SAFETY FACTORS

SAFETY FACTORS

Corrosion Fatigue: Corrosion fatigue is a failure mode where cyclic stresses and a corrosion producing environment combine to initiate and propagate cracks in fewer stress cycles and at lower stress amplitudes than would be required in a more inert environment. The corrosion process forms pits and surface discontinuities that act as stress raisers to accelerate fatigue cracking. The cyclic loads may also cause cracking and flaking of the corrosion layer, baring fresh metal to the corrosive environment. Each process accelerates the other, making the cumulative result more serious.

Surface or Contact Fatigue: Surface fatigue failure is usually associated with rolling surfaces in contact, and results in pitting, cracking, and spalling of the contacting surfaces from cyclic Hertz contact stresses that cause the maximum values of cyclic shear stresses to be slightly below the surface. The cyclic subsurface shear stresses generate cracks that propagate to the contacting surface, dislodging particles in the process.

Combined Creep and Fatigue: In this failure mode, all of the conditions for both creep failure and fatigue failure exist simultaneously. Each process influences the other in producing failure, but this interaction is not well understood

Factors of Safety.—There is always a risk that the working stress to which a member is

subjected will exceed the strength of its material. The purpose of a factor of safety is to minimize this risk.

Factors of safety can be incorporated into design calculations in many ways. For most calculations the following equation is used:

S𝑤 = S𝑚 ⁄ 𝑓𝗌

where

𝑓𝗌    is the factor of safety, 

S𝑚   is the strength of the material in pounds per square inch,

S𝑤   is the allowable working stress, also in pounds per square inch.
 Since the factor of safety is greater than 1, the allowable working stress will be less than the strength of the material.

In general, S𝑚 is based on yield strength for ductile materials, ultimate strength for brittle materials, and fatigue strength for parts subjected to cyclic stressing. Most strength values are obtained by testing standard specimens at 68°F. in normal atmospheres. If, however, the character of the stress or environment differs significantly from that used in obtaining standard strength data, then special data must be obtained. If special data are not available, standard data must be suitably modified.

𝑓𝗌                                Application

1.3–1.5                 For use with highly reliable materials where loading and environmental conditions                               are not  severe, and where weight is an important consideration.

1.5–2                    For applications using reliable materials where loading and environmental                                           conditions are not severe.

2–2.5                    For use with ordinary materials where loading and environmental conditions are                                 not severe.

2.5–3                    For less tried and for brittle materials where loading and environmental                                               conditions  are not severe.

3–4                      For applications in which material properties are not reliable and where loading                                  and  environmental conditions are not severe, or where reliable materials are to be                              used  under difficult loading and environmental conditions.

Working Stress.—Calculated working stresses are the products of calculated nominal stress values and stress concentration factors. Calculated nominal stress values are based on the assumption of idealized stress distributions. Such nominal stresses may be simple stresses, combined stresses, or cyclic stresses. Depending on the nature of the nominal stress, one of the following equations applies:

𝘚𝑤 =K 𝝈               (2)

𝘚𝑤 =K τ                (3)

𝘚𝑤 =K σ′               (4)

𝘚𝑤 =K τ′              (5)

𝘚𝑤 =K σcy             (6)

𝘚𝑤 =K τcy             (7)

where 
K is a stress concentration factor; 
σ and τ are, respectively, simple normal (tensile or compressive) and shear stresses;

σ′ and τ′ are combined normal and shear stresses;
σcy and 𝜏𝖼𝗒 are cyclic normal and shear stresses.

Stress Concentration Factors.—Stress concentration is related to type of material, the nature of the stress, environmental conditions, and the geometry of parts. When stress concentration factors that specifically match all of the foregoing conditions are not available, the following equation may be used:

K = 1 + q (Kt – 1)

Kt       is a theoretical stress concentration factor that is a function only of the geometry of a part and             the nature of the stress; 
q        is the index of sensitivity of the material. 
If the geometry is such as to provide no theoretical stress concentration, Kt = 1.

Curves for evaluating Kt are on next. 
For constant stresses in cast iron and in ductile materials, q = 0 (hence K = 1). 
For constant stresses in brittle materials such as hardened steel, q may be taken as 0.15; 
for very brittle materials such as steels that have been quenched but not drawn, q may be taken as 0.25. 
When stresses are suddenly applied (impact stresses) q ranges from 0.4 to 0.6 for ductile materials; for cast iron it is taken as 0.5; and, for brittle materials, 1.

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