Tension-tension Fatigue Loading Research Articles

Effect of TiN monolithic and Ti/TiN multilayer coating on the fatigue behavior of titanium alloy under tension-tension

This paper investigates the fatigue failure mechanism of mono- and multilayer coatings on the fatigue performance of TC11 titanium alloy under tension-tension. The morphology, phase composition, mechanical properties were measured by scanning electron microscope, X-ray diffractometer and nanoindentation. Electron back scatter diffraction was employed to investigated the failure mechanism. Fatigue limits obtained of uncoated TC11, TC11 with TiN coating, TiN/Ti multilayer (ML-6, ML-3, ML-1) and after 1 × 107 cycles are 855 MPa, 550 MPa, 525 MPa, 500 MPa and 400 MPa. Under fatigue loading, the hard-coating/TC11 substrate experiences fatigue failure through coating cracking hastens the substrate's fatigue failure. EBSD analysis results indicate that the main slip system of TC11 titanium alloy under tension-tension fatigue load is α phase (10-10)[-12-10]. After 1 × 107 cycles at fatigue limits, the average dislocation density on the surface of the TC11 with TiN coating is lower than that of TC11. Due to the surface defect induced by coating preparation and high crack propagation velocity, the hard coating significantly deteriorates fatigue property of TC11 by reducing fatigue crack initiation period. Therefore, instead of approaching from the perspective of coating structure design to increase the fatigue crack propagation cycles, it is more effective to reduce the surface roughness of the coating and enhance the fatigue crack initiation cycles.

Microstructure analysis of carbon-fiber-reinforced polymer laminates subjected to self-heating and fatigue strengthening under tension-tension fatigue loading

In this study, tensile-tensile fatigue testing of unidirectional (UD) carbon-fiber-reinforced polymer (CFRP) prepreg laminates was performed. Two types of specimens (open-hole and unopened-hole ones) were examined at different stress levels to elucidate the impact of fatigue damage on their static strength at different numbers of cycles. The effect of internal heat generation during fatigue on the life of unopened-hole CFRP laminates was analyzed by observing the changes in the microstructure of specimens. The results revealed that the difference in fatigue life between unopened specimens with and without cooling at 70% ultimate tension stress (UTS) level was about 88 times. In the open-hole CFRP laminates, the residual tension strength (RTS) after increases and then decreases with the number of fatigue cycles, and the interface debonding was a significant cause of fatigue strengthening. Open-hole specimens were shown to withstand continuous cycling under severe loads at 5% above their ultimate stress levels. Therefore, the fatigue damage extension of the open-hole laminates can be slowed down under intensive cycling at high stress levels, ensuring good fatigue performance of the material.

Tensile fatigue behavior and damage evolution of z-pinned carbon/phenolic woven composite using in-situ monitoring technology

Carbon/phenolic composite, as structural materials for aerospace, undergoes static and dynamic loading which can affect the structural integrity of components. This paper aims to comprehensively investigate the mechanical properties and damage evolution mechanisms of z-pinned carbon/phenolic woven composite subjected to static tensile and different levels of tension-tension fatigue loading. The essential correlation between damage modes and acoustic emission (AE) signals of carbon/phenolic composite was first established. The AE signals were processed using the Hilbert-Huang transform method to obtain the percentage of damage modes at various load levels. The results showed that the peak frequency ranges of the three damage modes, matrix cracking, debonding, and fiber breakage, were 100–200 kHz, 200–300 kHz, and over 300 kHz, respectively. Moreover, the damage evolution mechanisms can be revealed using the OM-AE monitoring method. It was found that the dominant damage modes were all matrix cracking. The fatigue damage occurs at the edge of the specimen, and the three damage modes interact with each other on a microscopic scale. Cracks expand from the edge to the center at low stress. The developed OM-AE online damage monitoring system allows real-time damage monitoring and accurate recognition of damage modes.

