Delamination of surfaces
Approach to detecting delamination
To detect delamination defects in composite materials that are typically beneath a thickness of a few millimetres of the material, a thermal imager will observe surface temperature changes over a period of time. To reveal defects the temperature of the material needs to be changing, and thermal imagers typically choose to observe materials as they cool.
In many building structures such as concrete delaminated area defects can be relatively deep i.e. several centimetres rather than a few millimetres.
Since concrete has a low thermal diffusivity compared to other materials and the defects tend to be contained within a significantly larger volume of material, a heating/cooling period of several minutes to hours is generally required to ensure that thermal waves of sufficient energy have propagated within the structure. The observation of spatial temperature variations on the surface of the material for a period of several minutes to hours is required for both active and passive inspection techniques. The time required relates to the conductivity of the material in question.
These spatial variations can be an indication of internal flaws, such as unbonds and delaminations, that tend to increase the thermal impedance of the structure. The enhanced thermal impedance due to a defect can result in localized surface temperature differentials ranging from less than 0.5 K to more than 3 K, depending on the flaw depth and the thermal characteristics of the structure.
Fibreglass delamination in boat hull following active imaging
Concrete road delamination
Near surface delamination in railway tunnel
Passive Thermal Imaging
‘Passive' thermal imaging relies on detectable thermal differences in the structure as a result of natural environmental heating and cooling cycles. For building thermography this is a more practical approach due to the scale and mass of the structures involved. Imaging around dusk is typically a good time to observe changes as structures cool following solar heating in daylight hours.
The absence of an artificial heating/cooling source causes this approach to be considered as a passive technique, though in reality the principals are identical.
Active Thermal Imaging
'Active' thermal imaging relies on the application of an external heating/cooling source to simulate heat flow within the material while the surface is monitored during the inspection period for spatial temperature variations as it returns to thermal equilibrium.