Cyclic Loading Conditions Research Articles

Fatigue life assessment of notched laminated composites: Experiments and modelling by Finite Fracture Mechanics

In this paper, the coupled Finite Fracture Mechanics criterion is extended to assess the finite fatigue life of orthotropic notched laminated composites. The approach is validated through a comprehensive experimental program conducted on laminated composites under tension-tension cyclic loading conditions with two distinct lay-ups. For a given loading ratio, fatigue tests on plain and cracked specimens are first performed to provide the model inputs, the critical cyclic stress and stress intensity factor amplitudes. Fatigue tests on samples weakened by circular holes of two different radii are then used for blind predictions. Accurate predictions of the number of cycles to failure are achieved without the need for inverse calibration of material properties or deviation from standard testing procedures. Finally, a parametric study is performed to investigate the hole radius effect. It is worth mentioning that the proposed approach is general and can be applied to any notched geometry.

Evaluating the Long-Term Plastic Strain Accumulation in Reclaimed Asphalt Pavement (RAP) Application for Road Subbase Construction: A Cyclic Triaxial Loading Study

Reclaimed Asphalt Pavement (RAP) has gained significant attention in recent years as an environmentally sustainable solution for road construction. However, the main concerns are regarding its long-term performance and the potential impact on the road subbase. This study aims to investigate the behavior of RAP under cyclic triaxial loading conditions, specifically focusing on the accumulation of plastic strain over time. The research methodology involved laboratory testing using a cyclic triaxial apparatus to simulate the stress conditions experienced by RAP in road subbase applications in reference to AASHTO code. Several RAP specimens with varying degrees of compaction were subjected to repeated cyclic loading while monitoring the associated plastic strain accumulation. To provide a comparative analysis, conventional subbase materials were also tested under the same conditions. The findings indicate that RAP exhibits a notable disadvantage in terms of long-term plastic strain accumulation when compared to conventional subbase materials. The cyclic triaxial loading tests revealed a substantial increase in plastic strain accumulation over extended periods, suggesting the potential for long-term deformations and compromised performance of the road structure. This accumulation of plastic strain could lead to increased maintenance requirements and decreased service life of the road. Based on the results obtained, it is recommended that careful consideration be given to the utilization of RAP in road subbase construction in areas subjected to heavy cyclic loading conditions. Additional research and development efforts are necessary to explore potential mitigation strategies and optimize the use of RAP, ensuring sustainable and resilient road infrastructure.

The asymmetric evolution of grain-scale stresses in notched specimens under cyclic loads

Engineering components are often exposed to various cyclic loading conditions. The development of microscale stresses under such conditions depends on the material texture, local microstructure, the degree of crystals elastic and plastic anisotropy, as well as the presence of geometrical irregularities. In this study, the three-dimensional synchrotron X-ray diffraction (3D-XRD) technique is used and coupled with a crystal plasticity finite element (CPFE) model to deconvolute the contribution of such parameters on the magnitude of localized stresses. Special attention is paid to the notch geometry, where two double-edge-notched α-zirconium specimens with different geometries but the same texture were deformed in-situ while grain-scale stresses were measured both in the vicinity of the notches and in the far-field zones. Deep and shallow notches were cut to the dimensions comparable with the specimens’ average grain sizes. For each specimen, the center of mass, orientation, elastic strain, stress, and relative volume of more than 6,000 grains were measured and studied. Post-deformation analysis with Electron Backscatter Diffraction (EBSD) was conducted to further validate the observed trends. It is shown that the distribution of grain-scale stresses is asymmetric within each specimen. This asymmetry develops from the early stages of the applied cyclic load and remains intact with further loading. A new method is introduced for quantifying the effects of grain neighbourhood on load sharing, where it is shown that such effects relax the stresses acting on “hard” grains, altering grain-scale stress concentration factors.

