Compressive Fatigue Research Articles

Mean Stress Correction in Low Cycle Fatigue Life of High Strength Steels

Fatigue strength of welded joints varies depending on the shape, filler metal, welding procedure, and post-treatment method. In addition, mean stress affects the fatigue behavior in terms of stress ratio. Typically, it is well known that tensile mean stress tends to decrease fatigue life while compressive mean stress increase fatigue life. There are various methods to incorporate the mean stress effect in fatigue strength, but there are relatively few correction methods in compressive mean stress, especially in low cycle fatigue regime.</br>In this study, a load-based fatigue test was conducted to evaluate the effect of mean stress on the welded joints of High Yield strength steel(HY Steel) with T-joint shape. The effect of the mean stress on the welded part of HY Steel with a T-joint shape was investigated using stress-based mean stress correction methods and strain-based mean stress correction methods. When the effective mean stress approach, which can correct compressive mean stress and tensile mean stress, is applied to HY Steel with a T-joint shape, it shows accurate correction result than stress-based mean stress correction method and strain-based mean stress correction method. Sensitivity parameters to consider mean stress for HY steel are presented.

Fatigue properties of SiC graded ceramic lattice structures with a triply periodic minimal surface manufactured by laser powder bed fusion

SiC graded ceramic lattice structure (GCLS) offers superior mechanical strength and its impact on fatigue warrants investigation. In this work, SiC triply periodic minimal surface (TPMS) GCLSs were prepared via laser powder bed fusion technology and liquid silicon infiltration process, alongside SiC TPMS uniform ceramic lattice structure (UCLS) as a reference. Fatigue properties, fatigue failure, and strengthening mechanisms were systematically investigated through compression fatigue tests, finite element (FE) simulations, and theoretical analysis. Fatigue failures of SiC UCLSs and GCLSs are influenced by cyclic ratcheting and fatigue damage, with cyclic ratcheting dominance. The fatigue strength ratios of UCLS and GCLS are 0.7 and 0.74, indicating that GCLSs exhibit stronger deformation resistance and superior fatigue performance. FE results show that surface tensile stress level in GCLS is lower than in UCLS, resulting in slower fatigue crack initiation. Stress redistribution and higher crack thresholds contribute to enhanced fatigue resistance in SiC TPMS GCLSs.

Fatigue and monotonically-loaded reinforced concrete specimens subjected to partial area loading

The degradation of concrete structures is often due to the continuous application of compressive fatigue loads. Understanding of concrete behaviour in different environmental conditions and under especial fatigue loading arrangements, particularly in cases where the loaded area is smaller than loaded element, herein referred to as partially loaded area, is of utmost importance to make a realistic prediction of the concrete fatigue life for a reliable design of civil engineering infrastructure. This paper focuses on investigating the fatigue performance of reinforced concrete specimens subjected to partially loaded areas, considering the influences of steel fibres, reinforcement arrangement, and the environmental factor of water submergence. A typical example of a partially loaded concrete component subjected to fatigue loading is circular spread footings used as concrete foundations for typical wind turbines. In these types of elements, the compressive strength of the concrete can be enhanced due to the confinement provided by way of lateral restraint stemming from surrounding concrete and reinforcement. However, design codes primarily address static loading and lack specific criteria for fatigue loading in these partially loaded regions. To address this research gap, results from a comprehensive experimental study, which in total included 111 concrete specimens subjected to monotonic loading and 25 specimens tested under cyclic loading, were used to study this problem. The investigation aimed to evaluate the effects of steel fibres and reinforcement arrangement on both static and fatigue capacity. Additionally, the impact of submerging concrete structures in water on their fatigue performance was examined, revealing a decrease in their fatigue life. The experimental results demonstrated that the presence of steel fibres and splitting reinforcement significantly increased both static and fatigue capacity. Most current design codes neglect the environmental factor of water submergence, leading to reduced fatigue capacity in concrete structures. Consequently, the validity of the design codes DNV-OS-C502 and fib Model Code 2010 was evaluated, and a new environmental factor for partially loaded areas subjected to cyclic loading is proposed. To evaluate the experimental results and understand the mechanical response, nonlinear finite element analyses were conducted using a smeared-crack continuum modelling procedure coupled with high cycle fatigue material models to assess its applicability for estimating fatigue performance of partially loaded reinforced concrete elements. It was found that a plane stress modelling procedure was capable of estimating fatigue capacity for dry concrete components, however, overestimated the fatigue capacity of wet concrete components.

