鋳造アルミニウム合金AC4CHの疲労挙動に及ぼす温度の影響

抄録

Fully reversed axial fatigue tests were conducted using plate specimens of a cast aluminium alloy, AC4CH, at three different temperatures of room temperature (R.T.), 150°C (423K) and 250°C (523K), and crack initiation and small crack growth were studied in detail by means of replication technique. At 150°C and 250°C, fatigue strength was nearly the same as, or significantly lower than, at R.T., respectively. The fatigue strength characterized in terms of fatigue ratio, σ/σ_B, was the same at all temperatures in high stress region, but slightly lower at 250°C in low stress region. Cracks invariably initiated from a casting porosity at R.T., while crack initiation due to slip deformation became dominant at elevated temperatures. At 250°C, a number of cracks were initiated due to slip deformation and the coalescence of the main crack and other cracks occurred very frequently, particularly in low stress region. At a fixed maximum stress intensity factor, K_max, small crack growth rates were faster with increasing test temperature. When characterized in terms of K_max/E (E : Elastic modulus), crack growth rates became similar at R.T. and 150°C, but still faster at 250°C. It was indicated that such enhanced crack growth at 250°C was attributed to most frequent coalescence of the main crack and other cracks, resulting in the lowest fatigue strength at this temperature.