Studying Carbon Fiber Plastics Strength and Heat Resistance under Conditions of Periodic Thermal Cycling

Testing of the Flying Vehicles Composite Structures Elements


Аuthors

Nikolayev V. N.1*, Petrov M. G.1**, Gusev S. A.2, 3***

1. Siberian Aeronautical Research Institute named after S.A. Chaplygin, 21, Polzunov St., Novosibirsk, 630051, Russia
2. Institute of Computational Mathematics and Mathematical Geophysics of the Russian Academy of Sciences, Novosibirsk, Russian Federation
3. Novosibirsk State Technical University, 20, prospect Karla Marksa, Novosibirsk, 630073, Russia

*e-mail: nikvla50@mail.ru
**e-mail: mark-stein@list.ru
***e-mail: sag@osmf.sscc.ru

Abstract

The possibility of the polymer composite carbon plastics application in supersonic passenger aircraft structures requires an assessment of the airframe structural elements ability to maintain operational characteristics under deterministic and random thermal and mechanical loads. As the result of these actions, cyclic expansions and contractions of carbon fiber plastics originate, which cause internal mechanical stresses under the impact of temperature, humidity, solar radiation, and other external factors. This leads to the microcracks forming inside the carbon fiber elements, and presents the reason for a considerable decrease in the strength of materials and operational reliability of the aircraft. For computer simulation of thermal stresses in carbon fiber under conditions of temperature drops, the authors tackle a one-dimensional uncoupled thermoelasticity problem, representing a system of two boundary-value problems under conditions of the flight mode operational parameters and the air environment outside the aircraft. The flight temperature profile was experimentally modeled in the SM–60/90-500 TVX climate chamber. After completing a certain number of thermal cycles, the AFM 102-C130UD carbon fiber elements were tested for strength and heat resistance by the longitudinal bending method. 
In this case, thermal stresses are being determined by the Duhamel–Neumann relation, while the method of Green’s functions is used to solve boundary value problems. The authors applied a Gaussian random process with independent increments to model trajectories via the Euler–Maruyama scheme, defined on a time interval by the uniform grid. The hyperbolic equation of the boundary value problem, with a term on the right hand side in the form of a temperature gradient, is solved by determining the Lebesgue inner integral with representation of the Ito stochastic integral.
Computation of solutions to the boundary value problems and their gradients, as well as deterministic integrals, was performed analytically, i.e. by the exact formulas, while stochastic integrals were computed numerically. When summing series, finite sums were used. The number of terms in the sums was being selected each time experimentally, based on the uncertainty computing of 0.1 % of the calculated value.
Computations in the deterministic case revealed that the temperature values at the opposite boundaries of the carbon fiber composite element differ slightly, since perturbation of the zero solution in the one dimensional inhomogeneous thermoelasticity problem occurs only due to the temperature gradient across the thickness. Computational experiment demonstrated that random temperature perturbations with relatively small amplitudes were leading to the significant random values in the stresses.
In solving the thermoelasticity problem, the ASM 102-C130UD carbon plastic plates were experimentally subjected to periodic heating according to the program corresponding to the adopted temperature change profile during the flight of a supersonic passenger aircraft. The tests were conducted with a SM 65/150 166 TXB thermobaric chamber, but at the atmospheric pressure, based on the capabilities of this equipment. Tests for strength and heat resistance by the longitudinal bending method were performed in the initial state of the material and after each series of thermal cycling. The plates securing method was selected to be a hinged support of the ends.
At strength tests, loading was brought about by moving the movable support of the testing machine at a constant speed of 0.001 m/s at room temperature. Compression forces and movements of the movable support were recorded herewith. 
During heat resistance tests, the samples were loaded with longitudinal bending by moving the movable support of the testing machine by 0.03 m and fixed in this position. To accelerate the process, the samples were preheated to 353 K (below the binder softening temperature), and held until the sample temperature equaled the furnace temperature. Then, the temperature was being increased at a rate of 0.017 K/s, and the load and temperature were recorded at the moment of the sample failure.
The tests demonstrated that performing a temperature program simulating the structure heating during the flight increased the heat resistance of the samples. The load at failure is on average somewhat lower due to the temperature dependence of the material stiffness if 1179.9 N in the initial state and 1143.7 N after the thermal cycling, but with significantly less variability. The same applies to the initial load at room temperature, indicating a decrease in the samples stiffness. Thus, the structural state of the samples clearly changed as a result of the temperature program, but it became more stable.
According to the technical specifications for the durability of the supersonic civil aircraft technology complex demonstrator, the service life is set at 100 flights and, accordingly, 100 thermal cycles during the tests. The assessment of changes in the strength and heat resistance in the studied thermal cycling range up to 120 thermal cycles suggests the possible application of this carbon fiber at temperatures from 220 to 420 K.

Keywords:

random thermal stresses modeling in carbon fiber composites, one-dimensional uncoupled thermoelasticity problem, longitudinal bending strength tests method, strength and heat resistance tests under thermal cycling

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