Fatigue Behaviour of High-Performance Green Epoxy Biocomposite Laminates Reinforced by Optimized Long Sisal Fibers.

In recent decades, in order to replace traditional synthetic polymer composites, engineering research has focused on the development of new alternatives such as green biocomposites constituted by an eco-sustainable matrix reinforced by natural fibers. Such innovative biocomposites are divided into two different typologies: random short fiber biocomposites characterized by low mechanical strength, used for non-structural applications such as covering panels, etc., and high-performance biocomposites reinforced by long fibers that can be used for semi-structural and structural applications by replacing traditional materials such as metal (carbon steel and aluminum) or synthetic composites such as fiberglass. The present research work focuses on the high-performance biocomposites reinforced by optimized sisal fibers. In detail, in order to contribute to the extension of their application under fatigue loading, a systematic experimental fatigue test campaign has been accomplished by considering four different lay-up configurations (unidirectional, cross-ply, angle-ply and quasi-isotropic) with volume fraction Vf = 70%. The results analysis found that such laminates exhibit good fatigue performance, with fatigue ratios close to 0.5 for unidirectional and angle-ply (±7.5°) laminates. However, by passing from isotropic to unidirectional lay-up, the fatigue strength increases significantly by about four times; higher increases are revealed in terms of fatigue life. In terms of damage, it has been observed that, thanks to the high quality of the proposed laminates, in any case, the fatigue failure involves the fiber failure, although secondary debonding and delamination can occur, especially in orthotropic and cross-ply lay-up. The comparison with classical synthetic composites and other similar biocomposite has shown that in terms of fatigue ratio, the examined biocomposites exhibit performance comparable with the biocomposites reinforced by the more expensive flax and with common fiberglass. Finally, appropriate models, that can be advantageously used at the design stage, have also been proposed to predict the fatigue behavior of the laminates analyzed.

Assessing and Forecasting Fatigue Strength of Metals and Alloys under Cyclic Loads.

Within the scope of this research, patterns of changes in the fatigue life and limit of metals under cyclic stress were identified and the most informative parameters were determined as the basis for developing a method for the universal transformation of experimental data on the fatigue of metals and alloys for their subsequent generalization. Experimental data on metal fatigue, obtained by a large number of authors for a wide range of grades of steels and alloys, under the influence of various combinations of factors, were systematized. A generalized dependence of the recalculated parameters of fatigue life and limit was obtained, its characteristics were assessed, and a sensitivity analysis was performed, confirming the universal nature of the obtained dependence. A system of parameters has been proposed making it possible to consider and forecast high-cycle fatigue processes for a wide range of metals and alloys, under the conditions of various combinations of operating factors, from unified positions and a more general point of view.

Effect of Corrosive Environment on the High-Cycle Fatigue Behavior of Reinforced Concrete by Epoxy Resin: Experimental Study.

Large engineering structures made of various materials, including concrete (e.g., bridges, dams, buildings, and multilevel car parks), steel (e.g., power towers, ships, and wind turbines), or others, are often subjected to severe vibration, dynamic, and cyclic loads, which lead to crack initiation, crack growth, and finally structural failure. One of the effective techniques to increase the fatigue life of such structures is the use of reinforced materials. In the meantime, environmental factors, such as corrosion caused by corrosive environments, also affect the fatigue behavior of materials. Therefore, the main purpose of this paper is to study the influence of corrosive environment on the high-cycle fatigue (HCF) behavior of concrete reinforced by epoxy resin. For this purpose, five corrosive environments with different intensities, including fresh air, water: W, sea water: SW, acidic: AC, and alkaline: AL, were considered and the laboratory samples of conventional concrete (CC) and polymer concrete (PC) were immersed in them for one month. Next, axial fatigue tests were performed under compressive-compressive loading with a frequency of 3 Hz on cylindrical specimens. Moreover, to achieve reliable results, for each stress amplitude, the fatigue test was repeated three times, and the average number of cycles to failure was reported as the fatigue lifetime. Finally, the stress-life cycle (S-N) curves of different states were compared. The results showed that polymer concrete can resist well in corrosive environments and under cyclic loads compared to the conventional concrete, and in other words, the epoxy resin has performed its task well as a reinforcer. The results of fatigue tests show that the load bearing range of 10 tons by CC has reached about 18 tons for PC, which indicates an 80% increase in fatigue strength. Meanwhile, the static strength of samples in the vicinity of fresh air has only improved by 12%.

