Nonlinear Ultrasonic Techniques for Nondestructive Evaluation and Fatigue Life Prediction

工程材料在循环荷载作用下的破坏通常分为裂纹孕育、裂纹扩展和断裂三个阶段。在裂纹孕育阶段，材料没有可检测到的裂纹，现有的无损检测方法无法对该阶段的疲劳损伤进行量化。不幸的是，在许多实际情况下，孕育阶段可能占用材料总疲劳寿命的80%。因此，开发能够量化裂纹孕育阶段疲劳损伤程度的新技术至关重要。为了应对这一挑战，该项目正在开发一种基于非线性超声的方法，该方法能够无损地量化金属材料的早期疲劳损伤，从而预测疲劳寿命。具体来说，该项目将(1)开发一个基于物理的模型，定量地将二阶谐波的振幅与材料中疲劳引起的累积塑性应变联系起来，(2)开发创新和稳健的测量技术，以准确测量裂纹孵化阶段疲劳损伤产生的二阶谐波。本项目开发的方法将使我们能够获取和使用有关土木和机械结构的信息，以提高其安全性、可靠性、成本和性能。

Failure of engineering materials under cyclic loading typically consists of three stages, crack incubation, crack growth and fracture. In the crack incubation stage, the material has no detectable cracks so that fatigue damage cannot be quantified in this stage by existing nondestructive testing methods. Unfortunately, in many cases of practical interest, the incubation stage may take up as much as 80% of the total fatigue life of the material. It is therefore critically important to develop new techniques that are capable of quantifying the degree of fatigue damage in the crack incubation stage. To meet such a challenge, this project is to develop a methodology based on nonlinear ultrasound that is capable of nondestructively quantifying early fatigue damage in metallic materials, thus predicting fatigue life. Specifically, the project will (1) develop a physics-based model that relates quantitatively the amplitude of the second order harmonic to the fatigue-induced cumulative plastic strain in the material, and (2) develop innovative and robust measuring techniques to accurately measure the second order harmonic generated by fatigue damage in the crack incubation stage. The methodology developed in this project will enable us to acquire and use information about civil and mechanical structures to improve their safety, reliability, cost and performance.