Page 291 - Materials Science and Engineering An Introduction

8.5 Principles of Fracture Mechanics • 263

Table 8.2
Defect Size
A List of Several Technique Defect Location Sensitivity (mm) Testing Location
Common
Nondestructive Scanning electron Surface 0.001 Laboratory
Testing Techniques microscopy (SEM)
Dye penetrant Surface 0.025–0.25 Laboratory/in-field
Ultrasonics Subsurface 0.050 Laboratory/in-field
Optical microscopy Surface 0.1–0.5 Laboratory
Visual inspection Surface 0.1 Laboratory/in-field
Acoustic emission Surface/subsurface 0.1 Laboratory/in-field
Radiography (x-ray/ Subsurface 2% of specimen Laboratory/in-field
gamma ray) thickness

However, if stress level and plane strain fracture toughness are fixed by the design
situation, then the maximum allowable flaw size a c is given by

Computation of 1 K Ic 2
maximum allowable a c = p a sY b (8.7)
flaw length
A number of nondestructive test (NDT) techniques have been developed that
3
permit detection and measurement of both internal and surface flaws. Such techniques
are used to examine structural components that are in service for defects and flaws that
could lead to premature failure; in addition, NDTs are used as a means of quality con-
trol for manufacturing processes. As the name implies, these techniques do not destroy
the material/structure being examined. Furthermore, some testing methods must be
conducted in a laboratory setting; others may be adapted for use in the field. Several

commonly employed NDT techniques and their characteristics are listed in Table 8.2. 4
One important example of the use of NDT is for the detection of cracks and leaks
in the walls of oil pipelines in remote areas such as Alaska. Ultrasonic analysis is utilized
in conjunction with a “robotic analyzer” that can travel relatively long distances within
a pipeline.

DESIGN EXAMPLE 8.1

Material Specification for a Pressurized Spherical Tank
Consider a thin-walled spherical tank of radius r and thickness t (Figure 8.11) that may be used
as a pressure vessel.
(a) One design of such a tank calls for yielding of the wall material prior to failure as a result
of the formation of a crack of critical size and its subsequent rapid propagation. Thus, plastic
distortion of the wall may be observed and the pressure within the tank released before the
occurrence of catastrophic failure. Consequently, materials having large critical crack lengths

3 Sometimes the terms nondestructive evaluation (NDE) and nondestructive inspection (NDI) are also used
for these techniques.
4 Section M.5 of the Mechanical Engineering (ME) Online Module discusses how NDT are used in the detection
of flaws and cracks.