Test Program for Evaluation of FilmedWindows Subjected to Blast LoadsExecutive Summary | Blast Hazards | Damage Level Classification Window Size and Framing | Summary of Results | Conclusions
ExecutiveSummaryComparative tests were performed on plain annealed glass specimens to demonstrate the potential hazard from unprotected windows. Additionally, some tests were performed on glass specimens with a daylight application of safety film. Tests were also performed on 1/4 inch laminated glass in a standard frame to permit comparison of the relative protection provided by laminated glass and safety film in a daylight application. Testing was conducted August 26-29, 1996 at WBE's test range in EImendorf, Texas near San Antonio. WBE utilized a 3 ft x 3 ft shock tube to produce blast loads for a11 tests. This shock tube has the capability to produce peak pressure up to 3S psi with durations up to 40 ms. The primary test objective was determination of the maximum blast capacity of filmed windows at various hazard levels. A second objective was to demonstrate the reduction in glass fragment hazard produced by applying safety film to annealed glass. Currently, no standard exists in the United States for evaluating damage and hazard levels of window glass subjected to blast loads. For this reason, WBE used a classification method developed in the United Kingdom. The Explosion Effects Branch of the Safety and facilities Establishment, Department of Environment, England currently endorses this hazard assessment technique; however, it is not an adopted standard. Three levels of hazard are identified: Break Safe, Low Hazard, and High Hazard.ASTM 1' 1 64Z - 95 "Standard Test Method for Glazing and Glazing Systems Subject to Air blast Loadings" was used as a guideline for preparation of the test procedures. For this test series, 3 ft x 3 ft (nominal size) windows glazed with nominal 1/4 inch thick annealed glass were used. Clear window size was 32.5 inches x 36 inches.This report is a summary version condensed from the full report issued November 1996. It contains results for tests for plain annealed glass and anchored film. A summary of the test results is shown in Table I. The values in the table represent maximum pressure which can be applied without causing a higher damage level and are based on a limited amount of test data. Table 1. Summary of Test Results
N/D = No Data — this glazing system/damage level was not experienced in the test program * Standard 5/8 inch bite "Test Program for Evaluation of Filmed Windows Subjected to Blast Loads" Summary Report January 1997 WBE Project No. 817-001 BlastHazardsHigh explosive detonations are well characterized by experimental data and modeled by equations as a function of charge weight and standoff distance. Data also exists relating various explosive materials to an equivalent weight of TNT, thus allowing prediction of blast loads for threats involving materials other than TNT. The use of an equivalent weight of TNT is significant to window film testing because of number of charge weight-standoff distance combinations, which match the tested blast load, are possible. For a given standoff, a charge weight can be determined which will produce a peak pressure equivalent to that measured in the tests. Conversely, required standoff can be computed a specific charge weight so that the peak pressure and duration will match the test data and film performance can be predicted. Table 2 gives pressure and duration parameters for various combinations of charge weight and standoff. Table 2. Blast Load Parameters for High Explosive Detonations
Damage Level ClassificationCurrently, no standard exists in the United States for evaluating damage and hazard levels of window glass subjected to blast loads. For this reason, WBE opted to use a classification method accepted in the United Kingdom. The Explosion Effects Branch of the Safety and Facilities Establishment, Department of Environment, England currently endorses this hazard assessment technique; however, it is not an adopted standard. Three levels of hazard are identified: Break Safe, Low Hazard, and High Hazard. Figure 1 illustrates the locations of the thresholds between each hazard level for the test setup used in this point.Break/Safe (B/S) categorizes window systems that, after window failure, do not release the window, shards, or framing components beyond 1 m (3.3 ft) behind the window. Mild cuts and abrasions to the legs and lower body of persons in proximity to the window are typical injuries expected in this category. Low Hazard (L) classifies window systems that upon window fracture release the window, shards, or framing components which travel beyond the 1 m B/S threshold but do not strike the witness panel, located 3 m (9.8 ft) behind the window, at a height greater than 0.5 m (1.6ft). Possible injuries at this hazard include cuts and abrasions of the upper and lower body of persons in proximity to the window. As the