Effect of the loading frequency on the compressive fatigue behavior of plain and fiber reinforced concrete

This paper presents the recent experimental results aimed at disclosing the loading frequency effect on the fatigue behavior of a plain concrete and two types of fiber-reinforced concrete, using polypropylene and steel fibers. Compressive fatigue tests were conducted on 123 cubic specimens (100 mm in edge length). Four different loading frequencies, 4 Hz, 1 Hz, 1/4 Hz and 1/16 Hz, were employed. The maximum stress applied on the specimen was 85% of its compressive strength and the stress ratio was kept constant as 0.3. The results show that the loading frequency effect on the fatigue behavior of the plain concrete is pronounced. The fatigue life (the number of cycles to failure) at lower frequencies is less than that at higher frequencies. However, the fibers do improve the fatigue behavior significantly under low loading frequencies. Such trend can be attributed to the effectiveness of the fibers in bridging cracks, and thus inhibiting the crack extension under cyclic loads.     

On the strengthening effect of increasing cycling frequency on fatigue behavior of some polymers and their composites: Experiments and modeling

Effect of cycling frequency on fatigue behavior of neat, talc filled, and short glass fiber reinforced injection molded polymer composites was investigated by conducting load-controlled fatigue tests at several stress ratios (R = −1, 0.1, and 0.3) and at several temperatures (T = 23, 85 and 120 °C). A beneficial or strengthening effect of increasing frequency was observed for some of the studied materials, before self-heating became dominant at higher frequencies. A reduction in loss tangent (viscoelastic damping factor), width of hysteresis loop, and displacement amplitude, measured in load-controlled fatigue tests, was observed by increasing frequency for frequency sensitive materials. Reduction in loss tangent was also observed for frequency sensitive materials in DMA tests. It was concluded that the fatigue behavior is also time-dependent for frequency sensitive materials. A Larson–Miller type parameter was used to correlate experimental fatigue data and relate stress amplitude, frequency, cycles to failure, and temperature together. An analytical fatigue life estimation model was also used to consider the strengthening effect of frequency in addition to mean stress, fiber orientation, and temperature effects on fatigue life.     

Increase of bending fatigue resistance for tungsten inert gas welded SS400 steel plates using friction stir processing

To improve the fatigue resistance of tungsten inert gas (TIG)-welded SS400 steel plates, friction stir processing (FSP) was performed on TIG weld beads. Although the tensile properties of the TIG-welded steel plates with FSP were similar to those without FSP, their bending strength exhibited about 1.4 GPa at room temperature, which was 40% higher than that without FSP (about 1 GPa). Similarly, FSP produced about 170% increase in the number of cycles to failure at an applied stress amplitude of 270 MPa during three-point bending fatigue at room temperature. A fine-grained FSP region (grain sizes of about 1–2 μm in diameter) enhanced grain-boundary strengthening, leading to the higher bending strength and bending fatigue resistance.     

Elevated temperature low cycle fatigue of grey cast iron used for automotive brake discs

This paper evaluates the fatigue life properties of low carbon grey cast iron (EN-GJL-250), which is widely used for automotive brake discs. Although several authors have examined mechanical and fatigue properties at room temperatures, there has been a lack of such data regarding brake discs operating temperatures. The tension, compression and low cycle fatigue properties were examined at room temperature (RT) and at brake discs’ working temperatures: 500 °C, 600 °C and 700 °C. The microstructure of the material was documented and analysed. Tensile stress–strain curves, cyclic hardening/softening curves, stress–strain hysteresis loops, and fatigue life curves were obtained for all the above-mentioned temperatures. It was concluded, that Young’s modulus is comparable with both tension and compression, but yield its strength and ultimate strength are approximately twice as great in compression than in tension. All the mechanical properties remained quite stable until 500 °C, where at 700 °C all deteriorated drastically. During fatigue testing, the samples endured at 500 °C on average at around 50% of cycles at room temperature. Similar to other materials’ properties, the cycles to failure have dropped significantly at 700 °C.     

