TYPES OF LOADING

The primary factors to contemplate when specifying the sort of loading to which a machine part is subjected are the way of variation of the load and also the resulting variation of stress with time.

Stress variations are as followed:

1. Maximum stress, smax

2. Minimum stress, smin

3. Mean (average) stress, sm

4. Alternating stress, sa (stress amplitude)

The maximum and minimum stresses are usually computed from known information by stress analysis or finite-element methods, or they’re measured using experimental stress analysis techniques.

Then the mean and alternating stresses are often computed from sm = (smax + smin)/2 (5–1) sa = (smax – smin)/2 (5–2a) sa = (smax – sm) (5–2b)

The behavior of a cloth under varying stresses depends on the style of the variation.

One method want to characterize the variation is termed stress ratio.

Two sorts of stress ratios that are commonly used are defined as follows:

Stress ratio R = minimum stress maximum stress = smin smax

Stress ratio A = alternating stress mean stress = sa sm (5–3)

STATIC STRESS

When a component is subjected to a load that’s applied slowly, without shock, and is held at a continuing value, the resulting stress within the part is named static stress.

An example is that the load on a structure thanks to the dead weight of the building materials.

Because smax = smin, the strain ratio for static stress is R = 1.0.

Now you’re asked to continue this design exercise by selecting a fabric from which to create the 2 bent circular bars that are welded to the rigid support.

Also, you want to specify an acceptable diameter for the bars when a specific load is applied to the test material.

Static loading can even be assumed when a load is applied and is removed slowly so reapplied, if the quantity of load applications is tiny, that is, under some thousand cycles of loading.

REPEATED AND REVERSED STRESS

Pure Oscillation A stress reversal occurs when a given element of a load carrying member is subjected to a particular level of tensile stress followed by the identical level of compressive stress.

If this stress cycle is repeated many thousands of times, the strain is termed repeated and reversed or pure oscillation.

Because smin = -smax, the strain ratio is R = -1.0, and also the mean stress is zero.

An important example in machine design could be a rotating circular shaft loaded in bending.

In the position shown, part on the underside of the shaft experiences tensile stress while a component on the highest of the shaft sees a compressive stress of equal magnitude.

As the shaft is rotated 180° from the given position, these two elements experience an entire reversal of stress.

This is an outline of the classic loading case of repeated and reversed bending.

This type of loading is commonly called fatigue loading, and a machine is usually wont to test the fatigue behavior of materials.

The device shown is termed the R. R. Moore fatigue test device and therefore the material property thus measured is named endurance limit.

The shaft is supported by a control at each end while a yoke is supported on bearings.

A known loading is applied to the yoke leading to two concentrated loads being applied; one at each bearing that supports the yoke.

Note from the shearing force and bending moment that this kind of loading provides uniform bending moment between the yoke arms while the shearing force is zero.

Thus, pure bending occurs within the test section.

The shaft is machined to express dimensions with the center portion having a really gradual taper right down to atiny low diameter.

That diameter is often 0.300 in.

With the gradual taper, the strain concentration factor is virtually 1.0.

Furthermore, the shaft is polished to a fine surface finish in order that machining marks don’t affect the strain levels within the bar.

The shaft is rotated by an electrical motor while the system counts the quantity of revolutions.

It also contains a device to sense when the specimen breaks in order that there’s a known relationship between the strain level and also the number of cycles to failure.

Actually, reversed bending is simply a special case of fatigue loading, since any stress that varies with time can cause fatigue failure of part.

Many materials test laboratories are using computer-controlled, repeated and reversed axial loading rather than rotating bending to accumulate fatigue strength data.

It is described later that there are differences between these two methods in relevance the strength values obtained.

It is essential that care be exercised to work out what style of stress is employed to live the fatigue strength when using published data

8 Important Concepts That’ll Make You Better at Shaper & Planer Machine. (prajengineer.com)