Study on the fatigue life and toughness of recycled aggregate concrete based on basalt fiber

This paper studies the influence of basalt fibers on the mechanical properties, fatigue life, and toughness of Mechanically Recycled Aggregate Concrete (MRAC). Basalt fibers of varying volume fractions and lengths, along with quantified amounts of mineral powder and fly ash, are incorporated into MRAC to prepare Basalt Fiber Mechanically Recycled Aggregate Concrete (BFMRAC). The results of basic mechanical tests and fatigue tests showed that mineral powder and fly ash reduce cement consumption, achieve the effect of green construction, and basalt fibers improve the mechanical properties and toughness of concrete. The results of the response surface method (RSM) model indicate that the volume fraction of basalt fibers has a greater influence on the mechanical properties of BFMRAC compared to fiber length. The fatigue test results indicate that under different fiber volume fractions and lengths, the uniaxial compressive fatigue life (N) of specimens for each mix proportion is consistently improved. When the length of basalt fibers is 12 mm and the volume fraction is 0.3 %, the uniaxial compressive fatigue life of BFMRAC reaches its maximum (231638), which is twice that of MRAC. When the survival rate (P) is set to 0.5, with a basalt fibers volume content of 0.3 % and a length of 12 mm, the fatigue limit strength (Se) attains its maximum (0.8648 fc). When stress levels (S) reach 0.75 and the basalt fibers length is 12 mm, the relative CI Intensifying Coefficient achieves its maximum (1.133), resulting in optimal fatigue toughness. When the basalt fibers length is 6–12 mm, Se generally increases with the increase of fiber length, and the enhancement effect is best at 12 mm.

Magnetic Hyperporous Elastic Material with Excellent Fatigue Resistance and Oil Retention for Oil-Water Separation.

Oily wastewater has caused serious threats to the environment; thus, high-performance absorbing materials for effective oil-water separation technology have attracted increasing attention. Herein, we develop a magnetic, hydrophobic, and lipophilic hyperporous elastic material (HEM) templated by high internal phase emulsions (HIPE), in which free-radical polymerization of butyl acrylate (BA) and divinylbenzene (DVB) is employed in the presence of poly(dimethylsiloxane) (PDMS), lecithin surfactant, and modified Fe3O4 nanoparticles. The adoption of the emulsion template with nanoparticles as both stabilizers and cross-linkers endows the HEM with biomimetic hierarchical open-cell micropores and elastic cross-linked networks, generating an oil absorbent with outstanding mechanical stability. Compressive fatigue resistance of the HEM is demonstrated to endure 2000 mechanical cycles without plastic deformation or strength degradation. By exploiting the synergistic effect of hierarchical structures and low-surface-energy components, the resulting HEM also possesses excellent and robust hydrophobicity (water contact angle of 164°) and good oil absorption capacity, in which Fe3O4 nanoparticles lead to convenient magnetically controlled oil recyclability as well. Notably, the unique biomimetic microporous structure demonstrates superior oil retention capacity (>95% at 1000 rpm and >60% at 10,000 rpm) over the state-of-the-art porous materials for a diverse variety of oils to reduce the risk of secondary oil leakage, along with good recoverability by squeezing owing to the excellent compression resilience. These excellent performances of our HEM provide broad prospects for practical applications in oil-water separation, energy conversion, and smart soft robotics.

