Stress Intensity Factor Solutions for Fasteners in NASGRO 3.0

Abstract

Fasteners are widely used in aerospace and other industrial applications. Structural integrity analysis of such components is a critical necessity in many of the applications. The present paper describes some recently obtained stress intensity factor solutions for cracked threaded members and for cracked fillet areas under bolt heads. The cracks were assumed to be thumb-nail shaped and to originate at the thread roots or fillet radii. The assumed aspect ratios were based on observed shapes from crack growth tests. The tests were conducted on A286 steel, Ti-6Al-4V titanium alloy and 7075-T73 aluminum alloy. Crack growth was measured using marker bands that result from changing the load level after a certain number of cycles. The measured crack growth rates were converted to stress intensity factors using certain principles of similitude. Three-dimensional finite elements were used to verify the solutions. Finite-element analysis was also used to compute the stress concentration factors at the thread roots and fillets. A distinction was made between rolled and cut threads. In the case of rolled threads or fillets, the residual stresses cause a reduction of the stress intensity factor for small cracks. In the case of machined threads or fillets, the stress concentration factor at the thread or fillet root governs the magnitude of the stress intensity factor. Specific solutions were constructed and coded in for standard aerospace bolt sizes, but provision was made for other sizes that may be applicable for general industrial use. The solutions are valid for metric as well as for U.S. customary sizes. These solutions were developed for implementation into the NAS-GRO 3.0 software. NASGRO is a state-of-the-art software package for fatigue crack growth and fracture mechanics analysis.