Lecture 12 - Creep, Damping, Lightweight performance Flashcards
What is fatigue?
resistance to cyclic loading
How many percent of the failures in polymer based components is because of fatigue?
80%
What is the relation between Fatigue failure strength and static tensile ultimate strength?
Fatigue failure strength is typically ≈ 20-40% of the static tensile ultimate strength
What is the advantages with each of Strain and stress controlled machines
Stress: more affordable
Strain: more sensitive
What is creep?
when strain increases with time at constant load (stress)
What are the main parameters for creep?
Time and temperature
Which fiber can withstand creep most times?
In most of the cases carbon and glass fibres remain elastic and experience no creep.
What is damping?
control of the resonance frequency and avoiding unpleasant motions, noise and dynamics stresses
What is Celerity?
The speed of sound
What property is important when you want noise reduction?
Sound reflection
What property is important when you want noise insulation?
Sound absorption
What does a high loss tangent mean?
Good damping
What is a sandwich material?
A composite with a core material between the laminate (skins/facings)
What can the core material in a sandwich material consist of?
Cellular materials like honeycombs, foams and balsa
Which are main types of honeycombs?
- Nomex - refers to meta-aramid fibres impregnated with phenol-formaldehyde
- aluminium
- polymer impregnated glass or carbon fibre
- impregnated kraft paper
Name some of the important factors to consider before using Honeycombs
- expensive and difficult to handle during manufacturing of the sandwich but of great interest!
- Honeycomb sandwich panelling is the most lightweight option for compressive or bending loading specific applications
- Usually of hexagonal shape but can also be rectangular, square and TMs
- Honeycombs are anisotropic, however, the core acts nearly isotropically for in-plane loads when assembles in a sandwich construction
What properties does Nomex have?
- high toughness, fire retardance, abuse retardance (this referring to the ability to withstand overloads locally)
- Used frequently for aircraft cabins, walls, floors and ceilings.
- High cost
Which is the most used core material for sandwich composite?
Aluminium
What properties does aluminium have as a sandwich core material?
- Its price-quality ratio is high and makes it an attractive material
- high thermal conductivity, electrical conductivity and high energy absorption
- unfortunately, it has high thermal expansion
What applications are common for honeycombs?
are too expensive for marine construction, however find application in decks and bulkheads of weight-critical craft such as hydrofoils, hovercraft, high performance sailing racing boats (where robustness is sacrificed for performance)
What combinations can be used when we talk about polymer impregnated glass or carbon fibre?
- Glass fibre and quartz honeycombs. The first one is considered to be heat resistant, and having low thermal and electrical conductivities. It is usually used in electrical applications.
- Quartz honeycombs perform better than the glass fibre ones at higher temperatures. They can be attractive because of the low thermal expansivity
- Kevlar fibers are also used for cores. They have good mechanical properties, low density, low dielectric coefficient, high toughness, however, they are expensive.
- Carbon fibre, for example impregnated with cyanate ester (CE), is used to construct honeycombs -> negative thermal expansion of carbon fibres in the fibre direction! Operation at wide range of temperatures
- low dielectric constant and dielectric loss, low moisture absorption, and low shrinkage
What are foams/cellular structures?
They are multi-phase systems comprised of solid matrix (frequently polymer) and a fluid (frequently a gas).
Which are the main types of foams?
- Polyvinyl chloride
- Polyurethane
- Polystyrene
- Phenolic
- Polypropylene
- Polymetharylimide (PMI)
- Other foams: polyester, epoxy, ABS, PC, PES, PEEK, PEI
Name some properties of Polystyrene
• least expensive
• has relatively low mechanical properties
• commonly used for disposable packaging
• Has the strong disadvantage of being soluble in styrene (major part of UPs and VEs), and is
badly softened by gasoline and other solvents