Tension Fatigue Loading Research Articles

A novel machine learning model to design historical-independent health indicators for composite structures

Developing comprehensive health indicators (HIs) for composite structures encompassing various damage types is challenging due to the stochastic nature of damage accumulation and uncertain events (like impact) during operation. This complexity is amplified when striving for HIs independent of historical data. This paper introduces an AI-driven approach, the Hilbert transform-convolutional neural network under a semi-supervised learning paradigm, to designing reliable HIs (fulfilling requirements, referred to as 'fitness'). It exclusively utilizes current guided wave data, eliminating the need for historical information. Ensemble learning techniques were also used to enhance HI quality while reducing deep learning randomness. The fitness equation is refined for dependable comparisons and practicality. The methodology is validated through investigations on T-single stiffener CFRP panels under compression-fatigue and dogbone CFRP specimens under tension-fatigue loadings, showing high performance of up to 93% and 81%, respectively, in prognostic criteria.

Fatigue assessment of defective A357-T6 cast aluminum based on affected depth

Fatigue assessment of defective A357-T6 cast aluminum based on affected depth

Static and Fatigue Characterization of Large Composite T-Bolt Connections in Marine Hygrothermal Environments

Fiber-reinforced polymer composites have been highlighted as ideal candidates for structural applications in marine renewable energy devices, such as tidal turbines and wave energy converters. It is well understood that harsh marine environments can cause strength degradation of composite laminates, which has been extensively researched at the coupon scale; however, no research has investigated how this translates into larger-scale composite structures. This paper presents a subcomponent-scale study which investigates the effects of hygrothermal aging and subsequent static and fatigue characterization of thick composite T-bolt connections as part of a large, multilaboratory materials research effort. Of the glass-reinforced epoxy and vinylester-epoxy matrix composites tested, both showed measurable static strength degradation (4–36%) after being hygrothermally aged, even though the composite specimens were only partially saturated with water. Under tension–tension fatigue loading, the epoxy specimens performed very well in their dry states but exhibited significant degradation after hygrothermal aging. In comparison, the vinylester-epoxy specimens had much shorter fatigue lives in their dry states but exhibited no degradation after hygrothermal aging. Overall, this research demonstrates that hygrothermal aging can have significant effects on the ultimate strengths and fatigue lives of even partially saturated thick composite T-bolt connections, indicating that degradation of the outer plies on thick composite laminates can have pronounced effects on the whole structure. It discusses the challenges of building an understanding of the effects of harsh marine environments in large-scale composite structures.

Experimental and numerical investigation on tension–tension fatigue behavior of three-dimensional five-directional hybrid braided composites

This paper presents an experimental and numerical investigation on tension–tension fatigue behavior of hybrid carbon/glass three-dimensional five-directional (3D5D) braided composites. Fatigue tests were carried out under the stress ratio of 0.1. The experimental results show that the fatigue strength significantly decreases as the fatigue life increases and the stiffness gradually decreases as the loading cycles increase. The fatigue life at the stress level within 35 %–50 % ultimate tensile strength (UTS) is within 4 × 103–6 × 105 cycles. The stiffness degradation before failure increases from 39 % to 58 % as the fatigue stress decreases from 50 % UTS to 35 % UTS. Under tension–tension fatigue loading, matrix cracking inside the yarns and interfacial debonding between the yarns and the matrix firstly occur and propagate, then matrix cracking appears in the resin-rich regions and numerous fiber breakage happens until final failure. The fatigue performance and damage mechanisms of braided composites are affected by fiber hybrid mode and braiding methods. Additionally, a mesoscale representative volume element (RVE) model with periodic boundary conditions and a progressive fatigue damage model was developed for the 3D5D hybrid braided composites. The numerical simulation results of fatigue life, stiffness degradation and damage progression have good agreement with experiments, demonstrating the validity of the developed model.

Analysis of Stochastic Matrix Crack Evolution in CFRP Cross-Ply Laminates under Fatigue Loading

The present work aims at understanding the stochastic matrix crack evolution in CFRP cross-ply laminates under tension–tension fatigue loading. An experimental campaign was carried out on twenty-three specimens at different stress levels, while two optical techniques were used for the in-situ monitoring of the accumulation of transverse matrix cracks. The results showed a significant scatter in crack evolution among specimens. This stochastic behaviour was further investigated using image analysis and numerical modelling. It was found that transverse matrix cracks can be classified into the independent and dependent cracks based on a critical crack spacing. Furthermore, the severity of interaction among cracks was quantified by introducing a dependent crack ratio. Finally, a strength-based probabilistic model was proposed to describe the scattering regime of the crack evolution. The agreement between model and test results indicates that local strength variations of 90 plies are the dominant scattering source governing the initial fatigue resistance to cracking and determining the accumulation of transverse matrix cracks among specimens. These results may provide a new insight into the stochastic nature of matrix cracking in composite laminates and aid in the design of fatigue resistance properties.

