Abstract
In this work an image-based inertial impact test is proposed to measure the interlaminar tensile stiffness and strength of fibre-reinforced polymer composite materials at high strain rates. The principle is to combine ultra-high-speed imaging and full-field measurements to capture the dynamic kinematic fields and exploit the inertial effects generated under high strain rate loading. The kinematic fields are processed using the virtual fields method to reconstruct stress averages from maps of acceleration. In this way, the specimen acts like a dynamic load cell, with no gripping or external force measurement required. Stress averages are combined with strain measurements to construct stress–strain curves and identify the interlaminar stiffness and tensile strength. Special optimised virtual fields are also implemented to identify interlaminar stiffness parameters from complete maps of strain and acceleration. Interlaminar stiffness and tensile strength are successfully identified at average, peak strain rates on the order 3500 s^{-1} and 5000 s^{-1}, respectively. Results show an increase in stiffness between 30 and 35%, and an increase in strength of 125% compared to quasi-static values.
Highlights
There are many engineering applications where fibre-reinforced polymer (FRP) composite structures are subjected to dynamic loading
Typical experimental kinematic fields are presented for two time steps for a 1–3 plane and 2–3 plane interlaminar specimen
Since it is difficult to get a true appreciation for the dynamic nature of an impact test through still images, videos of all kinematic field for all specimens, as well as the raw grey level images can be found in the data repository detailed at the end of the manuscript
Summary
There are many engineering applications where fibre-reinforced polymer (FRP) composite structures are subjected to dynamic loading (e.g., impact, blast, crash, etc.). Since interlaminar properties are matrix-dominated, literature suggests that the interlaminar stiffness and strength should exhibit a strain rate dependency [1, 2]. The number of studies attempting to measure high strain rate properties in the interlaminar direction are relatively scarce and inconsistent [2]. Studies on the strain rate dependency of interlaminar tensile properties are comparatively fewer compared to compression and shear. The effect of strain rate is not well understood, as exemplified by highly scattered measurements in available studies [2]. A similar situation exists for tensile strength measurements at high strain rates. Much ambiguity remains regarding the effect of strain rate on interlaminar stiffness and tensile strength. Scatter is likely amplified by other factors complicating tensile tests, such as gripping, alignment and geometry (stress concentrations)
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