NBSIR 76-981 70

Examination of Steel Components

from the Yadkin River Bridge,

Siloam, North Carolina

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

November 1975

Failure Analysis Report

Prepared for

Bureau of Aviation Safety

National Bureau of Standards

Washington, D. C. 20594
I
NBSIR 76-981

EXAMINATION OF STEEL COMPONENTS

FROM THE YADKIN RIVER BRIDGE,

SILOAM, NORTH CAROLINA

T. Robert Shives

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

November 1975

Failure Analysis Report

"This document has been prepared for the use of

the Bureau of Aviation Safety, National Transportation Safety Board, Washington, D.C. Responsibility for its further use rests with that agency.

NBS requests that if release to the general public

is contemplated, such action be taken only after

consultation with the Office of Public Affairs

at the National Bureau of Standards."

Prepared for

Bureau of Aviation Safety

National Transportation Safety Board

Washington, D. C. 20594

•"•»»u o'

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-John son. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Acting Director

TABLE OF CONTENTS

Page

SUMMARY iv

1. GENERAL INFORMATION 1

1.1 Reference 1

1.2 Accident Information 1

1.3 Parts Submitted 1

1.4 Parts Designation 2

2. PURPOSE 2

3. RESULTS OF EXAMINATIONS, TESTS, AND ANALYSES 2

3 . 1 Relationship of Flaws and Bar Fabrication

Technique 2

3.2 Tensile Tests 3

3.3 Fractographic Examination 5

3.3.1 Submitted Field Fractures 5

3.3.1.1 Visual and Macroscopic

Examination

..

5

3.3.1.2 Scanning Electron

Microscope Examination 5

3.3.2 Laboratory Produced Tensile Fractures 6

3.3.2.1 Visual and Macroscopic

Examination 6

3.3.2.2 Scanning Electron Microscope

Examination 6

3.4 Metallographic Examination 7

3.5 Chemical Analysis 9

3.6 Hardness Measurements 9

i
TABLE OF CONTENTS (continued)

Page

4. DISCUSSION 10

5. CONCLUSIONS 12

6. ACKNOWLEDGEMENT 13

FIGURES

1. Components from the Yadkin River Bridge, Siloam,

North Carolina as received at NBS on May 20, 1975.

2. Components from the Yadkin River Bridge, Siloam,

North Carolina as received at NBS on July 28, 1975.

3. Part of bar XlA at the junction where two bars were forge

welded together.

4. Heavily etched longitudinal section through bar Yl

showing the junction between the two components of the

bar that had been forge welded together.

5. Fracture profile of bar B6 showing necking which

occurred during the laboratory tensile test.

6. Fracture profile of bar XlA.

7. Fracture profile of bar Yl.

8. Fracture profile of bar X1B

.

9. Opposing fracture surfaces of bar Bl.

10. Opposing fracture surfaces of bar B5.

11. SEM fractograph of area adjacent to the apparent

fracture origin in bar Bl.

12. SEM fractograph of area near the apparent fracture

origin in bar Bl.

13. SEM fractograph from bar Bl showing the fracture

surface away from the origin.

14. SEM fractograph from bar B5 (region A, figure 10)

showing mostly cleavage and some dimpled rupture.

15. SEM fractograph from bar B5 (region A, figure 10)

exhibiting mostly dimpled rupture and some cleavage.

16. Fracture surfaces produced in the laboratory at NBS.

ii
FIGURES (continued)

17. Surface of the laboratory produced fracture through

bar XIA.

18. SEM fractograph from laboratory produced tensile

fracture in bar B6.

19. SEM fractograph of bar XlA in the region containing

the pre-existing crack.

20. SEM fractograph of bar XlA in the remaining ligament

in the plane of the pre-existing crack.

21. SEM fractograph of the part of the fracture of bar X1B

that exhibited cleavage as the primary fracture mode.

22. SEM fractograph of bar XlB where the fracture was

following a flaw in the junction between two bars

that had been forge welded together.

23. Etched longitudinal section through bar B6 showing the

fracture profile.

24. Profile of the fracture through bar Bl

.

25. Profile of the fracture in the pre-existing crack

portion of the fracture through bar XlA.

26. Longitudinal section through bar XlA showing secondary

crack parallel to the fracture.

27. Part of the longitudinal section shown in figure 4 at a

higher magnification and at a different orientation.

