Vibrothermography is an inspection technique that detects cracks by observing vibration induced crack heating. Frictional crack heating in a vibrating specimen is directly linked to the resonant vibrational stress on the crack. In simple geometries we can measure the vibrational mode structure and intuit the dynamic vibrational stress field on the crack. This is used to establish a relationship between crack heating and vibration. Such a relationship will be critical for vibrothermography to be accepted as a viable inspection technology. We correlate stress to heating by exciting specimens in a well understood and repeatable resonant vibration mode. Our sample set consists of 65 Titanium and 63 Inconel specimens with low cycle fatigue cracks. Through knowledge of the mode shape, a single point surface velocity measurement is sufficient to calculate the deformed shape of the entire specimen. The loads and stresses within the specimen are calculated from the deformed shape and used to identify the relationship between crack heating and vibration. The observed relationship between normal stress, crack size, and crack heating is presented. This relationship may eventually prove viable for quantifying crack detectability in vibrothermography.