Figure 6.22 Complex crack configurations. (a) Crack emanating from hole; (b) circular corner crack; (c) elliptical corner crack emanating from through-thickness hole. B. (a) (c)
Design Against Fluctuating Loads
Machine elements are subjected to varieties of loads, some components are subjected to static loads, while some machine components are subjected to fluctuating loads, whose load magnitude tends to fluctuate. The components of a machine, when rotating at a high speed, are subjected to a high degree of load, which fluctuates from a high value to a low value. For the machine elements under the action of static loads, static failure theories are applied to know the safe and hazardous working conditions and regions. However, most of the machine elements are subjected to variable or fluctuating stresses, due to the nature of load that fluctuates from high magnitude to low magnitude. Also, the nature of the loads is repetitive. For instance, shafts, bearings, cams and followers, and so on.
Design Against Fluctuating Load
Stress is defined as force per unit area. When there is localization of huge stresses in mechanical components, due to irregularities present in components and sudden changes in cross-section is known as stress concentration. For example, groves, keyways, screw threads, oil holes, splines etc. are irregularities.
Cortical bone is decidedly anisotropic in its fracture
behavior. Crack propagation parallel to the long axis of a
bone is much easier than perpendicular to the axis
(i.e., “across” the bone). When fracture of a human tibia is measured in the parallel crack configuration, an average
value of KIC = 3.95 MPa-m1/2 is found. Imagine that a hole
is drilled through cortical bone in a direction perpendicular
to the bone axis. The hole is intended to be used for insertion
of a pin associated with repair of a nearby fracture. Imagine
that in the hole drilling process, a semicircular crack is
created emanating from the drilled hole aligned with the
osteon cement lines (see Figure 6.22c for clarification).
Assuming that the cortical bone is 3 mm thick, the modulus
can be estimated as E = 20 GPa, and that during the surgery
the worst-case tension strain that the bone will experience
perpendicular to the crack is 2 x 10-3, what is the maximum
tolerable crack radius to avoid fracture at the drilled hole?
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