In-depth analysis of mechanical and electrical responses of PDMS/MWCNT micro-composite films under cyclic loading conditions

This study has comprehensively examined the variations in the mechanical and electrical properties of PDMS/MWCNT micro-composite (PMMC) films under cyclic tensile loading. While previous studies have primarily focused on the electrical performance of wearable sensors, it is essential that wearable sensors need to meet both electrical characteristics and mechanical reliability throughout their designed operational lifespan. We conducted cyclic tensile tests for PMMC films with different MWCNT mass fractions of 3-wt%, 4-wt%, and 5-wt% and obtained cyclic mechanical properties and electrical conductivity. Each PMMC film exhibited distinct stabilization trends in both mechanical and electrical responses under cyclic loading. Additionally, the stabilised cyclic hyperelastic material properties were defined from the test results to enable predicting structural behaviour of PMMC films using finite element analysis. The results of this study provide valuable insights into the material properties and mechanical/electrical responses under cyclic loads for the structural integrity assessment of wearable sensors employing PMMC films.

Influence of rebar couplers on the cracking behavior of reinforced concrete beams

Mechanical connections of rebars are widely used and demonstrated high effectiveness in reinforced concrete structural elements, but the differences in the performances of different connection methods in reinforced concrete beams from the perspective of adhesion, bending, and load transfer are rarely documented. In addition, existing rebar connections are large and may lead to local cracking because of their size. In this study, we investigated the use of compact mechanical couplers by pull-out tests and bending tests of full-scale reinforced concrete beams. For pull-out tests, the influence of the couplers position in concrete was studied. Different rebar configurations were tested for beams: continuous, with a covering, or connected with compact mechanical couplers. The nonlinear behaviour of beams was analysed under monotonic and cyclic load conditions. The overall behaviour of beams is discussed in terms of load–displacement curves and concrete cracking distribution. To study the local behaviour, digital image correlation was set up, allowing to monitor strain distribution and crack opening.Experimental results show no significant impact of the compact couplers on the global or local behaviour of reinforced concrete beams compared to the beams using continuous rebars: failure load, overall stiffness, and crack distribution are similar, in the uncertainty range of tests. The regulatory calculation of crack openings and spacing fits the experimental results for most of the studied cases.

Многокритериальная оптимизация операционных режимов технологических процессов

The issues that are rarely combined in one small study are viewed. First of all, this is a well-known problem of engineering support of a specified quality of hardened steels surface layer using a blade tool made of superhard materials and the state of microclimate of operating areas of metal-cutting machines. It had a shot at creating a mathematical model that combines these two objective functions. As a mathematical tool that allows optimizing the modes of finishing and fine turning, the linear- programming technique was applied, which visualises through its graphic image, a picture of the influence of the engineering constraints included in the model on the optimal values of the cutting speed and feed. On the basis of analytical studies, the dependences of the formation of hydrogen sulfide vapors in the operating areas of metal-cutting machines using sulfofresols of various brands as lubricating and cooling fluids were obtained. Theoretically, it has been established that the maximal influence on the intensity of hydrogen sulfide vapor formation is exerted by the temperature in the cutting zone, which is more effected by the lathe turning speed. The paper presents empirical dependencies ratios that allows finding an average temperature in the cutting zone in the process of various steels treatment, as well as high-strength alloys and non-ferrous alloys. The roughness and the degree of cold work coefficient were taken as parameters of the surface layer condition for cylindrical surfaces, which decisively determine the durability of the products under conditions of friction and cyclic loads. Testing of the obtained mathematical models made it possible to obtain optimal values of feed and cutting speed for finishing and fine turning of hardened steels 45 and 65G.

Effects of Grain Boundary Misorientation Angle on the Mechanical Behavior of Al Bicrystals

This research article explores the effect of grain boundary (GB) misorientation on the mechanical behavior of aluminum (Al) bicrystals by means of molecular dynamics (MD) simulations. The effect of GB misorientation on the mechanical properties, fracture resistance, and crack propagation are evaluated under monotonic and cyclic load conditions. The J-integral and the crack tip opening displacement (CTOD) are assessed to establish the effect of the GB misorientation angle on the fracture resistance. The simulations reveal that the misorientation angle plays a significant role in the mechanical response of Al bicrystals. The results also evidence a gradual change in the mechanical behavior from brittle to ductile as the misorientation angle is increased.

