Material Properties

MATERIAL PROPERTIES TESTS OF WOVEN FABRICS
SRI International is performing research under contract to FAA to investigate the use of high-strength polymeric fabrics as ballistic barriers to protect critical aircraft components against fragments resulting from uncontained failure of a turbine engine. As part of this program, we are performing laboratory tests to characterize those material properties which are relevant to the fabric behavior that we have observed in impact and quasistatic penetration tests.

These tests include yarn tensile and transverse load tests, to characterize the deformation and failure of individual fabric yarns, and yarn pull-out tests, to characterize the frictional forces between yarns.

YARN TENSILE TESTS

TENSILE TESTS
High-strength polymeric fabrics, such as Zylon, Kevlar, and Spectra, consist of intersecting fill and warp yarns, each composed of several hundred fibers. The fill yarns are relatively straight, while the warp yarns are more crimped. When a single yarn is gripped tightly on both ends and pulled in tension, it stretches until the individual fibers fail at various points along the yarn. This type of failure, similar to the "remote yarn failure" mode observed in the fabric impact and quasistatic tests, is shown here.

ZYLON FIBER FAILURE DURING YARN TENSILE TEST

Stress-strain curves (from load cell and extensometer histories during tensile tests) of different Zylon yarns show that weaving reduces the tensile strength of a yarn. Yarns that are highly-crimped, such as the 40x40/in. warp yarn, have a significantly reduced strength and failure strain.

TENSILE TEST RESULTS FOR VARIOUS ZYLON YARNS

TRANSVERSE LOAD TESTS
When a high-strength fabric ballistic barrier is impacted by a fragment, the fabric yarns are not pulled along their axis, as in a tensile test, but are deflected sideways to their axis. Also the sharpness of the fragment's impact edge has a strong effect upon the ballistic resistance. So we developed a materials property test that would more closely simulate this type of loading and allow us to characterize the effect of impactor sharpness.

TRANSVERSE LOAD TEST WITH KNIFE-EDGE LOADER

In the transverse load tests, a loader is pushed at a constant rate into a tightly gripped yarn perpendicularly to the yarn axis. Loads and deflections are measured, and a videocamera records the yarn failure. When the loader has a very sharp edge, as depicted above, the yarns fail by "local rupture" of the fibers at the point of contact. When the loader has a rounded edge, as depicted below, the yarn fails by "remote failure" of the fibers, just as in the tension test.

TRANSVERSE LOAD TEST WITH
CYLINDRICAL LOADER

Stress-strain results for a yarn loaded in a transverse test with a cylindrical loader show nearly identical failure as in a tensile test. But when a knife-edge loader is used, the fiber failure occurs at much lower strains.

TEST RESULTS FOR UNWOVEN ZYLON YARN

YARN PULL-OUT TESTS
Frictional forces between intersecting warp and fill yarns can play an important role in the behavior of high-strength fabrics undergoing fragment impact and penetration. Impacted yarns slide along their intersecting transverse yarns during fabric deformation. Ungripped yarns can be pulled out of the fabric by motion of the impactor. And frictional forces are involved in the load transfer from impacted to adjacent unimpacted yarns during the "remote failure" mode. We devised a test to allow us to measure the force required to pull out an individual yarn from a fabric that is tightly gripped and pre-tensioned in the direction perpendicular to the yarn pull-out.

YARN PULL-OUT TEST

The axial force on the yarn, transverse load on the fabric, and yarn displacement are measured, and a videocamera records the yarn pull-out. First the yarn being pulled un-crimps (straightens) as the load increases, then pull-out begins and the load decreases.

TYPICAL PULL-OUT TEST RECORDS

Pull-out test results for a variety of high-strength fabrics are shown below. The frictional forces strongly depend on the transverse pre-load and the fabric mesh density (i.e. yarns/in.).

PULL-OUT TEST RESULTS FOR VARIOUS FABRICS

Data from the materials property tests are being used to develop and calibrate a computational model for simulating impact and penetration of fabric targets.

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