In situ biaxial loading and multi-scale deformation measurements of nanostructured materials at the CoSAXS beamline at MAX IV Laboratory

Characterization of the mechanical response of polymers and composite materials relies heavily on the macroscopic stress–strain response in uniaxial tensile configurations. To provide representative information, the deformation process must be homogeneous within the gauge length, which is a condition that is rarely achieved due to stress concentration or inhomogeneities within the specimen. In this work, the development of a biaxial mechanical testing device at the CoSAXS beamline at MAX IV Laboratory is presented. The design facilitates simultaneous measurement of small- and wide-angle X-ray scattering (SAXS/WAXS), allowing assessment of the microstructural configuration before, after and during the continuous deformation process at multiple length scales. The construction also supports multiple deformation conditions, while guaranteeing stability even at high loads. Furthermore, the mechanical experiments can be complemented with spatially resolved mesoscopic surface deformation measurements using 3D-surface digital image correlation (DIC). Polycarbonate (PC) was used to demonstrate the varied material response to multi-axial deformation, as PC is isotropic with a high glass transition temperature (∼150°) and high strength. As a result, a clear correlation between full-field methods and the microstructural information determined from WAXS measurements is demonstrated. When a uniaxial load is applied, homogeneous strain regions could be observed extending perpendicular to the applied load. When a secondary axial load was added (biaxial mode), it was observed that high strain domains were created near the centre of the sample and at the boundaries after yield. With increased strain, the deformation in the main deformation direction also increases. Mechanical reliability was demonstrated by carrying out static loading of polyacrylonitrile-based carbon fibre (CF) bundles. As a result, the nonlinear stiffening behaviour typically observed in CFs was seen, while no evidence of the creation of new voids during loading was observed. The results support the reliability and broad applicability of the developed technique.