  Knowledge of the state of plastic deformation in metallic structures is vital
to prevent failure. This is why non-destructive acoustic tests based on the
measurement of first order elastic constants have been developed and
intensively used. However, plastic deformations, which are usually
heterogeneous in space, may be invisible to these methods if the variation of
the elastic constants is too small. In recent years, digital image correlation
techniques, based on measurements carried out at the surface of a sample, have
been successfully used in conjunction with finite element modeling to gain
information about plastic deformation in the sample interior. Acoustic waves
can penetrate deep into a sample and offer the possibility of probing into the
bulk of a plastically deformed material. Previously, we have demonstrated that
nonlinear acoustic methods are far more sensitive to changes in dislocation
density than linear ones. Here, we show that the nonlinear Second Harmonic
Generation method (SHG) is sensitive enough to detect different zones of von
Mises stress as well as effective plastic strain in centimeter-size aluminium
pieces. This is achieved by way of ultrasonic measurements on a sample that has
undergone a three-point bending test. Because of the relatively low stress and
small deformations, the sample undergoes plastic deformation by dislocation
proliferation. Thus, we conclude that the nonlinear parameter measured by SHG
is also sensitive to dislocation density. Our experimental results agree with
numerical results obtained by Finite Element Method (FEM) modeling. We also
support the acoustic results by X-Ray Diffraction measurements (XRD). Although
intrusive and less accurate, they also agree with the acoustic measurements and
plastic deformations in finite element simulations.
