Investigation of cold expansion of short edge margin holes with pre‐existing cracks in 2024‐T351 aluminium alloy

The United States Air Force has requirements to inspect and cold expand potentially thousands of fastener holes for an aircraft fleet, and the presence of existing cracks at those fastener holes is expected. Fatigue experiments were performed to investigate the resulting fatigue crack growth life of a fastener hole that contained a representative ‘unknown’ crack at the time of inspection (approximately 0.050 in. in length) at a short edge margin hole that was then cold expanded and compare that to a non‐cold expanded hole and a cold expanded hole with no pre‐existing cracks. The United States Air Force analytical approach used to account for the benefit due to cold expansion was compared to the experimental data and does not consistently provide conservative predictions.     

Considerations in the Validation and Application of Models for Eddy Current Inspection of Cracks Around Fastener Holes

To improve the detection and characterization of cracks around fastener holes in multilayer structures without removing the fastener, model-based approaches are proposed to support the design of advanced eddy current (EC) NDE systems. This work demonstrates the validation and application of models to simulate EC inspection as part of the design process. The volume integral method (VIM) and finite element method (FEM) are both used to simulate eddy current inspection of fastener sites for fatigue cracks. Convergence studies, validation with existing models, experimental validation studies and validation through inverse method demonstrations are presented, providing a continuum of methods to ensure the quality of measurement models. Consideration concerning convergence and validation is also given with features sensitive to the sample geometry and flaw characteristics. A novel calibration technique is also presented to practically evaluate the transformation between model-based impedance calculations and experimental voltage data. A series of studies are presented concerning the detection of cracks around fastener holes demonstrating the quality of the simulated data to represent experimental measurements.     

含紧固件复合材料层压板雷击后损伤及力学性能退化试验

以含金属紧固件编织结构碳纤维/环氧树脂复合材料层压板为研究对象,通过人工模拟雷电流试验,并结合目视观察、超声损伤扫描、微米X射线三维成像及高倍显微镜等损伤检测手段,对其在雷电流直接效应作用下的损伤模式、机制及特征进行了分析,同时,采用材料力学性能试验系统,对其雷击后的静拉伸承载能力退化程度进行了评估。研究结果表明：由金属紧固件引起的"雷电流分散效应"导致含紧固件复合材料层压板雷击损伤以分层损伤为主,且损伤沿层压板整个厚度方向分布;雷击损伤的产生存在一个雷电流强度门槛值,当雷电流强度小于该值时,层压板不会出现损伤;紧固孔周围的雷击损伤在静拉伸过程中会产生"应力分散效应",导致含紧固件层压板静拉伸承载能力随雷击峰值电流的增加先提高后下降。     

Characterizing the performance of eddy current probes using photoinductive field-mapping

We present a new method for characterizing the performance of eddy current probes by mapping their electromagnetic fields. The technique is based on the photoinductive effect, the change in the impedance of an eddy current probe induced by laser heating of the material under the probe. The instrument we developed maps a probe's electric field distribution by scanning an infrared laser beam over a thin film of gold lying underneath the probe. Measurements of both photoinductive signals and flaw signals for a series of similar probes demonstrate that the impedance change caused by an electrical-discharge-machined (EDM) notch or a fatigue crack is proportional to the strength of the photoinductive signal. Thus, photoinductive measurements can supplant the use of artifact standards to calibrate eddy current probes. Furthermore, the shape and symmetry of the probe's field pattern can reveal defects in probe construction. By combining photoinductive measurements of a probe's field strength with a theoretical model, we are able to quantitatively predict the probe's performance under hypothetical conditions. To model commercial eddy current probes with ferrite cores, we developed a procedure to treat them as effective air-core probes. We obtained good agreement between the flaw signals calculated using this effective-coil approach and actual fatiguecrack signals measured with commercial probes. We also calculated probabilities of detection for target flaws in titanium alloys for a series of commercial probes. The results reveal how probe sensitivity can affect the reliability of an eddy current inspection.This article is dedicated to Professor Bertram A. Auld on the occasion of his 70th birthday and his retirement from Stanford University.     

