Scanning Electron Microscopy of Fractures in Eggshells Subjected to the Puncture Test

Transcription

1 Scanning Electron Microscopy of Fractures in Eggshells Subjected to the Puncture Test I. L. STEVENSON Chemistry and Biology Research Institute* PETER W. VOISEY Engineering and Statistical Research Institute 2 and Animal Research Institute, 3 R.M.G. HAMILTON Research Branch, Agriculture Canada, Ottawa, Ontario K1A 0C6 (Received for publication December 20, 1979) ABSTRACT Eggshells were subjected to puncture tests and the microstructure of the resulting fractures examined by scanning electron microscopy. The shell material fractured in a manner typical of brittle materials where a cone of material is forced inwards. According to established theory the fractures are due to tensile stress. This explains why the forces required to puncture the egg and to fracture the egg by quasistatic compression between flat surfaces are related; both tests respond to the resistance of the shell material to tensile fracture. Tests using a punch and die to impose shear fractures showed that the shear strength of the shell is greater than the tensile value. Furthermore, the shear strength of the shell material is directionally dependent with the shell showing greater resistance when punched from the outside. The findings show that comparative studies using the eggshell puncture test must be carefully interpreted, since unknown and confounded factors affect the puncture force. (Key words: microstructure of eggshell fractures, puncture test, scanning electron microscopy) INTRODUCTION The puncture test has been widely applied and considerable data reported in the measurement of eggshell strength (Voisey and Hunt, 1974). The test is simple and is claimed to be suitable for the measurement of both between and within egg strength variations (Hunt et al, 1977). Comparisons of the puncture test with the quasistatic compression test (Hunt et al, 1977; Voisey et al, 1979) showed that the forces to fracture the shell by the two methods are related. It was assumed that fracture in the puncture test was due to shear stresses, and theoretically, fracture occurs under tensile stress in the compression test (Voisey and Hunt, 1967). These facts led to the conclusion that tensile and shear fracture properties of the 'Contribution No Contribution No 'Contribution No Poultry Science 60:89-97 shell were related (Hunt et al., 1977). Tyler and Moore (1965), using light microscope methods, studied the fractures produced by puncture tests and observed that "a squat circular cone was pushed out from the inside of the eggshell." Similar observations were reported by Clark and Acree (1974). The observed depth of the cylindrical fracture made in the shell by the punch was: a) "no more than half the shell thickness" (Tyler and Moore, 1965), and b) an average of 32% of the shell thickness (authors' calculations from data of Clark and Acree, 1972); the fracture then expanded to a cone shape. For calculating the ultimate shear stress of the shell material (force/unit area), it can only be assumed that shear occurred within the cylindrical portion. The cone fracture surface area and its contribution to puncture resistance is not taken into account by such stress calculations. Clark and Acree (1972, 1974) based their calculations of shear stress on the maximum puncture force and the surface area of the cylindrical fracture. The resulting fracture shear stress (111,000 to 195,000 kpa) was about ten 89

