An investigation of the damage mechanisms and fatigue life diagrams of flax fiber-reinforced polymer laminates

In this paper we investigated the fatigue damage of a unidirectional flax-reinforced epoxy composite using infrared (IR) thermography. Two configurations of flax/epoxy composites layup were studied namely, [0]₁₆ unidirectional ply orientation and [±45]₁₆. The high cycle fatigue strength was determined using a thermographic criterion developed in a previous study. The fatigue limit obtained by the thermographic criterion was confirmed by the results obtained through conventional experimental methods (i.e., Stress level versus Number of cycles to failure). Furthermore, a model for predicting the fatigue life using the IR thermography was evaluated. The model was found to have a good predictive value for the fatigue life. In order to investigate the mechanism of damage initiation in flax/epoxy composites and the damage evolution, during each fatigue test we monitored the crack propagation for a stress level and at different damage stages, a direct correlation between the percentage of cracks and the mean strain was observed.     

Non-destructive assessment of the fatigue strength and damage progression of satin woven fiber reinforced polymer matrix composites

The aim of this study is to utilize infrared thermography to assess the critical damage states, and to capture the evolving damage processes, of 5HS and 8HS woven carbon fiber/epoxy composites subjected to uniaxial in-plane tensile quasi-static and fatigue loading. Quasi-static test results revealed that the dominant damage mechanisms were matrix cracks contained within the weft yarns, which initiated at the thermally-detected material thermoelastic limit and were confirmed through SEM observations. An established thermographic technique was also used to confirm the existence of a high cycle fatigue limit, which may in fact be a characteristic of all fabric reinforced polymeric composites. Temperature profiles captured during cyclic testing directly correlated with corresponding stiffness degradation profiles, providing support for thermography as an accurate fatigue damage metric. The infrared camera was able to detect the evolution of weft yarn cracking during the initial stage, as well as the initiation and growth of interply delamination cracking during the final stage of three-stage cyclic damage evolution. The reported results and observations provide an important step in the validation of thermography as a powerful non-destructive tool for assessing the development of damage, as well as predicting the critical damage states of fiber reinforced polymeric composite materials.     

Damage evolution and real-time non-destructive evaluation of 2D carbon-fiber/SiC-matrix composites under fatigue loading

Electrical resistance acquisition, acoustic emission (AE) monitoring and infrared thermography were employed to evaluate damage evolution of 2D carbon-fiber/SiC-matrix composite under fatigue loading. Damage evolution was discussed on the basis of the calculation results of the modulus and mechanical hysteresis variation. At lower stress levels, the majority of damage was produced in the first few cycles and then the rate of damage accumulation gradually approached a steady value as the cycles proceeded. When the applied stress exceeded the endurance fatigue limit, extensive damage took place and led to failure of the composite. Changes of composite electrical resistance, AE activity and surface temperature had fairly well agreement with the modulus and hysteresis responses. It can be concluded that it is possible to employ these real-time non-destructive evaluation methods as in-situ damage evolution indicators for this kind of composites under fatigue loading.     

Effects of local fibre waviness on damage mechanisms and fatigue behaviour of biaxially loaded tube specimens

Damage development in and final failure process of glass fibre winding specimens during biaxial fatigue loading are investigated. The phenomena in nominally defect-free tubes and specimens exhibiting local fibre waviness in one layer are compared. A subset of wound tubes is analysed using non-destructive testing methods, i.e. air-coupled guided waves, thermography, optical fracture analysis by a high-speed camera, and discrete damage monitoring. Air-coupled guided waves are employed for detection of fibre waviness and for monitoring the failure progress initiated by this waviness. Stiffness degradation due to fatigue damage corresponds to a decline in guided wave velocity. Using infrared inspection, the fibre waviness can be detected in an early stage of fatigue life. Non-destructive evaluation reveals that initiation of final failure in the specimens is caused by local fibre waviness. Finally, the effect of local fibre waviness on the S–N curves of the specimens is illustrated.     

An experimental analysis of fatigue behavior of AZ31B magnesium alloy welded joint based on infrared thermography

The fatigue behavior of AZ31B magnesium alloy welded joint during high cycle fatigue test was investigated by infrared thermography. Five stages of superficial temperature evolution were observed: an initial temperature increase, a temperature decline, a temperature equilibrium, an abrupt temperature increase and a temperature drop after the failure. The theoretical models were formulated to explain the observed temperature evolution. The mean temperature decline caused by thermoelastic effect was observed and discussed when the maximum stresses were below 30 MPa. The influence of weld reinforcement on fatigue behavior was also investigated. A good precision was achieved in fatigue strength prediction by means of infrared thermography.     

