Material Behavior under High Strain Rates

Luciano Avila Gray, M.Sc.

Research Team Leader

Tel.: +49 (89) 289 - 15203

Carbon Fiber Reinforced Plastics (CFRP) are increasingly used in the aerospace and automotive industries. Composite structures are often loaded dynamically (e.g. crash and impact scenarios), and hence it is necessary to investigate the strain-rate sensitive behavior of CFRPs. Ideally the material lay-out design should also be considered. Due to rapid developments in the field of high-speed photography and establishment of optical measurement methods, the accuracy of high dynamic testing has increased in recent years. At the LCC, high dynamic and strain-rate dependent material behavior is analyzed using Split-Hopkinson Pressure Bars (SHPB). Besides the classical experimental setup for compression tests, from mid-2013 a Split-Hopkinson Tension Bar (SHTB) and a Split-Hopkinson Torsion Bar (SHToB) will be available.

Split-Hopkinson Bar Method

The Split-Hopkinson Pressure Bar method (SHPB) is suitable for the identification of mechanical properties of CFRP under dynamic loading in the range of approximately 50 – 2000 s-1. The test is based on the creation and dispersion of elastic strain waves in metallic bars. The specimen is clamped between “incident” and “transmission” bars (figure 1).

Figure 1: Schematic representation of different variants of the Hopkinson bar

High-speed camera Photron SA5

Deformation of specimens is recorded by high-speed camera, allowing the failure-mode to be detected and the strain field to be evaluated (using Digital Image Correlation, see below). The maximum frame rate is 775,000 fps.

Figure 2: Failure mechanism of a UD carbon-epoxy matrix specimen under compressive loading in SHPB experiment [Koerber et al., Mechanics of Materials, 42 (2010)]

GOM ARAMIS® (Digital Image Correlation)

For analysis, state of the art optical measurement methods are used. The 2D strain-field is calculated by Digital Image Correlation (DIC). In comparison to conventional SHPB analysis or strain gauges, this method of contactless measurement of the strain-field provides more information about the deformation-state of the specimen.

Figure 3: Axial strain field of a textile CFRP sample under off-axis loading in the SHPB test