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Apr 30, 2024

Assignment Task

Questions

1. The total length of a chain can be calculated by summing the lengths of all the chain links. A 5-link chain consists of 5 links of the following dimensions:

  • 8.34 ± 0.05 mm
  • 8.35 ± 0.05 mm
  • 8.28 ± 0.04 mm
  • 8.18 ± 0.01 mm
  • 8.23 ± 0.02 mm

2. Calculate the total chain length and the resultant uncertainty in the measurement. Give your answers in mm: the total length to 3 s.f. and the uncertainty to 2 s.f. The Young`s modulus of a material is calculated from measurements of stress and strain. The measurements are as follows:

  • σ = 580 ± 26 MPa;
  • ε= 0.0044
  • 0.00018.

3. Calculate the resultant uncertainty in the Young`s modulus measurement. Give your answer in GPa to 2 s.f. Resultant uncertainty:

4. GPaThe original length of a cable is 100.0 ± 0.004 m and the change in length is 4.7 ± 0.007 m. Calculate the resultant uncertainty in the strain on this cable. Give your answer as a percentage of 2 significant figures. Uncertainty in strain: %

5. Calculate the resultant uncertainty in stress for a material that has a Young`s Modulus of 155 ± 6 GPa and is experiencing an elastic strain of 0.00045 ± 0.000025.

An aircraft accident

A passenger aircraft with a pressurised fuselage suffered a serious accident in which part of the fuselage lost during explosive decompression. It had been used to make short, local flights between neighbouring islands and had therefore undergone a high number of flight cycles (~90,000). The fuselage of the aircraft was made from sheets of an aluminium alloy attached to a space frame (see Figure 1). The sheets were riveted together using aluminium rivets at numerous lap joints and the joints were strengthened by using an epoxy film as an adhesive to bond the sheets together.

After the accident, examination of the fuselage at intact places revealed that there was extensive debonding in epoxy layer at many places, suggesting poor control of bonding procedure during manufacture. Furthermore, there was significant corrosion of the aluminium alloy at debonded areas, with the presence of salty water. The examination also revealed extensive fatigue crack formation in

6. Rivet holes contain cracks introduced during manufacturing. Rivet holes are subject to higher stresses because they are the sites where the loads are transferred between outer and inner skins through the rivets. Rivets holes are stress raisers. Rivets holes are susceptible to galvanic corrosion, which promotes crack initiation under cyclic loading. There are tensile residual stresses at the rivet hole, introduced by the manufacturing process.

7. Why did fatigue cracks formed and 90° and 270° from vertical and grew along the length of the fuselage (see figure 1c)?

8. Epoxy film near the top edge of the inner skin degraded due to the exposure to salty water, causing debonding near the edge, hence increasing the loading and promoting crack growth parallel to the edge.

9. The stringers in the space frame of the structure provide reinforcement in the horizontal direction, hence the stresses in the vertical direction are larger, so cracks initiate at a direction perpendicular to the maximum stresses.

10. The distance between rivet holes is greater in vertical direction than the horizontal rows, so the stress is therefore larger in horizontal direction, and hence easier for cracks to initiate.The aircraft fuselage is a pressurized cylindrical pressure vessel, so the hoops stresses are twice of those longitudinal stresses. The cracks will grow in a direction which is 90 degrees to the maximum stress.

11. The plate material is suspected to contain tensile residual stresses in the same direction of the tensile stress of 90MPa that the plate is subjected to. What magnitude of residual stress would cause the plate to fail (i.e. the reserve factor is reduced to 1.0) and what would be the failure mode?

  • 54MPa, brittle fracture
  • 54MPa, plastic collapse 68MPa, brittle fracture
  • 68MPa, plastic collapse
  • 82MPa, brittle fracture
  • 82MPa, plastic collapse

12. The following questions are about the Boston Tunnel collapse in Chapter 14 of your module material. The collapse occurred when ceiling panels fell from the roof of the tunnel.

13. Which of the following statement(s) about the disaster are true?

  • The epoxy resin grout had suffered some degradation
  • Creep was the only reason for the roof collapse
  • Driver errors contributed to the collapse
  • Substantial yielding of the metal ceiling anchors was the primary cause of failure Epoxy resins are a thermoplastic polymer
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