A　bistable　composite　tape-spring　(CTS)　is　stable　in　both　the　extended　and　coiled　configurations,　with　fibres　oriented　at　+/-　45　degrees.　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,　but　there　is　limited　research　concerning　the　mechanics　of　structural　failure:　here,　we　investigate　the　shear　failure　mechanisms　in　particular.　We　perform　in-situ　neutron　diffraction　on　composite　specimens　using　the　ENGIN-X　neutron　diffractometer　at　Rutherford　Appleton　Laboratory　(STFC,　UK),　and　shear　failure　is　characterised　at　both　macroscopic　and　microscopic　scales.　Elastic　and　viscoelastic　strain　evolutions　at　different　strain　levels　reveal　the　fundamentals　of　micromechanical　shear　failure,　and　their　temperature　dependency.　Multiscale　shear　failure　mechanisms　are　then　proposed,　which　will　benefit　the　optimisation　of　structural　design　to　maintain　structural　integrity　of　CTS　in　aerospace　applications.