Measurement of the Layers in Clad Metal Cookware
Background
Many common pots and pans used for home and restaurant cooking are constructed from multiple layers of metal. Stainless steel is easy to clean and does not react with foods, but it is not an ideal conductor of heat for cookware, especially in thin sheets. Aluminum and copper conduct heat quickly and efficiently, but can react with acidic foods like tomatoes or citrus fruits, and are also more expensive than steel. For that reason, clad metal cookware was developed, incorporating a stainless steel body with a base made from a relatively thick core of either copper or aluminum sandwiched between two thin layers of steel, normally less than 1 mm (0.040") thick. The thermally conductive core provides optimum heat distribution across the bottom of the pan, while the nonreactive stainless steel contacts the food while also presenting a good appearance.
For quality control purposes, manufacturers of clad cookware need to monitor the thickness of the three layers comprising the base. Traditionally this has been done through destructive testing, cutting sample pots and measuring layer thickness from a visible cross-section. However ultrasonic testing provides a quick, reliable way of measuring layer thickness nondestructively, with no need to cross-section parts.
Theory of operation
High frequency sound waves will reflect from boundaries between dissimilar materials. An ultrasonic pulse that is coupled into the bottom of a clad pan will generate reflections from the cladding/core and core/cladding boundaries as well as from the inside surface.
Ultrasonic gages precisely measure the time intervals between these echoes and then use a programmed speed of sound in each metal to calculate the individual layer thicknesses.
Equipment
For optimum capability, the Model 38DL PLUS thickness gage with the Multi-Measurement software option can simultaneously measure cladding thickness, core thickness, and total thickness. For measurement of cladding layer only, the Model 35DL gage can be used. Transducer selection will depend on the specific range of thicknesses to be measured, but will typically be either an M208-RM (20 MHz) or M202-RM (10 MHz) delay line transducer.
Typical Procedure: The waveforms below show simultaneous measurement of cladding and core thickness in a typical clad aluminum cooking pot, using an M208 transducer. In this case measurements are made between two echoes representing the outer surface and the cladding/core boundary, between the echoes representing the two surfaces of the core, and between the echo from the core/cladding boundary and the inside surface of the pot. 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.
