Fatigue resistance of aligned carbon nanotube arrays under cyclic compression

Structural components subject to cyclic stress can succumb to fatigue, causing them to fail at stress levels much lower than if they were under static mechanical loading. However, despite extensive research into the mechanical properties of carbon nanotube structures for more than a decade, data on the fatigue behaviour of such devices have never been reported. We show that under repeated high compressive strains, long, vertically aligned multiwalled nanotubes exhibit viscoelastic behaviour similar to that observed in soft-tissue membranes. Under compressive cyclic loading, the mechanical response of the nanotube arrays shows preconditioning, characteristic viscoelasticity-induced hysteresis, nonlinear elasticity and stress relaxation, and large deformations. Furthermore, no fatigue failure is observed at high strain amplitudes up to half a million cycles. This combination of soft-tissue-like behaviour and outstanding fatigue resistance suggests that properly engineered nanotube structures could mimic artificial tissues, and that their good electrical conductivity could lead to their use as compliant electrical contacts in a variety of applications.     

Effect of fatigue loading on the bond behaviour between UHM CFRP plates and steel plates

Extensive research has been conducted on static bond behaviour between CFRP and steel. However, very limited research is available on the effect of fatigue loading on the bond behaviour between CFRP and steel. This paper attempts to fill the knowledge gap in this area. A series of static and fatigue tests on UHM (ultra high modulus) CFRP plate and steel plate double strap joints were conducted. Five specimens were tensioned to failure under static loading as control specimens. The other 12 specimens were tested under fatigue loading with load ratios ranging from 0.2 to 0.6 (defined as the ratio of the maximum fatigue load to the average static bond strength of control specimens). After going through pre-set number of fatigue cycles, the specimens were tensioned to failure under static loading. The failure modes, residual bond strength and residual bond stiffness of such specimens were compared with those of control specimens, to facilitate the investigation of the effect of fatigue loading on the bond behaviour. Microscopic investigation was also performed to reveal the underlying failure mechanism.     

The fracture of wood under torsional loading

A series of experiments have been carried out on hardwood (red lauan) and softwood (sitka spruce) test pieces using static and cyclic torsional loading under displacement control. Measurements of the applied torque, the corresponding angle of twist and the number of cycles to failure were recorded. It was found that under static torsional loading, the strength of both hardwood and softwood reduced as the grain orientation of the sample to the axis of twist increased from 0° to 90° with a corresponding decrease of elastic modulus. Hardwood is stronger than softwood. In the fatigue test, when the torsional load is plotted against cycle number, the results showed that under displacement control stress relaxation occurs. The S–N curve for softwood has a shallower gradient than that of hardwood, indicating that the torsional strength of softwood is less affected by fatigue loading than hardwood. In both static and cyclic torsional loading tests, the failure mode of hardwood is slow and incomplete, whereas, softwood fails suddenly and completely. The crack growth is along the tangential direction in the hardwood cross-section and in the radial direction in the cross-section.     

Fatigue and damage accumulation in open cell aluminium foams

Open‐cell aluminium foams are a relatively new material with interesting uses in different engineering applications. This study investigates the fatigue behaviour and damage accumulation of metal foams via a fatigue analysis (Weibull E‐N model), a failure criterion (the relation among the prepeak compressive and tensile slopes, the reduction in the tensile stress, or the reduction in the compressive stress), and a mathematical approach (linear, quadratic, or exponential). As a result of combining the 3 mathematical approaches and 3 failure criteria, different approaches are obtained, analysed, and validated by using experimental data. Finally, the proposed approaches can be used to directly obtain the damage accumulation level for open‐cell metal foams under fully reversed cyclic loading as a function of the number of cycles applied, the total strain amplitude, and the initial damage accumulation condition.     

