Feasibility study of detecting railroad spike damage using a non-contact NDE system

In this paper, the feasibility and effectiveness of a guided-wave approach are investigated as a first step toward the development of an automated non-contact ultrasonic system for inspecting railroad spikes to locate cracks and breaks. An automated system is desired that can perform the inspection more effectively and reliably than current manual tapping or pulling methods. The goal of this research is to detect damage in railroad spikes in a well-controlled laboratory environment. A recent study using contact ultrasonic transducers for spike inspection revealed many limitations, including high signal attenuation, high sensitivity to transducer positioning, and the need to use an ultrasonic couplant. To circumvent these limitations and provide a more robust inspection method, an “airborne ultrasound” system comprised of an air-coupled transducer (ACT) for actuation and a laser Doppler vibrometer (LDV) for sensing is proposed and evaluated. The inspection system employs the concept of guided waves, which propagate with less attenuation at lower ultrasonic frequencies and, hence, a larger amplitude than that from the bulk waves typically used for ultrasonic nondestructive inspection (NDI). The detection of broken spikes is based on the premise that the wave energy will be attenuated in the transmitted wave inhibited by the damage and amplified in the reflected wave from additional waves scattered from the damage. Initial tests on cut spikes demonstrated that the proposed system performs with confidence of damage detection based on baseline comparisons with pristine spikes.