Evaluation of the prestressing effects on the fatigue life of composite box sections

Abstract

Fatigue tests were conducted on non-prestressed and prestressed composite box sections. The approach to prestressing of composite structures seeks to eliminate compressive stress excursions under fatigue loading. This approach increased the fatigue life of composite structures; the experimental gains in fatigue life were in agreement with theoretical prediction. The tensile prestress developed in the composite structure mitigate compressive stress excursions under fatigue load, which significantly enhance the fatigue life of composites. Flexural fatigue tests were conducted through repeated application of a constant deformation, with the maximum load level monitored throughout the tests. The repeated (constant) deformation was selected to produce 70% of the ultimate (quasi-static) flexural strength of each of the prestressed and non-prestressed composite box sections. Two prestressed and two non-prestressed specimens were tested under (displacement controlled) fatigue loading–unloading conditions. The fatigue life of non-prestressed composite sections was reached upon local buckling of top flange (which experienced the maximum compressive stress). For both non-prestressed and prestressed composite sections, failure was defined as the number of cycles after which the peak load (at constant repeated deformation) dropped to 70% of the value in the first cycle. The fatigue life of prestressed composite sections was about 100% greater than that of non-prestressed sections. The residual strength of prestressed sections after fatigue loads was, on the average, 56% greater than that of non-prestressed sections. The experimental results indicated that contribution of prestressing to the fatigue life of composite sections was significant.