Mechanical Performance and Failure Modes of Bolted Joints in Hybrid Plain‐Satin Woven <scp>GFRP</scp> Laminates

ABSTRACT Bolted joints are widely used in aerospace composite structures for assembly and maintenance, but stress concentration and bearing damage can limit reliability. The behavior of joints in hybrid plain‐satin woven glass fiber‐reinforced polymer (GFRP) laminates remains insufficiently understood, particularly when countersunk fasteners, insert‐containing designs, joint geometry, and hygrothermal conditioning are coupled. This study investigates single‐shear bolted joints in such laminates using seven room‐condition configurations and one conditioned counterpart. The effects of bolt‐head geometry, complete insert‐containing joint design, aperture‐based edge‐distance ratio (), and conditioning were evaluated through load–displacement response, nominal bearing strength, secant stiffness, failure morphology, and micro‐CT observations. For insert‐free joints, protruding‐head bolts provided higher peak load, nominal bearing strength, and displacement tolerance than countersunk‐head bolts. The insert‐containing designs increased peak load for both bolt types, especially countersunk joints, but reduced nominal bearing strength because the laminate aperture and nominal bearing area were enlarged. Their stiffness effect depended on bolt‐head geometry: secant stiffness increased in protruding‐head joints but decreased in countersunk‐head joints. For insert‐containing countersunk joints, gave the highest peak load among the tested ratios, indicating a non‐monotonic edge‐distance effect. Hygrothermal conditioning reduced the peak load by 19.8%, while the conditioned insert‐containing countersunk joint retained a stable post‐peak load‐carrying plateau. Overall, joint performance is governed by coupled interactions among bolt‐head geometry, insert‐mediated confinement, aperture size, edge distance, and environmental degradation.