Strengthening mechanism of abnormally enhanced fatigue ductility of gradient nanostructured 316L stainless steel

Structural materials with a good combination of high fatigue strength and high fatigue ductility are highly desirable in engineering applications. However, it remains a challenge to “break” the exclusive relationship between fatigue strength and fatigue ductility. The cylindrical 316L stainless steel specimens with a gradient nanostructured surface layer were prepared using surface mechanical rolling treatment (SMRT). Fully reversed strain-controlled tension–compression fatigue experiments indicated that the SMRT processed 316L specimens achieved simultaneous improvement of fatigue strength and fatigue ductility. The abnormal increase in fatigue ductility is attributed to the significant secondary cyclic hardening caused by the martensitic phase transformation and the transition in fatigue crack initiation mode. The martensitic phase transformation enhances the fatigue strength while reducing the proportion of plastic deformation in the controlled strain amplitude. The competition between the driving force and resistance of fatigue nucleation in different structural units results in the migration of fatigue crack initiation sites from the surface to subsurface, thereby prolonging the fatigue crack initiation life. This work reveals the synergistic strengthening mechanism of fatigue strength and fatigue ductility of a gradient nanostructured 316L stainless steel.

Fatigue damage evolution of modified recycled aggregate concrete

Basalt fiber (BF) and nano-silica (NS) are used to improve the properties of recycled aggregate concrete (RAC) and produce modified RAC (MRAC). The fatigue failure patterns of the MRAC were observed through a uniaxial compression fatigue test, and the reasons for the different patterns were explained. The effects of the stress level, replacement rate, and loading frequency on the fatigue life were investigated, and the effects of BF and NS on the fatigue strain, damage evolution, and energy dissipation evolution of MRAC were analyzed. The enhancement mechanism of BF and NS on RAC was revealed using microstructural analysis techniques, such as scanning electron microscopy (SEM) and computed tomography (CT). The results showed that the fatigue failure modes of the MRAC could be divided into split, shear, and mixed failures. The fatigue life and fatigue strain of RAC exhibited a pattern of initially increasing and subsequently decreasing with an increase in the BF content (from 0 kg m−3 to 3 kg m−3 and then to 6 kg m−3). At stress level of 0.80, the average fatigue life of the specimens increased by 76.15 % compared to RAC when the BF doping was 3 kg m−3; however, when the BF doping was increased to 6 kg m−3, the average fatigue life of the specimens increased by only 47.68 % compared to RAC. The mixed BF and NS improved the fatigue life of RAC more than the single mixed BF or NS. Fatigue equations and unified single logarithmic fatigue equations, including fiber parameters, were established to accurately describe the relationship between the fatigue stress level, fatigue life, and failure probability. The fatigue strain gradually decreases with increasing NS content (0–2 %); with increasing loading frequency (from 1 Hz to 10 Hz and then to 15 Hz), the fatigue strain gradually decreases. The fatigue life of the MRAC decreased dramatically at low loading frequencies (1 Hz) and then steadily increased as the loading frequency increased. BF and NS have an improving effect on the porosity of microstructure, improve the fatigue life of RAC, and BF exhibits a more significant improvement than NS. A composite fatigue damage model that comprehensively considers the contribution rates of NS and BF was established, and the fatigue damage evolution of the MRAC was predicted.

Simulation of fatigue crack initiation life under compressive loading based on material meso-characteristics

The fatigue problem under compressive loading is common in the key components of aeroengine. Most of the fatigue life prediction methods studied in the past are macroscopic phenomenological methods based on a large amount of experimental data. Due to the difficulty in carrying out the fatigue test under compressive loading, the S-N curve or Manson-Coffin formula commonly used on the fatigue life prediction of material under compressive stress were obtained through fatigue tests under conditions where the average stress was greater than or equal to zero. Although the average stress can be corrected by some formulas, whether the life estimation formulas fitted by those fatigue test data are applicable to the fatigue life analysis with average stress less than zero has not been solved in engineering. And there were few studies on the influence of compressive stress on fatigue life at home and abroad. On the basis of Tanaka-Mura fatigue crack initiation theory, a mesoscopic model suitable for fatigue characteristic analysis under compressive loading was established in this article. The model verified the feasibility in predicting the fatigue initiation life using test data of 7075-T651 aluminum alloy under the compressive fatigue load.