28. Transverse section through one of the components of bar

B6 exhibiting a Widmanstatten structure.

29. Longitudinal section through bar B5.
SUMMARY

The Bureau of Aviation Safety, National Transportation

Safety Board, submitted six bar segments from the collapsed

bridge over the Yadkin River in Siloam, North Carolina, to

the NBS Mechanical Properties Section for examination.

Three of the six bars had fractured in the field. All six

of the bars were found to consist of two components that had

apparently been forge welded together. Two of the three

bars that had fractured in the field had partially fractured

through forge welded junctions between two bars.

Laboratory tensile tests were performed on two asreceived bars which already contained flaws at the junctions.

These tests resulted in fracture at the junction in one bar,

and away from the junction in the second bar. Another test

of the second bar resulted in fracture at the junction. All

of the failures at junctions (two in the field and two in the

laboratory) plus the one field fracture not occurring at a

junction exhibited very little ductility.

Three other laboratory tensile tests resulted in

fractures not at junctions that exhibited good ductility.

The field fracture that did not pass through a junction

may have been due to impact loading. The other field

fracture that was examined had passed through a junction.

This fracture appeared to be either a high strain rate

fracture or a fracture due to stress corrosion cracking.

Stress corrosion cracking was the probable mechanism

of formation for a pre-existing crack adjacent to a junction

in a bar that was tested in tension in the laboratory.

Variations in the chemical composition, microstructure

,

and hardness, and poor forge welding in the bars indicate

a lack of control over the material and the fabrication

process

.

iv
Examination of Steel Components from the

Yadkin River Bridge, Siloam, North Carolina

1. GENERAL INFORMATION

1 . 1 Reference

Bureau of Aviation Safety, National Transportation

Safety Board (NTSB) , Washington, D.C. This investigation

was conducted at the request of Mr. Michael L. Marx by

letters dated May 20, 1975 and August, 197 5.

1 . 2 Accident Identification

The submitted components were from a bridge over the

Yadkin River in Siloam, North Carolina, that collapsed on

February 23, 1975.

1 . 3 Parts Submitted

On May 20, 1975, parts of three fractured steel

components from the collapsed Yadkin River Bridge were

submitted by the NTSB to the NBS Mechanical Properties

Section for examination. These parts are shown as

received by NBS in figure 1. (The NTSB had sectioned one

of the components before sending it to NBS.) The components

were identified as follows:

Bar L4E-U5E B6

Bar L3E-U4E B5

Bar U8L8W Bl

On July 28, 1975, an unbroken bar segment from each of

three additional steel components from the bridge was

submitted for examination. Two of these bar segments were

attached to a turnbuckle. These parts are shown as received

in figure 2. They were identified as follows:

Bar Yl L4W-U5W

Bar XI (A) L6E to U5E

Bar XI (B) L6E to U5E

Except for the threaded regions, the bars were essentially

square in cross section and measured about 7/8 to 1 inch

on a side.

In addition, 0° and 90° radiographs of the latter three

bars were submitted. Radiographs of two bars other than

those submitted were also included.

- 1

-
1 . 4 Parts Designation

In this report, the designations of the components as

submitted will be shortened to the following NBS designations:

As submitted designation NBS designation

L4E-U5E B6 B6

L3E-U4E B5 ' B5

U8L8W Bl Bl

Yl L4W-U5W Yl

XI (A) L6E to U5E XlA

XI (B) L6E to U5E XlB

2 . PURPOSE

The Bureau of Aviation Safety requested that the NBS

Mechanical Properties Section perform tensile tests on bars

B6, Yl , XlA, and XlB. Tests at high speed were suggested

to give results characteristic of a high strain rate

fracture process. The modes of the fractures produced by

these tests and the modes of the field fractures of bars

Bl and B5 were to be determined. The field fracture

surface of bar B6 had been examined by the NTSB before it

was submitted to NBS and was not included in this investigation. The nature of the flaws indicated by the radiographs

of bars XlA and XlB was also to be determined. If deemed

necessary, other tests or analyses such as chemical analysis,

cross-sectioning, and tensile testing could be conducted.

3. RESULTS OF EXAMINATIONS, TESTS, AND ANALYSES

3 . 1 Relationship of Flaws and Bar Fabrication Technique

Features on the submitted radiographs indicated the

presence of a flaw (flaws) in bars XlA and XlB at an angle

of about 35° to the longitudinal axis of the bar. In

addition, there was a flaw indicated in bar XlA that was

about 90° to the longitudinal axis of the bar. This flaw

terminated at the 35° angle flaw. The flaw in bar XlA

that was 90° to the longitudinal axis could be seen at the

surface of the bar (figure 3) and will be discussed later.