Tension–compression fatigue of a SiC/SiC ceramic matrix composite at 1200 °C in air and in steam

Tension–compression fatigue behavior of a non-oxide ceramic composite with a multilayered matrix was investigated at 1200 °C in laboratory air and in steam. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had pyrolytic carbon fiber coating with boron carbide overlay applied. Tension–compression fatigue behavior was studied for fatigue stresses ranging from 80 to 200 MPa at a frequency of 1.0 Hz. Presence of steam significantly degraded the fatigue performance. Specimens that achieved fatigue run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Constant and variable amplitude fatigue testing of aluminum alloy 2024-T351 with ultrasonic and servo-hydraulic equipment

Constant amplitude (CA) and variable amplitude (VA) fatigue lifetimes of the aluminum alloy 2024-T351 were measured with servo-hydraulic (8–70 Hz) and ultrasonic testing equipment (20 kHz) at positive load ratios. Experiments in the high cycle fatigue regime served to identify influences of frequency and testing method on lifetimes. CA tests showed similar numbers of cycles to failure for both methods. Ultrasonic tests were performed in pulsed mode. In ultrasonic VA tests vibration amplitude of successive pulses of 2000 cycles length is varied. Servo-hydraulic VA tests are performed by varying the load of successive blocks. Servo-hydraulic VA tests with block length 2000 cycles delivered lifetimes similar to the ultrasonic tests. No frequency effect is found in CA and VA tests. Cracks are preferentially initiated at secondary phase particles at both frequencies. Lifetimes in servo-hydraulic VA tests are reduced when block length is decreased from 2000 to 200, 20 and single load cycles. Varying the load for each successive cycle at 50 Hz is realized with a feed-forward optimization of control parameters. Lifetimes differ by a factor 6 for different block lengths indicating a strong load sequence effect.     

Off-axis tensile fatigue assessment based on residual strength for the unidirectional 45° carbon fiber-reinforced composite at room temperature

The tensile fatigue behavior of unidirectional carbon fiber-reinforced thermoplastic and thermosetting laminates was examined at room temperature. Tension-tension cyclic fatigue tests were conducted under load control at a sinusoidal frequency of 10 Hz to obtain stress-fracture cycles (S-N) relationship. The fatigue limits of carbon fiber-reinforced thermoplastic laminates (CF/PA6) and thermosetting laminates (CF/Epoxy) were found to be 28.0 MPa (48% of the tensile strength) and 56.2 MPa (63% of the tensile strength), respectively. Two types (in constant and incremental loading way) of loading-unloading low cycle fatigue tests were employed to investigate the modulus history of fatigue process for announcing the fatigue mechanism. The residual tensile strength of specimens that survived fatigue loading maintained with the increase of fatigue cycles and applied stress. Examination of the fatigue-loaded specimens revealed that the more flexible/ductile trend of resins and the formation of micro-cracks at the interface between fiber and matrix was facilitated during high fatigue loading (⩾fatigue limit stress), while no interfacial/matrix damage in resins was detected during low fatigue loading (<fatigue limit stress), which was consider to be the governing mechanism of strength maintain during fatigue loading.     

Fatigue properties and fracture analysis of a SiC fiber-reinforced titanium matrix composite

Tension–tension fatigue properties of SiC fiber reinforced Ti–6Al–4V matrix composite (SiC_f/Ti–6Al–4V) at room temperature were investigated. Fatigue tests were conducted under a load-controlled mode with a stress ratio 0.1 and a frequency 10 Hz under a maximum applied stress ranging from 600 to 1200 MPa. The relationship between the applied stress and fatigue life was determined and fracture surfaces were examined to study the fatigue damage and fracture failure mechanisms using SEM. The results show that, the fatigue life of the SiC_f/Ti–6Al–4V composite decreases substantially in proportion to the increase in maximum applied stress. Moreover, in the medium and high life range, the relationship between the maximum applied stress and cycles to failure in the semi-logarithmic system could be fitted as a linear equation: S_max/μ = 1.381 − 0.152 × lgN_f. Fractographic analysis revealed that fatigue fracture surfaces consist of a fatigued region and a fast fracture region. The fraction of the fatigued region with respect to the total fracture surface decreases with the increase of the applied maximum stresses.     

