Quasistatic Fabric Tests

Under contract to FAA, SRI International is performing laboratory tests of high-strength polymeric fabrics for use as ballistics barriers against fragments resulting from uncontained failure of a turbine engine. The laboratory tests include the push tests, described below, as well as gas gun impact tests and yarn material properties tests.

PUSH TESTS

QUASISTATIC PENETRATOR (PUSH) TESTS

A penetrator is pushed at a constant rate into and through a fabric target. The measured force–deflection history can then be correlated with the videotaped images and acoustic emissions of the fabric deformation and failure. This gives us a clearer understanding of the evolution and phenomenology of fabric target deformation and failure during fragment penetration. For example, we have observed in impact tests that a fabric target gripped on 2 edges absorbs significantly more energy than a target gripped on all 4 edges. Results from push tests, as shown below, can explain this.

EFFECT OF BOUNDARY CONDITIONS

Although the peak load is much higher for the Zylon targets gripped on 4 edges, full penetration (i.e., load drops to zero) occurs immediately following the peak load. The 2-edge-gripped target, on the other hand, continues to resists penetration long after the peak load. Full penetration doesn't occur until a deflection of roughly twice that at the peak load, and the energy absorbed is more than twice that for the 4-edge-gripped target. Different modes of fabric failure were observed for these two different gripping geometries.

We have identified three different fabric failure modes:

• Local Yarn Rupture – all of the fibers within the yarn break at the point of fragment contact.
• Remote Yarn Failure – the fibers break at different points along the yarn length (not at the point of fragment contact), and the load is transferred to adjacent yarns.
• Yarn Pull-out – yarn fibers do not break, but one end of the yarn is pulled out of the fabric mesh (ungripped yarns only).

YARN FAILURE MODES IN ZYLON PUSH TESTS

All three modes were observed in this test of a Zylon fabric. The resultant load-deflection curve is shown below. Large sudden drops in load correlate with multiple yarn failures, while gradual decreases in load correlate with yarn pullout.

LOAD DEFLECTION CURVE AND YARN
FAILURE MODES

Although the fabric deformation rates in the push tests are much lower than those in fragment impact tests, each of the three failure modes observed in the push tests have also been observed in the impact tests. The push tests have therefore contributed greatly to our understanding of fabric deformation and failure and to our development of a realistic fabric computational model . We have performed push tests which varied the fabric material, the fabric boundary conditions (gripping geometry), the penetration rate, and the penetrator orientation and sharpness. Detailed results of these tests can be found in the reports .

Contact Us
Don Shockey
Director, Center for Fracture Physics
Phone: 650-859-2587
Email: donald.shockey@sri.com