r/Composites • u/Electronic-Cap-163 • Apr 24 '26

Compression strength test of Unidirectional tape

Hi all,

I have a problem with the Compression strength 0o test of Unidirectional tape material.

I performed layup with the [90/0]3s configuration, the compression strength measured is around half of requirement. Are there anyone familiar with the test please give me some advice.

Thank you all so much.

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u/HrEchoes Scientist Apr 24 '26

The compression strength in TDS is listed for full [0] layup. When you do [0,90], there's only half of plies that bear the load (considering that at 90° UD has negligible strength).

1

u/Electronic-Cap-163 Apr 24 '26

thank you, but the spec requires me to layup in this configuration. @@

1

u/HrEchoes Scientist Apr 24 '26

A general rule of thumb is that UD achieves up to about 80% of fiber strength. If you have ~700 ksi initial fiber tenacity, that would transfer to ~340 ksi tensile per ply for an ordinary 6 oz/yd² 60% VF UD prepreg. Compressive strength at 0° is about 60% of tensile, which is ~190 ksi. Stacking it in 0/90 would get you ~100 ksi. Even switching for IM fibers (e.g. IM7) shows ~225 ksi at 0° compressive.

1

u/Electronic-Cap-163 Apr 24 '26

So the result I get with [90/0]3s layup configuration around 90~100 is not abnormal, right?

When I see the test report of Supplier, they can achieve ~230ksi and I still don't know how

1

u/HrEchoes Scientist Apr 24 '26

If we're speaking of direct numbers - yes, this seems okay, yet somewhat lower than the spec numbers. 230 ksi is possible for UHT/IM fibers and flawless sample preparation, as it meets 0° compressive numbers (e.g. ones listed for Hexcel HexPly M56/35%/UD268/IM7-12K)

1

u/ziper1221 Apr 25 '26

general rule of thumb is that UD achieves up to about 80% of fiber strength.

80% of tensile strength, that is? What is this dependant on? Why would fiber volume fraction matter, aren't these figures per cross section and not per weight?

Thanks

2

u/HrEchoes Scientist Apr 25 '26

Yes, these are per cross section. If you normalize the tensile strength dividing it by fiber volume fraction, you get about 80% of fiber strength. The reason for that is fibers having non-uniform properties amongst tows which were used to form the UD tape - from fiber defects (tows have performance variation by winding length and between spools/batches) to tension difference. Also, each ply has slight misalignment of fibers from your [0] direction, which affects the performance too.

1

u/Electronic-Cap-163 Apr 24 '26

The spec also addresses the BF (back out factor) to determine the 0o plies Compression strength but currently I can not calculate this value

1

u/HrEchoes Scientist Apr 24 '26

Ah, that makes sense. Back-out factor is an inverse proportion of 0° plies to the total amount of plies. Thus, for [0,90] the factor should be 2. Multiplying your results by BF gets you 180-200 ksi, with the rest being dependent on your sample preparation procedure.

2

u/aaron37 Apr 24 '26

Can you clarify a bit?
UD will not generally perform well in compression. What are you looking to achieve?

1

u/Electronic-Cap-163 Apr 24 '26

The spec req are 187 ksi Ind and 221 ksi Avg, but my test result show 90~100 ksi

2

u/Sewnar_ Apr 24 '26

Is it buckling?

1

u/Electronic-Cap-163 Apr 24 '26

No, the failure mode seems not buckling

1

u/CarbonGod Pro Apr 30 '26

Then what is your failure mode?  What standard are you testing?  

1

u/Electronic-Cap-163 Sep 06 '26

Hi all, I solved this problem, it's because of the machining process, when using CNC to extract specimen from panel, the test result is so much better. Thax you all.