distance of the person from the window increases, these injuries tend to the lower body and legs. High Hazard (H) identifies window systems ~~ after window fracture release the window, shards, and framing components which travel the full 3 m (9.8 ft) striking the witness panel at a height greater than 0.5 m (1.6 ft). Likely injuries include severe cuts and abrasions and possibly puncture wounds to the upper and lower body of persons located near the window. The increased risk of injury to the upper body, compared with Low Hazard, indicates an increased risk of fatality since the potential of injury to vital organs increases.Criteria in a draft standard produced by the European Commission TC 33 provides four load levels or ratings for classification of performance. To meet a particular rating, no debris from the door or frame may be created and the doors must remain in the frame. The load levels provided in this standard are substantial. Safety film applications are realistically appropriate for this window size at the lowest level which is 0.5 bar (7.5 psi) pressure with a duration of 20 ms.Window Size and FramingFor this test sense, 3 ft x 3 ft (nominal size) windows glazed with nominal 1/4 inch thick annealed glass were used. Clear opening size was 32.5 inches x 32.5 inches. Two of the annealed glass tests with no safety film applied were constructed with 3/16 inch glass. Glass was installed in standard commercial flush glaze aluminum frames (Kawneer Trifab II 450) with rubber gaskets. The typical bite (depth of glass beyond the interior edge of the frame) was approximately 1/4 inch. Laminated glass specimens were constructed of two 1/8 inch panes with a 0.030 inch interlayer. A batten anchorage system utilizes a mechanical connector to secure the film edges to the frame as shown in Figure 2. Film is applied to the window and an extra length or "tail" of film on each edge is attached to the frame with adhesive tape. Batten bars are installed over the "tail" of film and secured to the frame. Test Setup WBE utilized a 3 ft x 3 ft shock tube to produce blast loads for all tests. The shock tube consists of three major components: driver, expansion sections, and target area. Air pressure in the driver control the peak pressure of the shock wave, while driver volume controls the load duration. An interior bulkhead controls the volume of air in the driver section. The shock tube generates a wave by bursting an aluminum diaphragm located at the front of the driver. As the diaphragm bursts, a shock wave travels down the expansion chamber to the target area where it loads the test item. Steel angles restrain the window frame to the end of the expansion section. Peel Test The U.K. blast community has developed a test to evaluate the performance of film adhesive. The test consists of cutting a 1 inch wide vertical strip on filmed window. A 700 gm weight is attached to the top of the strip. For acceptable performance, the film must not peel. An additional 800 gm is then added. As the film beings to peel away from the glass, the distance down that it travels in 60 seconds is measured. If it does not travel more than 12 inches in 60 seconds, it is deemed to provide acceptable adhesion. This test was performed on a sample window unit with 14 mil film. The total distance traveled in 60 seconds was 3 inches. This application provided adequate adhesion. Summaryof ResultsTable 3 presents summary information for each test. Columns 2-4 indicates the peak pressure, duration, and peak impulse measured for each test. Impulse is the integral of the pressure-time history. The third column describes the test specimen. Daylight film was not anchored to the frame. Columns 5-6 shows the percent glass lost during each test with filmed glass. Column 5 indicates the total percentage of glass lost from the window unit, including glass within the bite. This percentage was obtained by comparing pretest weight of the specimen with the combined weight of the test frame and filmed glass following the test. Column 6 reports the percentage of glass lost from the film. For Column 6 computations, only the amount of glass originally bonded to the film was considered in the calculation of percent glass lost from the film. This excludes glass which was inside the frame but was not bonded the film prior to the test. This is a direct indicator of the ability of the film to restrain glass during a blast and is a measure of the adhesive performance under dynamic loads. Column 7 reports the damage level for each test determined in accordance with the U.K. criteria.
Table 3. Summary of Test Results
B/S - Break Safe N/P - Non Penetrating A — Total percentage of glass not retained on film including bite B — Percentage of glass lost off of film (excludes bites and 3/4 inch perimeter at radius of film) n/a — not applicable ConclusionsTesting performed during this project yielded the following conclusions:
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