Laboratory Measurements of Eggshell Strength

Laboratory Measurements of Eggshell Strength. 2. The Quasi-Static Compression, Puncture, Non-Destructive Deformation, and Specific Gravity Methods Applied to the Same Egg1 PETER W. VOISEY Engineering and Statistical Research Institute, R. M. G. HAMILTON Animal Research Institute B. K. THOMPSON Engineering and Statistical Research Institute, Research Branch, Agriculture Canada, Ottawa, Ontario, K1A 0C6 (Received for publication July 20, 1978) ABSTRACT Specific gravity, non-destructive deformation, puncture force, and compression fracture force measurements were obtained from 860 eggs collected from three flocks of Single Comb White Leghorn hens. Compression fracture and puncture force measurements indicated that tensile and shear fracture resistances of the shell material were related on a within egg basis and that the correlation between fracture shear stress and shell thickness was low. Linear relationships were found between specific gravity and both puncture force and fracture shear stress while there was an apparent curvilinear relationship between these force measurements and non-destructive deformation. Stepwise regression analyses indicated that non-destructive deformation accounted for the most variation in predicting compression fracture force (54%). It was found possible to measure puncture and compression fracture force, in that order, on the same egg. Deformation measurements obtained using horizontally applied forces closely matched those obtained with the forces applied to the egg vertically. 1979 Poultry Sci 58:288-294 INTRODUCTION H u n t et al. ( 1 9 7 7 ) c o m p a r e d t h e quasistatic compression and p u n c t u r e l a b o r a t o r y tests for measuring shell strength b y testing alternate eggs from each bird b y each m e t h o d . T h e t w o readings were correlated, suggesting t h a t t h e tensile and shear strength of t h e shell were related. A l t h o u g h t h e eggs tested b y each m e t h o d were similar, t h e results o b t a i n e d could have been influenced b y the t w o m e t h o d s having been applied to different eggs. T h e p u r p o s e of t h e w o r k reported here was t o c o m p a r e the quasi-static compression and p u n c t u r e tests o n t h e same egg and t o d e m o n strate a new test a p p a r a t u s (Voisey and MacDonald, 1978). F u r t h e r , deformation readings obtained u n d e r vertical and horizontal force applications were c o m p a r e d . It was h y p o t h e s i z e d

that p u n c t u r i n g

the

1 Contribution No. 1-38 from Engineering and Statistical Research Institute and No. 774 from Animal Research Institute.

shell at t w o diametrically o p p o s i t e equatorial sites should n o t affect subsequent resistance t o quasi-static compression applied at points o n t h e diameter perpendicular t o t h e p u n c t u r e axis. Since t h e p u n c t u r e points lie on t h e axis of s y m m e t r y , t h e y should influence neither t h e shell deformation nor t h e stress causing fracture which are localized at t h e c o n t a c t p o i n t s with t h e compression surfaces (Carter, 1 9 6 9 ; Voisey and Hunt, 1967). METHODS T o o b t a i n a range of shell quality, eggs were collected from flocks of Single C o m b White Leghorn hens. T h e flocks were selected o n t h e basis of differences in age, dietary t r e a t m e n t and strain. Flock A contained 110 h e n s from t w o experimental strains, 2 6 4 days of age at t h e start of t h e e x p e r i m e n t and fed a laying ration containing 3 . 1 % calcium from 1 4 0 days of age. T h e birds in Flocks B and C were 4 8 9 days of age and from eight strains (three commercial, four two-way strain crosses, and a c o n t r o l strain). F l o c k B consisted of 525

