DETERMINATION AND MONITORING OF STRENGHT PROPERTIES OF AIRCRAFT COMPOSITE STRUCTURES BY MAGNETIC MICROWIRES

The submitted paper deals with the application of magnetic microwires as sensors. The aim is to summarize the engineering view of the given issue and to inform about necessary steps for practical application of such a new sensor in the aircraft industry. The paper talks about the formation of composite samples necessary for tensile strength tests, the aim of which is to obtain material properties of composite materials. The obtained material properties of fiberglass samples are processed into graphical and tabular form. Subsequently, the material properties are used in the strength calculations of the prismatic beam, in which magnetic microwires are applied in the experimental part of the work. There are described in the submitted article two application of magnetic microwires as a stress monitoring sensors.


INTRODUCTION
Due to their advantages, composite materials are widely used in the aviation industry. When applying composite materials, it is necessary to know their behavior, and predict mechanical properties. The aim of the introductory chapters of the article is to provide the reader with information about the possibility of obtaining the properties of composite materials and choosing the most suitable matrix composition from the strength point of view. The output is the material properties, which are applied to the experimental aircraft structure of the prismatic beam in the following chapters of the article. In the conclusion there are summarized possibilities of prediction of experimental investigation of aircraft structures. One of the progressive tools to monitor the mechanical properties of composite structures are magnetic microwires. A number of numerical simulations have to be carried out to observe the internal mechanical stresses for application of magnetic microwires in real construction during flight operation. With numerical computer simulations it will be possible to realistically assess and apply this measurement methodology to aviation practice. Obtaining material properties is a necessary step in strength calculations. In terms of Fem calculations, the main disadvantage of composites is their material, which is orthotropic, each material layer may have other mechanical and material properties. Composite materials have a greater dispersion of properties compared to conventional materials, and statistical analysis is therefore essential in the evaluation of properties.

MECHANICAL PROPERTIES OF COMPOSITE MATERIALS
The mechanical properties of the composite are determined experimentally and the determination must be simple and straightforward. It is based on the most widespread uniaxial pull test. In this test, a fiberglass specimen clamped in the jaws of the tearing machine is prepared. The sample is stretched according to the prescribed rules until it tears. The force size and its corresponding elongation, resp. in our case tensile stress and sample elongation [1]. ISSN 1) for better grip into the pneumatic jaws of the tearing machine. Sample production is done by manual wet lamination [1].

Figure 1 Composite sample sketch
An epoxy resin layer is evenly applied to the polished plate with a roller with hardener. The fiberglass reinforcing fabric is placed at different angles on the resin layer. Again, a layer of the prepared mixture is applied to the fabric by means of a roller so that a further layer of fabric is also partially saturated. The process is repeated for different compositions, resulting in seven sheets of fabric composition graded α = ± 0 °, ± 15 °, ± 30 °, ± 45 °, ± 60 °, ± 75 °, ± 90 °. The fabric surface must be as saturated as possible with epoxy resin. This process is repeated twelve times to form a twelve-layer fiberglass board. Twelve layers provide the desired sample thickness of approximately 2.4 mm. The layers are stacked symmetrically about the longitudinal axis. This process is repeated for all tracks from 0 [°] to 90 [°], creating seven boards with different characteristics. Subsequently, the samples according to Fig. 1. Curing is carried out for 24 hours at 22 ° C. After curing, the duralumin pads are bonded to the samples using epoxy resin and one mat layer from glass fiber. The hardening of the samples results in large interlaminar stresses that need to be removed, therefore the samples are subjected to heat treatment. The samples are warmed up for 6 hours at 60 ° C [1].

Strength tests of composite samples
Tensile tests are performed on a Zwick Roell Z030 universal test machine with a maximum force of 30 [kN]. The dependence of the tensile stress and the relative strain is called the working diagram resulting from the experimental tests. From the proportional deformation it is possible to calculate the elongation of the test sample and then the Young's modulus, which is once from basic constants in strength calculations.  3). In the same way, mechanical tests are performed for all prepared samples with tracks α = ± 15 °, ± 30 °, ± 45 °, ± 60 °, ± 75 °, ± 90 ° [1].