Out-of-Plane behaviour of dowel type precast Panel-to-Foundation connections

Precast concrete wall panels are a common structural system, particularly in low-rise industrial and commercial buildings. In past earthquakes the connections between precast concrete panels and other structural members have been found to perform poorly. Fourteen panel-to-foundation assemblages were tested to investigate the out-of-plane performance of common connection detailing. These connection details included both dowel starter bars formed from conventional reinforcement, such as hooked bars, as well as starter bars connected to the panel via the use of cast-in threaded inserts. Variations in panel strength and connection strength were incorporated across the range of tested assemblages, and both cyclic and monotonic loading conditions were considered within the test program. It was found that conventional starter bars performed well because the presence of the hook returns of the starter bars within the joint region elevated the joint strength. Conversely it was found that panels having threaded inserts performed unsatisfactorily due to flexural cracking that propagated vertically behind the insert heads, leading to separation of the panel from the starter bars. It was determined that because the panels were close to the minimum reinforcement ratio, that the drift demands were very sensitive to the assumed cracking moment of the panel. Due to the brittle behaviour of the panel-to-foundation connection, it is advised that existing panels utilizing this connection be limited to less than 2% drift in the out-of-plane direction in order to avoid joint failure.

Fatigue life of plain concrete subjected to low frequency uniaxial stress reversal loading

Most concrete structures sustain cyclic loading conditions during their service life. The accumulated damage caused by this type of loading can be regarded as fatigue and leads to sudden failures. Therefore, knowledge regarding the behavior of concrete under repeated loading is critical in structural design. It becomes even more critical when the fatigue stresses range from tension to compression. Since the fatigue behavior of plain concrete under cyclic sequential application of tension and compression, known as stress reversal, could differ from the behavior under pure tensile or compressive fatigue, concrete fatigue analysis under different cases of stress reversal is pursued in this study. A low frequency of 0.1 Hz is chosen for this study to replicate the slow recurring nature of certain loading conditions. Analysis indicates that stress reversal generally causes more severe deterioration and reduces the number of cycles to failure when compared to tensile fatigue. Results also indicate that adding some level of compression largely contributes to the energy that is required to fail the specimens under stress reversal fatigue. A macroscopic cross-section failure analysis is also followed in this research, revealing that specimens with highest concentration of fractured aggregates achieved longer fatigue lives.

Mechanical Responses of Ductile Aluminum Alloy under Biaxial Non-Proportional Tensile Reverse Loading Patterns

This paper deals with the study of the mechanical responses of ductile metals under biaxial non-proportional cyclic loading tests. The biaxially loaded HC specimens manufactured from 4 mm thick aluminum alloy sheets (EN AW 6082-T6) are subjected to various loading paths, including monotonic and cyclic loading conditions. The aim is to investigate the plastic, damage, and fracture behavior of the material under these different loading scenarios. In terms of numerical aspects, a modified anisotropic two-surface cyclic plastic–damage continuum model is used to predict the material behavior in the load-displacement field and different strain fields. Numerically predicted stress states are analyzed in detail to gain a better understanding of the damage mechanisms. Moreover, the scanning electronic microscopy (SEM) pictures taken from the fracture surfaces confirm the dependency of the damage mechanisms on the loading histories. The present work indicates the importance of considering different loading conditions for the accurate prediction of material responses.

Features of processes of high-speed milling with a complex profile tool in the processing of aluminum alloys and composite materials

