NATIONAL BUREAU OF STANDARDS REPORT

99H

FRACTOGRAPHY OF A 7178-T6 ALUMINUM FAILED WING PLANK

TO

NATIONAL TRANSPORTATION SAFETY BOARD

BUREAU OF AVIATION SAFETY

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

3120412 9911

FRACTOGRAPHY OF A 7 1 78-T6 ALUMINUM FAILED WING PLANK

by

D. B. Bal lard

To

National Transportation Safety Board

Bureau of Aviation Safety

IMPORTANT NOTICE NATIONAL BUREAU OF ST I

for use within the Government,

and review. For this reason, the whole or in part, is not authori Bureau of Standards, Washingtoi

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015

s accounting documents intended

subjected to additional evaluation

listing of this Report, either in

Office of the Director, National

i the Government agency for which

pies for its own use. <NB^>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

by

D . B . Ballard

Introduction

Three sections which had been cut from the top surface of a fractured

right wing plank adjacent to the forward edge of the access door (in

the area of station 119) of Braniff Electra N-9707 were submitted by

Mrs. R. Mayner of the National Transportation Safety Board, Bureau of

Aviation Safety on 24 June 1968 for f ractographic analysis. Two additional

pieces of wing plank from the same accident (location unknown) were submitted by Mr. W. L. Holshouser on 22 July 1968 for comparison of fracture

surface with the first three specimens.

Observations

Photomacrographs of the first three fracture sections (1-1, 1-2, 1-3)

are shown in Figs. 1, 2, and 3» The area examined by Electron Microscopy

is indicated by the circle on each figure. Figures 4 and 5 show the

additional fractured sections of wing plank and the location of fracture

surfaces. Photomacrographs of the replicas of these fractures (#4, #5,

#6) and areas examined by Electron Microscopy are shown in Figs. 6, 7, and

8. In all cases except fracture (1-2) a particular effort was made to

place the replica on the microscope grid so as to observe the topography
2

of the outside surface in the vicinity of the fillet, the origin at the

edge of fracture, and the fracture surface 0.5 to 1.0 mm away from the

edge of fracture.

The fractures were ul trasonical ly cleaned in acetone to remove loose

contamination and oils. At least seven cleaning replicas were stripped

from each fracture to obtain a clean metal surface. Three final replicas

were made from each area examined. Negative one-step replicas were made

using cellulose acetate softened with a mixture of acetone and dibutyl

phthalate. After drying the replicas were stripped and rotary shadowed

with palladium at 45° and carbon at 90°.

Fractographs of specimen (1-1) are shown in Figs. 9a thru 9f* The

original magnification is indicated by the length of the micro meter mark

included in each electron photomicrograph. The two photomicrographs 9a

and 9b were taken adjacent to the fillet on the fracture surface. The

general surface topography is typical of intergranular fracture which

usually indicates stress corrosion cracking. Superimposed on the grain

facets can be seen a fine dimple-like surface indicating low ductility

fracture. There is a small increase in surface roughness in Fig. 9c

which was adjacent to Figs. 9a and b but more distant from fillet edge.

The first evidence of t ransg ranu 1 a r ductile fracture is shown in Fig. 9d

as narrow bands of dimpled rupture parallel to the long axis of the

fracture between areas of low ductility. These features are shown again

in Figs. 9e and 9f and are typical of the fracture surface approximately

0.5 mm away from the edge of fracture.
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-

Visual examination of the section (1-2) showed that the fracture

edge adjacent to the fillet was mechanically abraded and all original

fracture surface was destroyed. Therefore an undamaged area was chosen

at a location approximately half the distance across the fracture.

Figures 10a through 1 Of show microstructure typical of this area. In

general the surface shows an elongated t ransgranu 1 a r fractured grains

of low ductility bounded by narrow bands of dimpled rupture. The amount

of dimpled rupture shows a significant increase of ductility over the

fracture edge of (1-1).

The surface of fracture section (1-3) was coated in places by a

dark brown debris. This was removed by successive cleaning replicas

until a clean metal surface in the area of interest was obtained. The

outside exposed surface in the area of the fillet is shown in Figs. 11a

and b. Identification of the lamellar structure is unknown. (It may

be the partially corroded anodized coating). The fillet edge of the

fracture is shown in Figs. 11c, d, e. Severe corrosion or oxidization

has changed the surface and altered the original fracture surface features.

A smaller amount of surface corrosion appears to have occurred on the

areas shown in Figs. 1 1 g and 1 1 h . This surface area was approximately

1 mm distant from the fillet edge and appears to retain some of the

original fracture topography. At a distance of 1 .5 mm the surface appears

as in Figs. Ilk and 11 j6. Little or no corrosion is noted and the small

amount of dimpled rupture is evident.