正交复合材料层板在静态与拉—拉疲劳下的损伤破坏

本文研究了正交铺层([0/90]s)碳纤维增强环氧复合材料在静态与拉—拉疲劳下的损伤破坏,探讨了它的破坏机理。通过采用显微观测,声发射技术及红外热象分析等多种手段测试其力学性能、损伤的产生和扩展,得到了不同加载条件下材料微结构的损伤断裂与声发射表征量及温度变化的关系。声发射累积量的突然增大可预报材料的断裂,温升的大小可供判别材料损伤的部位和程度。     

A new application of the infrared thermography for fatigue evaluation and damage assessment

A new method, based on the infrared thermography, was proposed and applied to rapidly determine the fatigue behavior of Q235 steel. The temperature evolution due to localized microplasticity was considered as the fatigue damage indicator, and good predictions were confirmed between the predicted values and the traditional values. The temperature patterns of the hot-spot zone on the specimen surface were tightly linked with the physical evolution of the fatigue damage in order to identify the damage status for safety evaluation. Based on the limiting energy theory, an energetic damage model was established to predict the residual fatigue life.     

Application of infrared thermography for the characterization of damage in braided carbon fiber reinforced polymer matrix composites

The focus of this study is to assess, using infrared thermography, the fatigue behavior and the corresponding damage states of a textile polymeric composite plate, as a prerequisite step in the development of damage based life prediction models for such advanced composite materials. Monotonic (quasi-static) loading test results confirmed that the dominant damage mechanism is cracking in the braider yarns, which was monitored using thermographic images and confirmed by edge replication microscopic observations. Fatigue results confirmed that the saturation of braider yarn cracks during cyclic loading corresponded to changes in the stiffness degradation rate as well as the surface temperature profile. This was confirmed by edge replication and scanning electron microscopic analysis. The reported results and observations provide an important step in the validation of thermography as a powerful non-destructive evaluation tool for monitoring the development of fatigue damage as well as predicting the damage states of laminated composite materials in general, and braided polymeric composite materials in particular.     

A multianalysis thermography‐based approach for fatigue and damage investigations of ASTM A182 F6NM steel at two stress ratios

Infrared thermography allows an alternative energy‐based approach for studying the fatigue behaviour of materials to better understand damage phenomena. In particular, the methodology of infrared thermography can explain the complex dissipative mechanisms promoted by the input parameters, such as the loading ratio, can rapidly provide information about the fatigue strength, and has low cost. In this work, analysis of the thermographic sequences of ASTM A 182 grade F6NM steel obtained during fatigue testing provided four thermal indexes that were used to investigate the thermoelastic and plastic behaviour of material. Fatigue tests at two opportunely chosen loading ratios (R = ?0.1, R = 0.5) were performed to investigate the relation between the material behaviour and each index at a specific loading ratio. Finally, estimation of the fatigue strength by means of suitable analysis procedures allowed for an investigation of the damage behaviour of materials under specific loading conditions.     

基于锁相红外热成像技术对铝合金铆接结构件疲劳极限的快速测定

该文将锁相红外热成像技术应用到2A12铝合金材料制成的铆接结构试件疲劳极限的快速测定上,将锁相红外热成像技术获得的结果与现有的该种材料在不同应力集中系数和应力比下疲劳极限试验数据进行插值得到的结果进行了对比,并与通过传统的阶梯法获得的试验结果进行了对比,验证了对于过盈量不大的普通铆接结构件来说,可以将锁相红外热成像技术应用到疲劳危险点处于多轴应力状态的金属铆接结构试件疲劳极限的快速测定上。     

Monitoring mechanical damage in structural materials using complimentary NDE techniques based on thermography and acoustic emission

This work deals with nondestructive evaluation (NDE) of the fracture behavior of metallic materials by combining thermographic and acoustic emission (AE) characterization. A new procedure, based on lock-in infrared (IR) thermography, was developed to determine the crack growth rate using thermographic mapping of the material undergoing fatigue. The thermography results on crack growth rate were found to be in agreement with measurements obtained by the conventional compliance method. Furthermore, acoustic emission was used to record different cracking events. The rate of incoming signals, as well as qualitative features based on the waveform shape, was correlated with macroscopically measured mechanical parameters, such as load and crack propagation rate. Additionally, since the failure modes have distinct AE signatures, the dominant active fracture mode was identified in real time. The application of combined NDE techniques is discussed for characterizing the damage process which leads to catastrophic failure of the material, thereby enabling life prediction in both monolithic aluminum alloys and aluminum alloy/SiC particle (SiC_p) reinforced composites.     

