Ply-Splicing Requirements

[PLMKY]

I'm looking at designing a structural laminate for some load testing. The part I am looking to have designed is about 3ft wide and 12ft long. Since I will require the laminate to have +45/-45 plys I will need to incorporate ply splices in order to maintain a balanced and symmetric architecture. The laminate I am looking to achieve will be [0/45/-45/0/45/-45/0/45/-45/0}s. I am planning to cut sections of a roll of +45/-45 double bias fabric and rotate it 90 degrees to achieve the -45/+45 plys I will require for my symmetry condition. Due to the length of my part being significantly longer than the width of the double bias fabric rolls I am able to acquire, this will require I splice the -45/+45 plys.

This is my first foray into composites design and I am not sure what the industry practice would be for an acceptable splice dimension. From the limited information I have been able to find I believe a half inch long overlap splice will suffice and I must ensure that the splices are staggered through the thickness of the laminate. Will there be issues with the strength of the splice since I will be joining two layers of a double-bias fabric?

Any advice on the issue or recommendations for related reading materials will be greatly appreciated! Thanks in advance.

 

[Compositepro]

This where uni-directional tape has a great advantage. No splices are required in the length direction. In the 90 degree direction, butt-splices have no structural or functional issues. With fabrics a splice results in cut fibers and an interruption of the load. A 0.5" overlap will generally transfer the load through the splice with little reduction in strength. But the extra thickness of the splice will be clearly seen on the bag-side of the laminate, even if there are 20 plies, or more. A caul, or pressure plate can help to conceal this ridge. If the strength reduction at the splice can be tolerated, then a butt splice can be used. This is often done when making composite tooling (molds), where strength is not an issue but cosmetics, stiffness, and coefficient of thermal expansion are most important.

Splices also affect how air and resin flows when vacuum-bagging, which needs to be considered.

 

[SWComposites]

A 0.50 inch minimum overlap splice for 3K plain weave (PW) fabric (~ 0.008 inch thick) is sufficient. Thicker fabrics should have proportionally longer overlaps. Not sure what your "double bias" fabric is. Yes, you need to stagger parallel splices so they do not occur at the same location (it is generally ok/unavoidable that they cross).

 

[RPstress]

Double bias fabric seems to be two plies, one at + 45 and one at - 45, stiched together, the two together can be called a 'layer' of double bias fabric. It's basically two plies of UD. See jloncomposite.com and merrittsupply.com .

If this comes on a roll then I don't see why it needs splicing. If +45/-45 is a layer then turn it over for -45/+45 .

Or have I misunderstood 'double bias fabric' in your case?

As stated above, if you have to splice it then with good resin, such as epoxy or even polyester, 0.5" should be enough (some conservative types might go for 0.75").

 

[mighoser]

Or you could just buy the laminate and cut it to size. Here is one.

http://www.matweb.com/search/datasheet.aspx?matguid=137f96e53ec6473caba99f5f57a0bf04&ckck=1