VENT 7 - STRUCTURE

The essential thing in design of underwater vehicles is to use materials with a high strength-density ratio. Because we are dealing with sea-water, and more specifically vent fields, there are several environmental conditions taht go into choosing a material to use for the auvs and rovs. Metalals, and other materials tend to corrode in the sea-water due to electro-chemical mechanisms. There are five main ways corrosio can occur:
The chemicals present in sea-water that lead to corrosion are nitrates, acids, sodium and other chlorides, steam

Ways of reducing corrosion: Eliminate the corrosive environment e. gelimination of Cl- iond by ion exchange from aqeuos solutions.
CAthodic protection system: more reactive metallic element cating that reacts in preference to vehicle material
Improvements in internal structure - avoiding pools of stagnant water within the vehicle.

Plastics for the three sisters:
The thre sisters are auvs that are internally flooded, so the outer fairing does not have to provide structural support, or be a pressure vessel.Also, the three sisters will avoid the high temperatres near the vent, by swimming at a fairly constant vertica distance above the vents. Therefore we chose to use ABS, a plastic polymer for the outer fairing in these vehicles. Before we can consider its pros and cons, it is important to understand the fundamental properties of plastics.
There are two types of plastic :
Thermosetting: A substance that does not soften on heating
Thermoplastic: A substance that softens on heating

PLatics are moulded into shape by a process called thermoforming, where athermoplasdtic sheet is clamped onto a frame, heated in order to make soft and pliable, then made to conform to the shape of the mould by applying differential pressure. THe pressure can be provided mechanically ( eg matched molfd fomring), via air pressure.
ABS is thermoplastic, which it makes it shock absorbant. Unofrtunateley, it can become deformes if it gets to hight temperature regions. An advantage of ABS is that it has the same acoustic properties as water and so allows sonar signals to enter and leave the auv, without much distortion.

Below are some of the properties of ABS and its cehmical name:
ABS stands for Acrylonitrile Butadiene Styrene.

Physical Properties

Physical PropertyValues
Density g/cc1.05
Hardness, Rockwell103
Tensile Strength, Yield, MPA46.9
Elongation % break25
Impact Strenght J/m2.99
Vicat Softening Point C98

Metallic Alloy for ROVs
Titanium is a bad choice from a corrosion point of view and at high temperatures near vent it would be toast from a structural point of view. Any titanium alloy that we would have to use would be at it's limit structurally and not good from an environmental cracking point. The vent chemistry is quite noxious. The best options are a composite (metal layer type) of some kind where the environment-facing material is a high strength Ni-base alloy, one that might be used in a sour gas environment, but backed up by something lighter. One can bond Ni-Base materials to Ti.
The properties of Ni-base corrosion resistant alloy (containing chromium, amognst other elements.)



The properties of titanium alloy are

Physical PropertyValues
Density g/cc8.19
Hardness, Rockwell38
Tensile Strength, Yield, MPA1276
Elongation % break23
Reduction of area, %25
Physical PropertyValues
Density g/cc4.7
Modulus of elasticity, GPa110
Poissopns Ratio0.33
Sheer Modulus,GPA42
Melting point1550 C
Thermal Conductivity, W/m-k7
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