Ultimate Strength Of Specimens Research Articles

Low-cycle fatigue of unidirectional composites:: Bi-linear S– N curves

Low-cycle fatigue (LCF) behavior of polymer matrix composites (PMCs) is investigated in an experimental study of unidirectional glass/epoxy composites subjected to axial tensile loading along longitudinal 0° orientation of fibers. Under high LCF loads, fatigue life of PMCs is found to be less than 10 4 loading cycles due to the high property degradation rates that are noticeably higher than those seen during high-cycle fatigue (HCF). In PMC response, unique LCF features have been identified and linked with damage accumulation patterns in unidirectional composites. At high loads near the ultimate strength of specimens, large strains and finite strain rates are found to be significant under semi-rectangular loading so the LCF behavior is affected. Lower and upper limits for the LCF life impose some restrictions on the S– N curves that are obtained for the LCF life assessment. A bi-linear S– N curve is used to approximate the data in the LCF and HCF regions. The bi-linear S– N relationship and the associated fatigue model are described by a proposed analytical formula. The concept of pre-LCF damage state is introduced.

Effect of oxidation on the strength of silicon nitride ceramics

1. Oxidation of materials of the type NKKKM-79 commences at 800°C and it proceeds in accordance with reactions (1) and (2). Silicon dioxide forming in this way in the temperature range 800–1100°C is in the solid state, and oxidation continues until complete closing of pores. At temperatures above 1100°C the oxide layer is in the liquid state, and in a short time all pores are closed at the specimen surface, creating in this way a film which hinders internal oxidation of the material. 2. Oxidation in the temperature range 900–1050°C for 3 h causes an increase in the ultimate strength of these materials: at room temperature by about 35–40%, and at 1400°C by about 25%. The ultimate strength at 1200°C almost remains constant independent of prior oxidation temperature. 3. The ultimate strength of unmachined specimens previously oxidized at 1450°C increases at room temperature and it remains commensurate with the ultimate strength of unoxidized machined specimens. At 1400 and 1200°C the ultimate strength of specimens oxidized at 1450°C decreased by about 25% compared with the ultimate strength of unoxidized specimens.