tutorial 1 and 2 - sustainability and material properties Flashcards
why is the yield stress more important to engineers than the ultimate stress
- yielding: structure will begin to permanently deform and gives warning that the structure will collapse soon
- once yielded people have time to evacuate and respond to failure
- if designed at ultimate stress, bubildings would yield and permanently deform arbitrarily at loads below the design load, structures would lose their capacity to function properly due to lack of dimensional stability or collapse with no warning
when testing a steel specimen in tension, what is the difference between engineering stress and true stress
- stress = F/A
- engineering stress uses the inition cross-sectional area
- true stress uses the actual cross-sectional area at the the time when necking occurs, the area decreases and the stress increases
- since Aeng> A true ; Stresseng< stresstrue
what is the difference bewteen stress and force
Stress = F/A
what is the difference between deformation and strain
deformation = pysical length change (delta L)
strain = dimensionless proportion of deformation to original length, e = delta L/L
what happens to the ductility of steel in the following conditions:
- increase the rate of applied strain
- increase material temp
- increase defree of triaxiality
- higher strain rate: less ductile, more brittle
- higher temp; more ductile, less brittle
- more triaxiality: less ductile, more brittle
quote the definition of sustainability
“sustainable dev. is the dev that meets the needs of the present without compromising the ability of future generations to meet their own needs
things we can do to be more sustainable
- choice of materials
- source of materials
- durability
- mode of construction
- design choices
- energy used choices
- maintenance regime
- reduce, reuse, recycle (in order of efficiency)
the three components of the total energy footprint of contrustion materials
- indirect energy
- direct energy
- recurring energy
indirect energy
extraction, raw material transportation, process and manufaction of materials
direct energy
energy fro transportation and construction of infrastructures using construction materials
reoccuring energy
energy for repair maintenant refurbishment or replacement in infrastructure
% of GHG production of cement
8%
% od GHG of residential and commercial buildings operations
17%
microstructure lenth
10^-9 to 10^-4
4 types of engineering materials
- metals and alloys
- polymers
- ceramics and glasses
- composites
metals and alloys properties
- homogeneous or reoccuring microstructure
- electrically conductive
alloys: - combo 2+ metals - properties depend on proportion and on cooling rates
ceramics and glasses - properties
- typically non metallic inorganic material (silicate)
- electrically insulating
- brittle
- homogeneous microstructure
polymers/ plastics properties
- infinite chains of self repeating molecule
- usually composed of hydrocarbons
- cross linking possible (mostly long chains)
- high degree plasticity (easy shaped or moulded)
- good electrical insulation
composite materials properties
- 2 + engineering materials used in conjunction
- BIG variation in macroscopic mechanical properties
- reinforce concrete
- fibre reinforced polymers
reinforcing concrete does what
provides compression strenth
the fibres provide what
provide axial resistance
macrostructural length
10^-3 to 10^4
what is difference between stress and force
- stress is force divided by area
- diff stresses in diff axes (uniaxiality vs triaxiality) (shear vs normal stress)
- tension or comopression
what is deformation
- length of change compared to undeformed surface
what is strain
- deformation per unit
what is elasticity/ elastic deformation
ability for deformation materials to return to undeformed state
what is plastic deformation
- permanent
- unloading occurs elastically but with permanent deformation
- brittle material break before they deform (so not plastically)
- continuing deformation without corresponding increase in force
what is strain rate and why does it occur
- rate external applied loading
- material trying to get to equilibrium
what happens to materials when a slower strain rate i applied
- slow rates = material more ductile bc more time to adjust - more plastic deformation
- faster rates = less time - less plastic deformation - more brittle
what happens to the material when exposed to higher Temp and lower temperatures
- high T = more ductile
- low T = more brittle
what happens to material when the degree of triaxiality (loading in one or multiple axes) is high and low
- high degree => material has no room to expand since all axes are loaded so negation of the poisson effect - more brittle
- low degree => poisson effect allows more space to expand in axes not loaded - more ductile
parts of the stress strain curve
yield stress, ulitimate stress, rupture stress, elastic, strain harden8ing, necking, rupture stress
microstructural length
10^-9 to 10^-4 m
when is stress higher than average stress
- in localized zones like around corners, in necking, around load applications, etc
when testing a steel specimen in tension, what is the difference between engineering stress and true stress
- stress = f/a
- engineering stress uses the initial cross-sectional area
- true stress uses the actual cross-sectional area at the time when necking occurs, the are decreases and the stress increases
since Aeng > A true
then Stress(eng) < stress(true)
why is the yield stress more important to engineers than the ultimate stress
- at yielding, structure will begin to permanently deform, which gives warning that the structure is likely to collapse soon
- once yielded people have time to evacuate and respond to failure
- if we design buildings at ultimate stress they would yield and deform permanently deform randomly at loads below the design load (cant give warning)