2 90 STEVENSON ET AL. times that of calcium carbonate (14,800 kpa). Hunt et al. (1977) and Voisey et al. (1979) assumed that the fracture shape was a cylinder through the shell thickness and obtained fracture shear stresses of 35,400 kpa and 52,000 kpa, respectively. These data suggest that the total puncture force is not accounted for by the area of the cylindrical fracture. The purpose of the present study was to examine the type of fracture resulting from the puncture test and to re-examine the factors affecting the potential of this test as a comparative measure of the material properties of eggshells. The stress required to cause fracture of the shell, a property of the shell material, has the advantage of eliminating eggshell geometry in comparisons of eggshell strength. This measurement is potentially useful as both structural and material properties of the eggshell govern its resistance to fracture (Voisey and Hamilton, 1976). MATERIALS AND METHODS Eggs were collected from three strains (1,4, 7) of Single Comb White Leghorn hens when they were 578, 611, and 715 days of age. These strains were from a long-term selection study described by Gowe (1977) and were force-molted by the Washington State University method described by Swanson and Bell (1974). The hens received ad libitum a laying diet containing 15% protein and 3.2% calcium. Eggs were selected for the puncture tests after determining the specific gravity (SG) of each egg by Archimedes' principle or the flotation procedure (Voisey and Hamilton, 1977). Eggs with SG < and > (representing the extremes of shell quality) were punctured at the equator with the egg tester described, by Voisey and MacDonald (1978). Hardened steel punches.2,.3, and.4 mm in diameter were used to puncture the eggs at 20 mm/min. In addition, 30 eggs were also tested with the.4 mm punch moving at 5 mm/min. Two portions of shell were then cut from each egg equator and subjected to punch (.4 mm diameter) and die (.5 mm diameter) tests with the punch applied from the outside or the inside of the shell. The die supporting the shell during penetration by the punch was expected to confine the fracture to a cylindrical surface corresponding to the punch-die diameters throughout the shell thickness, that is, to impose a shear fracture parallel to the punch axis. During the tests, force resisting penetration by the punch was precisely recorded on a strip chart. A high frequency recorder (30 Hz) was used so that any minor force fluctuations were accurately recorded (Voisey, 1971). When the recorder indicated, by a rapid reduction of force, that fracture had occurred, the punch motion was immediately reversed to minimize the post fracture damage to the shell. Sections (1.0 cm 2 ) were cut from around the puncture sites. Shell membranes were removed by the enzymatic procedure described by Stevenson (1980) to prevent damage to the fracture surfaces and to expose them for examination. Advantage was taken of radial cracks resulting from the puncture test that bisected the puncture sites to obtain cross sectional views of the hole punctured in the shell. For electron microscopy, shell samples were air dried, cemented to aluminum specimen mounts at appropriate angles, gold coated (in vacuo) on a rotating stage, and stored in a desiccator. Samples were examined with an AMR 1000A scanning electron microscope operating at 5 or 10 kv. RESULTS AND OBSERVATIONS Scanning electron microscopy showed that, FIG 1. View of shell-puncture site at a radial break illustrating the conical section of shell forced inward by the punch (.4 mm). The fracture planes extend obliquely from the shell surface through the palisade layer to the mammillary region (65X). FIG. 2. Isolated conical section forced out by the.4 mm diameter punch showing that the depth of cylindrical fracture is confined to the cuticle and outermost portion of the shell with the main fracture extending obliquely to the punch direction (73X). FIGS. 3,4. Inner-shell surfaces with shell membranes and conical plugs removed to illustrate the fracture surfaces and irregular shapes and areas of the fractures resulting from penetration by the.4 mm diameter punch (Fig. 3, 42X ; Fig. 4, 30X). FIG. 5. Oblique view of the inner-shell surface with shell membranes and plug removed to show the abrupt termination of the oblique fracture at intact mammillae (62X).

3 MICROSTRUCTURE OF EGGSHELL PUNCTURE FRACTURES 91 'CX^,,.

4 92 STEVENSON ET AL. regardless of the punch diameter (.2,.3,.4 mm) or of the punch velocity (20 mm/min and 5 mm/min), a conical section of shell was consistently forced inward by the puncture test (Fig. 1). Examination of the conical fractures showed that these fracture surfaces were oblique to the punch direction, extending at angles of 22 to 37 and averaging about 30 from close to the shell surface to that point where individual mammillary cones occurred at the inner shell surface (Figs. 1, 5). It was clear that a cylindrical fracture, corresponding to the punch diameter, penetrated only the cuticle and a small portion of the outer "spongy" shell layer (Figs. 1, 2). The craters resulting from the punch test showed that the cone bases at the inner shell surface were not circular but irregular in shape and random in both shape and area (Figs. 3, 4). In these studies there was no apparent relationship between SG of the test eggs and the angle of the fracture surface or the size and shape of the conical fractures. Fractures obtained with the punch and die test, with the punch penetrating from either the outer or inner shell surface, showed that the fractures were confined to an approximately cylindrical surface (Figs. 6, 7). The exit hole of the shell fragment dislodged by the test exceeded the die diameter by up to 25% (Fig 12). This was possible because the punch motion was reversed before the fragment was forced into the die. Inspection of holes in both internally and externally punched shells clearly indicated that a cylindrical shear fracture predominated throughout most of the shell thickness (Figs. 8, 9, 10). A typical shell plug isolated after the punch and die test further illustrates this point (Fig. 11). Figure 13 illustrates the typical force-deformation tracings recorded for' a shell undergoing a) the puncture test (Fig. 13A), b) the punch and die test with the punch applied to the inner shell surface (Fig. 13B), and c) the punch and die test with the punch applied to the outer shell surface (Fig. 13C). The relationship between force and deformation for the punch and die tests was nonlinear (Fig. 13B, C) as opposed to the linear relationship normally observed (Voisey, 1975) in the puncture tests (Fig. 13A). Furthermore, the rounded peaks obtained (Fig. 13B, C) at the instant of maximum force (fracture) with the punch and die suggest that the fracture was not abrupt (brittle) as was the case in the puncture test (Fig. 13 A). When the punch and die test was applied with the punch contacting the inner surface, compression crumbling occurred at the individual calcified tips of the mammillae. This was reflected in the force-deformation record obtained during the test where a transient peak occurred during the period of increasing force (Fig. 13B). Table 1 shows a comparison of the forces required to fracture 30 eggshells by the three tests outlined above. The force required to penetrate the shell with a punch was considerably less than required for a punch and die. Furthermore, with the punch and die, the force to puncture from inside the shell was less than that to puncture from the outside. DISCUSSION The conical fractures observed in the puncture test are similar to those observed by Roesler (1956) and Benbow (1960) in their experiments on controlled fractures in brittle materials (Benbow and Roesler, 1957). According to these authors this phenomenon was first observed by Hertz in The theory, FIG 6. Inner shell surface with membranes removed showing the nature of the fracture resulting from the punch and die test with the punch penetrating from the outer shell surface. Compare the extent of the fracture crater to those of Figures 3 and 4 where no die was used (72X). FIG. 7. Outer shell surface illustrating the fracture crater from the punch and die test with the punch penetrating from the inner shell surface (55X). FIG. 8. Cross section of a shell where the punch penetrated from the inside of the shell. Note the near cylindrical shear extending through the greater portion of the shell thickness as opposed to the oblique fractures observed in Figures 1 and 2. (81X). FIG. 9. Enlarged portion of the shear face illustrated in Figure 8 from a slightly different angle. (228X). FIG. 10. Cross section of a shell through the puncture in the punch and die test when the punch penetrated from the outer-shell surface. As in Figure 8 the near vertical shear extends through the greater part of the shell thickness. (133X). FIG. 11. Isolated shell plug from the punch and die test where the punch penetrated from the outer shell surface. The upper portion of the intact fragment shows near vertical shear fracture with the lower portion flaring out in the mammillary region on the die side of the fracture. (100X).

