A team of researchers from Khalifa University and STRATA has investigated the ways in which parts manufactured for the aerospace industry may deform during the manufacturing process using simulation tools to predict these deformations before they happen.
PhD student Mariam Ahmed Al-Dhaheri, Dr. Kamran Khan, Associate Professor of Aerospace Engineering, Dr. Rehan Umer, Associate Professor of Aerospace Engineering, and Prof. Wesley Cantwell, Director of the Advanced Research and Innovation Center (ARIC) and Associate Dean for Research, with Frank van Liempt, STRATA Manufacturing, reached accurate results in predicting the process-induced deformations of composite sandwich structures, with less than five percent error. Their research was funded by Mubadala Aerospace and published in Composite Structures.
A composite material combines various material ingredients to achieve specific structural properties. Polymer-matrix composite (PMC) materials are one such example used in aerospace applications.
PMC materials can enhance performance in an aircraft while reducing weight. Their high strength, stiffness, and toughness combined with low density make them the structural material of choice for aircraft components.
“The use of high-performance polymer-matrix composite materials in the aerospace industry has increased significantly in recent years, due largely to their high strength-to-weight ratio, immunity to corrosion, and excellent fatigue resistance,” Al-Dhaheri said. “They were first used in secondary structures, but more than half of the recent Airbus A350 is made from PMCs, and the Boeing B787 uses PMCs in the nose structure.”
However, PMCs are not immune to faults. One major issue is the potential for process-induced deformations (PIDs) that come from the manufacturing process itself, without any contribution from external factors. In aerospace manufacturing, these PIDs represent a significant concern during the design phase as they can cause difficulties during the final assembly.
“Aerospace composite structures are typically cured in an autoclave at high temperatures and pressures, involving a complex thermochemical cycle that cures the polymer matrix until it reaches a solid state yielding the required mechanical properties,” Al-Dhaheri said. “Although the curing process strengthens the composite structure, it also introduces residual stresses that remain in the structure upon cooling.”
When these parts are removed from the machinery that makes them, the material ‘relaxes’ as it is removed from its strained condition on the tool. This can cause structural deformation. Parts that aren’t quite right will be challenging to assemble, forcing aircraft technicians to apply greater levels of force to make them fit, which could create internal stresses or even damage the overall structure. The parts may even be deemed unsuitable and fail the airworthiness tests.
“Process-induced deformation is one of the main concerns during the manufacturing process of composite aerostructures,” Al-Dhaheri explained. “This is because residual stresses in the manufactured parts cause instability. Ultimately, this could lead to the part being rejected by the customer or even an expensive part being completely scrapped. Predicting these deformations numerically in the early design stages can help ensure conformity with the design and quality requirements.”
PIDs can be compensated for by accounting for these deformations during the design stage, but this is a trial and error approach, which is expensive and inefficient. Alternatively, process modeling or computer simulation approaches can streamline the production process.
“PIDs can be minimized by controlling specific parameters that contribute to the development of residual stresses,” Al-Dhaheri said. “Previous research has studied the effects of these parameters on process-induced deformations, but they haven’t considered composite sandwich structures, mainly the warpage in flat panels and the spring-in of curved structures, which is what our research focused on.”
Residual stresses are the internal stresses that develop during composite part processing. During curing, the materials undergo shrinking, and when cured into curved shapes, the angle between the two curved sides is reduced. This change in the angle is known as ‘spring-in’. Spring-in causes considerable difficulty and expense for composite manufacturers as it can vary with material, cure temperature, structure, and other manufacturing factors. This means that what worked once may not necessarily work the next time. If spring-in from residual stresses could be consistently and easily predicted, the manufacturing process could be tuned for specific part characteristics.
A large proportion of current aerospace composite components are light sandwich structures, where thin composite laminates constitute the ‘bread’, and honeycomb cell walls make up the ‘sandwich filling’. While light and strong, these structures are susceptible to damage and repairing them can be complicated. To date, there is little research into PIDs in sandwich structures, which the KU research team sought to rectify.
They used a simulation tool that can predict the occurrence of process-induced deformations in sandwich structures with a high degree of accuracy. They constructed a 3D finite element model of the composite part and simulates the curing process, as well as the interactions between the manufacturing tool and the part. From their simulations, the research team recommended that an aluminum core and aluminum tools should be used for manufacturing curved structures with reduced PIDs. They also recommended the sandwich design configuration to avoid large deformations in the final cured composite structure, compared to other designs.
While further experimental studies are needed to further validate the simulation findings, this represents a crucial tool to understanding the effects of various parameters on PIDs and improving the design and production process of airworthy composite parts.
14 November 2022