锁相红外热成像技术测量结构的应力分布

通过试验研究了运用锁相红外热成像技术进行结构应力测量时,加载频率的改变对于测量结果的重要影响,给出了合理确定加载频率这一重要参数的试验方法。将锁相红外热成像技术应用到航空工业飞机机翼蒙皮与骨架之间铆接结构应力分布地研究中,显示了锁相红外热成像技术在结构应力测量方面相对于传统应变片电测技术的明显优越性。     

红外热成像技术在零件无损检测中的发展和应用现状

红外热成像技术是一种通过外加激励获得零件表面温度场分布,并从热像图中提取零件损伤信息的无损检测技术,它具有快速、实时、非接触等优点,研究和应用前景广阔。综述了红外热成像无损检测技术的发展现状和应用实例,通过对比3种常用的红外热成像技术:脉冲红外热成像、超声红外热成像和锁相红外热成像,认为超声锁相红外热成像技术具有其他几种技术无法比拟的优势,并从检测工艺和信号处理等方面提出了改进此技术的具体措施。     

几种常用的金属材料疲劳极限试验方法

金属疲劳试验用于测定金属材料的许用疲劳应力,绘制材料的疲劳曲线,进而在交变应力下测定金属材料的疲劳极限。疲劳研究的试验方法有很多,该文根据有关国家标准和现有文献资料对一些常用疲劳试验方法进行了综述,包括单点疲劳试验法、升降法疲劳试验、高频振动疲劳试验法、超声波法疲劳试验、红外热像技术疲劳试验方法,并对每种疲劳试验方法的试验目的、适用条件、试验试样、所需仪器、具体步骤和数据处理进行了介绍。     

红外热像技术测量Ⅰ型裂纹应力强度因子的研究

目前工程中出现的裂纹形态主要是I型,大多数工程构件中裂纹的应力强度因子难以直接接触测量,建模计算也因受力条件复杂而耗时和困难。红外热像技术以非接触式测量方法为研究材料和结构的断裂问题提供新的实验手段。通过研究在不同频率循环拉伸条件下304不锈钢试件的热-力响应,得到材料体积应变与温度变化的定量关系,进而对紧凑拉伸试件进行循环加载,同步测量裂纹尖端的温度场,获得I型材料裂尖附近应力强度因子。实验结果表明:在近绝热条件下,材料热弹性区的温度变化与体积应变成定量线性关系;不同荷载条件下实测裂纹尖端附近应力强度因子与理论值对比,误差均在2%以内,说明利用红外热像实验方法测量工程实际中循环荷载情况下的应力强度因子是可行并且可靠的。     

小子样测定叶片S-N曲线试验方法研究

根据试验要求,对叶片在样本较少情况下测定其疲劳极限σ-1和S-N曲线。通过振动疲劳试验的方法进行研究,试验采用升降法选择所施加振动应力的水平,采用异方差回归分析方法进行疲劳试验数据的分析。最后获得叶片材料的均值S-N曲线和置信度为95%、可靠度为99.9%的P-S-N曲线,并得到各自曲线上的疲劳极限σ-1。此方法保证试验的顺利进行并且具有较高的可靠性,对于小样本测定材料的S-N曲线具有较好的实用价值。     

铆接界面端三维奇异性应力场的研究

针对铆接结构的特点,应用特征函数扩展技术分析柱坐标下接触界面端的应力奇异性问题。建立了柱坐标下圆柱体端面接触边缘附近的三维渐近位移场和应力场渐近表达式,并根据铆钉/被铆接件接触界面端的位移和应力边界条件,建立一个非线性特征方程组。据此方程组可求解界面端邻域的应力奇异性指数、位移和应力角分布函数的数值解。通过与有限元方法计算结果相对比,验证了该方法的有效性。分析了平头、沉头以及半圆头铆钉构成的铆接结构的应力奇异性问题,考察了铆钉材料、几何形式和摩擦系数对接触界面端应力奇异性指数和应力场角分布的影响。     

基于热红外成像技术的300M钢疲劳极限测定

对经过热处理和未经过热处理的超高强度300M钢在室温下的常规疲劳性能进行了研究,用红外成像仪测量了疲劳试验过程中合金表面的温度变化;根据疲劳试验过程中温度-应力关系及试样表面温度分布差异,确定疲劳断裂位置和疲劳极限。结果表明:根据试件表面温度变化可以预测疲劳断裂位置;以此法确定的热处理300M钢疲劳极限为833MPa,与常规疲劳试验测得的疲劳极限吻合较好;加载初期300M钢升温到一定温度后形成平稳段,在断裂前温度快速升高。     

基于锁相红外热成像理论的复合材料网格加筋结构的无损检测

基于锁相红外热成像理论, 对复合材料网格加筋结构的几类典型缺陷进行无损检测, 采用法国Cedip公司开发的锁相红外热成像系统对检测结果进行分析。讨论了加载频率、 输出电压偏移量对检测的影响。结果表明, 相位图比幅值图含有更多的缺陷信息。不同的加载频率会产生不同的检测结果, 选择恰当的加载频率是检测的关键; 增加输出电压偏移量有利于检测。该方法可用于对复合材料未知缺陷的检测。      

