Lecture 12 - Creep, Damping, Lightweight performance

Question 1

What is fatigue?

Answer 1

resistance to cyclic loading

Question 2

How many percent of the failures in polymer based components is because of fatigue?

Answer 2

80%

Question 3

What is the relation between Fatigue failure strength and static tensile ultimate strength?

Answer 3

Fatigue failure strength is typically ≈ 20-40% of the static tensile ultimate strength

Question 4

What is the advantages with each of Strain and stress controlled machines

Answer 4

Stress: more affordable
Strain: more sensitive

Question 5

What is creep?

Answer 5

when strain increases with time at constant load (stress)

Question 6

What are the main parameters for creep?

Answer 6

Time and temperature

Question 7

Which fiber can withstand creep most times?

Answer 7

In most of the cases carbon and glass fibres remain elastic and experience no creep.

Question 8

What is damping?

Answer 8

control of the resonance frequency and avoiding unpleasant motions, noise and dynamics stresses

Question 9

What is Celerity?

Answer 9

The speed of sound

Question 10

What property is important when you want noise reduction?

Answer 10

Sound reflection

Question 11

What property is important when you want noise insulation?

Answer 11

Sound absorption

Question 12

What does a high loss tangent mean?

Answer 12

Good damping

Question 13

What is a sandwich material?

Answer 13

A composite with a core material between the laminate (skins/facings)

Question 14

What can the core material in a sandwich material consist of?

Answer 14

Cellular materials like honeycombs, foams and balsa

Question 15

Which are main types of honeycombs?

Answer 15

• Nomex - refers to meta-aramid fibres impregnated with phenol-formaldehyde
• aluminium
• polymer impregnated glass or carbon fibre
• impregnated kraft paper

Question 16

Name some of the important factors to consider before using Honeycombs

Answer 16

• 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

Question 17

What properties does Nomex have?

Answer 17

• 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

Question 18

Which is the most used core material for sandwich composite?

Answer 18

Aluminium

Question 19

What properties does aluminium have as a sandwich core material?

Answer 19

• 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

Question 20

What applications are common for honeycombs?

Answer 20

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)

Question 21

What combinations can be used when we talk about polymer impregnated glass or carbon fibre?

Answer 21

• 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

Question 22

What are foams/cellular structures?

Answer 22

They are multi-phase systems comprised of solid matrix (frequently polymer) and a fluid (frequently a gas).

Question 23

Which are the main types of foams?

Answer 23

• Polyvinyl chloride
• Polyurethane
• Polystyrene
• Phenolic
• Polypropylene
• Polymetharylimide (PMI)
• Other foams: polyester, epoxy, ABS, PC, PES, PEEK, PEI

Question 24

Name some properties of Polystyrene

Answer 24

• 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

Question 25

Name some properties of Phenolic

Answer 25

• very good fire-resistant properties
• can have very low densities
• has relatively low mechanical properties (max. recommended temperature for continuous use is 150°C)

Question 26

Name some properties of PMI

Answer 26

• much more expensive than the other types of foams
• Has greater mechanical properties
• used primarily in aerospace and recreation product applications.

Question 27

Name some properties of PVC

Answer 27

• 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!

Question 28

Name some properties of Polyurethane

Answer 28

• 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)

Question 29

What two kinds of foams are there?

Answer 29

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

Question 30

Tell us about balsa

Answer 30

• 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

Question 31

What other wooden cores can be used other than balsa?

Answer 31

spruce, mahogany, redwood, pine, fir, etc. Plywood is sometimes used in boat construction. Similarly the deficiency is water penetration.