When Kevlar is spun, the resulting fiber has a tensile strength of about 3,620 MPa,
and a relative density of 1.44.
Kevlar maintains its strength and resilience down to cryogenic temperatures
(-196 °C); in fact, it is slightly stronger at low temperatures. At higher
temperatures the tensile strength is immediately reduced by about 10–20%,
and after some hours the strength progressively reduces further. For example
at 160 °C (320 °F) about 10% reduction in strength occurs after 500 hours.
At 260 °C (500 °F) 50% strength reduction occurs after 70 hours.
Kevlar® KM2 fiber is a transversely isotropic material. Its tensile
stress-strain response in the axial direction is linear and elastic
until failure. However, the overall deformation in the transverse
directions is nonlinear and nonelastic, although it can be treated
linearly and elastically in infinitesimal strain range. For a linear,
elastic, and transversely isotropic material, five material constants
are needed to describe its stress-strain response. In this paper,
stress-strain behavior obtained from experiments on a single Kevlar
KM2 fiber are presented and discussed. The effects of loading rate
and the influence of axial loading on transverse and transverse
loading on axial stress-strain responses are also discussed.
Kevlar KM2, 600 denier, was Instron tested in quasi-static, uniaxial tension to determine its
strength. Specimens included both single yarns and 68-yarn-wide, single-ply strips of plainwoven
fabric (Style 706). Never-woven single-yarn specimens were tested with varying degrees
of initial twist. Strength of the untwisted, never-woven yarn was 2.66 ± 0.04 GPa. The twist
multiplier was 1.2. Single warp-oriented and fill-oriented yarns were extracted from Style 706
fabric, tested, and found to have strengths of 2.06 ± 0.01 and 2.20 ± 0.05 GPa, respectively. The
single-ply fabric specimens of both warp and fill orientations were tested and found to have
strengths of 2.23 ± 0.04 and 2.67 ± 0.04 GPa, respectively. The strength effects of weaving,
finishing, yarn extraction, and inter-yarn contact are discussed.
Table 1. Strength of Kevlar yarns.
Yarn Type sfail
Kevlar 29 2.9
Kevlar 49 2.9
Kevlar 68 3.1
Kevlar 119 3.1
Kevlar 129 3.4
Kevlar 149 2.3
Finally, 68-yarn-wide, single-ply specimens of plain-woven, 600-denier KM2 fabric were tested
in quasi-static, uniaxial tension. Warp-oriented and fill-oriented fabric specimens had strengths
of 2.23 ± 0.04 and 2.67 ± 0.04 GPa, respectively. These fabric strengths are larger than those
found for extracted warp and fill yarns. The mechanism of frictional contact being adjacent
yarns was speculated to account for the added strength in the case of woven fabric.
All these properties give Kevlar many advantages over other polymers.
It has a tremendously high tensile strength and is five times stronger
than steel. Underwater, Kevlar is up to 20 times stronger than steel!
Temperature-wise, Kevlar exceeds the performance of many other materials.
It can withstand temperatures up to 300°C while retaining its strength
properties. Even at -196°C Kevlar shows no signs of embrittlement or loss
of strength. Almost all solvents are ineffective at degrading Kevlar except
the few powerful acids.