4.6.2. Disc safety factor coefficients

In practice the following safety factor coefficients are more often considered:

- long-term safety factor coefficient K_^T;

- safety factor coefficient of the destroying rotational speed К_В;

- low-cycle fatigue safety factor coefficient.

The long-term safety factor coefficient is calculated by the formula

where _^Т  is a long-term strength on a base of t hours (usually =100 hours or =1000 hours) and at temperature Т.

The factor К_^Т changes along the disc radius. Therefore, when evaluating disc strength it’s necessary to construct dependence = f(r) in a number of sections with data on values calculations being used. From this dependence dangerous section can be found from value.

The minimum long-term strength safety factor coefficient must be multiplied by the value К_Z to keep the record of disc loading recurrence

where К_Z is a factor, taking into account decrease of long-term strength due to loading recurrence. The factor К_Z is equal to 0,8...0,9 in calculations (the smaller of these values is for plastic materials).

Many ratings of the disc loading can be taken into consideration, determining safety factor coefficient by the formula of hypothesis of linear summation of damages

(4.12)

where n is the number of ratings in a working cycle (usually only maximum, nominal and maximum cruise ratings are taken into account); (K '_^T)_i min is own minimum safety factor coefficients on i-th ratings of a working cycle; m is an exponential equation index, which describes the characteristics of the long-term strength (m = 6...8).

The disc is considered serviceable within the limits of its safe life being checked, if the minimum long-term strength safety factor coefficient makes К_^Т_min = 1,3...1,5 (where t=100 hours).

We evaluate the temperature safety factor coefficient at project calculations of disc, which works at high temperatures

where Т_limit is a limiting temperature in the disc dangerous section, at which material destruction will take place under stress _е during time ; Т_design is a design temperature in the same section.

The value К_Т usually makes 70...120С.

The rotor destroying rotational speed safety factor coefficient is determined from the ratio

where n_limit is a limiting rotor rotational speed, which causes the disc destruction at a given temperature; n_design is a design rotor rotational speed.

The simpler way to determine limiting rotor rotational speed is grounded on the theory of limiting equilibrium. According to this theory a full equalization of the stresses along radius take place when rotor rotational speed is increased up to n_limit. It happens at the expense of redistribution of stresses being affected by plastic deformations. And at the moment of disc destruction the circumferential stresses _t reach short-lived strength limit _B^T at temperature Т. Thus, in all disc sections _t=_B^T.