Vibrothermography is a method for finding cracks based on the heat generation due to vibration-induced rubbing of the crack surfaces. This technique has shown substantial promise for industrial use finding flaws in gas turbine parts, but the underlying physics remains unclear. Two long standing questions are the dependence of crack heating on excitation frequency, and the dependence of crack heating on loading mode (normal vs shear). With our broadband excitation system, we are able to excite the specimen at different resonances, and correlate the heating with dynamic vibrational stress and frequency of vibration. By exciting the specimen in different modes that load a particular crack either in pure normal stress or pure shear stress, we can measure how crack heating depends on loading mode. Our data shows that similar amount of heating occurs due to normal and shear stresses. In addition, when we fit a simple power law to the heating versus dynamic stress, we found that characteristics of different types of loading seem to be quite similar. A more general physical model is then proposed, which incorporates the dependence of vibrational frequency and dynamic loading types.