Influence of Wood Grain Direction on Linear Welding

Wood grain orientation differences in the two surfaces to be bonded yield bondlines of different strengths in linear wood welding. End-grain-to-end-grain welds of good strength were obtained for both beech and oak woods. The tendency to defibration in end-grain-to-end-grain welding indicated that for wood densities higher than or equal to the density of beech wood, end-grain-to-end-grain welding is possible and yields sufficient joint strength. A higher density seemed to yield a stronger joint. Wood pieces having other grain directions were also vibration welded. These were: (1) with the grain perpendicular to the wood longitudinal grain direction, (2) with the grain of both wood pieces at 45° to the wood longitudinal grain direction and (3) with the grain of both wood pieces at 45° to the wood longitudinal grain direction but at 90° to each other to form a fishbone-like pattern. The first of these yielded results comparable to end-grain-to-end-grain welding. The other two yielded much lower strength of the joints, indicating that fibre orientation in the interphase composite formed during welding had considerable influence on joint strength. These differences in joint strength have been explained by the very marked effect that anisotropy of the interphase composite has on fibre/matrix composites.     

Measurement of Mode I and Mode II Fracture Properties of Wood-Bonded Joints

In this work, the fracture characterisation of wood-bonded joints under pure mode I and mode II loading was performed. The tested material was maritime pine (Pinus pinaster Ait.) bonded with an epoxy adhesive. Two fracture mechanical tests were chosen: the double cantilever beam (DCB) for opening mode I loading, and the end-notched flexure (ENF) for sliding mode II loading. The compliance-based beam method (CBBM) was used for both mode I and mode II fracture, since the Resistance-curves can be obtained directly from the global mechanical response of the specimens (load–displacement curve), without crack monitoring during propagation. This data reduction scheme was validated by direct comparison with the modified experimental compliance method (MECM).     

A New Test Methodology for Simultaneous Assessment of Monotonic and Fatigue Behaviors of Adhesive Joints

Adhesive joints are normally subjected to different working conditions in their service life. This may involve both static and cyclic loadings. In many instances, a combination of various loading conditions occurs that can be further provoked by exposure to hostile environments. This, in turn, leads to the need to characterize the joint behavior under different combinations of working conditions. Extensive experimental tests are needed in order to evaluate the joint performance under such variable working conditions. This implies the development of low cost and efficient test technique, the one that is simple and reduces the operator time as well. With this objective in mind, a novel technique in mechanical evaluation of adhesive joints was developed in the present work. Alternative monotonic and variable-amplitude cyclic loads were applied on the same double cantilever beam (DCB) specimens under cleavage mode. DCB specimens were made from aluminum bars joined together by a two-part toughened structural adhesive. On one face, a series of crack detection sensors were bonded to control the test machine for switching between monotonic and cyclic loadings. The test machine had two aligned hydraulic actuators which applied bending forces on the upper and lower arms of the DCB specimen. The effects of test frequency and applied load history were also investigated within a range of 4–20 Hz for a nominal adhesive thickness of 0.5 mm. The fatigue performance of each configuration was represented by a power-law relationship and was compared for different test conditions. The test results revealed that the fatigue damage occurred at relatively lower load levels (35%) when compared with monotonic fracture load. The power-law constants for the tested adhesive were influenced by test frequency but were not sensitive to loading order.     

Fracture mechanics testing of the bond between composite overlays and a concrete substrate

This paper presents a methodology for assessing the bond strength of composite overlays to concrete utilizing a fracture toughness test. The principles and practices of existing ASTM standards for determining the fracture toughness of adhesive bonds between double cantilever beam (DCB) metallic and composite specimens (D 3433-93 and D 5528-94a) have been extended to cover the case of an elastic composite layer bonded to a rigid concrete/masonry substrate. In the theoretical section, the dominant loading conditions, relevant ASTM standards, and the development of energy release rate concepts for analyzing a disbonding composite layer modeled as an elastic cantilever beam are presented. The experimental section covers specimen fabrication and preparation, experimental setup, test procedures, post-test evaluation of the specimens, and data processing. The discussion of test results focuses on explaining the variability in measured strain energy release rate, and identifies trends between the measured strain energy release rate and the fraction of the fracture surface retaining cement paste after disbonding. It was found that good-quality composite-to-concrete bond is associated with high fracture toughness of the adhesive and location of the crack path in the concrete substrate. Strict enforcement of surface preparation and adhesive handling procedures was found to play an important role in promoting good bond strength and high fracture toughness. The fracture toughness test developed in this paper can be used for screening various composite-repair systems, to assess the effect of different environmental attacks, and as a quality control tool.