Designs for Different Types of Loading

Designs for Different Types of Loading - Designs for...

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1 Designs for Different Types of Loading Chapter 5 Material taken from Chapter 5 Mott, 2003, Machine Elements in Mechanical Design Loading Types ± The manner of computing the design stress depends on the manner of loading and on the type of material. Loading types include the following: – Static – Repeated and reversed – Fluctuating – Shock or impact – Random Types of Loading and Stress Ratio ± The primary factors to consider when specifying the type of loading to which a machine part is subjected are the manner of variation of the load and the resulting variation of stress with time. ± Stress variations are characterized by four key values: – Maximum stress, σ max – Minimum stress, σ min – Mean (average) stress, σ m – Alternating stress, σ a (stress amplitude)
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2 Types of Loading and Stress Ratio ± The maximum and minimum stresses are usually computed from known information by stress analysis or finite- element methods, or they are measured using experimental stress analysis techniques. ± Then the mean and alternating stresses can be computed from: ¾ σ m = ( σ max + σ min ) / 2 ¾ σ a = ( σ max - σ min ) / 2 Types of Loading and Stress Ratio ± The behavior of a material under varying stresses is dependent on the manner of the variation. One method used to characterize the variation is called stress ratio. ± Stress ratio R = minimum stress = σ min maximum stress σ max ± Stress ratio A = alternating stress = σ a mean stress σ m Static Stress ± When a part is subjected to a load that is applied slowly, without shock, and is held at a constant value, the resulting stress in the part is called static stress. ± Because σ max = σ min , the stress ratio for static stress is R = 1.0.
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3 Static Stress Mott, 2003, Machine Elements in Mechanical Design Repeated and Reversed Stress ± A stress reversal occurs when a given element of a load-carrying member is subjected to a certain level of tensile stress followed by the same level of compressive stress. ± The stress is called repeated and reversed. ± Because σ min = - σ max , the stress ratio is R = -1.0, and the mean stress is zero. Repeated, Reversed Stress
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4 Fluctuating Stress ± When a load-carrying member is subjected to an alternating stress with a nonzero mean, the loading produces fluctuating stress. Fluctuating Stresses Mott, 2003, Machine Elements in Mechanical Design Shock, Impact, and Random Loading ± Loads applied suddenly and rapidly cause shock or impact. ± Examples include a hammer blow, a weight falling onto a structure, and the action inside a rock crusher. ± When varying loads are applied that are not regular in their amplitude, the loading is called random.
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5 Endurance Strength ± The endurance strength of a material is its ability to withstand fatigue loads. In general, it is the stress level that a material can survive for a given number of cycles of loading. If the number of cycles is infinite the stress level is called the endurance limit. ± Endurance strengths are usually charted on a
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Designs for Different Types of Loading - Designs for...

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