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Background: Fiberglass Reinforced Plastic (FRP) is used extensively in the boating industry, both work boats and recreational craft. Boat manufacturers and marine surveyors need to obtain accurate hull
thickness measurements, and traditionally, unless there was a place to put a micrometer, there was no way to determine thickness but to drill a hole in the hull. Nearly 30 years ago, Panametrics was the first company to develop
a thickness gage and transducer (the Model 5227 and M2008 respectively) that were specifically designed to make digital thickness measurements from one side of the fiberglass. Since then, aside from many commercial
applications, our instruments have been used by Olympic organizations to certify boats for international competition.
Ultrasonic thickness measurements are particularly useful with fiberglass mat/roving lay-ups. Variations in layer thickness make it necessary to periodically check thickness. Sometimes boat hulls are made of fiberglass skins
sandwiched around an inner core of balsa wood, cork, or foam. The core material makes it impossible to make total hull thickness measurements, but individual fiberglass layers can be measured very effectively with ultrasonic
equipment.
Equipment: The Panametrics-NDT Model 35DLHP is a portable gage designed to make digital thickness measurements on fiberglass. It is commonly used with a M2008 transducer (500 KHz) to maximize
penetration of ultrasound in thick or attenuating fiberglass, providing a thickness measurement range of approximately from 0.125 to 3 inches (3 to 75mm), depending on the type of fiberglass. The Model 35HP may be used in
situations where data logging is not required. In especially challenging cases, the Model 25HP PLUS with waveform display is recommended, to permit on-site operator adjustment of setup parameters,
With other transducers such as the M1036 (2.25 MHz) or M109 (5 MHz), the 25DLHP can measure down to approximately 0.050 inch (1.25mm) in fiberglass. The standard Model 35DL can often be used for thickness
measurements on hulls less than 0.500 inch (12.5mm), particularly if the material is of good quality.
Procedure: The 25HP PLUS, 35DLHP, and 35HP gages all have pre-programmed default setups for the two transducers most commonly used for fiberglass measurement, the M2008 and M1036. In many cases these default
setups can be used with no further adjustment, just the two-point velocity and zero calibration that is recommended for all ultrasonic thickness measurements.
The fiberglass used in boat construction is generally of high quality, and unlike some other fiberglass laminates it usually does not produce high amplitude internal reflections that can cause false readings. However, if false
readings are observed in a particular case, the situation can usually be corrected by small adjustments of initial gain and TDG slope as outlined in the gage operating manuals. The waveforms below show a typical hangup on
internal reflections, and a correct reading of total hull thickness following a simple gain adjustment.
False reading due to hangup on internal reflections
Correct reading following gain adjustment
Any delaminations that may occur in the hull will appear as strong, well-defined reflectors accompanied by a complete loss of the backwall echo. While ultrasonic flaw detection should always be performed with instruments
specifically designed for that purpose, such as the Panametrics-NDT Epoch series, a trained operator can use the waveform display of the Model 25HP PLUS gage to make initial judgments about the
presence of delaminations in a hull.
Gelcoat measurement A closely related application is measurement of smooth gelcoat applied over fiberglass. For detailed information on ultrasonic measurement of gelcoat thickness, please refer to the separate
application note Measurement of Fiberglass Gelcoat.
For further information on any of the points discussed here,
contact Olympus NDT.
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