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Material properties for part design
/ l$ A7 O+ [1 j5 K; yPlastics are sensitive to operating conditions
! Y6 a& C1 W/ _' C5 fThe plastics molding processes allow parts designers more freedom than working with
7 N, S4 H/ W4 W6 P" G, _# Jmetals because plastics materials are so versatile. Unlike metals, however, the
6 i2 ^; j" s+ w) N+ J* n' I1 dmechanical properties of plastics are very sensitive to the type, rate, duration, and
" \; r2 b& d. e1 C4 G+ bfrequency of loading, the change in operating temperature, and in some cases, relative - J# T, W! M& |/ {3 Y' m
humidity. The plastics part designer must take a material's response to these conditions ! r0 W! A: e( u/ I+ Z6 j. |. z' F
into account. The table below lists the five typical loading and operating conditions, * X ^' S9 | S" s3 G
together with the relevant material properties a designer needs to consider.
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5 n% x( q& }1 i5 _6 e2 I/ l/ ~. HTABLE 1. Typical loading/operating conditions together with the relevant # z2 |' M6 C- L G
material properties : b& o2 }; F! r
Loading/operating conditions. Relevant material properties. % Y4 J: g+ n* B: I6 N& {+ b
Short-term loading. Stress-strain behavior. 6 \! _0 C& n3 b( H+ e( v
Long-term loading. Creep and stress relaxation. 0 E" q4 V6 o6 P& z
Repeated loading. Fatigue. % `3 x0 @3 {) g9 L2 U
High velocity and impact loading. Impact strength.
$ Y4 E+ k3 n* Z/ [8 c; {' K9 hLoading at extreme temperatures. Thermal mechanical behavior. & ^! }& E1 d ?9 ]2 d
Stress-strain behavior 7 r3 X9 A, M7 y0 E5 n+ U( P v: p
Part strength " M: k3 ?) m1 C; F; x k+ z
The stress-strain behavior of a material determines the material contribution to part strength (or
0 t3 e7 u/ i- hstiffness), the relationship between load and deflection in a plastic part. Other factors that affect part
2 i8 R1 Z9 X! i/ hstrength include part geometry, loading, constraint conditions on the part, and the residual stresses : Q9 C' c- \# S! j( f
and orientations that result from the molding process. There are various types of strength, such as
8 ^0 B* \0 v, [% R* [tensile, compressive, torsional, flexural, and shear, depending on the load and restraint conditions
5 [* O+ j1 [* C" v+ athe part is subjected to. These types also correspond to the primary load state present in the part. The
0 q, D& [7 N+ F# ~8 d* L2 Tstress-strain behavior of the material in the same mode as the primary load state in the part is most
% m0 L2 w7 H6 `8 \" Z, u$ i4 hrelevant in determining part strength. |
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发表于 2007-5-31 11:12:28