Camera-Based Deformation Measurements During Vibration-Based Fatigue Testing

High cycle fatigue (HCF) is an important design factor in key aerospace applications, for example gas turbine engines. However, HCF tests are notoriously slow, requiring many hours or days to obtain a single point on an S-N curve. One promising method to reduce HCF testing times is to use a vibration-based technique in which a rectangular plate is cantilevered on top of a mechanical shaker, which excites the plate to high-frequency resonance, thus accumulating many fatigue cycles much more quickly. In particular, the “two-stripe” mode – so named for its two nodal lines which run from the cantilevered edge to the opposite free edge of the plate – is especially desirable because it applies high frequency uniaxial bending along the free edge of the plate at rates on the order of 1500 Hz. When performing such vibration-based tests, the maximum strain is monitored at the center of the free edge using a strain gauge. However, strain gauges have a fatigue life of their own and often fail before the plate specimen. Thus, in order to maintain a consistent strain amplitude over the full duration of a vibration-based test, the plate must be additionally monitored by a laser vibrometer, which is non-contacting and thus remains undamaged by the test. The vibrometer provides feedback control for the shaker, and strains are extrapolated by a linear calibration curve relating the measurements of the vibrometer and strain gauge.

As an alternative to strain gauges, Digital Image Correlation (DIC) is a non-contacting, camera-based technique that obtains full-field displacement measurements by comparing digital images taken before and after a specimen is deformed. Strains are then commonly computed by taking spatial derivatives of the DIC strains. The purpose of this work is to apply DIC to the two-stripe vibration-based fatigue method, thereby more directly obtaining strain measurements long after strain gauges have failed. There are two main challenges associated with applying DIC to this vibration-based method. First, one must maintain sufficient lighting because high vibration speeds require short exposure times to reduce motion blur, and large out-of-plane displacements require small apertures to increase depth of field. Second, images must be captured at the point of maximum deflection within the period of oscillation. The second challenge is often accounted for by utilizing a high-speed camera; however, high-speed cameras have significantly lower resolution than other “slower” cameras. Here, the authors employ a phase-locking technique to enable the precise triggering of strobe lights and low-speed, high-resolution cameras. The images captured are then post-processed using DIC to re-create a high-resolution mode shape. This work analyzes the ability of DIC with phase-locking to capture the mode shape of the specimen by comparing the measured mode shape to a finite element analysis (FEA) simulation of the experiment.