7376113
Description
Flashcards by Martin Sejas, updated more than 1 year ago
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Created by Martin Sejas
about 8 years ago
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| Question | Answer |
| What was the material of choice for low pressure distribution mains until the 1950's? And what were the pipes subjected to? | Cast Iron. They were sealed by jute packing plus cement or molten lead. Leaks developed in the packing over time due to the overhead traffic, freeze-thaw cycles, shifting soil, and the switch to dryer natural gas |
| What negative properties cast iron has making it unsuitable for modern distribution mains? | Cast iron has the problems of: Corrosion Brittleness Leakage Expertise needed to replace pipes |
| What are desirable properties for a material used in a distribution main? | Chemically Resistant Toughness High Yield Stress High Stiffness Ease of fusion Resists Pressure requirements Creep Resistance Cost |
| It is essential that pipes "yield before break". To determine this fracture toughness equation is used. Explain the terms on it. | K = stress intensity factor K(1)<-- Represents the type of loading K1c = Critical stress intensity factor (after this point cracked structure will fail) Y is dimensionless and depends on shape a is related to the length of the crack |
| What are the terms in the Materials Index called "Ductility factor"? | Its to prevent plain strain fracture, the crack exceeding a cricial length. Hence its Critical Stress intensity factor/ yield stress |
| What is the general objective of a gas pipe? | To deliver low pressure natural gas, safely with low gas leakage for 50 years at a low cost. |
| What is the main disadvantage of PE Pipes used in gas distribution? | Methane diffuses through PE pipes, however the leak is very small and controlled (in comparison to cast iron) |
| What impacts does the external environment have on PE gas pipes? | Exposure to UV radiation and oxygen degradation influence heavily on the material properties of PE pipes. |
| What does PE stand for? | Polyethylene |
| How are polymer crystals arranged? | They are always separated from each other by amorphous layers. |
| What is an amorphous layer? | A layer in which atoms are not arranged uniformly in a 3d lattice (not crystalline). Therefore even crystalline polymers are actually only semi-crystalline. |
| Name 2 types non-crystallizing polymers, and some examples of them. | Glassy Polymers: Polystyrene, PMMA, polycarbonate, (transparent) Elastomers (rubbers): Polyisoprene, or butyl rubber, filed with particles to ncerase stiffness and resistance to wear. |
| What does LDPE stand for? | Low density Polyethylene (Used for films, bags, packaging). |
| What does HDPE stand for? | High Density Polyethylene (Used in pipes and tanks) |
| What does LLDPE stand for? | Linear low density polyethylene, processed at lower temperatures than LDPE, with higher tensile strength, impact and puncture resistance than LDPE. |
| What does MDPE stand for? | Medium density polyethylene, processed by mixing LDPE and HDPE. |
| What does UHMWPE stand for? | Ultra High Molecular Weight Polyethylene. Has high toughness and difficult to form crystal structure. Only processed by compression or machining. |
| What does MFI stand for and what is it used for? | Melt Flow Index, empirical method to determine melt viscosity of polymers. |
| What affects the MFI of a polymer? | Molecular weight distribution, presence of co-monomers, degree of chain branching, crystallinity and heat transfer. |
| What is the difference in creep in metals and creeps in a polymer? | When metals suffer creep they experience permanent strain, while polymer its temporary, and they might recover from the strain if no further strains near yield point occur. |
| What is a severe stress cracking agent? | A potentially hazardous liquid absorbed by a polymer in a short period. |
| What is Runaway Crack Growth, and when does it occur? | Its fracture energy, and occurs when its bigger than the critical value G1c |
| What does PP stand for? | Polypropylene, its more rigid than PE and used in lids, tapes, caps, plastic bottles and car parts. |
| Is PP a thermoplastic or a thermoset? | PP is a thermoplastic. |
| What is PU and what is it used for? | Polyurethane, used for elastic seals, its an elastomer and best used especially when cost is a factor. |
| How is PU made? | Usually made during the manufacture of the project, its also a thermoset. |
| What does PDMS stand for? | Poly dimethyl siloxane, its a silicone with methyl groups. |
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