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Stress intensity factor : ウィキペディア英語版
Stress intensity factor

The stress intensity factor, K, is used in fracture mechanics to predict the stress state ("stress intensity") near the tip of a crack caused by a remote load or residual stresses. It is a theoretical construct usually applied to a homogeneous, linear elastic material and is useful for providing a failure criterion for brittle materials, and is a critical technique in the discipline of damage tolerance. The concept can also be applied to materials that exhibit ''small-scale yielding'' at a crack tip.
The magnitude of K depends on sample geometry, the size and location of the crack, and the magnitude and the modal distribution of loads on the material.
Linear elastic theory predicts that the stress distribution (\sigma_) near the crack tip, in polar coordinates (r,\theta) with origin at the crack tip, has the form
:
\sigma_(r, \theta) = \frac ( \theta) + \,\,\rm

where K is the stress intensity factor (with units of stress \times length1/2) and f_ is a dimensionless quantity that varies with the load and geometry. This relation breaks down very close to the tip (small r) because as r goes to 0, the stress \sigma_ goes to \infty. Plastic distortion typically occurs at high stresses and the linear elastic solution is no longer applicable close to the crack tip. However, if the crack-tip plastic zone is small, it can be assumed that the stress distribution near the crack is still given by the above relation.
== Stress intensity factors for various modes ==

Three linearly independent cracking modes are used in fracture mechanics. These load types are categorized as Mode I, II, or III as shown in the figure. Mode I, shown to the left, is an opening (tensile) mode where the crack surfaces move directly apart. Mode II is a sliding (in-plane shear) mode where the crack surfaces slide over one another in a direction perpendicular to the leading edge of the crack. Mode III is a tearing (antiplane shear) mode where the crack surfaces move relative to one another and parallel to the leading edge of the crack. Mode I is the most common load type encountered in engineering design.
Different subscripts are used to designate the stress intensity factor for the three different modes. The stress intensity factor for mode I is designated K_ and applied to the crack opening mode. The mode II stress intensity factor, K_, applies to the crack sliding mode and the mode III stress intensity factor, K_, applies to the tearing mode. These factors are formally defined as:〔

\begin
K_ & = \lim_ \sqrt\,\sigma_(r,0) \\
K_ & = \lim_ \sqrt\,\sigma_(r,0) \\
K_ & = \lim_ \sqrt\,\sigma_(r,0) \,.
\end


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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