Fatigue-resistant Materials Research Articles

Enhancing the fatigue property of high entropy alloy by regulating characteristics of grains and precipitates

High entropy alloys (HEAs) are a new class of alloys with many excellent mechanical properties, having the potential as structural materials. One advantage of HEAs is the vast space for designing compositions and microstructures, which provides more possibilities to develop fatigue-resistant materials. In this study, a multi-phase fine-grained microstructure was engineered through cold rolling and annealing treatment in Al0.3CoCrFeNi HEA to obtain an enhanced fatigue property. This alloy consists of FCC matrix phase with an average grain size of ∼ 1.13 ± 0.84 μm, and B2 and σ precipitates with sub-micron scale. Their phase fractions are ∼ 90 %, ∼ 9 % and ∼ 1 %, respectively. Stress-life (S-N) data indicate that fatigue endurance limit and ratio in terms of stress amplitude are ∼ 281 MPa and ∼ 0.3, based on stress ratio of 0.1. Good fatigue property is related to high ultimate tensile strength derived from good work-hardening ability and fine microstructure. In addition, activation of deformation twins, blunting of microcracks and deflection of fatigue cracks associated with precipitates are beneficial for enhancement of fatigue resistance. The above results provide theoretical guidance on how to enhance fatigue resistance of HEAs by regulating microstructures.

Compression fatigue properties of bioinspired nacre-like composites compared with natural nacre: Effects of architectures and orientations

Bioinspired architectures mimicking natural nacre demonstrate a remarkable efficiency in toughening materials, yet their fatigue properties remain to be explored, specifically regarding the effects of architectural types and orientations. Here the compression-compression fatigue properties of bioinspired ceramic-polymer composites with nacre-like lamellar and brick-and-mortar architectures along with natural nacre were investigated by cyclically loading along different orientations. The fatigue performance and damage mechanisms of these materials were revealed to be closely related to their architectural types and orientations. The nacre-like architectures played a role in hindering the propagation of fatigue cracks by inducing crack deflection, promoting the contact and mutual friction between cracking faces, and via the plastic deformation of polymer phase. This work may provide experimental basis for the application of nacre-like composites and give insights for designing new bioinspired fatigue-resistant materials.

Understanding the Fatigue Notch Sensitivity of High-Strength Steels through Fracture Toughness

This study presents an innovative approach for selecting high-strength materials for fatigue dimensioning parts, considering both fracture toughness and fatigue performance. Warm and hot forming processes enable the construction of high-strength parts above 1000 MPa with complex geometries, making them suitable for lightweight chassis in automotive and freight applications. This research reveals that high-strength steels can experience up to a 40% reduction in fatigue performance due to manufacturing defects introduced during punching and trimming. Fracture toughness has been proposed as a good indicator of notch sensitivity, with a strong correlation of 0.83 between fracture toughness and fatigue notch sensitivity. Therefore, by combining fracture toughness measurements and fatigue resistance obtained through the rapid fatigue test, it becomes possible to quickly identify the most fatigue-resistant materials to deal with defects. Among the nine materials analysed, warm-formed steels show promising characteristics for lightweight chassis construction, with high fatigue resistance and fracture toughness exceeding the proposed fracture threshold of 250 kJ/m2.

Bioinspired Cellular Single-Walled Carbon Nanotube Aerogels with Temperature-Invariant Elasticity and Fatigue Resistance for Potential Energy Dissipation

Elastic and fatigue-resistant materials with a wide service temperature range are extremely needed in the crash cushion, automobile safety, and personal protective equipment. By combining radial freeze casting with carbon welding, single-walled carbon nanotube aerogels (SWCNTAs) with cellular structures are fabricated from a facile, scalable sol–gel method. The prepared SWCNTAs show a negative Poisson’s ratio and high energy loss efficiency under the uniaxial compressive strain. Furthermore, the SWCNTAs exhibit both outstanding fatigue resistance with negligible plastic deformation even after 105 compressive cycles and superior elasticity ranging from −100 to 300 °C in air, making the aerogels viable candidates for energy dissipation in extreme environments.

