The interface between the residual limb and the socket of a person with an amputation experiences complex loading conditions during daily activities. Currently, the effect of such loading conditions on tissue health and comfort is poorly assessed. This is largely due to the lack of appropriate sensing technology to facilitate measurement of strains/stresses at this interface. This paper will outline a novel method to simultaneously measure the normal and shear strains using fiber Bragg grating sensors embedded into a foam liner that sits at the prosthetic interface. Furthermore, this method enables the measurement of normal and shear strains when the material properties of the embedding material are unknown. As a validation to the new technique, an application of this process using Pe-Lite foam is presented. A comparison of the derived load based on this process to an applied load measured using a material testing machine is performed. Results show a very good agreement of measured normal and shear strains/stresses for loads below 20 N. However, discrepancies were evident above this load. This paper has provided a proof of concept for using fiber Bragg grating sensors in materials with unknown or inconsistent properties. This method provides a pathway for embedding sensors in prosthetic and orthotic applications.