# Estimate a probability with Monte Carlo¶

In this basic example we are going to estimate a probability by means of a simulation algorithm.

This model is a simple beam, restrained at one side and stressed by a traction load F at the other side.

The geometry is supposed to be deterministic: the diameter D is fixed to D=20 mm.

It is considered that failure occurs when the beam plastifies, i.e. when the axial stress gets bigger than the yield stress:

Therefore, the state limit G used here is:

Two independent random variables R and S are considered:

• R (the strength):

Stochastic model:

• F ~ Normal(75e3, 5e3) [N]
• R ~ LogNormal(300, 30) [N]

Theoretical results:

This two dimensional stochastic problem can be solved by calculating directly the failure probability:

If R and S are independant, then:

The numerical application gives:

In [10]:

from __future__ import print_function
import openturns as ot

In [11]:

# create the joint distribution of the parameters
distribution_R = ot.LogNormalMuSigma(300.0, 30.0, 0.0).getDistribution()
distribution_F = ot.Normal(75e3, 5e3)
marginals = [distribution_R, distribution_F]
distribution = ot.ComposedDistribution(marginals)

In [12]:

# create the model
model = ot.SymbolicFunction(['R', 'F'], ['R-F/(_pi*100.0)'])

In [13]:

# create the event we want to estimate the probability
vect = ot.RandomVector(distribution)
G = ot.RandomVector(model, vect)
event = ot.Event(G, ot.Less(), 0.0)

In [14]:

# create a Monte Carlo algorithm
algo = ot.MonteCarlo(event)
algo.setMaximumCoefficientOfVariation(0.05)
algo.setMaximumOuterSampling(int(1e5))
algo.run()

In [15]:

# retrieve results
result = algo.getResult()
probability = result.getProbabilityEstimate()
print('Pf=', probability)

Pf= 0.02858192505510653