Introduction of a New Parameter to Quantify the Fatigue Damage in Asphalt Mastics and Asphalt Binder

This study involves the quantification of fatigue damage in asphalt materials by introducing a new fatigue damage parameter denoted as the F parameter. One waste filler, i.e., red mud and an asphalt binder were chosen to blend the asphalt mastics at three filler contents of 10, 20, and 30% respectively with respect to the volume of binder and tested at temperatures of 5, 15, and 25 °C. The proposed parameter incorporates the effect of both peak shear stress as well as the failure strain, and hence, can better represent the fatigue damage. A lower value of F is recommended for a better fatigue resistant material. The F parameter was found increasing with the increment in filler content, which signifies higher degree of damage with a high level of stiffening. On the other hand, it consistently decreased with the increment in temperature. The behavior of the materials under the action of increasing shear strain was clearly justified by using the F parameter corresponding to different filler contents and the testing temperatures. In addition to that, the observations from the F parameter were also complemented by the fatigue diagrams. Hence, the proposed parameter is envisaged to be a promising fatigue damage indicator in future works.

Temperature-Invariant Superelastic and Fatigue Resistant Carbon Nanofiber Aerogels.

Superelastic and fatigue-resistant materials that can work over a wide temperature range are highly desired for diverse applications. A morphology-retained and scalable carbonization method is reported to thermally convert a structural biological material (i.e., bacterial cellulose) into graphitic carbon nanofiber aerogel by engineering the pyrolysis chemistry. The prepared carbon aerogel perfectly inherits the hierarchical structures of bacterial cellulose from macroscopic to microscopic scales, resulting in remarkable thermomechanical properties. In particular, it maintains superelasticity without plastic deformation even after 2 × 106 compressive cycles and exhibits exceptional temperature-invariant superelasticity and fatigue resistance over a wide temperature range at least from -100 to 500 °C. This aerogel shows unique advantages over polymeric foams, metallic foams, and ceramic foams in terms of thermomechanical stability and fatigue resistance, with the realization of scalable synthesis and the economic advantage of biological materials.

Stretchable and fatigue-resistant materials

In developing a material for a load-bearing application, attention inevitably falls on the resistance of the material to the growth of a crack, characterized by toughness under monotonic load, and by threshold under cyclic load. Many methods have been discovered to enhance toughness, but they do not enhance threshold. For example, stretch-induced crystallization and inorganic fillers have made the toughness of natural rubber well above 10000 J/m 2 , but have left the threshold of natural rubber around 50 J/m 2 . Here we describe a principle of stretchable and fatigue-resistant materials. To illustrate the principle, we embed unidirectional fibers of a soft and stretchable material in a matrix of a much softer and much more stretchable material, and adhere the fibers and the matrix by sparse and covalent interlinks. When the composite is cut with a crack and subject to a load, the soft matrix shears readily and delocalizes the high stretch of a fiber over a long segment. A threshold of 1290 J/m 2 is reached, below which the composite does not suffer any mode of failure (fiber break, kink crack, or matrix fracture). The principle of stretchable and fatigue-resistant materials is applicable to various materials, layouts, and methods of fabrication, opening an enormous design space for general applications.

Reduced-order microstructure-sensitive protocols to rank-order the transition fatigue resistance of polycrystalline microstructures

The transition fatigue regime between low cycle fatigue (LCF) and high cycle fatigue (HCF) is often addressed in the design and performance evaluation of load-bearing components used in many structural applications. Transition fatigue is characterized by elevated levels of local inelastic deformation in significant regions of the microstructure as compared to HCF. Typically, crystal plasticity finite element method (CPFEM) simulations are performed to model this phenomenon and to rank-order microstructures by their resistance to crack formation and early growth in the regime of transition fatigue. Unfortunately, these approaches require significant computational resources, inhibiting their use to explore novel materials for transition fatigue resistance. Reduced-order, microstructure-sensitive models are needed to accelerate the search for next-generation, fatigue-resistant materials. In a recent study, Paulson et al. (2018) extended the materials knowledge system (MKS) framework for rank-ordering the HCF resistance of polycrystalline microstructures. The efficacy of this approach lies in the reduced-dimensional representation of microstructures through 2-point spatial correlations and principal component analysis (PCA), in addition to the characterization of the HCF response with a small set of performance measures. In this work, these same protocols are critically evaluated for their applicability to rank-order the transition fatigue resistance of the same class of polycrystalline microstructures subjected to increased strain amplitudes. Success in this endeavor requires the formation of homogenization linkages that account for the significantly higher levels of local inelastic deformation and stress redistribution in transition fatigue as compared to HCF. A set of 12 α-titanium microstructures generated using the open access DREAM.3D software (Groeber and Jackson, 2014) are employed for this evaluation.

