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& j! Q- f8 z3 k! DMaterial properties for part design
) ]0 V: f" G- x3 {& g) M* w8 RPlastics are sensitive to operating conditions
/ V, Y7 l7 O4 T& g5 O: JThe plastics molding processes allow parts designers more freedom than working with
) [+ G3 E, C. u! jmetals because plastics materials are so versatile. Unlike metals, however, the - S) N2 |5 N e
mechanical properties of plastics are very sensitive to the type, rate, duration, and
, d% l9 ?; n' a. }% W- H! rfrequency of loading, the change in operating temperature, and in some cases, relative
( f: x- `0 Y5 whumidity. The plastics part designer must take a material's response to these conditions ' }7 O$ x' f3 Y" q5 D3 m
into account. The table below lists the five typical loading and operating conditions,
+ l! d5 @, t8 [) \# z( ~7 J8 S2 rtogether with the relevant material properties a designer needs to consider.
# j+ S6 u3 V( E9 @/ L1 S. # w! I% H; f- J4 f2 g
TABLE 1. Typical loading/operating conditions together with the relevant ; h9 E- \% m1 p: e1 g2 B* V
material properties ; }/ E" k7 H0 h* O3 |! o% [3 b
Loading/operating conditions. Relevant material properties.
% Q* I3 S8 ]4 ?$ rShort-term loading. Stress-strain behavior.
/ E1 e' K+ T0 d! L, z6 v6 K% WLong-term loading. Creep and stress relaxation.
: n; M6 a: W0 w3 t4 S. cRepeated loading. Fatigue.
3 ^, Q' x u( r2 i. |- l$ g6 |High velocity and impact loading. Impact strength.
7 y) _- U9 V: WLoading at extreme temperatures. Thermal mechanical behavior. 4 t) Q; B! v' n% f
Stress-strain behavior # T, x# z# D3 |' O) p" V8 D$ e
Part strength
& l8 n4 b4 H9 S$ \2 K6 L K7 `7 GThe stress-strain behavior of a material determines the material contribution to part strength (or . L5 y( n- u! J! ]0 O, D
stiffness), the relationship between load and deflection in a plastic part. Other factors that affect part
5 G8 ~( L7 _( t% Ystrength include part geometry, loading, constraint conditions on the part, and the residual stresses
8 O& }$ O/ h: a) [and orientations that result from the molding process. There are various types of strength, such as
- Z3 b7 A7 V/ g+ y2 h' Ctensile, compressive, torsional, flexural, and shear, depending on the load and restraint conditions % i. g( _' H. A1 C
the part is subjected to. These types also correspond to the primary load state present in the part. The 5 {6 b+ X, A4 L
stress-strain behavior of the material in the same mode as the primary load state in the part is most 1 }5 H9 e. P( Z7 f; h
relevant in determining part strength. |
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发表于 2007-5-31 11:12:28