Lightweight Wheel Disc with Carbon Aluminium Foam Sandwich
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KeywordsAluminium Foam Metal Foam Foam Core Metallic Foam Technology Demonstrator
Harmful emissions from motor vehicles can be lowered by combining new process technologies with smart material systems a direct optimisation of technical systems. In the automotive application as a wheel a direct improvement in driving characteristics is also achieved by reducing the unsprung weight. Through the focused utilisation of material-side benefits in the composite material, the lightweight construction wheel puts a symmetric sandwich with aluminium foam core and cover layers made of thermoplastic fibre-reinforced plastics as a structural element in application.
Configuration and Preparation of the Composite Sandwich Structure
For the wheel disc of a car wheel has a sandwich built-up in this way a very high performance. As a bending claimed structural element it is a central part of the component stiffness and also offers excellent potential for lightweight construction. For the production of this component, the boundary conditions in the manufacturing process and a precise knowledge of the material behaviour of the individual materials and the combination of materials are required. Therefore, the structure of the material composite and the technology have been investigated for the preparation and then verified with respect to the automotive application.
The stage thermoformed composite core for the hub of the three-piece lightweight wheel is manufactured near net shape. The semi-finished thermoplastics of the top layers are thereby pressed in one process step to the closed cell core. A special feature is the combination of measures, to counteract adhesion failures before and during manufacturing process. Initially, the aluminium foam core with a very thin and closed outer side, the interface to the outer layers, allows a chemical or mechanical modification to improve the bond strength. In the use of carbon fibres as a reinforcement of the top layers further an additional buffering layer is applied on the interface with glass fibre reinforcement, which harmonises the high gradient of the mechanical characteristics of all the individual materials. Nevertheless while the production with the thermal pressing method, particularly in the cooling phases high residual stresses occurs. By a drastic increase in pressure, these are intensified, which further contributes as the pretension to an improvement in performance.
Application as a Wheel Disc
Simplified Simulation of Circular Bending
The civil use and their regulatory handling of wheels for passenger cars on public roads are governed by various national and international regulations. As safety-related component of the suspension geometry a wheel must bear especially the abuse load cases and need to have also in the subsequently caused damage a residual functionality. The load cases are based on scenarios such as the larger potholes or the curb driving and the impact of strong load cycles at the maximum load. The inspections to be carried are specified for the various wheel concepts. Multipiece wheels with demountable rim have in addition to material testing, and a corrosion test, a circular bending test, a rolling test, an impact test and an Alternating to be subjected.
The wheel was initially analysed for the three directions of load at maximum load.
While the rolling and the impact tests at today’s relevant environments are performed with mounted tires, the circular bending is in addition to the alternating torsion tests an important component test procedure for a wheel without a tire, in which the side forces are simulated, which act in a turn on the wheel.
As for the real circulating bending test according to ECE R124 the inner rim ring is firmly clamped at the horn. The load is applied in each case via a net-like modeled arm with an infinitely rigid hub connection.
By choosing a macroscopic approach, the outer layers of the core network are set up as a shell element and are material-configured with eight unidirectional continuous fibre-reinforced individual layers, which are stacked with different fibre orientation over the other. The buffer layer which was integrated while the preliminary studies to minimise the manufacturing associated residual stresses will neglect and a layer structure with each at 45° to each other twisted fibre orientations implemented. Because the area around the hub terminal near the clamping point can be expected with the occurrence of high stresses, three more individual layers are additionally provided to simulate a local reinforcement and thus corresponding to a load-oriented and graded structure at this location.
Results of the Simulation
As for the real circulating bending test according to ECE R124 the inner rim ring is firmly clamped at the horn.
Manufacturing of a Technology Demonstrator
Summary and Outlook
Both the preliminary investigations and the performed work at the core composites in preparation for an application as a wheel disk of a wheel for an automotive application have demonstrated the enormous potential property in view of the use as claimed bending, plate-shaped component. Especially the high energy absorption of this construction for use as a design agent for ostensibly bending and impact stress components made of composite materials designates. In addition, the different versions of the interface modification show a direct influence on the subsequent failure image, while the production is essential for assignment to a possible manufacturing strategy as a mixed and hybrid composite. Especially for lightweight applications in the automotive mass production are multi-material designs of high interest wherein the compound of thermoplastic fibre-reinforced plastics with metal has particularly for high volume applications special lightweight construction potential. The combination as a core composite in hybrid and mixed construction was examined for the first time on a series suitability and thereby verified as a sandwich structure with a functionally graded structure and a new manufacturing technology.
As a result, the development of the core composite structure for a lightweight construction wheel in the described design could protect legally registered as utility models (DE 20 2014 005 111 U1) and a patent (DE 10 2014 009 180 A1) by the German Patent and Trademark Office. The application for protection refers wheels with an inner and an outer rim and a center wheel disk, which are interconnected. The wheels are characterised especially by their low weight. The center wheel disk in a symmetrical sandwich construction material is a composite of a closed metal foam core between layers of thermoplastic fibre-reinforced composites.
Circular Bending Test According to ECE R124
For the circular bending test according to ECE R124 the inner rim is clamped to a base plate of a testing device, while a revolving bending moment is introduced through a load beam in a defined length at the hub of the wheel connection surface. The torque is generated by a tumbling weight around the rotational axis. For a test to obtain an authorisation on the public a total of four wheels must be examined. The test provides different scenarios for aluminium wheels of the vehicle class M1. For the short-term test, the wheel is stressed with 75 % of the maximum bending moment in 2·105 load cycles games, while the long-term test, only 50 % of the maximum bending moment at 18·105 load cycles stipulates whose frequency should be as high as possible, but outside the complex resonance frequency. The wheel should not show a technical incipient crack and is allowed to have increased by less than 10 % load beam displacement against the determined shift after about 10.000 cycles. The allowed reduction of the tightening torque applied to the wheel studs shall not exceed 30 % [ECE R124].
This work was performed within the Federal Cluster of Excellence EXC 1075 “MERGE Technologies for Multifunctional Lightweight Structures” and supported by the German Research Foundation (DFG). Financial support is gratefully acknowledged.
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