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Two bent transparent plastic spoons, one undeformed "control"
spoon, two bent 3/8-inch (9.5m) diameter aluminum rods and
a fractured plastic spoon and a fork were submitted to the
Metallurgical Laboratory for examination. It was reported
that one of the aluminum rods, both bent spoons and the
broken fork were the result of an undisclosed warm-forming
process while the broken spoon and one of the aluminum rods
were manually broken or bent at ambient temperature. This
report documents the test results and observations made
on the plastic as well as the metal parts.
All of the objects were examined with the unaided eye as
well as using a binocular microscope with magnifications
of 7 to 25X. The plastic parts were also examined using
polarized light, however, no additional information was
gained above that obtained by conventional lighting techniques.
In all cases, observations on the bent spoons were compared
to the No. 1, (see Data Section for listing of code numbers)
The handle of spoon No. 2 was bent slightly down, Figure
1, as compared to spoon No. 1. The bent region exhibited
transverse "crazing" (fine internal cracks), while
the adjacent bowl showed longitudinal crazing, Figures 2
Figure l. Curvature of the Handle of Spoon No. 2 Compared
to Spoon No. l.
Figure 2. Transverse Crazing in the Handle of Spoon No.
2 Compared to Spoon No. 1, Convex Sides up.
Figure 3. Longitudinal Crazing in the Bowl of Spoon No.
2 Compared to Spoon No. 1, Convex Sides Up.
handle of spoon No. 3 exhibited a compound curvature and
was bent up as compared to spoon No. 1, Figure 4. There
was extensive transverse crazing on the handle as well as
some longitudinal crazing at the bowl, Figures 5 and 6.
Figure 4. Curvature of the Handle of Spoon No. 3 Compared
to Spoon No. l.
Figure 5. Transverse Crazing in he Handle of Spoon No. 3
Compared to Spoon No. 1, Convex Sides Up.
Figure 6. Longitudinal Crazing in the Bowl of Spoon No.
3 Compared to Spoon No. 1, Convex Sides Up.
The fractured spoon (No. 4) and the fork (No. 5) are shown
in Figures 7 and 8. Both fractures occurred near the circular
die mark on the handles. The fracture in the spoon propagated
across the handle on a plane roughly normal to the long
axis of the spoon while the fork exhibited a fracture that
propagated partly along a plane normal to the long axis
as well as along about a 45-degree angle to the axis of
the handle. This difference in fracture paths may have been
due to the handle of the spoon being flexed in only one
direction, up and down, while the fork may have been subjected
to a combination of up and down as well as sideways and
twisting loads. The central regions of the fractures which
to the axes always exhibited a smooth, flat, cleaved central
region surrounded by jagged cleavage fractures. The 45-degree
fracture consisted entirely of cleavage steps. It should
be pointed out that "cleavage" is a correct description
of the fracture mode in these plastic parts. Modern polystyrene,
which is assumed to be the spoon and fork material, is a
crystalline plastic and a crystalline structure is a necessary
condition for true cleavage to occur.
Figure 7. Fractured Handles of the Spoon (Bottom) and Fork
(Top), Convex Sides Up. Arrow Points to a Secondary Crack
in the Fork.
Figure 8. Transverse Crazing in the Handles of the Fractured
Spoon (Bottom) and the Fork (Top), Convex Sides Up. Arrows
Point to Approximate Location of Fracture Initiation
The characteristic patterns of the cleavage steps were used
to establish the local fracture direction and thus trace
the principal catastrophic fracture back to its initiation
region. The locations of the principal fracture initiation
areas are shown in Figure 8. In both cases, the fractures
initiated at a flat facet (see Fractographic Analysis).
Small, secondary cracks, which produced the crazing present
at the broken handles, intersected the main fracture surfaces.
The fork exhibited a prominent longitudinal secondary crack
at the principal fracture origin, Figure 7.
The bent aluminum rods are shown in Figure 9. The warm-
and the manually-formed rods exhibited approximately a 180-degree
bend, with a radius of curvature of 4.3 inches (109.2mm)
for the warm and 3.4 inches (86.4m) for the manually-formed
rod. Aside from the slight difference in radii of curvature,
both bent rods had a similar appearance and surface texture.
Figure 9. Warm- (W) and Manually-Formed (M) Aluminum Rods.
The fracture surfaces of the spoon and fork were examined
in the scanning electron microscope. The fracture appearance
in the principal crack initiation region in the spoon is
shown in Figure 10. The flat fracture initiation zone had
a very smooth texture, some moderate roughening was evident
only at a very high magnification, Figure 11. The area within
the cleavage steps had a rougher appearance with numerous
low, irregular, plateau-like structures, Figure 12.
Figure 10. Fracture Initiation Facet (I) in the Broken Spoon.
Open Arrows Indicate Local Crack Propagation Direction.
Slender Arrow Points to Location of Fracture Surface Shown
in Figure 12.
Figure 11. Surface Detail in Fracture Initiation Region
Figure 12. Fracture Appearance at Cleavage Step. Note the
Irregularly Shaped Plateaus.
