Anyone who has had to start from scratch to perform materials' testing knows the amount of design effort required to execute a single test. For plain-woven fabrics that can consist of composite materials with complex microstructures, the task is more difficult. Measuring multi-axial and shear, material properties, seam strength and joint strength raises the difficulty level even further. However, plain-woven fabrics are frequently used as structural materials in air-inflated structures and quick set-up temporary structures such as shelters and bridges. As a result, the Naval Undersea Warfare Center Div. Newport (NUWCDIVNPT) was very interested in verifying the mechanical properties of these materials. To accomplish this testing, NUWCDIVNPT designed and patented a multi-axis technique and developed a fixture to implement the approach. Patent #6,860,156 was issued for a combined in-plane shear and multi-axial tension or compression testing apparatus.
To perform in-plane shear and multi-axial tension or compression, the tester has four upper linkage arms and four lower linkage arms connected by pivoting sleeves. Load transfer plates secure the specimen and provide pivot mounting points for the linkages. When the vertices are under compressive or tensile forces, each linkage can rotate toward the other linkages and apply compression or tension to the specimen. The testing machine can rotate the vertices for shear testing and apply a torsional load to the specimen. The fixture enables materials testing in several modes including uniaxial tension, uniaxial compression, biaxial tension both with and without shear, biaxial compression with and without shear, and shear only.
Both the Army and the Navy have used the fixture to successfully design air-inflatable composite structures.