The ASTM D4502 method describes the test to estimate the resistance of adhesive-bonded joints to degradation induced by high temperature and moisture. The degradation rate of adhesives depends on temperature, moisture, chemicals present, and stresses on the joint. The test is meant primarily for wood to wood joints, but can also be used for joints of wood to other materials. Since adhesives degrade by physicochemical mechanisms, degradation speed can be analyzed using chemical kinetics. The effect of temperature on degradation kinetics can be modeled by the Arrhenius temperature and reaction rate relationship. The ASTM D4502 test provides data to assess the durability of various adhesives and can be used to forecast expected service life. Comparative assessment of established adhesive-adherend combinations with new or unknown combinations can be conducted by this test, to support research and development of new adhesives. The effects of stress or cyclic variations in temperature or moisture levels are not covered by this test. The test subjects groups of specimens to various controlled temperature and moisture environments, followed by measurement of adhesive properties, to assess degradation kinetics and the relationship with operating conditions.
The ASTM D4502 test needs several test specimens, with each specimen made from two wooden blocks of the same length and width, but not necessarily the same thickness, bonded together in a slightly staggered (lapped) configuration. The unaged properties of specimens are first estimated by control tests. Typically, in ASTM D4502 testing, the shear strength is the property that is measured, using the block shear test, using a guillotine type tool. Next, several groups of specimens are exposed to elevated temperatures, to accelerate the aging process. Periodic testing is performed by removing a group and testing the adhesive properties, duly recording the property values and time of aging. This is repeated at several time intervals, with the remaining specimen groups, under the same temperature, to assess the effect of exposure time on accelerated aging. The effect of temperature is then assessed using the same methodology, at several elevated temperature values. A complete dataset is therefore obtained relating time and temperature to the aging of adhesives and the resulting degradation of properties. This data set can be extrapolated to normal working temperatures using measured trends and the Arrhenius relationship, to assess service life at normal working temperatures.
Video 01: Adhesive lap joint shear strength test
The ASTM D4502 test method usually employs a block shear specimen. However other shear strength or tensile strength specimens may also be used in accordance with the respective standards. The specimen size for block shear can comprise of two adhesively bonded blocks, each about 45 mm long by 50 mm wide (1.75 inches by 2 inches), staggered 6 mm (0.25 inches). The specimens can be smaller than this as well, considering that several specimens are needed.
Shear strength, which is typically the property of interest, is reported in megapascals (Mpa).
The ASTM D4502 method describes the test to estimate the resistance of adhesive-bonded joints to degradation induced by high temperature and moisture.