Tension-tension fatigue testing of pultruded carbon fibre composite profiles

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Insinööritieteiden korkeakoulu | Master's thesis
Mechanical engineering
Degree programme
Master’s Programme in Mechanical Engineering
Carbon Fibre Reinforced Plastics (CFRP) are composite materials commonly used in advanced lightweight structures. Their anisotropic nature, combined with the high strength of carbon fibre, results in negligible degradation in strength over fatigue life. Hence, CFRP structures are increasingly used for fatigue load intensive applications. For uniaxial fatigue loading applications, unidirectional (UD) CFRP composites are commonly used. Prior to their use in a practical application, validation of fatigue performance is required through a fatigue test setup. However, fatigue testing of the UD CFRP composites is difficult, as these composites often fail under the machine grips due to high-stress concentration at the gripping area. The main objective of this master thesis is to design an in-house test setup to evaluate the tension-tension fatigue behaviour of the UD CFRP pultruded profile of a rectangular cross-section with dimensions of 5.1×2.56 mm. The pultruded profiles comprised UD CFRP composites of fixed cross section manufactured through the pultrusion process. The testing of the pultruded UD CFRP profile is even more difficult due to a limitation in the available gripping area and low out-of-plane compressive strength. The guidelines for the test setup was obtained from the fatigue testing standard available for UD CFRP laminate. However, the standard specimen produces high-stress concentration at the gripping area, causing failure of the specimen under the grips. Hence, to reduce the stress concentration, a new specimen design was created based on state-of-the-art research. The specimen design was further modified for the specific pultruded profile to reduce stress concentration. The newly designed specimen was manufactured and primarily tested under static tension loading. The tension test results indicated improvement in the strength for the newly designed specimen. Further, the newly designed specimen was tested for two different load levels under tension-tension fatigue loading. Despite an initial debond between the tab and profile, 9 of the 12 specimens completed more than 105 cycles resulting in only minor damage to the UD CFRP profile. Based on the observed damage progression during fatigue testing, the thesis suggests that the design of the new specimen could be improved by selecting a new tab adhesive and adhesive front.
Remes, Heikki
Thesis advisor
Wallin, Markus
fatigue, pultruded profile, carbon fiber, stress concentration
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