True Stress, True Strain, Engineering Stress, and Engineering Strain 

engineering stress  

true stress  

engineering strain  

true strain 
P  load 
A_{0}  crosssectional area of specimen before deformation has taken place 
A  crosssectional area of specimen at which the load is applied 
δ  total elongation 
L_{0}  original value of the gage length 
L  successive values of the length as it changes 
(Eq1) 

(Eq2) 

(Eq3) 

Δε = 

ε_{t} = ∑Δε = ∑ 

ε_{t} =  ∫ 

 = ln 

(Eq4) 

ε_{t} =  ∫ 
 = ln 
 = ln 

True stress continues to increase after necking because, although the load required decreases, the area decreases even more. 
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