Multi-scale characterization and progressive failure analysis of stitched aerogel sandwich structures for lightweight thermal protection systems

Stitched aerogel-based sandwich structures are promising candidates for lightweight thermal protection systems (TPS), but their mechanical characterization remains challenging because of the large stiffness mismatch between the aerogel core and stitching threads, the weak strain response of some deformation modes, and the sensitivity of inverse identification to measurement noise. This study develops a multi-scale experimental–numerical framework to characterize the macroscopic equivalent properties and in-situ constituent behavior of stitched aerogel TPS sandwich structures under bending. First, full-field strain data obtained by Digital Image Correlation (DIC) are integrated with Finite Element Model Updating (FEMU) to identify the macroscopic equivalent elastic constants of the stitched core. To improve the robustness of the inverse problem, a multi-step decoupled identification strategy is proposed by exploiting the orthotropic response of the core, in which the transverse shear modulus (G₁₃), transverse normal modulus (E₃₃), and longitudinal normal modulus (E₁₁) are extracted sequentially. Virtual experiments and noise-sensitivity analyses demonstrate that this strategy mitigates the ill-posedness associated with the weak ϵ₁₁ signal and enables stable identification under realistic measurement noise. Second, a Representative Volume Element (RVE) homogenization model is established to relate the identified macroscopic out-of-plane stiffness to the in-situ compressive modulus of the ceramicized quartz stitching threads, which is determined as 1365.8 MPa for the investigated material system. Finally, progressive failure simulations are performed to evaluate the effects of stitching geometry on damage evolution and bending tolerance. For the investigated stitched aerogel TPS sandwich structure under quasi-static three-point bending and within the studied range of thread diameter d=1.5–4.0 mm and stitch pitch p=10–20 mm, the macroscopic out-of-plane stiffness increases approximately linearly with stitch volume fraction. Increasing the thread diameter is found to be more effective than solely reducing the stitch pitch in suppressing crack propagation and improving global damage tolerance, whereas pitch reduction exhibits diminishing returns beyond a certain density. The proposed framework provides a quantitative basis for mechanical characterization, progressive failure analysis, and design optimization of stitched sandwich composites for lightweight TPS applications.