Question

QUESTION 5A (a) There are FOUR (4) factors that need to be considered in service condition. Interpret how all the four factors influence the properties of materials during service condition. (8 marks) (b) (i) Sketch a graph showing the relationship between impact energy and temperature for metal with body centered cubic (BCC) crystal structure. Indicates on the graph the point of ductile to brittle transition temperature. (3 marks) In the history of Titanic and Liberty Ship, the disaster occurred due to the materials failure. Interpret the problem related to the materials. What was the main problem and how it can be prevented? (4 marks) (c) You are requested to produce a ceramic vase for your lab project. Propose a suitable ceramic fabrication process and explain it by using appropriate figures. (5 marks)

Answer 1

(a) The four factors that need to be considered in service condition are

(i) Material chemical composition: It's the most important property which control the property, utility and cost of construction of the device.

(ii) Design of the device.

(iii) Operating procedures.

(iv) Maintenance of the device used in service.

(b) (i) The graph showing the relationship between impact energy and temperature for metals with body centred cubic structure (BCC) is presented;

BCC material Impact Energy (J) Brittle behavior Ductile behavior Temperature (T) Ductile to brittle Transition (DBTT)

(ii) The disaster happened with Titanic and Liberty ship occured due to material failure. Both of these ships failed due to brittle fracture.

Titanic Ship: The materials used in the construction of titanic ship were basically low grade metals,which were basically brittle in nature. Thousands of mild steel plates were joined by wrought iron rivets. The mild steel plates along with the wrought iron rivets were found to be much brittle as compared to high strength steel, which have failed at the time of impact.

Cause: The ship failure occured due to brittle fracture. The use of wrought iron and mild steel rivets caused the disaster to happen. The causes of brittle fracture include low temperature, high impact loading and high sulphur content of the steel. Brittle fracture occured as the temperature of water was below freezing point so that the steel went from ductile to brittle transition behavior. Also, the titanic was travelling at a high speed on impact with iceberg and due to high sulphur content, brittle fracture occured in the hull of the ship.

Prevention: The disaster could have been prevented if the following methods were followed:

(a) Use of high strength steel plates and steel rivets.

(b) Nickel addition should be done in the steel used. It improves the toughness of the material. However, Nickel was missing.

(c) Lowering ductile to brittle temperature of the steel by improving steel quality (reducing sulphur and phosphorus content).

Liberty Ship: The Liberty ships produced in great numbers during WWII were the first of all welded ships. The materials used were low carbon steel. A significant number of ships failed by catastrophic failure.

Causes: The use of low carbon steel in the ships led to the failure of the ship catastrophically. Fatigue cracks occured at the corners of square hatches and propagated rapidly by brittle fracture. The steels used in those ships were low in carbon content. It was thought to be ductile but it undergo brittle transition. Some of the ships deployed in north atlantic oceans, where the temperature is very low, these steels undergo ductile to brittle transition at such temperatures. Weld defects and discontinuities were introduced by inexperienced operators.

Prevention:   

(a) Lowering ductile to brittle temperature of the steel by improving steel quality (reducing sulphur and phosphorus content).

(b) Rounding off corner hatches.

(c) Installing crack arrest devices such as rivets, and strong weld seams.

(d) Improving welding practices.

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