The flaws at 35° to the longitudinal axis were actually

discontinuities in junctions between two bars that had

apparently been forge welded together. (Henceforth in this

report, the term "forge welded" implies "apparently forge

welded.") The evidence for the existence of the junctions

- 2 -
was revealed in a metallographic examination (Section 3.4

of this report) . An example of one of the forge welded

junctions appears in figure 4 where a heavily etched

longitudinal section through bar Yl is shown. The junction

goes from the lower left to the upper right in the figure.

No flaw was evident on the radiograph of bar Yl . The

junction was essentially continuous or sound except for

small regions such as that indicated by the arrow in

figure 4. Presumably, these flaws were too small to be

detected by radiography.

All six of the submitted components consisted of two

bars that had been joined by forge welding.

3 . 2 Tensile Tests

Uniaxial tensile tests were conducted on bars B6,

X1A, XlB , and Yl. Each of the four bars was tested with

the full as-received width and thickness. No machining

was done except for the removal of some burrs and paint

near the ends of the specimens to facilitate gripping in

the testing machine.

The junctions in bars XlA and XlB were located about

seven inches from a turnbuckle to which they were attached

when submitted, as evidenced by the flaws shown on the

radiographs. For the tensile testing, it was assumed that

the junction in bar Yl was in a similar location (as

measured from the end of the threads) . At this point in

the investigation, the metallographic work revealing the

junction in bar Yl had not been performed. In the tensile

tests of each of these three bars, the junction (or the

assumed location of the junction) was positioned

approximately midway between the grips of the tensile

testing machine.

Bar B6 had fractured in the field through its junction

therefore, for the tensile test, the approximate center of

the submitted piece was placed about midway between the

testing machine grips.

All tests were conducted at a machine cross head speed

of 16 inches per minute, the maximum cross head speed of

the machine employed.

- 3 -
The maximum tensile loads sustained by the bars are

as follows:

Bar Maximum load, lbs

B6

X1A

48,000

9,100

48 ,000*

26,300*

49,100

X1B (first test)

(second test)

Yl

* Bar X1B failed first near the bottom grip and away from

the junction. The remainder of the bar containing the

junction was tested in tension a second time. The second

test resulted in failure at the junction. The maximum

load sustained in the second test was much less than that

sustained in the first test.

Bar B6 (figure 5) necked considerably in the vicinity

of the fracture, indicating that the material possessed good

ductility. Reduction in area at the fracture was approximately

45%. The fracture was essentially perpendicular to the

longitudinal axis of the bar except for the 45° shear lip

around the perimeter.

Bar X1A failed at the junction through the flaws

indicated by the radiograph. The part of the fracture that

was perpendicular to the longitudinal axis of the bar passed

through a pre-existing crack (the flaw that intersected the

surface of the bar as shown in figure 3) . A significant

portion of the fracture followed the path of the two-bar

interface (about 35° to the longitudinal axis of the bar)

.

There was essentially no necking or reduction in area in

the vicinity of the fracture. The fracture of bar X1A can

be seen in figure 6.

Bar Yl failed in a ductile manner, exhibiting considerable

necking in the vicinity of the fracture (figure 7) . The path

of this fracture tended towards 4 5° to the longitudinal axis

of the bar. The fracture did not occur at the assumed location of the junction in the bar.

As mentioned above, bar XlB fractured the first time it

was tested in a region away from the flaw indicated by the

radiograph. There was considerable necking in the vicinity

of this fracture, indicating good ductility for the material

in this part of the bar (figure 8a)

.

- 4 -
The remaining part of bar XlB containing the junction

was retested. The bar then failed at the junction through

the flaw indicated by the radiograph. A large part of this

fracture followed the path of the junction (about 35° to

the longitudinal axis of the bar) . There was essentially

no necking in the region of the fracture indicating a low

ductility fracture. This fracture can be seen in figure 8b.

3 . 3 Fractographic Examination

3.3.1 Submitted Field Fractures

3.3.1.1 Visual and Macroscopic Examination

The fracture surfaces of bars Bl and B5 are shown in

figures 9 and -10, respectively. Both of these bars had

failed in the field. The fracture surfaces of both bars

were corroded, but those of bar B5 appeared to be considerably

more corroded than those of bar Bl. Neither bar exhibited

any visible reduction in area at the fracture.

