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8 H" R( S5 i# S3 W# k4 |Material properties for part design : c4 X3 T2 L( S( J
Plastics are sensitive to operating conditions 8 A# R+ G8 J4 ^3 H% W0 F1 q
The plastics molding processes allow parts designers more freedom than working with . }2 a3 Z: t3 x( M7 f
metals because plastics materials are so versatile. Unlike metals, however, the
4 Y1 T/ g6 j: V4 o" zmechanical properties of plastics are very sensitive to the type, rate, duration, and
7 P7 Y& t6 M% H/ qfrequency of loading, the change in operating temperature, and in some cases, relative
7 c2 H% |* Y! ]# _humidity. The plastics part designer must take a material's response to these conditions . M, P. }$ C s! g% I! Q
into account. The table below lists the five typical loading and operating conditions,
/ f" h" I, k2 S! jtogether with the relevant material properties a designer needs to consider. . G8 c$ B0 E, e) n) l
.
6 ~5 L* e2 S/ r" y: ?* qTABLE 1. Typical loading/operating conditions together with the relevant + C, l' C& j' R
material properties , ]4 f% |3 C: r- [' z4 d& ]
Loading/operating conditions. Relevant material properties. 6 a2 _1 M9 I. T9 @. N
Short-term loading. Stress-strain behavior.
4 ]3 `, y4 [! a% m7 I4 WLong-term loading. Creep and stress relaxation.
! h5 Q! k3 Q# U& N3 M' lRepeated loading. Fatigue. ; e4 ]; ?5 ]5 C
High velocity and impact loading. Impact strength.
3 r# P& M' ~1 @1 fLoading at extreme temperatures. Thermal mechanical behavior.
/ G6 G! k% H1 r; s) |# l: gStress-strain behavior
: n3 G e4 A3 \* E( wPart strength
6 d( Q8 T& h2 V% @ S% ~The stress-strain behavior of a material determines the material contribution to part strength (or
' n. ^3 D j) f6 }% P3 X8 ]stiffness), the relationship between load and deflection in a plastic part. Other factors that affect part 5 A8 \0 _8 ]& D
strength include part geometry, loading, constraint conditions on the part, and the residual stresses 4 J4 N4 c+ |5 C/ g" Z: x
and orientations that result from the molding process. There are various types of strength, such as 2 S5 w$ ^0 @# c1 H3 J
tensile, compressive, torsional, flexural, and shear, depending on the load and restraint conditions
9 H) v& P( I3 sthe part is subjected to. These types also correspond to the primary load state present in the part. The
) Y, V0 z* V2 @) T2 Z( L7 hstress-strain behavior of the material in the same mode as the primary load state in the part is most % w& O: B( q: E7 d7 d m. x
relevant in determining part strength. |
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发表于 2007-5-31 11:12:28