Experimental and Numerical Investigation on Fatigue Properties of Carbon Fiber Cross-Ply Laminates in Hygrothermal Environments.

The fatigue properties of composite materials are degraded seriously in hygrothermal environments, so taking into account their influence is very important when evaluating the fatigue life of composite structures. Tensile fatigue experiments of carbon fiber reinforced resin composite cross-ply laminates were conducted in room temperature/dry (RTD), cool temperature/dry (CTD) and elevated temperature/wet (ETW) conditions. The S-N curves and fatigue failure modes of the cross-ply laminates were obtained in three conditions. On this basis, a finite element model was established to discuss the influence of temperature and moisture content on the fatigue properties, as well as a method for determining environmental factors of fatigue life of cross-ply laminates was established. The results show that the saturation moisture absorption and temperature have a significant influence on the tensile fatigue properties of cross-ply laminates. The high-cycle fatigue property is weakened significantly by the saturation moisture absorption and high temperature, but the low-cycle fatigue properties were strengthened in cool temperature conditions. The delamination failure mode in ETW is the most severe, presenting with an obvious necking phenomenon. The influence of temperature has a greater effect than that of moisture content, but moisture absorption would play its affect obviously when temperature exceeds 40 °C.

Study on the P-S-N Curve of Sucker Rod Based on Three-Parameter Weibull Distribution.

During the oil production process, sucker rods are subjected to cyclic alternating load. After a certain number of cycles, a sucker rod can experience fatigue failure. The number of cycles is called fatigue life (N), and the accurate relationship between maximum stress (S) and fatigue life (N) under a certain reliability (P), namely the P-S-N curve, is an important basis for the reliability analysis and fatigue life prediction of sucker rods. The Basquin model, based on log-normal distribution, is widely used for fitting the P-S-N curves of sucker rods. Due to the limitation of this model, it is difficult to extrapolate the conclusion obtained from a finite fatigue region to the high-cycle or ultra-high-cycle fatigue region, which makes it impossible to estimate the fatigue limit of the sucker rod. Compared to the log-normal distribution, Weibull distribution causes the sucker rod to have a minimum safety life, namely the safety life at 100% survival rate, which complies with the fatigue characteristics of the sucker rod and is more in line with the actual situation. In this study, the fatigue data for ultra-high-strength HL and HY grade sucker rods were obtained through experimental fatigue tests. A new fatigue life model was established and the P-S-N curves of two types of ultra-high strength sucker rods were obtained. For HL- and HY-type ultra-high strength sucker rods, the average error between the fitting result and fatigue test value is 1.25% and 4.39%, respectively. Compared to the S-N curve fitting result obtained from the Basquin model commonly used for sucker rods, the new model based on three-parameter Weibull distribution provides better fitting precision and can estimate fatigue limit more accurately, so this model is more suitable for estimating fatigue life and can better guide the design of ultra-high strength sucker rod strings.

Effect of Steel Fibre Reinforcement on Flexural Fatigue Behaviour of Notched Structural Concrete.

