# Main Types of Failure Distributions

**Model 1:** Exponential distribution, λ is considered to be a constant
![equation](R(t)=e^(-t/theta))
Corresponds to a Weibull distribution for which parameter β = 1

**Model 2 :** Normal distribution - the failure appears around a t mean
![equation](R(t)=1-1/(sigma*sqrt(2*pi))*int1/te^(-1/2*(t-mu)/sigma)dt)


Corresponds to a Weibull distribution for which parameter β = 3.6

**Model 3 :** Log-normal distribution - applies to the phenomena of mechanical wear/use
![equation](R(t)=1-1/(sigma*sqrt(2*pi))*int1/te^(-1/2*(ln(t)-mu)/sigma)dt)

Corresponds to a Weibull distribution for which parameter β = 2.5

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Main Types of Failure Distributions

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