288

SHELL STRENGTH MEASUREMENTS ON THE SAME EGG hens which received a 3.25% calcium ration from 145 days. Flock C was comprised of 475 hens fed diets containing .51% calcium from 1 to 144 days, 2.25% from 145 to 327 days, and 3.25% from 328 days of age. Thirty eggs were collected in the afternoon from each flock for 17 working days over a 29-day period. The eggs were stored overnight in an environmental chamber at 15.6 C and 72% relative humidity. The following morning the specific gravity (SG) of each egg was determined by flotation in saline solutions stored in the chamber (Voisey and Hamilton, 1976b; 1977a). The eggs were removed from the chamber and conditioned to room temperature for 3 hr. Subsequently, the eggs were candled and those with defective shells discarded. Twenty eggs were then selected from each flock, attempting in the process to obtain the widest possible range of SG values. Four equidistant sites were marked on the equatorial circumference of each egg and sequentially numbered 1 to 4. Non-destructive shell deformation was measured at the equator across sites 2 and 4 for a force change of .1 to 1.1 kg at a deformation rate of 20 mm/min. Two readings were obtained by using two instruments in the following order: a) an Instron Testing Machine, (Instron Canada Ltd., Burlington, Ontario) as previously described (Voisey and Hamilton, 1976a), and b) the new eggshell tester described by Voisey and MacDonald (1978). The two readings enable comparison of forces applied to the egg vertically and horizontally, respectively. The eggs were then candled and cracked shells discarded. The maximum puncture force was measured at sites 1 and 3 using the egg shell strength tester equipped with a .4 mm diameter punch moving at 20 mm/min. Candling and discarding of cracked shells were again performed. Next the eggs were compressed to fracture at 20 mm/min in the egg shell strength tester applying force at sites 2 and 4. Pieces of shell were removed from the membranes at the two puncture sites (1 and 3) and the compression fracture site (2 or 4) and their thickness measured with a dial guage comparitor. The fracture shear stress (S) at each puncture site was calculated from: S = F/7Tdt where: F = maximum force to puncture the egg; d = diameter of the punch; and t = shell

289

thickness. RESULTS AND DISCUSSION

Complete data were obtained for 860 of the 1020 eggs tested. During the second puncture test, 7.5% of the eggs cracked on the opposite side at the first puncture site, because the force required to puncture the shell was higher than that required to crack it. A further 8.2% of the eggs cracked at other stages prior to fracture by compression. The results are summarized in Table 1. A paired t-test showed no difference (P<.05) in shell thickness between the two puncture sites (1 and 3). There were, however, significant differences in the fracture shear stress and puncture force (P<.01) at the two sites, indicating that thickness was not the only trait governing resistance to puncture. This conclusion was also reached by Hunt et al. (1977) who found that shell thickness and puncture force were unrelated within eggs, although they found the measurements to be highly correlated within birds. They did not examine the relationships with respect to shear stress. The significant differences in shear stress and puncture force observed in the present study are surprising in that the two sites are essentially random points on the equator of the egg. Two explanations for these differences can be put forward. First, the puncture at site 1 may have weakened the shell so that eggs with higher puncture resistance at site 3 tended to crack under compression at site 1 prior to puncture at site 3. If so, the mean for site 3 is biased downwards because these eggs were discarded. Second, there may have been local surface damage at site 3 from the compressive forces required to support the egg during puncture at site 1 even though the average puncture force at fracture was about half that for compression fracture (Table 1). This is because under compression, the initial contact stresses are theoretically infinite and the shell surface must crumble to distribute the load (Voisey and Hamilton, 1976a). Hunt et al. (1977) obtained six puncture force measurements per egg, of which three could have been influenced by prior compressive insult, and yet, unlike the present study, no significant differences were observed among sites. This result is not surprising, however, because position was judged visually and prior shell damage is localized. For example,

290

VOISEY ET AL. TABLE 1. Summary of measurements taken from each of860z eggs

Trait d Specific gravity Puncture force 1 Puncture force 3 Mean puncture force Shell thickness 1 Shell thickness 3 Mean shell thickness (1 and 3) Deformation (Instron) Deformation (egg tester) Difference of deformation readings from Instron and egg tester (means) Compression fracture force Shell thickness (2 or 4) Fracture shear stress 1 Fracture shear stress 3 Mean fracture shear stress (1 and 3)

g g g mm mm mm mm mm mm g mm g,mm~2 g.mrrf 2 g,mm~2

Mean

SEM b

CV%C

1.082 1764 1714 1739 .334 .335 .334 .0620 .0683

.00022 11.91 11.10 10.26 .00099 .00097 .00097 .00045 .00046

.6 19.8 19.0 17.3 8.7 8.5 8.5 21.5 19.6

19.34 .00096 23.12 20.91 17.99

16.9 8.4 16.2 15.1 12.8

.0063 3356 .334 4186 4062 4123

Those eggs of the 1020 from which all measurements were obtained. Standard error of mean. Coefficient of variation. The numbers associated with the compression, puncture, and thickness traits indicate the four sites marked sequentially around the egg circumference.