One of the biggest challenges in facilitating the installation of concrete is the development of fibre-reinforced concrete. Although nowadays fibre reinforced concrete is relatively common, it is still necessary to deepen in the study on its behaviour, especially regarding its fatigue behaviour. This paper proposes a new methodology to analyse the bending fatigue behaviour of notched test specimens. From these tests, it was possible to verify that, despite carrying out the tests with load control, the presence of fibres extends the fatigue life of the concrete after cracking. This effect is of great importance since during the extra lifetime with the cracked concrete, the damage to the concrete will be evident and the corresponding maintenance measures can be carried out. Regarding the analysis of the results, in addition to obtaining a traditional S-N curve, two new criteria have been applied, namely energy and notch growth. From these two new approaches, it was possible to determine critical energy values that can be used as predictive indicators of the collapse of the element. Moreover, from the notch growth analysis, it was possible to determine crack growth rate as a function of the stress conditions for the concrete and the specific geometry. From the comparison among the results obtained from the different tests, a limit cracking index of 0.05 mm can be defined.

Fretting Fatigue Performance of Unidirectional, Laminated Carbon Fibre Reinforced Polymer Straps at Elevated Service Temperature.

The fretting fatigue performance of laminated, unidirectional (UD), pin-loaded, carbon fibre-reinforced polymer (CFRP) straps that can be used as bridge hanger cables was investigated at a sustained service temperature of 60 °C. The aim of this paper is to elucidate the influence of the slightly elevated service temperature on the tensile fatigue performance of CFRP straps. First, steady state thermal tests at ambient temperature and at 60 °C are presented, in order to establish the behaviour of the straps at these temperatures. These results indicated that the static tensile performance of the straps is not affected by the increase in temperature. Subsequently, nine upper stress levels (USLs) between 650 and 1400 MPa were chosen in order to establish the S-N curve at 60 °C (frequency 10 Hz; R = 0.1) and a comparison with an existing S-N curve at ambient temperature was made. In general, the straps fatigue limit was slightly decreased by temperature, up to 750 MPa USL, while, for the higher USLs, the straps performed slightly better as compared with the S-N curve at ambient temperature.

Optimized Planning and Evaluation of Dental Implant Fatigue Testing: A Specific Software Application.

Mechanical complications in implant-supported fixed dental prostheses are often related to implant and prosthetic design. Although the current ISO 14801 provides a framework for the evaluation of dental implant mechanical reliability, strict adherence to it may be difficult to achieve due to the large number of test specimens which it requires as well as the fact that it does not offer any probabilistic reference for determining the endurance limit. In order to address these issues, a new software program called ProFatigue is presented as a potentially powerful tool to optimize fatigue testing of implant-supported prostheses. The present work provides a brief description of some concepts such as load, fatigue and stress-number of cycles to failure curves (S-N curves), before subsequently describing the current regulatory situation. After analyzing the two most recent versions of the ISO recommendation (from 2008 and 2016), some limitations inherent to the experimental methods which they propose are highlighted. Finally, the main advantages and instructions for the correct implementation of the ProFatigue free software are given. This software will contribute to improving the performance of fatigue testing in a more accurate and optimized way, helping researchers to gain a better understanding of the behavior of dental implants in this type of mechanical test.

Application of the S-N Curve Mean Stress Correction Model in Terms of Fatigue Life Estimation for Random Torsional Loading for Selected Aluminum Alloys.

The paper presents the experimental fatigue test results for cyclic constant amplitude loading conditions for the case of the torsion of the PA4 (AW-6082-T6), PA6 (AW-2017A-T4) and PA7 (AW-2024-T3) aluminum alloy for a drilled diabolo type test specimen. The tests have been performed for the stress asymmetry ratios R = -1, R = -0.7, R = -0.5 and R = -0.3. The experimental results have been used in the process of a fatigue life estimation performed for a random generated narrowband stress signal with a zero and a non-zero global mean stress value. The calculations have been performed within the time domain with the use of the rainflow cycle counting method and the Palmgren-Miner damage hypothesis. The mean stress compensation has been performed with the S-N curve mean stress model proposed by Nieslony and Böhm. The model has been modified in terms of torsional loading conditions. In order to obtain an appropriate R = 0 ratio S-N curve fatigue strength amplitude, the Smith-Watson-Topper model was used and compared with literature fatigue strength amplitudes. The presented solution extends the use of the correction model in terms of the torsional loading condition in order to obtain new S-N curves for other R values on the basis of the R = -1 results. The work includes the computational results for new fatigue curves with and without the mean stress effect correction. The results of the computations show that the mean stress effect plays a major role in the fatigue life assessment of the tested aluminum alloys and that the method can be used to assess the fatigue life under random conditions.

