1. A method for measuring strain in a target body comprising: a. acoustically coupling a transducer to the outer surface of a target body such that the path of a beam emitted from the transducer defines a transducer axis; b. emitting first a pulse of ultrasound energy into the target body along the transducer axis; c. receiving a first reflected signal with the transducer; d. storing the first reflected signal; e. allowing the target to change dimensions along the axis defined by the transducer; f. emitting a second pulse of ultrasound energy into the target body along the transducer axis; g. receiving a second reflected signal with the transducer; h. storing the second reflected signal; i. selecting a portion of the first and second reflected signals; j. computing the frequency spectrum of each of the selected portions of the first and second selected signals; k. computing the shift between the computed spectra; and l. normalizing the computed shift to one of the computed spectra.