As can be seen in figures 1 and 9, the fracture

path in bar Bl is essentially perpendicular to the

longitudinal axis of the bar along most of the fracture

(region A, figure 9) , but then makes an abrupt change in

direction (arrow B, figure 9) and follows a path more

closely paralleling the longitudinal axis of the bar

(region C, figure 9). Region C of the fracture surface is

smooth and is probably more accurately described as part of

a junction in the bar. This junction was produced when the

loop in the end of the component was made by bending the bar

back upon itself, the seam being located where the loop was

closed. The loop can be seen in figure 1.

Except for a small region near the center, the

fracture path in bar B5 is essentially perpendicular to the

longitudinal axis of the bar. The features of one part of

the fracture (region A, figure 10) appear to be finer than

those of the re-st of the fracture.

3.3.1.2 Scanning Electron Microscope Examination

One fracture surface each from bars Bl and B5 was examined

with the scanning electron microscope (SEM) . In bar Bl, the

fracture crack appears to have initiated about 0.7 inch

from the end of the junction where the loop in the bar closed.

Arrows B and D in figure 9 indicate on the opposing fracture

surfaces the approximate location of the fracture crack

origin. Adjacent to the apparent origin, the fracture

surface features were masked with a film of what appeared to

be corrosion product. An SEM fractograph from this area is

shown in figure 11. There was a relatively small area

- 5 -
(region 1, figure 9) near the fracture origin that exhibited

apparent dimpled rupture as the dominant fracture mode. An

SEM fractograph from this area is shown in figure 12.

Cleavage was the predominant mode for the

remainder of the fracture surface of bar Bl. An SEM

fractograph from the region of the fracture surface

exhibiting cleavage is shown in figure 13.

Before examination of the fracture surface of

bar B5 with the SEM, corrosion product was removed by

replication techniques in order to reveal the fracture

features. The fracture mode in region A, figure 10, was

mostly cleavage (figure 14) with some areas of primarily

dimpled rupture (figure 15). There are several large holes

or pores that can be seen in figure 15. The mode of the

upper portion of the fracture as it is oriented in figure 10

is predominantly dimpled rupture (similar to figure 15) . The

step in the fracture is along the junction between the two

components of the bar.

3.3.2 Laboratory Produced Tensile Fractures

3.3.2.1 Visual and Macroscopic Examination

The fracture surfaces of the four NBS laboratory produced

tensile fractures that were examined are shown in figure 16.

The fractures of bars B6 and Yl had the appearance of ductile

overload. The fracture through bar X1B that did not pass

through the junction in the bar also had the appearance of

ductile overload. This fracture is not shown in figure 16

and it was not examined in detail.

The fracture surfaces of bars X1A and X1B

(the fracture at the junction in the bar) exhibited considerable

evidence of the discontinuities in the junctions through which

the fractures passed. As seen in figure 17, this evidence

is especially prominant in the center portion of the fracture

in bar X1A, where the fracture surface is characterized by

a wavy appearance with regions of very smooth material and

regions of sponge-like material. As mentioned earlier, the

fracture in bar X1A (figure 17) also passed through a preexisting crack (see figure 3) that was essentially perpendicular to the longitudinal axis of the bar. The pre-existing

crack penetrated to the junction, but did not follow its

path. The part of the fracture surface containing the

pre-existing crack was covered with corrosion product as can

be seen in figure 17.

3.3.2.2 Scanning Electron Microscope Examination

Dimpled rupture was the predominant fracture mode over

- 6 -
essentially the entire fracture surfaces of both bars B6

and Yl. An SEM fractograph from bar B6, which is

representative of both fractures, appears in figure 18.

This fracture mode is consistent with the significant

reduction in area of these bars at the fractures.

The fracture surface of bar X1A was examined

both in the area of the pre-existing crack and in an area

not containing the pre-existing crack. The area not

containing the pre-existing crack was on the same plane as

the pre-existing crack and adjacent to it. The fracture

surface was cleaned by repetitive replication with cellulose

acetate before examination in order to remove enough of the

corrosion product so that the fracture features could be

seen.

