9.6 Methods to Evaluate Degradation and Damage

2. Research trends

Research into methods to evaluate material degradation can

roughly be classified into two kinds: firstly, the development of

new methods mainly based on new principles; and secondly,

work to improve existing assessment techniques by making them

more sensitive and accurate. Here we will introduce examples of

the former: non-linear ultrasonic techniques and the terahertz

wave. Many scientists have focussed on these two techniques in

recent years, and both utilize the non-linear responses of ultrason-

ic waves.

In the conventional pulse reflection method that uses linear

ultrasonic waves, it is very difficult to detect and estimate the size

of tiny, closed cracks that have occurred as a result of residual

Fig. 9.6.2 Second and third harmonic wave intensity ratio and

stress or other factors. In the last couple of years a great deal of

cumulative event count during ultrasonic testing for fatigue in high-

research has been made into the development of methods that use

strength steel (Shiwa et al, NIMS)

non-linear ultrasonic wave responses on closed cracks and

incomplete junctions and bonded surfaces. Fig. 9.6.1 shows a dia-

cations. This is an example of the evaluation of the progress of

gram of an example of how the method works 1). It is known that

fatigue deterioration before cracks develop, and illustrates the

the incident ultrasonic wave causes the open and closed parts of

usefulness of non-linear ultrasonic techniques.

the crack surface to vibrate, producing a sub-harmonic wave that

Terahertz waves are electromagnetic waves that have a fre-

has half the frequency of the incident wave as well as a higher

quency of 0.1 to 100 THz. Until recently, terahertz waves have

harmonic wave of several times the frequency. This phenome-

been difficult to apply, due to the lack of a suitable signal source,

non can be used to evaluate the level of damage in an object.

but with advances in technology their use has now become possi-

Ohtani et al have produced images of a secondary higher har-

ble. Various effects exist in the terahertz band, including optical

monic wave of creep damage detected in the heat affected zone

phonon scattering in solids, dielectric properties such as ion

(HAZ) of a weld on a steam superheater pipe that had been in

polarization and orientation polarization, superconductive energy

service for 200,000 hours at a thermal power station, and demon-

gaps, and various kinds of vibrations of molecules and solids or

strated the usefulness of the technique 2). These kinds of higher

reciprocal effects between molecules. It is hoped that the use of

harmonic waves are also known to be caused by dislocation

terahertz waves will allow researchers to learn about what hap-

movement within materials, and this can be used to determine

pens inside materials in a way never before possible 3). Since tera-

how far fatigue damage has progressed. Fig. 9.6.2 shows the

hertz waves can pass through ceramics and plastics, and will not

change in the intensity ratio of the excitation frequency and sec-

harm the human body like X-ray does, they show promise for use

ond/third harmonic waves with respect to the number of cycle

in transmission imaging. Reported examples 3) of applications of

ratio in the ultrasonic fatigue test of high-strength steel. The same

terahertz waves in non-destructive evaluation include imaging of

chart also shows the AE cumulative event count. From the AE

the inside of the human body and the screening of postal items for

signal, the start of macroscopic cracking can be estimated as

drugs, analysis of semiconductor integrated circuits for faults, and

occurring above 1.18 x 106 cycles. Before that, the second higher

checking of the heat-proof tiles on the space shuttle. It is hoped

harmonic wave component rapidly increases, and the third har-

that the characteristics of terahertz waves will make them particu-

monic wave decreases. This is thought to be in response to a

larly useful when testing polymers, FRPs, living tissue and

decrease accompanying the separation and union of pinned dislo-

ceramic form.

In the creation of a sustainable society that is safe and secure

and in harmony with the natural environment, non-destructive

evaluation techniques that will allow us to ensure reliability by

detecting damage and assessing residual service lifetime, as well

as the efficient allocation of costs and resources, will become

more important than ever in near future.

References

1) M. Hiraou, Non-Destructives Testing (Japanese edition), 56

(2007) 292.

2) T. Ohtani et al.: Japan. J. Appl. Physics 46-7B (2007) 4577.

Fig. 9.6.1 Diagram of a non-linear response occurring at a close

3) M. Tonouchi: Terahertz Technology (Japanese edition),

Ohmsha, (2006).

crack 1)

Chapter 9. Diagnosis and Lifetime Prediction Technologies

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