ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control

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Ultrasonic theory predicts that constructive interference (increased amplitude) occurs when the transmitted and reflective

waves are in phase with one another (Ref 36, 37). Conversely, destructive interference (decreased amplitude) occurs when

the transmitted and reflected waves are 180° out of phase with one another. Destructive wave interference causes a drop

in signal amplitude, and the C-scan recorder will not record in the area of interference. An inexperienced operator might

misinterpret these areas as voids or unbonds. However, interference effects (caused by tapered adherends) are uniform

across the part, resulting in bands of light and dark on the C-scan recording (Ref 10). When destructive interference

effects are shown in C-scan recordings of bonded splices having tapered adherends, the bond joints are given another

inspection by a different method, such as the Fokker bond tester.

Destructive wave interference effects can also be seen in C-scan recordings of adhesive-bonded laminates made with an

adhesive having a mate carrier. It is less evident in laminates bonded with adhesive having a woven carrier. The

destructive interference effects are most evident near the outer edges of the assembly where the adhesive is pinched off to

thicknesses of 0.08 mm (0.003 in.) or less. The interference effects are frequently dependent and therefore will change

position if the test frequency is altered (Ref 36, 37). Less wave interference is experienced with through transmission

testing than with pulse-echo testing at similar frequencies. Bonded laminates exhibiting destructive wave interference

effects require inspection by another test method in the area in question to verify bond quality.

Correlation of Bond Strength and Fokker Bond Tester Readings

The ability of the Fokker bond tester to measure bond strength is completely dependent on the accuracy of the correlation

curves, which plot Fokker readings versus either lap shear strength (for metal-to-metal joints) or flatwise tensile strength

(for honeycomb joints). For metal-to-metal joints, the correlation is made between the Fokker A-scale readings and

tensile lap shear strength. For honeycomb structure, the correlation is made between Fokker B-scale readings and flatwise

tensile strength.

Flatwise tensile specimens are prepared in accordance with MIL-STD-401 (Ref 38), except that the adherends and

core should simulate those of the bonded test assembly being tested. Fabrication procedures should be identical to those

used in preparing the assembly to be tested. Specimens are prepared with various degrees of bond quality by the

introduction of porosity in the cured adhesive. The degree of porosity increases with the thickness of the adhesive layer.

Figure 53 illustrates the procedure for honeycomb-to-skin joints.

Fig. 53 Schematic of honeycomb sandwich test panel

Nondestructive testing of the panels is performed in accordance with procedures outlined in MIL-STD-860 (Ref 23) or as

specified in the Fokker bond tester training manual. Instrument readings should be taken within predetermined areas of

the test specimen. Each reading should be recorded and correlated with the probe location on the specimen. The

specimens are then destructively tested, and the tensile strength values are recorded. Flatwise tensile strength should be

plotted versus average ultrasonic readings for each specimen and a representative curve drawn through the plotted points.

Two correlation curves should be drawn for specimens with dissimilar thickness adherends. A typical correlation curve

for Fokker B-scale readings versus flatwise tensile strength is shown in Fig. 54.

Fig. 54 Typical Fokker B-

scale correlation curve versus honeycomb flatwise tensile strength. Cell structure:

0.64 mm (0.025 in.) face skin, 3.18 mm (0.125 in.) cell size, and 0.025 mm (0.001 in.) foil thickness.

Probe

used was No. 3414.

Effect of Porosity and Voids on Bond Testing. A relationship can be established between Fokker bond tester A-

scale frequency shift and lap shear strength as related to porosity. Tests have shown that the percentage of porosity

increases with adhesive thickness (single layer) (Ref 5, 10). Porosity first appears at an adhesive thickness of 0.25 mm

(0.010 in.) and becomes almost completely void at 0.51 mm (0.020 in.) in thickness.

The first step is to produce high-quality lap joints (25.4 mm, or 1.0 in., wide by 13 mm, or in., overlap) containing 100,

80, and 50% width bond lines, as shown in Fig. 55. The test specimens are approximately 178 to 203 mm (7 to 8 in.) long,

and a minimum of six specimens should be made for each percentage bond and thickness, that is, 1.0 mm/1.0 mm (0.040

in./0.040 in.) (in general, the t

thickness is equal to the t

thickness) and so on. Typical average lap shear values for 2024-

T3 and FM-73 adhesive are shown in Fig. 56.

