Effect of Angular Loading on Glue Line Strength

Kiln-dried boards of American beech (Fagus grandifolia) were planed to a uniform thickness of 26/32 inch and ripped into strips 2-1/2 inches wide and 27 inches long. These strips were glued face to face with a resorcinol formaldehyde adhesive to form blocks containing multiple glue lines. The glue blocks were pressed at room temperature under a pressure of 280 psi for 24 hours. After 2 weeks, these blocks were ripped lengthwise perpendicular to the glue line and planed into strips 1/2 inch thick. Test specimens were laid off on these strips at angles (0? to 60?) to the glue line. The specimens were notched on the lengthwise sides to make the portion of the glue line actually tested 1/2 inch wide by 1 inch long. The angular compression samples were tested in the apparatus similar to that used for standard compression tests but with offset blocking to cause shearing. The compression control specimens (0? angle) were tested in a standard shear block testing head. The tension sample were tested in a standard plywood shear head. The testing results showed the loading angle had a marked effect on the maximum shear stress of the tension samples but had little effect on the compression samples. The linear regression line for the compression samples was non-significant. It is assumed the compression shear strength of the glue line in dry beech is not affected by angle loading between 0 and 50 degrees but will tend to fail whenever the shear stress equals the shear strength of the glue line. The shear strength of the tension samples varied with loading angle, but the angular stress at rupture did not vary. This indicates that when a glue line in dry beech wood is stressed in tension at an angle, it will fail when this angular stress reaches a value of about 1400 psi. Both the shear component and the normal component of the stress contribute to the failure which occurs when their vector sum equals 1400 psi. Contrastingly, in compression test, only the shear component of the testing force contributes to the failure while the normal component aids in resisting failure. The test data showed that the glue line shear strength in compression is over twice as great as the shear strength in tension and that the percentage wood failure was conspicuously lower in the compression test. The explanation was that compressive forces on the glue line increase the grip of projecting fingers of glue by pressing them deeper into the wood while tension on the glue line tends to pull projecting fingers of glue out of the wood. The data also showed low variability of results in compression test at 30 and 40 degree angles, suggesting further study of this method as an alternate to the standard glue block shear test.

You must be logged in to download any documents. Please login (login accounts are free) or learn how to Become a Member