Effect of stress ratio on tension-tension fatigue behaviour of woven angle ply GFRP laminates

The fatigue behavior of woven angle ply glass fiber reinforced plastic laminates under tension–tension fatigue loading has been investigated. Rectangular specimens were subjected to fatigue loading of constant amplitude at three stress levels 80%, 70% and 60% of ultimate tensile strength for stress ratios 0.1 and 0.3. The fatigue life in woven angle ply GFRP composites was improved at all stress levels once the stress ratio was raised from 0.1 to 0.3. Moreover, it was determined that the fatigue life, fatigue stiffness, cyclic creep strain and dissipated energy were dependent on stress level and stress ratio.

Fatigue behavior of unidirectional fiber‐reinforced pultruded composites with high volume fiber fraction

AbstractPultrusion is a high‐volume manufacturing process for fiber‐reinforced polymer (FRP) composites ensuring high strength and quality products. However, research studies are very limited on the fatigue behavior of these products, especially with unidirectional fiber alignment. This study investigates the fatigue behavior of the unidirectional pultruded glass FRP (GFRP) composites with high fiber volume fraction ( ). Tension–tension fatigue loading was applied with different levels of applied stress, frequency, and gripping condition until failure. Results showed that the high composites have significantly low fatigue life cycle. Moreover, increasing the frequency showed better fatigue resistance, while the tabs geometry has insignificant effect on the fatigue behavior of the dog‐bone shaped specimens. Comparison with available literature showed that GFRP composites with high have lower fatigue performance than those with low . A fatigue capacity reduction factor is also proposed for unidirectional GFRP composites as a function of the .

The impact of the fiber volume fraction on the fatigue performance of glass fiber composites

The dependency of the fatigue resistance on the fiber volume fraction of a non-crimp glass fiber fabric reinforced polyester polymer matrix composite is investigated, in the range of 55% to 60% fiber volume. The experimental investigation cover tension–tension, tension–compression, and compression-compression cyclic loading cases. Both an overall and local fiber volume fraction are considered. It is observed that the high fiber volume fraction cases also have higher porosity densities. The tension–tension fatigue loading is found to be most dependent on the fiber volume fraction, while the compression-compression load case is found to have the least dependency.

Bayesian inference-based prognosis of fatigue damage for MPPO polymer using Zhurkov fatigue life model

In this study, the fatigue damage prognosis of a modified polyphenylene oxide (MPPO) polymer is performed using a Bayesian framework, and a Zhurkov model-based dynamic fatigue life model is employed to obtain the probabilistic stress–cycle (P-S-N) curve. Activation energy and tensile tests are performed to determine the aleatory uncertainty of the lethargy coefficient of the Zhurkov fatigue life model. This uncertainty is quantified by performing sequential statistical modeling using experimental data with embedded scattering. The P-S-N curve is estimated using these data, and the Zhurkov fatigue life model is validated via the fatigue test. Furthermore, damage data are obtained via a low-cycle fatigue analysis in conditions identical to those of the fatigue test conducted on the specimen scale. Based on computational damage data, the initial model parameters of the fatigue damage model are obtained using the least-squares method. These model parameters are estimated while considering scattering by applying the Markov Chain Monte Carlo and particle filter. Therefore, the remaining useful life (RUL) of the MPPO, which depends on the amplitude stress, is predicted under the tension–tension fatigue loading ( R = 0), and the prediction accuracy of the RUL is evaluated using prognostics metrics.

Fatigue life of thin pre-cracked aluminum alloy panel repaired with CFRP patch at elevated temperature

The fatigue life of a thin pre-cracked aluminum alloy panel, repaired with CFRP patches was investigated. Aluminum alloy 6061-T6 plate having a center crack (20 mm) was repaired by varying no. of CFRP patches made of several UD CFRP plies. Asymmetrically bonded CFRP patched specimen were prepared using VARTM (Vacuum Assisted Resin Transfer Moulding). Different configurations of CFRP plies were tested with changes in stiffness ratio and environment temperature mainly at 80°C and room temperature. Using a computerized axial fatigue test machine the specimens were loaded under tension – tension fatigue load. Fatigue testing was carried out at 50% of yield strength of skin made of Al 6061-T6 alloy. Frequency of 9 Hz and Stress ratio, R = 0.1 were maintained throughout the experimentation. A set of three specimens under each temperature condition and patch configuration were tested and results were recorded. Fatigue crack propagation cycles till final failure, Nf of the specimen was monitored. The numerical simulations were carried out to find SIF at various crack lengths. The results were compared with experimental results. The tested specimens at elevated temperature showed an increase in fatigue cycles compared to room temperature specimens.