5 MICROSTRUCTURE OF EGGSHELL PUNCTURE FRACTURES 93 &A I

6 94 STEVENSON ET AL. FIG. 12. Illustration of the fracture resulting from the punch and die test. In the test the shell fragment remains on the punch side of the die but for clarity it has been depicted as isolated from the shell cavity. which was confirmed experimentally with glass, states (Benbow, 1960) that at any instant during propagation of the fracture the cone shape (Fig. 14) is described by: where n = S = P = n = S/P 2 / 3 (1) [1] constant of the material diameter at the base of the cone force on the punch When stress is applied to a body, fracture will occur when the compressive, tensile, or shear stress at any point exceeds the ultimate value of one of these stress values the material can sustain. Typically, the ultimate compressive and shear values for brittle materials are greater than the ultimate tensile value; because of this, brittle materials are predisposed to fail under tensile stress. Examples of such materials are concrete, cast iron, bricks, limestone, and sandstone (Oberg and Jones, 1959). Rehkugler (1963) reported that the ultimate compressive stress for eggshells was larger than the tensile value. The cone fracture (widely observed in brittle materials) is progressive and will propagate in proportion to the force applied to the punch (Equation [1]) as demonstrated by Benbow (1960) by applying incremental forces. In effect, the stress has the same effect as a wedge applied to split the material. As the fracture progresses the force must increase to maintain the stress at the ultimate level over the increasing area of the cone base perimeter. The cylindrical surface fracture immediately adjacent to the punch was attributed to compression of the material under the punch tip by Roesler (1956). This is a surface contact phenomenon where the compressive stress at the perimeter of the punch is infinite (Mohsenin, 1970) until surface crumbling occurs to redistribute the applied force. Close examination of these fractures in the eggshells indicated that the shell material, in the cylindrical surface area, fractured in a similar manner. There was no evidence of shear slip planes (Figs. 1, 2, 5). Thus, the fracture previously attributed to shear (Hunt et al, 1977) was actually the collapse of the material under compression; in fact, none of the puncture fracture force can be attributed to shear (Fig. 14). The above findings regarding the cylindrical fracture differ from those of Tyler and Moore (1965) and Clark and Acree (1974) and the assumptions of Hunt et al. (1977) and Voisey et al. (1979). This is understandable since the depth of focus and resolution of scanning electron microscopy has permitted a more definitive observation of the fracture surfaces than heretofore possible. According to Roesler (1956), the cone angle A (Fig. 14) is constant for a uniform material. In the case of eggshells a considerable variation was noted in fracture shapes between and FIG. 13. Force-deformation records obtained with a high frequency recorder (30 Hz) during: A) the standard puncture test (as diagrammed in Fig. 14); B) the punch and die test with the punch applied to the inside of the shell surface (as diagrammed in Fig. 12 with the punch and die positions reversed); and C) the punch and die test with the punch applied to the outside of the shell surface (as diagrammed in Fig. 12).