In the part of the fracture containing the

pre-existing crack, the fracture mode was mixed, consisting

of cleavage, apparent intergranular cracking, and a small

amount of dimpled rupture (figure 19) . In the area not

containing the pre-existing crack, the fracture mode was

predominantly dimpled rupture (figure 20) .

The part of the fracture through bar X1A that

followed the junction was not examined with the SEM, but

the features of this part of the fracture were macroscopically

similar to those exhibited by bar X1B in a similar area.

The fracture of bar XlB is discussed below.

The upper part of the fracture of bar XlB, as

the bar is oriented in figure 16, exhibited mostly cleavage.

This part of the fracture did not follow the path of the 35°

angle junction and is essentially perpendicular to the

longitudinal axis of the bar. Figure 21 is an SEM fractograph of the part of the fracture where cleavage predominated.

For the most part, the remainder of the fracture

followed the flaw or junction in the bar and exhibited the

features of the flaw (figure 22) . In regions where the

forge welding appeared to be partially successful, the

fracture surface showed a mixture of dimpled rupture (similar

to figure 2 0 for bar X1A) where the junction was sound, and

the features of the flaw where the junction was not sound.

3 . 4 Metallographic Examination

Metallographic (or macrographic) examination of

longitudinal sections through each of the six bars revealed

that all contained a junction or interface where two

separate components had been forged together. The flaws

- 7 -
35° to the longitudinal axes of bars X1A and X1B that

were indicated on the radiographs were discontinuities

in these interfaces. Except for the field fracture

through bar Bl, all of the apparent low ductility fractures

followed the paths of the junctions in part. And in bar Bl

,

the fracture path partially followed a seam or interface

where the bar had been bent back upon itself to form a

loop.

An etched longitudinal section showing the fracture

path through bar B6 appears in figure 23. This fracture

path is typical of those in the apparent low ductility

fractures (except for that of bar Bl) . Where the fracture

path is about 35° to the longitudinal axis of the bar, the

fracture path essentially coincides with the junction or

interface between the two components of the bar. Where the

fracture path deviates from the interface, as in the lower

left in figure 23, the interface can be seen (arrows,

figure 23) .

The profile of the fracture in bar Bl is shown in

figure 24. The fracture appears to be primarily transgranular, but it may be intergranular in places. The

fracture profile in the pre-existing crack portion of the

fracture in bar XlA is shown in figure 25. The fracture

appears to be partially transgranular and partially

intergranular. The fracture profile of bar B5 (not shown)

in an area where the fracture did not follow the interface

between the two components also appeared to be partially

transgranular and partially intergranular.

There was at least one secondary crack close to and

essentially parallel to the fracture in bars XlA (figure 26)

and B5. Both of these secondary cracks appeared to be

somewhat intergranular in nature.

In bar Yl, which failed in a ductile manner, the

fracture did not pass through the junction. The radiograph

of this bar did not reveal the presence of flaws at the

junction because the junction was apparently sound over all

but a very small portion of its length, as was shown in

figure 4.

A photomicrograph of part of the section shown in

figure 4 exhibits a change in microstructure at the junction

(figure 27) . The junction is essentially vertical in the

center of figure 27. The microstructure on both sides of

the junction consists of pearlite and ferrite, but the

microstructure to the left of the junction in figure 27

appears to have a higher carbon content (more pearlite)

than the microstructure at the right.

- 8 -
The microstructure of all the bars consisted primarily

of ferrite and pearlite, but the relative amounts of these

two constituents varied considerably from bar to bar, and

in some cases, between the two components of a single bar.

Examples of some of these variations in microstructure have

been presented in previous figures. Another variation is

shown in figure 28 where some Wldmanst&tten structure is

evident.

In bars X1A and B5, there was a surface layer that

appeared to have much less carbon than the rest of the material

in the part of the component that was examined. This

condition in bar B5 can be seen in figure 29.

3 . 5 Chemical Analysis

Chemical analyses were made by a commercial laboratory

on samples of one component from each of three of the

submitted bars. Referring to figure 1, bar Bl was analyzed

to the left of the fracture, but to the right of the junction;

bar B5 was analyzed to the left of the fracture which was at

the junction; and bar B6 was analyzed to the right of the

fracture which was at the junction. The results (weight per

of these analyses are as fo Hows

Element Bar Bl B ar B5 Bar B6

Carbon 0. 19 0. 22 0. 41

Manganese 0. 48 0. 47 0. 61

Phosphorus 0. 013 0. 016 0. 016

Sulfur 0. 034 0. 027 0. 023

Silicon < 0. 05 < 0. 05 * 0. 15

Nickel < 0. 05 < 0. 05 0. 08

Chromium < 0. 05 < 0. 05 < 0. 05

Molybdenum < 0. 05 < 0. 05 < 0. 05

Copper < 0. 05 < 0. 05 < 0. 05

The chemical composition of the samples from bars Bl and B5

are very similar. The sample from bar B6 is significantly

different, especially in carbon and manganese.

