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Article Excerpt
Abstract
Edgewise bending fatigue performances of three wood-based composites (southern yellow pine plywood, oriented strandboard, and particleboard) were evaluated by subjecting them to zero-to-maximum constant amplitude and stepped cyclic bending loads. Results of zero-to-maximum constant amplitude cyclic load tests indicated that fatigue lives of 25,000 cycles each began at stress levels of 75 and 70 percent of modulus of rupture (MOR) values for the plywood and oriented strandboard evaluated in this study, respectively. Particleboard fatigue life did not reach 25,000 cycles until the stress level was reduced to 55 percent of its MOR value. Regression analysis of S-N data (applied nominal stress versus log number of cycles to failure) indicated a linear relationship between applied nominal stress and the logarithm of number of cycles to failure. It was observed that the S-N function relationship could be expressed with the form S = MOR (1 - H X [log.sub.10] [N.sub.f]). The constant H values in the equation were 0.05, 0.07, and 0.09 for plywood, oriented strandboard, and particleboard, respectively. It seems that the constant H is correlated to basic wood element sizes of composite raw material such as veneer and particles. Cyclic stepped load tests of full-size sofa back top rail specimens verified that the Palmgren-Miner rule is an effective method to estimate fatigue life of wood composites subjected to the edgewise cyclic stepped bending stresses using their S-N curves.
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Strength design of upholstered furniture frames should take into account information about member material fatigue strength properties since most service failures of the frames appear to be fatigue related (Eckelman and Zhang 1995). As more plywood and engineered wood composites are used for furniture frame structural materials, and furniture manufacturers continue to seek new materials in order to re-engineer their products, the information related to fatigue strength properties of various types of wood composites becomes increasingly essential.
In engineering, the term fatigue is defined as the progressive damage that occurs in a material subjected to cyclic loading (USDA 1999). This loading may be repeated (stresses of the same sign, that is, always compression or always tension, for example, zero-to-maximum complete repeated stressing refers to cases with a zero stress ratio) or reversed (stresses of alternating compression and tension, that is, a non-zero stress ratio). The stress ratio, R, is defined as the ratio of minimum stress over maximum stress per cycle (Dowling 1999). The stress range is the difference between the maximum and the minimum stress values. Half the stress range is called the stress amplitude. Averaging the maximum and minimum values gives the mean stress. Fatigue life is the number of cycles that are sustained before failure, while fatigue strength is the maximum stress attained in the stress cycle determining fatigue life. Fatigue strength versus life is a stress-life curve, also called an S-N curve. The stress amplitude or nominal stress is commonly plotted versus the number of cycles to failure in metals. The stress level (the percentage of the static strength) versus the number of cycles to failure is commonly seen in wood and wood composites research publications (Kommers 1943, Cai et al. 1996). The fatigue limit, or endurance limit, is defined as the stress to which a specimen can be subjected an infinite number of times with-out failure.
There are three major approaches to analyzing and designing against fatigue failures: the stress-based approach, the strain-based approach, and the fracture mechanics approach (Dowling 1999). The stress-based approach is to base analysis on the nominal (average) stresses in the region of the component being analyzed. The nominal stress that can be resisted under cyclic loading is determined by considering mean stresses and by making adjustments for the effects of stress raisers. The strain-based approach involves more detailed analysis of the localized yielding that may occur at stress raisers during cyclic loading. The fracture mechanics approach specifically treats growing cracks using the methods of fracture mechanics.
Factors influencing fatigue strength and life of wood and wood composites are frequency of cycling, repetition or reversal of loading, stress ratio, temperature, moisture content (MC), and specimen size (USDA 1999).
Creep, temperature rise, and loss of MC occur in tests of wood for fatigue strength (USDA 1999) at faster cyclic loading. Smaller rises in temperature would be expected for slower cyclic loading or lower stresses. Decreases in MC are probably related to temperature rise.
Fatigue study results of wood and plywood subjected to repeated and reversed flatwise bending stresses at 1,790 cycles per minute (Kommers 1943) indicated that the S-N curves (percentage of mean control static modulus of rupture [MOR] versus the number of cycles to failure) for wood does not exhibit a "knee" as do the curves of ferrous metals. No endurance limits were established for tested wood specimens, and the experimental data indicate that if an endurance limit for wood exists, it occurs above 50 million cycles. The fatigue strength for 50 million cycles of reversed stress is approximately 27 percent of the static MOR for the species investigated (yellow birch, yellow-poplar, Sitka spruce, and Douglas-fir) whether in the form of solid wood or...
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