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
Name some properties of Phenolic
- very good fire-resistant properties
- can have very low densities
- has relatively low mechanical properties (max. recommended temperature for continuous use is 150°C)
Name some properties of PMI
- much more expensive than the other types of foams
- Has greater mechanical properties
- used primarily in aerospace and recreation product applications.
Name some properties of PVC
- Linear PVC foams have high ductility and low mechanical properties
- cross-linked PVC is relatively brittle but has high strength and stiffness
- good resistance to water penetration
- good thermal-electrical insulation and acoustic damping.
- However, typically mechanical properties decrease by 50% at 40-60°C (max. recommended temperature for continuous use is 50°C)
- PVC has high smoke emission and acid emission in fire!
Name some properties of Polyurethane
- Relatively inexpensive
- used primarily in automotive applications, requiring moderate structural properties, and boats
- PUR foams are inferior at room temperature compared to PVC foam, but better retain strength and stiffness at higher temperatures (max. recommended temperature for continuous use is 100°C)
- PUR foams are inferior at RT compared to PVC foam, but better retain strength and stiffness at higher temperatures (max. recommended temperature for continuous use is 100°C)
What two kinds of foams are there?
Open cell (reticulated) foams:
- pressurised gas or explosives break the membranes
- hydrolysis, oxidation or thermal degradation is used
Closed cell foams:
- unbroken membranes, no flow of fluid (gas) between cells
Tell us about balsa
• moderately priced core material
• is a natural wood product
• density of balsa is less than one-half of the density of
conventional wood products
• However, balsa has a considerably higher density than the other types of structural cores
• main drawback: water penetration -> skin core debonding and rotting
• Even so, Balsa core with phenolic GRP skins has been
successfully used for internal decks and bulkheads not exposed to water
What other wooden cores can be used other than balsa?
spruce, mahogany, redwood, pine, fir, etc. Plywood is sometimes used in boat construction. Similarly the deficiency is water penetration.
