The reflective spectra from infra-red radiation may also be used to provide characteristics of polymeric materials. One such method is Fourier Transform Infrared Spectroscopy (FTIR ).
Figure 1 FTIR spectrum from fracture surface of a test joint on old Comet aircraft. FTIR spectra from areas of normal and discoloured adhesive. Source MTS Project Report 9 Part 2.
Analysis of fracture surface of failed adhesive joint for ions and characteristic molecular groups and signs of environmental degradation of the adhesive.
To identify molecules by the selective absorption or transmission of electromagnetic radiation.
The surface being examined is exposed to a focussed infrared beam. Infrared spectra are recorded from the reflected signal. The data is subject to a Fourier transform. The resulting spectra can provide and indication of the presence of particular characteristic such as –OH or alcohol groups and environmental degradation that has occurred to the adhesive in service.
The spectra can provide a fingerprint method to identify regions of normal and degraded adhesive. See Figure 1 above
FTIR was applied in the MTS programme to assess different regions of the adhesive surface of a fractured test joint from an old comet aircraft. The spot diameter of the infra-red beam in the FTIR instrument was 6mm. Thus information collected from the brown stained area included an overlap into the normal region.
The fingerprint spectra of the two different adhesive areas are given above in Figure 1. There is evidence of some differences between these traces which can be visualised by plotting a trace in which the normal sample spectrum is subtracted from the discoloured area spectrum as shown below in Figure 2. If the materials are identical the plot would be a straight line at 0. The most significant different is the increase of intensity at 3400cm-1 in the stained area. This spectrum band is associated with -OH and may indicate either the presence of alcohol groups or absorbed water (or both).
Other changes within the spectrum may be attributed to some sort of hydrolysis reaction but the analysis is not conclusive. The specimens had been exposed to the atmosphere for some weeks after fracture and absorbed atmospheric moisture may have contributed to the changes.
Figure 2 FTIR spectrum from different area of fracture surface of a failed joint in test on old Comet aircraft. Result of subtracting normal sample spectrum from the discoloured area spectrum. Source MTS Project 3 Report 9 Part 2
MTS Project 3 Report No 9 Forensic Studies of Adhesive: Forensic Studies of Adhesive Joints. Part 2 - Bonded Aircraft Structure
