Temperature dependence of high strain-rate impact fracture behaviour in highly filled polymeric composite and plasticized thermoplastic propellants

Abstract

The effect of temperature and strain-rate on the fracture behaviour during high strain-rate (∼ 10³ sec⁻¹) impact of two highly filled polymeric composite propellants (containing segmented polyurethanes based on hydroxy-term inated polybutadiene (HTPB) or glycidyl azide polymer (GAP) filled with ammonium perchlorate (AP) particles) and a plasticized thermoplastic (cast double base (CDB) nitrocellulose-nitroglycerine) propellant have been examined over a wide temperature range encompassing the ⋊ittle-ductile” transition. In the “elastic” region of the loaddisplacement curve, the yield stress and fracture toughness is highest for GAP/AP and lowest for HTPB/AP. In the “elastic” and post-yield “ductile” regions CDB is more fracture-resistant than GAP/AP and HTPB/AP over the temperature range −20 to 50° C, but below −40° C, where both CDB and GAP/AP are brittle, GAP/AP is more fracture-resistant than CDB (as observed in the “elastic” region). Although all the propellants are known to develop small cracks in the “elastic” and post-yield “ductile” regions of the load-displacement curve, the overall fracture behaviour is largely governed by viscoelastic properties (because the cracks close up in compression). The good mechanical properties of CDB, above the “brittle-ductile” transition temperature, can be attributed to the presence of a largeβ-transition loss peak. In the composites, the fracture behaviour is also influenced to a lesser extent by the degree of filler-binder interactions. Dynamic mechanical analysis indicates that GAP/AP has a slightly higher degree of filler-binder interactions than HTPB/AP. A temperature-strain rate reduction has been obtained for the yield stress and the composite curve can be expressed by the equation σ_y =K ₁ +K ₂ log (Ġea _T ) whereK ₁ andK ₂ are constants anda _T is a shift factor.K ₂ is a material constant which reflects the temperature and strain-rate sensitivity.