Glass fibre reinforced polypropylene (GF/PP) composite is a well-established material system for fabricating bistable composite tape-spring (CTS) structure. It is light-weight and multifunctional, and has attracted growing interest in shape-adaptive and energy harvesting systems in defence, civil and especially aerospace engineering. The factors governing its bistability have been well-understood; whist there is limited research concerning the micromechanics and microstructural failure. In this research, we investigate the failure mechanisms of the GF/PP composite. This is achieved by performing in-situ neutron diffraction on composite specimens using the ENGIN-X neutron diffractometer at Rutherford Appleton Laboratory. Shear failures are characterised at both macroscopic and microscopic scales. Elastic and viscoelastic strain evolutions at different levels reveal the micromechanical shear failure. The failure mechanisms are then proposed, which will benefit optimisation of structural design and structural integrity of the CTS in aerospace applications.