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! ^4 N8 n; V i* r, W2 XMaterial properties for part design
6 r. A$ z) M6 ], Y' B2 [$ m2 d" W0 QPlastics are sensitive to operating conditions 6 D) S: @- I9 K3 g! N7 D5 c W
The plastics molding processes allow parts designers more freedom than working with 6 C& a3 y7 t4 A% e+ ?4 b( J
metals because plastics materials are so versatile. Unlike metals, however, the & S7 ]+ i# _% n
mechanical properties of plastics are very sensitive to the type, rate, duration, and
% }, P; \1 a5 l% ?frequency of loading, the change in operating temperature, and in some cases, relative - X2 M5 x8 T E1 _& m: s* W3 w' E
humidity. The plastics part designer must take a material's response to these conditions
0 x3 M1 i1 y( p; L5 D$ z$ p& Iinto account. The table below lists the five typical loading and operating conditions,
+ B+ m7 D% N- k' t" Gtogether with the relevant material properties a designer needs to consider. 7 T& R* H! w1 |! j8 J' w/ m
. 6 @) Q( ?+ e5 h9 K5 d. }8 [
TABLE 1. Typical loading/operating conditions together with the relevant
; p3 I2 ]% K6 n! `material properties
& y9 ~* f. a6 G( `) \' G4 @0 ZLoading/operating conditions. Relevant material properties. 2 E$ @- w* h1 ^+ g6 D, X& ?
Short-term loading. Stress-strain behavior. # c, I1 Y- C& _1 }
Long-term loading. Creep and stress relaxation.
0 S$ S2 ~5 w) h* wRepeated loading. Fatigue.
, c. V+ ^; p$ @/ k: WHigh velocity and impact loading. Impact strength. 1 G! F2 b6 \. P1 W7 g
Loading at extreme temperatures. Thermal mechanical behavior. 9 s1 A- @: f7 U6 O6 K
Stress-strain behavior
1 v: G/ Q y, ePart strength # A- U5 M1 G$ H4 U3 J9 w
The stress-strain behavior of a material determines the material contribution to part strength (or 3 [$ [9 D0 B& m% z& b
stiffness), the relationship between load and deflection in a plastic part. Other factors that affect part
$ Q" _+ h+ g* o# Estrength include part geometry, loading, constraint conditions on the part, and the residual stresses
# p" y! U! c! Q& G0 |2 P( band orientations that result from the molding process. There are various types of strength, such as 1 u+ X( T3 U+ l4 Y9 i
tensile, compressive, torsional, flexural, and shear, depending on the load and restraint conditions 3 i! b: g+ a+ O
the part is subjected to. These types also correspond to the primary load state present in the part. The " X5 N q2 w" ?' [: [
stress-strain behavior of the material in the same mode as the primary load state in the part is most
q( v# X( r2 O, e9 Nrelevant in determining part strength. |
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发表于 2007-5-31 11:12:28