ENGT5258 Advance Solid Mechanics

Question:

Your marked coursework and feedback will be available to you on 22/12/2019 If for any reason this is not forthcoming by the due date your module leader will let you know why and when it can be expected. The Head of Studies should be informed of any issues relating to the return of marked coursework and feedback.

Note that you should normally receive feedback on your coursework by no later than four working weeks after the formal hand-in date, provided that you met the submission deadline.

1. Explain and discuss the following in terms of creep with practical examples

1. Stress relaxation

2. Creep rupture strength

3. Creep life

4. Factors affecting creep (15%)

2. A secondary creep rate is given in table below were obtained from creep tests carried out on Aluminium alloy at 100â—¦ C

Determine the parameters of the Norton power law for creep: is the secondary creep rate, σ is stress, and A & n are constants. The result should include MATLAB polyfit with clear steps. (10%)

The secondary creep rates are given in table below obtained from a series of tensile tests as various temperatures under the same load.

Consider the material complies with Arrhenius definition:

Given the molar gas constant as R = 8.314 Jmol−1K

Determine the activation energy and the constant for secondary creep and hence identify the material. The result should include MATLAB polyfit with clear steps. (10%)

In a cylindrical vessel a pressure of 1.5MP a acts on a circular cover plate. The cover plate has a diameter of 450mm and is held in a position by thirty bolts of 20mm diameter, equally spaced around the rim. Ignoring primary creep but account for elasticity with E=210GPa and a secondary creep rate of 32.2 ×

Where all symbols have their standard meaning, A and n can be found from creep rate. Determine

1. The initial tightening stress in the bolts in order that a safety factor of 1.6 is maintained after 8000hrs of creep relaxation at 500â—¦ C.

2. The time when the bolts should be re-tightened to prevent leakage around the plate. (10%)

2 Fatigue

5. Give a brief descriptions on the following:

1. Explain the three major fatigue life methods and their benefit and limitations.

2. Discuss endurance limit with practical examples, also include endurance limit modifying factors.

3. Briefly explain with examples the meaning of stress concentration and notch sensitivity. (15%)

The fatigue life of various stress levels of machine tool steel is given in the following table. The machine tool is subjected to reversed cycle loading at 120 cycle/day in a continuous sequence.

Which involves the following three stages:

1. 250 cycles at ±130 MPa

2. 250 cycles at ±120 MPa

3. 250 cycles at ±80 MPa

Estimate the life of the component in days according to

Apply to the semi-stress ranges of 130 MPa, 120 MPa and 80 MPa respectively. (15%)

7. The reverse stress fatigue limit for low-carbon steel is ±270 MPa, its Ultimate Tensile Strength, UTS is 670 MPa and its yield stress is 260 MPa.

1. Estimate the safe range of stress for a repeated cycle based on the Gerber and Goodman fatigue predictions.

2. Determine the peak stresses and safe range of stress for an alternating tensile cycle with mean stress 185 MPa based upon the Soderberg prediction

Important points for both sections

• The report should be concisely written with clear steps.

• The report presentation constitutes 10% of this coursework, 5% for each section. Appropriate referencing should be used where necessary.