Fracture toughness woods

The word "composites" has a modern ring. But using the high strength of fibres to stiffen and strengthen a cheap matrix material is probably older than the wheel. The Processional Way in ancient Babylon, one of the lesser wonders of the ancient world, was made of bitumen reinforced with plaited straw. Straw and horse hair have been used to reinforce mud bricks (improving their fracture toughness) for at least 5000 years. Paper is a composite so is concrete both were known to the Romans. And almost all natural materials which must bear load - wood, bone, muscle - are composites. 

Wood Density (Mg m- j Young s modulus IGPol Strength (MPaj II to grain Fracture toughness (MPa m j ... 

Fig. 26.7. The fracture toughness of wood, like its other properties, depends primarily on relative density. That across the grain is roughly ten times larger than that along the groin. Both vary as...

Because the fracture toughness depends both on cure time and temperature, the arbitrary selection of time and temperature for accelerated tests may not be appropriate for reliable prediction of longterm service life of joints (J7). In order to reduce test variability and improve the durability prediction of adhesive joints, it would be necessary first to control the cure temperature and time required to produce a level of fracture toughness that does not change further (14). The study is thus an excellent example of a multidisciplinary approach combining chemistry, fracture mechanics, and wood science in the investigation of the adhesive bonding of wood. 

A comprehensive account of the fracture toughness of cellular materials including foams, honeycombs, woods, and cancellous bone is presented by Gibson and Ashby [198], They have put emphasis on establishing and verifying relationships between the fracture parameters and the density of the material rather than the microstructural parameters such as the cell wall thickness. This is a practical approach since the density data for these materials are readily available or can be easily measured. 

Table 1 Fracture Toughness for Wood-Adhesive Joints... 

Very high fracture toughness values for wood adhesive joints can be attributed to a combination of adhesive plastic deformation and reduction of microcracking of the wood around the crack tip. A flexible adhesive layer, especially a thick layer, distributes the... 

Figure 12 Characteristic crack growth behavior and fracture toughness of wood adhesive joints (a) strong/unstable (b) strong/stable (c) strong/moderately unstable (d) weak/stable.

It is probable that very high fracture toughness values in wood joints bonded with thicker, lower-modulus adhesive layers may be due to the enhancement of an existing energy-dissipating mechanism such as microcracking of the wood as well as the adhesive. 

Lei and Wilson [35,67] developed a model for the fracture toughness K j of flakeboards bonded with PF resin adhesive. The model is based on the initial crack length a) in the specimen, the average size of the inherent flaw (S2) in the solid wood, the expected increase in crack length resulting from nonbonded interflake cracks and voids (Au), and the Kjc value of the wood used to make the flakes ... 

The best adhesive for improved fracture toughness (1) does not develop shrinkage stresses during cure, (2) has a modulus close to that of wood perpendicular to the grain, (3) has a modulus that changes in parallel with the wood modulus as moisture content changes, (4) penetrates small-lumen, thick-walled cells but does not overpenetrate large-lumen thin-walled cells, and (5) can infiltrate the cell wall to reinforce the weak interphase between cell-wall layers. 

In this chapter, an overview of performance characterization of FRP-wood bonded interfaces by conventional and fracture mechanics tests [4- 7] is presented. Modified ASTM standard tests (ASTM D2559 and D905) are first used to study the service performance and shear strength of the bond under moisture and/or mechanical loads, and then a contoured or tapered double cantilever beam specimen [8] is used to evaluate the fracture toughness of bonded interfaces under dry and wet conditions and cyclic loading. 

Once the performance of the interface bond was established by the previous delamination (durability) tests, contoured double cantilever beam (CDCB) specimens were designed to conduct mode-I fracture tests. In this study, bilayer CDCB specimens (see Fig. 6) were designed by the Rayleigh-Ritz method [8] and used for fracture toughness tests on bonded FRP wood interfaces under both dry and wet conditions. The critical strain energy release rate, G/c, which is a measure of the fracture toughness, is given as ... 

The TDCB specimen (Fig. 20) was used for evaluating fracture toughness of bonded FRP-wood interfaces under mode-1 loading [5], and the strain energy release rate G is given in Eq. (1). For one complete cycle, the maximum and minimum strain energy release rates are... 

Finally, the tapered double cantilever beam specimen [8] described in this study can be effectively used to obtain interface mode-I fracture toughness values under various environmental loadings (e.g., under dry and wet conditions [5,14,15]) and cyclic load conditions (fatigue fracture [7]) these data can be implemented in practical applications to assess the potential growth of a delamination crack at the FRP wood interface. 

Wood-based composites with a phenohc resin matrix have been transformed into silicon-infiltrated silicon carbide ceramics by carbonization at about 1 650°C [46]. The bending strength and the elastic modulus of these original ceramics were better than those of conventionally manufactured counterparts, whereas the fracture toughness was lower. 

Evidence for elastic-ligaments bridging and frictional pull-outs similar to those observed in other structural ceramics were found in all samples after fracture toughness and cyclic-fatigue testing. However, the excellent JGc and JQh values can be mostly attributed to the unique KB-induced lamellae (Figure 7.18b) that serve as very tenacious crack bridges, somewhat reminiscent of wood. 

K. Semrick, Determining Fracture Toughness by Orthogonal Cutting of Polyethylene and Wood-Polyethylene Composites, Oregon State University, Master Thesis (2012). 

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