Experimental determination of the micro-scale strength and stress-strain relation of an epoxy resin

• A method to determine a micro-scale stress-strain law of an epoxy resin used in polymer/fibre composites is presented.
• Before failure, the micro-scale strains in the epoxy reached 20% and the stress was estimated around 220–300 MPa.
• The micro-scale failure strain and stress of the epoxy were 3–4 times greater than the macro-scale values.
• An approach to compare strain fields around various notches by normalizing length parameters was experimentally validated.

An approach is developed for determining the stress-strain law and a failure stress appropriate for micro-mechanical models of polymer materials. Double cantilever beam test specimens, made of an epoxy polymer with notches having finite root radius, were subjected to pure bending moments in an environmental scanning electron microscope. The recorded images were used to measure strains around the notch with a 2D digital image correlation method. The strain in front of the notch was found to reach 20% before the failure initiation, which significantly exceeds the failure strain measured at the macro-scale (5–6%). The hardening exponent of a power law hardening material was obtained by the use of the J-integral, estimating the strain energy density around the notch. The hardening exponent was found to be within the range of 5–6 and the corresponding micro-scale failure stress was in the range of 220–300 MPa. Furthermore, the experimentally measured strains around the notch edge were compared with the strain field of the HRR-field. In addition, our experimental study shows that the strain fields between the notches with different notch root radii are comparable, if all length parameters are normalized with the width of deformed notch.

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