Fatigue behavior of high-entropy alloys: A review

Fatigue failures cost approximately 4% of the United States’ gross domestic product (GDP). The design of highly fatigue-resistant materials is always in demand. Different from conventional strategies of alloy design, high-entropy alloys (HEAs) are defined as materials with five or more principal elements, which could be solid solutions. This locally-disordered structure is expected to lead to unique fatigue-resistant properties. In this review, the studies of the fatigue behavior of HEAs during the last five years are summarized. The four-point-bending high-cycle fatigue coupled with statistical modelling, and the fatigue-crack-growth behavior of HEAs, are reviewed. The effects of sample defects and nanotwins-deformation mechanisms on four-point-bending high-cycle fatigue of HEAs are discussed in detail. The influence of stress ratio and temperature on fatigue-crack-growth characteristics of HEAs is also discussed. HEAs could exhibit comparable or greater fatigue properties, relative to conventional materials. Finally, the possible future work regarding the fatigue behavior of HEAs is suggested.

The influence of microstructure on the cyclic deformation and damage of copper and an oxide dispersion strengthened steel studied via in-situ micro-beam bending

Service materials are often designed for strength, ductility, or toughness, but neglect the effects of cyclic time-variable loads ultimately leading to macroscopic mechanical failure. Fatigue originates as local plasticity that can first only be observed on the micro scale at defects serving as stress concentrators such as inclusions or grain boundaries. Thus, a recently developed technique to perform in-situ observation of micro scale bending fatigue experiments was applied. Micro-beams fabricated from copper, single grained and ultrafine grained (ufg), and an oxide dispersion strengthened (ODS) steel were subject to cyclic deformation and subsequent damage. The elastic stiffness, yield strength, dissipated energy, and maximum stress were measured as a function of cycle number and plastic strain amplitude. From these properties, cyclic stress-strain curves were developed. Initial pronounced monotonic hardening and an increasing Bauschinger effect were observed in all samples with increasing strain amplitude. Cyclic stability was maintained until plastic strain amplitudes reached a critical value. At this point, dramatic cyclic softening and microcracking occurred. The critical strain amplitude was found to be approximately 5.4×10−3 for the copper with a refined grain structure and 1.2×10−2 for the steel specimen. Grain rotation and noticeable changes in sub-grain structure were evident in the ufg copper after a critical strain amplitude of εa=8.3×10−3. In-situ micro fatigue bending couples the cyclic evolution of bulk mechanical properties measurements with real-time electron microscopy analysis techniques of damage and failure mechanisms, which renders it a powerful method for developing novel fatigue resistant materials.

Additive manufacturing of fatigue resistant materials: Challenges and opportunities

This overview focuses on the current state of knowledge pertaining to the mechanical characteristics of metallic parts fabricated via additive manufacturing (AM), as well as the ongoing challenges and imminent opportunities in fabricating materials with increased fatigue resistance. Current experimental evidence suggests that the mechanical properties of laboratory AM specimens may not be representative of those associated with parts, due primarily to differences in geometry/size which influence the thermal histories experienced during fabrication, and consequently, microstructural features, surface roughness, and more. In addition, standards for mechanical testing methods, specimen design procedures, post-manufacturing treatments, etc., may need to be revised for AM parts. Standardizing the AM process may only be accomplished by strengthening the current understanding of the relationships among process parameters, thermal history, solidification, resultant microstructure, and mechanical behavior of the part. Having the ability to predict variations in mechanical behavior based on resultant microstructure, or matching the best conceivable properties of a part in accordance with the loading critical plane, are some possible solutions for making AM a more reliable means for producing functional parts. Developing microstructure-property models is arguably the first necessary step toward design optimization and the more efficient, accurate estimation of the structural integrity of AM parts.

Improvement of fatigue life by compliant and soft interlayers

Retardation of fatigue crack growth rate due to the introduction of thin, compliant and/or soft interlayers is investigated. The mechanism is the reduction of the crack driving force in the interlayer. Fatigue tests are conducted on composites made of high-strength aluminum alloy as matrix and technically pure aluminum or adhesive as interlayer material. The adhesive interlayer causes an increase in fatigue life by a factor 20 or more, whereas the aluminum interlayer yields only a moderate improvement. Numerical simulations based on the configurational force concept are utilized for understanding. The results show new possibilities for the design of fatigue-resistant materials.

Intrinsic tensile properties of cocoon silk fibres can be estimated by removing flaws through repeated tensile tests.

