Reduction of void content in laminated composites by mechanical vibrations

Abstract

Defects, such as voids and bubbles, are detrimental to mechanical properties of composite materials. Theory predicts and experiments show that mechanical properties deteriorate in the presence of voids, hence void management is critical to high quality composite parts. Eliminating common defects such as voids, bubbles, and poor adhesion at interfaces will increase the quality of laminated sandwich composite structure. There is evidence from previous research that mechanical vibrations applied during curing of a laminated composite can reduce the number of bubbles. Previous experiments provide guidelines for the exploration of vibrations as void eliminator in composites. This thesis reports experimental research results and analysis of the effect of mechanical vibrations applied to the curing system of composite materials production, particularly for minimizing void content. Range of frequency of vibrations covered was from 0 Hz to 40 kHz, for different period of vibrations (10-30 min), at different operating temperatures (20-55{u00B0}C), along with vacuum assistance. The composite laminates were made by hand lay-up using glass fibers and either vinyl-ester or epoxy resin, and examined under microscope to determine types and quantity of defects. The results showed remarkable reduction in void content when low frequency vibrations were applied (5-50 Hz). Void content was determined by optical microscopy. Details of internal structure were examined by scanning electron microscopy and computed tomography. Mechanical testing (flexural, tensile and shear) also supported these observations. After initial stage where electromagnetic shaker was introduced to the curing system, special apparatus (based on the Quickstep method) was built to 'explore suitable frequency range of vibrations for curing at elevated temperatures, supported with vacuum pump. Numerous tests were performed and conclusions are leading towards better mechanical properties of laminates cured with vibration assistance. Theoretical background is described taking into account various factors, such as temperature and viscosity, pressure variations, mass diffusion, and vibration itself as a complex process. Although there are still some unknown areas in the understanding of the process, some hypotheses made are supported and corroborated with experimental results. Show more