Tensile and fatigue behavior of superelastic shape memory rods

The tensile and fatigue behavior of superelastic shape memory alloy (SMA) bars heat-treated at three different temperatures were examined. Low cycle fatigue tests at variable load rates were carried out to determine the effect of stress and frequency on residual strain and energy dissipation in a fatigue cycle. The mechanism of energy dissipation was studied by monitoring the temperature changes in the fatigued samples as a function of applied stress and frequency of testing. Results from the tensile tests revealed that the stress for the Austenite to Martensite transformation decreased from 408 MPa to 204 MPa with an increase in temperature of heat treatment from 300 to 450 °C. The ultimate strength of the SMA increased from 952 MPa to 1115 MPa when the heat treatment temperature was increased from 300 to 450 °C. Fatigue testing prior to conducting the tensile test decreased the ultimate strength of the SMA and also reduced the failure strain. The energy dissipation in fatigue tests was found to decrease as test frequency increased from 0.025 Hz to 0.25 Hz and the change in sample temperature during the test at the lower test frequency was found to be considerably higher than at the higher frequency.     

The effects of fatigue and residual strain on the mechanical behavior of poly(ethylene terephthalate) unreinforced and nanocomposite fibers

Poly(ethylene terephthalate) (PET) control fibers (nominal diameter ～24 ± 3 μm) and PET fibers with embedded vapor-grown carbon nanofibers (PET-VGCNF) (nominal diameter ～25 ± 2 μm) were exposed to cyclic loading and monotonic tensile tests. The control fibers were processed through a typical melt-blending technique and the PET-VGCNF samples were processed with approximately 5 wt.% carbon nanofibers present in the sample. Under uniaxial fatigue conditions, the fibers were subjected to a maximum stress that was approximately 60% of the fracture stress of the sample at an elongation rate of 10 mm/min in uniaxial tension. The fibers were subjected to a frequency of 5 Hz. Subsequent to non-fracture fatigue conditions, the fibers were tested under uniaxial stress conditions for observation of the change in mechanical properties to assess the effects of fatigue loading. The elastic modulus, hardening modulus, fracture strength, work done, and yield strain of both PET control and PET-VGCNF samples in uniaxial tension subsequent to fatigue were shown to be dependent on the residual fatigue strains. Relative mechanical properties were used to quantify the difference in PET and PET-VGCNF samples as a function of residual strain. In most cases, the results indicated a strengthening mechanism (strain hardening effect) in the low residual strain limit for fatigued PET samples and not for fatigued PET-VGCNF samples. In comparison with the unreinforced PET sample, the PET-VGCNF fibers showed greater degradation of mechanical properties as a function of residual strain due to fatigue when cycled at 60% of the fracture stress. The effects of the fatigue process on the change in mechanical properties have been quantified and supported through existing qualitative, quantitative, and scanning electron microscopy (SEM) techniques.     

Ultrasonic fatigue of a single crystal Ni-base superalloy at 1000 °C

An ultrasonic fatigue testing system capable of operating at temperatures up to 1000 °C has been developed and utilized to study the fatigue behavior of a single crystal superalloy (PWA 1484) at a temperature of 1000 °C and loading frequency of approximately 20 kHz. The stress-life data generated from the ultrasonic testing system were comparable to those from conventional servo-hydraulic fatigue tests for similar single crystal alloys. Interior Ta-rich carbides were the major microstructural feature responsible for crack initiation in the alloy. Crack growth under ultrasonic loading frequency at 1000 °C for PWA 1484 occurred in a crystallographic manner on {1 1 1} octahedral slip planes, in contrast to the normal Mode-I growth mode typically observed for single crystal superalloys at high temperature (>850 °C) with conventional servo-hydraulic loading frequencies (<100 Hz).     

Fatigue of 2024-T351 aluminium alloy at different load ratios up to 1010 cycles

Fatigue properties of 2024-T351 aluminium alloy are investigated in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. Endurance tests are performed with ultrasonic equipment at 20 kHz cycling frequency at load ratios of R = −1, R = 0.1 and R = 0.5 up to 10¹⁰ cycles. Additional servo-hydraulic tests between 8 and 10 Hz at R = 0.1 show no frequency influence on fatigue lifetimes. Linear lines in double logarithmic S–N plots are used to approximate data. Slope exponents of approximation lines increase with increasing numbers of cycles for all load ratios. Failures above 5 × 10⁹ cycles (R = −1 and R = 0.1) or 10¹⁰ cycles (R = 0.5) occur, and no fatigue limit is found. Fatigue cracks leading to failures above 10⁹ cycles are initiated at the surface or slightly below at broken constituent particles or at agglomerations of fractured particles, which are probably Al₇Cu₂(Fe, Mn). Specimens stressed with more than 10¹⁰ cycles at R = −1 without failure show several cracks starting at constituent particles. Maximum crack lengths are 30 μm, which is considerably below grain size.     

