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模具英文资料5 ~& h0 ?! p% {6 Q( P: O1 S; _, f; D
Material properties for part design 4 C- D4 y1 [3 Y; E: l1 a
Plastics are sensitive to operating conditions
- d9 j8 H8 `/ E) B' O+ I+ @. Z' m; DThe plastics molding processes allow parts designers more freedom than working with
B/ O0 H! q' C7 wmetals because plastics materials are so versatile. Unlike metals, however, the
. G- Y$ t9 \9 K1 H% _6 }mechanical properties of plastics are very sensitive to the type, rate, duration, and , ]4 o# z7 }, B6 l( K! x
frequency of loading, the change in operating temperature, and in some cases, relative
, s) s9 {! o2 t8 y2 G+ J* Ahumidity. The plastics part designer must take a material's response to these conditions
* f7 `) r5 h( G! Y$ K! Ainto account. The table below lists the five typical loading and operating conditions,
6 x) E. g* k/ \together with the relevant material properties a designer needs to consider. ) G9 C+ z1 C! C, [. j/ R
. " [/ ^% a0 c7 Z8 x6 u4 _
TABLE 1. Typical loading/operating conditions together with the relevant * ^# Z4 E; ?( P& s% U# s
material properties
% ~$ \8 Z; S- {( L! V& M; ~1 |# iLoading/operating conditions. Relevant material properties. ) r( o$ Z" U! u p7 w0 Q/ a
Short-term loading. Stress-strain behavior. - l4 G3 P$ S, t! {' H! m1 y) i
Long-term loading. Creep and stress relaxation. 2 B$ r e4 b/ Z( {7 W4 u
Repeated loading. Fatigue. " @. ^ d5 X5 ?3 d* n5 c F) j
High velocity and impact loading. Impact strength. # X0 q# a( m& F, Z4 u) S6 f
Loading at extreme temperatures. Thermal mechanical behavior.
) C6 V: X, G5 t1 GStress-strain behavior
6 C& C [: _3 Y; S% \Part strength " p" Y2 g, t# P5 [) k8 |
The stress-strain behavior of a material determines the material contribution to part strength (or 1 |* {- t' ^7 c% _$ X
stiffness), the relationship between load and deflection in a plastic part. Other factors that affect part , N8 |6 _. r' s- v9 D: K1 s9 z
strength include part geometry, loading, constraint conditions on the part, and the residual stresses % |* m) U2 G' J# i9 m
and orientations that result from the molding process. There are various types of strength, such as
7 W* |2 p8 Q% E0 f4 ^8 r$ Utensile, compressive, torsional, flexural, and shear, depending on the load and restraint conditions
+ U. B5 v d, [0 z5 r* ^the part is subjected to. These types also correspond to the primary load state present in the part. The ! x1 I! f! f* }6 o6 V0 S% D8 V
stress-strain behavior of the material in the same mode as the primary load state in the part is most " P9 l7 a* T) g. g% q) o
relevant in determining part strength. |
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发表于 2007-5-31 11:12:28