An Engineering Appraisal of Egg Shell Strength Evaluation Techniques1

1708

A. H. HAMDY AND C. J. BLANCHARD

REFERENCES Fabricant, J., and P. P. Levine, 1962. Experimental production of complicated chronic respiratory disease infection ("Air Sac" disease). Avian Diseases, 6: 13-23. Gale, G. O., H. W. Layton, A. L. Shor and G. A. Kemp, 1967. Chemotherapy of experimental

mycoplasma infections. Ann. New York Acad. Sci. 143: 239-255. Gross, W. B., 1958. The role of E. coli in the cause of chronic respiratory diseases and certain respiratory diseases. Am. J. Vet. Res. 19: 448452. Gross, W. B., 1961. The development of air sac diseases. Avian Diseases, 5: 431-439. Hamdy, A. H., C. J. Farho, C. J. Blanchard and M. W. Glenn, 1969. The effect of lincomycinspectinomycin on Mycoplasma meleagridis during a natural outbreak. Avian Diseases, in press. Jungher, E. T., 1958. Symposium on chronic respiratory diseases. Am. J. Vet. Res. 19: 464-467. Layton, H. W., and G. A. Kemp, 1968. Chemotherapy of experimental mycoplasmosis in turkey poults. Proceedings of 7th Conference of Antimicrobial Agents and Chemotherapy. Pages 168-172. Markham, F. S., and S. C. Wong, 1952. Pleuropneumonia-like organisms in the etiology of turkey sinusitis and chronic respiratory diseases of chickens. Poultry Sci. 3 1 : 902-904. Olesiuk, O. M., H. Van Roekel and N. K. Chandrimani, 1964. Antibiotic medication of chickens experimentally infected with M. gallisepticum and E. coli. Avian Diseases, 8: 135-152. Sadler, W. W., 1967. How mixed infection affect disease and condemnations. Proc. Poultry Hlth. Symp., Davis, Californ;a, Univers'ty of California, Division of Ag. Sci. Agri. Extension Service, March 22, 1967 : 1-4.

An Engineering Appraisal of Egg Shell Strength Evaluation Techniques1 J. R. HAMMERLE Department of Biological and Agricultural Engineering, North Carolina State Raleigh, N.C. 27607

University,

(Received for publication May 17, 1969)

INTRODUCTION

1\ l\ ANY poultry researchers have stated J-'-l that losses due to thin and weak shells during production, processing, and marketing cost the industry millions of dol1 Approved for publication as paper number 2889 in the Journal Series of the North Carolina State University Agricultural Experiment Station, Raleigh, North Carolina.

lars annually, estimated at five to seven percent of United States production. Factors affecting the strength of egg shells in general are heredity, age, health, season, production rate, clutch position, diet, environment, and others more well known to specialists in poultry research. However, it is the strength of individual eggs, and the evaluation of strength itself, which has posed a difficult problem in research di-

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was given in the drinking water at 2 gm. per gallon for 3, 7 and 10 days; 3 gm. for 3 and 7 days; lincomycin or spectinomycin at 2 gm. for 7 days and tylosin at the manufacturer's recommended dose (2 gm. for 5 days). Lincomycin and spectinomycin combinations at 3 gm. for 7 days increased the survival rate of chickens from 40% to 100% in M.g. infection; from 33.3% to 80% in E. coli infection and from 26.7% to 80% in the combined infection. Exposed birds receiving the antibiotic combination had a lower score index for airsacculitis, reduced rate of pericarditis and perihepatitis and improvement of weight gain than those receiving lincomycin, spectinomycin, tylosin or the nonmedicated controls. The antibiotic combination was also active in vitro against the test organisms.

1709

EGG SHELL STRENGTH

DATA EVALUATION

With respect to materials testing there are several significant points to consider when applying engineering theories to biological materials. First, it is relatively immaterial if the specimens are tested to failure, thus rendering them useless. For most applications the test specimens are merely representative samples, used only to evaluate research work. It seems inconceivable that every egg marketed would be evaluated for shell strength, but rather that strength testing would be the tool of the research scientist. Second, when performing tests of a scientific nature it is imperative that the results of measurements be expressed in terms of fundamental physical units, rather than in arbitrary, qualitative, non-commonly used, undefined notations.

Quite often units of measurement have been reported erroneously or no comparison has been made between tests of the same nature but of different recorded data. To draw the many programs, techniques, and instruments used in evaluating egg shell strength together, an outline of test methods demonstrates the relationships among test methods by the following categorization: A. Physical properties 1. 2. 3. 4. 5.