3 . 6 Hardness Measurements

Diamond pyramid hardness measurements at a load of

2 1/2 kg (HV2 1/2) were made on a longitudinal section

through bar B6 in the vicinity of the fracture. These

measurements were made to determine whether the hardness

might change across the junction. The hardness of the

apparent low carbon component of the bar averaged about

131 HV2 1/2 (about 73 HRB) , whereas the hardness of the

- 9 -
apparent higher carbon component averaged 185 HV2 1/2

(about 90 HRB)

.

4 . DISCUSSION

Perhaps the most interesting feature revealed in the

examination of the forged bars from the collapsed Yadkin

River Bridge was that each of the six submitted bars was

actually composed of two separate components that had been

apparently forge welded together at an angle of about 35°

.

The junction between the two components of a bar was quite

evident in the results of the metallographic examination.

In some cases, the interface could be seen more easily

because of rather pronounced differences in the microstructure on either side of the interface. At the one

interface where hardness was measured, it too, was

significantly different on either side.

The flaws at about 35° to the longitudinal axes of

bars XlA and XlB that were indicated on the radiographs

were discontinuities in the forge welded junctions. The

flaw at about 90° to the longitudinal axis, of bar XlA

that was indicated on the radiograph was a pre-existing

crack. A comparison of laboratory tensile test results

from bar XlA with flaws at the junction with the results

from a bar with no significant flaw at the junction (bar

Yl) indicates that the flaws significantly reduced the tensile

load carrying capacity of bar XlA.

The maximum load sustained by bar XlA in the laboratory

test was much less than that sustained by bar XlB (fracture

through the junction) because the combined pre-existing

crack and junction discontinuity in bar XlA was a larger

flaw than the junction discontinuity in bar XlB.

The tensile tests of bar XlB led to some unexpected

results. In the initial test, failure occurred away from

the junction when the bar had sustained a load comparable

to the maximum load sustained by bars Yl and B6. The

fracture was quite ductile. The second test, however,

resulted in a low ductility fracture at the junction and at

a much reduced load. It is not clear why the bar sustained

the higher load initially nor why failure did not occur at

the junction. The first tensile test may have initiated

the fracture at the junction, and the second test completed

it.

Based on a comparison of the tensile test load carrying

capacities between a bar with an essentially sound junction

- 10 -
(bar Yl) and a bar segment with no junction (bar B6) , the

junction of the two components had no observable adverse

effect on the tensile strength of the bar if the junction

was sound.

The laboratory produced fractures that did not fail

at a junction exhibited good ductility, whereas both field

and laboratory fractures that passed through a junction

exhibited little ductility and tended to partially follow

the junction between the two components of a bar. The

fracture mode was predominantly cleavage, which is

consistent with the observed low ductility.

The fracture in bar Bl, the only submitted field

fracture that did not pass through a junction, was a low

ductility fracture. There did not appear to be as large

a buildup of corrosion product as there was on the fracture

surface of bar B5 (field fracture at a junction) , indicating

that the fracture in bar Bl may have been more recent. No

secondary cracking was noted in the longitudinal section that

was examined metallographically and the primary fracture

mode was cleavage. From these observations, it may be inferred

that the fracture in bar Bl was due to impact loading. The

strain rate in impact loading would be much higher than that

of the NBS tensile tests.

In bar XlA, both the pre-existing crack and a secondary

crack parallel to the fracture and about 1/8 inch from it had

an intergranular appearance in places, suggesting stress

corrosion cracking as a possible crack mechanism in this

bar

.

The fracture surface of bar B5 appeared to have

corroded considerably more than the fracture surface of

bar Bl, indicating that the fracture of B5 was probably the

older fracture. The appearance at the step in the fracture

indicates that there may have been a flaw at the junction

in this region through which the fracture passed. There is

some evidence of intergranular cracking at the fracture and

at a secondary crack parallel to the fracture suggesting,

as in bar XlA, stress corrosion cracking as a possible crack

mechanism for at least the initial part of the fracture.