Fracture #4 was, by visual examination, similar to (1-3); that is,

a flat semi lustre surface. The outside surface at the fillet is shown
- 4 -

in the upper left corner of Fig. 12a. The edge of fracture begins at

the diagonal , progresses to lower right, and exhibits a flat surface

with fine traces of corrosion. The large irregular shapes in Fig. 12b

may be a thick oxide or surfaces which have been deeply corroded. A

fine dimpled structure can be seen on all Figs, (a) through (f) indicating

a low ductility fracture. An indication of intergranular fracture is

shown in Fig. I2e. The mud crack pattern in Fig. 12f is probably the

result of rapid corrosion of an exposed large second phase particle or

an inclusion.

Macroexamination of fracture #5 revealed the presence of a small

flat brittle region between the fillet and the fibrous woody fracture

surface. Fractographs of this flat area and part of the outside surface

are shown in Figs. 13a through 13e. Two small areas of corrosion are

noted at arrows, these are adjacent to smooth areas which extend to the

outside surface. This may indicate the presence of a pre-existing stress

corrosion crack before overload. A steep sloping step is also noted by

a dark band that extends through the center of the photographs and

connects the fillet edge with the flat fracture. This may indicate the

fracture initiated by shear failure in these areas. Fig. 1 3 f is typical

of the fracture surface away from the edge. The fracture is transgranular

with a small amount of dimpled rupture and few small areas of initial

corrosion

Fracture #6 had occurred in the wing plank material on a plane

parallel to and nearly at same horizontal plane as the outside surface.

By visual examination it also resembled fracture section (

1 -3 ) in
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-

appearance. The upper left corner of Fig. 14a is the outside surface.

The wedge shape edge entering from upper right is intercrystalline

fracture with a small amount of dimple rupture, then changes in the

lower part of the photo to transcrystalline with a larger amount of

dimple rupture.

Figure 14b is an example of cleavage at the fracture edge as compared

with low ductility shear in Fig. 14c. Cleavage and evidence of corrosion

can be seen in Figs. I4d and l4e. Evidence of mud cracks can be noted

in both Figs. I4e and f. In Fig. l4f the prior fracture surface details

have been covered or removed by corrosion or oxidization.

Cone lusions

1. Dimpled rupture, as an indication of ductility, was found at

the fillet edge of all fractures except fracture section (1-3). This

section had evidence of severe corrosion at the fillet edge of fracture.

2. There is a possibility that a stress corrosion crack approximately

1.5 mm deep and of unknown length may have existed in section (1-3) prior

to final overload failure.

3. The occurrence of surface corrosion and/or mud cracks does not

constitute the positive identification of stress corrosion cracking in

a fracture surface of 7 1 78-T6 aluminum alloy extrusion. The corrosion

may have occurred on the fracture surface after failure as evidenced by

examination of two additional fractured samples of wing plank.

4. The precise location of replicas as related to the macro features

of a fracture is essential to the interpretation of the electron fractographs as fracture mode and topography can change radically with location

of electron microscope examination.
Fig. 1. Fracture Section (1-1)

Encircled area replicated for

electron f rac tography . X2

Fig. 3* Fracture Section (1-3)

X4

Fig. 2. Fracture Section (1-2)

X4

Fig. 4. Location of fracture

a reas #4 and #5 . X 1 /5
Fig. 7* Replica of fracture surface #5- Fig. 8. Replica of fracture surface #6

The outside surface is in the lower xb

sector of circle. Xb

A

Fig. 5* Fracture location #6

Xl/5

Fig. 6. Replica of fracture su rf ace #4
Fracture Section (l-l)

Fig. 9a- Adjacent to fillet Fig. 9b.

Fig. 9c. Fig. 9d.

Fig. 9e. Fig. 9f . Approximately 0.5 mm

distant from fillet.
Fracture Section (1-2)

Fig. 10a. Fig. 10b.

Fig. 10c. Fig. 1 0d

.

Fig. 10e. Fig. 10f.
Fracture Section (1-3)

Fig. 11a. Outside surface Fig lib. Outside surface

I^Edge of fracture
Fracture Section (1-3) continued

Fig. 1 1 g . Fracture surface approximately 1 . 0 mm distant from fillet.

Fig . 1 1 h .

Fig. Hi. Fig . 1 1

j

Fig. 11 l. Fracture surface approximately 1.5 nun from fillet edge.

Fig . Ilk.
Fracture Surface #4

Fracture surface Fig. 12b.

Fig. 12c.

Fig. I2e. Fig. 1 2f . Corrosion on second

phase, or inclusion.
Fracture Surface #5

Fig. 13b. Fig. 13a. Outside surface

Fig. 13c Fig. 13d.

Fig. 13e Fig. 1 3 f

-
Fracture Surface #6

Fig. l4e. Fig. I4f