Impact of Varying Set-Point Temperature Cycles

In contrast to other tools featuring a material with high thermal conductivity, resistively heated CFRP tools are actually capable of a diverse heat introduction according to the local part material and thickness by the application of different temperature cycles in different heating zones. However, the drawback of such an approach is a potentially more inhomogeneous part surface cure behavior and an investigation is required to assess the potential.

To investigate the impact of varying set-point temperature cycles, the thermally dimensioned simulation was adjusted to feature the three previously optimized set-point temperature cycles developed for a multi-zone application. The respec-tive temperature cycles are given in Table 6-7. The multi-zone temperature cycle for the approximate thickness of 30 mm was applied in the monolithic section (green and yellow zones on the right side in Figure 7-10 (d)), the cycle for an approximate thickness of 20 mm was applied on the transition section (dark green in Figure 7-10 (d)) and the cycle for an approximate thickness of 15 mm was applied on all remaining zones, namely the sandwich section and the GFRP spar.

Figure 7-12 (a) and (c) shows the temperature evolution at certain locations as well as the temperature distribution at the end of the second dwell.

Since different temperature cycles potentially increase the surface temperature deviations, the surface temperature evolution of Figure 7-12 had to be inves-tigated and compared to the case with a uniform temperature cycle applied, which is given in the previous section in Figure 7-11. The overall temperature distribution was more homogeneous. The part surface temperatures in this area increased during the second dwell up to∼5^◦Cabove set-point temperature. due to the exothermic reaction in the inner monolithic section. This was not the case in the sandwich section of the part, as the laminate in this section was very thin.

Due to the different cure cycle applied in the sandwich section, the surface

tem-a)

Time [s]

0 2000 4000 6000 8000 10000

Temperature [°C]

0 2000 4000 6000 8000 10000

Degree of cure [-]

Figure 7-12Temperature and degree of cure evolution in the thermally dimensioned model with multiple applied temperature cycles: (a) Transient temperature evolution, (b) transient degree of cure evolution, and (c) temperature contour plots for the surfaces and part middle plane.

perature was elevated to match the surface temperature of the monolithic section.

The temperature deviations apparent between sandwich and monolithic section were in line with the surface temperature deviations occurring in the monolithic section itself.

The single exception was the GFRP spar featuring a≈8 mm laminate thickness with a one-sided isolation by the sandwich core, which resulted in a temperature rise of ∼ 5^◦ C above the overall surface temperature at the bottom side. The bottom side was subjected to a lower convection coefficient leading to a more inhomogeneous surface temperature as it was more sensitive to the location of heat introduction due to exothermic resin reaction or heat zones.

However, the temperature cycle of the 8 mm GFRP spar was optimized to lead to a similar overall temperature and degree of cure profile compared to the 30 mm monolithic section in general and not in the surface plane only. The comparison of the temperature evolutions at Loc. 4.a (inner surface GFRP spar) and Loc. 2.b (center monolithic laminate) in Figure 7-12 (b) shows that the two different temperature cycles led to a close match in the degree of cure evolutions in the GFRP spar and the monolithic section in the part’s interior as well, despite the fact that the laminate thickness vary by a significant amount. Thus, the approach of a multiple temperature cycle application to ensure a more uniform in-plane part cure in the parts interior, which features varying laminate thicknesses, does work.

The optimization procedure developed was capable of providing the appropriate temperature cycles to do so.

Figure 7-13 illustrates these findings. The degree of cure contour plot in a cutting plane through the part is given and compared to the simulation with one overall temperature cycle applied at the end of the second dwell. The cutting plane is located on the bottom side of the upper spar (see Figure 7-13), between the middle plane and the part’s top surface.

t=6441 s Single Cycle Multiple Cycles

Figure 7-13Comparison of degree of cure contour plots between the simulations with one overall temperature cycle (left) and varying temperature cycle (right) at the end of the second dwell.

Since there are some local degree of cure deviations apparent resulting from the impact of the mechanical stiffeners, the mean degree of cure level on the bottom

side of the GFRP spar is similar to the degree of cure in the monolithic section in the same plane. In contrast, in the simulation with one overall temperature cycle applied, the cure of the GFRP spar is delayed, leading to a mean in-plane degree of cure offset of∼ −10 % between GFRP spar and monolithic section in the same plane.

While the improvement in the cure behavior induced by the application of the thermal dimensioning method and different cure cycles was illustrated at certain surfaces and times in this chapter, an overall comparison of the curing behavior in the simulation can be made by the evolution of the standard deviation of all curing elements in the simulation in every time step. Lower standard deviation of the degree of cure in the curing elements resembles a higher degree of ho-mogeneity in the curing process. Given that one degree of cure is given by one element with a distinct volumevi, a weighted standard deviation has to be used to consider the differences in element volume and gain a mesh-independent standard deviation of the degree of cureσ_α in one time step:

σα= Whereαdenotes the mean weighted degree of cure, which is calculated with the mean elemental volumeveaccording to:

α= 1

It has to be noted that, although these expressions enable a general comparison of degree of cure homogeneity of different setups, local degree of cure differences are considered similar to global degree of cure differences. However, the former lead to higher degree of cure gradients and, thus, worse laminate quality. Hence, investigation of the mean weighted degree of cure can only add to a general in-vestigation of the cure behavior and should not be the sole value of consideration in the assessment of the cure behavior of a part.

