Article by Tom Haxton
An Introduction to Defects in (FRP) Composites
Damage and failure mechanisms in Fibre Reinforced Polymers (FRPs) are very different to those found in conventional or homogenous materials and normally arise from defects within the FRP. The manufacturing process can sometimes allow small defects to go undetected and these can grow and cause problems when the product is in use. A more common occurrence is defects caused by damage to FRP from accidental impact, operations outside of the design limits and excessive environmental exposure. These can happen at any time during the product life cycle, and this is why regular Non-Destructive Evaluation (NDE) of deployed FRP infrastructure is essential. Defects are often hidden under the surface of the product and this can make them difficult to detect. These defects in FRPs are normally grouped into three categories – cracks (sometimes called matrix splitting), delamination and fibre breakage.
Cracks in the matrix material cause a number of issues. Firstly, they can expose the fibres to unfavourable environmental conditions by allowing water, air, light, dirt and other contaminants to attack the fibres greatly reducing their strength and increasing the risk of fibre breakage and delamination. Secondly, the rigidity provided by the matrix to the structure is lowered and this can lead to catastrophic failure. Cracks also tend to propagate through the material once established, particularly if there is a fatigue cycle or vibrations. Cracks can occur anywhere throughout the FRP matrix structure but tend to occur most often at impact sites, where the matrix is moulded with tight or complex shapes and where excessive vibration or movement is focussed. For this challenge, the minimum size of crack to be detected is 30mm long and 0.3mm wide.
Delamination occurs when the layers of woven fibres separate from each other thus reducing the ability to efficiently transfer loads. This lowers the compound strength provided by stacks of layers and affects rigidity, structural integrity and torsional strength. Once delamination starts, it can quickly spread along the boundary layer. Delamination is one of the most serious failure mechanisms and also one of the most difficult to detect as it is always below the surface of the product. The challenge requirement is to detect delamination zones of minimum diameter 2.5mm and layer separation of 0.5mm or higher.
Fibre breakage is normally associated with cracks and delamination but can occur in isolation, particularly if excessive tension or compression has been applied or a large impact has occurred. At the site of fibre breakage, the FRP loses its composite strength and only becomes as strong as the matrix material. Without fully effective fibres, the matrix quickly suffers damage and degrades while tensile strength in particular diminishes rapidly. The detection limits for the challenge require fibre breakage creating a gap of at least 0.3mm to be detectable across a 30mm section of matrix.
Repairing the Damage
If caught before it spreads too far, damage within FRPs can be quickly repaired or the section of infrastructure replaced. For repairs to be targeted effectively, the precise location, type and size of the damage must be determined. Detection is not enough, hence why we talk about Non-Destructive Evaluation (NDE) and not simply defect detection. In a future blog article, we will look at current testing and evaluation methods, but the diagram below gives an outline of the detection limits required for this challenge.
Become Part of the Innovation Process and Make a Difference
The types of defects found in FRPs and the associated failure mechanisms are well understood and there are many lab-based and static systems for detecting, locating, measuring and categorising these defects. However, there are not currently any highly mobile and quick evaluation systems that can survey large scale infrastructure in the field. Take pipelines, for example, there are literally thousands of miles that need periodic assessment. This is the problem faced by the Bureau of Reclamation and US Army Corps of Engineers who are continuing to replace and upgrade FRP infrastructure. This HeroX Imperfection Detection Challenge is therefore providing an innovation framework for these organisations to seek new solutions to address this capability gap.
The prize challenge is being run as a phased innovation process in order to maximise the benefits of crowdsourcing. Phase 1 is an Ideation Phase and has already been launched. In this phase solvers are encouraged to submit designs for rapid field evaluation of FRPs. The best designs will then be invited to progress to the next stage, the Prototype Development Phase. Solvers will have 10 months to build and test a prototype before submitting a report for judging. Up to three designs will then be taken forward to Phase 3 where they will be evaluated in a series of trials by the challenge setters and hopefully result in the creation of new and improved products for widespread rapid NDE of composite infrastructure in the field.