Thermochemical Part Response to Thermally Dimensioned

A cure simulation was conducted to identify the impact of the thermally dimen-sioned, resistively heated CFRP tool on the part’s curing process. The numerical heat zone control strategy developed in Chapter 4 was employed to obtain ac-curate temperature controlled heat introduction in the simulation model in each zone, with a set-point temperature cycle in accordance with the robust single-zone optimization results given in Table 6-7. Heat single-zone distribution and control temperature point placement is shown in Figure 7-10 (d). Figure 7-11 (a) and (b) shows the transient temperature and degree of cure evolution at several location within the part.

Overall, the temperature development in the part closely follows the 1D inves-tigations conducted in the previous Chapter. The largest temperature gradients in thickness direction is apparent at the end of the second dwell in the center of the monolithic section between laminate middle plane (Loc. 2.b) and the top surface (Loc. 2.c). Whereas the simulation accuracy experiment displayed a cure progress from the sides of the monolithic laminate to its center, the temperature evolution at the edge of the monolithic section does not deviate significantly from the center of the monolithic laminate in the middle plane (Loc. 1.a) as well as at the respective surfaces (Loc. 1.b). The temperatures between the foam core and the GFRP spar (Loc. 4.a) showed a significant temperature overshoot of similar value as the monolithic CFRP in the beginning of the last dwell only.

a)

Time [s]

0 2000 4000 6000 8000 10000

Temperature [°C]

0 2000 4000 6000 8000 10000

Degree of cure [-] Part top surface Part middle plane

Figure 7-11Temperature and degree of cure evolution in the thermally dimensioned model: (a) Transient temperature evolution, (b) transient degree of cure evolution, (c) temper-ature contour plots at the end of the second dwell, and (d) degree of cure contour plots at the beginning of the third dwell.

In Figure 7-11 (c), the temperature contour plot at the time of the largest devi-ations at the end of the second dwell is depicted. Given that the relative tem-perature overshoot of the resin at this point is the highest, the part’s surface temperatures at this stage are slightly above set-point temperature with the ex-ception of the locations close to the mechanical stiffeners. The impact of the tool stiffening structure on the part’s thermal behavior can be seen at this point, with local temperature deviations of up to -3^◦C from set-point value at the trailing edge of the blade. An exothermic temperature overshoot in the middle plane of the monolithic section is apparent, given that the applied cure cycle cannot fully prevent this effect from occurring.

It has to be noted that the tool bottom side was subjected to a lower convection coefficient and featured a slightly more deviating behavior of the surface tem-peratures. Since the heating zones completely compensate the mean convection influence, the overall energy introduction in the heating zones increases sig-nificantly with increasing convection coefficient. Thus, the temperature field in heating zones subjected to a lower convection coefficient are more sensitive to lo-cal changes leading to slightly higher surface temperature deviations originating from the thermal impact of the mechanical stiffeners.

Since the degree of cure is a result of the transient local temperature history, a degree of cure gradient was seen at the beginning of the third dwell on the surfaces as well as the middle plane. While a surface degree of cure appeared between 85 and 90 % at this stage in almost all areas except the trailing edge at the top side, the bottom side featured higher degree of cure deviations at the locations of the mechanical stiffeners in the sandwich section. The surface degree of cure at the GFRP spar is slightly elevated to 93 % at the bottom side, as the exothermic reaction leads to a higher surface temperature overshoot at this stage due to low convection impact. The middle plane features a degree of cure of

∼ 95% in the monolithic section. The sandwich material is not curing and the respective area remains white in the contour plot.

Overall, this study shows that manufacturing within the requirements of typical industrial applications is possible with resistively self-heated CFRP tools even for large and complex parts. The surface temperatures of all areas showed only a bigger deviation than 3^◦C, if significant exothermic reaction within the thick laminate occurred. Thus, close to homogeneous surface temperature is possible with these tools for parts with large thickness variations, where a varying heat introduction in space and time is required. It also shows that for a thermally di-mensioned resistively heated CFRP tool, overall surface temperature deviations in operation resulting from varying materials and thicknesses are so low, that an isolated mechanical stiffening structure has a thermal impact on the surface

temperature and degree of cure field which exceeds the general deviations orig-inating from the laminate material and thickness variations. Thus, in order to built a resistively heated manufacturing tool for large parts, development effort should be undertaken to reduce the thermal contact of a stiffening structure to an absolute minimum. If no stiffening structure is required in the tool, close to per-fect temperature distribution should be possible with a thermally dimensioned resistively heated CFRP tool using a limited amount of heat zones, independent of apparent laminate variations of the part.