Results of Strength tests of composite samples
The experimental determination of material properties results in the tensile strength, strain and calculated Young's modulus. In Tab. 1 shows the values for each sample type. For tensile tests, the number of 5 samples from each track was selected, which means 35 samples.

MODELING OF COMPOSITE BEAM IN ANSYS APDL
The obtained experimental properties of the composite materials can be used in the strength simulation of an aircraft structure beam. The same beam is subjected to experimental measurements using magnetic micro-wires. The aim is to test the possibility of using a new methodology for monitoring aircraft structures to enhance flight safety [1,2]  and four pairs of fabrics corresponds to a real-made beam that will be summarized in the following chapter. After applying the boundary conditions in the form of a loading force at one end and the fixation at the other, a numerical analysis is performed, where the maximum stress is examined, which will be compared with the measured experimental stress [1,2].

EXPERIMENTAL ANALYSIS
As in the numerical simulation in the experiment the beam is at one end fixed and on the other loaded with the same force as in the calculations. To monitor the maximal stress a magnetic microwire sensor is located in upper part of the beam (Fig. 6) [1, 3, 4].

Figure 6 Experimental composite beam
The sensing coil senses the dependence of the HSW switching field and the mechanical stress.
According to the resulting graph (Fig. 7), almost perfect copying of the applied load is evident on the beam. One cycle is shown in Fig. 7. The red color shows the mechanical stress calculated in the Matlab program and the blue color indicates the magnetic field of the magnetic field [1,3,4]. the model and applying maximal forces the maximal deformation and mechanical stress concentration area was obtained. From the point of view of the application of magnetic microwires as stress sensors, the results of the overall wing stresses are more important for us. Based on the Von Mises stresses, it will be possible to determine the appropriate location for the application of the microwires and the subsequent measurement of the mechanical stress using these sensors.
Maximal stress is concentrated in the lower part of composite beam. It would be suitable to implement sensor such as magnetic microwire into this area in order to monitor maximal stress inside composite beam between particular composite layers.
The mechanical stress of the wing is evident from Fig. 8  Failure Strength is the level of stress at which the material starts to deform plastically. After failure determination method selection (Distortion Energy -von Mises) and entering the cutoff stress limit for the method (Tensile Yield Stress = 400 MPa) it is possible to plot a Failure Index measure with a fringe plot based on the simulation results. The calculated stresses are compared to the cutoff stresses and the index is plotted. Less than 1 -material has not yielded. In this case the Failure Index is 0,8 max [5]. The BLF (Buckling Load Factor) is the magnification factor by which the loads applied in a previously specified static analysis would have to be multiplied to produce the critical buckling load (BLF ≥ 1 means that model has not buckled). BLF = 54 [5].
According to the results of composite wing hinge analysis that involves the static stress, displacement and buckling analysis shown in Figure 4 - [5].

CONCLUSION
The presented paper summarizes the study of the dependence of mechanical stress, mechanical properties and sensing the stress of composite structures by means of magnetic micro-wires. The aim of the article is to inform the reader about the research in the area of the possibility of monitoring modern aircraft composite structures and the necessary steps in the application of new sensors based on magnetic micro-wires.
In the introductory chapter is described the issue of obtaining the material properties of composite materials, whose output is numerical values determined for strength calculations. The obtained material properties are used in strength calculations, the results of which are summarized in the second chapter. The purpose of strength calculations of composite beam is predicting the stress of a simple aircraft structure. The resulting stress can then be compared to the experimental measurement using magnetic microwires. The output dependence of the mechanical stress and the switching field of the magnetic microwire can later be verified on the basis of the FEM calculation of the composite beam.
The next chapter shows the dependence of mechanical stress and switching a magnetic microwire array that is used as a sensor to monitor structural strength. Based on the results, it can be stated that the sensor is a suitable tool for increasing the safety of aircraft structures. This is a relatively new and very promising monitoring area, as it allows non-contact monitoring of the structure inside the material, without affecting the structure of the composite material. The last chapter discusses the possibilities of application of magnetic microwires in the aircraft industry.