# REVERSED STRESS & DESIGN:FINITE & INFINITE LIFE

# STRESSES AND DESIGN ANALYSIS: HOW THE PROCESS OCCURS?????????

**Reversed stresses** : Those stresses which have same magnitude of stresses but in opposite sense (i.e **σ1 = -σ2**).

Here, **σm=0** & **σv = σ1 or σ2**

**Fluctuating stresses** : Those stresses which have different magnitude of stresses and also induces in a specific direction.

**σm=** **σ1 +σ2/2 **& **σv = σ1 — σ2/2** where , **σ1= maximum stress σ2**= **minimum stress**

In fatigue design , we deal with two types are problems:

*Components subjected to completely reversed stresses*

*2) Components subjected to fluctuating loading stresses*

**For reversed stresses** ,since the Mean stress value is zero.

Therefore tensile stresses occupy the first half and Compressive stresses occupy the second half of the stress distribution.

**Whereas, for fluctuating loading stresses**, there comes a definite non zero mean stress value.

Therefore, the stresses here can be purely tensile, purely compressive or Mix of both tensile and compressive.

# DESIGN STRATEGY FOR REVERSED STRESS PROBLEMS:

For designing a component subjected to *completely reversed stresses*, there are further **2 sub conditions** under which study is done.

1) DESIGN FOR INFINITE LIFE

2) DESIGN FOR FINITE LIFE

## DESIGN FOR INFINITE LIFE

When a component has to be designed for infinite life, **the ENDURANCE LIMIT becomes the failure criteria.** Here, the stress amplitude (**σ**f) should be less than the Endurance Limit (**σ**e) to withstand infinite no. of cycles.

These equations are mainly used in this type of design analysis:

**σa=Se / FOS τa = Sse / FOS**

Where ;

σa = Stress amplitude in component (due to bending)

Se = Corrected Endurance Limit in reversed bending

τa = Stress amplitude in component (due to torsion)

Sse= Corrected Endurance Limit in reversed bending

## DESIGN FOR FINITE LIFE

When a component has to be designed for finite life, the **S N CURVE becomes the failure criteria.** Here, the stress amplitude (**σ**f) is usually greater than the Endurance Limit (**σ**e). It mostly finds applications in steels.

PLOTTING TECHNIQUE OF **S N CURVE** :

THE S N CURVE drawn here is a Straight Line AB drawn from **(0.9 Sut) at 103 cycles to Se at 106 cycles** on a LOG-LOG graph paper.

PLOTTING PROCEDURE :

1. Locate point A with coordinates **{log10(103),log10(0.9 Sut)}= {3, log10(0.9 Sut)}** .

2. Locate point **B** with coordinates **{log10(106),log10(Se)}= {6, log10(Se)}** .

3.Join **AB . **This is the criterion for failure for Finite life problems.

4. Depending upon the Value of the life N given to us, draw a *vertical line* passing through **log10N **on the abscissa, which further intersects AB at point F.

5. Draw a line *FE parallel to abscissa*. **The ordinate at point E i.e log10Sf gives the fatigue strength corresponding to N cycles.**

**To know more:**

https://mechomotive.com/reversed-stress-designfinite-infinite-life/