Experimental Mechanics

, Volume 20, Issue 6, pp 211–216 | Cite as

The relationship between tensile strength and flexure strength in fiber-reinforced composites

Flexure-and tensile-coupon data on unidirectional graphite-epoxy composites are compared to a weibull two-parameter statistical-strength model
  • J. M. Whitney
  • M. Knight


Tensile data on unidirectional composites generated from a flexure test usually yield a higher strength than observed from a standard tensile coupon. According to a statistical-strength theory based on a Weibull distribution, the presence of a stress gradient in the flexure-test results in an apparent increase in tensile strength as compared to the tensile test under uniform stress. In the present paper, this concept is explored by utilizing data from unidirectional graphite-epoxy composites to compare with theoretical results generated from a two-parameter Weibull distribution. A larger variation in tensile strength is observed from tensile-coupon data than from flexure data. Such differences are not in accordance with strength theories based on a uniform flaw distribution and raise questions concerning variability of the test methods, as well as sources of material variability.


Tensile Strength Tensile Test Weibull Distribution Stress Gradient Uniform Stress 
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List of Symbols


risk of rupture at stressS


width of test specimen, mm (in.)


length of tensile specimen or span of beam specimen, mm (in.)


thickness of test specimen, mm (in.)


maximum stress, MPa (psi)


characteristic bending strength, MPa (psi)


scale parameter in two-parameter Weibull distribution

\(\mathop S\limits^ \wedge _0 \)

maximum likelihood estimate ofS o

\(\bar S_b \)

adjusted maximum likelihood estimate for scale parameter under bending load, MPa (psi)


characteristic tensile strength, MPa (psi)

\(\bar S_t \)

adjusted maximum-likelihood estimate for scale parameter under tensile load, MPa (psi)


normalized strength


scale parameter for normalized strength


maximum likelihood estimate of normalized-scale parameter


shape parameter in Weibull distribution

\(\mathop \alpha \limits^ \wedge \)

maximum likelihood estimate of shape parameter

\(\bar \alpha _t \)

unbiased estimate of pooled-shape parameter for tensile loading

\(\bar \alpha _b \)

unbiased estimate of pooled-shape parameter for bending


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Copyright information

© Society for Experimental Mechanics, Inc. 1980

Authors and Affiliations

  • J. M. Whitney
    • 1
  • M. Knight
    • 1
  1. 1.Non-metallic Materials Division, Air Force Materials LaboratoryWright-Patterson Air Force Base

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