Egg specific gravity Shell specific gravity Shell thickness Shell/egg weight ratio Shape

B. Mechanical properties 1. Static (and quasi-static) loading 2. Dynamic loading The complexity of measurements which can be taken in these tests and the shapes, sizes, and other variables involved result in a combination which yields little possibility of cross-comparisons in test data. In many of the papers listed in the appended references2 physical properties have been evaluated and referred to as strength. Actually the strength of a material is independent of either shape or size, therefore it remains to be proven that any of the physical properties could be used as an estimator of the strength of shell material. This concept has been disproven for engineering materials and is undoubtedly invalid for biological materials also. Merely by increasing the thickness of shells the ability of the structure to resist loading can obviously be improved. Also, with spherical eggs, weak points due to geometry would be eliminated by having a uniformly shaped shell. Thus the structure 2

References are numbered for use in Table 1.

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rected toward improvement of egg shell strength—often referred to simply as egg shell quality. Numerous attempts have been made to evaluate shell strength, however most have merely resulted in a new technique or a new instrument rather than actually yielding useful data on the strength of shells. Usually the researcher interested in actually causing a change in egg shell strength has not been sufficiently acquainted with the science of strength of materials to handle the complexity of this specialized field of engineering, and in fact it was not until recently that engineers became involved with research concerned with egg shell strength. The purpose of this discussion is to review the literature published involving measurement of egg shell strength with a particular consideration to validity from an engineering standpoint, and to call attention to misconceptions regarding strength of materials theories and their applications. A suggested criterion for specifying strength—failure stress, is also discussed.

1710

J. R. HAMMERLE

mit comparisons among test results using the same techniques. No review of literature concerning shell strength would be complete without mentioning the text of Romanof and Romanof (1949) from which basic information on the structure of eggs can be obtained. Further information by Needham (1931) also proves helpful to the scientist concerned with egg shell strength. However neither of these references provides an insight as to the correct evaluation of shell strength. Munro (1968) discussed the microscopic structure of egg shells as it effects strength, and considerable work by Rauch (1959, 1958, 1956a, 1955, 1953) has been done in

TABLE 1.—Properties of egg shells Physical properties Egg Specific Gravity Shell Specific Gravity Shell Thickness Shell/Egg Weight Ratio Shape Mechanical properties

Reference numbers4 13, 14, IS, 20, 21, 28, 38, 44, 55, 59/40, 63, 84 14, 17, 25, 29/30/86, 33, 38, 42, 59/40, 84 2, 6, 7, 8, 13, 14, 15, 17, 20, 21, 23, 25, 28, 29/30/86, 34, 52, 54, 55, 58, 59/40, 65, 75, 76, 78, 82 14, 28, 38, 59/40 28, 40, 52, 55, 63, 78 Reference numbers

Static (and Quasi-Static)

Flat Surface (deformation) Needle (force) Needle (deformation) Cylinder (force) Cylinder (deformation) Sphere (force) Bending (force) Ring (force) Externally Distributed Load Hardness

1, 7, 8, 9, 11, 13, 14, 16, 17, 18, 19, 23, 26, 27, 28, 34, 38, 39, 52, 55 56, 63, 64, 72, 75, 80, 82, 83 7, 8, 52, 59, 60/27, 80 15, 44, 45, 71, 72, 73, 77 15,58 3, 10, 14, 35, 54, 67, 77 54, 59/40 34, 72, 73, 77, 85 2, 60/69/78 53 33 8, 9, 68

Dynamic Loading (Cracking) Flat Surface (momentum) Cylinder (momentum) Sphere (momentum) Sphere (no. blows) External Pressure Internal Pressure Egg-on-Egg Impact Other

25, 31(32), 44, 52 59/40, 77, 79, 81 s 7, 14, 20, 29/30/86, 65, 66, 76 72, 74, 75, 84 83 21/22, 61 52 4, 61, 62, 82

Flat Surface (force)

4 Slash ( / ) indicates same material covered in multiple publications, parentheses indicate a close relationship between publications. 5 Acceleration measured.

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of the egg shell—its size (thickness) and shape—must be considered independently of the material of which the egg structure is formed. It is the elusive shell material mechanical strength which must be evaluated to determine the ability of the egg to resist failure. In the following sections the segments of the previously mentioned outline will be discussed using the references which have been classified in Table 1. The last complete review of literature concerning egg shell strength was made by Tyler (1961); since that time over half of the pertinent references listed in Table 1 have been published. The references are grouped to per-

1711

EGG SHELL STRENGTH

evaluating the physical properties of eggs.

