Jack C. Smith

Stress-Strain Relationships in Yarns Subjected to Rapid Impact Loading Part VIII: Shock Waves, Limiting Breaking Velocities, and Critical Velocities

An impact velocity just sufficient to cause immediate breakage in a filament im pacted in tension is called a critical velocity. Theories, of von Karman and Taylor, required for calculating critical velocities are reviewed and extended to include case of shock-wave propagation. Estimates of critical velocity are then calculated for some textile yarns using stress-strain data obtained at impact speeds near 40 m sec. The velocities obtained range in value from approximately 100 m sec for cotton sewing thread and glass fiber yarn to approximately 300 m sec for some vinal, rayon-tire-cord. acrylic, and nylon yarns. Theory is developed for calculating a limiting breaking velocity from the specific breaking energy obtained by high-velocity impact tests. It is shown that the limiting breaking velocity is higher than the critical velocity if the stress-strain curve is predominantly comcave downward, lower if the stress-strain curve is predominantly concave upward, and equal to the cricital velocity if the curve is linear. Critical velocities are also estimated from stress-strain data obtained at 100% min straining rates and compared with calculations from high-velocity-impact data. Good agreement is found in many cases.

Stress-Strain Relationships in Yarns Subjected to Rapid Impact Loading Part IV: Transverse Impact Tests

If a textile yarn segment, clamped at each end, is impacted transversely at the mid point, the stress-strain curve for this yarn can be obtained from measurements on a high speed photographic record of the yarns motion. This paper describes the apparatus and procedure used. Stress-strain curves for high rates of straining, of the order 5000%/sec., obtained by this method are given for high tenacity nylon, Fortisan, and Fiberglas. Comparison with stress-strain data obtained at conventional rates shows that these mate rials have higher initial moduli, and that their stress-strain curves remain linear up to higher stress values, when the testing rate is high. The breaking tenacities are slightly greater and breaking elongation slightly smaller at these high test rates.

Stress-Strain Relationships in Yarns Subjected to Rapid Impact Loading Part VII: Stress-Strain Curves and Breaking-Energy Data for Textile Yarns

Stress-strain curves at rates of straining up to 440,000%/min. have been obtained for a number of textile yarns by a technique involving high-speed photography of the yarn following transverse impact. These curves and others obtained at conventional speeds are presented for samples of acetate, triacetate, cotton, polyester, glass fiber, human hair, vinal, nylon, acrylic, rayon, saran, and silk yarns. Also given are specific breaking energies obtained from the areas under the stress-strain curves and by direct measurements involving longitudinal impact speeds of the order of 50 m./sec. These data show how stress-strain curves depend upon rate of straining and provide ratings for the yarns with respect to ability to survive impact and to resist impact without appreciable deformation.

Stress-Strain Relationships in Yarns Subjected to Rapid Impact Loading: Part X: Stress-Strain Curves Obtained by Impacts with Rifle Bullets

When a yarn is struck transversely, a V-shaped wave of transverse motion is caused to spread outwards at a velocity Ū depending upon the impact velocity V. In this research data on Ū vs V for impact velocities between 10 m/sec and 700 m/sec are analyzed to determine stress-strain behavior in high-tenacity nylon and polyester yarns. The data are obtained by shooting rifle bullets at a yarn and recording the resulting configurations by microflash photography. A strain-rate-independent theory for transverse impact behavior is used to calculate stress-strain data. It was found that the time required to break a yarn depended upon the impact velocity. Nylon yarns broke 10 μsec after impact at 650 m/sec with a calculated breaking strain of 10.0% and tenacity of 77 g/tex. At 495 m/sec velocity the yarn broke within 100 μsec at 7.4% strain and 52 g/tex tenacity. The corresponding values in polyester yarn were, for a 10-μsec break, 620 m/sec, 10.5%, and 67 g/tex; for a 100-μsec break, 420 m/sec, 6.5%, and 40 g/tex.

Stress-Strain Relationships in Yarns Subjected to Rapid Impact Loading Part XI: Strain Distributions Resulting from Rifle Bullet Impact1

If a textile yarn, marked at intervals along its length, is struck transversely by a rifle bullet, a flash photograph taken shortly after impact will reveal a shifting of the marks caused by passage of a strain wave. Analysis of these shifts provides data on the dis tribution of strain and strain velocity in the wave. Tests were performed on specimens of a high-tenacity nylon and a high-tenacity polyester yarn, to determine strain-velocity distributions at various times after impact and at various impact velocities. The observed distributions were then compared with the predictions of a theory which assumed that stress-strain behavior was independent of strain rate. From discrepancies between theo retical and observed results, it was concluded that at strain levels up to 9%, significant creep and stress relaxation occurred within 30 μ sec after impact, but in the time interval 30 μ sec to 300 μ sec, creep and stress relaxation were negligible. For strains of the order of 1%, the 30 μ sec creep and stress relaxations at the point of impact were of the order of 15% and 5%, respectively.

