Repair of cracked holes using cold expansion

Fatigue tests have been conducted on open hole and bolted joint specimens with cold expansion applied at different stages of the life. The results showed that for the open hole specimens fatigue life enhancements would occur provided that cold expansion was carried out before a crack of 3mm had formed. The optimum enhancement occurred with cracks of less than about 1mm present. Directional expansion, where holes were preferentially expanded with the largest cracks at the mandrel outlet face, was shown to give much improved fatigue performance. Cold expansion of joints showed that life enhancements were dependent on load transfer and secondary bending characteristics. It was also shown that fatigue endurances of joints cold expanded when cracks of less than 2mm had formed were similar to those of joints where cold expansion had been carried out at manufacture.