High-strength self-healing hydrogels based on boronate-diol complexation cross-linking with good bioavailability

In this report, new self-healing hydrogels were obtained through simple one-pot free radical polymerization of acrylamide, phenylboronic acid, and 2-hydroxyethyl acrylate as copolymer monomers, potassium persulfate (KPS) as initiator, and tetramethylethylenediamine (TEMED) as promoter under low-temperature alkaline (pH = 9) conditions. The boronic acid group of phenylboronic acid (PBA) can interact with the hydroxyl group of hydroxyethyl acrylate (HEA) to form dynamic boronic ester bonds instead of the conventional bis acrylate crosslinkers. The effect of PBA content on the mechanical strength of the hydrogels was systematically investigated. The hydrogels exhibited higher stress, elongation, and self-healing properties due to the synergistic effect of H-bonds and boronate-diol complexation. Further studies revealed that the A–H–0.2–P–0.08 hydrogels exhibited excellent resistance to compression fatigue and also showed good biocompatibility through in vitro cytotoxicity tests.

A fatigue damage model of asphalt mixture considering tensile and compressive modulus decay

In practical asphalt pavement engineering, asphalt mixture experiences both tensile and compressive stresses, and the stress state plays a pivotal role in determining fatigue performance. This study thoroughly explores the fatigue damage characteristics of asphalt mixture, specifically emphasizing a meticulous analysis of the differences between tensile and compressive moduli. Initially, the compressive and tensile moduli of the asphalt mixture were calculated based on the dual modulus theory. Subsequently, differences in tensile and compressive moduli, along with fatigue performance characteristics, were evaluated through indirect tensile tests. Ultimately, a comprehensive fatigue damage model was proposed to ascertain the critical damage degree of the asphalt mixture, integrating both modulus decay and fatigue damage. The findings reveal that decay rates for the tensile modulus were higher than those observed for the compressive modulus. The tensile damage should be considered more in the design of pavement structures. Upon comparing the critical damage degree determined using the traditional modulus decay model with the comprehensive fatigue damage model, it can be deduced that the latter shows favorable applicability. The compressive fatigue modulus can better characterize the fatigue damage behavior. The model proposed in this study integrates modulus decay and fatigue damage information, offering a more comprehensive depiction of asphalt mixture damage behavior during fatigue processes.

Jute/PP composites for covering honeycomb panels: Designability and mechanical behaviors

An approach involving low energy consumption was used to transform jute/polypropylene prepregs into a sustainable sandwich structure for automotive application. Fiber orientation and stacking sequence were modified to investigate their influence on the physical-mechanical properties of the prepregs as well as cyclic flexural properties, compression fatigue behaviors and 3D microstructure of honeycomb sandwich panels. The mechanical properties of the face sheets were comparable with GF-SMC; but since they are derived from natural fiber they present a more sustainable and renewable alternative to GF-SMC. Stacking sequence and fiber orientation were found remarkably factors for the limitation of structure and performance design. The honeycomb panels occurred fatigue failures under the compression cyclic loading for 131–547 times. The flexural and shear buckling modes correlated with each other and all the materials failed in a combination of face wrinkling, partial crushing and debonding. It was found that the prepregs could provide more possibilities for optimum structural design of honeycomb sandwich panels, benefiting for the applications in the automotive lightweight field.

