Approximate Method for the Residual Strength Assessing of the Aircraft Composite Thin-Walled Structural Elements in the Presence of Low-Velocity Impact Damage

Аuthors

Garifullin M. F., Kazakov I. A.^*, Kireyev V. A.

Central Aerohydrodynamic Institute named after N.E. Zhukovsky (TsAGI), 1, Zhukovsky str., Zhukovsky, Moscow Region, 140180, Russia

*e-mail: normist@tsagi.ru

Abstract

The low-velocity impact damages significantly affect the residual strength with carbon fiber and epoxy matrix composite parts. The impact results in various defects such as fibers disruption, matrix cracking, delamination etc. Visual detection of such damage herewith is hampered. Thus, the “damage-tolerant design” concept has become widespread. 
Simplified models are employed to determine the residual strength of the composite structural elements in the presence of defects at early design stages. The presented article examines various versions of these models, highlighting their basic pros and contras. Based on the literature review, some comments on the effectiveness and scope of application of simplified models are made. The error of such models is 10-20% on average. It is noted that empirical formulas are used to reduce the required number of experimental data.
The authors proposed a model of a composite material properties degradation in the impact damage area. The stiffness degradation is being set by the reducing coefficients, changing in the defect zone by the predetermined law. A polynomial of a given degree is adopted as a function of the change of degradation coefficients. Thus, the damage model depends on the two parameters, which are minimum reducing coefficient and the degree of the polynomial.
The stress concentration computations at the defect zone boundary were performed with the finite element method. The damage was modeled by a zone with reduced stiffness. The article presents the graphs of the stress concentration dependence at various parameters of the properties degradation model. An algorithm for these parameters selecting based on the available experimental data is proposed.
To reduce the number of tests, an empirical model for the properties degradation grade determining inside the damaged zone is proposed. The dependence of the degradation coefficient on the relative defect area is well described by an exponential function. The empirical dependence parameters were determined with the Matlab Curve Fitting Toolbox. It is demonstrated that the coefficient in the empirical function can be set equal to the degree of the polynomial in the property degradation model as a first approximation. The error of the degradation coefficient determining herewith will be less than 20%.

Keywords:

residual strength of composite plates, low-velocity impact damages, stress concentration in the defects zone

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