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GR-PA and GR-PBT =0.0071
POM = 0.0082
ABS =0.0117
The other reduction factors in Equation 4.2 consider the following effects:
The factor Ast represents the effect of the time of static loading and can have following
values depending on time [I]:
time hours weeks months years
Ast 1.3 1.6 1.7 2
The factor Adyn takes the effect of dynamic loading into account and lies in the range of
1.3 to 1.6.
The factor AA, considers the influence of aging and has to be determined experimentally.
The reduction of strength caused by the absorption of moisture by the plastic can be
estimated from the factor Aw. For unreinforced polyamides Aw is roughly [ 1 ]
(4.4)
with /ranging from 0 < /< 3 where/= weight percentage of moisture. The value of Aw is
3.4 when/is greater than 3.
4.2 Part Failure
Usually stresses resulting from loading of the part are multiaxial. Because measured
material properties do not exist for combined stresses, the multiaxial state has to be
reduced to an uniaxial state by applying the principle of equivalence. According to HUBER,
VON MISES and HENKY [1] the governing equation for the equivalent stress is
where CT1, G1 and (T3 are normal stresses. The equivalent strain £ is defined by [3]
(4.5)
(4.6)
Materials, whose compressive stress is higher than the tensile stress, can be better described
by the conical or parabolic criterion [I]. The equivalent stress CT , according to the
conical criterion, is given as [ 1 ]