Correlation between uniaxial ratcheting and magnetic response under cyclic loadings in X80 pipeline steel

The objective of this research is to explore the correlation between uniaxial ratcheting behaviour and magnetic response under asymmetrical cyclic stressing in X80 pipeline steel. The magnetic field variations from cycle to cycle were recorded simultaneously during ratcheting. Analysis made in the present work shows that the magnetic hysteresis loop evolves systematically with the number of cycles in terms of its shape and position. The most prominent result here is that the magnetic field under a certain stress first decreases sharply in the preliminary stage of ratcheting, followed by a stable decreasing period with an approximately linear slope. Furthermore, it can be concluded that ratcheting influences the changes in magnetic field more significantly under tensile stress than compressive stress. The underlying reason of the strong non‐linearity of the magnetic field evolution was discussed.     

Damage accumulation in plasma-sprayed zirconia under cyclic loading

The stiffness and hysteretic response due to mechanical and thermo-mechanical cycling have been studied in plasma-sprayed yttria-stabilized zirconia (YSZ). Mechanical cycling of free-standing cantilevers of YSZ shows that the progressive decrease in stiffness is accompanied by monotonic increase in hysteretic energy dissipation per cycle and a permanent ratcheting displacement of ∼20 nm/cycle. Below a critical stress, it varies from coating to coating, ratcheting accompanied by a slow decrease in stiffness does not lead to failure even after ∼1000 cycles. In contrast, at higher stresses, the rates of ratcheting and decrease in stiffness increase rapidly, leading to the nucleation of macrocracks that lead to fracture failure. Prior thermal cycling of coatings on Inconel substrate up to 700°C, which induced an estimated cyclic thermal stress of ∼35 MPa, led to a pronounced reduction in stiffness and mechanical cycling life. During bending, damage accumulates in the tension side of the cantilever and the volume going through the compression cycle remains relatively undamaged.