Tensile fatigue of conventional and negative Poisson’s ratio open cell PU foams

In this paper, we present a comparative analysis between the cyclic loading tensile behaviour of conventional and auxetic thermoplastic PU foams. While the two types of foam share the same base material (open cell PU–PE), one batch is transformed into an auxetic one (i.e., negative Poisson’s ratio) using a special manufacturing process involving moulding and exposure to particular temperature profiles to stabilise the transformation of the microstructure. The effect of the stiffness degradation and accumulation of energy dissipation versus the number of cycles are discussed for different loading levels r. The results show that the fatigue behaviour until failure, subjected to cyclic loading depends on the loading levels and occur in three stages. The results obtained shows also that the auxetic foam have enhanced characteristics under static loading and tensile fatigue compared to the conventional parent phase form.     

Mechanical properties of open-cell foam synthetic thoracic vertebrae

This study presents comprehensive morphological and mechanical properties (static, dynamic) of open-cell rigid foams (Pacific Research Laboratories Inc. Vashon, WA) and a synthetic vertebral body derived from each of the foams. Synthetic vertebrae were comprised of a cylindrical open-cell foam core enclosed by a fiberglass resin cortex. The open-cell rigid foam was shown to have similar morphology and porosity as human vertebral cancellous bone, and exhibited a crush or fracture consolidation band typical of open-celled materials and cancellous bone. However, the foam material density was 40% lower than natural cancellous bone resulting in a lower compressive apparent strength and apparent modulus in comparison to human bone. During cyclic, mean compression fatigue tests, the synthetic vertebrae exhibited an initial apparent modulus, progressive modulus reduction, strain accumulation and S-N curve behaviour similar to human and animal vertebral cancellous bone. Synthetic open-cell foam vertebrae offer researchers an alternative to human vertebral bone for static and dynamic biomechanical experiments, including studies examining the effects of cement injection. Presented, in part, at the XXth Congress of the International Society of Biomechanics and 29th Annual Meeting of the American Society of Biomechanics, Cleveland, OH, July 31-August 5, 2005     

Experimental investigation of the fatigue strength of plain concrete under high compressive loading

Fatigue tests are conducted on plain concrete cylinders subjected to axial cyclic compression. The upper level of the cyclic stress ranges between 60% and 90% of the static compressive strength. A method based on volume strain measurements is used to predict the individual static strength of each tested sample. The results of fatigue tests are presented in a load versus cycles to failure diagram. The recorded values of the longitudinal strains are, plotted in terms of number of cycles. The test figures are discussed assuming that the failure is a result of both time and cycle dependent damage. For loading levels ranging up to 80% of the static strength, the number of cycles to failure shows hardly any variation with the effects of time. Above this level, the fatigue strength proves to be more sensitive to time dependent effects, as the level of loading increases.     

Polypropylene foam behaviour under dynamic loadings: Strain rate,density and microstructure effects

Expanded polypropylene foams (EPP) can be used to absorb shock energy. The performance of these foams has to be studied as a function of several parameters such as density, microstructure and also the strain rate imposed during dynamic loading. The compressive stress–strain behaviour of these foams has been investigated over a wide range of engineering strain rates from 0.01 to 1500 s⁻¹ in order to demonstrate the effects of foam density and strain rate on the initial collapse stress and the hardening modulus in the post-yield plateau region. A flywheel apparatus has been used for intermediate strain rates of about 200 s⁻¹ and higher strain rate compression tests were performed using a viscoelastic Split Hopkinson Pressure Bar apparatus (SHPB), with nylon bars, at strain rates around 1500 s⁻¹ EPP foams of various densities from 34 to 150 kg m⁻³ were considered and microstructural aspects were examined using two particular foams. Finally, in order to assess the contribution of the gas trapped in the closed cells of the foams, compression tests in a fluid chamber at quasi-static and dynamic loading velocities were performed.     

