quick question on shear strength

General Composites Discussions Engineering Talk
#1

canyon (or all),

I am currently trying to understand this whole lamination theory deal, so I am working some problems in a book I have. Instead of using the books strength and modulus numbers, I decided to use numbers from a prepreg manufacturer I may be getting some unidirectional material from. The question I am having is that I am not sure what to use as my shear stress term. I am given the short beam shear strength in the 0 degree direction and the 90 degree direction as per the ASTM D-2344 standard test. I thought that there was a certain congruency in shear stresses though, so only one value was used. Which one do I use, the 0 degree direction? or the 90 degree direction? or the lowest of the two? Or am I off on my theory and I actually need to use both of these values?

thanks,
Fleisch

#2

I know what you’re talking about but I’ve never been forced to answer this question. But from looking at the ASTM D2344 tests the composites are loaded out of plane so in one shear mode you’re shearing across the face of the fibers and in the second you’re shearing along the fibers.

What kind of analysis are you doing? For inplane loading you’d only need one value but out of plane loading requires another matrix (H) to completely describe the material. I can’t really say without knowing the loading conditions or what type of analysis you’re trying to run.

I’m glad you asked this question, it’s making me think! I’m really curious to see how your problem is solved. Mind sharing more?

#3

thanks for the quick response. I believe I’ll only be needing the in plane shear stress, as I am going to be performing calculations on a beam structure. After reading your response it make sense that the figure I need is the 90 degree shear strength then (as against the fibers indicates out of plane shear)?

I just realized I didn’t explain much of what I was working on. I’m doing my best to design a carbon downhill mountain bike frame. I have found a sample problem in the book I am using to try to teach myself this stuff, describing formulation of a layup schedule given a stress in one direction. I am planning on using the steps in this problem to write up a program on matlab which I will then use to define a layup schedule for my frame. This of course after I decide how I am going to model the loading system (which I got a lead on today, but I feel like I need to run it by a professor or two, so I will probably do that tomorrow or the next day)

EDIT: I think I’ve got it backwards. I’ll want to use the 0 degree shear strength…

#4

I have access to the ASTM standards today so I’m going to head over to the library and take a look in an hour or so. Let me look over it and I’ll see if I can give you any answers.

Cheers
Canyon

#5

Alright Fleisch,I’ve got the answer for you.

ASTM d2344 measures out of plane stiffness and the stiffness varies as a function of the angle of the fibers. This is great if you need that but for the basic problems you’re trying you need in plane shear which falls under ASTM 4255 - In Plane Shear Properties of Polymer Matrix Composite materials but the Rail Shear Method or D5379 - Shear Properties of Composite Materials by the V Notched Beam Method.

That’s the hassle with composites, it’s fun and all to put multiple components to get better materials but you get wonky effects like three shear values, one for each plane of loading

#6

thanks canyon. Looks like I’m out of luck unless there is some way to derive the in plane shear strength/stiffness from what I’ve been given. I really don’t understand why a company would only give you 3/4ths of what you need to know.

It is also weird that this company gives only short beam shear strength numbers if this is only used for stiffness. Maybe I need to give a tech rep a call or something…

#7

I stopped by the library to check out the ASTM standards myself, before I talked to the representative. Sounds like your right on the money with that one, between plane strength only. I called him up and he told me that essentially I was going to have to do the tests myself, but he will call me back if he finds anything out. Unfortunately, I’ve got a time constraint and doubt I will be able to do these tests myself.

Does anyone know of any prepreg manufacturers that supply the needed data with a given heat/pressure cycle to which I can try to replicate?

#8

I would call the weaver/manufacturer to see if they have any information. They’re usually more than happy to help out students if they can.