Experimental Investigation and Failure Criterion Development for CFR Composite Tubular Specimens Under Combined Axial and Pressure Loading

Abstract Carbon-fiber-reinforced (CFR) composites in aircraft structures are subjected to complex, multiaxial loading conditions that may induce fatigue damage prior to final failure. To ensure structural safety, reliable failure criteria must be established for both undamaged and fatigue-affected materials. This study presents experimental investigations of tubular CFR composite specimens subjected to combined axial force and internal pressure, generating complex stress states in the thin-walled gage section. The specimens were loaded to failure along various stress paths, enabling construction of a failure surface in principal stress space. Three distinct failure modes were observed: resin matrix puncture, longitudinal cracking, and circumferential cracking with specimen separation. A probabilistic approach was introduced to account for the large scatter in experimental data, replacing deterministic failure stresses with stress values corresponding to specified survival probabilities. The results indicate that the maximum principal stress criterion, formulated in three-dimensional principal stress space with axes aligned to fiber directions, provides a suitable framework for the investigated composite. Incorporating probabilistic assessment improves reliability in predicting composite failure under complex loading.