Breaking Four-Point and Three-Point Bending Tests

TL;DR

This paper uses phase-field fracture theory to analyze three-dimensional fracture behavior in three- and four-point bending tests, clarifying how to interpret their results and explaining differences in measured flexural strengths.

Contribution

It provides a comprehensive 3D fracture analysis of bending tests using phase-field theory, improving understanding of fracture nucleation and propagation in these tests.

Findings

Four-point bending tests generally produce smaller flexural strengths than three-point tests.

The analysis explains the fracture initiation points and crack paths in bending tests.

Results help practitioners choose appropriate testing methods and interpret data accurately.

Abstract

Since their initial standardizations in the 1930s and 1950s, the so-called four-point and three-point bending tests on unnotched beams have been embraced by practitioners as two popular methods to indirectly measure the tensile strength of concrete, ceramics, and other materials with a large compressive strength relative to their tensile strength. This is because of the ease that the tests afford in both the preparation of the specimen (a beam of rectangular cross section) and the application of the loads (simple supports pressing on the specimen). Yet, this practical advantage has to be tempered by the fact that the observations from both of these tests -- being \emph{indirect} experiments in the sense that they involve \emph{not} uniform uniaxial tension but non-uniform triaxial stress states throughout the specimen -- have to be appropriately interpreted to be useful. By making use of the phase-field fracture theory initiated by Kumar, Francfort, and Lopez-Pamies (2018), which has been recently established as a complete theory of fracture capable of accurately describing the nucleation and propagation of cracks in elastic brittle materials under arbitrary quasistatic loading conditions, the main objective of this paper is to carry out a thorough 3D quantitative analysis of when and where fracture nucleates and propagates in four-point and three-point bending tests and thereby establish how to appropriately interpret their results. As a corollary, the analysis provides an explanation for why four-point bending tests typically yield smaller flexural strengths than three-point bending tests, a source of constant headaches for practitioners who have been left to wonder which test -- if any -- would be more appropriate for their purposes.