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& H, c% x( p# H2 S! sMaterial properties for part design
4 G6 q p0 l7 O3 DPlastics are sensitive to operating conditions
8 W; c: r, T- }The plastics molding processes allow parts designers more freedom than working with $ e" @ W6 [$ \ K
metals because plastics materials are so versatile. Unlike metals, however, the # |! `. O! @6 o) S4 {
mechanical properties of plastics are very sensitive to the type, rate, duration, and
: d; {# r" H; W" b4 P+ Nfrequency of loading, the change in operating temperature, and in some cases, relative
' d6 `7 l% |! D( shumidity. The plastics part designer must take a material's response to these conditions c# S$ J! J( `! r/ N* @; O
into account. The table below lists the five typical loading and operating conditions,
5 J1 b: `) A8 Y; b" i- utogether with the relevant material properties a designer needs to consider.
& S+ ]" W/ Y8 d: f" d2 }1 i. + R4 I0 g5 Y/ J% p
TABLE 1. Typical loading/operating conditions together with the relevant 2 t, ?! g4 n# M. X& x; B# C
material properties 2 U& y0 A# S4 K. Q! F |( v3 e
Loading/operating conditions. Relevant material properties. 8 g- |- b: f4 g4 j4 L
Short-term loading. Stress-strain behavior.
2 e5 f# o( E6 q9 }6 q) zLong-term loading. Creep and stress relaxation.
M3 u. a6 s* A; s8 F. W9 QRepeated loading. Fatigue.
; m. e1 |6 `. S& a, o4 t. xHigh velocity and impact loading. Impact strength. , [* Z+ M1 K* z) a/ ]
Loading at extreme temperatures. Thermal mechanical behavior. 9 q$ z* Y: X, D) E# E/ w
Stress-strain behavior
1 q% `9 c9 [+ y) @! uPart strength
( P2 F# D# G4 R" w9 ?3 E" ?The stress-strain behavior of a material determines the material contribution to part strength (or
9 B+ R2 E4 s' i( }8 X Z/ j3 Lstiffness), the relationship between load and deflection in a plastic part. Other factors that affect part ; B+ H5 d8 }2 }; V/ d) \# l8 p$ u& d
strength include part geometry, loading, constraint conditions on the part, and the residual stresses
; V1 P/ E A2 W: @6 ^and orientations that result from the molding process. There are various types of strength, such as & D# f: a/ C/ Y: L N @+ {
tensile, compressive, torsional, flexural, and shear, depending on the load and restraint conditions
( Y3 ?* p4 y. Xthe part is subjected to. These types also correspond to the primary load state present in the part. The 1 V' V! _* y6 h
stress-strain behavior of the material in the same mode as the primary load state in the part is most + u8 N+ [, z; z6 j- j
relevant in determining part strength. |
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