Definición de discapacidad

The s m a ll strain dynamic m e c h a n i c a l p r o p e rt ie s were measured with pre-compression for a range of conventional and high-resiliency foam samples. The storage modulus, E'(a)), on a l o g a r i t h m i c scale, and the loss tangent, d( u>) , on a l i n e a r s c a l e h a v e b e e n p l o t t e d a g a i n s t logarithmic pre-strain. Figures 4*6 and 4*7 show typical results for type I and type II samples respectively. In each case the variation is drawn at frequencies of 0.07 , 1 and 10 Hz. All test pieces of the same type were found to h a v e p ro pe r ti es which v ar y with p r e - s t r a i n in a similar manner. Considerable differences may be noted, however, between the results for foam types I and II. Small Strain Dynamic Storage Modulus

For the type I foam test pieces, E'(o)) a ppears to 114

0-15 dfiol 0-05 0-01 0-05 _0-5 PRE-STRAIN F IGURE 4.6

V a r i a t i o n of E (&) and d ) with P r e - S tr ai n for a S a m p l e of C o n v e n t i o n a l PU Foam, at F r e q u e n c i e s of 0.07 Hz, 1 Hz and 10 H z .

IOHt, IHt< 0O7Hn 0-15 E'M /P a lOHr 00 5 0-01 _{0 0 5 0-1} PRE-STRAIN FIGURE 4.7

V a r i a t i o n of E 1 (^) and d (t*J) with P r e - S t r a i n for a S a m p l e of H i gh-R e s i 1 i e n cy PU Foam, at F r e q u e n c i e s of 0.07 Hz, 1 Hz and 10 H z .

decrease smoothly with increasing pre-compression, to a m i n i m u m at about 40-50$ strain. In al l the sam p le s measured no yield strain was apparent at low frequencies. This is c on tr a ry to the acc ep te d v ie w that b e l o w the b u c k l i n g s t r a i n , e-^, of the m a t e r i a l , the d y n a m i c p r op e rt i es are i n de p e n d e n t of pre-strain. A b o v e the minimum of the curve, E'(io) begins to increase once more. In all cases the variation with pre-strain has been found to be independent of frequency. The systematic increase with f r eq u e n c y of E'(u)) in figure 4*6 can be a tt r i b u t e d to the n o r m al f r eq u e n c y d e p e n d en ce of the v i s c o e l a s t i c properties of resilient materials, and this is consistent with an e l a s t o m e r i c m a t e r i a l w e l l a bo ve its t r a n s i t i o n

temperature [48].

In the case of type II foam s a mp le s a y i e l d strain is noted in the range 1 -4$« B e l o w the y i e l d strain E'(co) m a y be c o n s i d e r e d to be i n d e p e n d e n t of the s t a t i c deformation imposed. Above the yield value the storage m o d u l u s then f a l l s r a p i d l y to a m i n i m u m in the strain range 20-50$. E'(co) then rises, but less steeply than is the case for type I samples. Once again the curves have the same shape at different frequencies.

The r e s u l t s ha ve been i n t e r p r e t e d by d e f i n in g a shape fu nc ti o n in a s i m i l a r m an n er to that used by R usc h in the t reatment of n o n - o s c i 11 atory deformation. The s m a l l strain dynamic storage m o d u l u s has been f actored into frequency dependent and strain dependent parts

E 1(w) = E f (w) 5 (e) 4.9 where E (e) takes the form

5(e) = re~v + seW 4.10

The c o e f f i c ie n ts r, s, v and w are e mp i r i c a l curve fitting constants, and are defined in a similar manner to the Rusch c oe f f i c i e n t s of eq ua t io n 4*8. In order to fully characterise the curve a further three measurable quantities are defined; the yield strain, ey, defined as the strain at which E(e) has f a l l e n to 0 .9 5 , -(e)q» the m i n i m u m v a l u e of 5 (e), and e^, the s t r a i n at the m in im um •

Table 4»2 gives the values of these coefficients together with Ef(w) m e a s u re d at 0.07 Hz, for the foams tested in this way. The s l op e of ^(e) post y i e l d has a t e n d e nc y to be lower for type I foams. S ig ni f i c a n t d i ff e r e n c e s may also be noted b et w e e n the constant s r and s for the two foam types. The gradient of the shape function above the m i n i m u m shows no s y st e ma t ic v ar iation. This was e xpected as at high l e v e l s of p r e - s t r a i n both types of foam will tend to exhibit properties similar to the solid polymer. As noted p r e v i o u s l y no y i e l d strain has been o b s e r v e d with c o n v e n t i o n a l foams. In the case of high- r e s i l i e n c y foams no o b v i o u s d e p en d en c e on the foam density or cell size has been found.

The v a l u e s of strain at the m i n i m u m for type II foams