Crack growth in discontinuous glass fibre reinforced polypropylene under dynamic and static loading conditions

Crack propagation in single edge notched tensile specimens of isotactic polypropylene reinforced with short E-glass fibres has been investigated under both fatigue and creep loading conditions. Fatigue crack propagation (FCP) experiments have been performed at three different frequencies (0.1, 1, 10 Hz) and at a mean applied tensile load of 1200 N. Isothermal creep crack propagation (CCP) tests have been conducted under a constant tensile applied load of 1200 N at various temperatures in the range from 32 to 60 °C. Analysis of FCP data allowed an estimation of the pure fatigue and pure creep components of the crack velocity under the adopted cyclic loading conditions. Crack growth at low frequencies (0.1 and 1 Hz) is mainly associated with a non-isothermal creep process. At higher frequency (10 Hz), the pure fatigue contribution appeared more pronounced. Finally, the comparison of FCP and CCP as a function of the mean applied stress intensity factor confirmed the major contribution of creep crack growth during FCP process at low frequencies.     

Fatigue failure investigation on anti-vibration springs

Fatigue failure investigation on anti-vibration springs, involving both metal and rubber materials, is presented. Rubber-to-metal bonded springs are widely used in industry as anti-vibration components giving many years of service. Recently a need to improve time and cost efficiencies has caused an unexpected early fatigue failure of the component with no immediate explanation. The required total fatigue life was 1.25 million cycles but only 0.7 million cycles achieved. There was an urgent need to investigate the causes of the fatigue failure and to modify the component design accordingly to meet the customer requirement and the supply schedule.The investigation, based on the actual fatigue loads, is carried out on these failed and modified products using a method of continuum mechanics. To simplify the simulation, a non-linear quasi-static analysis is carried out and then the residual stresses are superimposed to obtain the effective stress range to predict the metal crack initiation. For the rubber parts a three-dimensional effective stress criterion is employed to predict the fatigue crack initiation. The fatigue failure is taken as visual crack observation (normally 1–2 mm).The fatigue crack initiation for the metal parts of the failed component is predicted at 225 K cycles under specified fatigue load against total metal broken at 700 K cycles from the test. For the modified part the minimum total fatigue life for the metal parts of the component, estimated conservatively, is 2.1 million cycles against 1.75 million cycles from the test without any crack observed. The rubber fatigue crack initiation is predicted at 90 K cycles against crack onset around 79 K cycles and crack length 40 mm at 145 K cycles from the test. From design point of view it is important to optimize the rubber profile under this very tight allowable space to provide the maximum support of the metal interleaves and at the same time to meet the minimum requirements of the manufacture process. It is shown that this approach can be employed at a design stage for both metal and rubber fatigue evaluations on anti-vibration springs.     

Influence of the laminate lay-up on the fatigue behaviour of SiC-fibre/BMAS-matrix composites

The influence of the laminate lay-up on the mechanical response and the damage development during fatigue loading of SiC fibre toughened BMAS glass-ceramic matrix composites was studied at room temperature. Uni-directional, cross-plied, angle-plied, and quasi-isotropic laminates were investigated. The different materials survived 10⁶ fatigue cycles for maximum fatigue stresses below a stress level dependent on the laminate lay-up. The thus defined fatigue limit decreased from 375 MPa for uni-directional to 70 MPa for angle-plied SiC/BMAS. Finally, the fatigue damage mechanisms were identified and the mechanical response related to the fatigue damage.     

