Fatigue and creep in chipboard. 3. The effect of frequency

Structural grade chipboard used as floor decking can be exposed to a combination of creep loading, and fatigue loading at a wide range of frequencies. Creep loads are produced by static masses such as machinery. Fatigue loads arise from intermittent loads such as fork lift trucks in motion, people in motion, or vibrating machinery. Four-point bend tests in fatigue and creep have been performed simultaneously on matched sets of four structural grade chipboard samples. The aim was to examine the effect of frequency on the fatigue performance of chipboard by following the rate of microstrain development in each case at the same peak stress. The ratio of the minimum to the maximum fatigue stress (the R ratio) was set at R=0.1. Fatigue tests were performed at three different frequencies classified as low, medium and high, determined by the rate of application of stress. The performance of chipboard in flooring applications is shown to be highly dependent on the loading frequency. Increasing the frequency increased the number of cycles to failure as did reducing the stress level. Using hysteresis loop capture to monitor damage accumulation during testing indicates that there is a fatigue limit for chipboard loaded at constant amplitude at some point just below 20% of its static bending strength.