Investigation of the stress–strain state of a composite blade in ANSYS WorkBench

Filippova Kseniya Anatol'evna Senior Lecturer; Department of Aircraft and Helicopter Engineering; East Siberian State University of Technology and Management 670013, Russia, Republic of Buryatia, Ulan-Ude, Klyuchevskaya str., 40 V ipq84@mail.ru Ayusheev Tumen Vladimirovich Doctor of Technical Science Associate Professor; Department of Engineering and Computer Graphics; East Siberian State University of Technology and Management 670013, Russia, Buryatia region, Ulan-Ude, Klyuchevskaya str., 40 V atv62@bk.ru Damdinova Tatiana Tsybikovna ORCID: 0000-0002-3597-3262 PhD in Technical Science Associate Professor, East-Siberian State University of Technology and Management 670000, Russia, Republic of Buryatia, Ulan-Ude, Klyuchevskaya str., 40 V dtatyanac@mail.ru Other publications by this author

Tsidipov Tsybik Tsirendorzhievich PhD in Technical Science Associate Professor; Department of Aircraft and Helicopter Engineering; East Siberian State University of Technology and Management 670013, Russia, Buryatia region, Ulan-Ude, Klyuchevskaya str., 40 V sssibik@mail.ru

Introduction

Composite materials have an advantage over traditional materials (metals and alloys) in the field of aviation - gain in weight, low sensitivity to damage, high rigidity, high mechanical characteristics. At the same time, the identification of vulnerabilities in a layered structure is a difficult task and in practice is solved with the help of destructive control. Modern software products, such as ANSYS WorkBench, allow comprehensive investigation of the layered structure.

The topic of this work is relevant, the computing power of modern computers makes it possible to conduct research on increasingly complex parts and systems, analyze and select the composition of composite materials and structures of the propeller blade. In the study ^[1], a helicopter blade is considered, which has a different blade design from the one under consideration, in the work ^[2], a blade design containing a spar in the form of a multilayer fiberglass pipe and a honeycomb filler is considered. In the study ^[3], bench tests were carried out to determine the destructive load, the peculiarities of the nature of the destruction of the butt part were revealed, which is confirmed by the results of numerical research in this paper, given in the conclusions.

Research objectives

1.        To assess the stress-strain state (VAT) of a composite blade with a classical assembly: the middle material is balsa, layers of unidirectional carbon fiber prepreg are laid on top at an angle of 0 ⁰, carbon fiber at an angle of -45 ⁰, carbon fiber at an angle of 45 ⁰.

2. Consider and calculate the VAT of blades with other types of median material: pine, aspen, and polyurethane foam.

3. Consider and calculate the VAT of a blade with a hybrid composite material: unidirectional fiberglass prepreg at an angle of 0 ⁰, carbon fiber at an angle of -45 ⁰, carbon fiber at an angle of 45 ⁰.

4.        To assess the safety margin according to the Tsai–Hill destruction criterion.

Materials and methods

 The UAV blade under study is a sandwich structure, inside of which there is a middle material - balsa, layers are laid from bottom to top on the upper part of the balsa: a layer of unidirectional carbon fiber at an angle of 0 ⁰ and two layers of woven carbon fiber at angles of -45 ⁰, 45 ⁰. On the lower surface of the balsa, the laying is performed similarly. The thickness of the composite woven layer is 0.16 mm, the thickness of the composite unidirectional layer is 0.08. The maximum length of the sample is 304.81 mm, the width of the sample varies from 15.26 mm to 52 mm. The mounting of the blade is cantilever. The centrifugal force of the Fsb and the lifting force of the Fpod act on the test sample.

The load calculation was carried out as follows:

Fcb=ml*?2*R,

Fpod=mvzl*k,

where the blade mass is ml = 0.06 kg,

the angular velocity of the blade ? = 6000 rpm,

the radius of rotation of the screw R = 300 mm,

take-off weight of the UAV mvzl = 25 kg,

The stock ratio is k = 1.5.

Based on the results of the calculation, the following data were obtained:

Fcb = 3553.058 N;

Fpod = 367.5 5 N.

Composite materials available in the ANSYS materials library were used for modeling: Epoxy Carbon Woven (230 Gpa) Prepreg woven carbon fiber in the form of a semi–finished prepreg impregnated with epoxy resin carbon fiber with Young's modulus E=230 GPa and Epoxy Carbon (230 Gpa) Prepreg is a unidirectional carbon fiber prepreg impregnated with epoxy resin with a Young's modulus E=230 GPa. The characteristics of balsa, pine, aspen and polyurethane foam are not available in the library of materials, so the characteristics ^[4] were added manually, such as density, axial elastic modulus, Poisson coefficients, shear modulus and tensile and compressive strength limits (Tables 1, 2)

Table 1 – Characteristics of wood materials

Table 2 – Characteristics of polyurethane foam

A model consisting of a blade and a balsa in the Parasolid format, built in the NX software package, is imported into ANSYS Workbench. The imported model is shown in Figure 1.

Figure 1 – Imported blade and balsa model

The model is divided by a grid generator into a grid of finite elements, the element size of 1 mm is selected as a parameter.

The laying of the composite material is set from top to bottom (Top - Down), the layers are modeled according to the following orientations at the corners of the laying: 45 ⁰, -45 ⁰, 0 ⁰( fig. 2).

Figure 2 – Layout of the composite blade

Boundary conditions are set for the butt part (due to symmetry, we study only half of the blade) - cantilever sealing (prohibition of movement along all axes). The centrifugal force Ftb is applied to the center of mass. The lifting force F is uniformly set along the lower surface of the blade along the line of the ? chord of the blade.

