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• The researchers set the fatigue strength at 107 cycles, for of frequency, temperature, and load direction in relation to
softwood (spruce, fir and Japanese cypress) it is 15–35 moisture content and heat generation on fatigue strength.
percent of the static strength.
Table 2: Effect of the combination of influential factors on
• The energy loss per load cycle is initially higher, then
wood fatigue.
decreases in a few cycles and remains approximately
constant until shortly before failure, where it increases Influencing Factor Influence on Fatigue
exponentially in just a few cycles. Frequency stronger influence with higher
humidity and lower frequency.
• Due to the influence of frequency, the waveform of the
High temperature stronger influence on fatigue strength
load cycle and the time under load must not be neglected. reduction at higher humidity.
• The square waveform of cyclic loading is the most
Load outside the strong influence on the reduction of
destructive. anatomical direction fatigue strength.
• The Goodman line describing the relationship between αm Compressive load higher fatigue strength.
and αa, is conservative for wood, the relationship is better High frequency, low load low heat generation.

described by a polynomial Gerber line. High load, low frequency high heat generation.
• At sufficiently low load frequencies (<0.5 Hz), the influence
Bending stress in better fatigue strength.
of creep begins to dominate over fatigue. tangential direction
• Damage from creep occurs earlier than damage from fatigue
if the criterion is time under load, while the accumulation Future Prospect
of damage from fatigue occurs faster. Certainly, unexplored areas themselves suggest directions
• Knowledge about fatigue cannot be transferred directly for future work. Although our overview is comprehensive, it

from tested samples to structural elements due to the does not cover all aspects. A separate study on the fatigue
size effect. of different types of glulam and wood composites compared
• The deviation of the load angle from the fibre direction to solid wood is needed.
should not exceed 10 degrees for elements subjected to Further investigation into the fatigue of joints would also
fatigue. be valuable. There are many potential applications in the field
• The process of quality control and the classification of of measurement methods.
wood into strength classes can damage the wood and In particular, it would be beneficial to test the integration of
reduce its resistance to fatigue. sensors. For example, ultrasound could be used to measure
• The dependence of fatigue on the frequency of cyclic the dynamic modulus of elasticity under cyclic loading, while

loading is greatest in compression and least in bending. deformation or stress could be measured using the DIC method.
• At lower frequencies, up to 100 Hz, the influence of Most importantly, wood fatigue testing methodology should
frequency on fatigue is greater, while at higher frequencies, be standardised, including predetermined specimen size ratios,
such as one kHz, it is negligible. waveforms, frequencies, load ratios, load directions (including
• It is not yet clear if engineered wood is more resistant specimen fixtures), moisture content, and density classes.
to fatigue because it has the positive properties of solid Overall, expanding research on wood fatigue will contribute
wood and greater homogeneity. to optimised structures and products, reducing the impact on

The findings presented in Table 2 are a direct result of the renewable resource, wood. FDM
the literature review and additionally summarise the effects ENQUIRY NO. 6301