ABSTRACT

This paper reviews and compares the results of ASTM Standard fabric tests and my Bi-Axial Rip Test (BART) to introduce a formula that can be further developed to provide for factors of safety (FOS) in designing tensile fabric structures that take into acount fabric tears and tear propagation.

Tensile ruptures of fabrics are rare. Actual failures occur at stress concentrations near fixed locations, along seams or are the result of tear propagation subsequent to a puncture or snag. In practice tensile fabric structures are typically designed using a single set of FOS against fabric tensile failure, regardless of fabric type, of four (4) for snow load, five (5) for wind load and eight (8) for prestress.

Using ASTM Standard test results, the calculated ratios of tensile strength per inch compared to the trapazoidal tear resistance (Sst/Rult) indicates that fiberglass fabrics are high (Sst/Rult varies from 6.5 to 12.5), low in polyethylene and ePTFE fabrics (2.0 to 2.8) and for polyester somewhere in between (2.8 to 6.5). The Trapezoid Tear Method ASTM D4857 (Old designation ASTM 5106) is typically conducted where a load is applied to a cut within the fabric plane at a test specified angle. A better test of the actual behavior of a fabric in a tensile fabric structure may be the mono-slit test in a uni-axially loaded cut fabric or even better a bi-axially loaded cut fabric. Comparing the mono-slit and bi-axial test results to the trap-tear tests may enable one to develop a formula for comparing fabrics when only given the strip tensile and trap-tear strengths of a fabric. This formula can then be used to chart the factor of safety required so that a tear of a certain length will not tend to propagate. Developing and using this formula should make for designing safer structures.

Refering to Table 1 (Mono-Slit Test), results indicate that to prevent an approximate four (4) inch slit from propagating requires a factor of safety with respect to tensile strength of approximately three (3) in Fabric A (polyethylene), while Fabric B (polyester) requires a factor of safety (FOS) of approximately five (5).

From the various test results it is possible that a formula can be developed for each specific manufactured fabric of each specific fabric type. In a tensile fabric structure what factor of safety should be used to prevent a given length cut? What cut length should be considered?

TESTING OF STRUCTURAL FABRIC

Four types of tests were performed in preparation of this paper.

• Strip Tensile: Fabric samples 0.75 inch, 1 inch and 2 inch wide were tested uni-axially for fabrics Q, R, S, T and X, Y, Z. Fabric is rated as pounds per linear inch of width (pli). Results are listed in Table 2 with the Bi-Axial Test 2 results.
• Mono-Slit Test: Fabric samples 15 inches wide were tested uni-axially in the tested direction called North-South (N-S) with various length cuts perpendicular to the tested tension direction we call East-West (E-W) and the results recorded in Table 1 as average pli in Fabrics A and B.
• Bi-Axial Rip Test 2: Fabric samples 24" inches wide by 24" long were cut into a "t" shape using 6 inch widths that follow the warp and fill directions of the fabric and tested bi-axially with various length cuts perpendicular to the tested tension direction. The tested direction we call North-South (N-S) and the perpendicular direction we call East-West (E-W). For each specific fabric series the tension in the E-W direction was kept constant and we varied the length of cut and tested the fabric in the N-S direction until failure and recorded the results in Table 2.
• Bi-Axial Rip Test 3: Similar to Test 2 but for each specific fabric series the tension in the EW direction was varied and the length of the E-W cut was always two (2) inches and tested the fabric in the N-S direction until failure and recorded the results in Table 3.

TEST RESULTS

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