A unified approach for high and low cycle fatigue based on shakedown concepts

ABSTRACT The purpose of this paper is to present a unified analysis to both high and low cycle fatigue based on shakedown theories and dissipated energy. The discussion starts with a presentation of the fatigue phenomena at different scales (microscopic, mesoscopic and macroscopic) and of the main shakedown theorems. A review of the Dang Van high cycle fatigue criterion shows that this criterion is essentially based on the hypothesis of elastic shakedown and can therefore be expressed as a bounded cumulated dissipated energy. In the low cycle fatigue regime, recent results by Skelton and Charkaluk et al. show that we can speak of a plastic shakedown at both mesoscopic and macroscopic scale and of a cumulated energy bounded by the failure energy. The ideas are also justified by infrared thermography tests permitting a direct determination of the fatigue limit.     

Application of full-surface view in situ thermography measurements during ultrasonic fatigue of cast steel G42CrMo4

In the present investigation a full-surface view in situ thermography method is adapted to an ultrasonic fatigue testing system. Full-surface view in situ thermography measurements were successfully performed in the high cycle fatigue and in the very high cycle fatigue regime on cast steel G42CrMo4 in the quenched and tempered state. The method enables the monitoring of the entire cylindrical specimen circumference during fatigue testing by infrared temperature field measurements with one thermocamera and two mirrors. Moreover, by correlating fractography and thermography the precise determination of the location of the crack initiation site and the time of final crack growth is possible. The technique is applied to study crack initiation at non-metallic inclusions in the investigated cast steel specimens. Moreover, the effect of a novel carbon-bonded metal melt filter coated with a functionalized spinel (MgAl₂O₄) coating is evaluated by ultrasonic fatigue testing in combination with the full-surface view in situ thermography technique and subsequent scanning electron microscopy.     

In situ monitoring in real time of fatigue-induced damage growth in composite materials by acoustography

There are growing concerns about the effects of accidental impact damage on the structural integrity of aerospace composites and about the possible growth of the damage due to in-service fatigue. There has been some success in the use of established methods (ultrasonic C-scan, thermography, X-rays) to monitor damage development during fatigue experiments by interrupting a test and removing the specimen for damage inspection but this stop-and-restart test procedure is far from satisfactory. Real-time damage monitoring in composite materials during fatigue has now become possible by the emergence of a new ultrasonic imaging technology, acoustography. The successful integration of acoustography and a servo-hydraulic fatigue test machine has resulted in a new measurement system which can be used for the in situ monitoring in real time of damage growth in composite specimens during long-term fatigue tests. Results are presented which show damage-area growth during fatigue cycling under high compressive loads. After an initial small enlargement (stage 1), damage grows at a constant rate (stage 2) until the third stage is reached when there is further growth at an increasing rate to final failure. However, a ‘fatigue limit’ has also been observed. At stresses below this fatigue limit, a zero damage-growth régime has been found in studies of >10⁶ fatigue cycles. The results obtained have important implications for the understanding of the effects of damage on fatigue life and for the design of ‘safe’ damage-tolerant structures.     

Monitoring of multiaxial fatigue damage evolution in impacted composite tubes using non-destructive evaluation

Damage evolution in wound glass fibre reinforced tubes due to impact (8.4 J and 14 J) and subsequent biaxial cyclic loading is studied. Nominally defect-free and impact damaged specimens are compared to investigate the effect of the impact damage on the fatigue life of multiaxial composites. Non-destructive inspection (air-coupled guided waves, thermography, high-speed photography, and microscopy) is applied to a subset of tubes. Air-coupled guided wave scans for characterisation of the delaminations due to impact agree well with visual inspection. Decline in guided wave velocity is consistent to a decrease in stiffness caused by fatigue damage. Using thermal imaging the impact is detectable during cyclic loading. Strong anomalies of the surface temperature in the vicinity of the impact at the end of the fatigue life correspond to the initiation spot of final failure observed by high-speed imaging. The considerable effect of impact damage on the durability of the specimens is discussed.     

