Diamond Light Source - Annual Review 2022/23

78 79 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 2 / 2 3 Quantifyingmicroscale strain in carbon fibre composites using synchrotron X-ray diffraction Technique Development – Materials Science – Material Engineering & Processes – Engineering & Technology – Composite Materials – Polymer Science Carbon fibre reinforced polymer (CFRP) design typically relies on material properties obtained from testing small samples or ‘coupons’ of thematerial at themacroscale (10mm+). Although coupon testing is cost-effective and convenient, the tests oversimplify the impact of micro defects, such as voids and wrinkles, that affect the global response of the structures. The current workaround for this issue is to use conservative safety factors and overengineer the composites, which reduces efficiency and performance. A better understanding of how load is redistributed around defects at the microscale length scales associated with these features will increase confidence in how these artifacts impact material properties and improve input parameters for numerical simulations. However, conventional X-ray diffraction analysis is challenging because carbon fibre is an anisotropic material with a semi-crystalline structure. As a newmethodology was required, researchers at the University of Bath worked with Diamond’s B16 beamline to investigate the feasibility of using synchrotron X-ray diffraction for fibre orientation and lattice strain mapping inside CFRPs. The highly flexible setup on B16 and its ability to capture X-ray diffraction and tomography data were crucial to this new approach. The tests used a sample shaped like a humpback bridge, a standard CFRP geometry with a known solution that induces significant shear stresses between laminates. Its use allows the generation of tensile and compressive stresses and facilitates easy validation with the general loading response of the geometry. Recording diffraction maps at different loads revealed the residual strains from the manufacturing process and the development of lattice strain in the fibres. The team developed a new two-scale modelling process (mesoscale to microscale) to facilitate numerical verification of the results. In general, the model was a good match to the experimental results, except at the edges where factors that could not be reliably modelled are known to exist. Their results offer the first quantification of micro-scale lattice strain in CFRPs. This study has provided validation of a powerful new approach to studying the microscale behaviour of CFRPs and other composite materials. Following further validation via comparative methods, which is ongoing, the researchers will be able to use this approach to substantially improve our understanding of the impact of defects and failure modes of CFRPs. That will allow optimisation of the design of these materials, reducing the overengineering of systems and increasing efficiency in numerous applications. Related publication: Srisuriyachot, J. et al. Carbon fibre lattice strain mapping via microfocus synchrotron X-ray diffraction of a reinforced composite. Carbon 200, 347- 360 (2022). DOI: 10.1016/j.carbon.2022.08.041 Funding acknowledgement: UKRI - EPSRC Grant ‘Certification for Design – Reshaping the Testing Pyramid’ /S017038/1 Corresponding authors: Alexander J.G. Lunt, University of Bath, [email protected] Igor Dolbnya, Diamond Light Source, [email protected] Jiraphant Srisuriyachot, University of Bath, [email protected] Richard Butler, University of Bath, [email protected] Optics andMetrology Group Beamline B16 Data collection and key experimental results obtained from in-situ loading of carbon fibre humpback bridge specimen. a) Example X-ray diffraction pattern showing {100} and {002} diffraction peaks which correspond to the axial and radial orientations with the carbon fibre. Lattice strain estimates in the b) axial and c) radial directions determined experimentally from peak shifts associated with these two peaks. Predictions from finite element models which show nominally identical trends in strain in the d) axial and e) radial directions, albeit with localised variations associated with defects in the sample.