Karakteristik lelah baja karbon rendah bertakik U akibat beban amplitudo fluktuasi

MULYADI, Rodesri, Ir. Jamasri, Ph.D

2005 | Tesis | S2 Teknik Mesin

Penelitian ini bertujuan menyelidiki karakteristik lelah baja karbon rendah dengan variasi takik U radius 8 mm dan 1,5 mm. Beban bending dengan amplitudo konstan dan berfluktuasi dibebankan pada spesimen untuk mengamati tingkat penurunan dan batas ketahanan lelahnya terhadap variasi takik serta mengidentifikasi penampang patahannya. Mesin yang digunakan adalah rotary bending fatigue machine yang menghasilkan beban dinamik beramplitudo konstan pada putaran 2850 rpm, serta dilengkapi peralatan beban kejut untuk menghasilkan amplitudo beban fluktuasi melalui pengaturan mekanisme kerja cam. Spesimen uji diberi perlakuan panas quenched dan annealed serta dibandingkan umur lelahnya terhadap spesimen non treatment. Selanjutnya ketahanan lelah hasil pengujian pada amplitudo beban fluktuasi dapat dibandingkan terhadap amplitudo beban konstan. Batas ketahanan lelah spesimen takik U radius 8 mm dan 1,5 mm juga dianalisa terhadap spesimen tanpa takik dan ciri-ciri patah lelahnya diidentifikasi melalui melalui penampang patahannya. Pengamatan terhadap kurva S-N untuk spesimen radius takik 1,5 mm yang diberi beban konstan dan perlakuan panas quenched, batas ketahanan lelahnya hingga 105MPa (rumus) dan konstanta fatik sebesar Sa=771.N–0,1511, untuk spesimen annealed 51Mpa (rumus) dan Sa=649.N–0,1691, untuk spesimen non treatment 86Mpa (rumus) dan Sa=654.N–0,1497, sedangkan pada spesimen takik radius 1,5 mm yang diberi beban fluktuasi serta perlakuan panas quenched, batas ketahanan lelahnya hingga 81Mpa (rumus) dan konstanta fatik sebesar Sa=750N–0,1710, untuk spesimen annealed 51MPa (rumus) dan Sa=633.N–0,1803, untuk spesimen non treatment 60MPa (rumus) dan Sa=649N– 0,1804. Perbandingan umur lelah pada spesimen bertakik 8mm dan 1,5mm terhadap spesimen tanpa takik menunjukkan penurunan sebesar 90% dan 85%. Pada pembebanan amplitudo berfluktuasi batas ketahanan lelah tersebut menurun hingga 70% dibanding pembebanan amplitudo konstan, dan kegagalan lelahnya terjadi diatas siklus 1,5.106. Sedangkan penampang patahannya menunjukkan dengan jelas ciri-ciri patah lelah berupa adanya initial crack, beach mark dan final failure

This research is aimed to investigate the fatigue characteristic of low carbon steel with variation of notch U radius 8 mm and 1.5 mm. Bending load with constant and fluctuated amplitude was applied on specimen to observe the degradation level and fatigue endurance limits on the notch variation and also to identify failured area. Machine that used in the research is generating dinamic load which has constant amplitude at 2850 rpm rotation. The mahine was also equiped with suddenly loads equipment to produce fluctuation amplitude loads through arrangement of cam work mechanism. Heat treatment of quenched and annealed was first given to the test specimen, then its fatigue life was compared to non treatment specimen. Fatigue endurance from the result of examinatiom on amplitude of fluctuation load could be compared to amplitude of constant load. Fatigue endurance limits of notch-U specimen use radius 8 mm and 1.5 mm is also analysed to specimen without notch and its fatigue damage is identified through failured area. Based on S-N curve, the fatigue endurance limits for the specimen with notch radius 1.5 mm in which amplitude of constant load was given 105MPa (formula) for the quenched specimen with the constant damage curve Sa=771N-0.1511; 51MPa (formula) for the annealed specimen with Sa=649 N–0.1691; 86MPa (formula) for non treatment specimen with Sa=654.N–0.1497, while for the specimen notch radius 1.5 mm in which amplitude of fluctuation load was given 81MPa (formula) for the quenched with the conctant damage curve Sa=750N–0.1710; 51MPa (formula) for the annealed specimen with Sa=633.N–0.1803; 60MPa (formula) for non treatment specimen with Sa=649N–0.1804. This result shows that the fatigue endurance limit decreases up to 70 % in fluctuated amplitude loading and the failure happens above 1.5 106 cycle. On the other hand, degradation of fatigue life for the specimen notch radius 8mm and 1.5mm is 90% and 85% due to specimen without notch, while the failured area shows fatigue damage characteristics in forms of initial crack, beach mark and final failure.

Kata Kunci : Baja Karbon Rendah,T akik, Fatik, Low steel carbon, notch, fatigue.