Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-09 GTBL042-Callister-v3 October 4, 2007 11:53

2nd Revised Pages

9.5 Principles of Fracture Mechanics • 301

Table 9.2 A List of Several Common Nondestructive Testing (NDT) Techniques

Defect Size

Technique Defect Location Sensitivity(mm) Testing Location

Scanning electron

microscopy (SEM)

Surface >0.001 Laboratory

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/

gamma ray)

Subsurface >2% of specimen

thickness

Laboratory/in-field

On the other hand, if stress level and plane strain fracture toughness are fixed by the

design situation, then the maximum allowable flaw sizeacis

ac=

1

π

(

KIc

σY

) 2

(9.7)

Computation of

maximum allowable

flaw length

A number of nondestructive test (NDT) techniques have been developed that

permit detection and measurement of both internal and surface flaws.^5 Such tech-

niques 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 control for manufacturing processes. As the name implies, these techniques

must 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 9.2.

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 9.1

Material Specification for a Pressurized Spherical Tank

Consider the thin-walled spherical tank of radiusrand thicknesst(Figure 9.11)

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 are desired. On the

(^5) Sometimes the terms nondestructive evaluation (NDE) and nondestructive inspection
(NDI) are also used for these techniques.