Prestressing of composite structures for enhanced structural efficiency

Due to their high strength-to-weight ratio, fiber reinforced polymer composites are finding growing applications in aerospace, automotive, infrastructure and other structural systems. Fiber reinforced polymer composites provide distinctly high tensile strength and modulus, which cannot be matched by their compressive performance. Reversible stress cycles are also detrimental to the fatigue life of composites. Hence, compressive stresses tend to govern the design of composite structures subjected to reversible stress systems, leaving their superior tensile attributes largely under-utilized. This undermines the structural efficiency of composite structures, and carries important weight penalties.;The primary purpose of prestressing is to introduce an initial stress system within the composite structure, which counteracts the critical (compressive) stresses developed in the structure under service loads. Control of critical stresses under service loads benefits the structural performance of prestressed composites, and enables design of structures with enhanced performance-to-weight ratios.;Initial proof of concept investigations focused on design and experimental validation of the benefits of prestressing to flexural performance of composite box sections under quasi-static and fatigue loading. Prestressing was used in this application to improve flexural strength and fatigue life by lowering peak compressive stresses. Theoretical models were developed for design of prestressed composite flexural members, and tooling and methodologies were developed for fabrication of prestressed composite box sections. Experimental results indicated that about 90% (based on one replicated test) gain in the flexural strength of a specific composite flexural elements could be realized with prestressing which carried a weight penalty of approximately 15%. Fatigue life of the composite flexural element was found to increase by over 100% (based on replicated tests on two prestressed and two non-prestressed specimens) upon prestressing.;More refined applications of prestressing were focused on PRSEUS composite structures which are stiffened composite panels with pultruded ros incorporated in their stiffeners for improved structural efficiency. Use of these pultruded rods as prestressing elements enables prestressing of PRSEUS composite structures with no weight penalty. Use of the unilaterally reinforced pultruded rods in conjunction with multiaxially reinforced constituents which govern failure of PRSEUS leaves the pultrued rods under-utilized at failure. Use of this reserve capacity of pultruded rods towards prestressing eliminates any weight penalties associated with the application of prestressing force. The contribution of prestressing to performance characteristics of an existing design of a rod-stiffened (PRSEUS) composite structure was investigated analytically and experimentally. Experimental results indicated 32% gain in average compressive strength resulting from prestressing of stiffened composite panels. The benefits of prestressing were validated in application to PRSEUS components of different size and complexity. The long-term stability of prestressing force was evaluated experimentally and improved under sustained exposure to elevated service temperatures and also under exposure to freeze-thaw cycles at elevated humidity.;Finite element modeling verified the contribution of prestressing towards enhancement of the structural performance of PRSEUS under compressive loads. The predicted failure mode and ultimate strength of the stringer obtained through finite element modeling agreed with experimental results.