stable and follows a single kinematic path that manifests as unrolling of the tape-spring. Figure 10 illustrates the transition from the free end that is mechanically held in the deployed state and the tape spring roll section. The unstable transition zone is where the deployment force is generated. Tape spring deployment torque is a weak function of the wrap thickness, which decreases as deployment progresses  and is at the minimum right before full deployment where the roll radius is equal to the  RH. The torque  value at this state can be roughly approximated by: (2) The above equations are used to observe the sensitivity of the stowed radius and deployment torque to  the laminate construction and initial geometry to aid in the design process. The predicted performance,  determined from either the rigorous calculations, such as described in Ref. 8, or through finite element  analyses are naturally only as accurate as the input parameters. For example, the results for thin flexible  composites have been shown [11] to be highly sensitive to the lamina thickness terms. Simply using the measured cured thickness can result in a significant over estimation of the available torque. The laminates are designed to be relatively insensitive to creep/relaxation effects; however, torque degradation should be taken into account. One method to approximate the long-term available torque is  to assume that the in-plane shear modulus terms ( G12) of the plain weave layers is several orders of  magnitude less than the initial value found from CLT. Accurate prediction of this degradation is one area  of active study in thin flexible composites.      Figure 10. Geometrics definitions of a cylindrical shell. Current and Future Research in Flexible Composites The sizeable body of research and development in flexible composite tape springs enables  implementation into space deployable architectures with an acceptable level of risk. The components are constructed with space-qualified traditional fibers and material systems to achieve high strains. Also,  space-qualified methods are followed to produce consistent, high-quality parts. Engineering methods to  predict bending stiffness/deployment torque have been verified [11] to acceptable accuracy. One  remaining critical area of current research is to develop the engineering methods to accurately predict flexible composite strength based on stress/strain state for a given laminate. Large deformation bending  of composite materials in flexure routinely yield strain levels not attainable by standard ASTM axial tensile  test methods. Traditional thick composites ASTM tests fail in compression at lower strains, where the fibers subjection to tensile loads are the failure mode in bending tests. AFRL Space Vehicles directorate  has developed a test and analysis methodology to determine stress/strain state at flexural failure [12].The method constructs a nonlinear constitutive model of the lamina material based on empirical data from  a combination of axial and bending tests. With an adequate understanding of lamina nonlinear  constitutive behavior, laminate bending response can be predicted and through thickness stresses and strains can be calculated as a function of curvature and at failure. Material failure criteria can then rationally be developed that are pertinent to the large strains achieved in thin flexures. Experience has shown that traditional CFRP laminates gradually lose strain energy (deployment force) or  change dimensions over time while packaged due to stress relaxation. The laminate can be designed to 

504