Measuring Glass Strength: Ceramic Enamel Glass Research and Requirements
By Bill Lingnell
The results of highly publicized testing have shown that, in some cases, the mean strength of freshly manufactured ceramic enameled glass is less than that of analogous uncoated fresh glass. With this in mind, some groups have proposed incorporating a strength reduction factor into ASTM E1300 “Standard Practice for Determining Load Resistance of Glass in Buildings” for the design of ceramic enameled glass. According to some testing, while the mean strength of freshly manufactured ceramic enameled glass appears reduced, so does the coefficient of variation (COV). Results show that the reduction in the COV, coupled with the fact that ASTM E1300 is based on the reduced strength of in-service glass, combine to compensate for the apparent reduction of the mean strength of freshly manufactured ceramic enameled glass. Given these results, there’s no need for a ceramic enamel glass strength reduction factor in ASTM E1300 for the heat-strengthened (HS) glass examined. The behavior of ceramic enameled glass is complicated and requires a substantial amount of additional research before changes to ASTM E1300 can be justified by technical considerations. This was presented in a paper at Glass Performance Days 2017 that I co-authored with Lynn Bea-son and Michael Brackin.
WHAT’S THE DIFFERENCE
There’s been a lot of controversy regarding the use of ASTM E1300 to specify uniform load resistance of ceramic enamel coated glass. This is apparently due to a lack of understanding that the glass thickness selection charts in the standard are based on the performance of in-service glass and not freshly manufactured glass. Results of in-service glass strength tests conducted by Beason show that its strength is significantly less than freshly manufactured glass. Additional testing since Beason’s confirms these results. The in-service strength reduction has been reported to be as much as 50 percent when com-pared to freshly manufactured glass, and is accepted by researchers and engineers throughout the glass industry.
The strength of in-service glass being significantly less than freshly manufactured glass was one of the primary drivers for the original development of ASTM E1300. The standard’s “Significance and Use” section states that using the practice assumes that “the surface condition of the glass is typical of glass that has been in service for several years, and is weaker than freshly manufactured glass due to minor abrasions on exposed surfaces.” Published papers relating to this fact have been in place for years. If the applicability of ASTM E1300 to ceramic enamel coated glass is to be challenged, it must be on the basis of comparing the strength of ceramic enamel coated glass to the in-service strength of uncoated glass, as defined in ASTM E1300, and not the freshly manufactured strength of uncoated glass.
FULL-SCALE TEST RESULTS
The most direct procedure to deter-mine any problems using ASTM E1300 is to first select the minimum thickness of ceramic enamel coated heat-strengthened (HS) glass. Next, test a representative sample of full-scale, ceramic enamel coated HS glass plates and compare the design load. This will determine the cor-responding design load in ASTM E1300.
Either ASTM E1300 over-predicts the strength of ceramic enamel coated HS glass or it under-predicts the strength of ceramic enamel coated HS glass. Any other comparison procedure involves projections and extrapolations.
In our research, we collected a group of data in which we subjected two sets of 40-by-60-by-¼-inch glass plates with four-sides of continuous support. We linearly increased uniform lateral loads to failure. One set of test specimens involved uncoated HS glass and the other set involved ceramic enamel coated HS glass with average thicknesses of 0.224 and 0.226, respectively. The enamel coating on the second set of glass was a full coat across the entire surface.
All of the glass specimens tested were taken from the same batch of freshly manufactured glass. The glass plates were tested with the ceramic enamel coated side in tension.
The test statistic of primary interest was the 3-second equivalent duration breakage load for each glass plate. Standard understandings of “static fatigue” were used to convert the failure strength data measured to equivalent 3-second durations. Statistical analyses were used to determine the equivalent mean 3-second breakage load, the associated standard deviation and COV. We also determined the equivalent 3-sec-ond duration failure load corresponding to a probability of breakage (POB) of 8 lites per 1,000. This was done for both the ceramic enamel coated and the uncoated specimens. Calculation of the equivalent 3-second duration failure load corresponding to a POB of 8 lites per 1,000 was done using standard normal distribution assumptions. The ratio of the measured design pressures (403 psf for the uncoated and 268 psf for the ceramic enamel coated specimens) to the ASTM E1300 load resistance (110 psf ) was 3.66 and 2.44 respectively.
The information gathered from the testing was extremely interesting. Consulting glass thickness selection charts in ASTM E1300, we found that the in-service design load corresponding to a POB of 8 lites per 1,000 for a 40-by-60-by-¼-inch annealed glass plate is approximately 55 psf. Then, if a factor of 2.0 is applied to this value, as directed for HS glass, it can be determined that the ASTM E1300 design load resistance for a 40-by-60-by-¼-inch HS glass plate is 110 psf. If this information is combined with the data presented, you find that the measured design load for uncoated, 40-by-60-by-¼-inch HS glass is about 3.7 times greater than is required by ASTM E1300. This is because of the inherent conservatisms discussed previously for fresh glass. In addition, while the ceramic enamel coated HS glass is weaker than freshly manufactured clear HS glass, as expected, it’s still more than 2.4 times greater than it has to be to comply with ASTM E1300. While there’s other supporting data and information, at this point ASTM E1300 appears to be adequate to determine the load resistance of glass subjected to uniform loads.
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