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Design and Fabrication of a HIC Compliant Bulkhead

STATIC TESTING OF THE EXISTING BULKHEAD

Static tests were conducted on a typical transport aircraft bulkhead to determine the load–deflection characteristics. The purpose of this test was to investigate whether the stiffness and crush strength of a particular bulkhead, that had met the HIC requirement in a dynamic sled test, were within the design heuristics required for HIC attenuation. The attachment points used with the existing bulkhead fixture were changed in an effort to simulate the constraint conditions of the bulkhead when fitted in an aircraft. The bulkhead was attached to the test fixture at four points, two at the top and two at the bottom. Figure 1 shows the side view of the attachment points of the bulkhead.

Figure 1. Fixture used for testing.

The bulkhead used for the test was a honeycomb structure sandwiched between two fiber sheets. The bulkhead was attached to the fixture at 4 points on the top and bottom support, which ran horizontally across the bulkhead. A hydraulic actuator was used for applying load on the bulkhead. The test was conducted in the Structures Laboratory at National Institute for Aviation Research. Figure 2 shows the actuator assembly with the bowling ball attached to one end was used to apply the load. The weight of the bowling ball used for the static test was 16 lbs. A maximum load of 800 lbs (approximately) was applied to the bulkhead in order to obtain the load deflection curve for the existing bulkhead.

Five vertical slits were provided on the front and back face sheets of the bulkhead in order to decrease the stiffness of the bulkhead. The load was applied normal to the bulkhead at a point where ATD head is most liable to strike.

Figure 2,3. Deflection in the bulkhead as the load is applied.

The load was gradually increased in small steps and to a maximum applied load of 770 lbf at which the actuator slipped and the load was no longer being applied normal to the bulkhead. Therefore the unloading data could not be obtained from the test. The loading data thus obtained is plotted. Figure 4 shows the loading curve that was obtained from the test that was conducted.

Figure 4. Load-deflection characteristics of the existing production bulkhead

The spike in the graph was due to the breakage of the bulkhead at that point. The maximum deflection that was measured from the static testing was 3.5 inches at 770 lbs. The stiffness of this bulkhead, obtained from the static test that was conducted, was
207 lb/in.

As per the bulkhead design guidelines and design charts for HIC attenuation [1], for the HIC to be less than 1000, the stiffness of the bulkhead needs to be less than 300 lb/in. The bulkhead, which was subjected to the static testing, was used in the sled test conducted at NIAR, No. 99109-002, in which a HIC of 810 was obtained. Hence the 207 lb/in stiffness of the bulkhead tested statically verifies the design charts produced in the earlier study. Hence in the required design of the bulkhead, one of the design requirements will be that the bulkhead stiffness should be below 300 lb/in for the HIC to be less than 1000.

Dynamic Testing (First Series Bulkhead)

Figure 4a. Pre-Test Condition for Sled Test (#01008-1)

Specifications:

PANEL: TEKLAM N510

CORE: Non-metallic Nomex Honeycomb

FACE SHEETS: Phenolic/7781 fiberglass

THICKNESS: 1 in. (2.54 cm)

DENSITY: 3 lb/in²

CARPET: No

Dynamic test (#01008-1 through 3) were conducted to measure the accelerations in the head of 49 CFR Part 572, Subpart B anthropomorphictest dummy (ATD) designed according to the design requirements at National Institute of Aviation Research (NIAR) Crash Dynamics laboratory. The test conducted on the first series bulkhead panel TEKLAM N510 Phenolic/Nomex Honeycomb sandwich panel. Figure 4a shows the test setup for tests conducted. Seat setback distances of 35 and 33-in were used for the test, so as to test for the performance of the bulkhead for larger and smaller seat setback configurations.

The first series of bulkheads with a crush strength of 300 psi were too stiff for HIC compliance. Second series of bulkheads with a crush strength of 295 psi which are less stiff were acquired for conducting dynamic tests for HIC attenuation.

Dynamic Testing (Second Series Bulkhead)

Dynamic test (#01008-12) were conducted to measure the accelerations in the head of 49 CFR Part 572, Subpart B anthropomorphictest dummy (ATD) designed according to the design requirements at National Institute of Aviation Research (NIAR) Crash Dynamics laboratory. The test conducted on the second series bulkhead panel TEKLAM 510(E) Phenolic/Nomex Honeycomb sandwich panel. Figure 1 shows the test setup for one of the earlier test conducted using the same test conditions.

