Fracture toughness is an indicator of the stress required to propagate a pre-existing flaw and is a critical property for materials such as laminated composites used in demanding structural applications.
Delamination of laminated composites reduces their stiffness and strength and can lead to loss of structural integrity. Hence, there is significant interest in improving the fracture toughness of laminated composites, particularly those used for aircraft construction.
Modern high performance aircraft uses a range of components made from laminated carbon-fibre/epoxy composites, but in less demanding aerospace applications the laminated wood composite plywood is still used.
Plywood consists of thin layers of wood veneer that are glued together, with adjacent layers having their wood grain rotated up to 90 degrees to one another.
The glue lines of plywood and also other laminated wood composites such as laminated veneer lumber (LVL) and glue-laminated lumber (glulam) often contain flaws and they can fail by delamination. Hence, there has also been interest in improving their fracture toughness.
The susceptibility of laminated composites to delamination depends on a variety of intrinsic and extrinsic factors, but the lack of reinforcement of adhesive bond lines is a fundamental reason why cracks propagate between laminae. Therefore, one obvious route to increasing the fracture toughness of laminated composites is to provide such reinforcement.
For example, numerous studies have shown that the susceptibility of laminated composites to delamination can be reduced by reinforcing adhesive bond lines with fibres that act as bridges to inhibit crack opening.
Subsequent studies reviewed this approach, and others used to improve fracture toughness of materials. Another approach to improving the fracture toughness of laminates, which we explore here, is to alter the geometry of the adhesive network to provide through-reinforcement across multiple laminae.
Such an approach is related although not identical to those used to provide through-thickness reinforcement of advanced composites, for example, 3D-weaving, stitching, braiding, embroidery, tufting, and z-anchoring.
We tested adhesive through-reinforcement across multiple laminae in combination with glass-fibre reinforcement of adhesive bond lines as a means of improving the fracture toughness of a model laminated birch wood composite.
Our results demonstrated that the fracture toughness of the wood composite can be significantly improved as a result of the introduction of adhesive through-reinforcement. Further significant increases in fracture toughness occurred when glass-fibre was added to the adhesive.
The introduction of adhesive through-reinforcement in the birch wood composite significantly improved fracture toughness. The improvement of fracture toughness was significantly greater in samples containing higher levels of reinforcement.
Fracture toughness was also significantly improved by reinforcing adhesive bond lines with glass-fibre. Analysis of variance indicated that there was no significant interaction of through-reinforcement and addition of glass-fibre to adhesive bond lines on fracture toughness.
Nevertheless, the addition of glass-fibre to adhesive bond lines was more effective at improving the fracture toughness of the birch composite containing through-reinforcement than it was at improving the fracture toughness of the control.
For example, the addition of glass-fibre to adhesive bond-lines improved the fracture toughness of the control by 42 percent, whereas comparable figures for composites containing through-reinforcement were 69 (24 reinforcements) or 67percent (39 reinforcements).
The effects of through-reinforcement and addition of glass-fibre to adhesive bond lines on five percent maximum fracture toughness values of specimens. These figures also show other critical fracture characteristics of tested specimens, and demonstrates the effectiveness of through-reinforcement, and also adhesive bond-line reinforcement on the fracture toughness of the birch wood composite.
Through-reinforcement of the birch composite appeared to arrest propagation of the crack induced during fracture toughness testing. This effect is suggested by load-displacement curves of specimens during testing, which show that load increased abruptly and then increased more slowly.
During the latter phase, there were increases in load as the crack encountered through-reinforcement. Load increases are sinusoidal in specimens with through-reinforcement at the lower level, but higher levels of reinforcement appear to smooth the sinusoidal variation of load.
The load carrying capability (-axis) of the specimens with through-reinforcement was generally higher than that of the control. Furthermore, specimens with through-reinforcement withstood greater mode I opening displacement (-axis) until ultimate failure than the control.
Crack resistance curves of specimens, also known as R-curves, accord with load displacement results and indicate the positive effects of through-reinforcement and adhesive modification on fracture toughness. They also support the suggestion that increases in toughness of specimens with through-reinforcement resulted from the ability of the reinforcement to arrest crack propagation.
Scanning Electron Microscopy
Scanning electron photomicrographs of fracture surfaces in laminated birch wood specimens after mechanical testing suggest how through-reinforcement and addition of glass-fibre to adhesive bond lines increased fracture toughness. They also cast some light on the bonding mechanism of polyurethane adhesives used with wood.
Polyurethane adhesive was clearly visible at the fracture surfaces of birch composites bonded with unmodified adhesive. Residual adhesive at fracture surfaces exhibited tearing and pull-out, which may have contributed to adhesive bond strength.
In addition, we observed pull-out of adhesive that had penetrated the cells in the rays of birch. The pillar-shaped structures that projected from the cells within rays had branches, some of which appeared to have fractured during testing. We also observed fracture, pull-out, and lateral displacement of adhesive columns that provided multiple through-reinforcement of the birch composite.
The same patterns of failure were noted at fracture surfaces of composites bonded with polyurethane adhesive containing glass-fibre. In addition, we observed pull-out of fibre-bundles at fracture surfaces. Glass-fibre was clearly evident at fracture surfaces and there was evidence of pull-out of the glass-fibres in horizontal glue-lines and also within the adhesive columns that provided through-reinforcement.