Fig. 55 Lap shear test coupon. Dimensions given in millimeters

Fig. 56 Variation in lap shear strength with t/ ratio and bond length using aluminum alloy 2024-

T3 and

adhesive FM-73. t

(mm): 1.0/1.0, 1.3/1.3, 1.6/1.6, and 1.8/1.8.

The second step is to make specimens with increasing thickness of the single adhesive layer. This will produce specimens

of variable quality resulting from porosity. To obtain the quality variation, spacer shims are used, as shown in Fig. 57.

The thickness of the spacers is increased in steps (0.10 mm, or 0.004 in., per step) from 0.10 to 0.51 mm (0.004 in. to

0.020 in.), resulting in a highly porous bond joint. The lap shear specimens are numbered and neutron radiographed. The

specimens are then Fokker bond tested, and the A-scale readings are recorded for each specimen. The lap shear coupons

are cut from each panel, and the adhesive thickness (for each specimen) is accurately measured. The adhesive will

generally be found to be slightly thicker than the associated shim. The lap shear test is performed in accordance with

Federal Test Method Standard No. 175, method 1033, or Federal Specification MMM-A-132. The neutron radiographs

indicate areas of nonuniformity of porosity, and results from these nonuniform areas should be discarded.

Fig. 57 Metal-to-metal lap shear test panel.

The third step is to prepare shimmed specimens using additional layers of adhesive. This will produce good-quality

specimens of thick adhesive. The panels are neutron radiographed, Fokker bond tested, cut into specimens, and lap shear

tested, with the data subsequently evaluated. Plots of lap shear strength versus adhesive thickness for the single porous

layer and multiple nonporous layers of FM-73 are shown in Fig. 58.

Fig. 58

Lap shear strength versus adhesive thickness for porous and nonporous adhesives. Adherends, 1.6

mm/1.6 mm; adhesive, FM-73 with mat carrier; primer, BR127

The fourth step is to plot lap shear strength versus Fokker A-scale readings for each adherend thickness (Fig. 59). If the

thickness of the adherends is not equal, the back (unprobed) thickness is plotted. Unequal-thickness adherend specimens

are tested from both sides and two quality curves are established. With reference to Fig. 59 and 60, the bond strength can

be determined for 100, 80, and 50% specimens having various thickness or t/ ratios. By evaluating all the data, the

following classes for single-layer (porous) and multiple-layer (nonporous) joints can be specified:

• Class A: Greater than 80%

• Class A/B: Greater than 65%

• Class B: Greater than 50%

• Class C: Greater than 25%

Fig. 59

Lap shear strength versus Fokker bond tester quality units. Bond tester, model 70; probe, 3814;

adherends, 1.6 mm/1.6 mm; adhesive, FM-73; primer, BR127

Fig. 60 Establishment of acceptance limits (sheet to sheet)

A-Scale Frequency Shift Versus Bond Quality. The large scatter in results for the 25% quality level specimens

can be attributed to the extensive porosity in these specimens. This does not pose a problem to the inspector, because

quality levels below 50% are usually not acceptable. The actual relationship between the A-scale frequency shift (as seen

by the inspector) and the bond quality (class) is illustrated in Fig. 60.

Cohesive Bond Strength Acceptance Limits. Finally, the acceptance limits are incorporated into a table for use by

the inspector. Table 5 lists the acceptance limits for single bond line joints with FM-73 adhesive and specific thickness

adherends. Establishing Fokker quality diagrams for multiple bond line joints is possible but complicated. Fokker-VFW

should be contacted to obtain the recommended procedures for developing such quality diagrams.

Table 5 Fokker cohesive bond strength acceptance limits for single bond line using FM-73 adhesive

Lower sheet

thickness

Lower limits

(a)

in.

Model 70

probe

Upper limit

(b)

Class A

(c)

80%

Class A/B

(c)

65%

Class B

(c)

50%

Class C

(c)

25%

1.0 0.040

3814 >R8 >L54 >L40 >L25

>L10

1.3 0.050

3814 >R10 >L48 >L36 >L20

>L9

1.6 0.063

3814 >R12 >L42 >L30 >L18

>L8

1.8 0.071

3814 >R16 >L40 >L28 >L18

>L7

2.1 0.081

3814 >R18 >L38 >L25 >L15 >L6

(a)

Skin/stringer and skin/frame bonds shall be Class minimum; doubler/skin bonds shall be Class A/B minimum. Additional adhesive layers shall

not yield readings less than the lower limits by the following factors: 2 layers (-15); 3 layers (-25); 4 layers (-30).

