Quality control of glulam: shear testing of bondlines

In quality control of glulam, shear testing of bondlines is required. However, in relevant standards the method of applying shear stress to the bondline is only exemplified by a principle schematic. Based on this schematic a considerable variety of test equipments is in use nowadays. Depending on the respective construction of the test equipment as well as on the execution of testing, the resulting stress in the bondline is not pure shear but rather a combination of shear and normal stresses. When the normal stresses are acting as tensile stresses perpendicular to the bondline, the registered shear strength values range considerably below the pure shear stress level, whereas compression stresses perpendicular to the grain lead to an overestimation of the shear strength of the bondline. To overcome this deficiency, a prototype of a modified shear test device was developed, which ensures a clearly defined state of shear loading of the specimens. Based on extensive comparative testing it is shown that by performing compression tests at an oblique angle to the grain of 14° the above mentioned problems can be avoided.