Silk fibres from silkworm cocoons have lower strength than spider silk and have received less attention as a source of high-performance fibres. In this work, we have used an innovative procedure to eliminate the flaws gradually of a single fibre specimen by retesting the unbroken portion of the fibre, after each fracture test. This was done multiple times so that the final test may provide the intrinsic fibre strength. During each retest, the fibre specimen began to yield once the failure load of the preceding test was exceeded. For each fibre specimen, a composite curve was constructed from multiple tests. The composite curves and analysis show that strengths of mass-produced Muga and Eri cocoon silk fibres increased from 446 to 618 MPa and from 337 to 452 MPa, respectively. Similarly, their toughness increased from 84 to 136 MJ m(-3) and from 61 to 104 MJ m(-3), respectively. Composite plots produced significantly less inter-specimen variations compared to values from single tests. The fibres with reduced flaws as a result of retests in the tested section have a tensile strength and toughness comparable to naturally spun dragline spider silk with a reported strength of 574 MPa and toughness of 91-158 MJ m(-3), which is used as a benchmark for developing high-performance fibres. This retesting approach is likely to provide useful insights into discrete flaw distributions and intrinsic mechanical properties of other fatigue-resistant materials.

The red-eared slider turtle carapace under fatigue loading: The effect of rib-suture arrangement.

Biological structures consisting of strong boney elements interconnected by compliant but tough collagenous sutures are abundantly found in skulls and shells of, among others, armadillos, alligators, turtles and more. In the turtle shell, a unique arrangement of alternating rigid (rib) and flexible (suture) elements gives rise to superior mechanical performance when subjected to low and high strain-rate loadings. However, the resistance to repeated load cycling - fatigue - of the turtle shell has yet to be examined. Such repeated loading could approximately simulate the consecutive high-stress bending loads exerted during (a predator) biting or clawing. In the present study flexural high-stress cyclic loads were applied to rib and suture specimens, taken from the top dorsal part of the red-eared slider turtle shell, termed carapace. Subsequently, to obtain a more complete and integrated fatigue behavior of the carapace, specimens containing a complex alternating rib-suture-rib-suture-rib configuration were tested as well. Although the sutures were found to be the least resistant to repeated loads, a synergistic effect was observed for the complex specimens, displaying improved fatigue durability compared to the individual (suture or even rib) constituents. This study may assist in the design of future high-stress fatigue-resistant materials incorporating complex assemblies of rigid and flexible elements.

A three-dimensional finite element solution of frictional wheel–rail rolling contact in elasto-plasticity

Wear, rolling contact fatigue, and plastic deformation are the major failure modes of railway wheels and rails. Proper analyses of the failure mechanisms as well as improvement in design and maintenance require an accurate evaluation of the stress and strain states. Solution of frictional rolling contact between wheel and rail in elasto-plasticity seems, however, still to lack in the literature. This paper presents a model for such a solution. A 3D finite element model is built up to simulate the rolling contact. The focus is on the tangential problem, namely the distributions of surface shear stress and micro-slip as well as the distinction of areas of adhesion and slip in the contact patch. With the presented model the assumptions of half space, linear elasticity and quasi-static or steady state, which are often employed in existing solution of rolling contact, are dropped. A bilinear elasto-plastic material with isotropic hardening is employed to examine the effects of plasticity on frictional rolling. It is found that when plastic flow occurs, the contact patch in the rail top changes from an ellipse into an egg shape; the cross influence between the normal and tangential contact problems become stronger: the normal solution is not independent of the tangential solution any more, and the tangential solution is greatly affected by the normal solution. The model can be applied to investigation of the relationships between material properties, plastic deformation, frictional work, wear, and crack initiation. Such relationships may help in better understanding the occurrence of corrugation, head checks, and squats, and may further be used for design of fatigue resistant materials and profiles of wheel and rail.

Microstructure-based multistage fatigue modeling of aluminum alloy 7075-T651

The multistage fatigue model for high cycle fatigue of a cast aluminum alloy developed by McDowell et al. is modified to consider the structure–property relations for cyclic damage and fatigue life of a high strength aluminum alloy 7075-T651 for aircraft structural applications. The multistage model was developed as a physically-based framework to evaluate sensitivity of fatigue response to various microstructural features to support materials process design and component-specific tailoring of fatigue resistant materials. In this work, the model is first generalized to evaluate both the high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes for multiaxial loading conditions, with appropriate modifications introduced for wrought materials. The particular microstructural features of relevance to fatigue in aluminum alloy 7075-T651 include micron-scale Fe-rich intermetallic particles and rolling textures. The model specifically addresses the role of local constrained cyclic microplasticity at fractured inclusions in fatigue crack incubation and microstructurally small crack growth, including the effect of crystallographic orientation on crack tip displacement as the driving force. The model is able to predict lower and upper bounds of the fatigue life based on measured inclusion sizes.