(b)

R, right-hand side.

(c)

L, left-hand side

References cited in this section

K.J. Rienks, "The Resonance/Impedance and the Volta Potential Methods for the Nondestructive Testing of

Bonded Joints,

" Paper presented at the Eighth World Conference on Nondestructive Testing, Cannes,

France, 1976

10.

D. Hagemaier and R. Fassbender, Nondestructive Testing of Adhesive Bonded Structures, SAMPE Q.,

Vol

9 (No. 4), 1978

23.

D.J. Hagemaier, Bonded Joints and Non-Destructive Testing: Bonded Honeycomb Structures, 2, Non-

Destr.

Test., Feb 1972

36.

B.G. Martin, et al., Interference Effects in Using the Ultrasonic Pulse-

Echo Technique on Adhesive Bonded

Metal Panels, Mater. Eval., Vol 37 (No. 5), 1979

37.

P.L. Flynn, Cohesive Bond Strength Prediction for Adhesive Joints, J. Test. Eval., Vol 7 (No. 3), 1979

38.

"Sandwich Construction and Core Materials; General Test Methods," MIL-STD-401

Nondestructive Inspection of Adhesive-Bonded Joints

Donald J. Hagemaier, Douglas Aircraft Company, McDonnell Douglas Corporation

References

1. M.T. Clark, "Definition and Non-Destructive Detection of Critical Adhesive Bond-Line Flaws," AFML-

TR-78-108, U.S. Air Force Materials Laboratory, 1978

2. R.J. Schliekelmann, Non-Destructive Testing of Adhesive Bonded Metal-to-Metal Joints, Non-

Destr.

Test., April 1972

3. R.J. Schliekelmann, Non-Destructive Testing of Bonded Joints--

Recent Developments in Testing Systems,

Non-Destr. Test., April 1975

P. Bijlmer and R.J. Schliekelmann, The Relation of Surface Condition After Pretreatment to Bondability

of Aluminum Alloys, SAMPE Q., Oct 1973

5. K.J. R

ienks, "The Resonance/Impedance and the Volta Potential Methods for the Nondestructive Testing

of Bonded Joints," Paper presented at the Eighth World Conference on Nondestructive Testing, Cannes,

France, 1976

6. J.C. Couchman, B.G.W. Yee, and F.M. Chang, Adhesive Bond Strength Classifier, Mater. Eval.,

Vol 37

(No. 5), April 1979

7. J. Rodgers and S. Moore, Applications of Acoustic Emission to Sandwich Structures,

Acoustic Emission

Technology Corporation, 1980

8. J. Rodgers and S. Moore, "The Use of Acous

tic Emission for Detection of Active Corrosion and Degraded

Adhesive Bonding in Aircraft Structures," Sacramento Air Logistics Center (SM/ALC/MMET),

McClellan Air Force Base, 1980

9. The Sign of a Good Panel Is Silence, Aviat. Eng. Maint., Vol 3 (No. 4), April 1979

10. D. Hagemaier and R. Fassbender, Nondestructive Testing of Adhesive Bonded Structures, SAMPE Q.,

Vol

9 (No. 4), 1978

11. R. Botsco, The Eddy-Sonic Test Method, Mater. Eval., Vol 26 (No. 2), 1968

12. J.R. Kraska and H.W. Kamm, "Evaluation o

f Sonic Methods for Inspecting Adhesive Bonded Honeycomb

Structures," AFML-TR-69-283, U.S. Air Force Materials Laboratory, 1970

13.