Chloride transport in high-cycle fatigue-damaged concrete

This paper explored the penetration of chloride into concrete damaged by high-cycle compressive fatigue loading. Two environmental conditions including immersion and wetting-drying cycles were prepared for experimentally investigating the transport performance of chloride in fatigue-damaged concrete. Additionally, a prediction model was developed with the damage index of residual strain. Test results revealed that both apparent diffusion coefficients and contents of chloride at each depth increased in fatigue-damaged concrete, indicating that the fatigue damage significantly degraded the resistance of concrete to chloride penetration. Furthermore, for fatigue damaged concretes with extremely different fatigue cycles and load levels but the same residual strain, the chloride ion content profiles were highly overlapped, which demonstrated the reasonability of selecting residual strains as fatigue damage indexes. The applicability of the transport model was firstly validated with the measured chloride contents and then the chloride transport behavior in gradient fatigue-damaged concrete was investigated numerically using the validated model. The numerical results indicated that the chloride transport was more sensitive to the residual strain at the exposed edge than to the residual curvature and the effect of the later can be neglected.

Wet-Stable Lamellar Wood Sponge with High Elasticity and Fatigue Resistance Enabled by Chemical Cross-Linking.

The excessive consumption of fossil-based plastics and the associated environmental concerns motivate the increasing exploitation of sustainable biomass-based materials for advanced applications. Natural wood-derived lamellar wood sponges via a top-down approach have recently attracted significant attention; however, the insufficient compressive fatigue resistance and lack of structural stability in water limit their wide applications. Here, we report a facile chemical cross-linking strategy to tackle these challenges, by which the cellulose fibrils in the lamellas are covalently bridged to enhance their connectivity. The cross-linked wood sponges demonstrate high compressibility up to 70% strain and exceptional compressive fatigue resistance (∼5% plastic deformation after 10,000 cycles at 50% strain). The interfibrillar cross-linking inhibits the swelling of cellulose fibrils and preserves the arch-shaped lamellas of the sponge in water, endowing the wood sponge with excellent wet stability. Such highly elastic and wet-stable lamellar wood sponges offer a sustainable alternative to synthetic polymer-based sponges used in diverse applications.

Nonwoven Reinforced Photocurable Poly(glycerol sebacate)-Based Hydrogels.

Implantable hydrogels should ideally possess mechanical properties matched to the surrounding tissues to enable adequate mechanical function while regeneration occurs. This can be challenging, especially when degradable systems with a high water content and hydrolysable chemical bonds are required in anatomical sites under constant mechanical stimulation, e.g., a foot ulcer cavity. In these circumstances, the design of hydrogel composites is a promising strategy for providing controlled structural features and macroscopic properties over time. To explore this strategy, the synthesis of a new photocurable elastomeric polymer, poly(glycerol-co-sebacic acid-co-lactic acid-co-polyethylene glycol) acrylate (PGSLPA), is investigated, along with its processing into UV-cured hydrogels, electrospun nonwovens and fibre-reinforced variants, without the need for a high temperature curing step or the use of hazardous solvents. The mechanical properties of bioresorbable PGSLPA hydrogels were studied with and without electrospun nonwoven reinforcement and with varied layered configurations, aiming to determine the effects of the microstructure on the bulk compressive strength and elasticity. The nonwoven reinforced PGSLPA hydrogels exhibited a 60% increase in compressive strength and an 80% increase in elastic moduli compared to the fibre-free PGSLPA samples. The mechanical properties of the fibre-reinforced hydrogels could also be modulated by altering the layering arrangement of the nonwoven and hydrogel phase. The nanofibre-reinforced PGSLPA hydrogels also exhibited good elastic recovery, as evidenced by the hysteresis in compression fatigue stress-strain evaluations showing a return to the original dimensions.