Effect of brittle TiN coating on fatigue performance of TC11 titanium alloy under rotating bending and tension-tension

In this paper, the effect of brittle TiN coating on fatigue performance of TC11 titanium alloy (Ti-6.5Al-3.1Mo-1.05Zr0.35Si) under rotating bending and tension-tension were investigated. TiN coating causes a reduction in high and low cycle fatigue performance of TC11 under both conditions. The median fatigue strength at 1 × 107 cycles of TC11 decreases from 582.5 MPa and 896.0–547.5 MPa and 531.0 MPa under rotating bending and tension-tension fatigue loading, respectively. In scenarios involving rotating bending fatigue, fatigue failure initiation for both TiN coated and uncoated TC11 specimens reside on the droplet in coating surface, irrespective of high or low stress conditions. As for tension-tension fatigue loading, both TiN coated and uncoated TC11 specimens experience surface-based fatigue initiation points under high fatigue stress. However, during low-stress tension-tension loading, the incorporation of a TiN coating prompts a shift in TC11's fatigue initiation source from the interior to its surface. Furthermore, under both fatigue loading condition, fatigue cracks initiate at droplets on the coating surface and propagate to the surface of the substrate and eventually its interior. And the fatigue failure of the TC11 substrate is accelerated due to the coating causing balanced tensile stress. The inherent compressive characteristics and lack of tensile strength in TiN ceramic materials render them more susceptible to premature cracking when subjected to tensile fatigue loads. This vulnerability leads to heightened damage to the fatigue performance of TC11.

Durability of Plant Fiber Composites for Structural Application: A Brief Review.

Environmental sustainability and eco-efficiency stand as imperative benchmarks for the upcoming era of materials. The use of sustainable plant fiber composites (PFCs) in structural components has garnered significant interest within industrial community. The durability of PFCs is an important consideration and needs to be well understood before their widespread application. Moisture/water aging, creep properties, and fatigue properties are the most critical aspects of the durability of PFCs. Currently, proposed approaches, such as fiber surface treatments, can alleviate the impact of water uptake on the mechanical properties of PFCs, but complete elimination seems impossible, thus limiting the application of PFCs in moist environments. Creep in PFCs has not received as much attention as water/moisture aging. Existing research has already found the significant creep deformation of PFCs due to the unique microstructure of plant fibers, and fortunately, strengthening fiber-matrix bonding has been reported to effectively improve creep resistance, although data remain limited. Regarding fatigue research in PFCs, most research focuses on tension-tension fatigue properties, but more attention is required on compression-related fatigue properties. PFCs have demonstrated a high endurance of one million cycles under a tension-tension fatigue load at 40% of their ultimate tensile strength (UTS), regardless of plant fiber type and textile architecture. These findings bolster confidence in the use of PFCs for structural applications, provided special measures are taken to alleviate creep and water absorption. This article outlines the current state of the research on the durability of PFCs in terms of the three critical factors mentioned above, and also discusses the associated improvement methods, with the hope that it can provide readers with a comprehensive overview of PFCs' durability and highlight areas worthy of further research.

Fatigue life prediction of aluminum 6061 alloy through experimental and numerical analysis under various stress ratios in axial tension-tension loading condition

Aluminum alloy 6061 is a versatile material widely used in aerospace, automotive, marine, and structural applications due to its mechanical strength, weldability, and corrosion resistance. Understanding and addressing fatigue is crucial for ensuring the reliable and safe performance of components made from this alloy. This study focused on predicting the fatigue life of aluminum 6061 at room temperature under axial loading conditions and evaluating the fatigue life at various stress ratios. The experimental S-N curve, derived from tension-tension fatigue loading tests, was instrumental in characterizing the material's fatigue behaviour under constant amplitude loading condition. The data obtained from these tests were then imported into ANSYS software, a widely recognized tool for finite element analysis and simulation. ANSYS was used to conduct a detailed numerical analysis to predict the fatigue life of aluminum 6061 at different stress ratios. The results from the experimental and numerical analyses exhibited good agreement, validating the accuracy of ANSYS software in predicting fatigue life at different stress ratios. This study demonstrated the effectiveness of employing a combined experimental and numerical approach for understanding fatigue behaviour in materials, particularly in terms of its applicability to different loading scenarios. The findings from this research provide valuable insights into the fatigue life of aluminum 6061 under diverse stress ratios, with the goal of enhancing material design and engineering processes. By leveraging the capabilities of ANSYS software in conjunction with experimental data, the study established a reliable methodology for predicting fatigue life, which is crucial for optimizing the performance and longevity of components subjected to cyclic loading in various industries.