Modeling of Multiple Fatigue Cracks for the Aircraft Wing Corner Box Based on Non-ordinary State-based Peridynamics

In the current research, we propose a novel non-ordinary state-based peridynamics (PD) fatigue model for multiple cracks’ initiation and growth under tension–tension fatigue load. In each loading cycle, the fatigue loading is redistributed throughout the peridynamic solid body, leading to progressive fatigue damage formation and expansion in an autonomous fashion. The proposed fatigue model parameters are first verified by a 3D numerical solution, and then, the novel model is used to depict the widespread fatigue damage evolution of the aircraft wing corner box. The modified constitutive damage model has been implemented into the peridynamic framework. Furthermore, the criteria and processes from multiple initiations to propagation are discussed in detail. It was found that the computational results obtained from the PD fatigue model were consistent with those from the test data. The angular errors of multiple cracks are within 2.66% and the number of cycles errors are within 15%. A comparison of test data and computational results indicates that the fatigue model can successfully capture multiple crack formations and propagation, and other behaviors of aluminum alloy material.

Investigation into the fatigue properties of flax fibre epoxy composites and hybrid composites based on flax and glass fibres

Natural fibre reinforced composites (NFRCs) offer a sustainable and environmentally friendly alternative to composites made from synthetic reinforcements, with their response to dynamic loading phenomena being of utmost importance for structural applications. Herein, flax fibre reinforced epoxy composites and their glass fibre reinforced hybrid counterparts were developed benefiting from synergic effects of hybridisation between flax and glass fibres with the aim of achieving optimised fatigue behaviour. In order to study the fatigue behaviour, three different types of composite laminates comprised of flax fibres only, and two hybrid flax-glass configurations were fabricated, with alternating flax/glass reinforced layers. The hybrid effects on the fatigue behaviour of all the composite laminates were investigated by using constant ratio R under a tension–tension fatigue loading. The investigated fatigue behaviour showed promising performance in the hybrid laminates, with a great effect of the hybrid configuration on the fatigue life. The hybrid laminate with alternating layers of flax and glass demonstrated the best performance under fatigue, showing a good damage accumulation indicator; about 7 times greater, as compared to the other two laminates. The statistical analysis performed indicated that the aforementioned laminate has a less probability of failure, i.e. it will fail in a greater number of fatigue cycles, as compared to the other two laminates. The influence of various parameters such as fabrication induced defects, fibre matrix interface on the fatigue life has been further characterised using imaging techniques. The findings of this study significantly contribute towards the exploitation of natural fibre reinforced composites and their hybrids to understand the damage mechanisms that take place during fatigue loadings and how the fabrication-related defects have an influence on the overall performance of those structures in load-bearing applications.

A novel fatigue life prediction methodology based on energy dissipation in viscoelastic materials

This paper introduces a new fatigue life prediction methodology for viscoelastic materials in the tension–tension fatigue loading region. The model was established based on the total amount of energy dissipated during fatigue loading, and offers two main advantages with respect to existing models in the literature, i.e. it considers the creep effect on fatigue behavior and requires less input data. The model was applied to three different materials - an angle-ply glass/epoxy fiber-reinforced polymer composite, a cross-ply glass/epoxy fiber-reinforced polymer composite, and an epoxy adhesive - to cover a wide range of structural viscoelastic materials used in the industry. It was observed that the model predicted the fatigue life of the studied materials well at different stress ratios including those close to 1.0 where the creep effect was considerable. The model was used to plot constant life diagrams (CLDs) by considering the cyclic-creep interaction to counter the lack of accuracy of existing models at high stress ratios. A new definition for the cyclic-creep interaction was also proposed, which suggests that the participation of the creep and cyclic parts in the cyclic-creep interaction is equal to the total amount of energy dissipated by each. Accordingly, the proposed model was employed to simulate cyclic-creep interaction and determine the cyclic- and creep-dominated regions in CLDs.

