Chapter 5 - Mechanical Properties Flashcards
What are the four key mechanical properties
elastic, plastic, creep, and cracking
What is the effect of neutrons in the primary metal components?
radiation enhanced cracking susceptibility and embrittlement
What are the effects of corrosion in metal components?
crack propagation under stress
What is the pressure balance initially with fuel and water? What does this balance cause?
pressure in coolant is higher than interior causing creepdown of cladding onto pellets
What is the pressure balance towards end of life between fuel and water? What does this cause?
p coolant is less than p interior cladding causing cladding liftoff. This means gap between cladding and fuel increases and fuel overheats since its not being cooled down the same
Why does fuel expand causing pressure increase? What happens with the cladding?
due to fission product swelling which eventually cladding and fuel come into contact again (Gap closure) and continued fuel swelling stresses on cladding.
There are two types of deformation cause by applied load (Stress)?
elastic (removable) or plastics (permanent)
What are the three main sources of stress in nuclear environments?
- Applied external
- Thermal stresses
- residual
What does strain defines?
fractional displacements as a result of applied stress
What are displacements?
changes in the position fo a point as a result of applied stress
What is the diffusion of an atom dependent on in substitutional diffusion?
Upon the presence of a vacancy on an adjacent site and the rate of diffusion therefore depends in how easily vacancies can form in the latice and how easy it is for an atom to move into a vacancy. This dependance upon the presence of vacancies makes substitutional diffusion slower than insterntial diffusion.
Name a candidate material for fast reactors?
HT-9
What metallic component is the first barrier to FP release?
zircaloy
What is the second barrier to FP release?
the primary pressure boundary made of the RPV the coolant piping and the steam generator tubes.
Describe the three steps in fuel cladding interaction throughout lifetime.
Initially the pressure in the coolant is higher than the interior of cladding causing creepdown of cladding onto pellets. Towards the end of life the fission gas release causes pressure to increase in the interior causing cladding liftoff meaning gap increases and fuel overheats. Afterwards, fuel pellets expand due to FP swelling which leads to gap closure and continued fuel swelling causing stress on cladding.
What are the two types of deformation resulting from applied load (Stress)?
elastic (removable) and plastic (permanent).
What is strain?
deformation to a body
What does uniform temperature changes cauases?
thermal expansion but do not generate stresses.
What does non-uniform temperature changes cauases?
thermal stresses and strains
What is the difference between engineering and true stress strain curves?
The engineering stress strain curves assume the cross section remains constant while the true stress strain curves accounts for changes in the cross section area as the experiment precedes.
What is the yield strength?
the stress at which a specific amount of plastic deformation is produced. In other words it indicates the limit of the elastic behaviour meaning the point where deformation is permanent.
What is the elastic modulus?
the quanity that measures an object or substance resisntance to being defomred elastically (non-permanently).
What is the ultimate tensile strenght?
the materials maximum resistance to fracture. It is the maximum engineering stress in a uniaxial stress strain test.
What is the UE
?
What is the TE
?
Where can residual stresses come from?
These are stresses that remain in a solid material after the original cause of the stresses has been removed. Sources include inelastic (plastic) deformation, temperature gradients (during thermal cycle) or structural changes (phase transformation).
What is the difference between strength and toughness?
Strength is the ability of a material to withstand an applied load while toughness is the ability to absorb energy without fracture
What are the three main regions in a UTS graph?
- Elastic
- Stable Plastic (work hardening)
- Unstable Plastic (necking)
What stress is higher: the true or the engineering stress and strain curve?
The true stress-strain curve because the cross sectional area as the experiment proceeds is taken into account meaning A goes down and sigma=P/A=goes up
What does elastic theory relates?
applied loads to non permanent displacements and strains.
How many components does the stress tensor have?
six; three normal and three shear
What does strains define?
The fractional displacements
What is graphed in a UTS?
stress vs strain
What are the three basic components of elasticity theory?
- equilibrium conditions ensuring force balance on a volume element
- displacement and strain relations that ensure the solid remains continuous as it defors
- constitutive equations that related stresses and straines
Elasticity theory applies only for stresses …
less than the yield stress and strain less than a few percent (removable, non-permanent deformation).
What is the youngs modulus?
mechanical property that measures the tensile stiffness of a solid material. It quantifies the relationship between tensile stress and axial strain in the linear elastic region, E = sigma/e where sigma is stress and e is strain. In other words it is the slope of the linear part of the stress strain curve.
What is the poissons ratio?
measures the phenomenon in whic ha material tends to expand in directions perpedincular to the direction of compression.
What is the poisson ratio of rubber and cork? What do they mean?
0.5 and 0 meaning cork has very little compression.
What is the shear modulus?
it is the ratio of shear tress to sheear strain.
What is the youngs or elastic modulus related to fundamentally?
The atomic binding
Does the E-modulus depend on the orientation of the crystal?
Yes
What is stiffness?
the extent at which an object resists deformation in response to an applied force.