Experimental study on axial compression fatigue of grouted connection in jacket offshore wind turbines

The grouted connection (GC) of the offshore wind turbine (OWT) jacket foundation mainly bears axial loads, the wind and wave load in the working environment make the structure prone to fatigue problems. The construction environment of the OWT foundation is relatively harsh, so there will be vertical and horizontal errors during the construction process. In this paper, the fatigue test of 9 specimens were designed to analyze the influence of vertical and horizontal errors, water ingression and other factors on the axial fatigue performance of the GC section. The static ultimate bearing capacity test was carried out to determine the load ranges of the fatigue test. Results of fatigue tests indicate that the fatigue performance of the GC considerably degenerates with the load eccentricity, especially at the end sections of the inner and outer tubes. Water ingression has a significant negative influence on the fatigue performance of the GC section. The presence of water contributes to the propagation and development of cracks in the grout layer. Load range, vertical and horizontal errors and grout material have negligible effect on the fatigue performance of the structure.

Antibacterial, Fatigue-Resistant, and Self-Healing Dressing from Natural-Based Composite Hydrogels for Infected Wound Healing.

The treatment of infected wounds faces substantial challenges due to the high incidence and serious infection-related complications. Natural-based hydrogel dressings with favorable antibacterial properties and strong applicability are urgently needed. Herein, we developed a composite hydrogel by constructing multiple networks and loading ciprofloxacin for infected wound healing. The hydrogel was synthesized via a Schiff base reaction between carboxymethyl chitosan and oxidized sodium alginate, followed by the polymerization of the acrylamide monomer. The resultant hydrogel dressing possessed a good self-healing ability, considerable compression strength, and reliable compression fatigue resistance. In vitro assessment showed that the composite hydrogel effectively eliminated bacteria and exhibited an excellent biocompatibility. In a model of Staphylococcus aureus-infected full-thickness wounds, wound healing was significantly accelerated without scars through the composite hydrogel by reducing wound inflammation. Overall, this study opens up a new way for developing multifunctional hydrogel wound dressings to treat wound infections.

Static-dynamic damage mechanism and self-heating effect of a clean elastic polyurethane grouting material for trenchless rehabilitation under high stresses

Compared with traditional excavation rehabilitation, trenchless grouting rehabilitation technology has the advantages of fast, precise, and low-carbon, which has a broad application prospect in wind turbine foundation rehabilitation. In this work, a non-toxic, environment-friendly, fast-setting, high-strength, and high-toughness elastic polyurethane (PU) grouting material was proposed, and the static-dynamic damage mechanism and self-heating effect under high stress were studied by DMA, SEM, static-dynamic compression, and self-heating monitoring tests. The results show that the static compressive properties increased in a power function relationship with increasing strain rate. The dynamic compression fatigue process can be divided into adjustment stage (Stage I), viscoelastic development stage (Stage II) and failure stage (Stage III) or cyclic hardening stage (Stage III*). At 41.57 MPa and 36.95 MPa, fatigue damage parameters increased slowly in Stage I and Stage II, and sharply in Stage III. The stiffness damage, loss factor, cumulative strain, and strain rate increased by more than 100% in Stage III. Fatigue failure dominated by vertical/slanting through-cracks, shed debris, drum deformation, etc. was closely related to temperature and loading. The failure temperature about 59.1 °C, which was higher than the glass transition temperature. At 32.33 MPa, fatigue damage parameters changed slightly and then stabilized, showing exponential relationships with cycles. The max temperatures were less than 15 °C. The slight temperature rise facilitated the adjustment of structure and cyclic hardening under cyclic loading. This work can guide the further engineering application and fatigue durability evaluation of PU grouting materials.

