic or periodic loading less than its ultimate strength. This phenomenon, fatigue, is usually considered in a metal, alloy and structures under repeated loading conditions. In underground structures, a static creep behavior rather than a dynamic fatigue behavior is mostly considered. However, when compressed air is stored in a rock cavern, an inner pressure is periodically changed due to repeated in- and-out process of compressed air. Therefore mechanical properties of surrounding rock mass and an inner lining system under cyclic loading/unloading conditions should be investigated. In this study, considering an underground lined rock cavern for compressed air energy storage (CAES), the mechanical properties of a lining system, that is, concrete lining and plug under periodic loading/unloading conditions were characterized through cyclic bending tests and shear tests. From these tests, the stability of the plug was evaluated and the S-N line of the concrete lining was obtained.

Grazing Effects on Compressibility of Kastanozems in Inner Mongolian Steppe Ecosystem

In Inner Mongolia, animal trampling is one of the main factors causing soil degradation manifested by altered mechanical strength or changes in water and gas fluxes. Soil samples were collected at two depths (4–8 and 18–22 cm) on the Stipa grandis steppe ecosystem in Inner Mongolia from two treatments characterized by different grazing intensities: ungrazed since 1979 (UG) and continuously grazed (CG). The following mechanical soil properties were determined under static and repeated loading conditions: precompression stress, Pc; coefficient of cyclic compressibility, cn, and compression index, Cc Air conductivity measurements were used to quantify the changes in soil functions due to application of repeated loading. The CG site showed significantly higher precompression stress values (111 kPa) than the UG site (64 kPa) at the first soil depth. The highest cn values were found in the topsoil of the UG site, while the CG site had significantly lower cn values. Repeated loading caused higher soil deformation compared to the static loading test. It was also found that the strain of soil samples from the UG site was higher than the CG site. We found a good fit between cn and precompression stress. The Cc values of the cyclically loaded samples were significantly lower at the CG site than the statically loaded ones. The air conductivity of the UG site remained constant for a wider stress range of repeatedly applied stress compared with the CG site, which reflects higher stability of the soil pore network at the UG site.

Performance of Thermally Sprayed Coatings under Reciprocal Sliding Test

This study aims to evaluate the behavior of thermally sprayed coatings with the HVOF technique (High Velocity Oxygen Fuel), subject to tribological testing of reciprocal movement with the SAE 52100 flat steel pin, in accordance with ASTM G 133. The carbon with Cr2C3-25NiCr and WC-12Co were evaluated under conditions of intense surface fatigue. The microstructure and surface modifications were analyzed by optical microscopy and scanning electron microscopy. By using these techniques were possible to compare the modifications of the coatings under repeated loading conditions, and to expand the knowledge about wear mechanisms, mainly the adhesive one, present in several mechanical components, particularly in bearing and internal combustion engines. The coatings tested showed lower wear rate when compared to SAE52100 steel and equivalent behavior among the different loading conditions applied.