New Penn State Scanner Probes Sub-Surface Over Broad Temperature Range

The new device, a modified Olympus UH3 Scanning Acoustic Microscope, is the first to enable researchers to peer nondestructively below a material's surface not only at room temperature, but also over a range up to 400 degrees centigrade.

The device was developed by Dr. C. Miyasaka, a visiting postdoctoral scholar, and Dr. Bernhard R. Tittmann, the Schell professor of engineering science and mechanics. They will describe the device and its first application Tuesday, Oct. 6, in a paper, "In-Situ Acoustic Microscopy of Healing Delaminations in GR/PEEK Composite," at the IEEE Ultrasonic International Symposium in Sendai, Japan.

Tittmann says the new device could potentially be used to study a broad range of materials. For example, microelectronics companies may be able to use the device to look for defects below the surface of computer chips or to study structural detail in integrated circuits. The device could also be used to study the grains and grain boundaries of complex materials or to optimize the distribution of liquid and solid in a semi-solid material.

The first material the Penn State team has studied is a laminate, called PEEK, composed of 8 layers of graphite-reinforced plastic and adhesive. PEEK is widely used in truck bodies, boats, planes, golf clubs, tennis rackets, and other consumer products. In the application the Penn State team investigated, PEEK was being considered by an international consortium for use as a material from which to manufacture the turbine blades in the first stage of aircraft engines.

The consortium hoped to decrease the weight of the engines while still maintaining strength and durability. However, during testing, the blades failed as the result of soft body or simulated "bird strike" impacts, but there were no telltale external marks on the blades to indicate that any damage was present.

At the request of the consortium, the Penn State researchers used their prototype acoustic microscope to image each of the eight layers in the turbine blades individually and to identify areas of delamination or separation that led to the failures. Also, by using the new temperature range feature of the modified microscope, the team was able to heat the sample and watch the delaminations close or "heal."

Tittmann says, "The resolution of the microscope at temperatures up to about 100 degrees is comparable to that achieved by optical microscopes. At higher temperatures and lower frequencies, the resolution is on the order of a 10th of a millimeter."

The Penn State researchers equipped the prototype device with several modifications that enable it to operate in its broad, nearly 400-degree temperature range. They surround the sample with a heat-tolerant fluid, racing car oil, which acts as a coupling medium. The acoustic waves are focused with a new type of low absorption, ceramic lens and they have added a specially developed thermal barrier between the hot container and the transducer.

Tittmann says, "The results clearly demonstrate the usefulness of scanning acoustic microscopy with temperature control. The variable temperature operation allows in-situ monitoring/imaging of materials undergoing changes in internal structure. The technique is completely nondestructive and lends itself for field development."