EFFECT OF WIRE-EDM CUTTING ON FATIGUE STRENGTH OF AZ61 MAGNESIUM ALLOY
Effect of wire electrical discharge machining (wire-EDM) cutting on fatigue strength of an extruded AZ61 magnesium alloy at room temperature was investigated. The surface condition of the samples after the wire-EDM cutting process is compared to the samples which have undergone careful polishing on their surfaces. A series of high cycle fatigue tests are performed on both the wireEDM as-cut and polished samples with constant amplitude stress at the frequency of 10 Hz. Fractographic analyses were conducted on the tested samples to identify the mechanism of fatigue fracture. The finding show that the fatigue limit of wireEDM as-cut specimen is 20 MPa lower as compared to smoothly polished specimens. Fatigue cracks are found to initiate and propagate to final failure from a cutting pit on the wire-EDM as-cut specimens.
Fatigue‐Life Prediction of Magnesium Alloys for Structural Applications
There has been growing interest in studies on fatigue strength and fatigue crack growth behavior of magnesium alloys. In the present study, fatigue-life prediction of magnesium alloys based on a fracture mechanical approach has been made, where the pit depth for an extruded AZ61 at high humidity and the inherent defect diameter for a die-cast AZ91D were assumed as an initial crack.
Stiffness-Thermal Expansion Relationships in High Modulus Carbon Fibers
Improved crystallite orientation in carbon fibers increases the axial stiffness while decreasing the axial coefficient of thermal expansion. Measurements of unidirectional Pitch 120 and Pitch 140 fibers, with axial tensile Young's moduli of 828 GPa (120 msi) and 966 GPa (140 msi), respectively, in epoxy and aluminum matrices are described. The axial CTE of these fibers was found to be -1.62 ± 0.04 × 10-6 C-1 in the range 0 ± 60°C. The positive deviation is attributed to high compressive stresses (P140/A1) while the most negative value is achievable by fiber misalignments of a few degrees from unidirectional. The stiffness-expansion relation changes for fiber stiffness above 680 GPa. This suggests a decoupling of the CTE dependence on preferred orientation with possible emphasis on defect structure.