Is the early wood the weak link to cause set?

[sleek]

I may have missed it, but can someone explain to the less initiated what MSI  AND GPA means?

Ok, I quit being lazy and looked it up. Just different units of measure for MOE and while the units are foreign to me, I at least can appreciate the trend in the charts.

[willie]

we normally assume that with wood being stronger in tension that the back does not stretch as much. Or perhaps my thinking is wrong and the back does stretch as much but is not under as much strain as the belly (not sure if that’s the correct use of strain/how close it is to breaking?). It would make sense to me that the back and belly experience set at different rates due to the differences in tension and compression.

Strain, in the engineering world, is simply a numerical percentage a materiel deforms (stretches or compresses) under a load. The deformation can be temporary (elastic) or permanent (plastic).  Other sciences define the word strain differently, so it's not always easy to visualize the exact meaning with the commonly used word. For instance, one often refers to a muscle or tendon strain to imply ligament damage. Permanent only if the body cannot rebuild the ligament. An "elastic" example would be the normal working of a muscle.

As for how much strain wood can sustain before breaking in tension, it may well be twice the amount wood can sustain in compression, when one considers the strain limit on compression to be the point where wood takes set. wood, can of course take quite a bit more compression before it actually breaks (collapses on the belly, moisture content being the big factor)

whether wood stretches and compresses the same under a given load is an interesting question I am hoping Alan can determine. Normal wood tests to not usually show these these differences , as typical bend tests do both at the same time.

Whether wood experiences set in pure tension is also an interesting question, to us bowyers at least. And whether some woods are better than others should be of interest to lam bow builders

[Selfbowman]

Thanks Willie muscles mean more to me than numbers.😁😁😁

[lonbow]

I just tested out if the wood stretches or compresses more, when a bow is braced. I took one of my elbs and marked a distance of 30 cm on the back and belly the mid limb area. Then I braced the bow and measured the two distances bethween the pencil lines again. I measured 30.15 cm on the back and 29.85 cm on the belly. The back elongiated about as much as the belly shortened, which means that the neutral plane should be in the middle bethween the back and belly.

This being said I must confess that the differences of the measured distances were so tiny that there might be measuring errors. It´s just a rough estimation. I also don´t know if the length ratios change when the bow is fully drawn.

lonbow

[Digital Caveman]

It was probably too small to measure accurately, but the concept is a really good one.

[sleek]

I just tested out if the wood stretches or compresses more, when a bow is braced. I took one of my elbs and marked a distance of 30 cm on the back and belly the mid limb area. Then I braced the bow and measured the two distances bethween the pencil lines again. I measured 30.15 cm on the back and 29.85 cm on the belly. The back elongiated about as much as the belly shortened, which means that the neutral plane should be in the middle bethween the back and belly.

This being said I must confess that the differences of the measured distances were so tiny that there might be measuring errors. It´s just a rough estimation. I also don´t know if the length ratios change when the bow is fully drawn.

lonbow

Unless you were able to measure around the verve vs the strait line between the two points, the test wasn't an accurate one.

[lonbow]

I used a tape measure and I followed the curves on the back and belly of the bow.

lonbow

[avcase]

I am ready to do some more testing. I had to make some adjustments to how the test samples were supported so that they didn’t slide around as the load was applied and released. This was becoming a problem with applying and releasing higher loads and larger deflections, so it was worth taking a little time to fix this.

I was also thinking about the method I am using to test these samples.  The amount of time I apply the load to a test sample is a variable that can make a big difference in short term and long term set or yielding.  The method I have been using is to apply a load for 5 seconds, take a reading, release the load and note where the indicator needle immediately returns to.  I then give it a minute or two to allow the sample to settle down and recover and note how close to zero the indicator needle is. Then I increase the load and repeat the process.

As the loads get higher, the deflection will continue to increase if I didn’t release it after this 5 second time.  This amount of time the load is applied can make a big difference when trying to determine when the material significantly yields.  I notice that the Forest Service uses some very slow strain rates in their tests.  I figured 5 seconds is reasonable because a bow is only held at full draw for a short time before release. 

Another thing to think about. The bow may also be kept braced for many hours. I am sure this can induce significant stress relaxation.  I see this with all my wood bows.  If kept braced for any length of time, it can take a little while after unstringing to return to its original profile.  I am not sure how to factor this in.  I think for now, I will just stick to this consistent method of applying the load and holding for 5 seconds and then releasing the load. I can follow this up with a test that applies a load for much longer duration to simulate the effect of keeping a bow braced.  Does this seem reasonable?

