TNO has developed an engineering model for flexible body armor, as one of their energy based engineering models that describe the physics of projectile to target interactions (weaves, metals, ceramics). These models form the basis for exploring the possibilities for protection improvement. This presentation describes how the main physical details of projectile to flexible weave (fabric) interaction can be modeled with a simple set of equations that are based on four well known relations for force, work performed, kinetic energy of a moving material and strain energy in a material and on the two relations for longitudinal wave velocity and cone diameter expansion. The evolution of an impact in the model is based on energy conservation, equal to the summation of kinetic energy in projectile and weave and the strain energy in the armor material. The effect of friction and delamination is excluded. The model allows plasticity of the projectile, strain rate effects on the fiber strength and fiber failure strength as a function of fiber damage due to transverse loading effects. The model is made time resolved, calculating projectile deceleration, weave displacement and straining in a series of calculation steps. The model has created a better understanding, how projectile kinetic energy is transferred into the flexible armour material and how fibre failure and energy absorption in the weave stack are shifted in time. The model has indicated that fibre failure in flexible weave under ballistic impact, is partly initiated by high lateral fiber stress at the location the projectile interacts with the weave stack. This phenomenon is currently investigated at TNO.