On the other hand, the features of the fracture surface

of bar B5 were somewhat like those from the laboratory fracture

through the junction in bar X1B , indicating that the fracture

through bar B5 may have been a high strain rate fracture.

- 11 -
The differences in chemical composition, microstructure

,

and hardness among various bars indicate that there was

little control over the materials employed or the condition

of the materials at the time of fabrication. The presence

of flaws at the junctions between two components of a bar

suggests that the forge welding process employed was not

always completely successful.

5. CONCLUSIONS

1. All six of the submitted bars consisted of two

components that had been apparently forge welded

together.

2. The flaws at about 35° to the longitudinal axes of

bars X1A and XlB that were indicated on the radiographs

were discontinuities in the junctions between two

components that had been apparently forge welded

together.

3. Two of the three submitted failed bars had fractured

through one of the interfaces or junctions between

two components

.

4. All three of the submitted fractures were low

ductility fractures.

5. Relatively high strain rate (16 inches per minute)

uniaxial tensile tests conducted in the laboratory

on full cross section bar segments resulted in high

ductility fractures where the fractures did not pass

through a junction or interface between two components

(bars B6, Yl , and XlB).

6. Similar tensile tests resulted in low ductility

fractures in bars XlA and XlB where the fractures

passed through a junction.

7. Based on the results of the NBS tensile tests, sound

junctions had no observable adverse effect on the

tensile strength of the bars containing them.

8. The flaws at the junction in bar XlA significantly

reduced the load carrying capacity of that bar.

9. The field fracture through bar B5 appeared to be

either a high strain rate fracture or a fracture due

to stress corrosion cracking.

- 12 -
10. The field fracture through bar Bl, the only field

fracture that did not pass through a junction in a

bar, may have been caused by impact loading.

11. There was a pre-existing crack adjacent to the junction

in bar X1A. The suggested mechanism for this crack is

stress corrosion cracking.

12. Variations in microstructure , chemical composition,

and hardness among the different components and the

flaws in some junctions indicate that control was

poorly maintained over the quality of the material

used in the bars, the condition of that material,

and the forge welding process employed.

6. ACKNOWLEDGEMENT

Mr. L. C. Smith of the NBS Mechanical Properties Section

performed the metallographic specimen preparation and the

photography and made the hardness measurements. Mr. W.A. Willard

of the Mechanical Properties Section and Mr. Smith conducted the

tensile tests.

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Figure 3. Part of bar XlA at the junction where two bars

were forge welded together. The arrow indicates

a flaw which intersected the surface of the bar

X 1 1/2

Figure 4 . Heavily etched longitudinal section through bar Yl

showing the junction between the two components of

the bar that had been forge welded together. The

junction goes from the lower left to the upper right.

In this section, the junction appears to be sound

except for the small flaw indicated by the arrow.

Etchant: 10% nitric acid X 1 2/3

Figure 5. Fracture profile of bar B6 showing necking which

occurred during the laboratory tensile test. X 1
I
Figure 6. Fracture profile of bar XlA. X 1

Figure 8

.

Fracture profile of bar X1B. XI

a) Fracture away from the junction.

b) Fracture through the junction.

Figure 9. Opposing fracture surfaces of Bar Bl. X 2

A is region of fracture perpendicular to the

longitudinal axis of the bar.

Arrow B indicates apparent fracture origin

and location of change in direction of

fracture path.

C is region of fracture following junction.

Arrow D indicates apparent fracture origin.

Region 1 is small area where the fracture mode

appears to be dimpled rupture.

Figure 10. Opposing fracture surfaces of bar B5. XI

The fracture features appear to be finer in

region A than on the rest of the fracture

surface

.

Figure 11.