7 MICROSTRUCTURE OF EGGSHELL PUNCTURE FRACTURES 95 TABLE 1. Comparison of the forces required to fracture eggshells at the equator in the puncture test (A mm punch at 5 mm/min) and of corresponding shell pieces fractured in the punch and die test (.4 mm punch at 5 mm/min;. 5 mm die) Test Direction of punch relative to egg Mean force* (N) S.D.* (±N) Puncture Punch and die Punch and die From outside From outside From inside 'Number of samples = 30. Differences between all the means were significant (P<.01). within individual eggs as indicated by the shapes of the cone base and cone angles (22 to 37 ). The patterns observed did not reflect differences in shell strength (SG), and it must be assumed that these variations are the result of the inherent heterogeneity of the shell material. As expected, the use of a die to support the eggshell under the punch radically changed the fracture patterns and predisposed the material to shear fractures. The increase in penetration force (Table 1) indicated that the shell material was stronger in shear than tension, which is typical of the behavior of brittle materials (Oberg and Jones, 1959). This further supports the above explanation of why the shell material fails in tension in the punch test. The difference in the punch and die force required when the punch penetrates from the inner and outer shell surface (Table 1) indicates that the resistance to shear fracture of the shell material is directionally dependent. It is of interest that nature has arranged the FIG. 14. Illustration of the section across the conical fracture resulting from the puncture test. greatest resistance to exterior forces where field insults originate and a lowered resistance to interior forces such as those applied by the chick during hatching. An explanation of the force difference in the two directions is difficult. In must be recognized that the force readings were affected by: a) the differences in surface contact stresses when the punch or die were placed in contact with either the smooth outer shell surface or the rough membrane-covered inner surface and b) the fact that punch and die were not the same diameter. The nonabrupt fracture indicated by the rounded peak in the punch and die tests is likely due to friction between the post fracture surfaces. The linear force-deformation curve obtained in the puncture test indicated that the entire shell structure deformed according to Hooke's Law as reported by Voisey (1975). There was no indication of the progressive nature of the cone fracture in the region of the maximum force, possibly because the speed of fracture propagation was too rapid to be recorded by the high frequency recorder used. Carter (1970, 1971) found that the inner one-third of the eggshell did not contribute to the resistance to puncture. This was confirmed by Hunt et al. (1977). These authors based their findings on a regression of puncture force on shell thickness that did not pass through the origin. In examining the microstructure of shells fractured by the puncture test it is clearly apparent that a number of factors contribute to the maximum puncture force required: a) heterogeneity of the shell; b) the high shear strength relative to the tensile strength of the shell; c) the presence of the shell membrances; d) the compressive surface crumbling adjacent to the punch; and e) the incomplete bonding of the calcite pillars of the mammillary region at the inner surface. Of the