for compressive damage to influence subsequent puncture resistance, the puncture axis must be positioned within the area of the .4 mm diameter punch. Even so, it would appear to be desirable to use an odd number of puncture sites, thereby avoiding the possibilities discussed above. This could not be done in the present work because the sites were to lie on the axis of symmetry during the compression test. There were small but significant differences (P<.01) between the non-destructive deformation readings obtained by compressing the egg vertically and horizontally, the horizontal readings being larger (Table 1; Instron vs. egg tester). Unless there were unknown systematic errors, the differences cannot be attributed to the mechanical or electronic performance of the two instruments as the same readout device, deformation rate and contact surfaces were used. Possible causes of the differences are: a) the first deformation test in the Instron creating surface crumbling (as in the puncture case above), thus affecting the second measurement (Voisey and Hamilton, 1975); and b) the different methods of supporting the egg, particularly the third surface supporting the egg for horizontal force application in the egg tester. These factors

are confounded; the only conclusions to be drawn regarding the effect of the method of support and direction of force application are that any differences are small and that the variance of both readings is similar. Coefficients of variation among eggs (Table 1) were similar for the force, deformation, and stress readings; the coefficient of variation (CV) for average puncture force was 17.3%, for mean fracture shear stress 12.8%, for compression fracture force 16.9%, and for non-destructive deformation 19.6% (egg tester) and 21.5% (Instron). The coefficient for shell thickness, however, was less (CV = 8.4%) and for SG much less (CV = .6%). The correlation (r = .70) between the compression fracture (where tensile stress caused fracture) and mean puncture force (where shear stress caused fracture) was high (see Figure 1A), supporting (on a within egg basis) the conclusion of Hunt et al. (1977) (on a within hen basis) that the shear and tensile fracture behavior of the shell are related. When thickness was taken into account by including shear stress rather than puncture force, the scatter was greater (Figure IB) and the correlation considerably smaller (r = .52). Similar results were observed in the plots for other variables when shear stress replaced

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puncture force (e.g., Figure 2, A cf. B). In particular, the correlation between shell thickness and shear stress was low (Figure 2B) suggesting that shear stress, a property of the shell material, may itself influence shell strength. The correlation between compression fracture force and mean puncture force was somewhat lower in the present study (r = .70) than in the study of Hunt et al. (1977) (r = .80). This result may seem surprising in that, unlike the earlier study, each pair of observations in the present study was taken from the same egg. However, the correlation reported by Hunt et al. (1977) was based on the mean; that is, each observation was the mean of measurements from each of eight eggs from the same hen. The correlation involving individual observations might be expected to be lower if

eggs from a hen were relatively similar and if there was considerable random variation within egg, due, for example, to irregularities in shell material or to measurement imprecision. Hunt et al. (1977) illustrated this point in their discussion of the relationship between shell thickness and puncture force. The implication in the present context is that because the correlation between measurements made on different eggs from the same hen is high, and because there appears to be considerable random variation, it may prove more useful to obtain repeated measurements from each hen, even if they are from different eggs, than to introduce the difficulties of taking all measurements from the same egg. Generally, the relationships among the various traits appeared linear (Figures 1A, IB; 2A; 3A; 4A, 4B), the only exceptions

VOISEY ET AL.

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being the relationships between non-destructive deformation and puncture force or shear stress (Figure 4C), where evidence of curvilinear relationships was observed. The nonlinear relationship between deformation and compression force was reported earlier by Voisey and Hamilton (1977b). The curvilinear characteristics were further demonstrated by considering the relationships between the reciprocals of deformation (D) and D 2 and the puncture force, shear stress and compression force (e.g., Figure 5). These relationships were found to have higher correlations than the original ones. No effort was made to establish more optimal transformations of D because this had been attempted previously for compression fracture force (Voisey and Hamilton, 1977b).

