JEC COMPOSITES MAGAZINE - Issue #76 - October/November 2012 - 68

thermoplastic
Test validation Multiple tests were performed on both configurations per the setup shown in Figure 6. The load vs. displacement curves of representative tests from each configuration are plotted together in Figure 16. material from outside the mould, and 2) migrating resin and 0° fibres from the flanges and web. The physics of these two mechanisms are described in the following sections. New 90° material As the molten laminate begins to experience pressure forces from the cavity and core, an internal pressure inside the laminate begins to build up. While the matrix remains above its melt temperature it behaves hydrostatically. The hydrostatic pressure acts radially into the vacant corner. The radial pressure forces the outside ply – that is the ply along the outer mould line (OML) adjacent to the cavity – into the corner. The outside ply, which must be a 90° ply to avoid coming apart during forming, experiences significant tension as it is forced to conform against the cavity from radial pressure. On the web side, the tension in the external 90° ply is reacted by the core coming in contact with the cavity. On the flange side, however, the only force on the 90° ply resisting the tension caused by radial pressure is friction against the cavity. The frictional force is a function of the draft angle, the forming pressure and the cavity surface treatment. Special coatings on the cavity allow the outside 90° ply to overcome frictional force and entirely conform to the cavity, thereby pulling in new material from outside the matched-metal region. This mechanism is illustrated in Figure 18 and demonstrated in the following micrographs.

Fig. 16: Test Validation

The 4.3-mm (0.17 in) constant-thickness stringer begins to yield at 7.20 kN (1619 lbs). The 1.5-mm (0.06 in) novel stringer does not yield prior to ultimate failure at 13.16 kN (2958 lbs), over 80 % higher than the constant-thickness stringer. There is no ultimate strength improvement, which is a function of web thickness after the radius fully crushes.

Producibility validation
A key question often asked is: “Where does the material come from to fill the radius?” The area in question is graphically illustrated with the question mark, ?, in Figure 17. The laminate being formed is constant thickness and no additional fillers are added to the radii, yet the nominal tool configuration defines a formed part which is more than 30 % thicker at the radius than the nominal tailored blank. If the final part is produced according to the tooled configuration without bridging at the external radius, then material must be supplied to fill the void.

Fig. 18: Physics of Material Migration

For this example 16-ply laminate, the net result is a 30 % cross-sectional area increase relative to a constant-thickness corner of the same internal radius. The boundaries of the area for relative comparison are defined by horizontal and vertical planes at the tangency of the internal radii, as shown by vertical lines in Figure 18. Migrating resin and 0° fibres This new 90° material accounts for some but not all of the material needed to fill the corner. The remainder of the material originates in the flanges and web, and is migrated as molten flow by the same hydrostatic forces. Approximately 50 % of this migrated material is resin, while the other 50 % is 0° fibres moving freely with the resin. This is observed in micrographs by the thickening of the 0° plies in the corner, with corresponding thinning in the webs and flanges. The flow of 45° fibres within the laminate is very limited, and no flow of 90° fibres has been observed. The flow of molten resin and 0° fibres is illustrated in Figure 18.

Fig. 17: Missing Material in Corner

Physics of material migration Migration of material into the corner is accomplished through careful tailoring of the laminates and proprietary tooling techniques. There are two principle sources of material to fill this area: 1) new 90° ply

68 jec composites magazine / No76 October - November 2012

JEC COMPOSITES MAGAZINE - Issue #76 - October/November 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #76 - October/November 2012