iMechanica - Characterising fracture toughness in epoxy using mechanical fatigue. Help! - Comments

Hi, thanks for

Kaiyuan — Wed, 22 Jul 2009 10:07:34 -0400

Hi, thanks for reply!

Yes, I will be creating the pre-crack through fatigue. Another way would be thorugh the use of a razor blade and manually sawing it right?

I have another question: Since i will be propagating a crack in a single specimen, how do i actually measure the growth of the crack experimentally? EG: from one crack to another. Is there any internal sensors? or external sensors must be used? The machine i will be working with is from Instron.

Am i correct to say that the value of K1c, the fracture toughness, can be derived from the experiments?

Once again, thanks for helping me out!

Student
Singapore (JC)

Characterising fracture toughness in epoxy using mechanical fati

mikllaw — Sun, 12 Jul 2009 21:53:43 -0400

Fatigue crack growth and fracture toughness are quite different things, but related by the Paris Law of crack growth (http://en.wikipedia.org/wiki/Paris'_law). dA/dN= CDK^m where a is the crack length, N is the number of load cycles, C and m are material constants, and ΔK is the range of the stress intensity factor. The stress intensity factor is calcuated in the same way as for fracture mechanics, but the coefficients C and m are unrelayted. Fracture mechanics is looking for a fracture toughness K1c.

Are you "fatigue pre-cracking" a specimen before you perform a convenntion fracture test- this is done to ensure a sharp crack tip.

Characterising fracture toughness in epoxy using mechanical fatigue. Help!

Kaiyuan — Fri, 03 Jul 2009 10:50:44 -0400

Hi all,

I have taken up this research project/attachment on
"characterising the fracture toughness of epoxy using mechanical
fatigue", and encountered lot's of problems while researching. I'm just
a student, not even in university, but rather still schooling (grade
10), so am completely new to this field, which explains my problem in
understanding all the lit reviews, lecture slides i found online.
Luckily, i found out this site, and would hope that someone can guide
me in the right direction!

So far, i have read some
articles/files on fracture mechanics, but sadly, its kind of hard for
me to understand. What i understand about my project is that i have to
propagate a crack using mechanical fatigue (cyclic loading) on an epoxy
specimen (SENB), so as to find out the fracture toughness of the
material.

I would like to ask, how do i go about designing the
experiment? For instance, how do i prepare the specimen, fatigue the
crack, how will the results be like and how do i analyse the results to
find out what i want?

I am supposed to subject the epoxy
specimen under cyclic loading, until the crack propagates. My mentor
told me that there will be 2 approaches, the displacement control
approach and the force control approacb for the cyclic loading. Which
is better and why? Also, what kind of sensors and detectors do i use to
measure the crack growth?

Another issue would be Kc and Gc. I find it hard to understand the relationship between these 2, can anyone explain it to me?

I
know my post may seem very easy and general to you guys here, but for
me, its new! Thus, i would appreciate if anyone can guide me along!
Will definitely learn and research more!

Thanks!