PSI - Issue 17

C P Okeke et al. / Procedia Structural Integrity 17 (2019) 596–601

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C P Okeke et al / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Polymers used in automotive lamps, if mass produced, are injection moulded. For ultra-low volume production, however, the cost of tooling for these traditional manufacturing methods is often very high. Hence, alternative approaches such as Silicone Tool – Vacuum casting are used. The vacuum cast Polyurethane components of automotive lamps are achieved by mixing in a liquid form, a colorless Isocyanate group of compounds and black Polyol organic compound at a given ratio and then vacuum casting into a silicone mould. The vacuum casting process follows three stages, preparation, casting and post-processing, this is explained in detail by Zhang et al (2016). Apart from being cost effective and time saving, the vacuum casting process has also been shown to accurately produce a complex geometry with good dimensional accuracy. Thian et al (2008) used vacuum casting technique to produce both micro-moulds and micro-gears of 38micron thickness and 1mm in diameter. The results show that vacuum casting process is able to consistently produce efficient micro-parts that are dimensionally accurate with respect to the master pattern. Cao et al (2009), developed a rapid fabrication process by combining stereolithography prototype, silicone rubber mould and vacuum casting process for zinc alloy moulds. They concluded that the complicated zinc alloy moulds with detailed patterns surface can be produced directly without etching or engraving by using vacuum casting technique. Denoual et al (2006), proposed microfabrication technique based on vacuum casting which offered significant benefits including replicating the desired part with high dimensional accuracy. Clearly, vacuum casting provides tremendous benefits over the conventional manufacturing techniques such as injection moulding. However, the parts produced using vacuum casting technique, which involves low pressure and low temperature manufacturing process, may have inferior dynamic performance. This can be a significant shortcoming as automotive lamps, like other automotive components, are subjected to vibration loading during vehicle operation. In order to ensure that the lamps are robust enough to withstand the exposure to this vibration loading over their life cycle, durability tests are performed during the design. Designing a robust lamp assembly essentially requires a good understanding of the dynamic behavior and fatigue life of materials involved. In this paper, the dynamic response and fatigue life of vacuum cast polyurethane are assessed. The specimens for dynamic response and fatigue testing were cut-out from a vacuum cast polyurethane plate. An instrumented beam was mounted on a shaker table and using sine sweep base excitation the dynamic response was measured. Using measured acceleration, response transmissibility was calculated. The degree of nonlinear behavior was investigated by varying the input amplitude. The bending fatigue properties were measured using a 4-point bending based resonance test apparatus. The apparatus with the test specimen was mounted on the shaker to generate base input. The beam was then excited at the first natural frequency but at varying base acceleration to find the fatigue properties. A series of tests were carried out to obtain average performance parameters. Harmonic dynamic response analysis is significant in understanding the mechanical vibration performance of a system. In order to design a robust structure, engineers seek to have a good understanding of the performance of the materials involved in the design. Harmonic analysis reveals the system’s resonance frequency which is an important parameter in the system design. The experimental characterization of the harmonic response of a system is normally done by sine sweep – from low to high frequency, covering frequency of interest. The parameters of harmonic response are very crucial in measuring the fatigue life of materials using resonance fatigue test system. The ratio of theoretical calculated response acceleration to the transmissibility forms the fatigue input loading. In this study harmonic response of every specimen at each load level was experimentally characterised. The theory governing sinusoidal harmonic response is given by: [ ]{ ̈} + [ ]{ ̇} + [ ]{ } = { } = (1) where { } , { ̇} and { ̈} are nodal displacement, nodal velocity and nodal acceleration vectors respectively. [ ] , [ ] and [ ] are mass, damping and stiffness matrices and { } is the applied force. The load represents the sinusoidal harmonic load. 2. Harmonic dynamic response