The need
LMAT offers software and consulting to improve composite products, such as those used to form the structures of wind turbine blades or aeroplane wings. Residual stresses during the curing process of thermoset-based composites often mean the final part deviates from the design specification, requiring costly additional machining to deliver the desired shape. LMAT solves this problem with a software package which uses a modelling technique based on Finite Element Analysis to simulate how composite materials will distort following curing. The model relies on knowing values for a number of input parameters that contribute to residual stress, such as coefficient of thermal expansion, cure gradients and interaction of moulds with the composite.
LMAT is confident of most of its inputs, but needed to measure reliable values for the shrinkage of the materials as a result of curing. They worked with NPL through the A4I programme to develop reliable measurements that they could input into their model, helping them provide more accurately designed moulds for their customers.
The solution
NPL worked with LMAT to develop a modified PVT cell to measure shrinkage of the composite. PVT is a technique which allows control of pressure, volume and temperature conditions.
The instrument was adapted from one previously used by NPL for characterising behaviour of thermoplastic materials used in injection moulded parts. The instrument was redesigned by NPL to take measurements to characterise shrinkage of thermoset resin systems, and manufactured by a contractor commissioned by LMAT.
NPL tested the cell on two materials provided by LMAT, one which cures at room temperature and one which cures at high temperature. The instrument is a cell, into which the composite material is placed, with a plunger located on top. As the material shrinks or expands during the curing process, the plunger is displaced in a way that follows the movement of the material, providing measurement of shrinkage.
The impact
As a result of the project, LMAT has access to tools for measuring cure shrinkage. Some refining is still continuing with NPL to improve accuracy further, and a route has been identified to reach the required level of precision.
This will allow LMAT to improve its material characterisation process, leading to better calculations of cure shrinkage properties that feed into their simulations, therefore improving the accuracy of the entire simulation.
It is expected that this will provide an important competitive advantage for LMAT, and improve customers’ productivity by reducing the need to rework or scrap parts.
Based on the project learnings, LMAT has also commissioned a new test rig which can be run on-site, which will be suitable for measuring most materials they encounter. With further refining, the PVT cell could also become a product which LMAT could manufacture and sell to its software customers to enable them to bring bespoke material characterisation in-house.
This work was done as part of the Analysis for Innovators programme.
