Manufacturing process and raw materials of bearing windshield

The windshield is an important part of the aircraft structure. In addition to providing a clear driving vision for the driver, it also needs to have certain strength and stiffness performance to meet the pressurization static load and impact dynamic load. At the same time, the windshield glass also needs to have a certain function of heating, anti-icing, and defogging. According to the application scenarios, the windshield can be divided into two types: bearing windshield and non-bearing windshield. Due to the requirement of weight, appearance, and security of the bearing windshield, it will become the mainstream design trend in the future. However, bearing windshield is composed of a wide variety of raw materials with different performances, the manufacturing process is complex, and the research and development verification tests are difficult. This paper introduces and summarizes the main difficulties and index requirements of the bearing windshield in terms of structural design, manufacturing process, and raw materials, and provides guidance and support for the subsequent airworthiness compliance verification of the bearing windshield.


Introduction
The aviation industry has entered a new period of rapid development.With the continuous development of various types of new design aircraft, the requirements for the shape of the nose are getting higher and higher, and the single area of the transparent parts of the windshield is getting larger and larger, which puts forward higher and higher requirements for the manufacture of the windshield [1].As a product integrating the function and structural strength of the system equipment, the windshield needs to have the function of anti-icing and defogging, and bearing the load of the body.In recent years, the bearing windshield has been widely used because of its outstanding advantages in aircraft weight reduction.It has been successfully applied to ERJ aircraft, CRJ aircraft, and some trunk large aircraft.
Aiming at the control and analysis of windshield heating systems and simulation research, Hu et al. proposed a research method of windshield heating system control based on airworthiness standards [2].Lin et al. proposed and established the numerical simulation technology of the windshield temperature control system according to the actual project [3].In addition, in terms of the strength of the windshield structure, Song et al. proposed and compared the bird impact problem of the windshield, the advantages, and disadvantages of the test cost, engineering method, and finite element analysis method, and gave a comparative opinion [4].He et al. proposed a process idea and a bird impact response analysis method for windshield bird impact verification [5].
On the basis of the above research results, this paper analyzes and demonstrates how to meet the performance indicators of windshield products from the source of manufacturing from the perspective 2. Load-bearing windshield structure According to the requirements of CCAR25 [6], the main windshield of the aircraft should have the function of anti-icing and demisting, the side windshield and the observation window should have the function of demisting, and the edge of the windshield should be connected with the surrounding window frame structure through bolts.According to the actual application scenarios, the windshield can be divided into bearing windshields and non-bearing windshields.Secondly, according to the connection form of the surrounding body structure, it can be further divided into three types: (non-bearing) pressure plate, non-bearing screw type, and bearing screw type [7].
Among them, the pressure plate or non-bearing screw-connected windshield structure, the windshield window frame is designed with a large opening, and the strength and stiffness of the window frame are relatively high.The windshield only bears its own local aerodynamic force and airtight load and does not participate in the transmission of the whole machine load.The bearing screw type, in addition to bearing its own local aerodynamic force and airtight load, also participates in the fuselage load transfer.The bearing windshield is mostly curved windshield glass, which transmits the load of the fuselage through the form of membrane tension, so that the force transmission path of the fuselage is continuous, and the weight of the surrounding window frame structure is effectively reduced [8].
According to the types of raw materials for manufacturing glass (transparent parts), the bearing windshield can be divided into three types: inorganic glass bearing windshield, organic glass bearing windshield, and composite bearing windshield.
The inorganic glass windshield is composed of tempered glass, an organic adhesive layer (PU, PVB adhesive), a heating system, FRP inserts, fasteners, and a metal plate, as shown in Fig. 1.The transparent area of the windshield adopts the structural form of three-layer toughened glass and organic adhesive layer bonding.The thickness of the toughened glass layer is constrained by static load, fatigue load, and bird impact conditions.In general, the outer glass does not participate in the fuselage force transmission, and the middle layer and the inner layer of glass are the main bearing layers.The organic glass windshield is composed of organic glass (PMMA), an organic adhesive layer (PU, PVB adhesive), bushing, metal plate, and fastener, as shown in Fig. 2. At present, the mainstream organic glass of the aircraft meets the requirements of MIL-P-25690B [9] material specifications.The organic glass is a directional stretch plate, and the raw material specification of the cast plate material is MIL-P-8184F [10].The transparent part of the glass adopts a two-layer organic glass laminated structure.The thickness of the organic glass is constrained by static load, fatigue load, and bird impact conditions.By directly drilling holes in the organic glass and installing bushings and bolts, the in-plane load of the windshield can be effectively transmitted [11].The composite load-bearing windshield is composed of tempered glass, organic glass, an organic adhesive layer (PU, PVB adhesive), bushing, metal plate, and fastener.Glass transparent parts can be constrained according to practical application scenarios, temperature, heating system layout, static load, fatigue load, and bird impact conditions, and the material type and thickness of each sandwich transparent part can be adjusted by test and analysis [12].This structure has the characteristics of the low density of organic glass, the lightweight of the windshield itself, and the strong bearing capacity of inorganic tempered glass.By adjusting the type and thickness of interlayer material of transparent parts, the windshield has the advantages of high bearing capacity and lightweight.The disadvantage is that the material of the windshield is complex, and the performance of different materials is different, which leads to the local thermal stress, deformation, and durability of the windshield after lamination, which affects the life of the product.The manufacturing process of composite materials is complex, and the cost of windshield products is high.
Compared with non-load-bearing windshields, the structure of load-bearing windshields is more complex.There are the following differences in design, manufacture, and verification: (1) The load-bearing windshield participates in the load transfer of the fuselage.In particular, the main bearing layer of the inorganic glass windshield is inorganic tempered glass, and the glass itself cannot be perforated.How to effectively transfer the fuselage load to the glass is the difficulty in the design of the connection area between the windshield and the window frame and the manufacture of the windshield.
(2) The windshield is composed of inorganic/organic glass, organic adhesive layer, glass fiber reinforced plastic, fastener, metal plate, and other structures.Different materials have different sensitivity to temperature.When verifying the windshield, it is necessary to consider the influence of temperature on the performance of the windshield material and the influence of temperature on the load transfer between the windshield and the aircraft structure.
(3) When verifying the performance of the windshield, it is necessary to consider the influence of manufacturing differences, material dispersion, and material performance attenuation on the windshield body and connection strength.It is necessary to consider simulating the fuselage load transmitted by the windshield in the verification test to show its structural compliance.
(4) Because the windshield has an independent temperature control system, in the actual flight use process, it is necessary to consider the heating process such as bird strike, and pressure relief test to verify the performance of the windshield.The simulation analysis also needs to carry out bird body mass, velocity, model verification, screening impact points, and response analysis [5].