TABLE 2.—Density-strength data for selected materials Material Grey cast iron (ASTM #20) Structural steel (SAE 1020 cold-rolled) Stainless steel (SAE 8650, 800°F. hard, and temp.) Cast aluminum alloy (19S-T4) White oak Nylon Molding Compound (Type 6/10)

Density (lb./in.a) .26 .28 .28 .10 .02 .04

Failure stress (psi.XlO 3 ) 20 60 214 32 7 9

Elastic modulus (psi.XlO 6 ) 1S.0 30.0 30.0 10.3 1.6 .2

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method has been much publicized, its usefulness is restricted since radioactive backPHYSICAL PROPERTIES scatter can only differentiate: The most popular physical properties as1. The relative thicknesses of several sociated with egg shell strength are shell materials of equal densities. thickness and shell and egg specific gravities. 2. The relative densities of several mateOften these two properties are erroneously rials of equal thicknesses. referred to as being synonymous with The probability of either uniform thick"quality" or "strength." By increasing the nesses or densities among test specimens is thickness of shell material the egg struc- poor, and the inadvisability of using either ture (shape and size) is certainly affected, density or thickness as a strength estimator however the resultant change in shell ma- has been discussed. terial strength is not apparent, as can be The use of shape as a single material evidenced by comparing strengths of ^in. strength evaluator is invalid as was menand ^ in. thicknesses of steel plate and tioned previously, however it can easily be plywood respectively. Size is therefore not seen that shape does influence the ability an indicator of material strength, since of eggs to withstand loading; a normal the higher strength of the thinner steel load applied to any body at a point of high plate results in the plate being able to re- curvature is more easily resisted than a sist loading better than the plywood. load applied to a flat surface (the principle Specific gravity (and therefore density) of the arch). The complex curvatures at is also a poor evaluator of material any point on an egg shell effect the strength, as can be seen from Table 2; spe- strength of the egg with the points of less cific gravity is primarily an indicator of curvature being weaker. A factor expressCaC0 3 /total egg weight. Two physical esti- ing the complex egg shape includes neither mators of strength commonly used in engi- thickness nor material strength evaluation, neering are failure stress and elastic modu- and therefore is only a partial measurement lus (both to be discussed later in the next of the ability of the egg to resist mechanisection). No relationship between either of cal loading. these strength measurements and density MECHANICAL PROPERTIES can be seen. A technique receiving much attention reResearchers have been interested in the cently is the use of radioactive materials, mechanical properties of eggs for a considreferred to by Wilson et al. (1968), James erable length of time. The earliest publicaand Retzer (1967) and James et al. tion referring to a mechanical testing (1967) as beta backscatter. Although the method was by Willard and Shaw (1909),

1712

J. R. HAMMEELE

ply supported beam, thereby causing inherent error. The method of relating material strength to surface hardness evaluated by indentation is open to considerable question. Any prepared surface is quite different from the material underlying the surface; by making a new surface, the material internal structure is changed. Sanding conditions the surface which is tested, as can easily be seen in photomicrographs. Brinell, Rockwell, Vickers, Shore scleroscope, and Tukon test equipment, procedures, and theories are discussed both pro and con in many engineering texts. However, because of highly complex states of stress in an indentation, there is no way of relating hardness measurements to each other or to other mechanical properties on the basis of theory. Furthermore, conversion tables from hardness readings to other material properties are valid only for the materials tested. This means that there is no direct conversion from hardness to strength of egg shell material based on some known conversion from hardness to strength for commonly used engineering materials. Basically it is difficult to relate the force or deformation required to cause failure in egg shell (by flat surface, needle, cylinder, or sphere) to shell material strength without completely eliminating induced variability resulting from shape (radii of curvature at contact) and size (shell thickness at contact) differences among specimens. Hardness is strictly a function of surface preparation, however as a testing technique it probably justifies further investigation. DYNAMIC LOADING In addition to the problems involving contact between bodies as discussed in the previous section, by placing the bodies in motion the complications of analysis are multiplied. Furthermore, in quite a few publications the units of measurement have

