Diagonal reenforcement of fabric with heavy thread

Have no idea if this would work, but how about two layers of fabric, each half the weight of the fabric you planned? Lay them at angles to each other so that stretch on the bias is cancelled out, then "quilt" with (machine) stitching to hold in place. Essentially, you'd be making fabric plywood.

Might be less work that hand-sewing all those kevlar threads.

David, thanks for the idea. Using two lighter layers would be a good idea for some applications, I think. For this, I see two shortcomings.

First, two layers of 0.9 oz/yard fabric would be far less resistant to abrasion than one 1.8 oz layer, I think. The Dacron fabric I chose has a low thread count and coarse 200D yarns. I think it is probably much more abrasion resistant than two layers of a 0.9 oz fabric.

Also, the Dacron fabric has a tensile strength in the thread directions (warp or weft) of about 95 lbs per linear inch. Using two lighter layers cuts the strength in half in those directions. If I add some very strong thread in the diagonal directions (I have 75lb test Vectran/Dyneema thread), just a few threads could give a surplus of strength on the diagonal with minimal increase in weight and no reduction in weave-direction strength.

I'm just wondering if anyone else has attempted it.

"then do a simple over-under stitch by hand along the lines, anchoring the thread securely at both ends."

In hand sewn leather work they stitch over under then stitch back using the same holes but on opposite sides of the fabric, which reduces perforations and makes it more of a locking stitch.

Glue which would bond the stretchy fibers together at the stress point might be useful.

Thanks, David. Those are good ideas. I hadn't thought about stitching back the other direction through the same holes.

Stay stitching like this doesn't work very well. It provides some reinforcement, but only directly on the line of stitching. if you're worried about it, put some sort of reinforcement there. You could bond a patch of the material on, with the force on the warp grain, or you could use a tape of some sort.

"Stay stitching like this doesn't work very well. It provides some reinforcement, but only directly on the line of stitching."

David, thanks for the feedback. Reinforcement between two points on the fabric (in a line) is what I need. I understand that this kind of reinforcement would not be very good at protecting against the chaotic stresses around a grommet, but I don't see why it wouldn't be perfect for alleviating tensile strain on the fabric between two points. Why do you suggest tape instead?

No, you're putting force on a two dimensional area — it's more than one thread wide. The stay stitching provides reinforcement for two threads, one on either side of the thread. What you're dealing with is mechanical stretch — the bias is stretchy because the fabric is mechanically distorted. One, or even a few, lines of thread aren't going to stop that. (I tried this, even though I didn't think it would work, on a shopping bag that was meant to fold up super small. The bottom sagged alarmingly, so I thought some stay stiches would be easy. didn't work, but a piece of cotton twill tape inserted into the corner seams, run across the bottom to the diagonally opposite corner, help a lot. Until the handle ripped out, at least.)

I suggested tape because it's a standard solution for stays like this, you're likely to already have something suitable, and there's probably a way to also make it a functional attachment. I think I'd actually prefer a reinforcement of the fabric by layering two plies of the bag's fabric. Or maybe figuring out how to change the design so there isn't a load like this, or changing how the panel is cut so that forces are put along the grain or crossgrain. That might not be possible, because it creates other problems.

"Why do you suggest tape instead?"

If I look at a line of stitching from the side it runs zigzag style from one side of the fabric to the other. So it would stretch somewhat if pulled.

A piece of non stretching tape is a straight line from point A to point B, running on only one side of the fabric. No zigzag shape to stretch.

I'm just guessing here but on first glance it seems to make sense.

Thanks Daryl and Davids (all three of you) for your responses. I'll give this a little bit of thought and maybe do some experimenting. I appreciate the feedback. I'll post updates if I come up with anything interesting.

This is a fascinating idea! It would make reinforcements pretty easy, and light.
I wanted to test it on sewing machine and tarp tie-outs.
Used normal polyester sewing thread to feed through the needle and window blinds cord for the bobbin (What are the correct names for sewing machine up and bottom side threads?)
It was quite ok to sew, and can take some pull force too. Photo of a very crude first trial:

https://picasaweb.google.com/lh/photo/wcOz8RGVmLsplRhJy35jhYbsAqoXbwEuQaFFz3o40w4?feat=directlink

Perhaps a bit thinner cord for the bobbin would be better, and sufficient.
Idea was to make a star or flower pattern as described in the thread 57578. Unfortunately I now need to pack for a skiing week in Lapland, so testing has to wait.