Figure 12. SEM fractograph of area near the apparent fracture

origin in bar Bl. Mode o± fracture appears to be

dimpled rupture. X 38 0

Figure 13. SEM fractograph from bar Bl showing the

fracture surface away from the apparent

origin. The predominant fracture mode

is cleavage. X 175

Figure 14. SEM fractograph from bar B5 (region A,

figure 10) showing mostly cleavage and

some dimpled rupture. X 210

Figure 15. SEM fractograph from bar B5 (region A,

figure 10) exhibiting mostly dimpled

rupture and some cleavage. There are

several large holes or pores. X 540

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Figure 17. Surface of the laboratory produced fracture

through bar XlA. The heavily corroded portion

near the top of the fracture surface is the

pre-existing crack. The center portion of the

fracture (about 35° to the longitudinal axis

of the bar) is essentially along the junction

of the two parts of the bar. X 1

Figure 18. SEM fractograph from laboratory produced tensile

fracture in bar B6. Dimpled rupture is the

primary fracture mode. X 4 90

Figure 19. SEM fractograph of bar XlA in the region

containing the pre-existing crack. The mode

of fracture is mixed, consisting of cleavage,

apparent intergranular cracking, and some

dimpled rupture. X 525

Figure 20. SEM fractograph of bar XlA in the remaining

ligament in the plane of the pre-existing

crack (see figure 17) . Dimpled rupture is

the primary fracture mode. X 2 000

Figure 21. SEM fractograph of the part of the fracture of

bar X1B that exhibited cleavage as the primary

fracture mode. X 2 00

Figure 22. SEM fractograph of bar X1B where the fracture

was following a flaw in the junction between

two bars that had been forge welded together.

X 550

Figure 23. Etched longitudinal section through bar B6

showing the fracture profile. The longitudinal

axis of the bar is vertical. The fracture

followed the junction between the two components

of the bar where the fracture is about 35° from

the longitudinal axis of the bar. Where the

fracture deviates from the junction at the lower

left, the junction can be seen (arrows)

.

Etchant: 10% nitric acid X 3

Figure 24. Profile of the fracture through bar Bl. The

longitudinal axis of the bar is essentially

vertical.

Etchant: 1% nital X 200

Figure 25. Profile of the fracture in the pre-existing

crack portion of the fracture through bar X1A.

The longitudinal axis of the bar is essentially

vertical

.

Etchant: 1% nital X 200

Figure 26. Longitudinal section through bar XlA showing

secondary crack parallel to the fracture. The

longitudinal axis of the bar is vertical.

Etchant: 1% nital X 40

Figure 27. Part of the longitudinal section shown in

figure 4 at a higher magnification and at

a different orientation. The junction

between the two components is vertical in

the center of the photomicrograph. The

microstructure is different on either side

of the junction.

Etchant: 1% nital X 50

Figure 28. Transverse section through one of the components

of bar B6 exhibiting a Widmanstaaten structure.

Etchant: 1% nital X 100

Figure 29. Longitudinal section through bar B5. The

longitudinal axis of the bar is vertical in

the figure. The surface of the bar is vertical

at the left. The arrows indicate a region along

the surface that is low in carbon compared to

the rest of the material in the component.

Etchant: 1% nital X 12

nDJ-l I4M IKt V. / / J)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

JNrJoxK / b y o

X

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

EXAMINATION OF STEEL COMPONENTS FROM THE YADKIN

RIVER BRIDGE, SILOAM, NORTH CAROLINA

5. Publication Date

November 1975

6. Performing Organization Code

7. AUTHOR(S)

T. Robert Shives

8. Performing Organ. Report No.

NBSIR 76-981

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 3120412

11. Contract /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Bureau of Aviation Safety

National Transportation Safety Board

Washington, D.C. 20594

13. Type of Report & Period

Covered Failure

Analysis Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The Bureau of Aviation Safety, National Transportation Safety

Board, submitted six bar segments from the collapsed bridge over the

Yadkin River in Siloam, North Carolina, to the NBS Mechanical Properties

Section for examination. Three of the six bars had fractured in the

field. All six of the bars were found to consist of two components that

had apparently been forge welded together. Two of the three bars that

had fractured in the field had partially fractured through forge welded

junctions between two bars.

Laboratory tensile tests were performed on two as-received bars

which already contained flaws at the junctions. These tests resulted in

fracture at the junction in one bar, and away from the junction in the

second bar. Another test of the second bar resulted in fracture at the

junction. All of the failures at junctions (two in the field and two in

the laboratory) plus the one field fracture not occurring at a junction exhibited very little ductility.

Three other laboratory tensile tests resulted in fractures not at

junctions that exhibited good ductility.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Brittle fracture; ductile fracture; forge welding; impact;

stress corrosion cracking

18. AVAILABILITY \Z2 Unlimited 19. SECURITY CLASS

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21. NO. OF PAGES

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22. Price

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USCOMM-DC 29042-P74