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44 MATERIALS NOV/DEC 2025 FDM ASIA | www.fdmasia.com

identifying localised damage. by considering elastic, viscous, and viscoelastic strain
While real-time processing at high fatigue frequencies components. However, they can be complex due to challenges
remains challenging, DIC has been successfully used to monitor in separating the energy that actually causes damage from
phenomena like humidity-induced swelling during fatigue tests. that which is recoverable.

Strain Gauges are widely used for measuring local (3) Number of Cycles Models: This approach uses linear
deformations. Their main limitation in wood is the material's fracture mechanics. For constant amplitude loading, Paris' law
heterogeneity; measurements from a small gauge may not relates the crack growth rate per cycle to the stress intensity
represent the global response. Despite this, they remain a factor. For variable amplitudes, the Palmgren-Miner rule sums
reliable and commonly used tool for both static and fatigue the damage fraction from each stress level. These models are
testing. simple but do not account for frequency or creep effects.
Other methods like Ultrasonic testing (for measuring (4) Damaged Viscoelastic Material (DVM) Model: This is
dynamic modulus of elasticity) and Linear Variable Differential one of the most comprehensive models for wood fatigue. It
Transformers (LVDTs for displacement) are also used but assumes an initial crack and defines damage as the energy

have specific limitations regarding sample size or real-time released during its propagation. The DVM model uniquely
monitoring capability. combines time-dependent (viscoelastic) behaviour with cyclic
Predictive models for wood fatigue generally fall into two loading effects, making it particularly suitable for wood. Its
categories: those based on the number of cycles to failure main limitation is the requirement of a pre-existing crack.
(S-N curve approach) and those based on the duration of load. Among these, the DVM model is considered the most
However, pure forms of these models are often inadequate applicable as it best adapts to the viscoelastic nature of
for wood, leading to the development of more sophisticated wood under cyclic loads.

alternatives: Finite Element Method (FEM) simulations are powerful tools
(1) Damage Accumulation Models: These models use for predicting the fatigue life of complex wooden structures.
a damage variable (α) that evolves from 0 (undamaged) to Successful simulation requires selecting appropriate damage
1 (failure). While simple, they typically represent only time- criteria and material models.
dependent damage and do not fully capture the effects of (1) Damage Criteria: Anisotropic failure criteria like Hashin
load oscillations. and Tsai-Wu, originally developed for fibre-reinforced composites,
(2) Energy Models: These models propose that failure are often applied to wood due to its similar microstructure.
occurs when the accumulated energy from loading cycles The Hashin criterion, for example, can distinguish between
reaches a critical value. They account for wood's viscoelasticity different failure modes like fibre fracture and matrix crushing.

(2) Advanced Constitutive Models: Sophisticated 3D
freepik.com material models have been developed that incorporate orthotropic
elasticity, plasticity, and isotropic damage. These models can
simulate the stress-strain response under variable-amplitude
cyclic loading and are calibrated using experimental data from
monotonic and cyclic tests.
(3) Cohesive Zone Models (CZM): These are effective for

simulating crack growth in quasi-brittle materials like wood.
CZMs describe the nonlinear process of damage formation