Noncontact Surface-Hardness Measurement Using Laser-Based Ultrasound

Abstract

A noncontact inspection system that uses lasers to both generate and detect ultrasonic waves has been developed at the Rockwell Science Center for the purpose of nondestructively inspecting metallic and composite structures. The laser-based ultrasound (LBU) system was initially developed to inspect large-area composite airframe structures, including the automated inspection of integrally stiffened and complexly curved composite structures. Furthermore the benefits offered by LBU, such as its noncontacting nature and ability to operate at elevated temperatures, have extended its usefulness to a wide range of applications—for example, in situ process monitoring where the capability to monitor a fabrication process or possibly inspect a part while still in a mold or autoclave could lead to significant cost benefits. Two manufacturing techniques that have shown promise and are presently under investigation are resin-transfer molding and compression molding. Since both of these processes typically occur at elevated temperatures, the noncontacting nature of LBU makes it an ideal candidate for an in situ sensor. More recently, measurement of the case depth in ground vehicle components has been investigated. This article describes current research that uses the LBU technique to determine surface hardness in steel-axle shafts.