A Novel Defect Inspection Method for Semiconductor Wafer Based on Magneto-Optic Imaging

The defects of semiconductor wafer may be generated from the manufacturing processes. A novel defect inspection method of semiconductor wafer is presented in this paper. The method is based on magneto-optic imaging, which involves inducing eddy current into the wafer under test, and detecting the magnetic flux associated with eddy current distribution in the wafer by exploiting the Faraday rotation effect. The magneto-optic image being generated may contain some noises that degrade the overall image quality, therefore, in this paper, in order to remove the unwanted noise present in the magneto-optic image, the image enhancement approach using multi-scale wavelet is presented, and the image segmentation approach based on the integration of watershed algorithm and clustering strategy is given. The experimental results show that many types of defects in wafer such as hole and scratch etc. can be detected by the method proposed in this paper.     

Finite element modelling of closed cracks in eddy current testing

The eddy current inspection of small fatigue cracks in Ti–6AL–4V is evaluated in both a finite element model and experiments. The crack was created in a fatigue process and an eddy current measurement was carried out as the resulting crack was subjected to different levels of static load. The signal showed a strong dependency of the time between the creation of the fatigue crack and the eddy current measurement. This dependency is proposed to be related to oxides forming on the crack faces. The oxide is favourable for the detection of fatigue cracks. The narrow width of the fatigue crack is important to consider in eddy current inspection and as static loads are applied across the crack faces, electrical connections arise within the crack, which has a strong influence on the eddy current signal. Four different models of the contact behaviour were implemented within the finite element model. It is shown that the electrical connections that arise within small fatigue cracks, as well as the influence from the narrow opening as tensile loads are applied, can be predicted by a finite element model of the eddy current method.     

金属材料缺陷的电磁超声/涡流复合检测技术研究

电磁超声检测和涡流检测因其非接触、检测速度快、对试件表面要求低等优点而被广泛应用于金属材料的缺陷检测中,但电磁超声检测存在近表面的检测盲区,涡流检测对内部深层缺陷灵敏度不高。基于电磁超声和涡流的复合检测方法,设计了能同时满足电磁超声检测和涡流检测的复合式探头,建立了电磁超声和涡流复合检测有限元模型,并对金属试件中不同类型的缺陷进行了检测实验。仿真和实验结果表明,该复合探头不仅能快速检测表面裂纹,而且可激发出具有明显指向性的纵波,一定程度上削弱了波形转换产生的干扰波,可实现对内部缺陷的准确定位、识别,为电磁超声和涡流复合式检测技术在板材的复杂缺陷检测中的应用提供了基础。     

The development of fatigue damage around fastener holes in thick graphite/epoxy composite laminates

This paper discusses the mechanisms by which damage develops and grows around countersunk fastener holes in composite laminates under fatigue loading conditions. Experiments have shown that the erosion of material between ply layers nucleates delaminations which then grow through the laminate under the action of the fatigue loads. The damage at the hole bore surface produces sites for the nucleation of delaminations. The morphology of the delaminations and ply cracking was mapped extensively and from these maps it was found that the volume of material around the fastener hole, damaged by the fatigue loading, adopted a characteristic shape; the volume of damaged laminate increased towards the faying surface of the laminate and (metal) fixture. This characteristic damage volume was generated by the fastener rocking under the fatigue loads. Growth of the delaminations has been shown to be preceded by intraply cracking and, as fatigue loading proceeded, more delaminations were generated at the hole bore surface. The interfacial region between the composite laminate and the fixture also provided the nucleation site for fatigue cracks in the fasteners. The effects of initial fastener-hole clearances on damage nucleation and growth did not appear to follow any clear pattern. However, coupons with excessive initial hole clearances did appear to exhibit greater than expected damage growth after only one sequence block.Several techniques for the measurement of damage growth (development) were investigated. Stiffness measurements of the test system were found to show only small changes with hole wear and fastener rocking (using shadow moire techniques) also showed only small changes with hole wear. Ultrasonic C-scanning methods were used to map the extent of damage around the fastener holes with fatigue loading.The experimental work has shown that damage development around fastener holes is a complex process, usually producing several delaminations in the region of the fastener hole which grow and may ultimately lead to the failure of the coupon.     

Methodology of short fatigue crack detection by the eddy current method in a multi-layered metal aircraft structure

The field of non-destructive inspection (NDI) is an integral part of aircraft maintenance and service. It is optimal to apply the eddy current (EC) method on the complex multi-layered metal aircraft structure to detect cracks and other damage. A range of standard eddy current probes giving satisfactory results with respect to crack detection are available. However, these EC probes are not always suitable for the detection of short fatigue cracks hidden under a rivet head. This article presents a new methodology for eddy current inspection of a critical area on the wing. The development of the new methodology was inspired by a catastrophic glider accident in 2010. Because of this accident, all glider service was prohibited. The critical area is located in the structure of the wing spar, which consists of six layers. The position of the critical area is hidden under the countersunk rivet head in the third layer of the spar flange and under two layers of metal sheets. Thus, the actual location of the area is in the fourth layer, created by the countersunk rivet head. In addition, another rivet of a smaller diameter is inserted into the rivet in the flange. This article presents the work related to the development of a special EC probe that is able to measure in an operating frequency range between 200 Hz and 100 kHz with a single-value interpretation of the individual EC signals.     