Inorganic glass thermoforming
There are many thermoforming methods for inorganic glass, such as the vertical bending method, the horizontal self-weight bending method, and so on.Different glass bending methods have different requirements for mold.The vertical bending method requires a yin-yang mold (solid mold), and the horizontal self-weight hot bending method uses a frame mold.Various bending-forming methods have different process characteristics.The vertical bending method is to heat the glass to a softening point, quickly remove it from the furnace, and place it in a matching pair of pressing molds.The glass is pressed according to the shape of the mold [16].Its characteristic is that the bending shape of the glass is accurate, but the cost is high.Optical distortion will be caused by the clamp marks and the extended part around the clamp, which will affect the visual effect.The horizontal self-weight hot bending method uses a hollow mold.When the glass on the mold is heated to the softening point, it is bent down along the periphery of the frame mold by the self-weight of the glass.The glass does not contact any position outside the mold.The shape of the glass depends on the distribution of the temperature field and the degree of softening of the glass surface, as shown in Fig. 3.The main difficulty lies in how to make the inorganic glass shape surface meet the requirements through the pre-deformation design of the mold and the precision control of different processing methods, without defects and micro-cracks.

Organic glass thermoforming
Organic glass is the main material of load-bearing windshields.It is difficult to form organic glass due to its large shape area, large curvature, and large radian.Usually, the production of spherical organic glass requires a special injection mold, but the injected organic glass has poor performance, poor durability, and is easy to break.The problem of poor performance of organic glass by injection molding can be solved by using flat organic glass by casting.However, it is difficult to make the flat organic glass into a hemisphere shape.In the process of blow molding organic glass, it is necessary to clamp the upper-pressure plate and the edge of the lower mold to form a closed cavity, as shown in Fig. 4. In the process of glass forming, the edge is clamped, and the margin of the edge cannot be moved to the middle, which will cause the middle part of the glass to be overstretched and thinned, causing the top of the ball to be thin and the edge to be thick.The top compression is easy to cause rupture, and the optical quality of the glass is poor because of the poor thickness uniformity.When the upper mold presses the lower mold, it is easy to cause the glass surface to press out the mold pattern and affect the appearance of the glass.By pre-pressing the upper and lower molds first, the edge margin of the glass can be repeatedly slid into the middle to reduce the amount of stretching in the middle of the glass, and then blown with compressed air to avoid the pattern pressed out of the mold, so that the glass thickness uniformity and optical performance are better.Improving the processing technology can effectively control the generation and diffusion of crazing [14].

