High Resolution Digital Image Correlation Strain Measurements of Adhesively Bonded Joints

Abstract

In tensile test setups strain gauges or extensometers are typically used to measure the strain. Strain gauges and extensometers provide average values over the size of the strain gauges or distance between the measurement points but no information for the remaining area of the speci-men. This is reasonable for monolithic isotropic materials with small deformations. However for anisotropic materials, like composites or bonded joints, the average values may not be sufficient. Important strain deviations may not be detected. The focus of the work presented are scarf repaired coupon and single lap shear specimens where the strain distribution along the bondline is highly uneven. This cannot be covered by single meas-uring points like strain gauges. With the digital image correlation (DIC) system ARAMIS 12M by GOM it is possible to gain detailed strain information over the complete measurement area. The advantage of DIC measurements on coupon specimens is that there are free edges where the scarf and the adhesive layer are visible. In this paper the challenges with the right combination of lenses, test setup and the right surface treatment especially for small measurement areas are ana-lysed and discussed. The aim is to use the full resolution of 12 mega pixel for the measurement area around the bondline. This possibility in combination with a suitable surface treatment allows a detailed view on the adhesive layer. After the test the definition of system parameters are critical for reasonable results. The results allow for detecting strain peaks caused by 0° plies, see Figure 1, or flaws in the bondline. Comparisons to Finite Element simulations are shown and discussed. Addi-tionally to scarf joints, the examinations are extended to a conventional lap joint configuration. For this case, the highly loaded overlap edges are investigated since they are most critical for the over-all joint failure.