Effect of vibration on the creep of tensile-test specimens of the aluminum alloy Al−Mg−Si (PA-4)
In this work are presented the results of research on vibrational creep at normal temperature of an aluminum alloy containing magnesium and silicon (designated PA-4). A uni-directional positive load is applied to a tensile-test specimen, such that the stress intensity in the specimen is of the type\(\sigma (t) = \sigma _m (1 + Asin\omega t)\) whereσ m is the mean (static) stress intensity andA =σ a /σ m is the ratio of the vibratory-stress intensity to the mean intensity. The results are given in the form of families of curves of plastic (i.e., permanent) deformation for various values ofA, namely,A=0.0000, 0.0065, 0.05500, 0.1000 and 0.2000.
Taking the creep limit for plastic strain as ε p = 1.8 percent, equations for this creep limit were deduced from experimental data.
Vibrations of small-stress-amplitude ratioA encourage creep, particularly with more lengthy tests.
For medium depths of vibration (i.e., medium-stress-amplitude ratios)A, the primary factor affecting the creep limit appears to be the meanstress intensity,σ m .
For large depths of vibrationA, creep is concentrated in the first 2000 to 3000 cycles. In this process a primary role is played by the maximum stress intensity.
KeywordsAluminum Silicon Experimental Data Magnesium Mechanical Engineer
distance from beam pivot to specimen end of beam (Fig. 1), cm
percent reduction of specimen area at 0.2 percent elongation under static load
σ a /σ m = ratio of vibratory axial-tensile stress intensity to mean axial stress intensity in test specimen=the stress-amplitude ratio
distance from beam pivot to statically loaded end of beam (Fig. 1), cm
a(k/I red )1/2=reduced value ofb, sec−1
amplitude ofz (Fig. 1), cm
percent reduction of specimen at fracture under static load
hardness, Rockwell B scale
reduced (effective) moment of inertia of pivoted beam with attached preloading massm, obtained experimentally by natural frequency measurements (Fig. 1), kg-cm-sec2
stiffness of spring applying load to the test specimen (Fig. 1), kg/cm
a2k=effective torsional stiffness of springk for angular oscillations of pivoted beam, kg-cm
mass applied to free end of pivoted beam to statically preload tensile specimen at opposite end (Fig. 1), kg
S·a=harmonic moment about beam pivot due to harmonic excitation of tensile-test specimen
kB-amplitude of the harmonic force applied to the specimen, kg
a particular alloy of aluminum, magnesium and silicon yielding pronounced rheological properties at room temperature (see text for its composition)
kBa/I red , sec−2
σmin/σmax ratio of minimum stress intensity to maximum stress intensity in the vibratory loaded specimen
stress intensity in specimen material at 0.02 percent elongation under static load, kg/cm2
stress intensity in specimen at 0.2 percent elongation under static load, kg/cm2
stress intensity in specimen at rupture (ultimate strength), kg/cm2
(x−z)k=instantaneous value of the harmonic load acting on the tensile-test specimen, kg
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