# Safe and Sustainable Design of Composite Smart Poles for Wireless Technologies

^{1}

^{2}

^{3}

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^{5}

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## Abstract

**:**

## 1. Introduction

#### 1.1. Wireless Outdoor Platforms

#### 1.2. Design Drivers for a Smart Pole Structure

#### 1.3. The Advantages and Sustainability of Composite Materials

## 2. Materials and Methods

#### 2.1. Pole Structure

#### 2.2. Impact Dynamics of the Shaft GFRP

#### 2.3. Finite Element Modelling and Mechanical Analysis

^{2}and 20 °C. The nominal element size (edge length) considered in the analysis was 5 mm, and each of the static analyses was roughly constituted by 250,000 elements.

#### 2.4. Experimental-Numerical CTE Determination

^{®}) with a Young’s modulus value of 4.0 GPa and Poisson’s ratio of 0.3 [32]. A CTE value of 1.17 × 10

^{−5}1/°C was used for the gauge based on the adoptable thermal expansion given by the manufacturer (i.e., the gauge follows corresponding expansion in terms of strain reading (zero)). The gauge was meshed by using parabolic (C3D20R) elements. The gauge model was attached to the coupon model by so-called tie-constraints. The model was run by a point load and thermal field ($\Delta T=36$ °C) in two steps.

#### 2.5. Signal Attenuation

#### 2.6. Heat Exchangers

#### 2.7. Computational Fluid Dynamics

## 3. Results

#### 3.1. Impact Dynamics of the Selected GFRP Shaft

#### 3.2. Finite Element Analysis

#### 3.3. Signal Attenuation at the GHz-Regime

#### 3.4. Thermal Management and Multiple Radio Analysis

## 4. Discussion

#### 4.1. Design Process and Interconnections of Results

^{3}, the mass of the entire pole would be 28.8–29.6 kg (from circular to diamond-shaped cross-section, respectively). For a similar steel pole, the mass would increase 380% (steel pole mass 153 kg). Because 5GPs are more deformation (sway) critical than strength critical, a steel pole could be made six times thinner in thickness in theory. If a practical minimum wall thickness would be two millimeters to prevent instability, the steel pole continue to be 60% heavier (i.e., GFRP leads to a minimum of 37% weight saving). The significantly lower mass of the GFRP pole directly makes its handling easier and lowers transportation emissions for a global 5GP usage.

#### 4.2. Assembly and Future Applications

## 5. Conclusions

- A full-composite glass fibre reinforced 5G pole was FE-modelled and analysed against standard wind and thermal load. The findings showed that a mechanically safe and functional (stiff) GFRP shaft results in significant weight savings (37–80%) compared to traditional steel shafts;
- RF signal attenuation at a GHz-regime (2.45 GHz) was found to increase significantly (90–175%) due to any paint layer while long-term UV degradation in the polymer structure led only to a nominal decrease of attenuation in terms of dielectric loss;
- Entirely integral one-piece heat exchangers were designed with CFD analysis of the fluid-solid interaction for heat transfer, and printed. It was found that a parallel liquid cooling of four radio units is rather insensitive to the flow rate (range 2…6 L/min) and as high as ≈60 °C inlet temperature can be allowed to keep the device surfaces at or below a critical 65 °C.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## Appendix A. Thermal Expansion of the GFRP Composite

^{−5}). For the combination of mechanical load and thermal load, the target deviation (error) was kept the same. The experimental strain developed slowly per temperature step, and an extrapolated value of ${\epsilon}_{g,residual}$ = −1.6 × 10

^{−4}was selected as the target. Finally, an iterated solution was met by using the values of CTE

_{axial}1.15 × 10

^{−5}1/°C and CTE

_{transverse}= 2.5 × 10

^{−5}1/°C (‘axial’ corresponding to the ‘3’ direction and ‘transverse’ corresponding to the ‘2’ and ‘1’ directions in the FEA). This iteration was deemed acceptable due to the fact that there are no non-linearities accounted for in the FEA that in reality might appear (e.g., at gauge-coupon interface or in the gauge). Additionally, Equation (1) does not take into account the deformation of the substrate (coupon) by the expansion of the gauge. These effects would lower the calculated strain with the selected CTE values [36]. It should be noted that the transverse CTE of GFRP affects the gauge strain (Equation (1)) due to the Poisson’s effect (in FEA). The iterated CTE values are in a good agreement with the literature values for GFRP [37,38].

**Figure A1.**The experimental and the finite element analysis results used for the determining of GFRP’s thermal expansion coefficients.

## Appendix B. Heat Exchanger Design

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**Figure 2.**A graph of the main analysis phases for an integral smart pole allowing current 5G services.

