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Fixture that Can Stretch & Twist Materials in Multiple Dimensions Simultaneously

The US Navy seeks to commercialize through licensing a suite of advanced, reliable, and versatile test fixtures for material tensile and shear testing that allows for much more accurate determination of strength and stiffness properties. The base patent accommodates a choice of independent or simultaneous application of in-plane shear and multi-axial loadings on a swatch of material. This innovation is most suitable for use in conjunction with axial-torsion testing machines; however, some of the benefits can be accrued to users with only uniaxial test machines. Both the Navy and the Army have used the base fixture to glean invaluable material design insights for airinflatable composite structures.

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Fixture that Can Stretch & Twist Materials in Multiple Dimensions Simultaneously
The Technology: 

Traditionally, the tensile and shear properties of materials are evaluated separately using uniaxial test machines. In practice, most materials are subjected to forces imposed simultaneously in multiple directions resulting in coupled responses that are quite different from the uncoupled behaviors observed from uniaxial tests. The current innovation was developed to permit the evaluation of both strength and stiffness properties of materials such as weaves (shown in the left hand figure), braids, and knits subject to combined biaxial tension and shear loadings. For fabrics constructed of two principle fiber directions, the fixture utilizes two rhombus-shaped frames connected with rotary joints. The center and right hand figures illustrate this device with a plain-woven fabric under biaxial tension and shear modes respectively. When testing a specimen with biaxial loading, four ends of the specimen are rigidly secured by the clamping mechanism. The vertices of the apparatus are attached to the crossheads of a conventional uniaxial tensile/torsion machine. Upon a movement of the vertices of the linkages toward each other, their lateral links will extend outward thereby increasing the distance between the corresponding load transfer plates of each linkage. This movement applies planar tension to the specimen. Additionally, by rotating one linkage with respect to the other, the specimen will be subjected to in-plane shear. An extension to the base patent teaches a design where differential force ratios within and between axes are possible. The data obtained by fixtures incorporating this suite of innovations will allow higher-fidelity design and analysis of structures fabricated from a variety of materials including fabrics, composites, elastomers, metals, biological tissues and others.

Benefits: 
  • Performance – provides coupled mechanical properties of materials much more akin to those observed in actual use
  • User friendliness – compatible with both conventional tension/torsion and uniaxial test machines
  • Versatility – this one fixture efficiently does the job that normally would require the inventory of many fixtures. Can be readily modified for 3-D fabrics by adding more rotating frames and clamping mechanisms.
The Opportunity: 

Application Examples

  • Composite preforms – helps designers model fiber placement methods to avoid fiber wrinkling during resin transfer molding
  • Tires – many of the benefits realized by the inflatable structure practitioners are transferrable to the tire community
  • Biomedical – connective tissues on human joints are subjected to multi-axial forces resulting in complex stress states that can be better evaluated using this tool
  • Inflatable structures – establishes effects of pressure and shear on mechanical properties of textiles and structural integrity of seams

Status:
Used by the Navy and Army to design inflatable structures; two issued U.S. patents and one pending

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