Smart Fasteners and Their Application in Flanged Joints

Fasteners are widely used in many industrial applications. Their function in many cases is to provide a leak-proof joint at the flange interface. To accomplish this function, fasteners must be clamped with appropriate force. In practice, it is difficult to measure such forces, intermittently or continuously. While measurement using load cells or strain gages is an available option, it tends to be expensive or infeasible due to the constraints imposed by the application. When tightening the fasteners initially or during maintenance, a less accurate method of specifying the bolt-tightening torque for achieving the necessary bolt force is widely followed in industry. These torque values are calculated using published design correlations [1]. Many factors affect such calculations: friction between the threads and collar and flange, age of the fasteners, assumptions about rigidity of the clamped components, being a few among those. Since specific values applicable for individual application are not always be known, fasteners are often over-tightened, resulting in increased stresses in the assembly elements or under tightened, resulting in leakages at the flange interfaces. In the current paper, we introduce the concept of smart fasteners that can visually indicate the tension they are subjected to, and validate it for a widely used industrial fastener size. Results from numerical and experimental studies conducted are presented for UNC 1/2 -13 × 4 1/4^″ smart fasteners. Lastly, relationship between bolt-tightening torque and color intensity of the smart fasteners is provided.