PDA

View Full Version : How can the long term modulus be negative?

benutella
2004-08-24, 04:13
Hello, Jorgen

i have two questions:

1. how can the long term moduli be negative?

i defined the instantaneous moduli by table, at 30C,the elastic moduli is 4.789e9 Pa, and at 50C, the elastic moduli is 2.23e9 Pa, the shear moduli according to these two elastic moduli are 1.84e9 Pa and 0.858e9. but the long term moduli i got from Marc. menat is negative, how come?
i know this is because the temperature effects are not defined properly, but how to solve this propblem?

2. what is the differience between instantaneous moduli and Young's moduli? may i get the Young's modulus from the relaxation test? does it mean that at time=0, the instantaneous moduli is equial to Young's modulus?

Thanks a lot!
benutella

Jorgen
2004-08-24, 04:35
Hmm, you get a negative long-term modulus, that does not sound right.

1. Typically, to specify the relaxation behavior you need to give the instantaneous modulus and a Prony-series for the reduction in modulus with time. These two together specify the long-term modulus. Hence, if you specify an inproper Prony series, that reduces the stress too much, you might end up with a negative long-term modulus.

2. For a viscoelastic material it is not easy to talk about Young's modulus; it is better to talk about the instantaneous modulus and the long term modulus. Although, the instantaneous modulus is often also referred to as the Young's modulus.

Jorgen

wshrr
2008-06-10, 12:19
I have plots from the manf. for a viscoelastic material "surorhane"
one for frequency vs shear modulus "I assume this the G'(w)"
the other one is frequency vs tan delta.

from these 2 plots i can
get G''(w)

Is my approach correct?

Now I have to implement these data to ABAQUS.

I only have these two information but not the long term or instantaneous modulus.

How to apply these data into ABAQUS.

Any help is appreciate regarding this issue.

Jorgen
2008-06-11, 20:46
Sounds like a fine approach. You might be able to estimate the long-term modulus from the G' data as w goes towards 0, or the instantaneous modulus from the G' data as w goes towards infinity.

- Jorgen