Application: Measurement of individual layers and total thickness in multilayer polymer aircraft windshields.

Background: Multilayer plastic construction is commonly employed by the aerospace industry for aircraft windshields, in which two relatively thick layers of acrylic or a similar hard plastic are typically sandwiched around a thinner layer of a soft material such as polyvinyl butyral (PVB) which provides thermal stress relief. The hard plastic layers are typically about 0.4 in. to 0.5 in. (10 to 12.5 mm) thick and the soft inner layer about 0.050 in. (1.25 mm) thick.

Scratches in aircraft windshields are typically removed by polishing during routine maintenance, after which thickness measurement is required to insure that windshield thickness has not been reduced below a specified minimum. Additionally, manufacturers need to measure the soft PVB middle layer to insure that it is within specified thickness.

Equipment: The Model 38 DL PLUS thickness gage with the Multi-Measurement software option is recommended for measurement of individual layers and total part thickness in this application. With this software, the 38DL PLUS gage is capable of using separately programmed setups (including sound velocity, gain, and blanking settings) for each layer being measured to optimize performance. In this test, the gage is normally used with either an M110-RM (5 MHz) or M112-RM (10 MHz) contact transducer.

Typical Procedure: The waveforms below show simultaneous measurement of all three layers of a typical multilayer aircraft windshield using an M110 (5 MHz) contact transducer. The outside layer is measured in Mode 1, to the first negative echo. The thin middle layer is measured in Mode 2, negative peak to positive peak. The bottom layer is also measured in Mode 2, positive to negative. The small echo just to the left of the detected backwall echo in the bottom screen print is not another layer, but rather a second multiple of the backwall echo from the outer layer.


outside polycarbonate layer


middle PVB layer


bottom polycarbonate layer


As with any ultrasonic thickness measurement, accuracy is dependent on proper sound velocity calibration. Velocity calibration must be performed for each material being measured, on samples of known thickness.