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Olympus Inspection and Maintenance Systems

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Metrology

Ultrasonic Inspection for Aircraft Composite Panels

Olympus 35RDC Ramp Damage Checker helps prescreen for composite panel damage.

Published: Wednesday, July 29, 2009 - 13:03

(Olympus NDT Inc.: Waltham, MA) -- Solid laminate aircraft composites can be internally damaged by impacts in a way that isn't visible on the outer surface. The Model 35RDC Ramp Damage Checker is a hand-held ultrasonic instrument, developed by Olympus under a license of the Boeing Management Co. to nondestructively inspect the composite panels of an aircraft (such as the Boeing 787) for structural damage. This simple-to-operate unit is specifically designed to be used by airport personnel to quickly and accurately evaluate internal damage caused by passenger ramps or other devices that have been placed against the composite fuselage panels. These inspections need to be done quickly and reliably at the terminal gate before the aircraft flies to its next destination. For that reason, the 35RDC is designed for very simple operation and easy-to-read assessment of damage or absence of damage.

Airport ground crew can use the 35RDC with little or no training by simply holding the instrument in one hand and placing a small ultrasonic probe to the composite part. The rugged 35RDC features a backlit LCD that displays the word GOOD if no subsurface damage is found or the word BAD when it detects subsurface damage.

It is normally used with a 5 MHz broadband contact transducer, part number V109-RB, that can be placed or scanned on the area of composite material suspected of having impact damage. The range of composite thickness that can be tested is 0.025 in. to 0.900 in. (0.063 mm to 22.8 mm). Weighing only 0.24 Kg (8.5 oz), the 35RDC features simple one-button calibration and a long battery life, typically 150 hours. NDT

A unique feature of the 35RDC is the "beyond cal thickness" readout on the display. The readout indicates that the material being tested is thinner or thicker than it was calibrated for. This feature can avoid a bad reading when there is no evidence of internal damage.

Theory of operation

Ultrasonic sound waves travel through materials in a predictable way. When a high-frequency sound pulse generated by an ultrasonic transducer is coupled into a piece of solid laminate composite, it will travel through the material until it encounters an air boundary, at which point it will reflect back as an echo that can be detected by a gauge. In the case of structurally solid composite, that reflection will come from the back wall or the inside surface of the laminate. If, however, there is a delamination or subsurface cracking parallel to the surface, then the reflection will come instead from the damaged area and arrive earlier in time. The difference in the echo patterns from good vs. damaged material can be used to identify areas of internal damage that aren't visible from the surface.

The default alarm setting for the Model 35RDC will respond to delamination occurring in the composite material within a nominal two-ply thickness buffer zone from the back wall to accommodate for velocity variations, slight thickness variations, or ply drops.

Procedure

The Model 35RDC gauge isn't intended to scan large areas. Rather, it's used to screen areas of suspected damage.

The 35RDC gauge must first be calibrated for the specific thickness of material that is to be tested. The user normally identifies an area that shows impact damage and then moves to a point three to six inches (75 to 150 mm) away from this spot to an area that appears to be clean. The ultrasonic transducer is coupled to this known good area of the material and the operator presses a calibration key. The gauge identifies the back wall echo from the far side of the composite at the calibration point and establishes it as a reference. The gauge then switches into test mode and the user may move the transducer to the suspect area.

When the transducer is coupled to a test point, the gauge compares the new echo pattern to the reference pattern. If an echo is received earlier than the back wall echo, or if the back wall echo isn't present, that indicates probable damage and the gauge displays the indication "bad." If an echo is received earlier than the calibrated back wall echo window, or if the back wall echo isn't present, that indicates probable damage and the gauge displays the indication "bad." If only a strong back wall echo is received within the calibration window, then the gauge displays the indication "good." If only a strong back wall echo is received, but it's beyond the calibration window, then the gauge displays the indication as "beyond cal thickness."

Because this analysis is based on the reference echo, the 35RDC must always be recalibrated whenever switching to an area of different thickness or different material. This calibration procedure normally takes only a few seconds. If an attempt is made to measure material of a different thickness without recalibration, the gauge will display either "bad" or "beyond cal thickness" depending on whether the new material is thinner or thicker than that used for calibration. It's important to note that a "bad" indication should be investigated further by a trained professional who is familiar with composite inspection. Testing with the Model 35RDC gauge is for preliminary screening only and isn't equivalent to comprehensive testing by a trained inspector. 
  
 

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Olympus Inspection and Maintenance Systems

Olympus Corp. is an international company operating in industrial, medical and consumer markets, specializing in optics, electronics and precision engineering. The Olympus Inspection and Maintenance Systems site covers industrial testing instruments, such as ultrasonic testing (UT), eddy current testing (ECT), remote visual inspections (RVI) and high speed video (HSV).