Even as recent as the previous century, a lot of ships were being lost at sea owning to material weaknesses. Learn about a few material properties required for a strong ship construction and the role of classification societies
We have been talking about various marine diesel engine components and types of ships and have seen a variety of ship types and marine components. Yet there is an underlying similarity between them in terms of materials used for construction. The basic ship construction materials used for construction are the same like most other construction projects and include but not limited to Iron, Steel, Aluminium, plastics and so forth. Even these diverse materials have some common factors in terms of their properties which make them useful for the task. We will learn about the definition of a few types of mechanical properties of such materials in this article.
Properties of Materials
- Toughness – when a material is bent and it can withstand that bending without getting fractured, it is known as a tough material. It requires a lot of cycles of bending before the material actually fails. This property is exactly opposite to the property of brittleness where a material simply breaks upon application of bending force.
- Ductility – this refers to the property of the material to get deformed before it actually fails due to tension. This property is dependent on temperature and decreases with rise in temperature. Gold is one of the most ductile metals but since we are considering from the point of view of ship construction, the ductile materials use for this purpose include tin, zinc, lead, copper etc. A closely related term is notch ductility and this gives a measure of the relative toughness of steel which is measured using impact test.
- Malleability – a material tends to crack under compression or when its shape is changed due to operations such as extrusion, forging and so forth. The malleability of a metal increases with temperature as against ductility which follow the reverse trend. This property is useful in working on hot ingots or metal slabs which are used for building various parts of the ship.
Survey & Classification
In the earlier days there used to be lot of marine accidents which resulted in loss of life and property. A survey was carried out to determine the root cause of these accidents from material weakness point of view. The following table shows the reasons for accidents which are based on the data collected for accidents that occurred during the period 1970 – 1975.
As you would have noticed from the table, the root causes for failure are different for different parts of the ship and brittle failure and fatigue failure were responsible for maximum number of failures on the deck and engine side respectively.
Regulations became tighter regarding use of materials and the grades of steel were described as follows.
Apart from these grades there are another set of grades which are normally used by classification societies which recommend the use of higher tensile strength steel plates to decrease the thickness of plates in larger sized ships to reduce its light displacement. These grades are known as AH, BH, DH & EH respectively and can be used where grades A, B, D or E respectively were supposed to be used hence we can use
· AH = A
· BH = B
· DH = D
· EH = E
These modified grades have their tensile strength in the region of 490 – 620 MN/m2 as compared to the normal grades which have tensile strength in the region of 400 – 490 MN/m2.
Australian Maritime Safety Agency, AMSA