25,26,27- Questions and answers

25. Discuss the conditions that must be met in order to occurrence of the crystallization. Comment on them. Crystallization - a special kind of solidification in which the metallic liquid undergoes a crystalline solid state. • To achieve crystallization, a thermodynamic equilibrium is required. • At constant pressures, the thermodynamic equilibrium occurs at the equilibrium temperature of the free energy of the solid phase FS and the liquid FL. • Crystallization takes place at a lower temperature than equilibrium temperature because free-phase energy of the solid phase is lower than that of the liquid phase.

FIG. 1. Dependence of free energy of solid and liquid phases from temperature

26. What is safety factor/indicator ? In order to obtain a determined degree of certainty that a given part will not be damaged or deformed permanently, safety factors are introduced. The stresses that can remain in the material without fear of infringement of the strength and stiffness condition are called permissible stresses. Values of permissible stresses are determined according to the properties of the material and the nature of the load. Factors affecting the value of the safety factor: • Way of applying the load. • Homogeneity of materials. • Initial stresses • Inaccuracy of calculation methods. • Time and conditions of construction work.

27. What is losses of stability? Example on strut. Let’s consider a simple AB bar pivotally attached to the ends and an axially compressed by P force- so small that the bar remains straight. If the bar is statically stable with a force Q perpendicular to the axis of the bar, this force will cause its deflection. When the force is withdrawn, the bar will return to its straight form. On the other hand, when the nature of the force Q is dynamic, it will cause vibration of the bar. When the critical force P is exceeded, the bar rapidly takes on a new form of constant equilibrium with a bent axis. Bending out of the bar due to overrun by the compression force of the critical value Pkr is called buckling.