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

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Fig. 43 Schematic of (a) pulse echo acoustical holography system and (b) holographic reconstruction unit

Selection of NDT Method

A universally accepted single test method for evaluating bonded structures has not yet been developed. Therefore, a

combination of test methods is required for complete and reliable inspection. Generally, selection of the test method is

based on:

• Part configuration and materials of construction

• Types and sizes of flaws to be detected

• Accessibility to the inspection area

• Availability of equipment/personnel

The application of NDT methods in the production cycle for honeycomb sandwich panels generally includes the

following:

• Material property tests

• Surface preparation checks (wedge crack, T-peel, and so on)

• Verifilm (prebond) tooling check

• Visual inspection

• Hot-water leak check

• Radiographic check for water and other internal discontinuities

• Ultrasonic C-scan for high-resolution inspection of skin-to-core bond

• Manual inspection of doublers, close-out members, and so on, using acoustic methods such as pulse-

echo ultrasonic, Shurtronic harmonic bond tester, or Fokker bond tester

Built-in defects were produced in typical laminate and honeycomb specimens. These specimens were evaluated by state-

of-the-art NDT methods to determine which methods reliably detected the different defects. The results of this

investigation are given in Table 3 for adhesive-bonded laminates and in Table 4 for adhesive-bonded honeycomb

structures (Ref 6). The NDT methods described in this section are used to test sandwich structures having aluminum or

titanium skins and aluminum core. Most of the methods can be effectively applied to structures with metal facing sheets

and nonmetallic core and structures with graphite fiber, boron fiber, and fiberglass face sheets bonded to aluminum core.

Only a limited number of the methods are applicable for structures having both core and facing sheets made from

nonmetallic materials.

Table 3 Correlation of NDT results for built-in defects in laminate panels

NDE method

(a)

Radiography Ultrasonic bond testers Ultrasonic inspection

(f)

Laminate defects

Low-

kilovolt

x-ray

Neutron

radiography

Fokker

bond

tester

Sondicator

Harmonic

bond

tester

210

sonic

bond

tester

Contact

pulse

echo

Contact

through

transmission

Immersion

C-scan

pulse echo

Immersion

C-scan

through

transmission

Immersion

C-scan

through

reflector

plate

Coin

tap

test

Remarks

1. Void

(b)

D D D D D D D D

(c)

D D PD

(e)

(b)

(c)

(e)

(g)

2. Void (C-14

repair)

(b)

D D D D D D D D

(c)

D D PD

(e)

(c)

(e)

3. Void (9309

repair)

(b)

D D D D D D D D

(c)

D D PD

(e)

(c)

(e)

4. Lack of bond

(skin to adhesive)

(b)

ND D D D D D D D

(c)

D D PD

(e)

(c)

(e)

5. Manufacturer's

sheet (FM123-41)

(b)

(d)

D D ND PD ND PD D

(c)

(d)

(c)

(d)

6. Thick adhesive

(1, 2, 3 ply)

(b)

ND D ND ND D ND ND ND

(e)

(c)

(e)

7. Porous adhesive

(b)

D D ND ND D D D D D D ND

(b)

8. Burned adhesive

(b)

ND D ND ND D D D PD D D ND

(h)

9. Corroded joint D D D PD PD D D D PD D D PD

(i)

(a)

ND, not detected; PD, partial detection; D, detected.

(b)

Panels were made using FM-73, which is not x-ray opaque. With x-ray opaque adhesive, defects 1, 2, 3, 5, 6, 7 are detected.

(c)

Method suffers from ultrasonic wave interference effects caused by tapered metal doubles or variations in adhesive thickness.

(d)

Manufacturer's separator sheet not detectable but developed porosity and an edge unbond during cure cycle, which was detectable.

(e)

MIL-C-88286 (white) external topcoat and PR1432G (green) plus MIL-C-83019 (clear) bilge topcoat dampened the pulse-echo response.

(f)

Contact surface wave was tried but did not detect any built-in defects.

(g)

Minimum detectable size approximately equal to size of probe being used.

(h)

Caused by drilling holes or band sawing bonded joints.

