Лако-красочные материалы — производство

External loads are imposed on a bonded joint or structure by the dead weight of the structure and its contents, accumulated snow, wind, and people. The average stresses in a joint created by these loads can be calculated from structural analysis, but the maximum stress at joint edges is more difficult to determine. These stresses have been examined in some detail. Discussions by Walsh and others [63] and Glos and Horstmann [40] of the effects of external forces on the stresses in bonded wood lap joints are notable. Walsh and others applied plastic failure and fracture mechanics criteria to study the effects of the ratio of lap length (L) to primary member thickness (T) on the average failure stress of

double-lap joints. The authors conclude that the plastic failure criterion (uniform stress) governs failure of the joint only at a very low L/T ratio (< 1). The fracture mechanics criterion governs to an L/T ratio of 8. The authors developed the following conservative empirical design formula:

У

for L/T — 2 to 8 and t/T — 0.5 to 2, where anom is the allowable axial stress in the adher — ends, ty the design shear stress in the joint, and t the lap adherend thickness.

Glos and Horstmann [40] systematically studied the effects of various joint design factors on the fracture of double-lap joints. These factors included (1) grain angle between two side-grain to side-grain members, (2) length of overlap, (3) shape of bonded area, (4) wood density, (5) type of adhesive, and (6) end distance. (End distance is defined as the distance to which the unloaded portion of the bonded members extends beyond the joint.) The authors found that all factors had a strong effect on joint fracture except the shape of the bonded area. Most interesting was the finding that creating a finite end distance increased the strength of joints with lap length/member thickness (L/T) ratios of 3 or less, but decreased the strength for L/T ratios above 3. The explanation given is that in short joints (those governed by the plastic failure criteria), the critical stresses perpendi­cular to the grain at the end of the overlap are spread over a larger area than in joints in which the adherend ends abruptly at the end of the joint. In long joints (those governed by fracture mechanics criteria), creating a finite end distance increases the sharpness of the notch from 90° compared to 0°. The increase in notch sharpness increases the stress concentration at a given load and thus reduces the average stress in the joint at which fracture occurs.