Лако-красочные материалы — производство

A. Structural Joints

There are no standard design methods for adhesive-bonded wood joints, let alone design methods based on fracture mechanics. This is obviously due in part to the complexity of the fracture behavior of wood joints and materials. The lack of adequate design methods has obviously been a hindrance in furthering the use of adhesives in structural assembly joints for wood structures. However, studies have demonstrated the power of fracture mechanics for developing generalized methods for predicting the behavior of adhesively bonded joints and materials. Conventional strength tests have not been able to predict such behavior.

Komatsu and others [64] found that the strength of double-lap joints was porpor — tional to the bond area for relatively short overlaps. However, for long overlaps, stress concentrations and fracture mechanics controlled the strength of the joint. The authors developed the following fracture-based design equation:

smax — V GcExS

where amax is the shear strength of the joint, Gc the critical strain energy release rate, Ex the elastic tensile modulus of wood adherends along the grain, and S the geometrical joint parameter.

Wernersson and Gustafsson [65] developed a nonlinear fracture mechanics relation­ship based on pure shear for predicting the performance of lap joints of varying geometry and adherend properties based on the adhesive brittleness ratio:

tL

Gf where tf is the ultimate shear stress of the bondline obtained from the uniform stress test method, and Gf is the total fracture energy of the bondline. Wernersson [66] used this brittleness ratio to show how the failure of different types of joints is controlled by various criteria. Joints with a low brittleness ratio exhibit ductile behavior with uniform plastic deformation along the bondline. The joint strength is proportional to the local bond strength. Joints with a high brittleness ratio exhibit brittle behavior, with strength inde­pendent of the local bond strength. Joint strength is governed by fracture energy. The strength of joints with an intermediate brittleness ratio is affected by the local strength but also by the fracture energy and the shape of the stress-strain curve of the materials.

Based on his analysis, Wernersson proposed that the optimal adhesive properties, in terms of producing the strongest joint, are not necessarily those that produce the highest wood failure. However, Wernersson also acknowledges that this conclusion does not take into account the effects of time, temperature, or moisture. When long-term effects are considered, it is still too early to reject the long-standing requirements for high wood failure and maximum allowable cyclic delamination as indicators of the probable perma­nence of structural joints.

Komatsu [38] also applied fracture mechanics to the design of bonded cross-lapped knee joints that experience a torsion shear loading. Specimens were tested with the angle of the knee at 90°, 120°, and 150°. The crack initiation energies for the three angles were, respectively, 480, 600, and 1100 J/m2. Failures at 90° and 120° were largely brittle (tension perpendicular to the grain) and had a greater correspondence to the lower torsional shear fracture toughness values than failure at 150°. The 150° joints showed a fairly uniform distribution of five different types of fracture. Some of the difference in toughness and type of fracture is no doubt due to the greater proportion of sliding shear forces in the 150° joint. Overall, Komatsu concluded that the fracture mechanics analysis gave a better prediction of strength than a method based on elastic torsional theory.