Comprehensive creep compliance characterization of orthotropic materials using a cost-effective automated system

TL;DR

This paper introduces an automated, climate-controlled system for comprehensive measurement of creep compliance in orthotropic materials like Norway spruce, capturing directional and climatic effects to improve modeling accuracy.

Contribution

It presents a novel automated creep testing device capable of measuring all orthotropic compliance components under varying climatic conditions, addressing limitations of previous simplified methods.

Findings

Measured all nine creep compliance components of Norway spruce.

Found loading asymmetry effects up to 16%.

Identified four key components representing full orthotropy.

Abstract

Determining the creep compliances of orthotropic composite materials requires experiments in at least three different uniaxial and biaxial loading directions. Up to date, data respecting multiple climates and all anatomical directions are sparse for hygro-responsive materials like Norway spruce. Consequently, simulation models of wood frequently over-simplify creep, e.g., by proportionally scaling missing components or neglecting climatic influences. To overcome such simplifications, an automated computer-controlled climatized creep rack was developed, that experimentally assesses moisture-dependent viscoelasticity and mechanosorption in all anatomical directions. The device simultaneously measures the creep strains of three dogbone tension samples, three flat compression samples, and six Arcan shear samples via Digital Image Correlation. This allows for ascertaining the complete orthotropic compliance tensors while accounting for loading direction asymmetries. This paper explains the creep rack's structure and demonstrates its use by determining all nine independent creep compliance components of Norway spruce at 65% relative humidity. The data shows that loading asymmetry effects amount up to 16%. Furthermore, the found creep compliance tensor is not proportional to the elastic compliance tensor. By clustering the compliance components, we identify four necessary components to represent the full orthotropy of the compliance tensor, obtainable from not less than two experiments.