Effect of patch length and elevated temperature on fatigue behaviour of repaired aluminium panels with a CFRP patch

ABSTRACT Thin panels of aluminium alloy 6061-T6 with a centre pre-crack were repaired with a one-sided asymmetrical CFRP patch through co-curing of epoxy at the room temperature to have minimal residual stresses. The specimens were tested with a tension-tension fatigue load. The numerical simulation as well as the experiments showed that the fatigue life was improved considerably with the increasing patch length when tested at room temperature. In the asymmetrical repair with a single patch, a bending moment was induced whose magnitude decreased with the increasing patch length, resulting into longer fatigue life. The specimens failed through the growth of the pre-crack in the skin. However, the crack in the skin did not grow in case of the specimen with the longest patch length and the failure occurred at substantially higher number of fatigue cycles through a different mechanism in the bare portion of the aluminium alloy skin. Experiments were also conducted at the elevated temperature of 80°C. Thermal compressive stresses were developed in the aluminium alloy panel due to the vast difference in the coefficient of thermal expansion of the aluminium alloy and the CFRP patch. Consequently, the stress intensity factor of the crack in the aluminium panel was reduced considerably, resulting into the increase of the fatigue failure life significantly.

Relationship between hysteresis loops and cracking evolution/closure in ceramic-matrix composites under tension-tension fatigue loading

Relationship between hysteresis loops and cracking evolution/closure in ceramic-matrix composites under tension-tension fatigue loading

Finite element modeling of fatigue properties of polymer nanocomposites

This paper presents a 3-dimensional finite element modeling of fatigue properties of polymer nanocomposites with a Representative Volume Element (RVE) approach using ANSYS. Epoxy was used as a matrix, and alumina nanoparticles of spherical shape (diameter 33 nm) were used as reinforcement. Convergence analysis was conducted to select the proper size of RVE. A geometric model of a flat dog bone specimen was created to model the fatigue properties. MATLAB code was used to generate randomly distributed nanoparticles. The sample was loaded in uniaxial tension-tension fatigue loading with a stress ratio of 0.1. S-N data for epoxy and alumina were given as input to determine the fatigue life of polymer nanocomposites. The results obtained from modeling were compared with experimental data. The fatigue life was determined by taking the average life at five different locations in the gauge region for a sample. The average of five different samples was calculated for each wt.% (0.5, 1.0, and 1.5). Reinforcement of nanoparticles up to 1 wt.% caused an increment in the fatigue life of nanocomposites, and fatigue life decreased at 1.5 wt.% of nanoparticles. The finite element modeling was able to predict the trend of improvement provided by the alumina nanoparticles (similar to experimental data).

Fatigue characteristics of sandwich composite joints in ships

The aim of this study is to investigate the fatigue characteristics of sandwich composite joints in ships based on the experimental and numerical methods. Sandwich composite L-joints are subjected to tension-compression, tension-tension and compression-compression fatigue loading at several load levels. Residual stiffness, strain, and surface temperature were measured during fatigue tests. The experimental results show that the sandwich composite L-joints have lower fatigue lives than expected and the fatigue load amplitude has a perceptible effect on the fatigue lives of the joints. In order to illustrate the influence of stress concentration on fatigue life of the joints, a quasi-static numerical analysis was conducted. Moreover, the fatigue damage mechanism and damage growth were revealed by experimental observation, stiffness curves and numerical stress distribution. Finally, a damage model based on the maximum displacements is proposed for the fatigue life prediction of the sandwich composite joints, and the results show that the predicted fatigue lives are in good agreement with the experimental ones.

Fatigue life assessment and damage evolution in Z-pinned laminates

An experimental investigation has been carried out to elucidate the damage evolution and failure mechanisms in unpinned and Z-pinned quasi-isotropic carbon fibre/epoxy laminates manufactured via a pre-hole pin insertion technique and subjected to in-plane tension-tension fatigue loading. X-ray computed tomography was used to analyze the internal configuration of samples subjected to several stress levels of the ultimate tensile stress (UTS) in order to identify and understand the internal damage evolution. The obtained results indicate that, at low cycles, the Z-pinned laminates exhibited poor fatigue behavior owing to the low ultimate tensile strength as compared to the unpinned laminates. The stiffness degradation plots and Micro CT images suggest that the Z-pin bridging inhibited extensive edge debonding which resulted in higher normalized stiffness for the Z-pinned laminates. Matrix microcracking initiated around the Z-pins and propagated through resin rich zones leading to poor load bearing ability and subsequent fibre splitting and rupture. It is worth noting that the failure modes of the quasi-isotropic laminates evolved from delamination dominated failure to the on-axial fibre failure.

