Practical Applicatio

ENG302 : The Class : Forms and Formats : Formal : Practical Applicatio
Practical Application Report

THE THEORY/APPLICATION REPORT
The theory/application report uses a two-part structure superficially like that of the problem/solution report. However, in the theory/application report documented information is primarily in the first section, which functions lik the review of the literature section in the report of original research.

Once the writer documents the theory, he moves on to apply it, point by point, to a particular situation. Like the problem section in the problem/solution report, the application section is usually personal or job oriented. Thus, the' theory/application report centers information in the theory section and opinions in the application section. Structurally, then, the theory/application report is the opposite of the problem/solution report.

The following article, which describes some sophisticated test equipment originally appeared in a specialized engineering journal and was later reprints in a less specialized journal. The authors appeal to three audiences in this article: engineers, who want to see how this particular equipment works experts, who want to know the theory behind the equipment; and administrators, who want to know whether this equipment can solve their problems

Acoustic Emission Testing of FRP Equipment-I

T. J. Fowler and R. S. Scarpellini

Monsanto Co.

Acoustic emission testing is a recently developed method for determining the structural adequacy of fiber-reinforced plastic (FRP) structures. It is particularly valuable because, many of the nondestructive test methods used for metals are unsuitable with FRP. In many cases, it has been able to identify defects not detected by other methods and to furnish insight into failure mechanisms.

Acoustic emission offers a number of advantages over conventional visual inspection methods, which tend to be subjective. For testing of in-service equipment, visual examination requires an empty, decontaminated vessel stripped of external insulation. In contrast, plant acoustic emission tests are normally carried out with process fluids flowing, and plant downtime is reduced or eliminated. The need for decontamination is eliminated and only minor removal of insulation is required. In addition, acoustic emission testing has the advantage of providing information on the structural adequacy of the entire piece of equipment.

To facilitate exchange and development of nonproprietary information on the application of acoustic emission to FRP equipment, the Committee on Acoustic Emission of Reinforced Plastics (CARP) has been formed under the auspices of The Society of the Plastics Industry (spi). It includes FRP equip-ment manufacturers, research organizations, resin and glass suppliers, acoustic emission instrumentation suppliers, and FRP users.

WHAT IS ACOUSTIC EMISSION
Acoustic emission is the term used to describe elastic stress waves produced in solids as a result of the application of stress. The waves are generated by rapid release of energy within the material. In FRP composites, acoustic emissions are generated by cracking of the matrix, debonding of the matrix from the fibers, laminate separation, fiber pullout and breakage of the fibers. The acoustic emission generated during stressing of equipment is detected by sensitive piezoelectric transducers attached to the surface. Measurement is accomplished in a number of ways, as shown in Fig. 1:

Counts-Acoustic emission is normally measured in counts, the number of times the amplitude of the signal from the transducer exceeds a set threshold. The number of counts is the measure of the total acoustic emission.

Events-Acoustic emission occurs in 'bursts' of continuous counts, called events. Each event can be recorded and analyzed.

Amplitude-The maximum signal amplitude during an individual event is normally measured in decibels and is referred to as the event amplitude.

BEHAVIOR OF FRP

Fig. 2 is a representative acoustic emission plot for a composite material, showing total counts vs. load-ing, in accordance with ASTM-D638 test procedures. The curve illustrates a number of important facets Of FRP acoustic emission behavior. As with most materials, the total emission count increases at an accelerating rate with the addition of load. The massive emissions that occur near failure are of little practical interest; and for field tests, emissions at service loads are of much greater value.

The onset of emission will normally occur in the strain range of 0.001-0.005, the exact value de-pending on type of resin, and the construction and quality of the laminate. This strain range corre-sponds with the onset of fiber/matrix debonding and resin microcracking [ 1 -4 ], and is well below the ultimate strain of either the resin or glass. The initial cracking is due to stress magnification between and around the fibers [5 1.

As a general rule, the percentage of ultimate load corresponding to initiation of emission is in the25-50% range, depending on the type of construction. The fraction of ultimate load for first emission, or for a given number of counts, will decrease with an increasing amount of random glass. Random glass tends to emit sooner than unidirectional fibers (either parallel or perpendicular to the direction of stress). Combinations of different types of construction (for example, random and longitudinal) and woven roving, will tend to emit sooner than constructions having only random fibers.

CREEP

Above a particular level of load, acoustic emission will continue when the load is held constant (see Fig. 3). The continuing emission at 1,200 and 1,600 lb should be noted. Emissions during a load-hold are indicative of creep, and a time plot can be used to determine if the creep deformation is becoming unstable. Creep is the result of continuing damage resulting from stress redistribution caused by visco-elastic flow of the matrix [6].

AMPLITUDE DISTRIBUTION

A histogram of the amplitude of events can help to define defects in equipment [7]. The two histograms shown in Fig. 4 are for the load ranges 4% to 50% of the failure load, and 4% of failure load to failure. The load was applied in a series of steps to 25, 50,

75 and l00% of the failure load, with intermediate unloadings to a nominal stress of approximately 4% of ultimate. The equipment threshold was set at 40 dB.

For the lower load range, the events tend to be of low amplitude. It is believed that these correspond to fiber/matrix debonding and matrix cracking. For the higher load range, high-amplitude events occur that are believed to correspond to fiber breakage. As would be expected, an increasing proportion of high-amplitude events occur as the specimen approaches failure. An amplitude histogram can be used to estimate the percentage of ultimate load being carried by a structure.

