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School of Engineering/Aerospace Engineering and Aviation May 2020 RMIT Classification: Trusted — AERO2566 Aerospace Materials Assignment 2 Family Name: Given Name(s): Student #: ALLOWABLE MATERIALS AND INSTRUCTIONS TO CANDIDATES 1. Write your full name and student number on the first page of this paper. 2. Answer all the questions within the provided space. 3. You have until Wednesday 13th of May 11:59 pm to attempt the questions and submit the assignment. Please submit a PDF copy of the assignment. 4. Show all equations and workings. Marks will be deducted for missing or incorrect units. 5. Phrases in bold are for emphasis. 6. The maximum possible score is 20 marks and this assessment will contribute 20% to the final course mark. Page 2 of 8 RMIT Classification: Trusted Q1: You are in the process of selecting materials for the design of a load-carrying structure. Upon contacting an established supplier, they are unable to provide you with the yield strength of a specific metallic alloy product, A which they have in stock. The supplier explains that, while the new alloy has the same composition as their previous products (B and C), some stages of the metallurgical processes have recently been changed. The supplier still has the mechanical and material data for the discontinued products, B and C. Alloy B has a yield strength of 135 MPa and an average grain particle diameter of 0.005 cm, respectively. On the other hand, alloy C has a yield strength of 260 MPa and an average grain particle diameter of 0.008 mm, respectively. The supplier goes on to explain that even though they have not measured the yield strength of the new material, A, they have determined that its grain particle size is half that of product B. (a) Given the above information, determine the yield strength of material A. [3 marks] Page 3 of 8 RMIT Classification: Trusted (b) With the aid of diagrams, describe two separate mechanisms that explains the higher yield strength for alloy C compared to alloy B. [2 marks] • . • Page 4 of 8 RMIT Classification: Trusted Q2: You are an aerospace engineer designing a tie cable system with a rectangular cross-sectional area for attaching heavy loads to a composite structure. One end of the tie cable is fixed to the composite panel using bolts and nuts while the loads are suspended from the free end. You ordered an age-hardened aluminium alloy of length 2 m, width 65 mm and thickness 50 mm. Before assembly, you evaluated the performance of the tie cable and observed that the cable could lift 1000 N and elongate by 5 mm before failing (e.g. tie cable snapping into two pieces). However, when integrating the tie cable onto the composite structure, you discovered that you could only manage a tie cable thickness of 48 mm. To reduce, the thickness of the aluminium tie cable, you repeatedly rolled it between pressure drums. After a few iterations, you managed to achieve the desired 48 mm thickness. To your surprise, the tie cable could now lift loads greater than 1000 N despite the elongation-to-failure being lower than 5 mm. (a) Sketch the stress-strain curves (on the same axes) that describe the mechanical behaviours of the aluminium tie cable before and after the thickness was reduced to 48 mm. You can use PowerPoint or other free hand drawing software to sketch the stress-strain curves before copying and pasting as a picture in this Word document. [1 mark] Page 5 of 8 RMIT Classification: Trusted (b) Using bullet points, explain the mechanisms supporting the highlighted differences in the stress-strain behaviours of the 48 and 50 mm-thick alloys. [2 marks] (c) To further improve the bonding between the aluminium tie cable and the composite structure, the aluminium alloy base was coated using silver nanowires and annealed at 375°C for 2 hours before being adhesively bonded to the composite structure. It was, however, found that the cable failed at a relatively lower strength than the as-received alloy. Also, the tie cable consistently failed closer to the annealed section. Using bullet points explain this observation. [2 marks] Page 6 of 8 RMIT Classification: Trusted Q3: You are designing the sub-floor of an aircraft using carbon fibre composite beams. The carbon fibre supplier asks you what carbon fibre processing conditions they should use in manufacturing your fibres in reference to the chart below. (a) With a sound justification, nominate the processing temperature for the carbon fibres that are suitable for your design. [1 mark] (b) Upon receiving the carbon fibres, filaments were randomly extracted from fibre bundles and mechanically tested under tensile loading. The failure