Complex computational and experimental studies substantiate rational modes of milling of complex contour equiaxed surfaces with high accuracy of shape, dimensions and roughness parameters. Bars made of nanostructured carbide composite (produced by extrusion of WC-Co-Al2O3 bimodal powder mixtures) with increased strength, crack resistance and heat resistance were used as a workpiece material for the manufacture of new original tool designs. The combination of these properties is a necessary prerequisite for the effective operation of the developed designs of multi-blade cutters at high cutting speeds and under conditions of variable cyclic loads. A more complex kinematics of the joint rotational movement of the tool during milling dictates the need for new approaches when assigning rational cutting modes. To obtain reliable calculation formulas, numerical experiments were previously carried out, including simulation of the processing process using the VisualStudio integrated development environment, which supports WindowsForms technology. The ability to display graphical 3D objects was implemented using an additional software product in the form of the Open CASCADE geometric core. Numerical experiments using MathCAD software products and based on the analytical provisions proposed in the work made it possible to evaluate the influence of cutting conditions, geometric parameters of the cutting part of the tool (profile and number of teeth), kinematics of relative movement in the “tool – part” system on the shape of surfaces and contour parameters (roughness obtained during milling. A technique, algorithm and program for the automated calculation of cutting conditions has been developed, which has been verified during full-scale experiments and the manufacture of complex profile parts from aluminum alloys for drives of aerospace products (in the form of a RC profile and parts of a pinion transmission of guidance mechanisms). At the same time, on the basis of a 3D model of products, control programs for CNC machines were created using MasterCAM. The practical significance and technical and economic efficiency of the proposed design and technological solutions is to increase productivity and reduce the complexity of processing (in comparison with the basic options) through the use of new multi-edge carbide tools for high-speed milling (including when processing composite materials).

Investigating in-vitro degradation, fatigue behavior, and fracture toughness of electrical discharge-processed Mg alloys for biodegradable implant applications

Biodegradable magnesium (Mg) alloys hold great potential for revolutionizing the field of biomedical engineering by offering temporary support during tissue healing and degrading without leaving permanent residues. However, their clinical applications have been limited due to their relatively high degradation rate. This study focuses on evaluating the in-vitro degradation, fatigue resistance, and fracture toughness properties of Mg alloys under cyclic loading conditions, mimicking real-life scenarios. Wire Electrical Discharge Machining (WEDM) was used to prepare spark-processed Mg samples with complex surface texture, and fine-polished Mg samples were used for comparison. The structural characterization, electrochemical corrosion behavior, degradation assessment, and mechanical integrity of the samples were comprehensively analysed. The results show that the Electrical Discharge processed (EDed) Mg sample exhibited uniformly distributed overlapped craters on the surface, which led to a lower charge transfer resistance and higher corrosion potential compared to the Pristine Mg sample. The rough surface topography and alkaline pH microenvironment of the EDed Mg sample facilitated rapid apatite mineralization, but the resulting Ca-deficient apatite compromised its structural stability. Both EDed and Pristine Mg samples exhibited a significant reduction in fatigue life and lower fracture toughness with prolonged immersion. These findings provide valuable insights into the performance of Mg alloys and their potential applications in biodegradable implants, guiding the design of robust implant materials for enhanced patient outcomes.

A review of the performance of polymer parts printed on fused deposition modelling under cyclic loading

Fused deposition modelling (FDM) is one of the most promising and widely used additive manufacturing (AM) technologies. FDM is commonly used to fabricate simple to complex 3D parts from polymeric materials. Different 3D-printed polymeric components manufactured using FDM technology exhibit various mechanical and load-bearing properties. They are utilized in various industries, particularly the medical industry, for load-bearing applications in bio-implants. Therefore, it is crucial to understand the mechanical characteristics, especially the fatigue behaviour, of these 3D-printed parts under cyclic loadings. This paper focuses on the dynamic behaviour of 3D-printed polymeric parts produced through the FDM process and under different loading modes. First, the FDM mechanism, the various polymeric materials used in FDM, and the effective FDM parameters are briefly introduced. Then, the paper comprehensively covers one of the most important mechanical characteristics of 3D-printed parts for load-bearing applications: their fatigue life under different printing conditions and cyclic loadings. The most influential factors on the fatigue behaviour of polymers include the type of polymeric material, surrounding environmental conditions, cyclic loading conditions, the type of testing specimen, and the FDM printing parameters (such as raster angle, infill density, nozzle diameter, and nozzle temperature). In addition to the anisotropic behaviour reported by various studies on 3D-printed parts using FDM, it has been concluded that the layered structure (with a large number of interfaces between the raster) and the presence of microholes are two factors that weaken the fatigue strength of 3D-printed structures compared to other fabrication processes. These conditions can be exacerbated by choosing inappropriate printing parameters, and often, the failure mechanism in 3D-printed parts fabricated with FDM is delamination.

