Tensile Strength and Tensile Strain

[EpiGen]

Are these two things the same? I'm looking at bioabsorbable copolymers with similar properties to the human acetabular labrum, but I can only find tensile strength values for the copolymers and tensile stress at break for the labrum.

I've looked at the definitions of each it sounds like the values would be very similar - is this true?

Edit: Title should read 'Tensile Strength and Tensile Stress"

 

[timthereaper]

Well, the title of your post contains "tensile strain", but your post talks about "tensile stress". Just so you know, stress and strain are different concepts. They are somewhat related, but not the same concept.

However, in response to your post, I'm not sure about what the acetabular labrum is. From what I gathered, it's a ring of cartilage that stops the femur from pulling out of the socket of the hip joint. Cartilage seems to me to be a fairly non-ductile material, so I would think that the tensile stress at break (I'm interpreting this as fracture) would be close to the ultimate tensile strength. For the copolymer, I'm assuming the "tensile strength" is referring to the ultimate tensile strength. I would think the two would be fairly similar. I would like some confirmation on this from other smart people on the forums though ;-)

 

[Studiot]

Your source of data for the copolymer will probably have another column in the table labelled modulus, or bulk modulus or youngs modulus or some such.

Knowledge of this this will help us help you relate stress and strain. So if it is not obvious just post all the data you have on the copolymer (and the cartilege).

 

[EpiGen]

Your source of data for the copolymer will probably have another column in the table labelled modulus, or bulk modulus or youngs modulus or some such.

Knowledge of this this will help us help you relate stress and strain. So if it is not obvious just post all the data you have on the copolymer (and the cartilege).

So if I know the strain, could I just divide the modulus by the value and the result would be stress?

 

[Studiot]

Like tim, I am not well up on anatomy so you will need to help us to help you.

Both he and I asked some questions, designed to elicit the information needed for this.

Please answer them rather than asking further questions. It should be obvious, from my last post, that there is more than one type of modulus. It will be necessary to to assess the structural situation to suggest the most appropriate formula and modulus to use.

 

[EpiGen]

Like tim, I am not well up on anatomy so you will need to help us to help you.

Both he and I asked some questions, designed to elicit the information needed for this.

Please answer them rather than asking further questions. It should be obvious, from my last post, that there is more than one type of modulus. It will be necessary to to assess the structural situation to suggest the most appropriate formula and modulus to use.

The project I am working on is to find a bioabsorbable copolymer that we can use to repair acetabular labrum tears (acetabular labrum is what forms the seal around your hip joints, and keeps synovial fluid inside.) When someone tears their acetabular labrum they often describe a 'clicking' in their joint as the synovial fluid inside is free to move around (there's a damaged seal.) This often results in the femoral head scraping against the inside of the socket, which is very painful.

Once I find a copolymer with similar properties in tensile strength and tensile modulus to the acetabular labrum, we will get a piece of kevlar (a material can be tolerated inside the body) and attach micropores (bubbles) of the copolymer on each side (2-3 pore-layers thick.) The idea is that new tissue will grow into the space between the spheres, and as the copolymer is absorbed by the body more and new tissue can move into seal the tear (the body basically regenerates itself.)

The main thing is that the properties of the copolymer have to have same tensile strength, tensile modulus and strain to the acetabular labrum for this idea to work (an exact match isn't going to be possible, but very close should work.)

 

[Studiot]

That sounds like a worthwhile objective indeed.

In general, modulus = Stress divided by strain

Picking any two of these fixes or determines the third.

In the abscence of further information I would hazard a guess as follows.

From Tim's description of the labrum thingy it is like an O ring. So to pull the lump on the end of the femur pulling through will need to stretch the ring. This will cause an increase in tension in the ring.

So long as your copolymer has a tensile stength equal to or greater than that of the cartilege it will perform the same function. A close match of modulus will ensure that the amount of stretch (= strain) is the same for both materials. You do not need to know any further detail to state this.

How does all that sound?

 

[EpiGen]

That sounds like a worthwhile objective indeed.

In general, modulus = Stress divided by strain

Picking any two of these fixes or determines the third.

In the abscence of further information I would hazard a guess as follows.

From Tim's description of the labrum thingy it is like an O ring. So to pull the lump on the end of the femur pulling through will need to stretch the ring. This will cause an increase in tension in the ring.

So long as your copolymer has a tensile stength equal to or greater than that of the cartilege it will perform the same function. A close match of modulus will ensure that the amount of stretch (= strain) is the same for both materials. You do not need to know any further detail to state this.

How does all that sound?

Sounds good! Thanks for your help

 

[meldraft]

Just keep in mind that tissue typically exhibits non-Linear elastic behavior, so it's not really a matter of getting the same modulus, as much as a matter of getting a similar curve (if it's important for your project that is ;) )

 

[Studiot]

Just keep in mind that tissue typically exhibits non-Linear elastic behavior

Good point, but polymers too typically exhibit non linear behaviour.
Also I think in this particular application (cartilege substitution) the mechanical range is quite restricted so a linear approximation will probably suffice. I don't see the ring being pulled out like an elastic band.

go well