Fracture Mechanics, Metal, Concrete and Masonry, Timber and Composites

Section 1: Fracture Mechanics

a) What is stress concentration in a material? Why do stress concentrations develops? What type of locations to have maximum increase in stress concentrations? Explain with the assistance of a sketch or sketches. (6 marks)

 

b) A cantilever is shown in Figure Q1. There are two examples of the cantilever: one is made of a low alloy steel, and the other of concrete. A load F is applied at the free end of the cantilever. Both the low alloy steel and the concrete cantilever have transverse scratches of depth 0.== mm (The == are the last two digits of your student number.), near the fixed end of the cantilever, on the tensile side.

 

Calculate the stress needed to cause fast fracture. Will the element fail by yielding or by fast fracture?     

 

What is the critical crack depth for a surface crack for both steel and concrete before fast fracture occurs?

 

If the element fails by yielding, what is the maximum load it can carry before yielding?

Formulas and data tables are given at the end of the paper. If there is a range of values in the tables you can take an average value or a suitable value within the range. Justify the value you have taken.

2. Metal:

a) What is embodied energy? Why is the embodied energy of steel much higher than concrete?

b) What are the three main phase of steel present in the Fe-C diagram, and what are their crystal structures?  

c) Explain the effect of quenching and recrystallization on the strength, ductility and grain size of metals. Use bullet points and sketches as necessary.  

 

d) What is the function of shape memory alloys? Use sketches to explain and give an example.

 

e) A mild steel containing 0.==%C (== to the last two digits of your student number) is cooled very slowly from 1000°C to 200°C. Draw and label the microstructure at: 1000°C, 800°C, and 200°C.

a. From an environmental perspective of a construction material, what is the key step to transfer the material life cycle from cradle-to-grave life cycle to cradle-to-cradle life cycle, explain.                                           

b. Would you consider concrete to be a continuous and homogeneous material? Why or why not?

c. What is the possible cause of creep in hardened concrete? Explain elastic and creep strain with a schematic diagram

 

d. What are the major issues with concrete durability in relation to embedded reinforcement and in the concrete matrix itself? Explain the chlorine attack. 

 

e. What are the key functions of mortar in the properties of masonry?

a) If a piece of wood with a Moisture content (MC) of ==% (==  12 + the last two digits of your student number) was dried to an Equilibrium moisture content (EMC) of 12%, what types of water will be removed from wood? In which direction would the wood experience the greatest dimensional change? What type of defect would you expect this drying to cause? Explain your answers with reference to the microstructure of the wood. 

 

b) What are the advantages and disadvantages of using bamboo as a construction material?

 

c) You are requested to select a specific type of reinforcement bar for a marine concrete infrastructure. The available options are:

(i) steel,

(ii) bamboo,

(iii) glass fibre reinforced polymer (GFRP), and

(iv) carbon fibre reinforced polymer (CFRP). Explain which one you will select considering advantages and disadvantages in terms of durability and structural integrity.