飞机多层金属板隐藏缺陷远场涡流探头设计及试验研究

由于飞机多层金属板结构厚度大、复杂等特性,现有检测方法无法发现原位内部缺陷,对于多层板金属缺陷的检测一直都是航空无损检测的难题.远场涡流检测技术因打破趋肤效应的限制,涡流能量可穿透较厚的被测试件,对金属板结构中隐藏缺陷的检测具有潜在优势.该文针对飞机多层金属板隐藏裂纹的原位检测,建立多层金属板构件隐藏裂纹平面远场涡流检...     

Mixed mode fatigue growth of curved cracks emanating from fastener holes in aircraft lap joints

The finite element alternating method is extended further for analyzing multiple arbitrarily curved cracks in an isotropic plate under plane stress loading. The required analytical solution for an arbitrarily curved crack in an infinite isotropic plate is obtained by solving the integral equations formulated by Cheung and Chen (1987a, b). With the proposed method several example problems are solved in order to check the accuracy and efficiency of the method. Curved cracks emanating from loaded fastener holes, due to mixed mode fatigue crack growth, are also analyzed. Uniform far field plane stress loading on the plate and sinusoidally distributed pin loading on the fastener hole periphery are assumed to be applied. Small cracks emanating from fastener holes are assumed as initial cracks, and the subsequent fatigue crack growth behavior is examined until long arbitrarily curved cracks are formed near the fastener holes under mixed mode loading conditions.     

Analysis technique for interaction of high-frequency rhombicinducer field with cracks in metals

In the nondestructive evaluation (NDE) involving the ac field measurement (ACFM), a current carrying structure is required to induce the eddy current in the work-piece and a probe to sample the field. Due to its flat profile, slender shape, and other advantages, the rhombic wire loop is a suitable inducer for developing linear flexible arrays for the ACFM inspection of large surfaces of ferrous and nonferrous metals. This paper introduces an analysis technique for the evaluation of the interaction of the field of the rhombic inducer carrying a high-frequency current, with long surface cracks of uniform depth in flat metal plates. The technique is accurate and very efficient computationally. It uses the two-dimensional Fourier transform together with a special boundary condition at the metal surface. The boundary condition takes into account the thin-skin nature of the eddy current in the metal as well as the flux leakage at the crack mouth. The analysis technique benefits from the use of scalar potential functions and can be extended to simply or multiply connected wire loops. Also, it is applicable to high-frequency (thin-skin) eddy current problems. Using the analysis technique, the tangential field below a rhombic inducer along its long diagonal when the inducer is located above the surface of aluminum and steel is given in the presence and absence of a crack. This field was found to have a nonuniform phase distribution. Near a crack, the phase change is significant, even for shallow cracks. The role of the nonuniform phase in the detection sensitivity is addressed. Also, simulated ACFM crack responses using an inducer with a linear probe attached along the long diagonal are presented and discussed. To support the validity of the analysis technique, experimental results obtained for some of the simulations are also reported. In addition to its application in predicting crack responses, the technique can be used for model-based inversion of crack signals     

On statistical moments of fatigue crack propagation

A new statistical theory is proposed for the analysis of fatigue crack propagation, based on the concepts of fracture mechanics and random processes. Focus is centered on conceivably more useful information of the random time at which the crack size grows to any specific value. Given an initial crack size, recursive relationship is obtained for the statistical moments of this random time for a rather general class of material behaviors, and examples are given for the case where the crack propagation rate is governed by a power law. A procedure to estimate the parameters in the power-law model is also illustrated, using the experimental data of some 7475-T7351 aluminum fastener hole specimens subjected to the excitation of a certain bomber load spectrum.     

