Comfort, continued - Investigators Find Flaws in Army Body...

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Unformatted text preview: Investigators Find Flaws in Army Body Armor Tests Armor 10/16/09 10/16/09 The Government Accountability Office Government report says the Army strayed from established testing standards and established concludes several of the designs that passed would have failed had the tests been done properly. … most significant departure from testing standards was the incorrect measuring of the amount of force a plate can withstand. the Comfort, continued Comfort, Thermal and Moisture Issues Laboratory Tests for Stretch and Recovery: “Give” Recovery: – Stretch fabric using a small force, not the Stretch small same as the elongation measured at rupture during tensile testing rupture – Measure % stretch and % recovery – Stretch and Recovery Depend on: Fiber – wool and synthetics more elastic than Fiber cellulosics cellulosics Yarn – crimp, texture Fabric Fabric – Popularity of knits – Trend to use sufficient elastomeric fiber to Trend provide comfort stretch provide Electrostatic Propensity Electrostatic Electric charge build up due to a difference in electric Electric charge between two materials. Depends on moisture content and fiber electrical resistance problem with synthetics (nylon, polyester, acrylic) problem (and wool in very dry conditions) (and nuisance in clothing (static cling) Attracts soil and lint problem in carpeting in computer rooms danger in hospital carpeting (sparks when oxygen danger is in use) is Manufacturing problem if fabrics cling to Manufacturing equipment equipment Standard tests Standard AATCC AATCC 76 – Electrical resistivity of fabrics AATCC fabrics measured using electrical resistance meter to check flow through fabric to AATCC 84 - Electrical resistivity of yarns AATCC yarns AATCC 115 – Electrostatic cling of fabrics AATCC to a metal plate, simulating the body. Time measured for the charge on the fabric to decay so that it no longer clings. decay AATCC 134 – Carpet testing with special AATCC shoes, recording voltages shoes, Air Permeability Air Defined as the rate of air flow passed vertically through a fabric when there is a pressure differential between the two sides of the fabric Affects breathability in clothing, efficiency of filtration in vacuum cleaners and industrial anti-pollution filters, ability of parachutes, automobile air bags, and sails to hold air Factors influencing – Fabric cover – knit vs. woven; openness of either – Surface texture, such as nap – Yarn diameter – bulk or texture reduce air flow; however, high twist allows close packing of high fabric count plain weave – Yarn crimp allows extensibility, opening up fabric – Finishing techniques such as calendering flatten yarns and fill spaces between them Test Method Test AATCC 737 uses an air permeometer which forces air through a known area of a fabric specimen. Rate of air flow is adjusted until a prescribed pressure differential, 125 Pa(12.7 mm/ .5 in.) of water, is achieved. Does not accurately simulate normal conditions because of use of forced air, but gives comparative data Thermal Regulation Thermal Conductive protection – Fiber physical structure rather than chemical most Fiber influences ability to insulate influences – Fibers have a high surface to volume ratio, providing Fibers many small spaces for “dead” air many – Dead air limits heat conduction and increases thermal Dead insulation so long as air volume is not large enough to allow convection allow – Fabric thickness is a good measure of conductive Fabric protection – thicker fabric insulates more protection Influences on dead air Influences Fiber structure: wool has crimp; textured fibers Fiber have surface irregularity; hollow fibers are ideal; very fine fibers have more surface area (Thinsulate is a microfber) (Thinsulate Yarn structure: spun or textured yarn entraps Yarn more air than smooth filament; low twist more than high than Fabric construction: tightness of weave or knit; Fabric knit often entrap more air than woven; pile or nap Porosity – a way to calculate “dead air” Porosity Considers pores, or interstices within a Considers fabric fabric Calculated using fabric area, thickness, Calculated weight, density weight, Important in air and moisture transfer Related to “cover factor” – fabric count, Related yarn density, yarn type, yarn twist all affect yarn Fabric Thickness Fabric The distance through the fabric from one surface The to another when a specific pressure is applied. to Dependent upon pressure applied to a specimen Related to thermal conductive insulation, fabric Related bulk, drape, sound insulation, padding abilities bulk, Also used to measure changes due to abrasion Also or shrinkage or Thickness Test Thickness ASTM D1777 Useable for most fabrics as a measure of Useable bulk and warmth bulk A pressure gauge is lowered onto a fabric pressure surface at a specified pressure – less pressure for blankets, pile fabrics, napped fabrics More Thermal Protection More Convective heat loss – Movement of air, e.g. wind – Prevented by tightly woven fabrics Radiation heat loss – Least important in clothing – Smooth, flat, reflective surfaces transfer heat Smooth, this way this – Reflective drapery linings – Some jacket outer fabrics have metallic Some linings to reflect heat back at wearer linings Measurement of Heat Transfer Measurement ASTM 1518 Guarded hot plate measures ASTM thermal resistance by all methods thermal Kawabata (same as for hand evaluation) has a Kawabata thermal transmittance measurement machine thermal Heated manikin, “copper man” used by US Heated Army Army – Advantage: garments drape and fold so Advantage: layers of fabric and air realistically simulate wear conditions wear – Disadvantage: air spaces between garment Disadvantage: and manikin cannot be controlled for measurement of convective heat transfer measurement Moisture Management Moisture Water conducts heat better than air, meaning Water presence of water cools a person faster presence Perspiration which cannot escape from the skin Perspiration condenses as sweat, making a person clammy condenses Evaporation of perspiration cools “evaporative Evaporation heat loss” heat Textiles which maximize evaporation improve Textiles comfort comfort ...
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