Artificial neural networks and phenomenological degradation models for fatigue damage tracking and life prediction in laser induced graphene interlayered fiberglass composites

The mechanical properties of fiber reinforced polymer matrix composites are known to gradually deteriorate as fatigue damage accumulates under cyclic loading conditions. While the steady degradation in elastic stiffness throughout fatigue life is a well-established and studied concept, it remains difficult to continuously monitor such structural changes during the service life of many dynamic engineering systems where composite materials are subjected to random and unexpected loading conditions. Recently, laser induced graphene (LIG) has been demonstrated to be a reliable, in-situ strain sensing and damage detection component in fiberglass composites under both quasi-static and dynamic loading conditions. This work investigates the potential of exploiting the piezoresistive properties of LIG interlayered fiberglass composites in order to formulate cumulative damage parameters and predict both damage progression and fatigue life using artificial neural networks (ANNs) and conventional phenomenological models. The LIG interlayered fiberglass composites are subjected to tension–tension fatigue loading, while changes in their elastic stiffness and electrical resistance are monitored through passive measurements. Damage parameters that are defined according to changes in electrical resistance are found to be capable of accurately describing damage progression in LIG interlayered fiberglass composites throughout fatigue life, as they display similar trends to those based on changes in elastic stiffness. These damage parameters are then exploited for predicting the fatigue life and future damage state of fiberglass composites using both trained ANNs and phenomenological degradation and accumulation models in both specimen-to-specimen and cycle-to-cycle schemes. When used in a specimen-to-specimen scheme, the predictions of a two-layer Bayesian regularized ANN with 40 neurons in each layer are found to be at least 60% more accurate than those of phenomenological degradation models, displaying R 2 values greater than 0.98 and root mean square error (RMSE) values smaller than 10−3. A two-layer Bayesian regularized ANN with 25 neurons in each layer is also found to yield accurate predictions when used in a cycle-to-cycle scheme, displaying R 2 values greater than 0.99 and RMSE values smaller than 2 × 10−4 once more than 30% of the initial measurements are used as inputs. The final results confirm that piezoresistive LIG interlayers are a promising tool for achieving accurate and continuous fatigue life predictions in multifunctional composite structures, specifically when coupled with machine learning algorithms such as ANNs.

A Robust Numerical Methodology for Fatigue Damage Evolution Simulation in Composites.

Composite materials, like metals, are subject to fatigue effects, representing one of the main causes for component collapse in carbon fiber-reinforced polymers. Indeed, when subject to low stress cyclic loading, carbon fiber-reinforced polymers exhibit gradual degradation of the mechanical properties. The numerical simulation of this phenomenon, which can strongly reduce time and costs to market, can be extremely expensive in terms of computational effort since a very high number of static analyses need to be run to take into account the real damage propagation due the fatigue effects. In this paper, a novel cycle jump strategy, named Smart Cycle strategy, is introduced in the numerical model to avoid the simulation of every single cycle and save computational resources. This cycle jump strategy can be seen as an enhancement of the empirical model proposed by Shokrieh and Lessard for the evaluation of the fatigue-induced strength and stiffness degradation. Indeed, the Smart Cycle allows quickly obtaining a preliminary assessment of the fatigue behavior of composite structures. It is based on the hypothesis that the stress redistribution, due to the fatigue-induced gradual degradation of the material properties, can be neglected until sudden fiber and/or matrix damage is verified at element/lamina level. The numerical procedure has been implemented in the commercial finite element code ANSYS MECHANICAL, by means of Ansys Parametric Design Languages (APDL). Briefly, the Smart Cycle routine is able to predict cycles where fatigue failure criteria are likely to be satisfied and to limit the numerical simulation to these cycles where a consistent damage propagation in terms of fiber and matrix breakage is expected. The proposed numerical strategy was preliminarily validated, in the frame of this research study, on 30° fiber-oriented unidirectional coupons subjected to tensile–tensile fatigue loading conditions. The numerical results were compared with literature experimental data in terms of number of cycles at failure for different percentage of the static strength. Lastly, in order to assess its potential in terms of computational time saving on more complex structures and different loading conditions, the proposed numerical approach was used to investigate the fatigue behavior of a cross-ply open-hole composite panel under tension–tension fatigue loading conditions.

