초록
Ti-6Al-4V 재에 다양한 표면처리방법을 통하여 나노스킨화된 피로시험편을 제작하고 회전굽힘피로 시험과 축하중 인장압축피로시험을 수행하였다. 특히 S-N 곡선에서 초음파나노표면개질(UNSM)법은 국내특허기술로 다른 처리법들보다 우수한 결과를 보였고, UNSM 처리된 직경 5 mm 재는 6 mm 티타늄재를 대치 가능할 성능을 얻었다. UNSM 처리된 $10^6$ 사이클 이상의 장수명영역에서 피로강도가 크게 향상되는 시험결과를 얻었다. 균열발생 패턴의 분석을 통하여 Ti-6Al-4V 재는 응력의 작용형태에 따라 표면균열발생형(surface originating crack type)과 내부균열발생형(interior originating crack type)인 어안균열(fish eye crack)이 발생하는 메커니즘을 분석하였다.
Nanoskins were fabricated on a Ti-6Al-4V material by carrying out various surface treatments, i.e., deep rolling, laser shot peening, and ultrasonic nanocrystal surface modification (UNSM). These surface treatments are newly developed techniques and are becoming more popular for industrial applications. Fatigue tests were carried out using material test system (MTS); these tests included the axial loading tension-compression fatigue test (R = -1, RT, 5 Hz, sinusoidal wave) and rotating bending fatigue test (R = -1, RT, 3200 rpm). The analysis of the crack initiation pattern in the UNSM-treated material indicated that the crack was interior originating in the axial loading tension-compression cycle, and was surface originating in the bending fatigue test. UNSM treatment significantly improved the fatigue strength for the regime of above $10^6$ cycles that S-N curve of rotating bending stress clearly show the performance of a 5 mm titanium specimen after UNSM treatment is similar to that of an untreated 6 mm titanium specimen.
