Creep Behavior of Materials

When a metal or alloy is under a constant load or stress, it may undergo progressive plastic deformation over a period of time, even though applied stress is less than the yield strength at that temperaure. This time dependent strain is called creep (above definition is taken from AMIE study material) More information is taken from Wikipdeia and shown below.

In materials science, creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to high levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near melting point. Creep always increases with temperature.

The rate of this deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function — for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade. Creep is usually of concern to engineers and metallurgists when evaluating components that operate under high stresses or high temperatures. Creep is a deformation mechanism that may or may not constitute a failure mode. Moderate creep in concrete is sometimes welcomed because it relieves tensile stresses that might otherwise lead to cracking.

Stages of Creep

In the initial stage, or primary creep, the strain rate is relatively high, but slows with increasing strain. This is due to work hardening. The strain rate eventually reaches a minimum and becomes near constant. This is due to the balance between work hardening and annealing (thermal softening). This stage is known as secondary or steady-state creep. This stage is the most understood. The characterized "creep strain rate" typically refers to the rate in this secondary stage. Stress dependence of this rate depends on the creep mechanism. In tertiary creep, the strain rate exponentially increases with strain because of necking phenomena.

General creep equation

where is the creep strain, C is a constant dependent on the material and the particular creep mechanism, mb are exponents dependent on the creep mechanism, Q is the activation energy of the creep mechanism, σ is the applied stress, d is the grain size of the material, k is Boltzmann's constant, and T is the absolute temperature. and

Creep in materials must be taken into consideration before designing machine components which work in high temperature / high stress environments. Other components in which creep is important design consideration include Bulb filaments, Crown Glass, Metal Paper clips, ...

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Factors controlling Fatigue Strength

       The fatigue limit of material is defined as stress that would cause failure after a specified number of stress reversals. The factors affecting the fatigue strength of materials and the ways of improving the fatigue strength are discussed here.

Factors controlling Fatigue strength:

• Stress Concentration: Fatigue strength is reduced by presence of stress raisers
• Surface Roughness: Smoother the surface finish of metal sample, higher the fatigue strength
• Surface Treatment: Carburizing and nitriding increase fatigue life. Decarburzing lowers the fatigue life.
• Environment: corrosive environment accelerates rate at which fatigue cracks propagate

Improving Fracture Limit

• Good design, avoiding sharp corners, avoiding regions of stress concentration.
• Polishing to give good finish & thereby removing surface irregularities helps
• Short peening of metals introduces compressive stresses at surface and helps in raising the fatigue limit
• A fine grain size improves the fatigue resistance
• Carburzing and Nitriding will be highly helpful

The above information is taken from Material Science Study Material provided by IEI and from Material Science and Engineering by Raghavan. Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

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Ductile Fracture

In ductile fracture, extensive plastic deformation takes place before fracture. The terms rupture or ductile rupture describe the ultimate failure of tough ductile materials loaded in tension. Rather than cracking, the material "pulls apart," generally leaving a rough surface. In this case there is slow propagation and an absorption of a large amount energy before fracture.

Many ductile metals, especially materials with high purity, can sustain very large deformation of 50–100% or more strain before fracture under favorable loading condition and environmental condition. The strain at which the fracture happens is controlled by the purity of the materials. At room temperature, pure iron can undergo deformation up to 100% strain before breaking, while cast iron or high-carbon steels can barely sustain 3% of strain.

Because ductile rupture involves a high degree of plastic deformation, the fracture behavior of a propagating crack as modeled above changes fundamentally. Some of the energy from stress concentrations at the crack tips is dissipated by plastic deformation before the crack actually propagates.

The basic steps are: void formation, void coalescence (also known as crack formation), crack propagation, and failure, often resulting in a cup-and-cone shaped failure surface.

The steps are clearly shown in the figure given here. The above info is taken from http://www.websters-online-dictionary.org/. Please do refer to them for further info. 

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Impact hardness

  We have been speaking about tensile toughness. Tensile toughness can be defined as the resistance offered by material to plastic deformation i.e. the ability to resit indentation and penetration or abrasion. Here, the load is applied slowly and the strain rate is quite slow too.

But in real life materials are also subjected to sudden blows. The resistance offered by materials to such blows (or impacts) can be called as impact toughness.

Hard, strong materials with good tensile toughness also falter under sudden impacts and exhibit brittle nature and undergo brittle fracture. The brittleness of materials and the reliability of materials under impacts can be studied using Charpy test and Izod test.

The tests are described in the further sections of this blog. Please do take time to go through the same.

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Knoop Hardness Test

      The Knoop hardness test  is a microhardness test - a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes

    A pyramidal diamond point is pressed into the polished surface of the test material with a known force, for a specified dwell time, and the resulting indentation is measured using a microscope. The geometry of this indenter is an extended pyramid with the length to width ratio being 7:1 and respective face angles are 172 degrees for the long edge and 130 degrees for the short edge. The depth of the indentation can be approximated as 1/30 of the long dimension. 

The Knoop hardness HK or KHN is then given by the formula:

where:

L = length of indentation along its long axis

C_p = correction factor related to the shape of the indenter, ideally 0.070279

P = load

        The advantages of the test are that only a very small sample of material is required, and that it is valid for a wide range of test forces. The main disadvantages are the difficulty of using a microscope to measure the indentation (with an accuracy of 0.5 micrometre), and the time needed to prepare the sample and apply the indenter.

