Abstract
Among the challenges in manufacturing of wood products such as glue-laminated timbers is the difficulty in controlling process variables and assessing the bond quality. This research project investigated an ultrasonic method as a means of monitoring bonding processes and assessing the quality of the cured bonds in wood laminates. The effects of curing phases and bond types on ultrasonic transmission were studied using paired specimens of clear Douglas fir with a single adhesive bond. Monitoring was performed simultaneously at normal and angular (5° nominal) transmission to the bond plane. Acoustic measurements were supplemented with destructive cure monitoring, gel time measurement and microscopic examination. Angular transmission provided greater sensitivity to bond quality and curing status than did normal transmission. The method was sensitive to curing phases (spread, penetration and hardening) and had a reasonable correlation with bond strength development. Different curing curves were observed for thick (0.5 and 1.0 mm), normal and kissing bonds. Thick bonds caused a greater increase in transmission than did normal bonds, whereas kissing bonds showed the smallest signal increase. In angular transmission, thick-bond curing curves showed a characteristic inflection that may be used to identify thick bonds and measure curing rate.
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