Materials and Fatigue / Fracture Analysis

 

 

CONFIDENTIAL

 

 

EXAMINATION PAPER

 

 

 

 

CODE:         MECHGM03 / MECHM004

 

TITLE:              Materials and Fatigue / Fracture Analysis 

 

VERSION:  FINAL

 

 

SESSION:        2016/17

QUALIFICATIONS: MSc/MEng
 

EXAM DATE:  16/03/2017

 

TIME ALLOWED: 2 hours

 

 

Rubric Answer 3 questions

All questions carry equal weight

No more than two questions from any section should be attempted

There are two sections to this paper

Reference Material to be provided by the Examinations Section ☐ Chemistry data book

☐ New Cambridge Statistical Table

☐ SI Units

☐ 1mm graph paper

☐ MCQ Card

Reference Material to be provided by the Department Specialist charts (replace ‘specialist’ with type of chart) ☐ Specialist tables  (replace ‘specialist’ with type of table)

Specialist papers (replace ‘specialist’ with type of paper)

Electronic Calculators ☒ Permitted

☐ Not permitted

☐ Provided by Department

Contact details of module coordinator for Exams Office to contact  
Additional information  

 

 

            

Answer THREE questions.

 

You must not attempt more than TWO questions from SECTION A or B

 

All questions carry equal weight.

 

SECTION A

 

 

This question concerns basic theory on strengthening mechanisms in metallic alloys, focussing particularly upon steel.  Answer all parts.

 

  1. Detail the underlying micro-level mechanisms responsible for the following hardening effects in metallic alloys;

 

  1. work hardening
  2. solid solution strengthening
  • second phase strengthening
  1. grain size hardening

 

[8 marks]

 

  1. Explain the difference between the terms “strength” and “toughness”, using the stress-strain curve for a ductile material to assist you. Use your explanation to detail why brittle materials often exhibit a distinct difference between their behaviour under tensile loading, compared to that under compressive loading.

 

[5 marks]

 

  1. Martensite is a brittle phase that can appear in steels. What steels are susceptible to the formation of martensite?  Explain the purpose of the process by which Martensite is deliberately formed and then “tempered”.  What are engineers trying to achieve?

 

[4 marks]

 

  1. Using TTT diagrams, describe what engineers mean by the term “hardenability” as applied to steels. Show how the hardenability of a steel can be qualitatively determined from its TTT diagram.

[3 marks]

 

  1. HSLA steels offer good strength, high toughness and excellent weldability. Such a combination of properties is difficult to achieve in conventional steels. Explain why and detail how HSLAs manage to deliver this.  Are there any engineering issues which need to be considered when making use of the benefits that HSLAs provide?

[5 marks]

 

 

 

This question concentrates on welding, especially of steels, and the metallurgical problems/issues that arise.  Answer all parts.

 

  1. What do you understand by the term “carbon equivalent value” (CE). Why is it relevant to welding metallurgy and procedures?  For a given steel, how is the CE value related to the steel’s TTT diagram and the steel’s hardenability?
    • marks]

 

  1. Residual stresses are normally present in metals after welding fabrication. Why do these form? In a butt weld between two metal plates (say in a shipbuilding application) draw sketches to illustrate the typical distribution of residual stresses.  Use your sketches to suggest where cracking is most likely to be observed in and around the weld, and what the orientation of the cracks might be.
    • marks]

 

  1. With reference to your answers in a) and b) above, detail the mechanism of failure in a weld known as “cold cracking”. Describe the various ways, which engineers and welding technicians employ, to avoid cold cracking, and detail the underlying principles behind these.
    • marks]

 

  1. Other alloy systems can suffer from metallurgically related problems after being welded. In this respect, briefly comment on the issues that can occur in aluminium alloys, titanium alloys, and in stainless steels when they are welded.

[7 marks]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This question concentrates on aspects of materials in bioengineering.  Answer all parts as fully as you can.

