polyimides LLDPE trounds BQundm VQp12 Flexural Fastflight weldable belite prestressing durability sigmaQundb flexural Maraging steels Compressive Valluvakkonathiri RQundA milliaria gridshell castability LDPE stiffness isogrid rudr kratos Sumirago toughness hardenability machinability sturdiness Cascaded superhuman fleetness brittleness nondirective