Damage Tolerance Substantiation and Certification of Multiple Fastener Joints

Damage tolerance (DT) requirements for rotorcraft structures are specified by FAA FAR 29.571 and 29.573 (Ref. 1). In broad terms, DT requires flight critical structure to tolerate certain damage without the damage becoming the source of crack growth and failure. The requirements can apply to multiple fastener joints (MFJ), such as riveted structures widely used in airframe applications, engine mounts, driveshafts, control tubes, etc. (both composite and metallic). Damage in such structures may be benign due to the multiplicity of fasteners to redistribute loads, and multiplicity of holes to stop the growth of damage. DT requirements for metallic MFJ structures are met by analysis of crack growth from the critical hole to failure. Stress intensity factors and material data support this type of analysis (Ref. 2-5). However, traditional analysis neglects the effects of adjacent holes and surrounding structure, which can produce inaccuracies. This paper presents a method for quickly simulating progressive growth of a crack from a rivet hole to an adjacent hole, and beyond. The method is more accurate than traditional methods based on stress intensity factor (SIF) solutions, and much more efficient than finite element methods. A test program was developed to validate the analytical approach, as well as to investigate potential improvements to MFJ designs for damage tolerance and fatigue. The testing employs a crack growth monitoring system that allows detection of multiple crack initiation locations and documentation of crack propagation in a continuous manner, on both sides of the specimen.