INTRODUCTION

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.

PROCEDURE AND RESULTS

  Visual Examination

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) undeformed spoon.

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 and 3.


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.

The 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 occurred normal 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.

Fractographic Analysis

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.


MAGN. 24.3X

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.


MAGN. 8,600X

Figure 11. Surface Detail in Fracture Initiation Region of Spoon.


MAGN. 8,500X

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.


MAGN. 25.5X

Figure 13. Fracture Initiation Facet (I) in the Broken Fork. Open Arrows Show Local Fracture Propagation Direction.


MAGN. 66.4X

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.


MAGN. 9,000X

Figure 15. Detailed Surface Appearance of Streaks (Dark Bands) in the Light Colored Area. Note the Random Pattern of Small Ridges Outside the Streaks.


MAGN. 9,000X

Figure 16. Fracture Appearance at the Cleavage Step. Note Similarity to Figure 12

Metallography

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.


ETCHANT: KELLER'S                MAGN. 200X

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.


ETCHANT: KELLER'S                MAGN. 20OX

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.


ETCHANT: KELLER'S                MAGN. 278X

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.


ETCHANT: KELLER'S                MAGN. 10,80OX

Figure 20. Microstructure of Warm-Formed Aluminum Rod. Open Arrows Point to Grain Boundaries, Solid Arrows Show Pits.


ETCHANT: KELLER'S                MAGN. 285X

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.


ETCHANT: KELLER'S                MAGN. 11,100X

Figure 22. Microstructural Detail of Manually-Formed Rod. The Microstructure is Essentially Identical to that Shown in Figure 20.

Hardness Testing

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 determined.

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 Aluminum Rods.

SIGNIFICANCE OF DATA

1. The bending of both spoons resulted in transverse crazing (cracking) in the handles and longitudinal crazing in the bowls.

2. The fracture in the spoon occurred across the handle while the fork had fractured primarily at about a 45-degree angle.

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 established.

6. No significant differences in microstructure or hardness were noted between the warm- and the manually-formed aluminum rods.

1.	CCN lW2WVA951
2.	Laboratory Worksheet No. 124828
3.	Specimen Codes:
	Specimen Number	Item	Description
	1	Spoon	Control
	2	Spoon	Manual Bend
	3	Spoon	"Klause 3/16/81"
	4	Broken Spoon	"Broken by Bill Houck 3/15/81"
	5	Broken Fork	"Chuck Allen 3/14/81"

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