N.B. Miller and V.H. Boruff, "Evaluation of Ultrasonic Test Devices for Inspection of Adhesive Bonds,"

Final Report ER-1911-12, Martin Marietta Corporation, 1962

14. J. Arnold, "Development of Non-Destructive Tests for Structural Adhesive Bonds," WADC-TR-54-

231,

Wright Air Development Center, 1957

15. R. Schroeer, et al., The Acoustic Impact Technique--A Versatile Tool for Non-

Destructive Evaluation of

Aerospace Structures and Components, Mater. Eval., Vol 28 (No. 2), 1970

16. H.M. Gonzales and C.V. Cagle, Nondestructive Testing of Adhesive Bonded Joints, in Adhesion,

STP 360,

American Society for Testing and Materials, 1964

17. D.F. Smith and C.V. Cagle, Ultrasonic Testing of Adhesive Bonds Using the Fokker Bond Tester,

Mater.

Eval., Vol 24, July 1966

18. H.M. Gonzales and R.P. Merschell, Ultrasonic Inspection of Saturn S-II Tank Insulation Bonds,

Mater.

Eval., Vol 24, July 1966

19. R.J. Botsco, Nondestructive Testing of Composite Structures With the Sonic Resonator, Mater. Eval.,

Vol

24, Nov 1966

20. R. Newschafer, Assuring Saturn Quality Through Non-Destructive Testing, Mater. Eval.,

Vol 27 (No. 7),

1969

21. "Fokker Ultrasonic Adhesive Bond Test," MIL-STD-860, 1978

22. R.J. Schliekelmann, Non-Destructive Testing of Adhesive Bonded Joints, in

Bonded Joints and

Preparation for Bonding, AGARD-

NATO Lecture Series 102, Advisory Group for Aerospace Research

and Development, 1979

23. D.J. Hagemaier, Bonded Joints and Non-Destructive Testing: Bonded Honeycomb Structures, 2, Non-

Destr. Test., Feb 1972

24. D. Wells, NDT of Sandwich Structures by Holographic Interferometry, Mater. Eval.,

Vol 27 (No. 11),

1969

25. R.M. Grant, "Conventional and Bead to Bead Holographic Non-

Destructive Testing of Aircraft Tires,"

Paper presented at the 1979 ATA NDT Forum, Seattle, WA, Sept 1979

26. P.R. Vettito, A Thermal I.R. Inspection Technique for Bond Flaw Inspection, J. Appl. Polym. Sci. App

Polym. Symp. No. 3, 1966

27. C. Searles, Thermal Image Inspection of Adhesive Bonded Structures, in

Proceedings of the Symposium

on the NDT of Welds and Materials Joining, 1968

28. W. Woodmansee and H. Southworth, Detection of Material Discontinuities With Liquid Crystals,

Mater.

Eval., Vol 26 (No. 8), 1968

29. S. Brown, Cholestric Crystals for Non-Destructive Testing, Mater. Eval., Vol 26 (No. 8), 1968

30. D.H. Collins, Acoustical Holographic Scanning Techniques for Imaging Flaws in Reactor Pressure

Vessels, in Proceedings of the Ninth Symposium on NDE (San Antonio), April 1973

31. A Sample of Acoustical Holographic Imaging Tests, Holosonics Inc.

32. "Adhesive Bonding (Structural) for Aerospace Systems, Requirements for," MIL-A-83377

33. "Adhesive Bonded Aluminum Honeycomb Sandwich Structure, Acceptance Criteria," MIL-A-83376

34. "Inspection Requirements, Nondestructive: For Aircraft Materials and Parts," MIL-I-6870 (ASG)

35. "Nondestructive Testing Personnel Qualification and Certification," MIL-STD-410

36. B.G. Martin, et al., Interference Effects in Using the Ultrasonic Pulse-

Echo Technique on Adhesive

Bonded Metal Panels, Mater. Eval., Vol 37 (No. 5), 1979

37. P.L. Flynn, Cohesive Bond Strength Prediction for Adhesive Joints, J. Test. Eval., Vol 7 (No. 3), 1979

38. "Sandwich Construction and Core Materials; General Test Methods," MIL-STD-401

Nondestructive Inspection of Boilers and Pressure Vessels

Introduction

DURING THE FABRICATION of a boiler, pressure vessel, and such related components as boiling water reactor piping

or steam generator tubes, various types of nondestructive inspection (NDI) are performed at several stages of processing,

mainly for the purpose of controlling the quality of fabrication. In-service inspection is used to detect the growth of

existing flaws or the formation of new flaws. This can be done while the vessel is in operation or down for servicing. The

inspection methods used include visual, radiographic, ultrasonic, liquid penetrant, magnetic particle, eddy current, and

acoustic emission inspection, as well as replication microscopy and leak testing. The assurance of component quality

depends largely on the adequacy of NDI equipment and procedures and on the qualification of personnel conducting the

inspection. In many cases, nondestructive inspection, both prior to and during fabrication, must be done to sensitivities

more stringent than those required by specifications. The use of timely inspection and rigid construction standards results

in the reduction of both the costs and delays due to rework.