Four different variants of the simulation model introduced in this chapter were compared via the mean weighted degree of cure:

• Case 1is the simulation accuracy experiment, which contains of the heat-zone distribution made by the manufacturer (see Fig. 7-7) and a cure cycle based on the resin manufacturer’s recommended cure temperature (see Fig. 6-5 (a)).

• Case 2constitutes of the heat zone distribution made by the manufacturer and the numerically optimized temperature cycle originating from a single zone examination (see Table 6-7).

• Case 3 represents the simulation model variant investigated in the previ-ous section, with thermally dimensioned heat-zone distribution and the numerically optimized temperature cycle originating from a single zone examination.

• Case 4 is the simulation model variant investigated in this section, with thermally dimensioned heat-zone distribution and a set of complementary multiple temperature cycles (see Fig. 6-8 (a)).

The evolution of the mean weighted degree of cure during the manufacturing process is given in Figure 7-14 and illustrates, that the biggest increase in the degree of cure homogeneity can be achieved in the application of an optimized temperature cycle (comparison of Case 2 to Case 1) and the thermal dimensioning technique (comparison of Case 3 to Case 2).

Time [s]

0 2000 4000 6000 8000 10000

<,[!]

Figure 7-14Comparison of the mean weighted standard deviation of the degree of cure in the curing elements of different simulation variants.

The application of different complementary temperature cycles led to a further decrease of the maximumσ_αby 7.7 % (comparison of Case 4 to Case 3), although the geometry and material combination of the generic rotor blade does not favor a σα comparison: Due to the non-curing sandwich core, a large portion of the curing volume is located in the monolithic section, which is subjected to the same temperature cycle as in Case 3. The complementary temperature cycles are applied in the sandwich section, where they do lead to overall improved cure be-havior, but the corresponding curing element volume is low in comparison to the monolithic section leading to an overall moderate improvement of theσαvalue.

Nevertheless, a distinct improvement in theσα-value is seen, indicating an

over-all improved cure behavior through the application of a set of complementary temperature cycles in the different heat zones.

7.6 Summary and Discussion

A case study featuring a generic rotor blade was conducted to investigate the thermal and cure behavior of a large part with industrial complexity level man-ufactured by a resistively heated CFRP RTM tool. A comparison of the part temperatures during the manufacturing process in experiment and simulation showed that the model is capable to predict the specific thermal characteristics apparent in the use case. The application of the thermal dimensioning strategy on this simulation model and investigation of its results led to the following conclusions:

1. Although the mechanical stiffeners were thermally isolated by the tool manufacturer to a certain extend, they still had a significant impact on the overall thermal and cure behavior of the part. Effort has to be undertaken to decouple the mechanical stiffeners as much as possible in case of resistively heated CFRP tools.

2. Manufacturing within the thermal requirements of most industrial appli-cations is possible with resistively heated CFRP tools even for large and complex parts.

3. It is beneficial for the overall cure behavior to apply different temperature cycles onto part sections featuring different material or laminate thickness, if the respective cure cycles are carefully chosen. The temperature cycles can be optimized to lead to small in-plane degree of cure deviations in the laminate’s interior instead of homogeneous degree of cure at the surface only. The procedure to derive these cure cycles, which was developed in Chapter 6, resulted in improved part cure at the surface as well as in the inner laminate.

The last-mentioned statement can, in some cases, result in an improvement of the part’s cure behavior induced by a resistively heated CFRP RTM tool in compar-ison to the cure behavior induced by traditional RTM tools made of aluminum or steel. Application of a non-uniform surface temperature onto a curing part in cases where higher temperature gradients are required is challenging with tools made of traditional tool material due to the higher heat conductivity.

Future Work

Although CFRP tools possess a variety of advantages, they are known for their thermal sensitivity and resulting temperature deviations during part production, which in some cases prohibit their widespread application. This work aims to provide the required knowledge and methods to achieve homogeneous part cure in composite manufacturing with resistively heated CFRP tools and, thus, contributes to their application in the industry. The following conclusions and contributions are a result of the presented work:

1. The potentially inhomogeneous temperature field generated by resis-tively heated CFRP tools can be predicted computationally efficient with appropriate accuracy.

The temperature response within one heat zone is controlled at a distinct control point. The power introduction within the whole heat zone is con-trolled to result in set-point temperature at the control point. Thus, temper-ature gradients may occur if the heat balance is location-dependent in the zone. Variations in part thickness and material as well as environmental impacts on the heat flow, such as convection, are potentially contributing to a diverse temperature field within one zone. Since the cure behavior of the part is governed by the tool temperature field, this temperature vari-ation can have a significant impact on part cure behavior and needs to be considered.

Contribution: A computationally efficient numerical surface heat flux con-trol method was developed and implemented in Abaqus utilizing the cure simulation platform provided by COMPRO/CCA. Validation results show that temperatures of resistively heated CFRP tools can be predicted within thermocouple accuracy even if large temperature gradients within one zone occur.

