Prediction of notched strength for cylindrical composites pipes under tensile loading conditions

What is it about?

In this paper, an experimental study has been performed on thick,±55° filament wound glass/epoxy tubes using quasi-static tests to examine their offshore applications. A new design for the experimental test is developed in which the end tabs and fixture system are made in a way that the stress concentration at the edges of the composite pipe can be reduced. Split disk uniaxial tensile test has been performed on composite pipes with 86mm of the internal diameter and the thickness of 6.2 mm, in order to evaluate the mechanical properties and to quantify the effect of geometric non-linearities. These pipes can be subjected to different phenomena which can result in introducing a defect in the structure. Two types of pipe specimens are tested which include the composite with notch and the other without notches. The objective of the using a notched specimen is to examine the evolution of the damage and to study the mechanical properties as a function of this damage. The experimental results are described as the influence of the number and the size of notches on the mechanical behavior. These results have shown that the yield stress decreases and yield strain increase with increasing of the notch size and number. The evolution of damage showed that the presence of notches plays an unfavorable role in the integrity of the structure.

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Why is it important?

The mechanical properties of the glass fiber reinforced epoxy composite pipes were experimentally investigated using Split-disk test. The objective of this study was to quantify the effect of the presence of geometric nonlinearities in these structures. The effect of the number and size of the notches had been analyzed. Several parameters reflecting the mechanical behavior were determined among which the maximum load, the material stiffness and the displacement associated with the maximum load had shown some interesting phenomena. Subsequently, microscopic observations were used to understand the rupture mechanisms. Through this experimental study, the following conclusions are obtained: • SDT Force–displacement curves were generally linear up to the point of failure. For the specimens having a geometric defect, a degradation in the mechanical properties was detected and the failure occurred predominantly by matrix cracking and delamination. • The maximum load decreases with increasing the number of notches and resulted in the reduction of rigidity of pipes. Increasing the notch size led to the reduction of the maximum load. • Maximum load and the displacement at break depended strongly on the notch size and the number of defects in the composite pipes. • In almost all the SDT tests, failure occurred between D-disk sections and becomes important for the notched pipes. Fiber fracture, delamination and fiber-matrix debonding were observed to be the dominant failure mechanisms for pipes having±55° of angle. For pipes without defect, the delamination of layers was detected and randomly distributed. For notched pipes, the delamination follows the direction of fibers. • SDT is an effective method to determine mechanical properties of composite pipes. Reliable results were observed with low standard deviations. Tests revealed that the geometric defect can affect the mechanical behavior of pipes.

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