Stability of sandy soils against internal erosion under cyclic loading and quantitatively examination of the composition and origin of eroded particles

Internal erosion refers to the movement of fine particles within soil framework due to subsurface water seepage. Existing criteria for assessing internal erosion usually are based on static loading, and the effect of cyclic load is not considered. Additionally, there are limited studies to examine the particle-size distribution and origin of eroded fine particles. This study presents an experimental investigation that examines the impact of cyclic loading on internal stability through a series of seepage tests. The composition and origin of lost particles are quantitatively studied using particle staining and image recognition techniques. With increasing hydraulic gradient, particle erosion progresses from top layer to bottom layer, with a gradual increase in the maximum particle size of eroded particles from each layer. After significant loss of particles, the specimens reach a state of transient equilibrium, resulting in a gradual slowdown of both particle loss rate and average flow velocity. The results indicate that cyclic loading promotes massive particle loss and causes erosion failure of specimens that are considered stable according to existing criteria. The reason is that under cyclic loading, local hydraulic gradients is oscillating, and a larger than average hydraulic gradient may occur, which is responsible for the internal instability. The analysis suggests that existing criteria can provide a reasonable assessment of the relative stabilities of specimens under static loads but fail to capture the stabilities under cyclic loading conditions.

A novel self-gripping long-term resorbable mesh providing temporary support for open primary ventral and incisional hernia

A novel synthetic fully long-term resorbable self-gripping mesh has been recently developed to reinforce soft tissue where weakness exists during ventral hernia repair open procedures. This resorbable mesh is a macroporous, knitted, poly-L-lactide, poly-trimethylene carbonate copolymer monofilament mesh with the ProGrip™ technology, providing grips on one side of the mesh. A new poly-L-lactide, poly-trimethylene copolymer was developed to provide the required features for mechanical support during at least 20 weeks covering the critical healing period, including resistance to fatigue under cyclic loading conditions, as it occurs in patients. The yarns and mesh initial physical and biomechanical properties were characterized. Then, the mesh mechanical strength was evaluated over time. The mechanical properties of the proposed mesh were found to be above the generally recognized threshold value to mechanically support the repair site of a hernia over a 20-week period during in-vitro cyclic loading test. The mesh performance was evaluated in vivo using a published preclinical porcine model of hernia repair at 4-, 12- and 20-weeks post implantation. The burst strength of the hernia repair sites reinforced with the new mesh were higher at 4 & 12 weeks and comparable at 20 weeks to the one of the native abdominal walls. At all time points, the mesh was well tolerated with moderate inflammation and was fast integrated in the abdominal wall at 4 weeks. Particularly, the grips were nicely engulfed in the newly formed connective tissue. They must facilitate the anchoring of the mesh by their extension from the mesh and their mushroom shape. The preclinical data of the self-gripping resorbable mesh suggests that it has all the favorable characteristics for future clinical use during ventral hernia repair open procedures.Graphical

Performance of fibre-reinforced landfill mined soil like fraction as an environment friendly fill material

Landfill mining is emerging as an effective strategy towards mitigation of the problems associated with legacy landfills. About 40–80% of the landfill mined waste is soil-like fraction (landfill mined soil like fraction, LMSF) that is obtained after removing the combustibles and recyclables. However, due to its lower calorific value and presence of organic content, LMSF has not found many applications except landfill cover material and filling of low-lying areas. In this context, the present study explores the strength properties of LMSF with and without fibres (coir and polypropylene fibres) through unconfined compression, direct shear, UU triaxial, cyclic UU triaxial, indirect tension and bending strength tests. Overall, the shear strength properties of LMSF are similar to sand with some cohesion. Further, the performance of LMSF with optimum fibre proportion (2% for coir fibres and 1.5% for polypropylene fibres) is quite superior under monotonic and cyclic compressive loading conditions, as well as indirect tension and flexural loading conditions. An assessment of 1 m replacement of soft soil with LMSF with/without fibre reinforcement infers substantial increase in the allowable bearing pressure of LMSF. Overall, though LMSF may need assessment/pre-treatment (as required) due to presence of organic content and any possible heavy metal concentration, utilization of LMSF would encourage the adoption of landfill mining process. The current research insights contribute in the direction of achieving sustainable infrastructure development by reduction in the landfill area that would reduce the depletion of conventionally utilized natural resources for fill applications such as sand and gravel, and contribute positively towards environment.

