The effects of various bolt preloads, viscoelasticity, and external applied static and dynamic loads on bolt load relaxation in a carbon/epoxy composite bolted joint have been studied. Both phenomenological modeling and finite element analysis (FEA) of bolt-connected three-point bending specimens were employed in the studies. Relaxation of 1.25-4.25% over a period of 30 h was observed depending on the initial preload and applied external loads. Both static and dynamic applied loads were considered. It was observed that for any magnitude of external load the bolt load relaxation decreases with increasing initial preload. These findings emphasize the importance of the magnitude of the preload. Comparing the bolt load relaxation in steel and composite joints for the duration of 30 h, it was concluded that only about 1/3 of the relaxation in composite specimens is due to viscoelastic behavior of the polymer matrix in the composite, and the remaining 2/3 of the relaxation is due to other mechanisms such as bolt thread slip, plasticity and/ or external excitation.
In this paper experimental and numerical results concerning the dynamic response of composite sandwich beams with curvature and debonds are reported. Sandwich beams made of carbon/epoxy face sheets and polyurethane foam core material were manufactured with four different radii of curvature and debonds between the top and bottom interface of face sheet and foam core. Dynamic response was obtained using the impulse frequency response technique under clamped-clamped boundary condition. Experimental results were compared with numerical finite element model results. A combined experimental and numerical FE approach was used to determine the material properties of the skin and foam core materials based on modal vibration and static flexure tests. Results indicate that the fundamental frequency increases with increasing curvature angle, however, for higher frequencies; the natural frequencies are not significantly affected. Also, it is found that face/core debond causes reduction of the natural frequencies due to stiffness degradation.
The aim of this research was to investigate the curvature and face/core debond influence on the vibration behavior of curved composite sandwich beams made up of carbon/epoxy laminate skins over a polyurethane foam core. Flat and curved sandwich beams with debond were prepared by keeping the arc length of the sandwich beams equal to the length of the flat sandwich beams. Sandwich specimens with three different curvatures and debond between the top and bottom face/core were manufactured. Systematic vibration and static experiments were carried to determine the response and to study the effect of curvature and debond. Flexural stiffness and strength of the sandwich beams were determined by four-point bending tests. The natural frequencies and damping loss factors of the sandwich beams were determined using impulse frequency response technique under free-free boundary conditions. Four-point bending test results indicate that stiffness of the beams decrease with presence of debond. Vibration test results show that face/core debond causes reduction of the natural frequencies whereas damping loss factor values increase with presence of debond. The effects of debond and curvature on damping loss factor is higher than that on natural frequency.
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