A Progressive Damage Model for Predicting Tensile Performance of Composite Single-Bolt Double-Shear Joints with Prefabricated Hole-Edge Delamination: Unveiling Size-Dependent Delamination Behavior
This study combines experimental testing and a validated 3D progressive damage model to demonstrate that prefabricated hole-edge delaminations significantly influence the tensile performance and failure propagation of composite single-bolt double-shear joints, revealing a size-dependent behavior where ultimate load decreases by 5.7% as delamination diameter increases from 0 to 28 mm.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Modern aircraft are increasingly built from layers of carbon fiber and resin, a material known for being incredibly strong yet remarkably light. This composite material allows planes like the Boeing 787 and the Airbus A350 to fly further and burn less fuel than their older metal counterparts. However, these lightweight structures still need to be held together, and the most reliable way to do this is with bolts. When engineers drill a hole to insert a bolt, the process can sometimes cause the layers of the material to separate slightly from one another, a defect known as delamination. This separation often happens right at the edge of the hole where the bolt sits. While a small separation might seem harmless, it can act as a hidden weak spot that grows under the stress of flight, potentially leading to a catastrophic failure. Understanding exactly how these hidden flaws affect the strength of a joint is critical for designing safer, more efficient aircraft.
Researchers at Nanjing University of Aeronautics and Astronautics and the AVIC Chengdu Aircraft Design and Research Institute set out to solve this puzzle by studying a specific type of connection: a single bolt holding together two overlapping pieces of composite material, a setup known as a single-bolt double-shear joint. They wanted to see how the size of a pre-made separation at the hole's edge would change the joint's ability to hold a load. To do this, they created a series of test specimens using a high-strength carbon fiber composite. Some of these specimens were perfect, with no defects, while others had a circular separation introduced between specific layers of the material, with diameters of 12, 20, and 28 millimeters. They then pulled these joints apart in a controlled test to see how much force they could withstand before breaking.
The team did not stop at physical testing; they also built a detailed digital twin of the experiment. Using advanced computer modeling, they simulated the complex way the material breaks, layer by layer, and how the separation spreads. This digital model allowed them to track the invisible cracks and stress points that are difficult to see in a physical test. The computer predictions matched the physical test results with remarkable accuracy, differing by less than ten percent. This success gave the researchers a powerful tool to explore scenarios they could not easily test in a lab, such as changing the shape of the flaw or moving its location within the material.
The physical tests revealed a clear and expected trend: the larger the pre-made separation, the weaker the joint became. The perfect specimens held an average of 16.44 kilonewtons of force. When the separation diameter grew to 28 millimeters, the average strength dropped to 15.51 kilonewtons, a reduction of 5.7 percent. While this percentage might seem small, in the world of aerospace engineering, every bit of strength counts. The researchers observed that the failure was not a sudden snap but a gradual process. As the load increased, the material around the bolt hole began to crush and the fibers to buckle, while the separation between the layers grew. The digital model showed that this damage started on the side of the hole where the bolt was pushing against the material and spread outward in a semi-circle.
Perhaps the most surprising discovery was how the size of the flaw changed the way it behaved. When the pre-made separation was small, the damage did not stay contained; it spread far beyond the original defect, racing outward through the material. However, when the separation was large, the damage stayed mostly within the boundaries of the pre-made flaw, with very little extra spreading. This suggests a critical threshold: small defects are dangerous because they trigger a chain reaction that destroys the surrounding healthy material, while large defects are dangerous simply because they are large, limiting the material's ability to share the load but not necessarily triggering a massive runaway failure.
The researchers also used their computer model to test how other factors influenced the joint's strength. They found that the location of the separation mattered significantly. A flaw located near the middle of the material's thickness was more damaging than one near the surface, because the layers in the middle are less constrained by the bolt's pressure and can spread more easily. The shape of the flaw also played a role; a square-shaped separation caused more damage than a circular or elliptical one, likely because the sharp corners created more intense stress concentrations. Similarly, the orientation of an elliptical flaw mattered, with a specific alignment causing the greatest loss of strength.
Finally, the study looked at how the assembly process itself could help or hurt the joint. They found that tightening the bolt more firmly, known as increasing the preload, actually made the joint stronger by squeezing the layers together and holding the separation in check. Conversely, if the bolt was slightly smaller than the hole, creating a gap, the joint became significantly weaker. The thickness of the metal parts holding the joint together did not change the maximum strength much, but it did affect how stiff the joint felt under a light load. These findings provide a clear roadmap for engineers, showing that while pre-made flaws are inevitable, their impact can be managed through careful design, proper assembly, and an understanding of how the material behaves when it is damaged.
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