5.5.3. Lattice Core-PCM ITPS

The lattice core/PCM solution faces milder thermo-mechanical loads compared to the corrugated core ITPS (cf. Section 5). The maximum top face sheet temperature reaches 618 K, whereas the inner face sheet remains at the initial temperature of 300 K (see Figure 15). Due to the direct bonding of different materials, their difference in coefficient of thermal expansion (CTE) leads to high thermo-mechanical stresses. To calculate these stresses, the temperature field through the thickness of the ADD is required. Therefore, in a first step, we calculate the temperature field by a steady-state heat transfer simulation in Abaqus® 2020.

The thermo-mechanical model consists of linear shell elements to model the three face sheets and linear beam elements for the lattice struts. The structural analysis is split in two load steps:

First, the previously calculated temperature field is applied to the model with respect to its stress-free initial state at a temperature of 298 K (room temperature). The CTEs of the materials are assumed constant over the entire temperature range. For the FE model of the lattice-PCM ITPS solution, the same constraints are used as in the simulations of the other two TPS concepts (see Figure 7).

In a second load step, the dynamic pressure during re-entry is applied to the outer face sheet of the sandwich structure in addition to the persisting thermo-mechanical loads. The resulting deformation is plotted in Figure 19, showing a maximum displacement of 42 mm.

The results of the structural analysis for both load cases are summarised in Table 5. It can be observed that the greater part of the deformations and stresses arises from the thermal gradients, not from the additional pressure load. The comparison with the allowable yield stresses leads to the conclusion that the thermally optimised design is not feasible from a mechanical design point of view.

In all layers made from Inconel 718 (i.e., face sheets and inner lattice core), stresses do not exceed the allowable to a level that could not be managed by further optimization of geometric parameters.

**Figure 19.** Deformation of the outer face sheet in millimetres on the lattice/PCM model for combined thermal and pressure loading. Cutouts represent the areas with local effects around the nodal constraints that were ignored in stress evaluations.

**Table 5.** Results of the mechanical simulation for the lattice core/PCM solution (see evaluated area in Figure 19).


The lattice in the outer core is made from an additively manufactured CuCr1Zr alloy with a yield stress of 310 MPa. The yield stress in the CuCr1Zr struts is exceeded by a factor of more than two in both load cases. It can therefore be concluded that the design resulting from the thermal optimization is not feasible from a mechanical engineering point of view. This requires the choice of a different material for the outer lattice core that has high strength and at the same time good thermal conductivity, e.g., tungsten.
