New insights into the interpretation of the results of four point bending tests on float glass

Abstract

The four point bending test is one of the most commonly used and standardised tests to determine the mechanical properties of materials. For its use on float glass there are both the ASTM C158-02R17 and European EN 1288-3:2001 standards. However when testing float glass the results tend be a statistical muddle. This impacts the reliability of the design strength of float glass determined using four point bending tests. In an attempt to resolve this problem a series of four point bending tests were conducted which were designed to be more systematic than those previously reported in the literature and which use newly developed digital microscopy techniques for pre- and post-test analysis. By systematically testing, the test results can be divided into different groups based on air side, Sn side and source of failure, allowing the data to be divided into clear and separate statistical groups. Secondly the results can potentially be used to validate the lower bound glass strength theory proposed by Ballarini et al. (J Eng Mech 142(12):04016100, 2016). The glass specimens were industrially cut, ground, chamfered and flat polished on the long sides. Specimens were checked using advanced digital microscopy before and after testing. The results suggest that uni-modal Weibull behaviour only applies above a critical failure stress. Failures at stresses below this critical failure stress as a group have a separate and steeper Weibull slope. This supports Ballarini’s theory for a lower bound failure strength although there are important differences between the air side and Sn side of the specimens which this theory does not currently allow for. These differences seem to be inherent to the differences between the glass and the Sn sides. The results also show that the strength of cut, ground, chamfered and flat polished glass can be high but that inconsistency in process control and the irregular occurrence of surface failures are the main causes for the statistical spread. Digital microscopy can reliably measure the quality of the various surfaces and intersections of surfaces of a glass specimen but there is no absolute relation between the size of a detected defect and the probability that this defect actually leads to failure.