Fatigue damage and hysteresis in wood-epoxy laminates

Wood-epoxy laminates were subjected to constant amplitude fatigue tests in tension-tension (R = 0.1), compression-compression (R = 10) and reverse loading (R = –1) in order to follow property changes and fatigue damage accumulation. Hysteresis loops were captured during these tests and the form of stress versus number of cycles to failure (S-N) curves was established. Reversed loading is the most damaging mode of cyclic stress application. In terms of static strengths, the wood laminate is weaker in compression than in tension. However at low levels of stress, following many fatigue cycles, the fatigue life is greater in compression-compression than in tension-tension. The shape of captured hysteresis loops is strongly influenced by loading mode. As subcritical damage develops, loop area increases and dynamic modulus falls. In reversed loading, loop bending and distortion is observed depending on whether the damage is tension- or compression-dominated or both. Maximum and minimum fatigue strains, the dynamic modulus and loop area have been plotted as a function of the number of fatigue cycles. The majority of damage occurs towards the end of the sample life but property changes can be detected throughout fatigue tests. Normalisation of fatigue data demonstrates that the fatigue behaviour of wood-epoxy laminates is consistent.     

关于残余应力的静载松弛与最佳喷丸残余应力场的研究

本文研究了喷丸残余应力场在疲劳加载初期的静载松弛现象及机理,并对最佳喷丸残余应力场进行了探讨。试验结界表明,残余应力在疲劳过程中的静载松弛是工件表层材料剧烈塑性变形的结果。残余应力的静载松弛会使晶界、相界等障碍物处形成一定数量的微裂纹,给工作表层材料带来损伤,降低疲劳裂纹的形核寿命。为了避免疲劳初期由于残余应力的静载松弛所造成的损伤,在喷丸后采用应力松弛低温回火工艺,预先降低残余应力场中的最大残余压应力值,建立最佳残余应力场。这种通过热激活的方式使残余应力发生的松弛属子非损伤性松弛,因而能够有效地提高材料在S—N曲线上的较高交变应力区的疲劳寿命。     

Creep–Fatigue Interaction of Inconel 718 Manufactured by Electron Beam Melting

Electron beam melting of Ni-base superalloy Inconel 718 allows producing a columnar-grained microstructure with a pronounced texture, which offers exceptional resistance against high-temperature loading with severe creep–fatigue interaction arising in components of aircraft jet engines. This study considers the deformation, damage, and lifetime behavior of electron-beam-melted Inconel 718 under in-phase thermomechanical fatigue loading with varying amounts of creep–fatigue interaction. Strain-controlled thermomechanical fatigue tests with equal-ramp cycles, slow–fast cycles, and dwell time cycles are conducted in the temperature range from 300 to 650 °C. Results show that both dwell time and slow–fast cycles promote intergranular cracking, gradual tensile stress relaxation, as well as precipitate dissolution and coarsening giving rise to cyclic softening. The interplay of these mechanisms leads to increased lifetimes in both dwell time and slow–fast tests compared to equal ramp tests at higher strain amplitudes. Conversely, at lower mechanical strain amplitudes, the opposite is observed. A comparison with results of conventional Inconel 718 indicates that the electron-beam-melted material exhibits superior resistance against strain-controlled loading at elevated temperatures such as thermomechanical fatigue.     

Behaviour of RC beams strengthened by partial jacketing under cyclic loading

This work presents a study on the behaviour under cyclic loading of reinforced concrete beams strengthened in bending by the addition of concrete and steel on their tension side using expansion bolts as shear connectors, denominated here as partial jacketing. The experimental program included tests on six full scale reinforced concrete beams, simply supported, with rectangular cross section 150 mm wide and 400 mm high, span of 4,000 mm and total length of 4,500 mm. All the beams, after receiving two cycles of static loading in order to create a pre-cracking condition, were strengthened in bending by partial jacketing and then subjected to cyclic loading until the completion of 2 × 10⁶ cycles or the occurrence of fatigue failure. Following the cyclic loading, the beams that did not fail by fatigue were subjected to a static load up to failure. The main variables were the flexural reinforcement ratio in the beam and in the jacket, the beam–jacket interface condition (smooth or rough) and cyclic load amplitude. On the basis of the obtained test results and the results of previous study of similar beams tested only under static loading, the behaviour of the strengthened beams is discussed and a proposal for their design is given.     