Inorganic glass tempering process
According to the performance requirements of the windshield product, it is necessary to have a certain stiffness performance, so that when subjected to static load and dynamic load, the overall deformation of the windshield is controlled within a certain range to achieve the purpose of matching the structural deformation of the window frame.Song et al. used Patran to study the response speed, acceleration, and displacement of each layer of the windshield under bird impact conditions.The results show that under the condition of a bird strike, the displacement and velocity response of the windshield is gradually attenuated from the outer layer to the inner layer, but the displacement of each layer of glass is still large [4].Therefore, it is necessary to strengthen the inorganic glass to improve the overall stiffness of the glass.The basic principle of inorganic glass toughening is to introduce compressive stress on the surface of the glass, which can be realized by physical and chemical methods, called the physical strengthening method and chemical strengthening method respectively.Physical tempering Physical tempering is a method of slowly heating the glass to around 700℃ until softening, then rapidly cooling to room temperature, as shown in Fig. 5. Due to the large curvature, lightweight, and thin single-layer thickness, the special-shaped curved glass cannot be physically tempered, and only chemical tempering technology can be used to improve the strength of the glass.Low-temperature chemical tempering is usually used to strengthen the glass.Low-temperature chemical tempering is when the ion exchange is lower than the glass transition point.The glass is immersed in a molten salt containing alkali ions with a larger radius than the alkali metal ions in the glass.Through ion exchange, the surface is squeezed by the radius difference between the two alkali metal ions to form a compressive stress layer on the surface [16].The thickness of the stress layer exceeds the maximum depth of the microcrack, and β-eucryptite with a small expansion coefficient is introduced on the surface to achieve the strengthening effect [17], as shown in Fig. 6.

Heating coating process
Windshield products are generally used between (inorganic/organic) glass interlayers, and the metal film layer is heated by electroplating to achieve an electric heating function.Because the windshield is composed of tempered glass and a PU layer, the thermal physical parameters of each material are different.Therefore, when there is inconsistent heating or a high-temperature concentration area in the heating process of the windshield, it is possible to cause a burst [3].At present, most transport aircraft use the electric heating method to heat the windshield, that is, by arranging conductive film or resistance wire between the windshield glass interlayer, the electric energy is converted into internal energy after energization, and the heat is transmitted to the inner and outer surfaces of the windshield through each layer of medium [2].However, with the increasing opening area of the windshield and the increasingly strict curvature of the aerodynamic shape, when the windshield adopts the curved surface special-shaped design, the ordinary uniform coating technology will cause the electric heating to be uneven and the local temperature to be too high, which may cause the windshield to be stratified or cracked.For organic glass materials, the vacuum degree is required during coating.There are MATMA-2023 Journal of Physics: Conference Series 2691 (2024) 012057 bubbles in the coating process, which will affect the light transmittance and electric heating uniformity of the glass [19].Gradient coating technology is to coat different thickness films on the same glass to meet the requirements of temperature uniformity and light transmittance.Gradient coating technology is adopted, and its heating uniformity can be less than 4℃, which meets the requirements of relevant standards (no more than 11℃), and solves the problem of electric heating uniformity of shaped glass, as shown in Fig. 7.

Multi-material lamination process
Multi-layer glass, film, thermistor, woven belt, glass fiber reinforced plastic, and other materials are selected for load-bearing windshield laminated glass to control the relative position through manual arrangement and laminated tooling in a reasonable order.At room temperature, the air between each layer of materials is discharged by an air pump on the bonding tooling to realize the arrangement of the heating and temperature control system and the fixed position between glass, film, glass fiber reinforced plastic, and other main materials.The concave surface of the glass is placed on a special hot-pressing mold and sent to the autoclave for hot pressing of glass.The autoclave is vacuuming, and the temperature in the autoclave is slowly increased in a vacuum environment, and the pressure is maintained to make the film melt.Keeping a certain time, the film gradually changes from a high elastic state to a viscous flow state.By continuing to heat up and pressure up, the glass becomes a transparent plate.After cooling and pressure relief, the optical quality and good agreement of the glass can be guaranteed, as shown in Fig. 8. Due to the complexity of the types of manufacturing materials and the large differences in the properties of the materials, the temperature, environmental sensitivity, and degree of influence are inconsistent, and strict requirements are placed on the selection and control of temperature during the processing.

