Fatigue in material science is a phenomenon where cyclic loading can cause failure at far lower stress levels than the strength of the material. Crack initiation and crack propagation are longtime not seen until the part fails. Engineers must design their parts with enough safety margin to prevent fatigue failure. But how do biological organisms mitigate fatigue? Here we present the wood tissue of living trees that relies on the prevention of fatigue upon repeated loading.
 Trees are exposed to cyclic loads, mainly by wind, causing the stem to bend back and forth, leading to alternating compression and tension in the wood tissue. Fatigue experiments on construction wood samples showed that, under compression, cell walls kink close to the boundary between early and late wood, leading to buckling of early wood tracheids [1]. Under tension, these creases initiate cracks, which propagate with each loading cycle [2]. In contrast to the dried construction wood, the wood in a living tree is water saturated and continuously grows throughout its life as new cell layers are added each year. In addition, exposure to external loadings influences this tissue growth as the tree adapts locally the amount of newly formed material to minimize stress concentrations [3]. Therefore, we explore experimentally how continuous fully reversed loading influences stem tissue growth and how the addition of new cell material can counteract crack formation and protect the wood tissue from fatigue failure
Understanding these mechanisms in living trees will give inspiration to engineers for their design of synthetic functional materials resistant against fatigue failure.

References
[1] M. Gong, I. Smith, Journal of the Institute of Wood Sciences, 2000, 15, 204–210. 
[2] K.T. Tsai, M.P. Ansell, Journal of Material Sciences, 1990, 25, 865–878.
[3] C. Mattheck, K. Bethge, Naturwissenschaften, 1998, 85, 1–10.