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followed by an unusual exchange of letters by Herrasti (1916) and Miller (1916). The mechanical properties in Table 1 are divided between response to loads which are applied statically (or very slowly), and response to loads which are applied very rapidly (sometimes referred to as "cracking" in literature concerned with egg shell strength). Loading in most cases has been done by flat surface, needle, cylinder or sphere—usually each researcher using equipment of different weights, shapes, sizes, and materials, thereby rendering cross-comparisons of data almost impossible. Furthermore, the contact problem, as it is called in Engineering Mechanics, is very difficult to analyze—the behavior of two dissimilar materials (one viscoelastic, one elastic) of different contact radii under static or dynamic loading has been a classical problem in engineering for almost a century. Mohsenin (1966) has reviewed the problem with particular application to biological materials, using the mathematical analyses of Boussinesq (188S) for contact between a cylinder and a flat or curved surface and of Hertz (1896) for contact between two curved bodies. A preliminary review of any of these publications should convince even the most avid researcher of the complexity of such a problem as the static or dynamic loading of an egg shell with different radii of curvature in perpendicular directions at a point by a flat surface, a cylinder, or a sphere. Static and quasi-static tests usually have been reported using either the force at failure or the deformation at failure. Differences between shell radii of curvature and shell thicknesses at the contact points have sufficiently influenced the results so as to make comparisons between tests almost impossible. Bending and ring loading techniques have some merit, however none of the methods used have accounted for the curvature across the ring or across the sim-

1713

EGG SHELL STRENGTH

v = \/2gh

[1]

where: v = velocity, cm./sec. g = 981 cm./sec.2 h = drop height, cm. The momentum of the falling object is obtained from: M

[2]

where: M = momentum, g.-cm./sec. m = mass of falling object, g.

Momentum and energy (kinetic) being both functions of only mass and velocity for the simple case of rectilinear motion, actually represent the same data since: 1 Mv — mv2 or E = 2 2

[3]

where: E = energy, dyne-cm. In a dynamic test either energy or momentum must be used to report the data (assuming same size, shape, and material of impacter and specimen from test to test). However it is well known that viscoelastic material behavior is quite dependent upon loading rate, therefore impact velocity must be the same for test to test comparisons. FAILURE STRESS At this point further discussion of the two properties which should be the best estimators of strength (failure stress and modulus of elasticity) is included. Stress is the internal resistance, at a point within a body, to applied loading, expressed in dyne/cm.2 (lb./in. 2 ). The methods of cal-

TABLE 3.—Table of units of measurement Measurement

Symbol

fps system

cgs system

Pressure

p

dyne/cm

Force

F

dyne

lb./in.' slug-ft.

\

sec.

2

lb. / / lb.-sec.2 \

g.

slug

Acceleration

cm./sec. 2

ft./sec. 2

Velocity

cm./sec.

ft./sec.

Mass

Momentum

m

M

g.—cm./sec.

slug-ft. /sec.

/ g.-cm. Energy

E

dyne-cm \

sec.2

2

\

/ slug-ft.2 \

/

\

sec.2

/

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been reported erroneously, thereby compounding the problem of comparing data. For this reason, a short discussion of physical measurements leading to the expressions for impact data is included. Table 3 shows the units of measurement used in both cgs and fps systems. Quite often such simple measurements as force or pressure are interchanged, whereas pressure is actually a distributed force. When dropping a sphere, cylinder, flat plate, etc. onto a test specimen (simple one dimensional motion) the velocity at time of impact can easily be determined from:

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J. R. HAMMERLE

STRESS

STRAIN

lem. Two new reports not yet available3 show promise and may provide new insight to the evaluation of shell strength. The basic assumptions of isotropy, homogeneity, and continuity are often considered invalid by some scientists when applied by engineers to biological materials. However, just as egg shells contain pores and other imperfections, engineering materials are also less than perfect; on a macroscopic level the basic assumptions are valid, and without them there is no basis for analysis.

FIG. 1. Stress-strain relations for a brittle material.

CONCLUSIONS There is a great disagreement among research techniques because of insufficient control of parameters of influencing the behavior or response of the test specimens. Coordination of test methods and data recording and reporting is imperative. Measurements which actually evaluate the mechanical strength of egg shells must be made and should be reported in a valid engineering manner. This means that if whole eggs are evaluated, two variables, shape and size, must be held constant or their variability removed from analysis of the most important variable—strength of the shell material. It is entirely conceivable that, within a short time after revision and reorganization of testing procedures, meaningful data can be obtained and replicated at different locations by different researchers. It is suggested that a concerted effort be devoted to the technique of experimental stress analysis for the refinement of mechanical testing as the standard egg shell strength evaluation method, and that standard units of measurements and techniques be prepared and followed. Perhaps a group of experienced engineers and poultry scien3

Manceau, J. R., and J. M. Henderson. Physical Properties of Egg Shell, ASAE Paper 69-386 and Stress Analysis of Egg Shell, ASAE Paper 69-387. Presented June 24, 1969, at the annual meeting of A.S.A.E., University, West Lafayette, Indiana.

