This paper describes the development of an opto-mechanical simulation of a complete shearography system, including the shearography instrument, the samples and the test environment. This simulation is applied to the measurement of 3D strains in engineering samples. The samples are a cylinder loaded by internal pressure and a flat plate under axial load. Finite elements models are used to obtain the displacements fields. A 3D shearography instrument consisting of a laser and four cameras has been simulated using the optical model. Combining the finite elements and optical simulations allows phase maps to be generated, which are the predictions for measurements using the complete test setup. Errors due to sample material properties, loading inaccuracy and dimensional tolerances are included in the model and this allows the calculation of phase maps at the minimum and maximum error limits. The simulation through path lengths and the simulation by inverted shearography processing provide similar results and the difference is associated with the approximation introduced by the sensitivity vector.