Minutes of the August 14, 2003 Tracker Meeting on Composites Issues

Present: Ben Rodini, Chris Fransen, Erik Swensen, Bernie Graf, Robert Johnson, Jim Martin

The discussion began with sidewall issues. Ben noted that the fiber volume that we expect is rather low. Erik said that it is the COI recommendation. COI says that these fibers become difficult to work with if the resin content is lower. Ben said that he calculates 53% fiber volume for the K13D and 45% for the YS90. The layup of the two together will be somewhere in between. Erik said that although their initial analysis assumed 60% fiber volume, all of the final analyses assumed the values recommended by COI.

Ben cautioned that measuring the fiber volume content in the hybrid layup (K13D and YS90) can be tricky. However, he noted that the similarity of the two densities should help a lot in avoiding errors.

Ben noted that Sandro’s Powerpoint presentation does not use the correct values of laminate density when deriving the ply thickness. He will send the details to Robert, but the net result is that with Ben’s changes to the calculation the expected ply thicknesses agree nearly exactly with the COI projections. Sandro’s measurements of the interior thicknesses of the panels also agree well with the COI predictions.

There was a lengthy discussion of pull testing. Sandro had informed us that the coupons pulled by Plyform all broke at the grip. Ben referred us to ASTM D-3039. This specification allows fiberglass tabs to be used in the clamping regions of the pull specimens. The document shows in detail how to implement the tabs correctly. Ben said that if not used correctly they can do more harm than good, but the basic idea is to reinforce both sides of the coupon in the clamping region with about 1/16 inch of fiberglass bonded to the carbon-fiber laminate. This prevents the grips from damaging fibers. The fiberglass needs to taper to reduced thickness toward the specimen, to avoid a stress riser at the edge of the tab. The tabs on opposite sides of the laminate need to be accurately aligned. The edges of the specimen need to be carefully and cleanly machined to avoid edge damage that could propagate inward. Finally, Ben thought that 30 plies for tensile testing is on the thick side, requiring more clamping force than would be needed for pulling a thinner specimen.

Ben mentioned that compression strength tests are probably more important to us than tension and can also be problematic to carry out correctly. He asked if we were planning to compression tests. Robert replied that Plyform did do some tests, but only one value was reported so far, which was a factor of 3 lower than expectations.

Ben requested photographs of the tension and compression test samples that were already tested.

Ben asked what was planned for acceptance tests. Robert said that the procedure had yet to be completely documented. Ben suggested that flexure tests or short-beam shear tests could be used to avoid having to deal with issues of pulling against grips. More analysis work would have to be done to correlate those tests with the parameters of interest, however.

There was a discussion of void content. Ben considered the Plyform measurements to be high. He suggested that the process could probably be altered to lower it. One suggestion was to apply a vacuum bag and pump out the air after each application of several plies. Void content may also be affected by the cure cycle. Ben asked whether Plyform is using a COI recommended cure cycle specific to the COI prepreg. The cure cycle can also affect the squeeze-out at the laminate edges. The width of the edge effect is rather large in the existing panels. Ben said they normally cut away about 2.5 cm of material, but in Sandro’s measurements the edge thinning extended inward by about 7 cm. Ben said that cyanate resins tend to have very low viscosity when hot. There are a number of possible variations to the cure cycle. For example, instead of doing a linear ramp all the way up to a plateau at, for example, 350 degrees F, it is common to ramp up first to an intermediate temperature, hold there for some time while the resin flows, and then ramp up to 350 F.

Ben asked to see the new drawing of the sidewalls. BJ needs to update it first, to remove the extra K13D layers that were added to the drawing.

Jim posed two questions for the meeting (unfortunately the conference system crashed and we lost the Swales connection at about this point):

1. Can we proceed with the prepreg order based on what we know?
2. What plans should we make for the unused prepreg available at Plyform?

The proposed answers to these questions are as follows:

1. Everybody seems to be in agreement that none of the problems encountered in the first attempt at sidewall fabrication are due to the prepreg. The problem with the missing YS90 layers is being resolved by clarifying the drawing and specifications. The ply thickness problem is clearly an edge effect that can be resolved by using only the interior of the laminate for sidewalls and for coupons. All agree now that the original COI thickness estimates are close to the mark and that with the existing design the walls will be close to the specified 1.5mm. Therefore, there is no reason not to order the prepreg exactly as was done in the previous iteration. SLAC needs to determine in the next day or two whether Plyform actually put in an order to COI before holidays. If not, then the SLAC order to COI should be released, in order to obtain new prepreg at the beginning of September.
2. With the unused prepreg already at Plyform we should produce 1 or 2 panels, depending on the amount available, of the actual design:
1. The revised drawing, including the clarified stackup, should be finalized by BJ, reviewed, and sent to Plyform.
2. We need to clearly specify that there should be zero resin loss in the panels. Edges with resin bleed must be cut away (about 7 cm if nothing else is improved in the process).
3. We should discuss with Plyform the cure schedule. If possible it should conform to COI recommendations.
4. We should discuss with Plyform process details that might lower the void content.
5. Coupons should be cut from the panels to perform ASTM tensile and compressive strength tests.
6. The ASTM D-3039 guidelines should be followed to incorporate tabs and pull the coupons in such a way as to obtain valid strength measurements.
7. If there is enough material, joint coupons (M2.5 and M4 inserts) should also be produced and sent to Hytec to be pulled.

There was a short discussion of goals for Ben’s visit to Hytec. The conclusions:

1. Study the damage to bottom tray SN001.
2. Discuss and review the repair procedure.
3. Review the Tracker sidewall drawings.
4. Review the plans for sidewall test coupons and testing.

Robert mentioned Erik’s concerns that the CLA results that we have in hand were obtained with a very different Tracker model from what we have now (the grid and other things included were probably also different). In particular, the bottom tray and the tower/grid interface is much stiffer now (the resonance frequency is doubled). Erik suggested analyzing the grid plus tracker at SLAC using the CDR reduced Tracker model (which is currently up to date). The goal would be to extract flexure loads to check that they have not increased with respect to the present CLA results.

Erik said that their working concept for qualifying the flexures is to carry out a static test with just two of the corner flexures detached. The test will be along the diagonal direction (xy), and the two corner flexures lying on the diagonal perpendicular to the pull direction would be detached. They estimate that this should not overload the composite structure. SLAC is preparing an increment to the contract for doing a complete analysis of this, as well as the actual test. The analysis will have to wait until the beginning of September, due to conflicting work load at Hytec.