Environmental effects on fatigue behavior of adhesively-bonded pultruded structural joints

The fatigue response of adhesively-bonded pultruded GFRP double-lap joints has been investigated under different environmental conditions. Tests were performed at ?35 °C, 23 °C and 40 °C. A fourth set of fatigue data was collected from tests on preconditioned specimens in warm (40 °C) water. The tests were performed at 40 °C and at 90% relative humidity. Specimens were instrumented with strain and crack gages to record fatigue data. In addition to the S–N curves, stiffness fluctuations and crack initiation and propagation during fatigue were monitored. The dominant failure mode was a fiber-tear failure that occurred in the mat layers of the GFRP laminates. In the presence of high humidity, the failure shifted to the adhesive/composite interface. Although the testing temperature was lower than the glass transition temperature of the adhesive, its influence on the fatigue life and fracture behavior of the examined joints was apparent and was aggravated by the presence of humidity.     

Damping characteristics of carbon nanotube reinforced aluminum composite

A multi-walled carbon nanotube (MWNTs) reinforced 2024Al composite was successfully fabricated by a procedure of mixing 2024Al powders and CNTs, cold isostatic press and hot extrusion. The damping behaviors of the composite were investigated with frequency of 0.5, 1.0, 5.0, 10, 30 Hz, at a temperature of 25–400 °C. The experimental results show that the frequency significantly affects the damping capacity of the composite when the temperature is above 230 °C; meanwhile, the damping capacity of the composite with a frequency of 0.5 Hz reaches 975 × 10^− 3, and the storage modulus is 82.3 GPa when the temperature is 400 °C, which shows that CNTs are a promising reinforcement for metal matrix composites to obtain high damping capabilities at an elevated temperature without sacrificing the mechanical strength and stiffness of a metal matrix.     

Frequency effect and influence of testing technique on the fatigue behaviour of quenched and tempered steel and aluminium alloy

Influences of testing technique and frequency on the fatigue behaviour of 50CrMo4 and EN AW-5083 were investigated. To clarify the effect of test frequency on the fatigue behaviour, tests with 20 kHz and f < 400 Hz were performed. The frequency effect can be caused by temperature, environment and strain rate. For the aluminium alloy, the influence of environment is responsible for the dependence of fatigue lifetime on the frequency. The fatigue lifetime of the steel showed in both environments similar frequency dependency, i.e. the strain rate is assumed to be responsible for the differences in fatigue lifetime.     

Fatigue strength of a single crystal in the gigacycle regime

The objective of this work is to investigate the fatigue behavior of a single crystal nickel-based superalloy in the gigacycle regime. Testing from 10⁶ to 10⁹ cycles at 593 °C was performed using an ultrasonic fatigue system operating at 20 kHz. Multiple tests were performed at stresses near the fatigue limit to determine the variability in fatigue life in this regime. Endurance limit results were compared to similar data generated on conventional servohydraulic test systems to determine if there are any frequency effects. Scanning electron microscopy was then used to determine the initiation sites and the failure mechanisms. Initial results indicate little or no frequency effect on the fatigue strength or failure mechanisms of PWA 1484 at 593 °C.     

Dislocation-based interpretation on the effect of the loading frequency on the fatigue properties of JIS S15C low carbon steel

Fatigue properties of low carbon steels are known to be particularly sensitive to the loading frequency. Indeed, literatures related to this field usually point out an increasing fatigue life with an increase of the loading frequency. The authors of the present paper have already reconfirmed such a general phenomenon in the case of JIS S15C (0.15%C) low carbon steel. In that paper, S–N properties under usual frequencies of 0.2–140 Hz can be successfully normalized by the lower yield stress at the individual frequency. Nevertheless, some irregularities have been detected on the fatigue property at 20 kHz. In order to clarify the physical meaning of such irregularities, we will compare fatigue properties at usual frequencies and ultrasonic frequency.In this work, the former experimental results were reintroduced and new discussions were developed by performing additional experiments and analyses paying an attention to microstructure and dislocation structure. Thus, it was found that the loading frequency effect at ultrasonic frequency is due to a particular behavior of B.C.C. ferrite under high strain rate. Such a behavior causes strain inhomogeneities at grain boundaries, and then facilitates the intergranular crack initiation mode rather than the usual intragranular one often reported at lower loading frequencies. Longer ultrasonic fatigue lives at ultrasonic frequency are directly related to this transition of the crack initiation mode. In addition, effects of the pearlitic volume fraction on the fatigue behavior have been also discussed in the present work.