Mechanical Properties of Norway Spruce: Intra-Ring Variation and Generic Behavior of Earlywood and Latewood until Failure

TL;DR

This study investigates the mechanical properties of Norway spruce's growth rings, showing how earlywood and latewood differ in strength and behavior, with humidity affecting parameters but not significantly due to variability.

Contribution

It provides detailed insights into the intra-ring variation and generic failure behavior of earlywood and latewood in Norway spruce under different humidity conditions.

Findings

Mechanical parameters decrease with increasing humidity.

Earlywood exhibits greater strain at failure than latewood.

Failure occurs in the middle lamella for both tissues.

Abstract

The alternating earlywood and latewood growth ring structure has a strong influence on the mechanical performance of Norway spruce. In the current study, tensile tests in the longitudinal and tangential directions were performed on a series of specimens representing one growth ring at varying relative humidities. All tested mechanical parameters, namely modulus of elasticity and ultimate tensile stress, followed the density distribution in the growth ring, with the minimum values in earlywood and the maximum values in latewood. The samples were conditioned at three the relative humidities 50%, 65% and 95%. With increasing relative humidity, the values of the mechanical parameters were found to decrease. However, due to the high local variability, this decrease was not statistically significant. The test in the tangential direction on a set of earlywood and latewood specimens at 65% relative humidity revealed a similar limit of linear elasticity for both early- and latewood. Where the strength of both tissues was equal, the strain at failure was significantly greater for earlywood. Furthermore, the portion of the non-linear stress/strain behavior for earlywood was significantly greater. A Weibull analysis on the ultimate tensile strength revealed a tissue-independent Weibull modulus, which indicates similar defect distributions. For both, the failure occurred in the middle lamella.