For some types Of FRP construction, the failure histogram is bimodal, with one peak at the threshold level and another corresponding to fiber breakage in the 70-to-80 dB range. It has been reported [81 that in addition to distinguishing between fiber breakage and other failure mechanisms, an amplitude distri-bution plot can detect delamination failures. Delaminations tend to give a cluster of events centered in the 50-to-60 dB range.

SPECTRAL ANALYSIS
As is the case with amplitude distribution analysis, spectral analysis of frequencies can be used to provide information regarding the mode Of FRP fracture [9-12]. However, attenuation is dependent on frequency and thus poses difficulties when spectral analysis techniques are applied to full-scale equip-ment. During the course of a test, the ratio of total low-frequency (50 kHz) to total high-frequency (150 kHz) counts declines, as shown in Fig. 5. A number of authors [9, 101 have reported that interfiber fail-ure has a spectral peak at a lower frequency than does fiber fracture. This is confirmed by Fig. 5.

FELICITY EFFECT
An unusual feature Of FRP emission relates to the Kaiser effect, which occurs during the unload/reload cycle. It is the phenomenon whereby emissions do not occur until the previously attained maximum load is reached. FRP exhibits the Kaiser effect up to a percentage of ultimate load. However, above this load, emission will begin at loads lower than the previously attained maximum. This is known as the Felicity effect.

If a FRP specimen is held at load for a long period of time, the Felicity effect will be present. This occurs because of the redistribution of residual internal stresses during the unload period. The result of the redistribution is that additional microfailures will occur during reloading. Redistribution begins as soon as load is removed, and for initial loads close to ultimate, the Felicity effect is observed without a hold in the unloaded condition.

Because of the Felicity effect it is possible to test in-service vessels without exceeding the maximum operating load, thus eliminating the risk of permanent damage.

The ratio of the load at onset of emission to the previously attained maximum load is known as the Felicity ratio. This is a measure of the total amount of damage. The lower the number, the greater the damage [13].

ATTENUATION

Wave propagation and attenuation studies conducted on FRP show that high-frequency emissions are attenuated faster than low-frequency emissions [14 ]. Apparently, transmissibility increases with an increasing percentage of glass, and with the hardness of the resin. In addition, continuous fibers transmit better than chopped fibers. Because continuous fibers provide good transmissibility in one direction, a filament-wound vessel will have different attenuation characteristics in different directions.
Part II (in the Nov. 17 issue) will present acoustic emission test procedures for FRP equipment.

REFERENCES

1. Owens, M. I., and Smith, T. R., Proc. 6th Inter. Reinforced Plastics Conference, British Plastics Federation, London, 1970.
2. Howe, R. J., and Owen, 1. M., Proc. 8th Inter. Reinforced Plastics Conference, British Plastics Federation, London, 1972.
3. Garrett, K. W., and Bailey, J. E., The Effect of Resin Failure Strain on the Tensile Properties of Glass Fiber Reinforced Polyester Cross-Ply Laminates, 1. of Mate-rials Science, Vol. 12, 1977.

4. Norwood, L. S., and Millman, A. F., Strain Limited Design Criteria for Reinforced Plastic Process Equip-ment, 34th Annual Technical Conference, Reinforced Plastics/Composites Institute, The Society of the Plas-tics Industry, New Orleans, 1979.
5. Kies, J. A., U.S. Naval Research Laboratory Report No. 5752,1962.
6. Rotem, A., and Baruch, J., Determining the Load-Time History of Fiber Composite Materials by Acoustic Emission, Technion-Israel Institute of Technology, Haifa, Israel, MED Report No. 44, March, 1974.
7. Rotem, A., and Eliezer, A., Fracture Modes and Acoustic Emission of Composite Materials, 1. of Testing and Evaluation, Vol. 7, No. 1, January, 1979.
8. Wadin, J. R., Listening to Composite Materials. Acoustic Emission Applications, Dunegan/Endevco, San Juan Capistrano, Calif., April, 1979.
9. Wolitz, K., Brockmann, W., and Fischer, T., Evaluation of Glass Fiber Reinforced Plastics by Means of Acoustic Emission Measurements, 4th Acoustic Emission Sym-posium, High Pressure Institute of Japan, Tokyo, Sep-tember, 1978.
10. Crostack, H. A., Basic Aspects of the Application of Frequency Analysis, Ultrasonics, Vol. 15, p. 6, Novem-ber, 1977.
11. Egan, D. M., and Williams, J. H., Jr., Acoustic Emission Spectral Analysis of Fiber Composite Failure Mecha-niSMS, NASA Contractor Report 2983, 1978.
12. Henneke, E. G., Signature Analysis of Acoustic Emis-sions from Composites, NASA Grant NSG 1238, 1978 '
13. Fowler, T. I., and Gray, E., Development of an Acoustic Emission Test for FRP Equipment, Preprint 3583, ASCE Convention and Exposition, Boston, April, 1979.
14. Pollock, A. A., and Cook, W. J., Acoustic Emission Testing of Aerial Devices, Southeastern Electric Ex-change, Engineering and Operating Division in Annual Conference, New Orleans, April, 1976.

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