Effects of environmental conditions on the axial tension–compression fatigue behavior of carbon/epoxy plain-weave laminates containing flaws

The objective of this work is to investigate the effects of environmental conditions on the axial fatigue behavior of a carbon/epoxy plain-weave laminate with an embedded flaw subjected to a partially reversed cyclic load (stress ratio R = −0.1) in tension–compression. This specific material is more commonly used in aerospace engineering for the manufacturing of aircraft structural parts, which are directly exposed to various environmental conditions during service. Specific environmental and loading conditions that are appropriate to simulate real-life conditions are considered to observe and collect information about the material's behavior. For the investigation, dry and wet coupons were submitted to room temperature, 82 and 121 ℃ under loading frequencies of 7 and 15 Hz. A maximum allowable strain increase criterion is used to monitor the flaw growth threshold or delamination onset, during fatigue testing. The ultrasonic imaging (C-scan) technique is used to verify and confirm the delamination onset. Results show that the delamination onset strain increase criterion, along with fatigue life, generally decreased as the operating temperature and humidity were increased and that frequency had little effect on the delamination onset fatigue life. The S– N curves obtained from the tension–compression fatigue data were then compared to those of a previous work carried out in tension–tension fatigue loading. Results show a clear degradation in the delamination onset fatigue life of the coupons tested under tension–tension cyclic loading when the minimum tensile component of the cyclic load was replaced with a compressive load of the same magnitude.

An experimental study of the damage growth in composite laminates under tension–fatigue after impact

The behavior of damaged laminates under cyclic load is crucial to the damage tolerance of composite structures. In this experimental study, we investigate impact damaged laminates under tension–fatigue loading. For that purpose, a tension after impact (TAI) specimen is designed, which exhibits a similar impact behavior like a specimen of the established compression after impact (CAI) standard. A set of quasi-static TAI tests is conducted as a reference for the investigation under cyclic load. A 30J impact was chosen as the baseline, causing a delamination-dominated barely visible impact damage. For the cyclic test, fifteen TAI specimens were distributed over three load levels. The tests reveal a widespread fatigue effect, edge delamination, and a significant growth of the impact-induced delamination. The damage propagates mainly in the load direction and in the interfaces close to the impact side. The observed delamination growth can be explained through to the fiber cracks caused by the impact. Moreover, the growth rate exhibits severe outliers, whose damage propagates exceptionally fast. These outliers can be traced back to the morphology of the impact damage. The qualitative pattern of fiber fracture massively influences the delamination propagation. Based on this dependency we outline the need for a qualitative evaluation of impact damage and its scatter.

A new criterion for determination S-N curve of CFRP under tension

The article is devoted to development of fatigue failure criterion of the CFRP specimen under fatigue tension loading. The main goal of the current study is to develop a quantitative failure criterion for fatigue damage of a specimen with open hole under tension. In response to this problem, the influence of the following parameters on the stability of the criterion and on its application was determined experimentally: manufacturing technology of a material, the loading conditions during testing. The usage of the failure criterion of thickening in the hole zone made it possible to obtain S-N curve while using the classical criterion does not allow to get a S-N curve at some cases. The application of developed failure criterion of thickening made it possible to reduce the standard deviation of the logarithm of SlgN durability. The dependence of the matrix damage area around the hole from the endurance at different loading levels was obtained according to the results of NDT method.

Residual strength of additive manufactured ABS parts subjected to fatigue loading

This study investigates the effect of tension fatigue loading on the residual strength degradation of additive manufactured ABS components formed by fused deposition modeling. Sinusoidal tension-tension fatigue loading with nine treatment combinations of cycling stress and duration were followed by tensile tests for 0°, +45/−45°, and +30/−60° mesostructures. Normalized residual strengths of the bidirectional mesostructures were significantly higher than that of the 0° parts for all factor combinations studied. A proposed phenomenological model, based on a strength degradation model for carbon fiber-reinforced composites, successfully described 92–96% of residual strength variability for the 0° specimens, and 77–92% for the bidirectional mesostructures tested.

Fatigue Analysis of Pre-cracked Aluminium Alloy Thin Sheets Repaired with a CFRP Patch at Elevated Temperature

The effect of fatigue life was studied at an elevated temperature of 80 °C of a centre pre-cracked specimen of aluminium alloy 6061-T6, repaired with a unidirectional carbon fibre-reinforced plastic (CFRP) patch. The patch was bonded only on one face of the aluminium alloy skin and tested under tension–tension fatigue load with a stress ratio of 0.1 and a frequency of 10 Hz. The performance of the two kinds of CFRP patches was studied with three-ply and five-ply patches. The fatigue life of pre-cracked bare specimen was improved substantially by bonding the CFRP patches. The performance of five-ply patch was significantly better than the three-ply patch. At low applied loads, the repaired specimen of both kinds failed with the growth of the crack tips all the way to the specimen edges. At high loads, the initial growth of fatigue crack was observed for few millimetres and the specimen failed due to the separation of the patch. At low loads and elevated temperature, the fatigue life was increased substantially over those of tests conducted at room temperature. However, at high loads, the failure life at elevated temperature was decreased significantly.