IDENTIFICATION OF DISTRIBUTED FATIGUE CRACKING BY DYNAMIC CRACK-CLOSURE
IDENTIFICATION OF DISTRIBUTED FATIGUE CRACKING BY DYNAMIC CRACK-CLOSURE
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动态裂纹闭合识别分布式疲劳裂纹
DOI：
10.1007/978-1-4615-1987-4_253
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
Gabor Blaho
中科院分区：
文献类型：
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作者：
P. Nagy;Gabor Blaho
早期疲劳损伤是影响结构多部位的典型分布现象。多点裂纹萌生和扩展发生在应力集中的许多点上，例如机翼面板上的铆钉孔。这种较大裂纹的合并可能导致突然断裂，并且可能在明显低于考虑单个裂纹抗断裂能力的载荷下发生[1,2]。在实验室条件下，多种无损检测技术可用于早期疲劳损伤检测和表征。这些技术包括声发射、线性和非线性超声、振动分析、涡流检测、磁学、热成像、x射线等。遗憾的是，这些方法大多不能直接适用于老化飞机的内场检测。其中一个原因是，最灵敏的超声波和涡流方法本质上是短距离的局部测量。在大型机身结构上，它们的使用仅限于对少数疑似高应力集中区域的检查，但当应用于分布和多点裂纹时，它们往往会非常迅速地失去灵敏度。大规模、全面的检查不可避免地降低了对分布式多位点裂缝的敏感性，特别是在存在导致误报警的人工制品的情况下。强伪信号是由检测过程中的不确定性（例如，超声波检测中的耦合或涡流检测中的上升）、固有几何特征（例如，附近的孔和边缘）和额外的不规则性（例如，不均匀加工、机械磨损、腐蚀等）引起的。似乎需要专门的远程超声波和涡流技术，这些技术可以更好地从固有的结构和材料变化中区分真正的疲劳裂纹。这些技术需要对疲劳裂纹所表现出的特征进行微调，并且只对疲劳裂纹进行微调。首先，我们将证明外部变形下的裂纹闭合可以显著提高远程超声和涡流检测方法的阈值灵敏度。其次，我们将论证用商业冷冻喷雾局部冷却产生动态裂缝闭合的可行性。
Early fatigue damage is typically a distributed phenomenon affecting the structure at many locations. Multiple-site crack initiation and growth occur at numerous points of stress concentration, e.g., at rivet holes in a wing panel. Coalescence of such relatively large cracks can lead to sudden fracture, and may occur at loads significantly below what would be expected from considering the fracture resistance of single cracks [1,2]. Under laboratory conditions, a great variety of NDE techniques are available for early fatigue damage detection and characterization. These techniques include acoustic emission, linear and nonlinear ultrasonics, vibration analysis, eddy current inspection, magnetics, thermal imaging, X-ray, etc. Unfortunately, most of these methods cannot be directly adapted to infield inspection of aging aircraft. One reason for this is that the most sensitive ultrasonic and eddy current methods are essentially short-range, localized measurements. On a large airframe structure, their use is limited to the inspection of a few suspected areas of high stress concentration, but they tend to loose sensitivity very rapidly when applied to distributed and multiple-site cracking. Large-scale, overall inspection has inevitably lower sensitivity to distributed multiple-site cracking, especially in the presence of artifacts causing false alarms. Strong artifact signals are caused by uncertainties in the inspection procedure (e.g., coupling in ultrasonic testing or lift-off in eddy current testing), inherent geometrical features (e.g., nearby holes and edges), and additional irregularities (e.g., uneven machining, mechanical wear, corrosion, etc.). There seems to be a need for specialized long-range ultrasonic and eddy current techniques which do a better job at distinguishing real fatigue cracks from inherent structural and material variations. These techniques need to be fine-tuned to characteristic features exhibited by fatigue cracks and only fatigue cracks. First, we shall demonstrate that crack-closure under external deformation can be exploited to achieve significant improvements in the threshold sensitivity of long-range ultrasonic and eddy current inspection methods. Second, we shall demonstrate the feasibility of producing dynamic crack-closure by localized cooling with a commercial freezing spray.