Influence of the matrix constituent on mode I and mode II delamination toughness in fiber-reinforced polymer composites under cyclic fatigue

Mode I and mode II fracture behaviour under static and dynamic loading was analyzed in two composites made up of the same reinforcement though embedded in two different matrices. Specifically, the delamination energy under static and dynamic loading was obtained for both materials and both fracture modes, i.e. the number of cycles necessary for the onset of fatigue delamination. Subsequently, the crack growth rate (delamination rate) was obtained for different percentages of the critical energy rate. The main goal of the study was to ascertain the influence of the matrix on the behaviour of the laminate under fatigue loading.From the experimental results for the onset of delamination, similar fatigue behaviour was observed at a low number of cycles for both matrices and both fracture modes, while in fatigue at a high number of cycles, a higher fatigue limit was obtained in the composite with the modified resin (higher toughness) for both fracture modes. From the point of view of crack growth rate, both materials behaved similarly for different levels of stress under fatigue and the two fracture modes for small crack lengths (initial growth zone < 5 mm), although the growth rate increased for large crack lengths. This behaviour was the same in both loading modes.     

Compressive Fatigue Behavior of Bovine Cancellous Bone and Bone Analogous Materials under Multi‐Step Loading Conditions

In this study the compressive cyclic behavior of bovine cancellous bone and three open‐cell metallic foams including AlSi7Mg foams (30 and 45 ppi) and CuSn12Ni2 foam (30 ppi) has been investigated. Multi‐step fatigue tests are carried out to study the deformation behavior under increasing compressive cyclic stresses. Short multi‐step tests, with steps of 300–500 cycles, are used to identify the cyclic yield stress (σ_cy) and the stress at failure (σ_fail). The residual strain accumulation, or cyclic creep, is observed during these tests. Long multi‐step tests, with 5000 cycles at selected stress ranges (0.4σ_cy, 0.6σ_cy, 0.8σ_cy, and σ_cy), are also carried out to study further the compressive fatigue behavior of the materials. Scanning electron microscopy (SEM) has been used to characterize the microstructure of the foams and the bone prior to and post mechanical testing. Particular attention is paid to the role of cyclic creep and buckling in the failure processes. The results show that residual strain accumulation seems to be the predominant driving force leading to failure of foams and bones. Although foams and bone fail by the same mechanism of cyclic creep, the deformation behavior at the transient region of each step is different for both materials. The maximum strain ε_max of foams decrease suddenly during the change of each step. This behavior may be explained by the rapidly developing microdamage in the cell struts that occur at the transient region of each step. Bones show more gradual decrease of ε_max, where microdamage may be accumulated progressively during the fatigue test.     

Effects of mechanical properties of concrete constituents including active mineral admixtures on fatigue behaviours of high performance concrete

In order to explore the links between the macroscopic and microstructural characteristics of concrete with admixtures of active mineral additions, four series of concrete prisms, of mortar matrix prisms and mortar-aggregate Interfacial Transition Zone (ITZ) are prepared and tested under monotonic and cyclic loads. Five static mechanical parameters (compressive and bending strength, fracture energy, elastic modulus, Poisson ratio) and bending fatigue performance (fatigue life, critical maximum displacement and strain, fatigue damage) of such materials are experimentally evaluated. The results show that degradation laws of concrete properties under both monotonic and cyclic loads vary with the different cohesive strength ratio and elastic modulus ratio of ITZ and mortar matrix. The single or double additions of ground slag and fly ash with optimized mass fractions remarkably enhance the static and bending fatigue properties as well as change the failure mechanisms of concrete.     

Zwillingsbildung bei der thermischen Ermüdung der Mg‐Basislegierung AZ31

Twinning at thermal fatigue of magnesium alloy AZ31 In this paper results of thermal fatigue tests of the magnesium base alloy AZ31 carried out in a temperature range between ‐50 °C and +290 °C are presented. Specimens were loaded under constant total strain and uniaxial homogeneous stresses. The resulting materials behaviour is described by stress amplitudes, plastic strain amplitudes and mean stresses as a function of the number of thermal loading cycles. It is well known that AZ31 shows different stress‐strain behaviour during tensile and compressive loading resp. at lower temperatures due to the fact that mechanical twinning depends on the loading direction. However untwinning processes may occur during unloading and reloading in the opposite direction. As a consequence, during the first thermal loading cycles, typical consequences of the formation and the dissolution of twins are observed. The interaction of deformation, recovery and recrystallization processes, characteristic for individual temperature ranges are discussed in detail to analyze the damage progress during thermal fatigue.     

