Non Destructive Evaluation (Nde) Techniques

Thermography

Infrared thermography measures the variations in heat emitted by an object and displays them into visible images, usually by monitoring infrared emissions using a thermal imaging camera. It is a rapidly developing technology for NDE in many applications. The imaging equipment used for thermography has improved a lot recently, which enables more rapid data acquisition and higher spatial resolution.

Thermography is used to isolate structural anomalies based on very small differences in thermal properties. The component being inspected is typically heated from one side and viewed from either the same side or the opposite side depending on access to the structure and the laminate schedule (marine sandwich laminates have very low throughthickness heat conduction). The heat application must be relatively uniform throughout the area being inspected. Uniform heating is achieved by using hot air guns, heat lamps or flash lamps in a controlled fashion.

The external application of heat creates a thermal gradient within the laminate. The movement of heat energy from the heated surface into the cooler  component is a function of the material’s thermal diffusivity. Thermal diffusivity is defined as the ratio of a material’s thermal conductivity to its thermal capacitance. Heat will diffuse uniformly throughout a laminate until it encounters a discontinuity.

Although each method is dependent on different basic principles in both application and output, repeatable and reproducible NDE results depend on specific understanding and control of the:

• Material composition (magnetic, non-magnetic, metallic, non-metallic ...)

• Part thickness, size and geometry

• Material condition (heat treatment, grain size ...)

• Inspection scanning rate

• Fabrication method (casting, forging, weldment, adhesive or brazing bonded ...)

• Surface condition (rough, plated, bright, scaled ...)

• Nature or use of the part (critical, non-critical, high or low stress ...)

• Human factors

No NDE process or procedure produces absolute discrimination of anomalies but the end output of a procedure may be quantified and the anomaly or flaw detection capability may be measured, analyzed, quantified and documented.