Evaluating quasi-static and fatigue performance of IN718 gyroid lattice structures fabricated via LPBF: Exploring relative densities

Additive manufacturing (AM) technology has unlocked the possibilities of fabricating complex structures of components such as lattice structures. Based on the literature review, the significance of the thickness effect on lattice structures has not been completely investigated yet. Therefore, this research aims to study the wall thickness effect on the mechanical performance of sheet-based lattice structures under quasi-static compression and fatigue loading conditions. For this aim, four geometries of lattice structures with designed relative densities of 12.5%, 25.0%, 37.5%, and 50.0% were fabricated via Laser Powder Bed Fusion (LPBF) process using Inconel alloy 718. The specimens were heat-treated to eliminate residual stresses and then tested under monotonic and cyclic compressive loading conditions to obtain the mechanical responses for each respective relative density. The experimental results from the quasi-static and fatigue compression tests on different geometries were compared and analyzed with the use of Scanning Electron Microscopy (SEM) to observe the fatigue failure locations and their dependency on the surface morphology of those failure locations. In addition, the actual relative densities were also calculated to evaluate the geometrical deviations and correct the experimental results. Overall, the experimental data revealed that there are strong associations between the lattices with various wall thicknesses and corresponding quasi-static and fatigue performance.

Correction to: Mechanical responses of Chrysopogon zizanioides roots under cyclic loading conditions

Correction to: Mechanical responses of Chrysopogon zizanioides roots under cyclic loading conditions

A hypoplastic model for cemented sand under monotonic and cyclic loading

In this study, a hypoplastic model is developed to describe the mechanical behaviors of cemented sand under both monotonic and cyclic loading conditions. A state variable is proposed to qualify the bonding strength, and it is incorporated into the model to reflect the influence of cementation on the strength, stiffness, and dilatancy of sand. To reflect the bonding degradation, this variable evolves during the shearing following a simple evolution rule and may vanish after large deformation. The critical void ratio and friction angle are related to the initial cemented content to consider the variation of the critical state induced by the cementation. The model is subsequently extended to account for cyclic loading by incorporating the intergranular strain, fabric change effect, and semifluidized state. The capability of the model is demonstrated by simulating the behavior of cemented sand under both monotonic and cyclic loading conditions.

Experimental assessment of the functional fatigue in biocompatible Ti67Zr19Nb11.5Sn2.5 shape memory alloy in the vicinity of drilled holes

Shape memory alloys (SMA) exhibit desirable and unique functionalities that are suitable for a plethora of applications in the automotive, aerospace, and biomedical industries. The TiNbZrSn SMA has been specifically developed for biomedical applications owing to its enhanced biocompatibility compared to the widely available and heavily investigated NiTi-based SMA. In this work, the effects of mechanical rolling, heat treatment, and grain size on the functional fatigue properties of TiNbZrSn are investigated under cyclic loading conditions. Superelasticity was observed for samples rolled to reduction levels exceeding 90 %. However, the recovery magnitudes improved significantly following 96.6 % rolling due to the optimization of the grain size. The reduction in grain size, from about 400 μm in the homogenized conditions to 50 μm for the 96.6 % rolled state, increases slip resistance which consequently promotes superelasticity. The functional superelastic properties were also evaluated for samples with geometric stress concentration (drilled hole). Using full-field measurement techniques, the applied, recoverable, irreversible strains, and their evolution under cyclic loading conditions were carefully measured. The use of virtual extensometers located in multiple locations around the geometric stress concentration provides means to not only evaluate the functional degradation of local superelastic properties, but also the transition to structural damage through the initiation and propagation of fatigue cracks.