Okay, I will now load an osage flat grain and edge grain sample up to significant yielding and see how they compare.

[willie]

This amount of time the load is applied can make a big difference when trying to determine when the material significantly yields.  I notice that the Forest Service uses some very slow strain rates in their tests.

I have some test data from the Canadian Forest Service which shows MOE in impact testing to be half again higher than static bend testing. The static test loads a 2 x 2 steadily over the course of 2 minutes (pine at elastic limit) compared to bouncing a 50 lb weight dropped from a foot or so.

Of course, snapshooters know this already 

sugar maple and yellow birch out perform hickories in the the drop test, and doug fir is right up there with the hickories.

[avcase]

Today I picked an average flat grain osage and edge grain osage sample and ran them through a much higher bending stress level.  It seemed both were right about at the edge of taking on some set at this point. I probably should have taken it up one more notch but I am sure it would have shown both yielding.

This flat grain sample had a pretty continuous late growth ring on the compression side, with a slight runout between a late and early growth ring on the tension side (toward one end).

This chart is a little different than the last one. This chart plots bending stress versus bending strain. This allows direct comparison regardless of the thickness, width, or length of the test sample.  Based on this first test, it appears 14,000-15,000 psi is about the maximum bending stress for a bow made with this osage before getting into some significant set.  Strain at this maximum stress level is between .75% and .80%. The surprise for me is that the edge grain sample did a little better, but this is only one data point so far.  I will do a few more.

0.8% on a 66” bow would be equal to the back stretching by .53” and belly compressing by .53” at full draw.

What do you think?  According to these results so far, it seems as though Arvin’s edge grain bow should have ended up a little lower mass than a comparable flat grain bow!

[sleek]

Thats really fascinating. If I'm not mistaken, Arvin had only .375 back stretch at full draw. That leaves .685 for the belly to compress. So the belly compresses 1.8 times further than the back stretches. I was running some  numbers and got a rough calculation of slightly more. Do you believe a thicker sample would influence those numbers, or better put, would stretch and compression increase as sample thickness goes up? I was expecting higher compression numbers than this is why I ask.

[avcase]

Thats really fascinating. If I'm not mistaken, Arvin had only .375 back stretch at full draw. That leaves .685 for the belly to compress. So the belly compresses 1.8 times further than the back stretches. I was running some  numbers and got a rough calculation of slightly more. Do you believe a thicker sample would influence those numbers, or better put, would stretch and compression increase as sample thickness goes up? I was expecting higher compression numbers than this is why I ask.

I should have wrote that the maximum strain possible strain for a bow 66” long is .53”.  Most bows will show less than this because there are many areas that are not pushed to the maximum strain. For example, the handle area, and bow tips, etc.

Alan

[Allyn T]

I was gonna say Arvin measured across the non bending handle and he didn't measure the belly side. Also I wonder how it affects the compression that we only tiller on the belly side while the back remains untouched.

[sleek]

Thats really fascinating. If I'm not mistaken, Arvin had only .375 back stretch at full draw. That leaves .685 for the belly to compress. So the belly compresses 1.8 times further than the back stretches. I was running some  numbers and got a rough calculation of slightly more. Do you believe a thicker sample would influence those numbers, or better put, would stretch and compression increase as sample thickness goes up? I was expecting higher compression numbers than this is why I ask.

I should have wrote that the maximum strain possible strain for a bow 66” long is .53”.  Most bows will show less than this because there are many areas that are not pushed to the maximum strain. For example, the handle area, and bow tips, etc.

Alan

So a better stretch test would be to measure only the working portion of a limb.

[avcase]

So far, I measured a bending stress where the Osage wood starts to yield. This was around 14,000 psi.  I wanted to get some idea of how this compared to the bending stress in real osage self bows. Is this too low, or is it almost inevitable that the typical osage bow is stressed considerably beyond this point?

As a quick sanity check, I measured Arvin’s edge grain bow and built a computer model using the Osage and gemsbok horn material properties. I braced and drew back the bow to 24” and monitored the bending stresses. The model calculated over 10,000 psi stress just to brace the bow and over 18,000 psi maximum bending stress at full draw.  This means that the actual bow is certainly taking on some set already.  The bow model gave a draw weight of 54.4#, and the last time the real bow was measured, it had a lower draw weight at the same draw length. I will need to measure this. A lower draw weight with the real bow hints at material changes.