Effects of PVA fibers and multi-walled carbon nanotubes reinforcement on uniaxial compression fatigue properties of Engineered geopolymer composites

As a new type of green and environmentally friendly cementing material, engineered geopolymer composites (EGC) have broad application prospects. In this study, uniaxial compression fatigue tests were conducted to investigate the fatigue properties of geopolymer material. A total of 137 samples were tested to investigate the effects of carbon nanotubes content, polyvinyl alcohol (PVA) fibers volume content, and maximum stress level on the fatigue life and deformation of geopolymers. The results showed that the fatigue life of geopolymers obeys Weibull distribution, and PVA fibers and multi-walled carbon nanotubes (MWCNTs) achieved multiscale enhancement of the fatigue life by up to 16.34 times. The total strain accumulation, residual strain, and fatigue stiffness of the geopolymers under fatigue loading exhibited a three-stage evolution rule, with stability within the secondary creep branch. In the process of EGC production, the innovative adoption of optimized MWCNTs surface modification and dispersion strategies improved the uniformity of MWCNTs distribution in the matrix. Scanning electron microscopy (SEM) confirmed the hybridization of MWCNTs and PVA fibers at the microscopic level, which can achieve multi-scale enhancement of geopolymers. This study successfully optimized the content of MWCNTs and PVA fibers, further expanding the application range of composite materials.

Flexural fatigue properties of concrete based on different replacement percentage of natural sand with manufactured sand

Compression and flexural fatigue tests were conducted on concrete specimens with varying strengths under different replacement percentages of natural river sand (R-sand) with manufactured sand (M-sand) at 0%, 30%, 70%, and 100%. The objective was to compare and analyse differences in fatigue damage of concrete across different replacement percentages. Findings were made that when the replacement percentage of M-sand was 30% and 70%, the concrete exhibited higher fatigue life, irrespective of any variations in compressive strength. The fatigue life of concrete with a compressive strength of 30 MPa could be increased by an average of 20%–30% compared to concrete with a compressive strength of 50 MPa, and this increasing trend reached its peak when the replacement percentage of M-sand was in the range of 30%–70%. In order to enhance the prediction of fatigue life for concrete subjected to varying replacement percentages of natural sand with M-sand and to elucidate the evolution process of fatigue damage in M-sand concrete, dynamic strain gauges and DIC methods were employed. This enabled determination of the cycle-varying maximum strain and residual strain during the fatigue process across different replacement percentages. The findings indicate that the addition of M-sand can significantly delay the degradation of dynamic elastic modulus (E), with this effect being enhanced by over 15%. The fatigue damage was defined based on fatigue deformation modulus; therefore, a new fatigue damage model was established. It was observed that this new model improved accuracy by up to 6.1% compared to the traditional fitted model in this test. Finally, the effects of different replacement percentages of natural sand with M-sand on the brittle transition of concrete were analysed at the microscopic level. This analysis involved a combination of microhardness testing and examination of the interfacial transition zone and hydride map of concrete under a scanning electron microscope across different replacement percentages of natural sand with M-sand.

Compression fatigue of elastomeric foams used in midsoles of running shoes

Due to their excellent specific mechanical properties, closed cell elastomeric foams are the main element in the soles of running shoes to absorb repetitive shocks from strides and to release a maximum of the absorbed energy. However, these cellular materials are gradually damaged. To enhance their mechanical durability by slowing their damage kinetics, it is critical to understand their mechanical behaviour in fatigue. The objective of this work is thus to clarify the link between the microstructure and the fatigue properties of five commercial elastomeric foams used in the midsoles of running shoes. The 3D cellular structures of each foam were finely analysed using X-ray microtomography. Foam samples were then subjected to cyclic compression which were close to running conditions. During cycling, samples exhibited a rapid densification associated with noticeable decreases of (i) the stress levels required to deform them as well as (ii) the cushioning and (iii) the rebound properties. We show that the two midsoles filled with micro-sized mineral fillers present the highest specific mechanical properties during the first compression cycle and during fatigue. However, their damage kinetics and rebound properties could probably be improved by tuning the fillers-matrix compatibility. The lightest foam, being very porous and presenting process-induced tortuous cell walls, is the best cushioning system but exhibits high damage kinetics. The densest foam presents poor specific mechanical properties, but very slow damage kinetics. Its double hierarchical architecture probably prevents the occurrence of micro-cracks in the cell walls.