We describe an investigation into the fatigue fracture behaviour under combined tension–torsion loading of a SiC whisker‐reinforced A6061 aluminium alloy fabricated by a squeeze casting process. Special attention was paid to the environmental effects on fatigue fracture behaviour. Tests were conducted on both the composite and its unreinforced matrix material, A6061‐T6, under load‐controlled conditions with a constant value of the combined stress ratio, α = τmax /σmax in laboratory air or in a 3.5% NaCl solution at the free corrosion potential. The corrosion fatigue strength of both the matrix and composite was less in the solution than in air. The dominating mechanical factor that determined the fatigue strength in air was either the maximum principal stress or the von Mises‐type equivalent stress, depending on the combined stress ratio. However, in the 3.5% NaCl solution, the corrosion fatigue strength of both materials was determined by the maximum principal stress, irrespective of the combined stress ratio. In the case of the matrix material, crack initiation occurred by a brittle facet normal to the principal stress due to hydrogen embrittlement. However, in the composite material, the crack was initiated not at the brittle facet, but at a corrosion pit formed on the specimen surface. At the bottom of the pit, a crack normal to the principal stress was nucleated and propagated, resulting in final failure. Pitting corrosion was nucleated at an early stage of fatigue life, i.e. about 1% of total fatigue life. However, crack initiation at the bottom of a pit was close to the terminal stage, i.e. about 70% or more of total fatigue life. The dominating factor which determined crack initiation at a pit was the Mode I stress intensity factor obtained by assuming the pit to be a sharp crack. Initiation and propagation due to pitting corrosion and crack growth were closely examined, and the fatigue fracture mechanisms and influence of the 3.5% NaCl solution on fatigue strength of the composite and matrix under combined tension–torsion loading were examined in detail.
Magnetic Hard Disk Drive (HDD) has become a primary choice as a cost effective means for digital audio players, portable computers, car navigation systems and other portable electronic products. A particular problem an HDD faces in such applications is periodic disturbances caused by the vibrations from the environment. The frequencies of these external vibrations are often not known a priori and/or vary in time. Typical feedback controllers have limitations on addressing such disturbances, and for unknown frequency cases, some sort of adaptive control scheme is called for. While there are algorithms proposed for HDD to address unknown periodic disturbances, ever-increasing demand for higher areal density requires further improvements on the performance. In this paper, a new adaptive method to identify and reject unknown periodic disturbances is applied to a mobile HDD and simulation and experimental results are presented in comparison with a scheme proposed in the literature.
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