By analogy, blade variants with other median materials are modeled and studied: pine, aspen, polyurethane foam, and a blade variant with a unidirectional fiberglass prepreg instead of a unidirectional carbon fiber prepreg oriented at an angle of 0 ⁰ is considered.

Results

The obtained results of the study of VAT of carbon fiber blades are presented in Table 3, VAT of glass-carbon fiber blades in Table 4, visualization of the study of VAT of the blade is shown in Figure 3. The maximum deformations of the blade are shown as a histogram for comparative analysis (Fig. 4).

 Table 3 - Calculation of the VAT of a carbon fiber blade: total deformation, deformations along the X, Y, Z axes

Table 4 - Calculation of VAT of a glass-carbon fiber blade: complete deformation, deformations along the X, Y, Z axes

            Figure 3 - Deformation (mm) of the carbon fiber blade (middle body – balsa).

1 - Initial position of the blade, 2 - Deformed blade

An essential feature of the assessment of the bearing capacity of composite structures is the layered VAT analysis of the composite structure, which consists in analyzing the VAT of each layer. In this work, the analysis was performed in layers: the maximum and minimum stresses were determined, and a mapping of vulnerabilities in each layer was obtained.

Figure 4 - Maximum blade deformations

The results of the layered study for the carbon fiber blade are presented in Table 5 and Figures 5-7, for the glass-carbon fiber blade in Table 6. For convenience, the layer encoding was used: layer 1 – layer oriented at an angle of 45 ⁰, layer 2 – layer oriented at an angle of -45 ⁰, layer 3 – layer oriented at the angle is 0 ⁰.

 Table 5 – Maximum and minimum stresses in layers for a carbon fiber blade

Table 6 – Maximum and minimum stresses in layers for a glass-carbon fiber blade

            Figure 5 – Stress pattern (MPa) in layer 1 (at an angle of 45 ⁰) of a carbon fiber blade (middle body – balsa)

            Figure 6 – Stress pattern (MPa) in layer 2 (at an angle of -45 ⁰) of a carbon fiber blade (middle body – balsa)

            Figure 7 – Stress pattern (MPa) in layer 3 (at an angle of 0 ⁰) of a carbon fiber blade (middle body – balsa)

To assess the strength, the inverse safety factor was used, which shows how many times the acting stresses are less than the permissible ones ^[5].

Destruction occurs when IRF>1.

,

where RF is the safety factor, it is defined as the ratio of the destructive load to the applied load.

Quite a lot of strength criteria have been developed for composites ^[6,7] and each of them reflects certain features of destruction. The above safety factor is based on the Tsai-Hill criterion, which is an analytical approximation of the test results of a unidirectional layer under various types of loading. This criterion has the following form:

where ?1, ?2, ?12 are normal and tangential stresses, 

 ?1, ?2, ?12 are their limit values.

Table 7 shows the inverse safety factors for a carbon fiber blade with different variants of the median material, Table 8 shows the inverse safety factors for a glass-carbon fiber blade with different variants of the median material (the value of the coefficient exceeding the permissible stresses is highlighted in italics, visualization of this value is shown in Figure 8).

Table 7 – IRF Summary table for carbon fiber blade layers with different variations of median materials

Table 8 – IRF summary table for layers of glass-carbon fiber blades with different variations of the middle materials

Figure 8 – Carbon fiber blade (the middle body is polyurethane foam).

IRF in the most stressed layer of carbon fiber (layer 3)

Conclusions

1.        The VAT assessment of a composite blade with a classic assembly was carried out: the middle material is balsa, layers of unidirectional carbon fiber prepreg are laid on top at an angle of 0 ⁰, carbon fiber at an angle of -45 ⁰, carbon fiber at an angle of 45 ⁰.

The middle material, balsa, is the best option in terms of maximum deformations, deformations along the x, y, z axes.

2. Calculated the VAT of the blade with other types of median material: pine, aspen, polyurethane foam.

Among the considered medium materials, polyurethane foam is the worst option. The indicators for the maximum deformation of the polyurethane foam blade exceed the same indicator of the balsa blade by 2.4 times.

The difference in deformations between a blade with balsa and a blade with aspen or pine as the median material is no more than 15%, which is a satisfactory result, given the cost and inaccessibility of balsa material, unlike common pine and aspen.

3. Calculated the VAT of the blade with a hybrid composite material: unidirectional fiberglass prepreg at an angle of 0 ⁰, carbon fiber at an angle of -45 ⁰, carbon fiber at an angle of 45 ⁰.

The composite material works in layers and stretches. and for compression. The best option for the middle material is balsa, however, the deterioration of mechanical characteristics (tensile stress, compression) in layers for pine and aspen materials does not exceed 20%, which is a satisfactory result, given the cost and inaccessibility of balsa material, unlike common pine and aspen.

4.        The safety margin of all blade variants has been evaluated.

The analysis of the most stressed sections in layers revealed that the strength of the blade is not provided with the option: carbon fiber blade, the middle material is polyurethane foam. A section with an IRF = 1.38 coefficient appears in the layer at an angle of 00, which exceeds the permissible voltage by 38%.

The load that caused the destruction of fibers in any layer of the composite structure is considered to be the limit for the entire structure. This makes it possible to determine the maximum possible bearing capacity of the composite structure under study. Thus, the VAT assessment of a layered KM structure is based on the VAT assessment of a unidirectional layer. It should be taken into account that the obtained simulation results need experimental verification.

The study was supported by a grant from the Ministry of Education and Science of the Republic of Buryatia (Agreement No. 413 dated 12/21/2023).