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模具英文资料5 m0 m# I9 F3 Y; S3 `0 @) a
Material properties for part design
^! w; A( S3 ?% w& |9 S) NPlastics are sensitive to operating conditions : k7 \( `+ l$ n# Y" m# J$ b
The plastics molding processes allow parts designers more freedom than working with 3 U, _2 G: f" h! s1 {. J6 }4 u# Q
metals because plastics materials are so versatile. Unlike metals, however, the % E1 }! P. k( B; [7 h. E# \8 e1 l
mechanical properties of plastics are very sensitive to the type, rate, duration, and * S7 c7 [- v& Y! l: y
frequency of loading, the change in operating temperature, and in some cases, relative
& U% y+ k/ y' S' Z2 lhumidity. The plastics part designer must take a material's response to these conditions
( O! ^/ ]2 A: f* ^) f4 i% U" Iinto account. The table below lists the five typical loading and operating conditions,
+ |8 t% h; D) Q9 S7 K( h8 |together with the relevant material properties a designer needs to consider.
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% ~7 Y: I* g9 u0 T yTABLE 1. Typical loading/operating conditions together with the relevant
! _( S+ y' y3 `4 c/ mmaterial properties
, m' J, l. o- R# d kLoading/operating conditions. Relevant material properties.
- D# L5 o9 U+ E6 G+ T1 D' q1 u' HShort-term loading. Stress-strain behavior.
+ d" v L" G) M! E8 M* K: iLong-term loading. Creep and stress relaxation.
5 s* v, g A9 I) {' W, }1 B# ARepeated loading. Fatigue.
( \7 Y6 e5 v& b" t8 y r& fHigh velocity and impact loading. Impact strength. - W! r1 s* ?7 f
Loading at extreme temperatures. Thermal mechanical behavior.
4 Z! p0 n+ j- U$ X7 ^Stress-strain behavior 3 w1 t9 x- a/ d4 _- j2 f
Part strength ' `" K3 I8 B2 o+ g* Y1 M
The stress-strain behavior of a material determines the material contribution to part strength (or
2 z5 C, Y' M9 Y& x( W& _& z7 e8 Y5 Gstiffness), the relationship between load and deflection in a plastic part. Other factors that affect part
2 }! @/ ?- h; y3 [/ sstrength include part geometry, loading, constraint conditions on the part, and the residual stresses 7 Z1 }: E8 o8 ~0 g$ \
and orientations that result from the molding process. There are various types of strength, such as
" K0 @8 ~2 t8 R# f+ Mtensile, compressive, torsional, flexural, and shear, depending on the load and restraint conditions 8 }2 U2 D. b1 ?/ k
the part is subjected to. These types also correspond to the primary load state present in the part. The
/ p# {; i- }2 j1 U$ y% q9 P/ w7 s, Jstress-strain behavior of the material in the same mode as the primary load state in the part is most ! n/ j' V- G8 c8 j: T! G
relevant in determining part strength. |
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发表于 2007-5-31 11:12:28