CHARACTERIZATION OF STRUCTURAL ADHESIVES FOR LOAD-BEARING COMPOSITE STRUCTURAL APPLICATIONS

Thermomechanical properties of structural adhesives qualified to a specific material specification were characterized using both thermal and mechanical testing equipment. Differential scanning calorimetry and dynamic mechanical analysis thermograms exhibited two prominent phenomena, such as the glass transition of the primary components and the crystalline melting of the adhesive carrier. Lap shear strengths generally increased with decreasing temperatures and/or increasing testing speeds. This observation indicated that the cured adhesives were not elastic, but exhibited viscoelastic behavior, especially at high temperatures. It was also observed that the failure modes shifted in a regular sequence with increasing temperature and/or decreasing testing speed. Quantitatively, each specific failure occurred in a specific strength region. After hot/wet conditioning, all the adhesives exhibited drastic losses in glass transition temperature. Of interest was the fact that the wet cured adhesives exhibited lap shear strengths higher than those of the dry adhesives at room temperature. At high temperatures, the lap shear strengths significantly decreased with increasing moisture uptake. These observations implied that the lap shear strength was affected by material strength as well as by toughness. Conclusively, for proper utilization of adhesives their viscoelastic characteristics must be accounted for in composite structural design in addition to traditional mechanical testing methods currently present in material specifications.