Fatigue Life Assessment of Filled Rubber by Hysteresis Induced Self-Heating Temperature.

As a viscohyperelastic material, filled rubber is widely used as a damping element in mechanical engineering and vehicle engineering. Academic and industrial researchers commonly need to evaluate the fatigue life of these rubber components under cyclic load, quickly and efficiently. The currently used method for fatigue life evaluation is based on the S-N curve, which requires very long and costly fatigue tests. In this paper, fatigue-to-failure experiments were conducted using an hourglass rubber specimen; during testing, the surface temperature of the specimen was measured with a thermal imaging camera. Due to the hysteresis loss during cyclic deformation, the temperature of the material was found to first rise and then level off to a steady state temperature, and then it rose sharply again as failure approached. The S-N curve in the traditional sense was experimentally determined using the maximum principal strain as the fatigue parameter, and a relationship between the steady state temperature increase and the maximum principal strain was then established. Consequently, the steady state temperature increase was connected with the fatigue life. A couple of thousand cycles was sufficient for the temperature to reach its steady state value during fatigue testing, which was less than one tenth of the fatigue life, so the fatigue life of the rubber component could be efficiently assessed by the steady state temperature increase.

Microstructural and Very High Cycle Fatigue (VHCF) Behavior of Ti6Al4V-A Comparative Study.

In this study, an investigation is carried out to evaluate and compare the material and physical properties of Grade 5 Titanium alloy (Ti6Al4V G5) samples of three different impeller manufacturers. The study aims to identify the efficient impeller core material from different Ti6Al4V G5 manufacturers. Ultrasonic fatigue test for Ti6Al4V samples of 100 horsepower (hp) centrifugal compressor impeller parts is performed before and after heat treatment. The effect of microstructure on Very High Cycle Fatigue (VHCF) behavior of Ti6Al4V is also analyzed and discussed in detail. Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) observation are carried out to investigate the microstructure of different Ti6Al4V material samples. The dynamic elastic properties are measured by the Impulse Excitation Technique (IET) at room temperature. The fracture behavior of the tensile specimens is analyzed by SEM. Post-heat-treatment analysis of Ti6Al4V is also carried out and presented which affects the grain size of the material sample and thus considerable effect in the mechanical properties. Chemical composition investigation of Ti6Al4V using SEM and Energy Dispersive X-ray Spectroscopy (EDS) also included in this study.

Very High Cycle Fatigue (VHCF) Characteristics of Carbon Fiber Reinforced Plastics (CFRP) under Ultrasonic Loading.

A liquid nitrogen cooling system was developed to ensure the successful ultrasonic testing of composite materials to characterize the very High Cycle Fatigue (VHCF) of carbon fiber reinforced plastics (CFRP). The fatigue failure of CFRP occurs even in the very high cycle range and there is no traditional fatigue limit. The S-N curve of the CFRP presents a step whose characteristics appear in the transition between high cycle and very high cycle fatigue. The damage evolution of CFRP in the same field of view is investigated. The morphology of damaged CFRP composites under ultrasonic loading is described by three characteristics: matrix damage at the intersection of fiber bundles, near fiber bundle parallel section matrix cavity and matrix penetration. With the increasing of cycles, the damage process is also presented in turn according to these three characteristics. The post-fatigue bending modulus changed significantly from the pre-fatigue values, indicating that the VHCF had a considerable impact on the mechanical properties of the composite. An evolution threshold was introduced from the S-N curve to determine the fatigue evolution law from the high cycle regime to the very high cycle regime.

Validation of Multiaxial Fatigue Strength Criteria on Specimens from Structural Steel in the High-Cycle Fatigue Region.