Inorganic glass
The main raw materials required for windshield glass are inorganic glass and organic glass.Among them, the inorganic glass materials for the aviation industry can be divided into three types: aluminum lithium silicon glass, high aluminum silicon glass, and sodium calcium silicon glass.
Among them, Al-Li-Si glass is widely used because the depth of the stress layer can reach 300 μm ~850 μm [21] after chemical tempering, and the surface strength is low, about 400 MPa~500 MPa [22].The surface strength of high alumina silica glass is 600~1000 MPa, and the depth of the stress layer is about 30~50 μm, so it cannot meet the requirements of a load-bearing windshield participating in the load transfer of the whole structure.The surface stress of glass reinforced by the soda-lime-silicate glass is 400 ~700 MPa, and the depth of the stress layer is 8~14 μm [23], so the bird impact performance is poor.The weight of windshield products made of this material is large.

Organic glass
In the manufacture of windshield products, the organic glass production process mainly includes the organic glass molding process, organic glass drilling process, bearing bushing processing process, organic glass edge connection process, sealing process, organic/inorganic glass lamination process, etc.In the subsequent airworthiness compliance test verification work for organic glass windshield products, it is necessary to increase the static pressure load by 1.5 times the load amplification factor according to AC25.775 [24], to cover the material, process, and environmental factors.Product uncertainty is used to indicate the compliance of windshield products from CCAR25.775 clauses.

Adhesives and auxiliary materials
The auxiliary materials required for the manufacture of bearing windshields mainly include PU/PVB film, adhesive, and sealant, glass fiber/glass fiber reinforced plastic, metal internal and external pressure plate, and other materials.Among them, PU / PVB film is mainly used as the intermediate layer bonding material in the field of aviation transparent parts.The tensile strength of PU film is ≥45 MPa, the tear strength is ≥50 KN/m, the elongation at break is ≥400 %, and the bonding strength is ≥10 KN/m [25].
During the use of aircraft, polymer materials such as adhesives and sealants are affected by sunlight, humidity, heat, and other environmental factors.For laminated structures, the edges are prone to moisture erosion, layered degumming, discoloration, and other phenomena [26].Therefore, it is necessary to prevent edge layered degumming, seal at the edge to prevent water erosion and aging and improve the stability of windshield products.
Glass fiber or glass fiber reinforced plastic is mainly used in the edge of inorganic glass-bearing windshields.The inorganic glass and glass fiber/glass fiber reinforced plastic strip is bonded together by an adhesive.Drilling holes are drilled at the glass fiber reinforced plastic strip, and bushings and rivets are installed, connected with the inner and outer metal plates of the windshield, and finally connected with the window frame structure to form a complete force transmission path.

Metal parts
When the load-bearing windshield is installed on the window frame structure, it is generally installed directly on the window frame through the external pressure plate of the windshield, and the metal pressure plate and window frame are connected by fasteners to form a complete force transmission path.The inner layer of the bearing windshield adopts a metal inner pressure plate to improve the overall stiffness of the windshield and maintain the shape of the windshield, which is convenient for the installation, sealing, and anti-corrosion treatment of the nut end of the connecting windshield fastener.The metal pressure plate used for the bearing windshield is a typical aluminum alloy machining part.

Conclusion
(1) The difficulty of inorganic glass hot bending technology is to design the mold pre-deformation, and improve the processing control accuracy, so that the inorganic glass shape meets the requirements, with no defects and micro-cracks.
(2) The technical difficulty of organic glass thermoforming is to reduce the deformation and displacement of organic glass in the mold, so as to improve the uniformity of glass thickness and the optical properties of the product.
(3) The technical difficulty of inorganic glass tempering is to select different tempering methods according to the different thickness requirements of the product, so as to form a suitable stress layer depth and provide the strength performance that meets the requirements.
(4) The difficulty of heating coating technology is to improve the stability of the electric heating gradient coating process while meeting the large curvature and large area product characteristics of windshield products.
(5) The difficulty of multi-material laminated technology is that it is necessary to strictly control the temperature, time, and humidity in the manufacturing process due to the different sensitivity of different materials to temperature and environment.
(6) The inorganic glass required for the manufacture of load-bearing windshields, the main assessment indicator is the stress layer depth and surface strength.The stress layer depth is required to be more than 300μm, and the surface strength is required to be less than 500 MPa.(7) The main difficulty of airworthiness verification for the organic glass required for the manufacture of load-bearing windshields is that the performance of the material that meets the performance requirements of MIL-P-25690 B still meets the requirements after manufacturing.