Fatigue Strength and the Sn Diagram

Introduction

Fatigue strength of a machine is referred to the strength of the machine associated with the amount of strength imparted upon it with a distinct magnitude and velocity with respect to time. In terms of this condition, due to the constant load being imposed on the machine it tends to crack and attain a fatigue breakdown. The limit thus attained in this situation determines the Fatigue strength of the machine.  This however is evaluated under various conditions. In this context, the fatigue strength of a machine will be discussed.

Fatigue failure

A case of fatigue failure occurs in the cases of the brittle fracture without a noticeable deformation; it begins with a minute crack which leads to a sudden breakdown. It so happens due to the difference in the flux of magnitude during pressure over time. 

Types of stresses

A varied cycle of stresses is practised which are at times irregular in nature but as per the SN diagram, it follows a sinusoidal curve because of the rotating machinery involved in certain machines. For the SN diagram, only the maximum and minimum values are essential to evaluate the fatigue strength and limit and not the pattern of the graph. The cycles are further sub-categorized into several sections:

• Completely reversed stress, which suggests that the values of extreme ends have an equal amount of magnitude with the mean value equals to zero.
• Repeated stress resembles the variation in the force imposition which ranges from zero to a specific maximum value.
• In cases of fluctuating stress, it is essential to note that although the amount of stress fluctuates over time. The minimum and maximum value is zero.
• In alternating stress, the stress pattern upon the object is altered over time. In this scenario, the nature of stress and its magnitude of extreme values are altered.

However, all these components are made of two components, in the case of the static component it is called mean stress (sm) and in the case of the variable component, it is called stress amplitude (sa).

Mean stress: σm = (σmax+ σmin)/2 and Stress amplitude: σa = (σmax – σmin) /2 

Stress range: σr = σmax – σmin

Fatigue strength and S-N curve

A test is conducted with the help of a rotating beam test that determines the fatigue properties of the substance. A polished circular object is used which undergoes high speed of rotation which undergoes several cyclic loads over time, the object is rotated at a high speed that causes it to experience variation in the stress cycle on the object. This stress is thus repeated over time using other materials. At first, the subject is made to undergo various variables of stress till it attains a fatigue break and thus the value is counted. In the first case, the subject is made to undergo a level of stress that is below the extreme for its tensile strength and over time the consecutive tests are performed under lower stress levels.

Results, as evaluated from the tests, are plotted between stress denoted as (S) and several cycles (N) ; this is generally defined in a log-log scale. The value on the coordinate of the S-N diagram resembles the fatigue strength. 

Fatigue limit is defined as the value of maximum amplitude a specimen can sustain within an unlimited number of cycles without fatigue failure, which is defined with the value Se

The fatigue strength of mild steel is 0.5 Sut

Conclusion

Experiments conducted to gather the evidential fatigue strength are determined by the S-N diagram. Thus, the conclusive data as derived is associated with the variable of stress as imparted upon a product which aids in determining the yield strength in the limit of an object which undergoes the stress of variable magnitude and velocity in respect to time. Fatigue strength is associated with the time that a machine needs to reach the fatigue limit till it breaks down due to the constant load of pressure as benign imparted upon a machine in the experiment over time.