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- n! f! e+ z- e3 e' p t* wMaterial properties for part design $ x: t# M5 d L( Q3 B4 y/ {
Plastics are sensitive to operating conditions % S+ _3 U5 M# p, a
The plastics molding processes allow parts designers more freedom than working with 6 g. _- k3 P# f1 G& X
metals because plastics materials are so versatile. Unlike metals, however, the
7 a# R, {" v9 ^; B5 a. \: O4 dmechanical properties of plastics are very sensitive to the type, rate, duration, and
6 a! g- c( S( s' j8 v2 |5 hfrequency of loading, the change in operating temperature, and in some cases, relative % W# x- o% Y# T1 T. U7 j! c: h
humidity. The plastics part designer must take a material's response to these conditions
" T6 m" y9 ?& y5 H0 z7 kinto account. The table below lists the five typical loading and operating conditions,
( x9 W0 ^0 G$ j/ G; N+ X9 B. Dtogether with the relevant material properties a designer needs to consider. ! Z3 X8 l: `: ^0 q4 U5 w) G
. : i2 _# ~! W* ?) d( I, C! P1 {/ `
TABLE 1. Typical loading/operating conditions together with the relevant 4 I) k6 r g5 [5 n7 i" g& W
material properties & Z+ N2 [3 y- z; a. @
Loading/operating conditions. Relevant material properties.
: x5 V% D% y2 l: Y1 X1 WShort-term loading. Stress-strain behavior.
6 t3 b" i5 g7 b4 t. XLong-term loading. Creep and stress relaxation.
8 K: _. M$ M* p/ C4 J6 o2 |7 u6 N5 jRepeated loading. Fatigue.
) j" h% x( E/ Z; {9 dHigh velocity and impact loading. Impact strength.
! `" f. \. `2 [) Q7 |! T6 SLoading at extreme temperatures. Thermal mechanical behavior.
! A, o% R1 o$ h) P1 A* q8 FStress-strain behavior
8 ` o3 K, l& ?Part strength
$ M0 _+ y) g- }- pThe stress-strain behavior of a material determines the material contribution to part strength (or : k+ B# ]9 S% s. V: _: K
stiffness), the relationship between load and deflection in a plastic part. Other factors that affect part
, L- C7 ]2 D1 k1 Fstrength include part geometry, loading, constraint conditions on the part, and the residual stresses
0 M. q3 @( [. U9 Q$ [( n- i% Fand orientations that result from the molding process. There are various types of strength, such as
2 s) I0 e- a+ V2 l+ V8 _tensile, compressive, torsional, flexural, and shear, depending on the load and restraint conditions
( r: r, _. B# a) U2 Y! O* Dthe part is subjected to. These types also correspond to the primary load state present in the part. The
6 i& N) L( ^) C( estress-strain behavior of the material in the same mode as the primary load state in the part is most ( X+ n' U7 \5 ]; o
relevant in determining part strength. |
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发表于 2007-5-31 11:12:28