Quality planning starts during the design stage. For inspections to be meaningful, consideration must be given to the

condition of the material, the location and shape of welded joints, and the stages of production at which the inspection is

to be conducted. During fabrication, quality plans must be integrated with the manufacturing sequence to ensure that the

inspections are performed at the proper time and to the requirements of the applicable standard. In the newest nuclear

plants, quality design planning includes:

•

Avoidance of complex weld geometries to facilitate attachment of ultrasonic transducers to the surface

at the best positions

• The increased use of ring forgings for pressure vessel components; this means th

at there are no

longitudinal welds that have to be inspected in service. The result is a reduction in the amount of in-

service inspection and man-rem exposures

• Incorporating large numbers of access points for introducing mechanized inspection equipment, w

hich

can be operated remotely, thus avoiding exposures to operators and enabling more accurate processing

than is possible with handheld inspection equipment

• The elimination of welds between cast austenitic components; inspection of welds through cast wel

ds is

difficult because they are opaque to ultrasonic inspection to a large degree (Ref 1)

Reference

Outlook on Nondestructive Examination, Nucleonics Week, 30 June 1988

Nondestructive Inspection of Boilers and Pressure Vessels

Boiler and Pressure Vessel Code and Inspection Methods

Pressure vessels--both fossil fuel and nuclear--are manufactured in accordance with the rules of the applicable American

Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Ref 2). For nuclear vessels, section XI of the

ASME code establishes rules for continued nondestructive inspections at periodic intervals during the life of the vessel.

One feature of the rules in section XI is the mandatory requirement that the vessel be designed so as to allow for adequate

inspection of material and welds in difficult-to-reach areas. Section III of the code describes the material permitted and

gives rules for design of the vessel, allowable stresses, fabrication procedure, inspection procedure, and acceptance

standards for the inspections.

Pressure vessels are constructed in various sizes and shapes, and some of the largest are those manufactured for the

nuclear power industry. Some pressure vessels are more than 6 m (20 ft) in diameter and 20 m (70 ft) in length and weigh

almost 900 Mg (1000 tons). Thickness of the steel in the walls of these vessels ranges from about 150 mm (6 in.) to more

than 400 mm (15 in.), although many pressure vessels and components are fabricated from much thinner material. Joining

of the many vessel sections is accomplished by welding. Welders of pressure vessels are qualified according to section IX

of the ASME Boiler and Pressure Vessel Code, and welding is done in accordance with qualified welding procedures.

Nondestructive inspection of welds is only a part of the inspection requirements; the materials themselves must be

inspected prior to welding. For pressure vessels that are not constructed according to the ASME code, it is a matter of

agreement between the manufacturer and the user as to whether NDI methods are to be employed and which method or

methods are to be used.

Nondestructive Inspection Methods. An appendix to each section of the ASME code establishes the methods for

performing nondestructive inspection to detect surface and internal discontinuities in materials. Four inspection methods

are acceptable: radiographic, magnetic particle, liquid penetrant, and ultrasonic inspection. All these methods are

mandatory for nuclear vessels, for section III, and for division 2 of section VIII of the code. Ultrasonic inspection is listed

in division 1 of section VIII of the code as nonmandatory. Leak testing, eddy current inspection, acoustic emission

inspection, and visual inspection are included in section V. Details as to which method is to be used and the required

acceptance standards are specified in the appropriate articles on materials and fabrication. All NDI personnel must be

qualified and certified to SNT-TC-1A procedures (Ref 3).

Radiographic Inspection. Methods of radiographic inspection are extensively detailed in the ASME codes;

radiography using either x-rays or radioisotopes as the radiation source is permitted. Radiography is the oldest inspection

method detailed in the codes and is probably the most understood and the most widely accepted. A principal reason for its