The paper describes results of fatigue strength estimates by selected multiaxial fatigue strength criteria in the region of high-cycle fatigue, and compares them with own experimental results obtained on hollow specimens made from CSN 41 1523 structural steel. The specimens were loaded by various combinations of load channels comprising push-pull, torsion, bending and inner and outer pressures. The prediction methods were validated on fatigue strengths at seven different numbers of cycles spanning from 100,000 to 10,000,000 cycles. No substantial deviation of results based on the selected lifetime was observed. The PCRN method and the QCP method provide best results compared with other assessed methods. The results of the MMP criterion that allows users to evaluate the multiaxial fatigue loading quickly are also of interest because the method provides results only slightly worse than the two best performing solutions.

Fatigue Life for Different Stress Concentration Factors for Stainless Steel 1.4301.

This paper presents the results of the static tensile and fatigue life tests under rotating bending of round 1.4301 (AISI 304) steel samples. The fatigue tests were carried out on smooth and notched samples with three different rounding angles with a shape factor of 1.4, 2 and 2.6. A fatigue life was determined for samples with different shape factors subject to identical loads. The results showed that the scatter of fatigue test results decreases with an increase in shape factor. To evaluate the cracking properties (cracking mode and mechanism), microstructure and fractographic tests of the fractured samples were carried out.

P-S-N Curve Description of Laser Metal Deposition Ti-6.5Al-2Zr-1Mo-1V Titanium Alloy after Duplex Annealing.

Fatigue tests were conducted on standard smooth laser metal deposition (LMD) Ti-6.5Al-2Zr-1Mo-1V titanium alloy specimens under three constant-amplitude stresses. The mixed failure behaviors and the influence of internal pores on the fatigue life were discussed in detail. The double-peak characteristics of the fatigue life distribution have been observed, and a bimodal lognormal distribution (BLG) with five variables can be used to describe the fatigue life variation. A parameter estimation method based on the maximum likelihood estimation (MLE) method was proposed to estimate the BLG distribution parameters, and the Newton⁻Raphson algorithm was used to solve the nonlinear equations derived from the likelihood functions. A P-S-N curve description of LMD titanium alloy was established based on the BLG and verified by fatigue life data obtained from a fatigue test.

ADM guidance-ceramics: Fatigue principles and testing.

BACKGROUND: Clinical failure of dental ceramics is usually reported as partial fracture of the restoration (chipping) or as catastrophic fracture of the whole structure. In contrast to metals, ceramics are linear-elastic, brittle materials exhibiting extremely low damage tolerance to failure. Well documented clinical and lab reports have shown this fracture event often occurs at loads far below their fracture strength due to intrinsic fatigue degradation via slow crack growth or cyclic fatigue mechanisms. The presence and development of surface flaws have a dominant role in damage accumulation and lifetime reduction of ceramic structures. AIMS: This ADM guidance document aims to summarize the aspects related to fatigue degradation of dental ceramics, reviewing the concepts of fatigue testing and furthermore aims to provide practical guidance to young scientists entering into fatigue related research. The description of fatigue strength is always accompanied by a clear understanding of the underlying fracture mechanisms.

Walker Parameter for Mean Stress Correction in Fatigue Testing of Al-7%Si-Mg Alloy Castings.

In this study, performance of two existing Walker parameter estimation models has been investigated. Results show that those developed mainly for steel did not provide reasonable fits to experimental Walker parameters for fatigue data for Al-7%Si-Mg alloy castings in the literature. A strong relationship between the Walker parameter and the structural quality, quantified by the quality index, QT, was observed and an empirical equation to estimate the Walker parameter for these alloys was developed. These findings indicate that the Walker parameter is not an intrinsic material property and the structural quality of the specimens must be taken into account for mean stress correction in fatigue testing.

A description of spinal fatigue strength.