Pulsed Eddy Current Technology: Characterizing Material Loss with Gap and Lift-off Variations

Ownership costs of operational aircraft have increased steadily over the years. One of the major cost drivers is structural deterioration due to corrosion. Beyond the economics, finding and characterizing corrosion is essential for the continued airworthiness of aircraft fleets. To this end, the pulsed eddy current technique holds the potential of becoming the primary means of detecting corrosion in multilayered structures. Its wide-band frequency spectrum allows the determination of a large number of parameters, such as defect size and location. Pulsed eddy current is still considered an experimental nondestructive technique because of realistic inspection problems (e.g., probe tilting, protrusion of rivets, and thickness variations in adhesive and paint) have not been addressed in the past. Recent advances change this situation and allow pulsed eddy current to be a credible field technique.     

The quantitative relations between true and standard depth of penetration for air-cored probe coils in eddy current testing

The true depth of penetration of eddy currents generated in a conducting sample by an air-cored probe coil, besides depending on the electromagnetic wave frequency and the magnetic permeability and electrical conductivity of the sample, also depends strongly on the coil dimensions and the sample thickness. The standard depth of penetration widely used as a guide for eddy current inspection purposes is calculated for a plane electromagnetic wave incident perpendicularly on a conducting half-space and is thus a material/test parameter rather than a true measure of penetration. In this paper the quantitative relations between true depth of penetration and standard depth of penetration are presented for three configurations of eddy current probe and test material. First an air-cored coil above a conducting half-space is considered, then the same coil above a conducting sheet, and finally the true depth of penetration is calculated in a conducting half-space covered with cladding for different ratios of condictivity between cladding and base material.     

Pulsed Eddy Current Technology: Characterizing Material Loss with Gap and Lift-off Variations

Abstract

Ownership costs of operational aircraft have increased steadily over the years. One of the major cost drivers is structural deterioration due to corrosion. Beyond the economics, finding and characterizing corrosion is essential for the continued airworthiness of aircraft fleets. To this end, the pulsed eddy current technique holds the potential of becoming the primary means of detecting corrosion in multilayered structures. Its wide-band frequency spectrum allows the determination of a large number of parameters, such as defect size and location. Pulsed eddy current is still considered an experimental nondestructive technique because of realistic inspection problems (e.g., probe tilting, protrusion of rivets, and thickness variations in adhesive and paint) have not been addressed in the past. Recent advances change this situation and allow pulsed eddy current to be a credible field technique.     

Boundary element analysis of cracks at a fastener hole in an anisotropic sheet

Stress intensity factors for cracks emanating from a fastener hole are obtained, using the principle of superposition and a Green's function for a point force applied in an anisotropic sheet with an elliptical hole. Various loading cases, such as a point load, a uniform pressure applied on an arc, and a cosine distribution of pressure acting over half the circumference of the hole are considered to simulate the bolted-joint load. Both cases of a single crack and of two cracks of unequal lengths are studied. The accuracy and simplicity of the technique are demonstrated by comparing numerical results for various loading conditions and crack lengths with their known isotropic counterparts.     

Effect of detail design on fatigue performance of fastener hole

In the present study, a series of tests were conducted on aluminum alloy 2024 to investigate the effect of detail design on the fatigue behavior of fastener holes in specimens. Two types of detail designs were concerned. One was the mode of fastener holes (countersunk rivet or countersunk bolt), the other was drilling process (traditional air-drilling process or one step compound cutting process). The fracture surfaces were observed by means of an optical microscope. Finite element method (FEM) was employed to analyze the distribution of stress around the fastener holes. The results showed that crack always initiated at the hole edge where the stress concentration occurred. Crack initiation was induced by stress concentration. Crack initiation life accounted for 80% of total fatigue life of fastener holes. The fatigue life of fastener hole using countersunk rivet was longer than that using countersunk bolt. Contrasted to traditional air-drilling process, the fatigue life of fastener hole could be improved by 44–55% using one step compound cutting process. However, the dispersibility of fatigue life became increasingly severe when fatigue life was prolonged.     

Effect of defects on fatigue life of plates with fastener holes

The behavior of defects (inclusions and cavities) on fatigue life of plates with fastener holes under tensile loading has been analyzed. Special attentions have been paid on the influence of the size and location of defects on fatigue life of plates with fastener holes. Thirty‐five different finite element models of plates with different size and location of defects are established and the nominal stress method is used to estimate the fatigue lives of this models. The results show that there is a region whose center is the maximum stress point of the hole without defects. When the defect is located in this region, the influence of defects on fatigue life of plates with fastener holes is obvious. When the defects are far away form this region' center, the defects hardly influence the fatigue life of plates with fastener hole. The larger the size of the defect is, the bigger this region is. In this region, the larger the size of the defect and the shorter the distance between the defect and the region's center, the shorter the fatigue life of plates with fastener hole is.