(i)

Moisture in bond joint (Armco 252 adhesive, Forest Products Laboratory etch)

Table 4 Correlation of NDT results for built-in defects in honeycomb structures

NDE method

(a)

Radiography Ultrasonic bond testers Ultrasonic inspection

Honeycomb

defects

Low-

kilovolt

x-ray

Neutron

radiography

Fokker

bond

tester

Sondicator

Harmonic

bond

tester

210

bond

tester

Contact

pulse

echo

Contact

through

transmission

Contact

shear

wave

lmmersion

C-scan

pulse echo

Immersion

C-scan

through

transmission

Coin

tap

test

Remarks

1. Void (adhesive

to skin)

(b)

D D D D D D D ND D D D

Replacement

(b)

standard

2. Void (adhesive

to skin) repair

with C-14

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

No void

improperly

made

3. Void (adhesive

to skin) repair

with 9309

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

No void

improperly

made

4. Void (adhesive

to core)

ND D D . . . D D ND D ND D D D

Replacement

standard

5. Water intrusion D D ND ND ND ND ND D ND ND D D

. . .

6. Crushed core

(after bonding)

(c)

PD PD

(e)

ND PD

(e)

ND ND PD

(e)

ND PD

(e)

(c)

(e)

7. Manufacturer's

separator sheet

(skin to adhesive)

(d)

D ND ND PD ND PD D ND D D PD

(d)

8. Manufacturer's

separator sheet

(adhesive to core)

(d)

D ND ND ND ND D ND ND D PD D

(d)

9. Void (foam to

closure)

D D D D D D D D ND D D D

. . .

10. Inadequate tie-

in of foam to core

D D D D D D ND D ND D ND D

. . .

11. Inadequate

depth of foam at

closure

D D D D ND PD

(f)

ND ND ND ND ND PD

. . .

12. Chem-mill

step void

(d)

D ND ND ND ND ND ND ND ND ND ND

(d)

(a)

ND, not detected; PD, partial detection; D, detected.

(b)

Panels were made using FM-73, which is not x-ray opaque.

(c)

The 0.13 and 0.25 mm (0.005 and 0.010 in.) crushed core detected by straight and better by angle shot.

(d)

Has been detected by x-ray when adhesive was x-ray opaque (FM-400).

(e)

Detects 0.25 mm (0.010 in.) crush core.

(f)

Discloses defect at a very high sensitivity

References cited in this section

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

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

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 Us

e of Acoustic 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, "Ev

aluation of 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. Appl.

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 Reacto

r Pressure Vessels,

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

31.

A Sample of Acoustical Holographic Imaging Tests, Holosonics Inc.

Nondestructive Inspection of Adhesive-Bonded Joints

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

NDT in the Product Cycle

Nondestructive testing serves as a basis for process control and serves to establish quality control standards. The

acceptance or rejection of a bonded assembly is directly related to the quality level desired by the designer. The quality

level, in turn, is based on the importance of the part or component in terms of performance and safety. The

frequency/severity flaw criteria and sensitivity of the test should be based on the desired quality level. To avoid

unwarranted inspection costs, it is advisable to divide the bonded joint into zones based on stress levels or criticality of

part function. Bonded joints are generally designed to withstand shear loads. Stress analysis of bonded joints reveals that

the outer edges of a bonded joint will be subjected to higher stress levels than the center of the joint. Therefore, higher

quality may be desired at the edges, and parts should be zoned accordingly. The zoned area should be dimensioned so that

the inspector can define the two different zones prior to inspection. Figure 44 shows an example of quality zoning for

bonded laminate skin splice. A definition of inspection grade numbers versus allowable void sizes is illustrated in Fig. 45.

Fig. 44

Schematic of bonded joint divided into quality zones based on design shear stresses. The example

shown is a longitudinal skin splice with an internal longeron. Quality grade 1 is within 13 mm (

in.) of the

edge of any bonded member, and grade 2 is the bond between any two grade 1 bonds.

Fig. 45 Definition of acceptance grades for voids or unbonds

For multiple bond line joints, it may not be possible to inspect to a high quality at the edge of all adherends, because of

the loss of test sensitivity at each successive bond line below the surface. If the edges of a bond joint are stepped or

staggered, high-quality inspection is possible at the edges of all bond lines. The width of the step should be large enough

to accommodate the test instrument probe. Joint edges that are not stepped should be inspected from opposite sides.