Figure 4b. Pre-Test Condition for Sled Test (#01008-9)

Specifications:

PANEL: TEKLAM N510 (E)

CORE: Non-metallic Nomex Honeycomb

FACE SHEETS: Epoxy/7781 fiberglass

THICKNESS: 1 in. (2.54 cm)

DENSITY: 2 lb/in²

CARPET: Yes

Dynamic tests (#01008-4 through 9) were conducted to measure the accelerations in the head of 49 CFR Part 572, Subpart B anthropomorphictest dummy (ATD) designed according to the design requirements at National Institute of Aviation Research (NIAR) Crash Dynamics laboratory. The test conducted on the second series bulkhead panel TEKLAM N510(E) Epoxy/Nomex Honeycomb sandwich panel. Figure 4b shows the test setup for one of the tests conducted.

Six tests (#01008-4 through 9) were conducted to verify the repeatability of HIC data.

The bulkheads were tested at both 33 and 35-in seat setback configurations to evaluate their performance for HIC compliance. The bulkheads performed well at larger seat setback distances but failed to produce potential solution at smaller seat setback distances.

MATERIAL FOR BULKHEADS (THIRD AND FOURTH SERIES)

Two additional materials were obtained for the design and testing of bulkheads, which are softer than the earlier two series of bulkheads, which could produce HIC values below 1000 even at lesser seat setback configurations. Figure 3 shows a sample material for the new series of bulkheads.

Figure 4c. Sample Material (Third Series Bulkhead)

Two additional materials were obtained for the design and testing of bulkheads, which are softer than the earlier two series of bulkheads, which could produce HIC values below 1000 even at lesser seat setback configurations. Figure 4c shows a sample material for the new series of bulkheads.

Figure 4d. Lay up for Third and Fourth Series Bulkheads

The new series of bulkheads will have an aluminum honeycomb core with fiberglass face sheets on either side. The face sheets on both sides will be covered with carpet typically used in aircraft installations. Figure 4 shows the lay-up for the third and fourth series bulkheads. Table 1 summarizes the properties of the bulkheads.

Table 1. Summary of Properties of Third and Fourth Series Bulkheads

Material	Third Series	Fourth series
THICkness	1.00-in	1.00-in
Facesheets	Fiberglass	Fiberglass
Facesheet material	7781 fiberglass	7781 fibgerglass
Core	Aluminum honeycomb	Aluminum honeycomb
Density	1.0 lb/in²	2.4 lb/in²
Carpet (Typically used in aircraft installation)	Yes (On both sides of face sheets)	Yes (On both sides of face sheets)

Dynamic Testing (Third Series Bulkhead)

Figure 5. Pre-Test Condition for Sled Test (#01008-14)

Dynamic tests (#01008-10 through12) were conducted to measure the accelerations in the head of 49 CFR Part 572, Subpart B anthropomorphic test dummy (ATD) designed according to the design requirements at National Institute of Aviation Research (NIAR) Crash Dynamics laboratory. The test conducted on the fourth series bulkhead panel TEKLAM Aluminum Honeycomb sandwich panel. Figure 5 shows the test setup for one of the tests conducted.

The test configuration is same as the third series tests but the bulkheads used had a density of 2.4 lb/in². Fourth series bulkheads were not accepted as a potential solutions for HIC attenuation, as they produced head impact criteria values above the threshold values of 1000.

Design methodology for HIC compliant Bulkheads

The initial bulkhead design is based on the aircraft cabin requirements and previous experience. The stiffness of the designed bulkhead is determined by three methods:

· Hybrid Analytical method

· Finite element Analysis

· Static testing

The stiffness value obtained is compared with those from the design curves (obtained from occupant model analysis tools). If stiffness is less than the threshold value (350 lb/in for 33-in. and 480 lb/in for 35-in. seat setback), full scale sled tests are performed to obtain the HIC value. If HIC from the sled test is below the threshold value of 1000 the bulkheads are fabricated and installed in the aircraft. If not the bulkhead design is modified for HIC compliance.