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culating stress resulting from any but the most simple of loading patterns are very intricate and are better left to the elastician or stress analyst; however the relationship between stress and modulus can be seen in Fig. 1 for a brittle material such as egg shell. The modulus is merely the constant which relates resultant stress arising from an applied strain. When stress at a particular point in a loaded body is plotted against strain (ratio of increase in gage length to original gage length) the modulus is the slope of the curve, as given by A for curve b. Three representative curves for perfectly elastic, brittle materials are given, and it can be seen that the failure stress (indicated by x) can be independent of both strain level and modulus. For this reason stress is the mechanical property which defines the ability to resist loading to failure, and the loading may take any form of static or dynamic application. There are very few engineers involved in egg shell strength research, however some significant progress has been reported beginning with Rehkugler (1964), followed by Sluka et al. (1967), Hammerle and Mohsenin (1967), and Hammerle (1968) who have used the theories of Engineering Mechanics as given by Flugge (1962) in solutions of the shell stress analysis prob-

EGG SHELL STRENGTH

tists might form the nucleus of a standardization effort which could not only analyze and correlate existing data, but would develop uniform testing procedures.

14.

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comparison of certain methods of estimating shell strength. Poultry Sci. 28: 339-343. Petersen, J., and C. Tyler, 1966. The strength of Guinea Fowl (Namida meleagris) egg shells. British Poultry Sci. 7: 291-296. Rauch, W., 1959. Mathematische—statistiche Bezrehungenzwischen Eiqualitatsmerk—malen. Arch. Geflugelkunde. 23: 108-121. Rauch, W., 1958. Vergleichende Untersuchungem zur Qualitatsbeurteilung von Frischeiern. Arch. Geflugelkunde. 22: 74-104. Rauch, W., 1956a. Vergleichende Untersuchungen zur Qualitatsbeurteilung verschieden Konservierten Huhnereier. Arch. Geflugelkunde. 20:371-382. Rauch, W., 1956b. Eine neue Eierwaage. Dtsch. Wirtschaftsgeflugelz. 8: 275-277. Rauch, W., 1955. Qualitatsprufungen von Geolten und Ungeolten Huhnereiern. Arch. Geflugelkunde. 19: 453-463. Rauch, W., 1953. Physikalisch-chemische Messemethoden zur Qualitatsbeurteilung von Huhnereiern. Arch. Geflugelkunde. 17: 6982. Rehkugler, G. E., 1964. Egg handling equipment design. Trans. ASAE. 7(2): 174-178, 187. Rehkugler, G. E., 1963. Modulus of elasticity and ultimate strength of the hen's egg. J. Ag. Eng. Res. 8: 352-354. Richards, J. F., and L. M. Staley, 1967. The relationships between crushing strength, deformation and other physical measurements of the hen's egg. Poultry Sci. 46: 430-437. Richards, J. F., and M. H. Swanson, 1965. The relationship of egg shape to shell strength. Poultry Sci. 44: 1555-1558. Romanoff, A. L., 1929. Studies of the physical properties of the hen's egg shell relation to the function of shell-secretory glands. Biol. Bull. 56: 351-356. Romanoff, A. L., and A. J. Romanoff, 1949. The Avian Egg. Wiley, New York. Schoorl, P., and H. Y. Boersma, 1962. Research on the quality of the egg shell. Proc. 12th World's Poultry Congress (Sydney): 432-435. Schuster, D., 1959. Relationship of shell strength to certam characteristics of chicken eggs. M. S. Thesis, Pennsylvania State Univ., University Park, Pa. Simons, P. C. M., C. Tyler and H. P. Thomas, .1966. The effect of sodium hydroxide and sodium sulphide on the snapping strength

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33.