Failure analysis of scarf adhesive joints modified with SiC-nanoparticles under fatigue loading at room temperature

Adhesive joints were fabricated by joining two woven carbon fiber composite adherends with scarf angle of 5° using Epocast adhesive system of 50-A1/946 modified with optimum weight percent of 1.5 wt% SiC-nanoparticles. The scarf adhesive joints (SAJs) were characterized through static tensile as well as tension-tension fatigue loading. The stress-number of cycles to failure curves are useful in determining the fatigue life, but they do not give any indication about the fatigue damage development during the joint's lifetime. In consideration of early detecting damage development in the SAJs, many dynamic parameters were estimated from the analysis of the stress-local strain cycles, with the aim to obtain more accurate predictions. These parameters include loss and storage moduli, potential and dissipated energies, and damping factor. It has been found that the tensile strength and fatigue limit of the SiC-SAJ were increased by 9.6% and 55% respectively with decreasing the bondline thickness from 0.25 mm to 0.17 mm. Incorporation of SiC-nanoparticles into the adhesive layer improves the tensile strength and the fatigue limit of the SAJs with bondline thickness of 0.17 mm by 26.1% and 22% respectively compared to the unmodified joint. At higher stress levels, the storage modulus and potential energy can be modeled for early prediction of the joint life and thereby, help in preventing their catastrophic failure. Zhang model showed good agreement with the measured potential energy compared to the storage modulus.

Process Evaluation, Tensile Properties and Fatigue Resistance of Chopped and Continuous Fiber Reinforced Thermoplastic Composites by 3D Printing

The aim of this article was to comprehensively evaluate the manufacturing process, tensile properties and fatigue resistance of the chopped and continuous fiber reinforced thermoplastic composites (CFRTPCs) by 3D printing. The main results included: the common defects of the printed CFRTPCs contained redundant and accumulation defects, scratch and warping defects; the continuous fiber contributed to the dimensional stability and accuracy of width and thickness; associations between mass percentage of fiber reinforcement and the averages of elastic modulus, strain at break and ultimate tensile strength were approximately linear based on tensile test results; the fatigue resistance improved with the increasing fiber reinforcement based on fatigue test results. As for specimens with four fiber rings, there was a good linear relationship between the stress level and logarithm value of cycles during the whole life while those of pure matrix and specimens with one and two fiber rings were piecewise linear, taking about 10,000 cycles as boundary. The micro morphology showed that the fatigue failure behaved as matrix fracture, large and small fiber bundles and single fibers extracted from matrix. Under the tension-tension fatigue load, the deformations where easily concentrating stress behaved as sunken surfaces along thickness and width directions, and the deformation along width direction was greater than that along thickness direction.

In-situ impact analysis during fatigue tests of open-hole carbon fibre reinforced polymer specimens

This paper presents the results for an experimental campaign of in-situ impact during tension-tension fatigue loading for open-hole carbon fibre reinforced polymer specimens. High-speed low energy impact was introduced to the specimen with the use of a canon, which was attached to testing bench enabling the impact without the need to remove the specimens from the test bench. Digital Image Correlation, C-scan and Acoustic Emission were utilized to record health monitoring data for damage diagnostics. A strain-based criterion was used to identify a common threshold for the timing of impact ensuring a fair comparison between the different tests. The results indicate that while an impact causes the total amount of damage to increase as one would expect, it does not necessarily increase the damage level in the critical area where final fracture occurs. A dependence on the moment of impact with the fatigue failure was found for specimens subjected to impact before the initiation of the fatigue loading. In contrast, impacting specimens in the presence of fatigue damage had no detrimental effect on the fatigue life, although it was observed that the damaged area was enlarged. Overall, the paper showcases the need to study systemically the effect of in-situ impact on the fatigue life in order to understand better the implications that may be introduced to the integrity of a composite structure.

Implications of free-edge effect at thin plain-woven carbon fiber reinforced plastic laminates with out-of-plane waviness under cyclic loading

The onset of damage caused by the free-edge effect in plain-woven carbon fiber reinforced plastic (CFRP) specimens with an out-of-plane waviness under tension-tension fatigue loading is investigated. Numerical calculations show that interlaminar and intralaminar stresses close to the out-of-plane waviness are higher than the equivalent stresses at the surrounding edge regions. Using submodels, the influence of the chosen out-of-plane waviness can be better assessed. The free-edge effect of the considered specimens, which originates from stress gradients between plies of different orientation, is altered by the change in the stress field caused by the out-of-plane waviness. Large interlaminar stresses between plies of the same orientation are obtained, which contrasts with existing literature. In experimental fatigue testing it is found that cracks at the free edge appeared at the predicted locations, and after reaching crack saturation, in regions close to the out-of-plane waviness, interlaminar and intralaminar stresses lead to additional cracks along the whole free edges. The experimental tests are supported by a three dimensional image correlation system (3D-DIC), a thermal-imager and a digital photographic camera, which allows detailed examination of selected areas. Visual observation during fatigue testing and post-mortem inspection show good agreement between experimental data and numerical calculations in relation to the location of the damage initiation. As a result, out-of-plane waviness at free edges must be considered as an additional significant fatigue damage initiation location in laminate analysis.

