Fatigue and Fracture of Materials and Structures, Stractural Integrity

Threaded fasteners are one of the most preferred joining methods today. Although there are many studies in this field, the number of researches on fatigue life as a result of the loosening behavior of fasteners is very limited. Within the scope of the study, the effects of preload loss due to transversal displacement for re-tightened bolted joints to their fatigue behaviors were investigated. The study was conducted by using M8 × 1.25 10.9 DIN 933 bolts and DIN 934 nuts with various preloads. Tightened bolts were subjected to repeated transversal displacement, and clamp load values were monitored and recorded for each cycle. Bolts subjected to preload loss were re-tightened to their desired clamp load values, and Junker vibration test was restarted. Increase on the tightening torque values after each usage was observed. This situation is caused by the friction coefficient change after reusing fasteners. Due to the increase in the friction coefficient, tightening torque values were increased as expected. Fastener subjected to repeated tightening failed during Junker vibration test due to fatigue. This phenomenon was mainly caused by high friction coefficient which prevents loosening and lead fasteners to take higher shear loads. A critical tightening-preload loss factor has been found for predicting early fatigue failure caused by loosening for re-tightened bolted joints.</p

Change in Free Radical and Antioxidant Enzyme Levels in the Patients Undergoing Open Heart Surgery with Cardiopulmonary Bypass

Objective. The purpose of this study is to determine the changes in oxidative damage and antioxidant parameters in open heart surgeries with cardiopulmonary bypass (CPB) in preoperative and early postoperative periods. Methods. A total of three consecutive arterial blood samples were obtained from the patients in the study group, in preoperative, early postoperative, and postoperative periods, respectively. Oxidative damage indicator (MDA) and antioxidant indicators (GPx, GSH, CAT, and SOD) were examined. Results. A statistically significant increase was observed in MDA level in postoperative period compared to preoperative and early postoperative periods. GSH levels and CAT activities increased significantly in early postoperative and postoperative periods. Analyses revealed an increase in GPx and SOD enzyme activities only in the postoperative period. Conclusion. Even though the increase in MDA level was suppressed by the increased GSH level and CAT activity like in early postoperative period, efficiency can be brought for the increases in insufficient significant antioxidant parameters in postoperative period by administering antioxidant supplements to the patients and thus the increase in MDA in postoperative period can be significantly suppressed