**Figure 3.**Testing of GFRP mechanics: (

**a**) the QS indentation of a half-circular specimen with a semi-rigid boundary; (

**b**) the drop-weight impact testing of a half-circular specimen with an ‘open’ boundary, and; (

**c**) the drop-weight impact testing of a tubular pole specimen.

**Figure 5.**Experimental arrangements and the finite element model of the test coupon to determine thermal expansion coefficients.

**Figure 6.**Attenuation measurements of radome/pole structure’s material samples representing signal windows; radome and measurement setup by using the SPDR method.

**Figure 7.**The cooling design for an integral device configuration in the smart pole(s): the estimated input data for thermal analysis of a heat exchanger by using CFD.

**Figure 8.**The mechanical testing of a panel specimen: the DIC-resolved displacement field during the indentation loading of the half-circular specimen at the moment of maximum load.

**Figure 9.**The performance of the GFRP laminate and tube: the contact force-displacement response based on experiments and FEA (linear material [27]). The inset graph on the right shows the detailed data at the beginning of the tests/analyses and predicted displacement level for damage onset.

**Figure 10.**FEA for the failure index when comparing two different pole shaft laminates and a diamond-shaped cross-section (5 mm and 8 mm-wall thickness); corners (hotspot) at a root cutout are shown.

**Figure 11.**FEA for the deformation when the pole shaft is subjected to the standard wind load (

**left side image**) and a thermal load (

**right side image**). Details of the highest deformation (pole top) are shown on the right.

**Figure 12.**Measured attenuation in terms of DL; results over the 72-day accelerated UV-ageing period.

**Figure 13.**CFD analysis of a 3D printed heat exchanger and the cooling strategy: (

**a**) simulated heat exchanger (outside) surface temperature (color range represents $\Delta T=10$ °C) when water flows into and out from a 3D printed heat exchanger via embedded rows of channels; (

**b**) the highest allowed inlet temperature as a function of mass flow rate to maintain below 65 °C surface temperatures per cooling strategy.

Constant | Parameter | Value (Units) |
---|---|---|

Axial Young’s modulus | ${E}_{11}$ | 35 GPa |

Transverse Young’s modulus | ${E}_{22}$ | 7.0 GPa |

Out-of-plane Young’s modulus | ${E}_{33}$ | 4.5 GPa |

Poisson’s ratios | ${\nu}_{12},{\nu}_{23}$ | 0.3 |

Poisson’s ratio | ${\nu}_{13}$ | 0.05 |

Shear modulus | ${G}_{12}$ | 3.6 GPa |

Shear modulus | ${G}_{13}$ | 3.6 GPa |

Shear modulus | ${G}_{23}$ | 2.0 GPa |

**Table 2.**Input selections used for the CFD analysis of heat management with 3D printed heat exchangers.

Fluid Domain | Solid Domain |
---|---|

Turbulence model used: k-$\omega $ SST | |

Inlet: | Heated surface: |

Velocity 0.4 m/s | Fixed temperature gradient |

Turbulent intensity 4.00% | Zero pressure gradient |

k = 0.00038 m^{2}/s^{2}, ω = 51.57 1/s | Fluid-solid walls: |

Temperature 300 K | Temperature calculated |

Pressure with zero gradient | Zero pressure gradient |

Walls: | Outer surfaces: |

Velocity—no slip | Zero temperature gradient (adiabatic) |

Turbulent variables—wall functions | Zero pressure gradient |

Zero pressure gradient | |

Temperature calculated | |

Outlet: | |

Zero velocity gradient | |

Zero gradient of turbulent variables | |

Fixed pressure (atm pressure) | |

Zero temperature gradient |

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**MDPI and ACS Style**

Di Vito, D.; Kanerva, M.; Järveläinen, J.; Laitinen, A.; Pärnänen, T.; Saari, K.; Kukko, K.; Hämmäinen, H.; Vuorinen, V.
Safe and Sustainable Design of Composite Smart Poles for Wireless Technologies. *Appl. Sci.* **2020**, *10*, 7594.
https://doi.org/10.3390/app10217594

**AMA Style**

Di Vito D, Kanerva M, Järveläinen J, Laitinen A, Pärnänen T, Saari K, Kukko K, Hämmäinen H, Vuorinen V.
Safe and Sustainable Design of Composite Smart Poles for Wireless Technologies. *Applied Sciences*. 2020; 10(21):7594.
https://doi.org/10.3390/app10217594

**Chicago/Turabian Style**

Di Vito, Donato, Mikko Kanerva, Jan Järveläinen, Alpo Laitinen, Tuomas Pärnänen, Kari Saari, Kirsi Kukko, Heikki Hämmäinen, and Ville Vuorinen.
2020. "Safe and Sustainable Design of Composite Smart Poles for Wireless Technologies" *Applied Sciences* 10, no. 21: 7594.
https://doi.org/10.3390/app10217594