The IMA-RS Fixture – evolutionary advancement of shear testing for anisotropic materials.
One of the responsibilities of test laboratories is the selection and optimization of test methods representing the actual material behaviour as optimally as possible. For fibre-reinforced plastics the in-plane shear properties are of high importance. Common test methods as Rail Shear, Iosipescu and Shear Frame test are available.
From IMA Dresden point of view, the V-Notched Rail Shear is the preferred test method, even though some disadvantages must be considered when using the original device defined in the standard and provided by Wyoming Test Fixtures Inc. (WTF). As a result of the mechanical specimen clamping by screws, a reproducible clamping force can hardly be achieved. Furthermore, the specimen changing and the alignment are time consuming. But the main disadvantage is the disabled transverse strain of the specimen, leading to additional stress components, which are measurable in terms of reduced strength and increased stiffness (5 to 10 % too high for carbon/epoxy). This effect is unknown to most users and is generally not considered.
All these reasons motivated the colleagues of IMA Dresden to turn upside down the original WTF device and to correct the known disadvantages. The IMA-Rail-Shear Fixture (IMA-RS) was developed – a test method for determining actual shear properties by means of hydraulic clamping, combined loading, linear guides and the free movement in the transverse direction.
Validation Test Results
For the validation of the new fixture a test program with several materials and test methods was performed, following we concentrate on the comparison between IMA-RS and WTF.
The properties shear modulus and strength were evaluated (the latter being determined at a shear deformation of 0.05). The comparison of the strength shows higher values with comparable scattering for the IMA-RS. This demonstrates the assumption that higher shear strengths can be determined by an unhindered shear deformation. Furthermore, the expected lower shear moduli (values are 10 % below the WTF-RS levels) could be confirmed.
Conclusion and Perspective
The validation results and results with further test methods confirm the theoretical approach and the advantages of the IMA-RS Fixture. This way the already very successful specimen geometry of ASTM D7078 can be combined with an improved fixture for free transversal movement and a reliable usability.
The invention is ‘patent pending’ under WO 2014090298 A1 and also published under DE 112012007218 A5. Furthermore the distribution of the IMA-RS Fixture by a long-standing and competent partner is planned.
IMA Materialforschung und Anwendungstechnik GmbH, Dresden
Dipl.-Ing. Frank Heinrich,
Head of department Synthetics and Composites,
phone +49 (0) 351 / 88 37-418,
Prof. Dr.-Ing. Jens Ridzewski,
Senior Principal Engineer,
phone +49 (0) 351 / 88 37-499,