Understanding fatigue failure of the spine is important to establish dynamic loading limits for occupational health and safety. In this study experimental data were combined with published data to develop a description of the predictive parameters for spinal fatigue failure. 41 lumbar functional spinal units (FSUs) from cadaveric spines (age 49.0 ± 11.9 yr) where cyclically loaded. Three different levels of sinusoidal axial compression (0-3 kN, 0-2kN or 1-3kN) were applied for 300,000 cycles. Further, published data consisted of 70 thoracic and lumbar FSUs loaded in axial compression for 5000 cycles. Cyclic forces ranged from lower peaks (Fmin) of 0.7-1kN to upper peaks (Fmax) of 1.2-7.1 kN. Based on Wöhler analysis, a fatigue model was developed accounting for three parameters: I) specimen-specific scaling based on the endplate area, II) specimen-specific strength dependency on age or bone mineral density, III) load-specific correction factors based on Fmax and Fmin. The most predictive model was achieved for a combination of Fmax, endplate area and bone mineral density; this model explained 61% of variation (p<0.001). A model including Fmax, endplate area and age explained only 28% of variation (p<0.001). Inclusion of a load-specific correction factor did not significantly improve model prediction of fatigue failure. This analysis presents the basis for the prediction of specimen-specific fatigue failure of the lumbar spine, provided the endplate area and bone mineral density can be derived.

Damage Evolution and Life Prediction of Cross-Ply C/SiC Ceramic-Matrix Composite under Cyclic Fatigue Loading at Room Temperature and 800 °C in Air.

The damage evolution and life prediction of cross-ply C/SiC ceramic-matrix composite (CMC) under cyclic-fatigue loading at room temperature and 800 °C in air have been investigated using damage parameters derived from fatigue hysteresis loops, i.e., fatigue hysteresis modulus and fatigue hysteresis loss energy. The experimental fatigue hysteresis modulus and fatigue hysteresis loss energy degrade with increasing applied cycles attributed to transverse cracks in the 90° plies, matrix cracks and fiber/matrix interface debonding in the 0° plies, interface wear at room temperature, and interface and carbon fibers oxidation at 800 °C in air. The relationships between fatigue hysteresis loops, fatigue hysteresis modulus and fatigue hysteresis loss energy have been established. Comparing experimental fatigue hysteresis loss energy with theoretical computational values, the fiber/matrix interface shear stress corresponding to different cycle numbers has been estimated. It was found that the degradation rate at 800 °C in air is much faster than that at room temperature due to serious oxidation in the pyrolytic carbon (PyC) interphase and carbon fibers. Combining the fiber fracture model with the interface shear stress degradation model and the fibers strength degradation model, the fraction of broken fibers versus the cycle number can be determined for different fatigue peak stresses. The fatigue life S-N curves of cross-ply C/SiC composite at room temperature and 800 °C in air have been predicted.

Effect of dental implant diameter on fatigue performance. Part I: mechanical behavior.

AIM: The purpose of this study was to evaluate the effect of the implants' diameter on the mechanical function and load-fatigue performance of dental implants. MATERIALS AND METHODS: Three groups of implants with different diameters (3.3 mm, 3.75 mm and 5 mm), were tested under static and cyclic compressive loading. A total number of 15 implants for the static test and 112 implants for the cyclic-fatigue test. In the cyclic test, the machine ceased operating when the structure collapsed or when it reached 5 × 10(6) cycles without apparent failure. The load versus the number of cycles was plotted as curves for biomechanical analysis (S-N curve) for each implant diameter. RESULTS: The S-N curve plotted for the 5 mm implants showed classic fatigue behavior with a finite life region starting from 620N. The same was observed for the 3.75 mm diameter implants, with a finite life region starting below 620N. By contrast, the 3.3 mm diameter implants failed to show predictable fatigue behavior and a fatigue limit could not be defined. CONCLUSIONS: The results of this study emphasize the importance of implant diameter on fatigue behavior. Narrow implants failed to show typical fatigue behavior which might be attributed to the implant design.