Page 32 - T. Anderson-Fracture Mechanics - Fundamentals and Applns.-CRC (2005)

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12 Fracture Mechanics: Fundamentals and Applications

design approaches developed in the U.S. and the British CTOD methodology have begun to merge
in recent years, with positive aspects of each approach combined to yield improved analyses. Both
parameters are currently applied throughout the world to a range of materials.
Much of the theoretical foundation of dynamic fracture mechanics was developed in the period
between 1960 and 1980. Significant contributions were made by a number of researchers, as
discussed in Chapter 4.

1.2.5 FRACTURE MECHANICS FROM 1980 TO THE PRESENT
The field of fracture mechanics matured in the last two decades of the 20th century. Current research
tends to result in incremental advances rather than major gains. The application of this technology
to practical problems is so pervasive that fracture mechanics is now considered an established
engineering discipline.
More sophisticated models for material behavior are being incorporated into fracture mechanics
analyses. While plasticity was the important concern in 1960, more recent work has gone a step
further, incorporating time-dependent nonlinear material behavior such as viscoplasticity and vis-
coelasticity. The former is motivated by the need for tough, creep-resistant high temperature materials,
while the latter reflects the increasing proportion of plastics in structural applications. Fracture
mechanics has also been used (and sometimes abused) in the characterization of composite materials.
Another trend in recent research is the development of microstuctural models for fracture and
models to relate local and global fracture behavior of materials. A related topic is the efforts to
characterize and predict geometry dependence of fracture toughness. Such approaches are necessary
when traditional, so-called single-parameter fracture mechanics break down.
The continuing explosion in computer technology has aided both the development and application
of fracture mechanics technology. For example, an ordinary desktop computer is capable of performing
complex three-dimensional finite element analyses of structural components that contain cracks.
Computer technology has also spawned entirely new areas of fracture mechanics research.
Problems encountered in the microelectronics industry have led to active research in interface
fracture and nanoscale fracture.

1.3 THE FRACTURE MECHANICS APPROACH TO DESIGN

Figure 1.7 contrasts the fracture mechanics approach with the traditional approach to structural
design and material selection. In the latter case, the anticipated design stress is compared to the
flow properties of candidate materials; a material is assumed to be adequate if its strength is greater
than the expected applied stress. Such an approach may attempt to guard against brittle fracture
by imposing a safety factor on stress, combined with minimum tensile elongation requirements on
the material. The fracture mechanics approach (Figure 1.7(b)) has three important variables, rather
than two as in Figure 1.7(a). The additional structural variable is flaw size, and fracture toughness
replaces strength as the relevant material property. Fracture mechanics quantifies the critical com-
binations of these three variables.
There are two alternative approaches to fracture analysis: the energy criterion and the stress-
intensity approach. These two approaches are equivalent in certain circumstances. Both are dis-
cussed briefly below.

1.3.1 THE ENERGY CRITERION

The energy approach states that crack extension (i.e., fracture) occurs when the energy available
for crack growth is sufficient to overcome the resistance of the material. The material resistance
may include the surface energy, plastic work, or other types of energy dissipation associated with
a propagating crack.