The fracture in the fork also initiated from a flat facet,
Figure 13. Some dark streaks, Figure 14 and 15, were observed
within the flat region. High magnification suggested that
the streaks were fine scratches, Figure 15. The flat facet
in the fork contained light-colored areas, Figure 14, which
exhibited a random pattern of small ridges, Figure 15, that
was not observed on the flat facet on the spoon. The uniform
gray regions on the flat facet on the fork had a surface
appearance identical to that of the spoon, Figure 11. The
fracture within the cleavage steps, Figure 16, was also
very similar to that on the spoon.
Figure 13. Fracture Initiation Facet (I) in the Broken Fork.
Open Arrows Show Local Fracture Propagation Direction.
Figure 14. Streaks (Arrows) Within the Fracture Initiation
Facet in the Fork. Detail of the Streak at (A) Within the
Light-Colored Area is Shown in Figure 15. The Fracture at
Cleavage Step (B) is Shown in Figure 16.
Figure 15. Detailed Surface Appearance of Streaks (Dark
Bands) in the Light Colored Area. Note the Random Pattern
of Small Ridges Outside the Streaks.
Figure 16. Fracture Appearance at the Cleavage Step. Note
Similarity to Figure 12
The aluminum rods were cross sectioned longitudinally at
the bends, and polished by standard metallographic practice.
The microstructure was revealed by etching with Keller's
etch (l HF - l.5 HCl - 2.5 HN03 - 95 H20).
When a rod is bent, the outside diameter is subjected to
longitudinal tensile stress while the inside diameter experiences
a longitudinal compressive stress. Both rods showed evidence
of superficial grain boundary cracking at the outside diameter.
Some typical cracks are shown in Figure 17. No cracks were
observed at the inside diameter.
Figure 17. Grain Boundary Cracking (Arrows) at Outside Diameter
of Bend in Warm-Formed Rod.
Examination of the cross sections in the light microscope
using conventional illumination as well as polarized light
and Nomarski phase interference contrast techniques revealed
no differences in microstructure between the bent rods.
A typical microstructure at the inside diameter of the warm-formed
rod exhibiting deformation bands, which are frequently observed
in plastically deformed metals, is shown in Figure 18.
Figure 18. Deformation Bands (Arrows) Near Inside Diameter
of Bend in Warm-Formed Rod.
In order to reveal the extremely fine microstructural detail
not visible in the light microscope, both cross sections
were also examined in the scanning electron microscope at
magnifications as high as 11,000X. Typical scanning electron
micrographs are shown in Figures 19 to 22. No significant
differences in microstructure were observed between the
warm- and manually-formed rods.
Figure 19. Typical Microstructure Near the Outside Diameter
of Bend in Warm-Formed Rod. The Dark Spots are Small Pits
Which Have Resulted From Inclusions Being Pulled From the
Surface During Polishing. Open Arrow Points to Region Shown
in Figure 20.
Figure 20. Microstructure of Warm-Formed Aluminum Rod. Open
Arrows Point to Grain Boundaries, Solid Arrows Show Pits.
Figure 21. Typical Microstructure Near the Outside Diameter
of the Manually-Formed Rod. Microstructure is Identical
to that in Figure 19. Open Arrow Points to Location Shown
in Figure 22.
Figure 22. Microstructural Detail of Manually-Formed Rod.
The Microstructure is Essentially Identical to that Shown
in Figure 20.
The two cross sections of the bent rods used for metallographic
analysis were also used for hardness testing. Conventional
Rockwell and Knoop (100g load) microhardness testing techniques
were used. In microhardness testing, a 100g load on the
penetrator resulted in about a 0.0047-inch (0.12mm) long
indentation in aluminum rod material. By using a microscope,
these tiny indentations can be precisely positioned, and
the hardness of a small local area can thus be accurately
Conventional hardness testing of the bent areas indicated
that both rods had an identical hardness of HRB 39-40. Knoop
microhardness testing in which the hardness indentations
were placed at precisely 0.020 inch (0.5mm) intervals in
a perpendicular, straight line from the outside to the inside
surface of the bends indicated that, within the accuracy
of the test, there was no difference in the hardness profiles
of the two bent rods, Figure 23.
Figure 23. Micrehardness Traverses of Warm- and Manually-Formed
1. The bending of both spoons resulted in transverse crazing
(cracking) in the handles and longitudinal crazing in the
2. The fracture in the spoon occurred across the handle
while the fork had fractured primarily at about a 45-degree
3. Both fractures initiated at a flat, subsurface facet.
4. The fracture mode in both cases was cleavage.
5. While the fracture modes were identical, there was some
fine detail in portions of the fracture initiation region
of the fork that was not present in the initiation region
of the spoon. The significance of this difference was not
6. No significant differences in microstructure or hardness
were noted between the warm- and the manually-formed aluminum
Worksheet No. 124828
by Bill Houck 3/15/81"
Materials & Processes - Metallurgy Design &
APPROVED BY ________________
R. A. Rawe, Branch Chief - Technology Materials &
Processes - Metallurgy Design & Technology