Лако-красочные материалы — производство

A natural crack initiated in the center of the adhesive layer in a symmetric joint between symmetric isotropic (metal) adherends will tend to propagate through the center of the adhesive layer. However, in wood joints, there is a strong tendency for the crack to travel in the wood near the joint. This condition should be expected in joints made with the lower-density species or in species with the low-density earlywood such as the Southern pines (Pinus spp.). However, wood failure is not uncommon in joints made with high — density species even when there is a starter crack in the adhesive layer before testing. There seem to be some rational explanations for this behavior.

First, a crack will deviate toward one or the other adherend if it is softer (lower in modulus) than the adhesive [51]. This is a common condition in wood joints bonded with rigid thermosetting adhesives. The tension modulus of the wood perpendicular to the grain is typically in the range 400 to 1200 MPa [54], while the tensile modulus of adhesives used with wood will be in the range 1200 to 4700 MPa at the same moisture level [55-57].

Second, shear forces that develop in the vicinity of the crack tip direct it toward one or the other adherend [52]. Shear forces arise in a cleavage specimen from unequal moduli of the two adherends and the adhesive. Unequal moduli of two wood adherends is vir­tually certain as a result of the variable morphology and density of any two pieces of wood. When a load is applied to the cracked joint, this inequality induces shear stress around the crack tip and thereby directs it toward one adherend or the other.

Once the crack enters the wood as a result of these mechanisms, it will travel pre­ferentially along the weak radial-longitudinal (RL) and tangential-longitudinal (TL) planes. Unless these planes again intersect the bondline, the adhesive will not be likely to fracture beyond that point. If the fiber direction in both adherends is oriented toward the bondline (this is done purposely in some fracture toughness test methods), the crack will be forced to remain close to the adhesive layer. In this case the local density and modulus of the two adherends seems to determine on which side of the adhesive layer the fracture occurs. Since these properties vary continually, it is not unusual for the crack tip to jump repeatedly from one adherend, across the adhesive layer, to the opposite adherend according to the mechanism of Wang and others [51] and Knauss [52]. Given a locale with earlywood on one side of the adhesive layer and latewood on the other side, the crack may not travel preferentially on the earlywood side. Pervasive adhesive penetration of the earlywood may raise the density and modulus to the extent that latewood on the opposite adherend is more amenable to crack growth.

2. Adhesive Layer Thickness

Shear strength studies of joints bonded with rigid thermosetting adhesives over many years has resulted in the prescription that the best joints are those with an adhesive layer in the thickness range 0.05 to 0.15 mm. Ebewele and others [3],

for example, found an optimal thickness between 0.07 and 0.08 mm (Fig. 13). Other research based on fracture mechanics [13, 28, 30] has helped to define this relationship, although not its cause. Apparently, below some minimum thickness, a joint is adhesive starved and the interphase is rife with voids. Above the optimum thickness, stress con­centrations are heightened by cure-shrinkage stresses in the adhesive layer. The narrow optimal thickness range disappears if the adhesive modulus is greatly reduced. In the study by Takatani and Sasaki [13], an epoxy adhesive was flexibilized by the addition of 20, 40, and 60 parts of polysulfide. These additions decreased the adhesive modulus from 2200 MPa to 1600, 670, and 160 MPa, respectively. The last two moduli are in the range of the tensile modulus of wood perpendicular to the grain used to test fracture toughness (beech, modulus of elasticity MOE = 590 MPa). Joints of the nonflexibilized adhesive had a slight optimum at 0.3 mm thickness; however, there was actually little difference in toughness (G1c = 220 J/m2) over the entire range of adhesive layer thickness from 0.1 to 1.5 mm. The addition of 20 parts of polysulfide removed the optimum at 0.3 mm thickness and increased toughness to 330 J/m2. The big change came with the addition of 40 to 60 parts of polysulfide. Although these additions failed to increase toughness of joints with the thin adhesive layer, toughness increased dramatically with each increment of adhesive thickness. In these joints the crack initiation energy increased from about 330 J/m2 to 1100 J/m2 in specimens with adhesive layers 1.5 mm thick.

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.