Advanced Ceramics

CASE STUDY: Improving Armor-Grade Ceramics

September 1, 2010
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Researchers at Rutgers University have reduced the time required to inspect armor-grade ceramic tiles.

Ultrasonic phased-array setup.


Many materials, such as the armor-grade ceramics used in critical applications, require 100% inspection. Conventional ultrasound inspection, a commonly used nondestructive method for evaluating ceramic integrity, may not be rapid enough for some commercial manufacturers. Researchers at Rutgers University have reduced the time required to inspect armor-grade tiles by mounting a phased array probe with 64 piezoelectric elements on a gantry machine.

The Techno Gantry System LCT automatically moves the probe over the ceramic while providing positional information so that the phased array instrument can measure material properties over large sample areas. “This approach has proven to be at least 10 times faster than conventional ultrasonic inspection with a single element transducer,” says Steve Bottiglieri, a graduate student at Rutgers.

“The precise motion of the gantry makes it possible to measure accurate property maps,” adds Andrew Portune, also a Rutgers graduate student. The Rutgers researchers worked closely with engineers from Techno Inc. to integrate Techno’s LC gantry with the OmniScan MX phased array module from Olympus.

Nondestructive Inspection

Dense ceramic materials are attractive candidates for armor applications because of their low specific areal density and high hardness. Unfortunately, the uniformity of ceramics is not something that can be assumed. During the manufacturing process, minor occurrences of anomalous defects can be found in the bulk of the material, and can lead to cracks and voids in the finished part. It is also possible for internal cracks to form in the plates during shipping, handling and use.

These flaws may significantly impact the performance of the tile in its application or cause it to fail unexpectedly. Since they normally originate inside the material, the flaws are also impossible to detect by manual inspection.

High-frequency ultrasound technology can be used to detect flaws in ceramic plates. First, an ultrasound transducer is passed over the object. Ultrasonic waves travel by exerting an oscillating pressure on particles in a medium. In a solid medium where particles are closely bound, the oscillation of one particle generates corresponding vibrations on adjacent particles; this causes ultrasound propagation.

The ultrasonic waves are captured by the transducer, which receives the waves that are reflected by the object. The waves can be reflected by lamination layers in the material or by large anomalous flaws. Measuring the time at which reflections are received makes it possible to determine the location of the flaw within the ceramic material. Another option (not used in this application) is to receive the waves after they pass through the object.

The historic method of performing ultrasound inspection on ceramic tiles was to manually move a single-element probe over the material. This method made it difficult for an operator to ensure 100% sample coverage while focusing on the flaw detector display. As a result, though effective as a mapping technique to highlight missed regions, the manual method was very slow.

Modern methods reduce the amount of labor required by driving a single-element ultrasonic transducer with a linear motion system. This process is effective for the inspection of single or a few parts, but it is not as feasible when inspecting large batches of parts.

Phased-array C-scan image of ceramic armor material. (Units are an arbitrary percentage of ultrasound energy.)

Phased Arrays

The speed of ultrasonic inspection can be increased through the use of phased-array technology, which combines a number of ultrasonic piezoelectric elements that all operate simultaneously. Phased-array systems pulse and receive from multiple elements of an array. These elements are pulsed in such a way that they cause multiple beam components to combine with each other and form a single wave front traveling in a desired direction. Similarly, the receiver function combines the input from multiple elements into a single signal.

Because phased-array technology permits electronic beam shaping and steering, it is possible to generate a vast number of different ultrasonic beam profiles from a single probe assembly. Beam steering can be dynamically programmed to create electronic scans, which enable software control of beam angle, focal distance and beam spot size. These parameters can be dynamically controlled at each inspection point to optimize incident angle and signal-to-noise for each part geometry.

Multiple-angle inspection can be performed with a single, small, multi-element probe and wedge, which can offer either single fixed angles or a scan through a range of angles. These capabilities provide greater flexibility for the inspection of complex geometries.

Integration of Gantry

Rutgers has further increased the speed of ultrasonic inspection by integrating the Olympus Omniscan MX phased-array ultrasonic system with the LC gantry from Techno. This approach significantly reduces collection time when compared to the conventional single element setup. The gantry moves the inspection system over the samples automatically-without requiring attention from an operator-so labor requirements are also dramatically reduced.

Rutgers researchers wrote software using the MATLAB programming language to control the motion of the Techno gantry that accepts as input basic parameters of the parts being inspected. The program then generates a G-code toolpath file to guide the machine through the intricate series of motions required to produce the scaffold.

“The Omniscan MX unit requires encoder input to make collected sample information correspond to a spatial location,” Portune says. “Techno was extremely helpful by modifying the linear motion controller to add a custom encoder output connector that interfaces with the phased-array Omniscan unit.”

The Techno Gantry System LCT is equipped with ball screws on all three axes with closed-loop servo-motor drives that provide an accuracy of ± 100 microns per 300 mm and a repeatability of ± 100 microns. The Techno machine provides a speed of 152 mm (6 in.) per second, which helps to achieve high inspection rates. The Gantry System LCT also features an X-Y travel of 30 by 24 in. Each axis is provided with two double-slide linear rails and four double-bearing blocks.

The X-axis bearings and drive screw are mounted below the work surface to protect them from dust and debris. In addition, the X-axis is provided with an aluminum dust cover and plastic lip seals to provide protection against contaminants. Heavy, cast aluminum side plates support the Y-axis and provide increased stiffness for positioning and cutting applications.

Top of the inspected material.

System Validation

The Rutgers researchers validated the new phased-array inspection system by inspecting glass-ceramic tiles of varying compositions. Each tile was approximately 4 in. in length and width, with a thickness of between ½ and 1 in. For laminated samples, ultrasound characterization focused on locating delaminations or irregularities in the bonding between tiles. Non-laminated samples were evaluated for compositional uniformity through elastic property mapping and spatial measurement of the ultrasound attenuation coefficient. The lamination amplitude map revealed the location and severity of every delamination, making it an excellent indicator of variances in the bonding between tiles.

A closer examination of lamination amplitude maps revealed additional information regarding the problem in the bonding layer between tiles. There appeared to be two kinds of deviations from the mean peak amplitude seen in delaminated areas. In each delaminated area, higher amplitude areas can be seen adjacent to and surrounded by lower amplitude areas.

Bottom of the inspected material.

The peak amplitude is directly related to the acoustic impedance mismatch between the two mediums found at the interface. Low peak amplitude means that the acoustic impedance mismatch was minimal. This could indicate a lack of bonding agent or two glass-ceramic tiles directly adjacent to one another. High peak amplitude indicates that the acoustic impedance mismatch was greater than the mean, which could point to the presence of air or water in the delaminated area.

“The integration of the gantry machine and the phased array provides a major advancement in ultrasonic inspection of ceramic material,” Bottiglieri says. “This new approach can inspect a large number of parts in considerably less time than is required by conventional methods. It also delivers excellent material property maps, making it possible to locate anomalous defects with a high degree of accuracy.”

For more information, contact Techno Inc. at 2101 Jericho Turnpike, New Hyde Park, NY 11040; (516) 328-3970; fax (516) 358-2576; e-mail support@technocnc.com; or visit www.technocnc.com. Additional information regarding Rutgers University is available at www.rutgers.edu.

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