Ultrasonic Characterization of Material Interfaces and Flaws

材料界面和缺陷的超声表征超声检测已成为工业中表征工程结构缺陷和评估材料性能的主要工具；这反映了超声诊断技术在生物医学领域的平行发展。在其最简单的形式中，中心频率在MHz范围内的短机械脉冲被发送到工程部件中，在那里它与缺陷、界面和其他类型的材料不连续相互作用。捕获衍射波和反射波，然后进行分析，以确定被检查部件的内部结构。然后，在进行下一次检查之前，可以使用数值建模来评估应力水平和部件失效的概率。更先进的测试模式包括共振测试、材料各向异性评估、粒度分布评估和非线性材料性能测量。这项技术正受到以下因素的挑战：(A)对产品安全的日益关注，(B)使用脆性材料，如低断裂韧性的陶瓷，(C)需要检查非常热的或放射性的组件，(D)给波传播带来复杂性的新型复合材料，以及(E)将工程材料推向其理论极限的日益增长的趋势，即使是最小的缺陷也可能是关键的。在这项研究计划中，我们正在开发超声波无损检测技术的几个关键要素，以应对这些挑战。每个项目都得到了业界的积极参与，并得到了政府授权机构和私人赞助商的财政支持：(1)开发用于高温和/或伽马辐射恶劣环境的新型超声波换能器。该项目的独特之处在于压电式元件的衰减衬垫元件--我们正在开发一种新型的多孔陶瓷材料，该材料具有这一特殊作用所需的高温稳定性、声阻抗和衰减。(2)新的数字信号处理策略，以增强超声图像的清晰度，使缺陷大小或裂纹深度可以测量到一毫米的几分之一。该项目采用了一个研究领域中常用的信号处理技术，如生物医学成像或地球物理，并将它们开发为工业无损评估领域的新应用。(3)材料界面的超声表征，特别是那些具有部分降解粘合力的界面--例如，环境退化的粘结剂；涡轮叶片中的冷关闭；挤压管中的研磨缺陷。在这类问题中，我们面临着一个界面区域，它的厚度远远小于传统换能器产生的典型超声波长~1 mm。要获得足够的成像分辨率来解决这类问题，需要多种技术的结合：甚高频换能器；提取高频分量的频谱外推技术；使用波长更短、对弱界面比压缩波更敏感的角度横波；波界面反射和传输的频率分析。(4)波在非均匀材料中传播的有限元模拟。这个问题不仅涉及到功能梯度材料，而且还涉及到具有很强温度梯度的材料，例如在线工业检测中遇到的材料。不仅声速会随温度变化，而且当波遇到温度梯度时，它会“歪曲”到弯曲的轨道上。

Ultrasonic Characterization of Material Interfaces and FlawsUltrasonic inspection has become the primary tool for industry to characterize flaws in engineering structures, and to evaluate material properties; this mirrors a parallel development of ultrasonic diagnostic techniques in the biomedical field. In its simplest form, a short mechanical impulse, with a central frequency in the MHz range is sent into an engineering component, where it interacts with defects, interfaces and other types of material discontinuities. Diffracted and reflected waves are captured, and then analyzed to determine the internal structure of the component under examination. Numerical modeling can then be used to assess the stress levels and the probability of component failure before the next inspection takes place. More advanced testing modes include resonance tests, evaluation of material anisotropy, grain size distribution assessments, and measurements of non-linear material properties.This technology is being challenged by factors such as (a) an increasing focus on product safety, (b) the use of brittle materials such as ceramics with low fracture toughness, (c) the need for inspection of very hot or radioactive components, (d) new composite materials that introduce complexities to wave propagation, and (e) a growing tendency to push engineering materials closer to their theoretical limit where even the smallest flaw can be critical. In this research program, we are developing several key elements of ultrasonic nondestructive evaluation technology to meet these challenges. Each project has active involvement from industry, with financial support coming from government granting agencies and private sponsors:(1) Development of new ultrasonic transducers for use in harsh environments of high temperature and/or gamma radiation. The key unique aspect of this project is the attenuative backing element for the piezoleectric element – we are developing a new form of porous ceramic material that has the high-temperature stability, acoustic impedance, and attenuation required for this special role. (2) New digital signal processing strategies to enhance the sharpness of ultrasonic images, such that defect size or crack depth can be measured to a fraction of a millimeter. This project takes signal processing techniques commonly used in one field of study such as biomedical imaging or geophysics, and develops them for new applications in the industrial nondestructive evaluation world. (3) Ultrasonic characterization of material interfaces, in particular those featuring partially degraded adhesion –e.g., an environmentally degraded adhesive bond; a cold shut in a turbine blade; a lapping defect in an extruded tube. In this class of problems, we are confronted with an interfacial region whose thickness is substantially less than the typical ultrasonic wavelength of ~1 mm produced by conventional transducers. A combination of techniques is required to achieve adequate imaging resolution for this class of problems: very high frequency transducers; spectral extrapolation techniques to extract high frequency components; use of angled shear waves that have a shorter wavelength and more sensitivity to weak interfaces than compression waves, frequency analysis of wave interfacial reflection and transmission. (4) Finite element modeling of wave propagation in non-homogeneous materials. This issue is concerned not just with functionally graded materials, but materials that have a strong temperature gradient such as encountered in on-line industrial inspections. Not only does the speed of sound change with temperature, but waves will “skew” onto a curved trajectory when they encounter a temperature gradient.