strength of the filaments ranged between 2400 and 2800 MPa. Explain the likely source of the large scatter in your test results. [2 marks] (c) The carbon fibre supplier offers to supply carbon fibres pre-impregnated (pre-preg) with the polymer resin. Two pre-preg carbon fibre polymer tapes are shipped to your company: one a thermoplastic-based polyether ether ketone (PEEK) and another a thermosetting epoxy system. In a rush to go for the long Christmas break, the technician forgets to stow away the shipment in a freezer. Upon manufacturing the two sets of composite panels, the carbon/PEEK system achieved the expected mechanical properties while the carbon/epoxy composite failed at a much lower strength than previously measured by the supplier. Using bullet points, explain this observation. [2 marks] Page 7 of 8 RMIT Classification: Trusted Q4: You are the Head of the Formula-SAE racing program at a university. One day you walk into the workshop to find first year team members enthusiastically making the chassis of the racing car using chopped strand mat. (a) How would you explain to these junior team members, in simple terms, that the resultant composite will not be fit for purpose? [2 marks] (b) Provide an alternative fibre-reinforced polymer composite system (that is not based on the chopped strand mat) that would meet the design requirements of a racing car. Explain why this system will perform as desired under in-service loads. [1 mark] (c) A team member suggests the use of a comparative vacuum system to monitor the structural integrity. How would that member explain to others how the technology can be used to locate and estimate the extent of in-service damage in the composite chassis structure. [2 marks] Page 8 of 8 RMIT Classification: Trusted — Title to go here 11 Micromechanics and Failure of Aerospace Composites Course learning outcomes: Evaluate the properties, performance and applicable domain of typical aerospace materials; Describe the main damage mechanisms affecting different types of aerospace materials and propose adequate mitigation actions; Critically discuss the influence of different materials in the initial and continuous airworthiness of aircraft, observing the applicable professional practices and existing standards; and, Select the best fabrication processes to meet the design and operation requirements of aircraft structures and components. 2 Scope This topic will cover the following principles: Micromechanics of composites - Constituent materials – fibres and polymers. Orthotropic and transversely isotropic materials. Rules of mixtures. Factors affecting strength and stiffness including voids and resin rich regions. Behaviour of laminate under conditions of plane stress. Transformation of stresses and strains within a lamina. 3 Micromechanics of Laminated Composites www.altairhyperworks.com wiccwavesgroup.weebly.com 4 Deflection at the wing tip: ???????? = ????4 8???? where ?? = ??ℎ 3 12 Thus, ?? = ???????????????? ???????? = ???? (???? 4 8???? ) Above equation is reduced to ?? = 8???? ??3 Model assumptions Cantilever beam configuration Length, L = 20 m Depth, h = 0.1 m Width, b = 2 m Aerodynamic forces are uniformly distributed and represented by ?? Euler-Bernoulli beam theory is applied An effective design has high stiffness, K Design Significance Design aircraft wing for high stiffness under aerodynamic loads is the objective 5 Micromechanics of Laminated Composites Three basic levels for modelling composites are generally considered: Micromechanical modelling The matrix and reinforcement material are considered as separate entities. Ply-by-ply modelling The layers are modelled separately, but matrix and fibres are considered a continuum. Macroscopic modelling The composite is modelled as a single orthotropic material or a single fully anisotropic material 6 Modelling of Composites Properties of fibre-reinforced polymer composites can then be estimated using the hierarchical modelling approach: unit cell → ply → laminate → structure Hierarchy of micromechanics-based analysis for composite structures, Mouritz, Introduction to Aerospace Materials 2012 7 What is Micromechanics? “aimed at providing an understanding of the behaviour of composites … in terms of the properties and interactions of the fibres and resin matrix. Approximate models are used to simulate the microstructure of the composite and hence predict “average” properties (such as strength and Young’s modulus) in terms of the properties and behaviour of the constituents”. From: Composite Materials for Aircraft Structures | BC Hoskin, AA Baker | AAIA Publication | 1986| 8 What is Micromechanics? “aimed at providing an understanding of the