Keisuke Ishikawa

Environmental effect of fatigue crack propagation of magnesium alloy

Fatigue crack propagation tests were carried out for a magnesium alloy in various kinds of environment. The presence of oxygen causes the production of oxide film on the fresh fracture surfaces made during the cyclic loading. Therefore, the fatigue crack behavior depends strongly upon the environment. The fatigue tests have been conducted in dry and wet argon gas as well as in air. The wet atmosphere, in particular, accelerated the fatigue crack propagation rate. The presence of the oxide film would restrict the deformation of the matrix beneath the hard film and promote hydrogen embrittlement in the wet condition.

Mechanical modeling and microstructural observation of pure aluminum crept under constant stress

Precise constant stress creep tests were conducted on five nine pure aluminum specimen between 293 and 623 K. A well-defined steady state creep was not observed under these conditions. The macroscopic stress response of the aluminum was examined by mechanical modeling, using combinations of the Maxwell and the Voigt elements. The plastic strain changes were well described by a serial combination of the two elements using experimentally determined viscosity coefficients for both elements. These results suggested that a dual phase substructure would form in crept aluminum under constant stress and this was confirmed by direct observation, which revealed a cellular structure. The viscosity coefficient is identified with the mobility of dislocations in both phases. The temperature dependencies of both coefficients were similar with the activation energy close to that for lattice self-diffusion in pure aluminum.

Creep behaviors of highly pure aluminum at lower temperatures

We have carried out experiments on the creep behaviors of pure aluminums at lower temperatures from a phenomenological viewpoint. The creep curves are classified into three regions; the transient, steady-state and terminal ones. The creep curve changes from a logarithmic to a constant strain rate curve at a higher applied stress. The creep curves yield a constant creep rate for a long period though lattice diffusion is less active at 293 K. The creep rate depends upon the applied stress. Cyclic stressing has an effect on the creep life of the materials. Both increasing and decreasing the applied stress reduce the rupture time. Since the steady-state creep is associated with the stable structure, is takes time to shift to the new structure built under the different applied stress. The hysteresis effect does some damage to the creep life of the pure aluminum.

Fatigue fracture toughness of AZ91D alloy at ambient temperature

Abstracts are not published in this journal

Creep Fracture of Binary and Ternary Commercial Aluminum Alloys

Binary and ternary commercial aluminum alloys were crept to the rupture under a condition of constant applied stress. The applied stress was guaranteed within 0.5% of the expected value. The commercial aluminum alloys have medium and high strength at the ambient temperatures. The binary alloy was A5083, the main alloy element of which was ca. 5% magnesium. Two kinds of ternary alloys were A6061 and A7075. The former contains 3% magnesium and 1% silicon. The latter consists of 3% magnesium and 7% zinc. The test temperatures were kept in the region of the solid solution of the alloys. The temperatures were guaranteed within 2K. The whole creep tests were carried out until the rupture of the specimens at the constant applied stress and temperature. The elongation was measured with the laser extensometer with the accuracy of 50 µm. The creep curves were converted into the true strain of the plastic elongation. The steady state creep was not observed for the whole alloys. The minimum strain rate was described by the following equation [1].

Fracture and Viscoelastic Behavior of Some Polymers at High Temperatures

Polymers are vital materials in better performance of specific strength. However their application can be restricted by the lower glass transition temperature, Tg. Some polymers have been developed as engineering plastics for the high temperature applications. We examined the high temperature strength of polymers at constant applied stress. The creep rupture and viscoelastic behavior were scrutinized for PC (polycarbonate) and PMMA (polymethyl methacrylate), which were quite different in the molecular structures. The former contains benzene rings and the latter is a single polymer. Tg is 423 K for PC and 378 K for PMMA. The large difference in the creep behavior was observed near Tg. The creep life strongly depends upon the applied stress just below Tg. The creep life is a function of the applied stress as follows. n life t − µ s . The stress exponent, n depends upon the temperature. Mechanical models were applied to evaluate the viscoelastic properties of the polymers at high temperatures. The viscosity rapidly decreased near Tg , regardless of the smaller decrease in the elastic constant. The results would be due to the difference in the molecular structures. The benzene ring could contribute to the higher resistance against the creep deformation through the higher viscosity.