ABSTRACT
The main assignment during the application of practical elements of fracture
mechanics to design is to protect the machines and structures from unsuspected
and uncontrollable structural failures. For this purpose it is prudent to assume that
all engineering materials contain some flaws and irregularities, which at the
appropriate stress levels can initiate the process of fracture. In particular, high-
strength and low-toughness materials can be subject to brittle behavior, and there-
fore a part of the designer’s effort should be devoted to estimating the correct
level of working loads that can be sustained without causing crack propagation.
In order to assure such a condition it is necessary to select the appropriate design
formulas and the certified mechanical properties. Such input is discussed in var-
ious portions of this book. It is also shown that a compound parameter such as
plane-strain fracture toughness involves the applied stress and the square root
of the crack size with the specified numerical multiplier. This parameter remains
of primary interest. It is also proper to comment here that although literally thou-
sands of K formulas have appeared in the handbooks and technical papers over
the past 40 years or so, it is surprising how often we tend to rely on a simple
expression such as [s(a)1/2], multiplied by a suitable constant term. Also the “theory and practice” shows that as long as this product is kept below, say, the
KIc magnitude for the selected material, the presence of crack a for the case at