Stress-Strain Relationships in Yarns Subjected to Rapid Impact Loading Part IX: Effect of Yarn Structure

Samples of high-tenacity deacetylated cellulose acetate yarn were braided or plied and twisted by various amounts. Stress-strain data were obtained on these yarns at conventional rates of straining and under conditions involving transverse and longi tudinal impact at velocities near 40 m/sec. Strain-wave velocities were also measured. The effects of twisting and braiding were the same at high rates of straining and at conventional rates of straining. The addition of twist or braid caused the following effects. The initial slope of the stress-strain curve and the strain-wave velocity de creased. The bend in the curve at the yield stress became less sharp so that the stress- strain curve became more linear. The breaking tenacity decreased and the breaking elongation increased. The work required to break a unit mass of yarn material re mained unchanged. The calculated value of the longitudinal critical velocity at which a specimen breaks immediately upon impact in tension remained unchanged. Calculated transverse critical velocities tended to decrease as the yarn was twisted or braided.

Wave Propagation in a Three‐Element Linear Spring and Dashpot Model Filament

Mathematical expressions are derived for the particle velocity, stress, and strain distributions in the wave that results when a semi‐infinite viscoelastic filament is subjected at one end to constant velocity tensile impact. The equations governing the stress‐strain‐time behavior of the filament are assumed to be those for a linear model consisting of a spring coupled in parallel with a spring and dashpot in series. These equations are so formulated that by changing a single parameter λ either the spring branch or spring and dashpot branch can be made to dominate. The general solution expressed as a series in λ is obtained. This solution is especially convenient for making calculations. Special solutions describing the behavior at the wavefront and at the point of impact are also derived. The application of these solutions in the interpretation of experimental data is discussed.

4—CHARACTERIZATION OF THE HIGH-SPEED IMPACT BEHAVIOUR OF TEXTILE YARNS

This paper discusses how the behaviour on impact of textile yarns may be characterized in terms of such parameters as tenacity-strain data, breaking energy density, limiting breaking velocity, and critical velocity. Methods are given for obtaining the parameters from tests involving speeds of impact of 50 m/sec or less. At greater speeds of impact, strain-wave phenomena become appreciable, but the behaviour of the yarn may be studied by transverse impact methods. The results of a wave theory for transverse impact are given. The theory is then applied in a method for measuring longitudinal strain-wave velocity, and in two methods for obtaining tenacity-strain curves from high-speed transverse impact tests.

Strain‐Wave Propagation in Strips of Natural Rubber Subjected to High‐Velocity Transverse Impact

If a flexible filament marked at intervals along its length is struck transversely by a flying projectile, high‐speed photography reveals a shifting of the marks caused by passage of a strain wave, and analysis of these shifts provides data on the strain and average strain velocity in the wave. Tests were performed on strips of lightly vulcanized natural rubber at transverse impact velocities up to 65 m/sec, and the resulting strain‐velocity distributions analyzed for viscoelastic effects. The analysis showed that although creep effects were small in the observation time interval of 1–8 msec after impact, significant creep must have occurred at the point of impact within the first millisecond, and additional significant creep occurs at times greater than 8 msec. The strain‐wave‐front velocity calculated from the quasistatic stress‐strain curve was 35.2 m/sec, but a value of approximately 60 m/sec was observed in the tests. The strain at the wave front, however, tended to attenuate as the wave propagated c...

Specific Breaking Energy for Human Hair: Corrected Values

Terylene film has Ixen niethanolyzetl on nimy occasions antl the tlerivetl niethyl esters esmiinetl by gasliqiiitl cIiron~atogr;qihy using a flitnie ionization detector antl tetra(p-cyniioetliy1) erythritol [2] on a support of 311-60 mesh ground porous tiles. No evidence of the presence of isoplitlialic acid has ever been olitainetl. the limit of detection Iieing 1% or lower I)asetl on the weight oi fiber. However. to ninke siicli an esaniination more rigorous a niethotl of niethanolysis was devised which incliitled a tenfold concentration of the dinietligl isoplitlialate antl permitted its detection clown to the 0.1% level. The technique was first established by application to synthetic mixtures of diniethyl tereplithalate, diniethyl isophthalate, and methanol in proportions similar to those present after niethanolysis. A reproducible recovery of over 95% of the isoplithalate initially present was obtained. Experiments with Terylene fiber containing 1 % of adtled diniethyl isophthalate confirnied these findings, wliile with tlie fiber alone dimethyl isoplithnlate could not be detected, and therefore, if present at all, mist have been below the 0.1% level. Similar results ivere obtained on esaniination of samples of “Dacron”1 61 antl “Dioleri” fibers. In Table \’I of