Fatigue crack initiation and propagation in click flooring systems: experimental and numerical analysis

The comprehensive innovative methodology is proposed for evaluating the fatigue performance of the tongue-and-groove joints of multilayer click flooring systems through a coupled experimental and numerical approach. Special simplified mini castor chair test was developed to reproduce the loading conditions of the ISO 4918:2016 castor chair test while reducing the tested surface area by approximately 56%. The click assembly was subjected to pulsating tensile cyclic loading (R = 0) at 4 Hz under different load configurations relative to the tongue-groove joint, allowing the most critical configuration for fatigue damage to be identified. Stiffness degradation was quantified from the normalized secant modulus and correlated with quantitative observations from interrupted X-ray tomography. Three-dimensional finite element simulations were used to characterize the local cumulative plastic deformation associated with crack initiation. Combining these complementary measurements revealed that loading near the tongue-and-groove joint produced the most severe damage, with reductions in normalized secant modulus of 20–34%, whereas loading directly above the joint resulted in reductions of less than 10%. X-ray tomography revealed two distinct crack types: transverse cracks propagating into the bulk material and longitudinal cracks developing along the click joint. Comparison with the standard castor chair test identified longitudinal cracking as the damage mode associated with functional failure in the standard castor chair test. Under the most critical loading configuration, longitudinal cracks were first detected at 10,000 cycles, following scans showing no detectable crack at 5000 cycles. At 20,000 cycles, the mini castor chair test reproduced both the location and millimeter-scale longitudinal cracking observed at functional failure in the ISO test, for which the crack lengths of 2.3–2.6 mm were measured in the side view of the joint. On the short side, crack lengths comparable to those observed at functional failure in the ISO test corresponded to a 20% reduction in normalized secant modulus in the mini castor chair test. Finite element simulations predicted maximum principal stresses in the experimentally observed crack-initiation zones, exceeding the tensile yield stress of the core material. Overall, the results establish a methodology for evaluating the fatigue performance of click-flooring assemblies that goes beyond the characterization of individual constituent materials by explicitly accounting for the geometry of the click profile and the mechanical response of the joint under representative fatigue loading.