Applying the Stressing State Theory and Network‐Free Normalization Method Reveals the Failure Law of the Compression‐Loaded Stiffened Composite Panels With Stiffener Runout

ABSTRACT Aircraft composite structures often feature stringers terminating near spars or cutouts, creating abrupt stiffness changes and stress concentrations prone to skin‐stringer debonding under compression. This study investigates the failure characteristics and damage evolution mechanisms of stiffened composite panels with stiffener runout (SCP‐SRs) from a thermodynamic perspective, based on stressing state theory. First, compression experiments were conducted on SCP‐SR specimens to obtain their strain responses and identify failure modes. Subsequently, the strain data were modeled as state variables of the structure. Using the network‐free renormalization approach, we extracted modal features and characteristic parameters that describe the stressing evolution process of the structure. By integrating the clustering algorithm, the phase transition loads at both partial and overall scales of the structure were identified. The stability of these phase transition loads was further verified using Wilson's theory. The results show that the standard deviations of the phase transition loads at the points of elastoplastic branching (EPB), failure starting (FS), and progressive failure (PF) were relatively low, with maximum values of 0.306, 0.458, and 0.416, respectively, indicating good repeatability and robustness of the method. The theoretical framework can provide a new engineering reference for strength design and failure analysis of large composite structures.