J. R. HAMMERLE

EGG SHELL STRENGTH

the measurement of some effects of various treatments. British Poultry Sci. 5: 19-28. 75. Tyler, C , and F. H. Geake, 1964e. Egg shell strength and its relationship to thickness with particular reference to individuality in the domestic hen. British Poultry Sci. 5 : 3-18. 76. Tyler, C , and D. Moore, 1965. Types of damage caused by various crushing methods used for measuring egg shell strength. British Poultry Sci. 6: 175-182. 77. Tyler, C , and F. H. Geake, 1963. A study of various impact and crushing methods used for measuring shell strength. British Poultry Sci. 4 : 49-61. 78. Tyler, C , and H. P. Thomas, 1966. A study of the snapping strength of egg shells and the effect of various factors on it. British Poultry Sci. 7: 227-238. 79. Voisey, P. W., and J. R. Hunt, 1967a. Behavior of egg shells under impact. J. Agr. Eng. Res. 12: 128-132. 80. Voisey, P. W., and J. R. Hunt, 1967b. Relationship between applied force, deformation of egg shells, and facture force. J. Ag. Eng. Res. 12: 1-4. 81. Voisey, P. W., and J. R. Hunt, 1967c. Physical properties of egg shells. 4. Stress distribution in the shell. British Poultry Sci. 8: 263-271. 82. Voisey, P. W., and J. R. Hunt, 1967d. Physical properties of egg shells. 3. An apparatus for estimating impact resistance of the shell. British Poultry Sci. 8: 259-261. 83. Voisey, P. W., and J. R. Hunt, 1964. A technique for determining approximate fracture propagation rates for egg shells. Canadian J. Animal Sci. 44: 347-350. 84. Wells, R. G., 1967. Egg shell strength. I. The relationship between egg breakage in the field and certain laboratory assessments of shell strength. British Poultry Sci. 8: 131139. 85. Willard, J. T., and R. H. Shaw, 1909. Analyses of eggs. Bull. Kansas State Ag. Coll. 159: 143-177. 86. Wilson, S. P., H. L. Marks and P. E. James, 1968. Associations among beta backscatter measurements and other measurements of shell strength. Poultry Sci. 47: 232-237.

AUGUST 9-14. THIRD INTERNATIONAL CONGRESS ON FOOD SCIENCE AND TECHNOLOGY, WASHINGTON, D.C.

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of egg shells. British Poultry Sci. 7: 309314. 61. Sluka, S. J., E. L. Besch and A. H. Smith, 1965. A hydrostatic tester for egg shell strength. Poultry Sci. 44:1494-1500. 62. Sluka, S. J., E. L. Besch and A. H. Smith, 1967. Stresses in impacted egg shells. Trans ASAE. 10(3): 364, 365, 369. 63. Stewart, G. F., 1936. Shell characteristics and their relationship to breaking strength. Poultry Sci. 15: 119-124. 64. Stuart, H. O., and C. P. Hart, 1938. The effect of different calcium intake levels on egg production, shell strength, and hatchability. Poultry Sci. 17: 3-7. 65. Swenson, T. A., and L. H. James, 1932, Oiling does not make shell eggs more brittle. U. S. Egg Poultry Mag. 38(11): 14-16. 66. Thornber Bros. Ltd., 1960. Private communication to C. Tyler, (see Reference 70). 67. Tully, W. C , and K. W. Franke, 1934. Comparative metabolism of several calcareous materials used in poultry feeding. S. Dakota State Coll. Ag. Expt. Sta. Bull. 287. 68. Tung, M. A., L. M. Staley and J. F. Richards, 1968. Studies on hardness of hen's egg-shell. J. Ag. Eng. Res. 13(3) : 12-18. 69. Tyler, C , 1968. The relationship between thickness and snapping strength in egg shells. British Poultry Sci. 2: 143-158. 70. Tyler, C , 1961. Shell strength: Its measurement and its relationship to other factors. British Poultry Sci. 2 : 3-19. 71. Tyler, C. and F. H. Geake, 1964a. The effect of water on egg shell strength including a study of the translucent areas of the shell. British Poultry Sci. 5: 277-284. 72. Tyler, C , and F. H. Geake, 1964b. A direct comparison of certain cracking and crushing methods used for measuring shell strength. British Poultry Sci. 5: 37-43. 73. Tyler, C , and F. H. Geake, 1964c. The testing of methods for crushing egg shells, based on paired readings from individual eggs and the measurement of some effects of various treatments. British Poultry Sci. 5: 2935. 74. Tyler, C , and F. H. Geake, 1964d. The testing of methods for cracking egg shells, based on paired readings from individual eggs and

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