Digital Image Correlation of Strains at Profiled Wood Surfaces Exposed to Wetting and Drying

Julian Mallet,Philip Evans,Shankar Kalyanasundaram

doi:10.3390/jimaging4020038

Julian Mallet, Philip Evans + Show 1 more

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https://doi.org/10.3390/jimaging4020038

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Abstract

We hypothesize that machining grooves and ridges into the surface of radiata pine deck boards will change the pattern of strains that develop when profiled boards are exposed to wetting and drying. Two wavy profiles were tested, and flat unprofiled boards acted as controls. Full-field surface strain data was collected using digital image correlation. Strains varied across the surface of both flat and profiled boards during wetting and drying. Profiling fundamentally changed surface strain patterns; strain maxima and minima developed in the profile ridges and grooves during wetting, respectively, but this pattern of strains reversed during drying. Such a pronounced reversal of strains was not observed when flat boards were exposed to wetting and drying, although there was a shift towards negative strains when flat boards were dried. We conclude that profiling changes surface strain distribution in deck boards exposed to wetting and drying, and causes high strains to develop in the grooves of profiled boards. These findings help explain why checks in profiled deck boards are mainly confined to profile grooves where they are difficult to see, and the commercial success of profiling at reducing the negative effects of checking on the appearance of wood decking.

Highlights

Wood surfaces exposed to wetting and drying develop unbalanced strains that cause cracks to form [1]
Strains reversed during drying, as noted above. To explain these observations we suggest that during wetting of profiled deck boards, profile peaks swell and compress areas within grooves resulting in negative strains in profile grooves, and positive strains at profile peaks, as we observed
Profiling increased the strains that developed at the surface of radiata pine deck boards exposed to wetting and drying, and profoundly altered their spatial distribution and direction

Summary

Introduction

Wood surfaces exposed to wetting and drying develop unbalanced strains that cause cracks (checks) to form [1]. Wooden decks are losing market share to wood plastic composites that are less susceptible to checking [2,3]. The decking market is a large and important end-use for treated timber, and in the US alone over 3.6 billion lineal feet (1.1 billion metres) of decking valued at 7.1 billion dollars is manufactured each year [3]. Most wooden decking is smooth on both sides, but in Asia, Australia, Europe and New. Zealand, profiled decking that has small ridges and grooves machined into the surface of boards is common [4,5,6]. Profiling makes the surface of wood less slippery [5], and it reduces the negative effects of weathering checks on the appearance of decking [7].

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