The use of polymeric composite materials, including fiber-reinforced polymers (FRP), has grown dramatically since inventors like Goodyear and Dunlop and Michelin began combining rubber with textiles to make the first tires for bicycles and wagons in the 1800s.
Today, applications for polymeric composites include everything from automobile parts, boats and aircraft to fixed infrastructure such as chemical storage tanks and wind turbines. Over the years, design principles and manufacturing practices have been codified and standardized to help engineers build better and better equipment made from these materials.
Design codes don’t apply to in-service equipment
However, once the equipment is in service, in most cases the design codes or construction standards do not apply and cannot be used to determine Fitness For Service. When failures occur, standard investigative approaches may yield inconclusive results, often resulting in blaming an “undetected manufacturer’s defect” for the failure.
In general, while design and construction of polymeric composite equipment has continued to improve, the profession’s understanding of how these materials change while in service, and how these changes can lead to failures, has not kept pace.
Fitness For Service a key predictor of future condition and behaviour
UTComp founder Geoff Clarkson and forensic engineering expert Daniel Couture are working to change that. Clarkson and Couture were featured recently at the 2024 Summer Conference of the National Academy of Forensic Engineers (NAFE), where they presented a paper titled, “Forty Years of Advances in Failure Analysis of Polymeric Composite Materials.”
The paper addresses the gaps in mathematical models used to predict the future condition and behaviour of polymeric composites. It discusses how non-destructive attenuation-based ultrasound (UAX) using the UltraAnalytix® system can bridge those gaps with reliable, data-based assessment of the material’s current condition.
Check the UTComp website in the coming weeks for links to the final draft of the paper, which is expected to be submitted for publication in the NAFE journal at a later date.
Applying the art and science of engineering to investigate failures
Forensic engineering involves applying engineering principles to investigate why a structure or machine or component has failed while in service. Forensic engineers are often called upon to share their expertise in legal settings to help resolve court cases or insurance claims.
How they figure out what went wrong depends on the type of material being investigated: whether a piece of equipment is made from metal, rubber FRP, wood laminate FRP or thermoset FRP, each family of material provides a different mix of information to be analyzed by the forensic engineer looking for answers.
Models focused on metals are insufficient
Over the decades, finite element models have been developed and refined to help engineers analyze and predict the behaviour of different materials in response to mechanical loads, heat, corrosion and other stresses encountered in normal service conditions. This allows them to compare data collected from material involved in a failure with its “as new” condition. These models and formulas are particularly well-developed for metals — allowing forensic experts to work backwards from the appearance of a crack or fracture to understand what caused a particular component to fail.
New approaches for investigating failures in polymeric composites
But the forensic techniques that work for metals aren’t necessarily helpful for composites. Due to the complex nature of polymeric composites including FRP, standard approaches may be insufficient to identify the origin of a fracture — especially in a large-scale incident, like a collapsed wind turbine, where extensive structural damage might obliterate any overt defects in the material that could be linked to the cause of the failure.
Incorporating the accumulated damage concept
However, even when details of the original design aren’t available, by incorporating some additional steps into standard forensic procedures, it is possible to compare as-new information with the conditions that existed at failure.
Extensive field work by Clarkson has demonstrated how UAX techniques can distinguish accumulated damage adjacent to fractures from areas that have not fractured. UltraAnalytix, UTComp’s patented UAX system, does this by evaluating changes in the key properties of the polymer such as flexural stiffness. This may provide insight into how and when the structure or component lost integrity.
How UAX can help forensic engineers
Forensic engineers are encouraged to consider these factors in cases involving polymeric composites, so that their investigation will successfully discover the cause of failure. Asset owners are also encouraged to incorporate non-destructive UAX testing into their maintenance programs to keep ahead of damage to their equipment and avoid costly failures in the future.