 

  1. Explain what is meant by a degradable and stable biopolymer.

[2 marks]

 

  1. Describe, in detail, what aspects should be considered in biomedical scaffold design.

[8 marks]

 

  1. Describe, in detail, one application for each of the following:

 

  1. a degradable
  2. a stable biopolymer

[15 marks]

 

SECTION B

 

 

  1. A shaft is subjected to a variable load, and the load history with time is recorded and shown in Figure 1. Count the load cycles using the Rainflow method, stating each maximum and minimum stress, and calculating each load range and mean stress.

 

 

Figure 1: A variable load history with time

 

[12 marks]

 

  1. A cylindrical pressure vessel with a diameter of 5.0 m and a wall thickness of 25 mm, was made of a steel having a Young’s modulus of 200 GPa, a yield strength of 1500 MPa, a Poisson’s ratio of 0.3 and a fracture toughness (GC) of 130 kJ/m2. The pressure vessel underwent catastrophic fracture when the internal pressure reached 18 MPa. Determine the reason that might have caused this failure.

[7 marks]

 

  1. A pressure vessel is to be fabricated from plate steel which may be either: (i) a maraging (18% nickel) steel with yield stress of 1900 MPa, KIC of 82 𝑀𝑃𝑎 m or (ii) a medium strength steel with yield stress of 1000 MPa, KIC of 50 𝑀𝑃𝑎 m .

 

Which of these two steels has the better tolerance to defects at the design stress?  Compare their fracture toughness if they are to have the same defect tolerance.

A factor of safety of 2 should be used for the design stress.

[6 marks]

  1. The analysis of the cyclic stresses on part of the landing gear of an aircraft shows that during each flight it is subjected to the following stress history (see Table 1 for the number of cycles in each stress range).

 

Stress history (MPa) Cycles per flight
50 100,000
100 10,000
150 500
180 200
185 100
200 50
210 10

 

Table 1: Cyclic stress history on a landing gear of an aircraft during a flight

 

If the S-N curve for the part’s material is given by;

 

σ=500N-0.064

 

where σ is stress in MPa, and N is the number of cycles to failure, estimate how many flights this component can withstand before fatigue failure occurs.

[15 marks]

 

 

  1. You have been given a set of compact tension test specimens, all of the same size and geometry. These specimens have been fatigue pre-cracked to various crack lengths. The stress intensity of this specimen configuration can be expressed as follows;

 

KI  f(a/w)

 

Where P is load, B is thickness, W is width, a is crack length, and f(a/w) is a dimensionless geometry correction factor.  Describe a set of non-destructive experiments you could perform to determine f(a/w) for this specimen configuration.

[10 marks]

 

 

  1. A shaft with the diameter of 25mm is subjected to a steady axial force of 50kN and a uniform bending moment M. If the yield strength of the shaft material is 300MPa and the fatigue endurance limit in reversed bending is 200MPa, calculate the maximum M to avoid fatigue failure in the shaft.

[6 marks]

 

 

  1. State the Paris Law used to analyse the fatigue life of material and prove that an equation for the calculation of the number of cycles to failure is given by:

 

N =               2                           1            1 

(m−2)CYm2 a0 2                                                                                         a f 2 

 

[5 marks]

 

  1. The blades of a turbine rotor are fitted into aluminium alloy discs on the rotor. A 0.1mm deep scratch is accidentally made in the surface of the disc during the assembly of the system.  The fracture toughness of the aluminium alloy is 35

𝑀𝑃𝑎 m .  The rotation of the turbine causes a stress of 350MPa at the plane of the scratch.  Estimate how many stress cycles the disc can withstand before fatigue failure occurs.

 

You may assume that the geometric correction factor Y = 1.12, and Paris coefficient C = 4.0-11, Paris exponent m = 3.54, when the crack length is expressed in metres and the fracture toughness in 𝑀𝑃𝑎 m .

[9 marks]

 

 

  1. A laser etched stainless steel femoral stem fractured in the femoral neck region after 3.5 years post-operative use, due to fatigue failure. The crack initiated and propagated from a site where the component was laser etched, creating a stress concentration in the material. If laser etching has to be included in the design, how might the neck design be changed to allow the etching but still meet the 20 year life requirement?

[5 marks]

 

MECHGM03 / MECHM004 2015/16

 

 

 

 

 

 

 

 

 

 

 

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