Mechanics of Expanding Grout Based Grips for Large Scale Composites Testing

Experimental testing of large composite specimens under tension or shear is inherently challenging because the large required gripping force and resulting local stress concentration tend to break specimens near gripping sites, rather than desired gage sections. A method using expansive grout materials has been proposed since the mid 1990’s. However, there has been no well-established design guideline due to lack of understanding of the gripping pressure developed by the expansive grout material. Key properties including the elastic modulus and linear expansion coefficient are difficult to measure because typical grout materials do not consolidate into coherent solid blocks for traditional property measurement. In this study, the elastic modulus and linear expansion coefficient of an expansive grout material have been indirectly measured through a carefully designed cylindrical system. The expansive grout material is let to expand within thick-walled steel pipes with one end capped and the other end free to the atmosphere. An analytical solution has been derived to correlate the hoop strain on the outer surface of the steel pipe (caused by grout expansion in the pipe) to the grout elastic modulus and linear expansion coefficient. By measuring the exterior surface hoop strains of two different steel pipes, the elastic modulus and linear expansion coefficient have been determined using the analytical solution of thick walled cylinders. With these parameters accurately determined, the interface friction coefficient has been estimated through analyzing actual composite specimen tests. Finally, based on the determined parameters and the analytical solution, an improved design procedure has been proposed.