VACUUM IMPREGNATION OF COMPACTED GLASS FABRIC

Vacuum Impregnation Of Compacted Glass Fabric: 

Vacuum Impregnation Of Compacted Glass Fabric CryoPrague 2006 ICMC’06 Tuesday 18th July 2006

Overview: 

Overview Introduction Magnet and NB3Sn manufacturing process Aims Sample preparation Material Tests Short Beam Shear, void and density measurements, microscopy Discussion Conclusion

Introduction: 

Introduction The current LHC design uses 10T NbTi final focussing dipoles. The planned upgrade to the LHC will require stronger 15T final focussing magnets. 15T is beyond the limits of NbTi, so these new dipoles would have to be made using Nb3Sn. Nb3Sn is a very brittle material and requires a specialist manufacturing to produce magnets.

Wind and React: 

Wind and React Due to the Brittleness of Nb3Sn magnets cannot be produced by conventional winding. Instead a precursor material, XX, is used. Glass fibre tape or braid is wound round the cable. The cable is then wound into the required shape and compressed in a mold to keep the required shape. This demands high tolerances to get the correct magnetic field. The precursor is then heat treated at ~600°C for ~10 days to form Nb3Sn. The magnet is then impregnated with a resin system to retain its shape.

Impregnation under stress: 

Impregnation under stress A high mechanical force is needed to keep the coils in place within the tool. This force is transferred to the glass fibre insulation which is then impregnated with resin. The resin is then cured at temperature. We looked at using at the effect this compaction stress had on the glass fibre-epoxy composites.

Aims: 

Aims Fibrous Composite structures are usually formed under pressure. This gives higher fibre contents and generally improved mechanical properties. However if the stress used is too great it can lead to low quality laminates. These tend to be opaque and have reduced mechanical strength. We intend to find out why this happens and to find the safe working limit of stress.

Sample preparation: 

Sample preparation Laminates were made by using stacks of 32 layers of E-Glass. Made by the vacuum infusion process. The vacuum draws the resin through the glass fibre stack. The load was supplied using a Testometric tensile testing machine set to load control. This automatically kept the machine load constant. Heat was applied using temperature controlled infrared lamps. The laminates were cured under stress.

Testing: 

Testing Carry out Short beam shear testing following ASTM 2344 at 77K. Density measurements Void content measurements Determined by loss on ignition Gives wt% of glass and fibres Theoretical density Void content calculated from difference between theoretical and actual density. Microscopy Sections of glass fibre-epoxy were cut using a diamond saw and polished using diamond compound.

Short beam shear results: 

Short beam shear results

Short beam shear results: 

Short beam shear results A sharp drop in shear strength occurs above 2MPa Correlates with the opaque appearance Thought to be due to voids due to entrapment of air during impregnation The Resin was unable to penetrate between the tightly packed bundles of filaments at higher compaction stresses

Density and WT% Glass results: 

Density and WT% Glass results

Density and Wt% Glass: 

Density and Wt% Glass . Density peaks at 3MPa The drop in density at higher applied stresses could be due to void content. Coincides with increased opacity of the laminate. The fibre content plateaus above 3MPa, ~ 82% glass content by weight probably due to packing limit of the glass fibres. G10 or G11 laminate typically 66-72 wt% glass.

Combined results: 

Combined results

0.1MPa sample: 

0.1MPa sample Approximately one third of the area is composed of resin-rich volumes between fibre bundles.

1MPa: 

1MPa At 1MPa the resin rich areas seen earlier are absent.

4MPa: 

4MPa This suggests that the bundles are not completely impregnated with epoxy. At 4 MPa many individual fibres were observed sprung out of the bundles.

Conclusion: 

Conclusion Applied mechanical stress up to approximately 2 MPa has a beneficial effect on short beam shear strength and glass content of epoxy-glass fibre laminates produced by vacuum impregnation. Beyond this applied stress the glass content does not increase and shear strength is reduced. At a stress of 10 MPa the laminate is not fully impregnated and shear strength is reduced to one third of a high-quality laminate. All results suggest that visual inspection is a good guide to laminate strength. These results provide useful data for magnet insulation systems.

Acknowledgments: 

Acknowledgments This work is supported in part by the European Community-Research Infrastructure Activity under the FP6 "Structuring the European Research Area" (CARE, contract number RII3-CT-2003-506395)