The change in properties of a propagating ultrasonic wave has been a mainstay characterization tool of the nondestructive evaluation (NDE) industry for identifying subsurface defects (e.g. damage). A variant of this concept could be applicable to 3D additive manufacturing where the existence of defects (e.g. pores) within a sub-layer could mark a product as non-qualifying. We have been exploring the utility of pulsed laser ultrasonic excitation coupled with CW laser heterodyne detection as an all optical scheme for characterizing sub surface layer properties. The all-optical approach permits a straight forward integration into a laser additive processing tool. To test the concept, we have developed an experimental system that generates pulsed ultrasonic waves (the probe) with high bandwidth (<<10MHz) and a surface displacement sensor that can capture the ultrasonic "return" signal with bandwidth close to 300 MHz. The use of high frequencies enables the detection of smaller defect sites. The technique is time resolved with the sensor and probe as point (>>30-200 microns) beams. Current tests include characterizing properties of spot weld joints between two thin stainless steel plates. The long term objective is to transition the technique into a laser additive manufacturing tool.
Two types of SiC plates, differing in their manufacturing processes, were interrogated using a variety of NDE
techniques. The task of evaluating the materials properties of these plates was a challenge due to their non-uniform
thickness. Ultrasound was used to estimate the Young's Modulus and calculate the thickness profile and Poisson's
Ratio of the plates. The Young's Modulus profile plots were consistent with the thickness profile plots, indicating
that the technique was highly influenced by the non-uniform thickness of the plates. The Poisson's Ratio is
calculated from the longitudinal and shear wave velocities. Because the thickness is cancelled out, the result is
dependent only on the time of flight of the two wave modes, which can be measured accurately. X-Ray was used to
determine if any density variations were present in the plates. None were detected suggesting that the varying time of
flight of the acoustic wave is attributed only to variations in the elastic constants and thickness profiles of the plates.
Eddy Current was used to plot the conductivity profile. Surprisingly, the conductivity profile of one type of plates
varied over a wide range rarely seen in other materials. The other type revealed a uniform conductivity profile.
A brief overview is provided on three common methods of nondestructive evaluation (NDE), Ultrasonic Testing (UT),
Radiographic Testing (RT) and Eddy Current (EC). These methods vary in the physics applied to the testing or
evaluation process while specific techniques vary in the way each method may be applied. Understanding the physics
behind each method and constraints of each technique helps us understand the capabilities and limitations of the
inspection. Recognizing the capabilities and limitation of these NDE Techniques helps us properly design the inspection,
collect and process data and draw appropriate conclusions.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.