Our results supported our hypothesis that introduction of through-reinforcement across veneers increases the fracture toughness of a laminated birch wood composite and show that improvements in fracture toughness depend on the level of reinforcement.
Our results also showed that glass-fibre reinforcement of adhesive bond lines significantly increases fracture toughness. They also suggest how the two different types of reinforcement increased fracture toughness. The addition of glass-fibre to adhesive bond lines appeared to provide reinforcement during crack propagation, hence consuming fracture energy.
This suggestion accords with the results of many studies that have examined the use of glass-fibre to reinforce and improve the toughness of other composite materials, for example, vinyl-ester polymer composites, polypropylene composites, and dental particulate composites.
There have been no previous studies to our knowledge of the use of adhesive through-reinforcement to increase the fracture toughness of laminated wood composites. Therefore, we cannot compare our findings with those of previous researchers.
However, the multiple through-reinforcement provided by adhesive columns running radially in specimens has some similarities with that provided by rays (radial ribbons of woody tissue), which are aligned in the same direction as the adhesive columns engineered here.
Our SEM images of fracture surfaces suggested that rays provided reinforcement, and another study of the fracture toughness of three hardwoods and the softwood, spruce, suggested that ‘the rays (in solid wood) can be considered as reinforcements in the radial directions.’ Furthermore, their load displacement graphs obtained during fracture testing of hardwoods have some similarities to those obtained by us.
Hence, we suggest that adhesive through-reinforcement provided reinforcement behind the crack tip, hindering crack propagation and absorbing fracture energy. This suggestion is supported by the wave-shape of the crack propagation resistance curves (R-curves) of specimens containing adhesive through-reinforcement.
The ‘wave peaks’ indicate abrupt increases in fracture energy when the crack propagated through adhesive columns. This bridging effect occurred at the macroscale, but it complements that provided by adhesive bond reinforcement with glass-fibre. Such a complementary effect was more pronounced in specimens with through-reinforcement than in the controls, possibly because the glass-fibre reinforced the adhesive columns in the birch wood composite, in addition to their ability to reinforce adhesive bond lines aligned in the x-y direction.
Birch plywood is a preferred material used for the construction of wooden aircraft, as mentioned above, and fracture toughness is an important property of composites used for this application. Nevertheless, as pointed out by other researchers, there are few studies of the fracture toughness properties of hardwoods such as birch.
The fracture toughness properties of composites depend on good adhesion between adhesive and the matrix. Adhesive bonding of wood involves a mix of physicochemical interactions including mechanical interlocking provided by penetration of adhesive into the porous microstructure of wood.
Penetration of adhesives into hardwoods occurs easily via the open pores (vessels) that are readily apparent in species such as oak. Hence, reviews of wood adhesion have focused on such an effect.
The penetration of adhesives into rays was mentioned by another study as possibly having a detrimental effect on adhesion because ‘radially oriented rays can allow excessive flow and over-penetration’ (of adhesive).
Our SEM photomicrographs showed pull-out of adhesive from cells in rays suggesting that penetration of adhesive into the rays located at the tangential surface of rotary peeled birch veneer improved adhesion.
The pillars of adhesive that pulled out from ray cells had side arms possibly produced by penetration of adhesive from ray cell lumens through pits (openings) into the lumens of adjacent cells. Some of these side arms were fractured, possibly indicating that they contributed to the fracture toughness of the composite.
This observation suggests that penetration of adhesive into rays can have a positive effect on adhesion and underscores the need for further studies of the effects of wood microstructure on the adhesive properties and fracture toughness of composites made from hardwoods.
The fracture toughness of the model composite we tested was clearly improved by adhesive through-reinforcement and also the addition of glass-fibre to adhesive bond lines. Hence, the addition of glass-fibre to adhesives could be a practical means of improving the fracture toughness of veneer-based wood composites used in demanding applications.
Clearly, our means of creating through-reinforcement, which also significantly improved the fracture toughness of our model composite, would be more difficult to implement in practice than adding glass-fibre to adhesive bond lines.
However, the approach is attractive because significant increases in fracture toughness were achieved without increasing the level of adhesive in the composite. Therefore, it would be worthwhile to explore approaches that are more practical than precision drilling as a means of perforating veneer.
One possible approach could involve passing veneer through a roller containing slitting knives, as has been done in the past to ‘reduce the tendency of rotary peeled veneer to distort when it is used to manufacture plywood’.
An alternative related approach to improving fracture toughness would be to nail the composite in the Z-direction using thermoplastic polymer nails.
Laminated wood composites that are bonded together with nails are available commercially, and such an approach might work with other types of wood composites. In support of this suggestion, the automated insertion of thin metal wires through laminates (Z-pinning) is highly effective at improving the fracture toughness of laminated synthetic composites and is used commercially to improve the properties of aerospace and automotive composites.
The Z-pins used in these applications are made from materials such as titanium alloy that are strong and stiff. This desirable property of Z-pins suggests that adhesives that are stronger than the polyurethane adhesive tested here might be better at increasing fracture toughness of laminated wood composites via adhesive through-reinforcement.