Acceptance/Rejection Criteria

Before inspection can be performed on a bonded structure, accept/reject criteria must be established by engineering.

These criteria are usually in the form of frequency (number of flaws per unit area) and/or severity (maximum allowable

size flaw data). To avoid unnecessary rejections, engineering must not specify unrealistic or overly conservative criteria.

The frequency and severity criteria should be based on sound engineering judgement as related to calculated levels. It

should be noted that flaws do not grow under cycle loads if good, durable adhesives are used and if generous overlaps

exist.

Military Guidelines. For military contracts, the guidelines or requirements for preparing such criteria are specified in

MIL-A-83377 (Ref 32). Specific acceptance criteria for adhesive-bonded aluminum honeycomb structures are specified

in MIL-A-83376 (Ref 33). Specification MIL-A-83377 states that the contractor shall prescribe nondestructive tests and a

complete NDT process specification when required by MIL-I-6780 in evaluating the quality of adhesive-bonded

structures (Ref 34). The NDT process specification must be submitted for approval by the government prior to bonded

assembly production. The contractor is required to specify the type and size of the defects that are acceptable. The size of

the defect must be consistent with the capability of the NDT method to be used, realizing that NDT methods and

instrumentation may have different capabilities. Application of the various NDT methods is limited by material types and

design configuration. For aluminum honeycomb sandwich, the allowables of MIL-A-83376 can be used, if determined by

the contractor to be acceptable minimums for the particular application.

Specification MIL-A-83377 also requires that the NDT of adhesive-bonded structures be performed only by qualified

NDT personnel. Personnel qualification requirements for NDT are specified in MIL-STD-410 (Ref 35).

Records and Specifications

It is recommended that four basic types of inspection records be prepared and maintained. They are specifications,

inspection log, detailed written test procedure, and rejection or nonconformance record.

Specifications. Test method specifications should be obtained for each production inspection instrument to be used--for

example, Fokker bond tester (Ref 4), 210 bond tester, harmonic bond tester, Sondicator, and so on). Each specification

should specify controls for the particular method and should define reference standard configurations. In addition, an

acceptance criteria specification should be prepared.

Inspection Log. The production inspection personnel should maintain a daily inspection log. This log should contain

the part number, part dash number, part serial number, fabrication outline serial number, inspector name or stamp, date

and method of inspection, and a remarks column to indicate pass or reject. The rejection tag number can be entered, along

with a description of the flaw, noting its exact location.

Detailed Written Test Procedure. Nondestructive testing inspections should follow a detailed written procedure for

each component tested. The written procedure must comply with the test method specification requirements and, as a

minumum, must include:

• A sketch of the part or configuration show

ing in detail the part thickness, alloy material and temper, and

the adhesive system used

• Manufacturer and model number of instrumentation to be used

• Model or designation number of probe to be used

• Any recording, alarm, or monitoring equipment and test fixtures or inspection aids that are to be used

• Applicable reference standard(s) serial number and design description

• Type of fluid couplant to be used

• Instrument calibration procedure, if required

• Testing plan. The part surface from which the test wil

l be performed, the testing technique(s) for each

joint, and the maximum probe index or shift per scan should be described

• Acceptance limits for each joint of the assembly

• Reference point or identifying mark, which is placed on each part prior to inspect

ion; this mark is used

for orientation when reviewing inspection results

• Outline of the areas where multiple layers of adhesive have been used

• Identification of section area changes not visible to the inspector

• Verification of the test procedure on a production part prior to approval

Rejection or Nonconformance Record. All nonconforming flaws should be described on a rejection tag. The

inspection agency should outline on the part surface the location and shape of unacceptable bond line flaws. If it is

necessary to mark the part permanently, the inspector must remove the couplant and wipe with solvent the area to be

marked, and then reapply the marks. Next, the marks are covered with a very light coat of clear epoxy enamel. Marking

the problem areas in this way facilitates their location after the part has been in service.

Reference Standards