What does no texture mean in terms of mechanical properties?
Isotropic mechanical properties meaning same properties in al direction.
What does texture mean in terms of mechanical properties?
anisotropic mechanical properties meaning different properties depending on direction
What does hooks law state?
it states that the force (F) needed to extend or compress a spring by some distance (x) scales linearly with respect to that distance—that is, F = kx where k is a constant factor characteristic of the spring (i.e., its stiffness), and x is small compared to the total possible deformation of the spring.
How does the thermal expansion coefficients vary with melting temperature
Exponentially go down.
Where do tubes rupture usually? Why
In length axis due to the azimuthal strain
What are the six type of failures in UTS tests?
Type I: Material with no YS Type II: Material with YS Type III: Upper and lower YS Type IV: Ideal plastic behavior Type V: Small Plastic Behavior Type VI: portevin le chatellier effect.
What is hardness?
Is a measure of the resistance of a materials against permanent shape change.
What is elastic deformationin terms of cyrstal planes?
It is the reversible displacement of crystal planes relative to each other. Original structure is recovered when the stress is removed.
What is plastic deformationin terms of cyrstal
is the permanent slippage of crystal planes, which
occurs above a critical stress. When many parallel atomic planes slip relative to each other, macroscopic deformation results
Why is the real critical shear stress many orders of magnitude lower than the theoretical value?
Because of dislocations?
Where will a single cystal plastically deform in a tension test?
The single crystal will plastically deform by shear on
the plane with the highest resolved shear stress.
What does the schmid factor describe?
The Schmid factor describes the relationship between external normal stresses and the shear stresses caused in a slip system!
What is the relationship between youngs modulus and melting temperature?
Melting temperature is also an indicator of atomic bonding strength and there is relationship with Young’s modulus. The general trend is that a higher melting temperature indicates a higher modulus and vice versa.
What is the relationship between the youngs modulus and the thermal expansion?
Young’s modulus, aka elastic modulus is more or less the stiffness of a material. In rough terms, it is the change in stress caused or divided by a change in strain. The bigger the modulus, the stiffer the material.
Coefficient of thermal expansion is the change in strain caused by a change in temperature. The two concepts tend to be inverse.
Why do some stress strain cruves show a peak at yielding?
A point at which Maximum load or stress required to initiate the plastic deformation of material such point is called as Upper yield point. And a point at which minimum load or stress required to maintain the plastic behavior of material such a point is called as Lower yield point.
Why is schmid law important and what are the implications for a polycrystalline material?
Since there is no define slip system due to the random orientation of grains in the pollycristalline system then it isusually stronger than signle crystals.
The crystal with the largest resolved shear stress (Tr) yields first and others later.
What is the hall petch relationship?
The Hall–Petch relationship tells us that we could achieve strength in materials that is as high as their own theoretical strength by reducing grain size.
What are lueders band attributed to?
Impurities like carbon and nitrogen can pint dislocations leading to an upper yield point. Once dislocations break loose from pinning points they start moving but still encountering pinning pints.
What are the four main failiure modes?
Ductile, Brittle, Fatigue, Creep
Describe ductile failiure. What does the stress strain grpah look like?
Necking, formation of pores at impurities/inclusions, joining of small pores to a crack, and final rupture at the highest shear stress in an angle (max shear stress).
smooth
Describe brittle failiure. What does the stress strain grpah look like?
Initial dislocation motion, moving dislocations fruther costs more energy than nucleating a crack, brittle fracture-cracking. Dislocations start to pile up.
sudden break withouth the plastic section.
What other tests is used for brittle materials since these cannot be tested easy in tension?
Compression tests.
What is fatigue?
weakening of am aterial caused by cyclic loading that results in progressive and localized structural damage and the growth of cracks. Once a fatigue crack has initiated, it will grow a small amount with each loading cycle, typically producing striations on some parts of the fracture surface.
What is strain aging?
When steel has been strained (deformed plastically) and then allowed to age, it has been subjected to what is known as strain ageing
What is aging?
Aging is an essential step that ensures that the materials in the alloy do not revert to their original configuration after a time period. Aging is performed under controlled conditions so that the resultant grain structure will create a greater tensile strength in the metal than in its former state.
Describe the le chattelier dislocation process.
In materials, the motion of dislocations is a discontinuous process. When dislocations meet obstacles during plastic deformation (such as particles or forest dislocations), they are temporarily arrested for a certain time. During this time, solutes (such as interstitial particles or substitutional impurities) diffuse around the pinned dislocations, further strengthening the obstacles’ hold on the dislocations. Eventually these dislocations will overcome the obstacles with sufficient stress and will quickly move to the next obstacle where they are stopped and the process can repeat
What is dynamic strain aging?
Dynamic strain aging (DSA) is a process where aging is sufficiently rapid to occur during straining and it produces a variety of inhomogeneous deformations which are characterized by terms such as Portevin-le Chatelier effect, serrated yielding, jerky or serrated flow, blue brittleness, etc.
