High -temperature duct thermal insulation debonding analysis for civil aircraft

The paper focuses on the problem of the composite thermal insulation of High-temperature Pneumatic ducts for civil aircraft debonding phenomenon. The performance of thermal insulation material is tested to explain the reason why debonding. A model based on the test and thermal kinetic analysis is proposed to quantify the life of the insulation. Based on the model, a certain duration of thermal insulation is calculated. This methodology can predict the life of thermal insulation.


Introduction
As the container of high-temperature and high-pressure air, the high-temperature duct of the pneumatic system is installed in the nacelle, pylon, fuselage, wing, etc. Various thermal, mechanical, and installation stresses are combined together to impact the duct design. [1]irworthiness regulation FAR 25.863 (a) required "In each area where flammable fluids or vapors might escape by leakage of a fluid system, there must be means to minimize the probability of ignition of the fluids and vapors, and the resultant hazards if ignition does occur" . [2]o, the high-temperature duct design should take into account the thermal insulation design [3][4][5][6] .To reduce the weight, usually, the insulation should choose the epoxy and fiberglass combined material [7] .

Thermal insulation design
Usually, the high-pressure duct consists of a metal tube, thermal insulation, a venting hole, and leak detect bracket as figure 1 shows. Skin temperature of the thermal insulation should not be beyond 204℃ [8] , in both normal and abnormal conditions;  High-temperature air should only leak through venting holes;  In order to be detected, venting holes should aim at the leak detection system.If debonding occurs and the tube rapture, some hot air will go through from the debonding area instead of the venting hole.So, the leakage cannot be detected at once, which will even lead to a catastrophic hazard on the aircraft.Unfortunately, this debonding phenomenon was found on aircraft, and this problem should be solved as soon as possible.

Thermal aging test
To meet the downstream systems' working conditions, usually, the hot air temperature should be set as 200℃ or 225℃, depending on the different cases.The transit hot air temperature should not be higher than 260℃ [9] .In order to quantify the phenomenon of debonding, an index of mass loss rate is proposed.A thermal aging test is carried out to establish the model of mass loss rate.Some pieces of samples were made and put into ovens, the test temperatures were set to 350℃, 260℃, 230℃ and 180℃.The samples were weighed and observed after a fixed interval.The test duration, temperature, weight, and whether or not debonding should be recorded.Figure 2 shows the photo of the sample, and Table 1 shows the test result.To be conservative, this paper uses 15% weight loss to identify the debonding phenomenon.

Thermal kinetic analysis
As the test conditions are not the same as the reality, a kinetic study as formula 1 to 6 was carried out to assess the rate conversion of A into B in the function of time: A solid→ B solid + C gas.
( 1 ) This study was performed in function of temperature in order to get the impact of temperature on the rate conversion.The conversion considered was: The rate reaction can be defined as: where k is the rate constant which is a function of temperature through Arrhenius law [10] : R=8.312 J/mol/K, E: Activation energy Taking both equations and in case of isothermal assumption, time (t) necessary to have a% of conversion at temperature T(K) is expressed as: (5)

Data
For each α, ln(t) in the function of 1/T is reported.By the aging test, the activation energy and constant ln ((g(α))/A) could be identified in Table 2.
For each α(t), and t, the constant is calculated at 230℃ in Table 3.Based on the flight data collection, the following duty cycle was assumed, and related weight loss was calculated in Table 5.
Table 5 Analysis shows that the thermal insulation would start debonding when the aircraft reaches 2064 flight hours, and the related ducts shall be replaced before this age.

Conclusion
A general thermal insulation form and function is introduced to meet related airworthiness regulations.Aiming at the problem of the non-metallic insulation debonding, a method based on the aging test and thermal kinetic analysis is proposed to calculate the time when the insulation starts to debonding.The result shows the life of the current insulation is only about 2064 hours and could not meet the life requirement of the duct.

Figure 1 .
Figure 1.Diagram of high-pressure duct.The main functions of the thermal insulation are:

Figure 3
Figure 3 summarizes the relationship between test duration and mass loss rate under different temperatures.

Figure 3 .
Figure 3. Results of the thermal aging test.

Table 1 .
Results of the thermal aging test.

Table 3 .
Constant for different conversion and time at 230℃.

Table 4 .Table 4 .
Time prediction at different temperatures and different conversions.

.
Life prediction of the insulation.