       The above information is taken from Wikipedia. Please do visit eh same for more information.

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Status Chapter 03, Phase Diagrams

Well hello everyone,

   I worked a little on chapter Phase Diagrams. I have put up some posts related to the same on this blog. The posts mentioned below

• Introduction
• Basics of Phase diagrams
• Eutectic reaction
• Tie Line
• Old Questions, Material Science, Chap 3
• Phase diagrams & Lever Rule
• Triple Points & Gibbs Rule
• Presentation on Binary Isomorphous System
• Eutectic Systems

There is lot more to be covered and shall do the same at the earliest. The schedule for exams is out also. So gonna hurry up from now on.

Hope your preparation is going on at good pace

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Determination of yield Strength

Hello Everyone,

  In the previous articles, I told what is yield strength is? Now how to determine it is always a problem. Many a ductile materials get deformed (elastic and plastic). But the boundaries of deformation cannot be strictly defined due to hell lot of reasons. 

   So the Americans devised a plan to find out the yield strength. They define the same as the stress at which a predetermined amount of permanent deformation occurs. To find yield strength, the predetermined amount of permanent strain is set along the strain axis of the graph, to the right of the origin (zero). It is indicated in Figure as Point (D).

 

A straight line is drawn through Point (D) at the same slope as the initial portion of the stress-strain curve. The point of intersection of the new line and the stress-strain curve is projected to the stress axis. The stress value, in pounds per square inch, is the yield strength. It is indicated in Figure 5 as Point 3. This method of plotting is done for the purpose of subtracting the elastic strain from the total strain, leaving the predetermined "permanent offset" as a remainder. When yield strength is reported, the amount of offset used in the determination should be stated. For example, "Yield Strength (at 0.2% offset) = 51,200 psi."

 Notes for the above article is taken from www.engineersedge.com. Please do refer to them for more info

Don't forget to grab a copy of Material Science and Engineering book, which is essential for preparing for AMIE, Material Science.

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Stress And Strain

Stress

Stress is defined as "force per area".

Direct Stress or Normal Stress

Stress normal to the plane is usually denoted "normal stress" and can be expressed as

σ = F_n / A         (1)
where
σ = normal stress ((Pa) N/m², psi)
F_n = normal component force (N, lb_f)
A = area (m², in²)

Shear Stress

Stress parallel to the plane is usually denoted "shear stress" and can be expressed as

τ = F_p / A         (2)
where
τ = shear stress ((Pa) N/m², psi)
F_p = parallel component force (N, lb_f)
A = area (m², in²)

Strain

Strain is defined as "deformation of a solid due to stress" and can be expressed as

ε = dl / l_o = σ / E         (3)
where
dl = change of length (m, in)
l_o = initial length (m, in)
ε = unitless measure of engineering strain
E = Young's modulus (Modulus of Elasticity) (Pa, psi)

Hooke's Law -  Modulus of Elasticity (Young's Modulus or Tensile Modulus)

Most metals have deformations that are proportional with the imposed loads over a range of loads. Stress is proportional to load and strain is proportional to deformation expressed by the Hooke's law like

E = stress / strain = (F_n / A) / (dl / l_o)         (4)
where
E = Young's modulus (N/m²) (lb/in², psi)

Modulus of Elasticity or Young's Modulus are commonly used for metals and metal alloys and expressed in terms 10⁶ lb_f/in², N/m² or Pa. Tensile modulus are often used for plastics and expressed in terms 10⁵ lb_f/in² or  GPa


Please note: The above article is taken from www.engineeringtoolbox.com. Please do refer to them for further info.


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Tensile Test, Part Two

Please read the previous part of this article here

The typical stress strain curve for ductile material is given here: 

The points to note from this picture are:

Elastic Limit

  The material is elastic till this point in the curve. The stress strain ratio is constant when the curve is linear within this zone. The material deforms in this zone but regains original shape and size once the load is removed. The working stress is always much below the Elastic Limit

Yield Point:

   Plastic deformation happens at this point. The deformation is permanent in nature and the original shape and size are not restored once the load is removed.

Creep:

   A small amount of creep may come into play due to sudden elongation of material. This effect of creep is not shown in this picture. Creep usually appears for negligible time and is not taken into account.

Ultimate Strength:

    Stress is necessary to obtain stain from the yield point onwards. Ultimate tensile strength (UTS), is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract. This is the highest point in the curve

Fracture Point:

    Once the Ultimate stress is crossed, the material starts necking (i.e. non-uniform reduction in area of cross section in specimen). The material then breaks apart.

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The picture shown is taken from http://invsee.asu.edu/. Please do contact them for more info

Poisson's Ratio

Poisson's ratio (ν), is the ratio, when a sample object is stretched, of the contraction or transverse strain (perpendicular to the applied load), to the extension or axial strain (in the direction of the applied load).

When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression. This phenomenon is called the Poisson effect. Poisson's ratio ν (nu) is a measure of the Poisson effect

In the above picture, the stress is acting in X axis, but change in object is evident in Y and Z axes also. Poisson's effect is all about this change and Poisson's ratio is a measure of this effect and is given by 

For ideal material, the ratio is 0.5. But in general it ranges from 0.25 to 0.40.

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PS: Some of the info here is taken from Wikipedia. Please do consult them for more info