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。     

双相型不锈钢角焊缝连接承载性能有限元分析

为研究双相型S22053不锈钢角焊缝连接的承载性能,对不锈钢母材和焊缝金属圆棒试样进行材性试验,分别获得了母材和焊缝金属材料的本构关系;对不锈钢正面和侧面角焊缝连接试件进行拉伸试验,测得试件极限承载力和焊缝变形发展情况;利用ANSYS有限元软件对不锈钢角焊缝连接试件单向拉伸试验进行了非线性有限元模拟,并将试验结果与模拟结果进行了对比。对比结果表明,考虑不锈钢母材和焊缝金属本构关系差异并使用材料真实应力-应变曲线的有限元模型能有效模拟角焊缝连接承载性能拉伸试验。最后,利用经试验验证的有限元模型对角焊缝承载力影响因素进行了参数分析,并将有限元模拟值与按照美国规范SEI/ASCE 8-02、欧洲规范EN 1993-1-4,以及中国不锈钢结构技术规程CECS 410:2015计算求得的规范设计值进行了对比,得出了承载力模拟值与各国规范设计值的比值,可为不锈钢设计规范的完善和工程应用提供参考。     

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.     

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.     

S‐N curve for riveted details in corrosive environment and its application to a bridge

A formula for stress‐life curve is proposed to predict the fatigue life of riveted bridges located in corrosive environments. The corrosive environment‐dependent parameters of the S‐N curve are determined based on the corrosion fatigue testing results of different types of steel specimens in air, fresh water, and seawater. Eurocode detail category 71 and UK WI‐rivet detail category represent the fatigue strength of riveted members. The proposed S‐N curve formula is compared with full‐scale fatigue test results of riveted joints, plate girders, and truss girders, which were tested in a corrosive environment. Thus, the validity of the formula is confirmed. The formula does not require any material parameter other than the code‐given fatigue curve of riveted details. The fatigue life of a riveted railway bridge is estimated by using the proposed formula, and the results are compared with conventional approaches. The applicability and significance of the proposed curve are confirmed.     

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.     

Crack‐Tip Stress Field and Fatigue Crack Growth Monitoring Using Infrared Lock‐In Thermography in A359/SiCp Composites

Abstract: This paper deals with the study of fracture behaviour of silicon carbide particle‐ reinforced aluminium alloy matrix composites (A359/SiCp) using an innovative non‐destructive method based on lock‐in thermography. The heat wave, generated by the thermo‐mechanical coupling and the intrinsic energy dissipated during mechanical cyclic loading of the sample, was detected by an infrared camera. The coefficient of thermo‐elasticity allows for the transformation of the temperature profiles into stresses. A new procedure was developed to determine the crack growth rate using thermographic mapping of the material undergoing fatigue. The thermographic results on the crack growth rate of A359/SiCp composite samples with three different heat treatments were correlated with measurements obtained by the conventional compliance method. The results obtained by the two methods were found to be in agreement, demonstrating that lock‐in thermography is a powerful tool for fracture mechanics studies. The paper also investigates the effect of heat treatment processing of metal matrix composites on their fracture properties.     

Fatigue life estimation of coach peel riveted joints using multi-axial fatigue criteria

This study investigates fatigue behaviour of riveted joints in a coach peel configuration under various load ranges and load ratios. Three well-known multi-axial fatigue criteria of Smith–Watson–Topper, Glinka and Fatemi–Socie are used to predict the fatigue life of the joint based on the stress and strain components obtained from a finite element analysis. A three-dimensional finite element model was developed with considering elastic–plastic behaviour of aluminium alloy 2024-T3 for the joint plates as well as frictional contact between all the contacting parts. Fatigue life estimation results agreed well with existing experimental data for the same joint. This verifies the finite element results and shows how well the multi-axial fatigue equations can predict fatigue life of the riveted joints. The finite element modelling approach may be then used with confidence for fatigue life estimations of different configurations of riveted joints.     

Modelling of the lock-in thermography process through finite element method for estimating the rail squat defects

Squats are a major problem on the world railways. The non-destructive evaluation technique is becoming increasingly attractive in the detection of near surface defects on track. Non-destructive thermal evaluation is one such method of inspection technique that can be used for the detection of near surface defects. Its sub-group of lock-in thermography is under analysis. Lock-in thermography utilizes an infrared camera to detect the thermal waves and then produces a thermal image, which displays the local thermal wave variation in phase or amplitude. There are few studies into the actual experimental representation of complex subsurface defects when concerning lock-in thermography processes. While this may be less of a concern given the purpose of numerical defect characterization to reduce the need for extensive experimental pre-tests, the necessity for (artificial) representations of a defect will inevitably be required for validation. The research outlined in this paper examines the use of 3D finite element modelling (FEM) as a potential flexible tool in simulating the lock-in thermography process for detecting squats in track. In addition, lock-in analysis proved that the correct frequency range had to be selected for the material to detect the defect. As maximum positive and negative phase angles were located at “optimum” frequencies, at certain frequencies lead to minimal phase angle difference to which the defects were not detectable (blind frequency) by using the incorrect testing. The 3D finite element method has advantage for determining the “optimum” thermal excitation frequencies compare with experimental investigation. The experimental results show that 3D FEM models can be used to defect the location and the depth of squats in the railway.