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capacity due to the increased probability of defects in larger damage mechanism. Sinusoidal loading is more critical than
volumes (the ‘weakest link’ theory). triangular, and both are more representative of real-world

While some studies on engineered wood like LVL have conditions than square waves.
not found a significant size effect in fatigue, others warn that Frequency has a substantial impact. Lower frequencies
results from small specimens cannot be directly transferred (e.g., below 0.5 Hz) allow creep to dominate, leading to shorter
to full-scale structures without consideration of this effect. fatigue lives. The influence of frequency is more pronounced at
The size effect in wood fatigue remains an area requiring higher load levels and higher moisture contents. At very high
further research. frequencies (e.g., 1 kHz), the influence becomes negligible,
but such frequencies can cause internal heating and drying,
Influence Of Load Type & Shape artificially improving fatigue life.
The load ratio R (min stress/max stress) and mean stress are

critical parameters. A positive mean stress (tensile) is more Wood & Wood-Based Materials
detrimental than a negative one (compressive). The relationship The natural variability of solid wood makes standardising its
between mean stress and stress amplitude (σa) for a given fatigue response challenging. Fatigue life appears to be largely
fatigue life is best described for wood by a polynomial Gerber independent of wood species when stress is normalised to
line, while the linear Goodman diagram provides a conservative static strength.
approximation. Knots act as stress concentrators, typically reducing
The waveform of the load cycle (sinusoidal, triangular, fatigue life, though their higher density can sometimes have

square) affects fatigue life. Square waveforms are the most a compensating effect.
destructive, as they combine creep and fatigue into a single Products like Glued Laminated Timber (GLT), Laminated