  Measuring stress levels in loaded structures is crucial to assess and monitor
their health, and to predict the length of their remaining structural life.
However, measuring stress non-destructively has proved quite challenging. Many
ultrasonic methods are able to accurately predict in-plane stresses in a
controlled laboratory environment, but struggle to be robust outside, in a real
world setting. That is because they rely either on knowing beforehand the
material constants (which are difficult to acquire) or they require significant
calibration for each specimen. Here we present a simple ultrasonic method to
evaluate the in-plane stress in situ directly, without knowing any material
constants. This method only requires measuring the speed of two angled shear
waves. It is based on a formula which is exact for incompressible solids, such
as soft gels or tissues, and is approximately true for compressible "hard"
solids, such as steel and other metals. We validate the formula against virtual
experiments using Finite Element simulations, and find it displays excellent
accuracy, with a small error of the order of 1%.