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293

the new machine matched the Instron quite closely because the two readings of deformation showed a strong linear relationship (r = .87). The correlation between compression fracture force and mean fracture shear stress was low (r = .46) indicating that other factors influenced these traits differently. For example, the effects of shell thickness on compression and shear fracture are known to be different (Hunt et al., 1977). It is unfortunate that complexity in the calculation of tensile fracture stress under compression (Voisey and Hunt, 1967) precluded a comparison of tensile and shear behavior on the basis of stress. Using compression fracture force as the standard for egg shell strength (Voisey and Hamilton, 1976a; Voisey and MacDonald, 1978), the correlation coefficients indicated that the traits linearly related most closely to shell strength were in order: deformation (r = —.73 and —.74 for egg tester and Instron, respectively), mean puncture force (r = .70), mean shell thickness (r = .63), and mean fracture shear stress (r = .52). Stepwise multiple regression analysis showed that deformation accounted for the greatest reduction in the sum of squares (54.3%) of compression fracture force. Given that the regression equation contained deformation, the inclusion of either puncture force or fracture shear stress reduced the sums of squares by an additional 6.1%. The only other nondestructive measurement (SG) gave a further reduction of 1.6%. These results indicate that it is not necessary to destroy the egg to

TABLE 2. Correlations among traits based on readings for 860& eggs° Trait c A B C D E F G H I J K

A

SG Puncture force 1 .62 Puncture force 3 .63 Mean puncture force .70 Deformation (Instron) -.82 Deformation (Egg tester) -.81 Compression fracture force .71 Mean thickness (punctures 1 & 3) .75 Thickness (compression 2 or 4) .75 Mean shear stress .46 Log deformation (Egg tester) -.79

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.88 -.64 -.63 .63 .64 .64 .77 -.61

-.71 -.70 .70 .70 .70 .89 -.68

.87 -.74 -.80 -.80 -.45 .99

-.73 -.76 -.76 -.46 .86

.64 .64 .52 -.71

.95 .33 -.77

.31 -.77

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Those eggs of the 1020 from which all measurements were obtained. All the values shown are significantly (P<.01) different from zero. The numbers associated with the compression, puncture, and thickness traits indicate the four sites marked sequentially around the egg circumference.

294 obtain a fairly strength related

VOISEY ET AL. reliable prediction of shell t o quasi-static compression.

REFERENCES Carter, T. C , 1969. The hen's egg: Variation in shell deformation under static load with relative humidity and age of egg. Brit. Poultry Sci. 10:311— 319. Hunt, J. R., P. W. Voisey, and B. K. Thompson, 1977. Physical properties of eggshells. A comparison of the puncture and compression tests for estimating shell strength. Can. J. Animal Sci. 57:329-338. Voisey, P. W., and R. M. G. Hamilton, 1975. Behaviour of egg shell under compression in relation to deformation measurements. Brit. Poultry Sci. 16:461-470. Voisey, P. W., and R. M. G. Hamilton, 1976a. Factors affecting the non-destructive and destructive methods of- measuring egg shell strength by the quasi-static compression test. Brit. Poultry Sci.

17:103-124. Voisey, P. W., and R. M. G. Hamilton, 1976b. Notes on the measurement of egg specific gravity to estimate egg shell quality. Rep. 7322—598, Eng. Res. Service, Agr. Canada, Ottawa, March. Voisey, P. W., and R. M. G. Hamilton, 1977a. Sources of error in egg specific gravity measurements by the flotation method. Poultry Sci. 56:1457— 1462. Voisey, P. W., and R. M. G. Hamilton, 1977b. Observations on the relationship between non-destructive eggshell deformation and resistance to fracture by quasi-static compression for measurement of eggshell strength. Poultry Sci. 56:1463— 1467. Voisey, P. W., and J. R. Hunt, 1967. Physical properties of egg shells. 4. Stress distribution in the shell. Brit. Poultry Sci. 8:263-271. Voisey, P. W., and D. C. MacDonald, 1978. Laboratory measurements of eggshell strength. 1. An instrument for measuring shell strength by quasistatic compression, puncture and non-destructive deformation. Poultry Sci. 57:860-869.