# How to express strengt to weight ratio of test samples

1. Feb 18, 2013

### Thurman

I have designed a test apparatus where I can apply known bending and torsion loads to thin sheets of foamed plastic.

All test samples will be identical sized sheets with various treatments to improve stiffness..with the added weight of the treatment recorded. I will use the information to design and test stiffer parts to be used as wings in a rubber powered model airplane where weight and strength are important.

The bending and torsion loads will be in grams and deflections will be in inches. So, how do I express the results? Example: A 3 gram load at 25% chord will deflect .5 inches at the tip of the cantelevered sample which weighs 6 grams. A treated sample deflects .3 inches but weighs 6.2 grams.

I want to construct a table for many, many samples and express their strength/weight scores.

I really need some help. Thanks to all,

Thurman

2. Feb 19, 2013

### CWatters

What you are measuring is the stiffness not the strength. A sheet of kevlar cloth (without resin) would bend a lot but it would be strong.

I think you need to define what constitutes "strength" for your purposes? Perhaps you need to measure the breaking deflection rather than the bending deflection? Perhaps you need both?

Perhaps calculate the load as the total torque about the fixing point...

T = (Ms * Ds) + (Mt * Dt)

where

Ms = Mass of sample
Ds = Distance from center of gravity of sample to fixing (50%?)
Mt = Mass of test weight
Dt = Distance from test weight to fixing (25%?)

3. Feb 19, 2013

### Thurman

Thanks for the reply! Yes, I want to measure stiffness. But I do not want to quantify the physical properties (density, yield, compression/ tensile aspects, etc.) of the plastic foam sheet I am using as these are avail. from material data sheets. Nor do I want to test to failure.

I only want to measure stiffness of a sample relative to all samples treated and measured. It is the weight increase of the treatment vs. the improved stiffness of the stock material - in an attempt to increase the stiffness to weight ratio, that I am interested in.

For example heat forming the sheet into a shallow arc (airfoil in this case) improves the bending stiffness of the sample at no weight penalty/cost, thereby increasing its stiffness to weight ratio vs. the same flat sheet sample. Likewise, adding a thin rectangular spar which has the same thickness dim. as the sheet adds great resistance (more than the shallow arc) to bending/stiffness. However there is a weight penalty due to the weight of the spar and the adhesive used to bond it to the sample assembly. Alternatively, a balsa wood spar of the same dimensions as the carbon spar, also improves stiffness at a smaller weight penalty.

Each of the three samples will be subjected to either a standard (weight) force at the tip and the deflections measured, or I could pick a deflection (dimension) and load the samples as required to reach the required deflection point and record the force (weight).

So, simply, how do I calculate the relative improvement in stiffness to weight ratio of the three samples (and many more) using either test technique - and record results in a meaningful table?

I also plan to construct and test samples using the current best practice (balsa wood structure with leading/trailing edges and wing ribs -covered with doped tissue paper). This will tell me the minimum stiffness required for my application as well as a benchmark of its stiffness to weight ratio and weight per sq. in. of wing surface.

The question then becomes whether an adequate structure can be designed using the foam material to replace the current practice at equal or less weight.

I ramble, but maybe this helps to clarify my question.

Best regards, Thurman

Last edited: Feb 19, 2013