Methods for automated crack length detection in fracture mechanical fatigue tests of unreinforced polymers

摘要

The continuous measurement of crack growth during fracture mechanical fatigue tests is of great importance for an accurate lifetime estimation of components. Different methods for an automatic crack length detection exist for linear elastic materials and electrically conductive materials like metals (ASTM International E 647-11, 2011; International Standard ISO 15850, 2002a) or fiber reinforced composites (ASTM International D5528-13,2013; International Standard ISO 15024,2001). However, there is no satisfying automatic system for the crack length detection in polymers. For polymers, it is still state of the art to measure the crack length manually by the use of a travelling microscope. Besides other drawbacks, this way of measuring provides only a limited number of data points. Therefore, the aim of this work was to develop methods for the automated crack length detection in these materials. Two measuring techniques were examined regarding their general applicability: (i) infrared thermography (IRT) and (ii) digital image correlation (DIC). For the validation, three types of thermoplastics (Polyvinylchloride (PVC-U), Polymethylmethacrylate (PMMA) and Polyoxymethylene (POM)), which show rather different mechanical behavior, were tested. Satisfactory correlations were obtained for both DIC and IRT method. In general, all tested systems showed good results and the determined average difference in the measured crack lengths was smaller than 5.5%. Hence, the investigated methods based on DIC and IRT are highly interesting for an automation of fatigue crack growth measurements of polymers.