Success in adhesive bonding of components requires careful control of the bonding process, surface condition and an understanding of the operating environment. Forensic analysis forms an important part both in the optimisation of bonding processes, understanding the factors that affect bond quality, and in the evaluation of bond failures.
The situations where forensic analysis may be used in practice include the following:
1. Failure investigations - Failure of bond
2. Following proof testing
3. As part of design, procedure and method optimisation
4. Following inspection and monitoring to evaluate bond
To understand the failure of an adhesive joint it is important to understand where the failure has occurred, if the preparation and bonding process was carried out satisfactorily and if the loading or environmental conditions seen by the joint went outside design conditions.
There are a number of forensic methods that may be deployed in the investigation. Broadly speaking these fall into a number of categories:
Click the following links to find out more detail on specific methods of forensic analysis
There are specific items of information that an investigator would like to know to understand how a failure of an adhesive joint may have occurred. These relate to the different stages in the bonding process and conditions in service. The procedure and techniques deployed will depend on how important each of these causes are to the failure that has occurred.
It is very common that an operator may need reassurance that an adhesive joint in operation is still OK and hasn’t gone awry. They will not want to break the component. In such circumstances non destructive testing NDT methods are commonly applied. Specific NDT methods have evolved to assess bond quality and adhesion, check for disbonding and to measure voids or porosity in adhesive layers. Detection and characterisation of kissing bonds, where the surfaces are in close contact or bonded with limited or reduced bond strength, poses particular challenges for NDT methods.
Mechanical testing may be used in a number of instances. In batch processes the manufacture may periodically test components to destruction or proof test; commonly used in car production lines. In service applications sacrificial joints may be produced to test.
There are a number of tests that may be applied during joint manufacture to assess surface condition including colourimeters. Such tools can also find application in forensic analysis.
Microscopy and surface analytical methods such as XPS and SEM can give information on the location of bond failure, adhesive interface or parent material and local faults such as lack of curing or lack of adhesion Interpretation may be difficult as there can be a lot of local variation on the un-adhered surfaces. Identifying what is relevant is not straightforward. Adhesives are rarely pure in composition so identifying contaminants from such analysis is not in our experience straightforward.
Sampling or sectioning may be a precursor to other examinations. For example core samples or cross sections may allow examination of bond quality and location of disbonding or lack of adhesion.
Where a joint has failed forensic analysis can be valuable in identifying how to make the joint better next time. Playing with and optimising surface preparation methods can prove particularly effective.
A key question for a failed bond is did it ever work at all? If so then it is important to determine what caused failure in this instance. If the joint adhesion has never worked properly then there is a need to identify what is required to improve bond integrity.
The bond surface can be revealed by mechanical test methods such as peel testing which also give values for bond strength. It may be difficult to distinguish subsequently what failure occurred in the peel testing and what was pre-existing.
Specific industry developed test such as the Boeing Wedge Test can find wider application in other industries.
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