The venting process of a fully pressurized (7.50psig) aircraft fuselage is quite rapid. Within seconds, the entire aircraft has decompressed. In the event of an on-board explosion, however, the processes not always fast enough to prevent structural failure of the plane. A ventilation process enabling the fuselage to more quickly decompress should help to limit structural damage to the craft. Scaled model testing of two types of venting methods was conducted, the hinged panel and the fracture panel methods. The hinged panel method consisted of a flap-type device hinged at one end. The fracture panel method consisted of a medium (in this case, tempered glass)which was shattered to release the internal pressure of the vessel used for testing.

An extensive bit of hinged panel testing was conducted by Rob Reichenbach, and is covered in his paper. However, a bit of data needs to be borrowed from his research for comparison in this paper. Mr.Reichenbach found that the delay time inherent in the hinged panel method was on the average 5.6230 ms.. At this point, the term "delay time" should be defined. Delay time for these tests refers to the elapsed time between the opening of the venting panel, and the start of the pressure decay as seen by the pressure transducer. Once venting began, the time of depressurization for each method was the same. Therefore, the delay time difference between the hinged panel and the fracture panel was the point of interest to us. The same pressure transducer was used for all the tests conducted on both the hinged and fracture panels, thereby making the response time of the transducer inconsequential.

A square piece (2.50 in. x 2.50 in. x 0.1875 in.) of tempered glass was chosen as the fracture panel because of its unique property to shatter into numerous small pieces upon the initiation of a small crack. This fracture panel was tested on a pressure vessel pressurized to 7.50 psig and an orifice hole 1.47 inches in diameter. Upon reaching 7.50 psig in the tank, a crack was initiated in the glass and data was taken as the pressure relieved the glass from the orifice and the tank depressurized.

The fracture panel setup was quite involved. First, the glass had to be secured to the tank. The least amount of contact area between the glass retainer and the glass itself was desired. This limited the amount of glass area in compression. It was found that larger areas of compression often inhibited glass relief, causing glass to block 30 to 40% of the orifice opening. Two retainer clips were machined, and were found to sufficiently hold the glass to the tank, while at the same time ensuring 100% glass relief.

Next, a system of breaking the glass had to be developed. After a great deal of trial and error, a system involving a piece of tungsten-carbide sharpened to a point and mounted into the bottom of a 10-32metal screw was used. A tapped screw guide was machined and attached to the orifice plate. The guide directed the screw into the corner face of the glass, and approximately half a turn of the screw was enough to initiate a fracture in the glass. In order to turn the screw while standing behind shielding (used for protection from flying glass), a wrench was attached to the hex screw head. A piece of string was attached to the wrench, and the string ran up to a pulley and then back down to the operator.

Photographing the relief process was the next step. In order to capture the glass relief in a still shot, the techniques of high-speed photography had to be employed. A break wire trigger was designed and implemented to trigger a Vivitar 283 flash unit at the instant of glass fracture. The trigger was a 3-VoltDC source shorted to ground through a conductive paint stripe applied to the glass plate. Upon fracture the conductivity of the paint stripe was broken and a low to high voltage transition occurred, triggering the oscilloscope to begin sampling. A further electronic circuit was added to control the flash delay time through the use of the RC time constant. This allowed us to capture the glass fragments at different positions after fracture. From these pictures we were able to confirm that the glass pieces behaved as expected and the process was repeatable.

Using the same methods as employed in the investigation of the hinged panel delay time, the decaying pressure curve from the oscilloscope was analyzed. The pressure decay curve was fit to a second-degree polynomial line of best fit, and a delay time was found for the fracture panel. Results indicate that the average delay time for the fracture panel was 1.7880 ms.. The delay time difference of 3.8350 ms between the fracture panel and the hinged panel was large enough to not simply dismiss as error. However, future in-depth research will need to be conducted in order to determine the exact benefits that could be gained from using this type of fracture panel on a full size aircraft