2. Close to homogeneous part surface temperatures can be achieved with resistively heated CFRP tools even for large parts with industrial com-plexity level.

With a sound alignment of multiple heat zones in a designated heating area the manufacturing tool can be tailored to the requirements of the part to be produced. Thus, variations in the part, such as thickness or material changes, can be accounted for, resulting in a close to uniform surface tem-perature.

153

Contribution: A numeric approach was developed to determine the alloca-tion of multiple zones in a resistively heated manufacturing tool. Results of its application show a significantly increased temperature homogene-ity of the tool and an improved degree of cure homogenehomogene-ity in the part.

Additionally, the investigation of two application cases with varying com-plexity level indicate, that transition zones between heated and unheated regions vastly increase the temperature uniformity in a designated heat-ing area. Finally, a justified set of guidelines for the positionheat-ing of control thermocouples was determined.

3. The application of adjusted temperature cycles onto different part areas is feasible with resistively heated CFRP tools, potentially leading to im-proved part cure.

The cure behavior of laminates varies with its thickness since the exother-mic resin reaction increases the local laminate temperature, which in turn increases the rates of chemical reaction. This can lead to slight variation in the part surface temperature, given that low thermal conductivity of the tool material results in a partially insulating behavior between set-point temperature plane and part surface temperature. Additionally, variation in the laminate thickness can amount to in-plane degree of cure gradients in the part’s interior.

Contribution: An optimization scheme was implemented and applied to gain a thickness-robust optimal temperature cycle for laminate manufactur-ing with the investigated resin system. An adjusted optimization technique was developed to determine a set of complementary temperature cycles to be applied in different heat zones of one tool. A comparison of results of the application of multiple complementary cycles in contrast to one overall cycle showed that further cure homogeneity improvement can be achieved.

The numeric case study of a generic rotor blade showed a more homoge-neous in-plane degree of cure distribution at the part’s surface as well as the part interior, if multiple complementary cure cycles were employed.

The content of this thesis contributes to a simulation-based thermal design of resistively heated tools and, thus, addresses one of the major concerns regarding the widespread application of resistively heated CFRP tools. In order to generate more complementary knowledge to pursue an industrialization of these manu-facturing tools the following topics should be content of future investigations:

1. While part material and thickness variations could be handled well in the thermal dimensioning of the case study, the mechanically stiffening rib structure caused local variations in the surface temperature, which were often of greater magnitude than the global variations. To support

the application of CFRP tools onto large structures, further research is required to provide a thermally decoupled stiffening technique of the tool shell laminate. At the same time, experimental investigations have to be conducted to quantify the thermal contact between tool and stiffeners, enabling more accurate simulation results.

2. The application of a reverse engineering approach to determine a con-vection coefficient as well as the thermal properties of the isolation of the mechanical stiffeners led to a close correlation of the simulation model with the experiment in the case study. While this is applicable if a general es-timation of the capabilities of CFRP tools is conducted, industrial thermal tool design requires this information a priori. Thus, experimental work is required to improve the quantification of the thermal behavior, contact, and heat transfer between the part, tool shell, mechanical stiffener and ambi-ent air. A potambi-ential tool-part separation in the RTM manufacturing process could not be determined in the temperature evolution of the measurement thermocouples but might occur in other cases. Through combination of this experimental work with a coupling of CFD and cure simulation, generally applicable rules or guidelines might be found to appropriately model the convection impact in a sole cure simulation without prior experiments re-quiring the whole manufacturing tool. Additionally, a full coupling of cure simulation with a mechanical analysis might give an indication of the oc-currence as well as thermal and cure impact of tool-part separation in a closed mold.

3. The current state of development of resistively heated tools does not in-clude controlled tool cooling, which would significantly contribute in the reduction of the risk of a temperature overshoot in case of thick laminate manufacturing. Controlled cooling of resistively heated tools would offer new possibilities in the temperature cycle choice and reduce cycle times, as well.

4. CFRP tools offer the advantage of low thermal expansion which, in general, is potentially beneficial for the reduction of process-induced deformations.

However, there might be limits in the applicability of this assumption as the tool material itself features mechanically directional laminate response and is subjected to a certain temperature variation through the cure cycle.

The impact of the tool material CFRP onto process-induced deformations of the part upon demolding should be content of future investigations.

5. While most researchers in literature agree that homogeneous temperature and degree of cure distribution results in improved laminate quality,

quan-tified information on the impact of temperature and degree of cure gra-dients on laminate load carrying capability is scarce. To gain information how restrictive the allowable for in-plane and out-of-plane temperature and degree of cure homogeneity has to be set, a variety of studies needs to be conducted. This includes an experimental determination of the resin strength as well as the interface strength as a function of temperature and degree of cure, micro-mechanical investigations on the occurrence of resin cracks and macromechanical validation of the calculated stress states in the process simulation.

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Im Dokument Thermal Design of Resistively Heated Tools for Composite Manufacturing  (Seite 176-200)