SBIR-STTR Award

The residual stresses that result from coldworking a hole, or using an interference-fit fastener, can significantly increase the life of a structure subjected to fatigue loading. Manufacturing practices cause variability in the effectiveness of cold-worked holes and interference-fit fasteners. Furthermore, there currently are no 100% inspection techniques that can be used in situ on aircraft. Thus, the beneficial effects of resultant residual stresses are not accounted for in the structural analysis used for aircraft inspection intervals. TEC proposes to use advanced x-ray diffraction (XRD) techniques and instrumentation to measure residual stresses at and around cold-worked holes and interference-fit fasteners. Measurements parameters will be developed to subsequently allow quick in situ inspections of aircraft. Validation of the parameters will be performed on standard samples with cold-worked holes. Residual stress data will be reviewed by VEXTEC to propose a life prediction methodology. A conceptualization of a health assessment system will be prepared. Development of the health assessment system would occur in a phase II activity.

Benefits:
The proposed work will validate advanced XRD techniques and instrumentation for residual stress measurement of cold-worked holes and interference-fit fasteners. This quick, accurate technique will allow the Air Force to take credit for beneficial residual stresses. This work will lead to a Phase II activity in which a health assessment/life prediction system will be developed. This unique system will result in tremendous savings to the Air Force in terms of aircraft safety, availability, and reduced maintenance.

Keywords:
Cold-worked holes, interference-fit fasteners, x-ray diffraction, residual stress management, life prediction, health assessment, structural integrity, fatigue modeling