Lock-in infrared thermography for the evaluation of the structural performance of corrugated paperboard structures

This paper uses Lock-in infrared thermography to evaluate the stress and damage states in sandwich structured corrugated paperboard packaging. To this end corrugated boxes with and without handholds were subjected to compression loading and the resultant Lock-in thermal images captured. The stress contours around a handhold obtained from the Lock-in images enabled the failure mode and the structural response of the boxes to be assessed. As a result of this study it is shown that Lock-in infrared thermography is both fast and reliable, and as such has the potential to be a valuable tool for evaluating the structural performance of paperboard boxes and for rapidly evaluating different designs and materials.     

基于有限元法和锁相热像法对含缺陷构件的应力分析与疲劳性能评估

基于有限元法研究含盲孔缺陷构件的应力集中系数Kt随盲孔深度h和盲孔直径的变化规律。利用锁相热像法的热弹性分析模式(E-Mode)研究盲孔附近的应力分布,预测不同深度盲孔的Kt,与有限元结果相比较发现吻合良好。通过Altair Li软件中的耗散模式(D-Mode)和Altair软件分别研究构件在疲劳过程中的固有耗散量和温度信号的变化规律,以评估疲劳损伤的演化过程。以固有耗散和温度信号的变化规律作为疲劳损伤的指标,快速预测带盲孔试件的疲劳极限,进而预测试件的疲劳缺口系数Kf。理论计算的结果证明了锁相热像法的有效性。     

Multiaxial fatigue behaviour of GFRP with evenly distributed or accumulated voids monitored by various NDT methodologies

The effect of evenly distributed voids (dv) and regions of large accumulated voids (lav) on damage evolution and fatigue life during biaxial cyclic loading is studied. Various non-destructive test methods (thermography, microscopy and optical fracture analysis with high-speed photography) are presented for monitoring the void formation, multiaxial fatigue damage mechanisms and final failure process in glass-fibre reinforced plastics (GFRPs) manufactured by a filament winding machine. In addition, air-coupled guided wave measurements (non-contact and single-sided configuration) are applied to a subset of composite tubes for detection of voids and for observing the development of fatigue damages initiated by voids. Thermal imaging during cyclic loading reveals large accumulated voids as well. Variations of stiffness degradation, matrix cracking and guided wave velocities caused by finely distributed or accumulated voids are shown. Finally, the detrimental effect of the two types of porosity on the durability of multiaxially loaded composites is illustrated.     

Dissipated thermal energy and damage evolution of Glass/Epoxy using infrared thermography and acoustic emission

In this paper, we present an experimental approach for characterizing energy dissipation and degradation evolution in a woven Glass/Epoxy (G10/FR4) laminate subjected to fully reversed bending fatigue test. During cyclic loading, a fraction of the input mechanical energy is converted to thermal energy, which results in an increase in the temperature of the specimen. By analyzing the surface temperature and its drop rate after halting the cyclic operation, the dissipated thermal energy (DTE) is estimated. Infrared thermography is used to assess the temperature evolution and to various damage states. Acoustic emission is also utilized to corroborate the thermography results in characterizing the degradation progression. The results of these two non-intrusive techniques show similar evolutionary response revealing the existence of degradation stages. Using calculated DTE, a damage growth model is developed that appropriately characterizes the three damage phases during fatigue process of Glass/Epoxy.     

Eco‐Friendly Laminates: From the Indentation to Non‐Destructive Evaluation by Optical and Infrared Monitoring Techniques

Abstract: In this work, the combined effect of indentation damage and of manufacturing defects of a hybrid laminate including jute hessian cloth (plain weave) and hemp fibres in an epoxy matrix has been investigated. With this aim, various non‐destructive evaluation (NDE) techniques have been employed, such as near‐infrared (NIR) reflectography, infrared thermography (IRT), holographic interferometry (HI) and digital speckle photography (DSP). In particular, two different methods of heating were applied during IRT data collection: pulse thermography and square pulse thermography (SPT). The first one using a mid‐wave infrared (IR) camera, while the second one using a long‐wave IR camera. In the same way, two different cameras working into the near‐ and short‐wave IR spectra were used, to compare different results from ～ 0.74 to 14 μm. Data were processed applying principal component thermography (PCT), correlation and the robust second‐order blind identification (SOBI‐RO) algorithms. The latter is used for the first time to our knowledge in this work. The defects found were enhanced by image subtraction between the reflectogram and the transmittogram, distance transform and image fusion. In particular, data fusion from IRT and DPS images allowed clearly defining the extension of the indentation damage.