Strain Acquisition Framework and Novel Bending Evaluation Formulation for Compression-Loaded Composites Using Digital Image Correlation

Abstract

Consistent and reproducible data are key for material characterization. This work presents digital image correlation (DIC) strain acquisition guidelines for compression-loaded carbon fiber composites. Additionally, a novel bending criterion is formulated which builds up on the DIC strain data so that it is able to completely replace state-of-the-art tactile strain devices. These guidelines are derived from a custom test setup that simultaneously investigates the front and side view of the specimen. They reflect both an observation and post-processing standpoint. It is found that the DIC-based strain progress matches closely with state-of-the-art strain gauges up to failure initiation. The new bending evaluation criterion allows the bending state-and therefore, the validity of the compression test-to be monitored analogously to the methodology defined in the standards. Furthermore, the new bending criterion eliminates a specific bending mode, caused by an offset of clamps, which cannot be detected by the traditional strain gauge-based monitoring approach.

Keywords: composite material characterization; digital image correlation; mechanical properties; mechanical testing.

Figure 1

DIC camera setup seen from the top side.

Figure 2

Progressive failure in a 0° loaded [45,-45,0]_s laminate observed from the front view (1–6 chronologically obtained images).

Figure 3

Progressive failure in a 0° loaded [45,-45,0]_s laminate observed from the side view (1–6 chronologically obtained images).

Figure 4

The investigated strain post-processing techniques applied to a compression specimen: 1. strain field measurement, 2. long extensometer, 3. short extensometer.

Figure 5

(a) Front view DIC and strain gauge comparison for a 0° oriented compression-loaded laminate.; (b) side view DIC and strain gauge comparison for a 0° oriented compression-loaded laminate.

Figure 6

Three possible bending states in a compression specimen: (a) ideal load introduction; (b) inclined clamps; and (c) offset clamps.

Figure 7

Comparison of the newly proposed bending criterion (upper plot) with the state-of-the-art (lower plot) on a UD [0]₆ laminate in 0° direction. Left side shows specimen 1, the right side presents data obtained from specimen 4 of the test series.