Fiber break model for tension-tension fatigue of unidirectional composites

Damage progression in unidirectional (UD) composites in tension-tension fatigue is vital to understanding fatigue in more complex laminates and loading conditions. We therefore developed the UD fatigue model that considers fiber breaks, fiber-matrix debond growth, fiber fatigue as well as interactions of these damage modes. The contribution of each mode of damage in fatigue is assessed and those that dominate fatigue damage are identified. The model accounts for Weibull statistics of fiber strength and stress redistribution around fiber breaks. The predictions for fatigue loading indicate a significantly higher density of fiber breaks and clusters compared to quasi-static loading at the same stress level. The model predicts S–N curves in pure uniaxial tension-tension fatigue loading and does not consider stress concentrations caused near the grips; the predictions therefore tend to overpredict fatigue life compared to experimental S–N curves available in the literature. The model has potential to be developed for different loading conditions.

Fatigue damage tracking and life prediction of fiberglass composites using a laser induced graphene interlayer

Fiberglass-reinforced composite materials are commonly used in engineering structures subjected to dynamic loading, such as wind turbine blades, automobiles, and aircraft, where they experience a wide range of unpredictable operating conditions. The ability to monitor these structures while in operation and predict their remaining structural life without requiring their removal from service has the potential to drastically reduce maintenance costs and improve reliability. This work exploits piezoresistive laser induced graphene (LIG) integrated into fiberglass-reinforced composites for in-situ fatigue damage monitoring and lifespan prediction. The LIG is integrated within fiberglass composites using a transfer-printing process that is scalable with the potential for automation, thus reducing barriers for widespread application. The addition of the conductive LIG within the traditionally insulating fiberglass composites enables direct in-situ damage monitoring through simple passive resistance measurements during tension-tension fatigue loading. The accumulation and propagation of structural damage are detected throughout the fatigue life of the composite through changes to the electrical resistance measurements, and the measurement trends are further used to predict the onset of catastrophic composite failure. Thus, this work results in a scalable and multifunctional composite material with self-sensing capabilities for potential use in high-performing, dynamic, and flexible composite structures.

Tension Fatigue Behaviour of Woven Bamboo and Glass Fiber Reinforced Epoxy Hybrid Composites

The aim of this paper is to investigate the influence of stacking sequence of multi layered woven bamboo and glass fibers reinforced with epoxy matrix composites under static tensile and tension-tension fatigue loading. Six layers of bamboo fiber and seven layers of glass fiber has been used to prepare the samples by hand lay-up technique with [0°/90°] and [±45°]. Extensive fatigue tests (frequency 3 Hz and stress ratio 0.1) were performed in accordance with ASTM D3479. In constant amplitude and load-controlled tests, a percentage of the ultimate tensile strength (UTS) was applied to the composite specimens. Five stress levels were applied from 90% of UTS, and decreased in steps of 10% of UTS. The obtained results were used to draw S/N Curve and [0°/90°] lay-ups show better fatigue strength than [±45] laminate.

A continuum damage model for composite laminates: Part III - Fatigue

The mesoscale continuum damage model for laminated composites that has been developed for the past 10 years at AMADE research group is extended for fatigue life and residual strength predictions. The same intra-laminar damage mechanisms are considered by means of the LaRC04 failure criteria, but a new set of functions are introduced to account for the degradation of the material under fatigue loads. The cycle jump approach is adopted within the finite element code to reduce the computational cost. In the follow-up paper part IV (Llobet et al., submitted for publication), the computational model is validated by simulating the progressive failure mechanisms of open-hole and double-edge notched specimens under static, tension-tension fatigue and residual strength load cases. All the experimental data is also described in detail in the accompanying part.

Effect of backing fibers on the thermoelastic stress analysis of multi-directional glass/epoxy laminates during fatigue loading

A preliminary study was conducted regarding Thermoelastic Stress Analysis (TSA) for use in monitoring stiffness degradation of multi-directional GFRP laminated specimens during tension-tension fatigue loading. The specimens were made from a non-crimp fabric consisting of UD fibers stitched to a layer off-axis backing fiber bundles. The thermoelastic responses of the UD surface and backing fiber surface were compared. It was observed that the thermoelastic response differed between two faces of the fabric and was influenced by the presence of backing fibers on the surface.