Performance in Resistance to Surface Fatigue for Cr3C2–25%NiCr Coatings by Plasma Spray and CDS Spray

Three Cr3C2–25%NiCr coatings, prepared by Air Plasma Spray (APS)-Ar/H2, APS-Ar/He and Continuous Detonation Spraying CDS, have been tested on a double-point-contact fatigue-test machine to examine their surface fatigue limit. Results show that the CDS coating exhibits the best performance in resistance to contact fatigue, while the other two coatings tested in our study are unacceptable as fatigue-resistant materials. On the basis of SEM observations, different failure mechanisms for the coatings are suggested. The fatigue limit of the Cr3C2–25%NiCr coatings decreases with the reduction of coating thickness, and the causes for the phenomenon have been discussed.

Tension–tension fatigue behavior of hydroxyapatite reinforced polyetheretherketone composites

Hydroxyapatite (HA) particle reinforced polyetheretherketone (PEEK) composites with 5, 10, 20, 30, and 40% (by volume) HA content were subjected to tension–tension fatigue under load-controlled mode. Results of quasi-static tensile tests indicated that the tensile stiffness of the HA/PEEK composites increases while their tensile strength decreases, with an increase in HA content, up to 40% (by volume). Stress-life (SN) and stiffness degradation data were obtained. The fact that all of the specimens survived cyclic load at 50% ultimate tensile strength (and below) and the high estimated fatigue strength at 1 million cycles suggest that HA/PEEK composite is a promising fatigue-resistant material for biomedical applications. Microscopic examination of cyclically loaded specimens revealed evidence of interfacial failure between the filler and the PEEK matrix, and subsequent initiation and propagation of micro cracks in the matrix that cause fatigue failure.

Fatigue Dissipation and Failure in Unidirectional and Angle-Ply Glass Fibre/Carbon Fibre Hybrid Laminates

The tensile fatigue behaviour of unidirectional 0degrees, [+/-10](4S) and [+/-45](4S) carbon fibre/glass fibre hybrid composite has been investigated. The dissipation was measured by the stiffness, hysteresis loss and temperature field of the specimen surface in an insulated testing chamber. The hysteresis loss correlates well with the increase temperature. Microscopic studies show frictional sliding of longitudinal crack faces between carbon and glass fibre bundles to be the main source of dissipation for on-axis specimens. With increasing off-axis angle the primary loss mechanism became cyclic shear deformation of the polymer matrix. With a finer dispersion of the constituents of the hybrid, the growth of these longitudinal cracks or of zones of inelastic matrix shear deformation. would be suppressed, which would result in a more fatigue resistant material. A localisation of heat generation sets in just prior to final failure. Damage and heat localisation lead to impending failure. If the parameters that control localisation were better understood, it would be possible to improve the fatigue resistance of the material by sensible microstructural design.

An Investigation into the Fatigue of a Hybrid Titanium Composite Laminate

Hybrid composite laminates consisting of layers of metal foils bonded together with fiber-reinforced resin show great promise as a fatigue-resistant material. The hybrid titanium composite laminate (HTCL) incorporates the mechanical advantages of existing hybrid composite laminates such as ARALL and GLARE while extending their applications to elevated temperatures. Titanium layers are bonded by prepreg layers of high-temperature resin reinforced with carbon fibers. The constituent materials used in this study are a metastable β titanium alloy (Ti-15-3), a high-temperature polyimide (LARC™-IAX), and an intermediate modulus carbon fiber (IM7). This study includes quasi-static and fatigue testing of HTCL materials at room temperature and 177°C (350°F). Experimental stress-strain response of HTCL laminates are compared with predicted results by a laminate analysis code called AGLPLY. A stress-based fatigue study was performed to determine the fatigue properties of HTCL laminates. The roles of residual stress in the mechanical behavior and fatigue properties of HTCL are addressed. The development of damage in HTCL specimens during fatigue is shown to include titanium ply cracking, interfacial debonding, and PMC layer failure. The influence of the fatigue properties of titanium layers on the fatigue of HTCL is discussed. The performance of HTCL laminates in fatigue is shown superior to that of the monolithic titanium alloy for room-temperature and elevated-temperature conditions.