Relaxation of residual stress in shot peened Udimet 720Li under high temperature isothermal fatigue

The extent of residual stress relaxation in turbine disc material Udimet 720Li was measured using laboratory X-rays with the sin²ψ technique, for fatigue samples as a function of temperature and number of fatigue cycles for strain controlled loading to 1.2%. Results showed that extensive relaxation occurs upon the initial fatigue cycle. The maximum compressive residual stress (RS) parallel to the loading direction is found to decrease by 50% for all testing temperatures. The extent of relaxation upon further cycling increased with temperature. In the plastically deformed near surface region, the diffraction peak width decreased with increasing testing temperature and number of fatigue cycles (and exposure time), indicating that the relaxation of cold work is controlled by both thermal and mechanical processes.     

Fitting stress relaxation experiments with fractional Zener model to predict high frequency moduli of polymeric acoustic foams

This paper presents a time domain method to determine viscoelastic properties of open-cell foams on a wide frequency range. This method is based on the adjustment of the stress–time relationship, obtained from relaxation tests on polymeric foams’ samples under static compression, with the four fractional derivatives Zener model. The experimental relaxation function, well described by the Mittag–Leffler function, allows for straightforward prediction of the frequency-dependence of complex modulus of polyurethane foams. To show the feasibility of this approach, complex shear moduli of the same foams were measured in the frequency range between 0.1 and 16 Hz and at different temperatures between ?20 °C and 20 °C. A curve was reconstructed on the reduced frequency range (0.1 Hz–1 MHz) using the time–temperature superposition principle. Very good agreement was obtained between experimental complex moduli values and the fractional Zener model predictions. The proposed time domain method may constitute an improved alternative to resonant and non-resonant techniques often used for dynamic characterization of polymers for the determination of viscoelastic moduli on a broad frequency range.     

Cyclic fatigue of hot isostatically pressed silicon nitride at elevated temperatures

Cyclic fatigue properties of a hot isostatically pressed silicon nitride were investigated at 1150, 1260 and 1370 °C in ambient air. The uniaxial tensile tests were conducted under various cyclic loading wave forms and frequencies. The correlation of stress-life relations between cyclic and static fatigue results was evaluated. At 1150–1370 °C, cyclic loading caused less damage than static loading, as evidenced by the longer failure time under cyclic loading versus static loading with the same maximum applied stresses. The cyclic loading effect was more pronounced in high frequency tests at 1260 and 1370 °C and might be related to the viscoelastic behaviour of the intergranular phase. Microstructural analyses and macroscopic cyclic stress-strain and strain-time relations indicated that cyclic loading/unloading may inhibit the normal accumulation of creep damage.     

循环荷载作用对CFRP筋力学性能的影响

应用于桥梁工程的碳纤维增强复合材料（CFRP）筋通常受循环荷载作用,导致其力学性能的退化,这将影响桥梁结构的受力。为了考察循环荷载作用对CFRP筋力学性能的影响,首先利用静载试验对CFRP筋的初始力学性能进行了检测,然后结合静载试验结果并利用疲劳试验考察了各阶段循环荷载作用对CFRP筋弹性模量、松弛及抗拉强度的影响。研究结果表明,极限拉力作用下CFRP筋的弹性模量较初始状态高约5%,对于CFRP筋构件的变形对结构内力有较大影响的结构,在设计阶段建议适当地考虑CFRP材料的非线性问题;桥梁工程中,经200万次正常使用设计循环荷载作用的CFRP筋,其弹性模量和松弛性能较初始状态未发生明显退化,经应力幅为4.3%的极限抗拉强度的循环荷载作用后,CFRP筋的抗拉强度提高了1.2%,但应力幅提高至7%的极限抗拉强度时,其抗拉强度与初始状态相比无明显变化。