8 96 STEVENSON ET AL. total shell thickness, the compression crumbling of the outer shell surface and the weak nonbonded tips in the mammillary region appear to contribute little to penetration resistance. The observed fractures suggest that the maximum resistance to penetration is predominantly generated in the organization of the bonded calcite columns in the palisade layer where a tensile conical fracture occurs. However, despite this evidence, it is still not known precisely which portions of the shell thickness contribute to puncture resistance. Because the eggshell behaves like a typical brittle material and is weaker in tension than shear and compression, fracture in the puncture test is due to tensile stress. The test is thus a measure of the tensile strength of the shell material oblique to the shell surface. These findings explain the previously observed relationship between puncture force readings and other measures of shell tensile strength such as the quasistatic compression test (Hunt et al. 1977, Voisey et al. 1979). From our viewpoint the ideal eggshell strength test should fracture the shell under controlled conditions to facilitate comparisons. This requires measuring the force resisting fracture of a specific area of shell. Force is then proportional to the ultimate stress of the shell material. In the puncture test it is evident that the variation in the nature of the fractures is a consequence of a number of inseparable factors that limit the test to a comparison of the compounded effects of material property (ultimate stress), shell geometry, and its heterogeneity and random chance in selecting the puncture site. Because of the complexity of the fracture surface shape, rigorous analysis of fracture stress from puncture force readings is complex. Consequently, puncture force readings should be interpreted with care. While there are other methods to measure ultimate tensile stress such as the tensile (Carter, 1971) and bending (Tyler and Coundon, 1965) tests, these methods are tedious and unknown damage is introduced during preparation of the shell specimen for testing. These observations demonstrate the need to reassess laboratory measures of shell strength by examining the fractures imposed at the microstructural level. Obviously the laboratory fractures should be similar to those imposed by field insults. Such studies will increase the understanding of the eggshell strength mechanism and which components of the shell contribute to its resistance to insults. ACKNOWLEDGMENTS The authors wish to acknowledge the contribution of D. C. MacDonald of the Engineering and Statistical Research Institute and of M. A. Ewen and A. F. Yang of the Chemistry and Biology Research Institute who conducted the experimental work. The authors are also indebted to G. R. Cowper of the National Research Council, Ottawa, for his advice in interpreting the fracture behavior of the eggshells. REFERENCES Benbow, J. J., Cone cracks in fused silica. Proc. Phys. Soc. 75: Benbow, J. J., and F. C. Roesler, Experiments in controlled fractures, Proc. Phys. Soc. 70: Carter, T. C, The hen's egg: Factors affecting the shearing strength of shell material. Brit. Poultry Sci. 11: Carter, T. C, The hen's egg: Variation in tensile strength of shell material and its relationship with shearing strength. Brit. Poultry Sci. 12: Clark, R. L., and R. L. Acree, Eggshell shear strength evaluation with micro-punches. Paper No , Amer. Soc. Agr. Eng. Annu. Congr., AR. Clark, R. L and R. L. Acree, Eggshell shear strength evaluation with micro-punches. Trans. Amer. Soc. Agr. Eng. 17:46-48, 51. Gowe, R. S., Multiple-trait selection in egg stocks: 1. Performance of six selected lines derived from three base populations; 2. Changes in genetic parameters over time in six selected strains. Pages in Proc. 26th Annu. Poultry Breeders' Roundtable, Kansas City, MO. Hunt, J. R., P. W. Voisey, and B. K. Thompson, Physical properties of eggshells: A comparison of the puncture and compression tests for estimating shell strength. Can. J. Animal Sci. 57: Mohsenin, N. N., Page 290 in Physical properties of plant and animal materials. Gordon and Breach, New York, NY. Oberg, E., and F. D. Jones, Pages in Machinery s handbook. The Industrial Press, New York, NY. Rehkugler, G. E., Modulus of elasticity and ultimate strength of the hen's eggshell. J. Agr. Eng. Res. 8: Roesler, F. C, Brittle fractures near equilibrium. Proc. Phys. Soc. 69: Stevenson, I. L., The removal of eggshell membranes by enzyme treatment to facilitate the study of shell microstructure. Poultry Sci. 59: Swanson, M. H., and D. D. Bell, Force molting of chickens. 2. Methods. Pub. AXT-411, University of California, Berkeley, CA.

9 MICROSTRUCTURE OF EGGSHELL PUNCTURE FRACTURES 97 Tyler, C, and J. R. Coundon, Apparatus for measuring shell strength by crushing, piercing or snapping. Brit. Poultry Sci. 6: Tyler C, and D. Moore, Types of damage caused by various cracking and crushing methods used for measuring egg shell strength. Brit. Poultry Sci. 6: Voisey, P. W., Modernization of texture instrumentation. J. Texture Stud. 2: Voisey, P. W., Factors affecting the measurement of the shear strength of shell material by the puncture test. Brit. Poultry Sci. 16: Voisey, P. W., and R.M.G. Hamilton, Factors affecting the non-destructive and destructive methods of measuring eggshell strength by the quasi-static compression test. Brit. Poultry Sci. 17: Voisey, P. W., and R.M.G. Hamilton, Sources of error in specific gravity measurements by the flotation method. Poultry Sci. 56: Voisey, P. W., R.M.G. Hamilton, and B. K. Thompson, Laboratory measurements of eggshell strength. 2. The quasi-static compression, puncture, non-destructive deformation, and specific gravity methods applied to the same egg. Poultry Sci. 58i288-2?4. Voisey, P. W., and J. R. Hunt, Physical properties of egg shells. 4. Stress distribution in the shell. Brit. Poultry Sci. 8: Voisey, P. W., and J. R. Hunt, Measurement of eggshell strength. J. Texture Stud. 5: Voisey, P. W., and D. C. MacDonald, Laboratory measurements of eggshell strength. 1. An instrument for measuring shell strength by quasi-static compression, puncture, and non-destructive deformation. Poultry Sci. 57: