4.1 It cannot be assumed that this test method measures the true shear strength of the adhesive bond. Many factors interfere or bias the measurement including the strength of the wood, the specimen, the shear tool designs themselves, and the rate of loading.
4.1.1 Wood failure is very common in joints made with strong adhesives. Although high wood failure is normally desired, when it occurs the measured strength is lower than the true adhesive bond strength.
4.1.2 Stress concentrations at the notches of the specimen tend to lower the measured strength. In a similar test for the shear strength of solid wood, Test Methods D143, these effects are self correcting so that the measured strength is close to the true shear strength of the wood. By analogy the same may be true in this test method, however, other factors are also involved and may alter the relationship.
4.1.3 The rate of loading affects the strength of an adhesive bond according to the adhesive's rheological properties. The more viscoelastic or plastic the adhesive, the greater effect. Thermosetting adhesives like urea- and phenol-formaldehyde are elastic. Their bond strengths can be measured over a range of loading rate from 0.038 to 1.27 cm/min [0.015 to 0.5 in./min] with no apparent affect. Thermoplastic adhesives like polyvinyl acetate, hot melts, and elastomer-based adhesives exhibit a broad range of elastic, viscoelastic, and plastic behaviors. Their bond strengths will be affected to varying degrees by changing the loading rate. Generally, increasing the rate, increases the measured strength.
4.2 This test method is suitable for product research and development, qualifying adhesives in accordance with certain product or performance specifications, and monitoring bonding process control. This test method may be suitable for comparing and selecting adhesives, however, such comparisons must be made with caution since the measured strength of some adhesives may be different in different types of joints. Strength values obtained by this test method are not suitable as design shear strengths values without adjustment by certain engineering design factors.
