Test: Properties of Materials - Civil Engineering (CE) MCQ

• Magnetic susceptibility is defined as the property that indicates the degree of magnetization of a material in response to an applied magnetic field
• Magnetic susceptibility indicates whether a material is attracted to or repelled out of a magnetic field. 
• Antiferromagnetism is a form of magnetism in which the magnetic moments of neighbouring atoms are arranged anti-parallel.
• As with ferromagnetism, there are materials that show magnetic ordering below a critical temperature.
• In Anti ferromagnetic materials, the susceptibility will decrease with an increase in temperature and they have relatively small susceptibility at all temperatures.
• Antiferromagnetism property depends on the spin of electrons 
• Manganese oxide, Chromium oxide, (Cr₂O₃), and Ferrous oxide  are examples of Anti ferromagnetic materials 

Additional Information

• Ferromagnetic materials are those substances which exhibit strong magnetism in the same direction of the field
• Superparamagnetic substances do not retain any net magnetization once the external field has been removed.  they have no magnetic memory.
• A ferrimagnetic material is a material that has populations of atoms with opposing magnetic moments, as in antiferromagnetism, but these moments are unequal in magnitude so a spontaneous magnetization remains. 

Diamagnetic materials create an induced magnetic field in a direction opposite to an externally applied magnetic field and are repelled by the applied magnetic field. The opposite behaviour is exhibited by paramagnetic materials.

Properties:

• These substances are repelled by a magnet Atomic orbitals of these substances are completely filled
• It develops weak magnetization in a direction opposite to the direction of the applied magnetic field As soon as the magnetizing field is removed,
• it loses its magnetization When placed in a non-uniform magnetic field, it tends to move from stronger to weaker regions of the magnetic field
• When placed in a uniform magnetic field, it aligns itself perpendicular to the direction of the magnetic field Magnetic susceptibility is a small negative value Relative permeability is close to one and always less than 1
• Magnetic permeability is slightly less compared to free space

Additional Information

• Ferromagnetic materials are those substances which exhibit strong magnetism in the same direction of the field when a magnetic field is applied to it.
• In antiferromagnetic materials, the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in a regular pattern with neighbouring spins pointing in opposite directions. 
• Ferrimagnetic materials display a weak form of ferromagnetism associated with parallel but the opposite alignment of neighbouring atoms. 

True stress is the stress determined by the instantaneous load acting on the instantaneous cross-sectional area of the object. It is formulated as,

If a body is strained then only its dimension changes, and volume remains constant. So,

By putting all the values in equation (i), we have

where, σ_t = True Stress, σ_e = Engineering Stress, ϵ  = Engineering Strain, δL = Change in Length of Object
L_i, d_i, A_i = Initial Length, Diameter, and Area of Object respectively
L_f, d_f, A_f = Final Length, Diameter, and Area of Object respectively
Calculation:
Given:
σ_e = 400 MPa, d_i = 12 mm, d_f = 10 mm

σ_t = 576 MPa

The tool wear curve (measuring the wear until the cutter is broken):

• Referring to the graph given below,
  

Tool wear:

• Tool wear occurs due to the physical and chemical interaction between the cutting tool and workpiece as a result of the removal of small particles of the tool material from the edge of the cutting tool.
• Tool wear takes place in three stages as shown in Figure. Tool wear developed rapidly in the initial stage and then grew uniformly until it reached its limiting value.
• In the third stage, the tool wear developed rapidly and caused tool failure.
• Accelerating wear rate is found in the Failure region.

When ferromagnetic material heated beyond curie temperature,  turns into paramagnetic material, as the ferromagnetic domains become random.
Curie temperature:
Curie temperature is the temperature at which the magnetic properties of a material change.
When the temperature is greater than the Curie temperature, ferromagnetic material becomes paramagnetic material. Here, X as (magnetic susceptibility) vs. T (temperature)

Additional Information
Ferromagnetism:
Ferromagnetism is the presence of magnetic domains which are aligned in the same direction in magnetic materials. The most common examples of ferromagnetic materials are metals such as iron, nickel, cobalt, and metal alloys.

Antiferromagnetism:

• Antiferromagnetism is the presence of magnetic domains that are aligned in opposite directions in magnetic materials.
• These opposite magnetic domains have equal magnetic moments which are canceled out (since they are in opposite directions).
• This makes the net moment of material zero. This type of material is known as antiferromagnetic materials.

Classification of magnetic materials:

Resistance Thermometer -

• Resistance Thermometer first developed by Siemen in 1871, also known as ‘Platinum Resistance Thermometer’ works on the principle of change in resistance of the thermometric material (platinum) with temperature.
• Resistance is the thermometric property used in these types of thermometers. 
• These thermometers are more rugged and have more or less linear characteristics over a wide temperature range.

The variation of Resistance of material with temperature is given by:

where, α and β are the constants having their values depending upon the nature of the material used.
 R_t and R_o are the resistance value at t°C and t_o°C respectively.

• Platinum, Copper, and Nickel are mostly used as thermometric materials for resistance thermometers.
• Due to the high cost of Platinum, in general Copper and Nickel are used in place of Platinum.

Tungsten: 

• The metal tungsten is used for the filaments in incandescent bulbs.
• It has a high melting point and retains its strength when heated.
• Filaments of the light bulbs are made up of the Tungsten element.
• Its symbol is ‘W’ because of its scientific name ‘Wolfram’ and its and the atomic number 74. 
• As the resistance is less, heat energy is produced is very low which is not sufficient for an electric bulb to glow so the resistance is kept high.
• Tungsten is very resistant to corrosion and has the highest melting point (melting point = 3380 K) and the highest tensile strength of any element. Therefore option 3 is correct.
• Tungsten is used for making bulb filaments of incandescent lamps because it has the highest melting point and does not melt even while it is glowing for long hours. 
• Light bulb filaments aren't resistive because of the tungsten.
• They're resistive because of their very long length, and very thin wire.​

Key Points

• Ships are made of iron and a part of them remains underwater.
• On the part above water also water drops keep clinging to the ship’s outer surface.
• Moreover, the water of the sea contains many salts.
  • The saltwater makes the process of rust formation faster. 
• ​Therefore, ships suffer a lot of damage from rusting in spite of being painted.
• Stainless steel is made by mixing iron with carbon and metals like chromium, nickel, and manganese.
  • It does not rust.

Additional Information

• Iron is used in making bridges, ships, cars, truck bodies, and many other articles, the monetary loss due to rusting is huge.
• The process of rusting can be represented by the following equation:
  • Iron (Fe) + Oxygen (O₂, from the air) + water (H₂O) → Rust (iron oxide Fe₂O₃) 
• For rusting, the presence of both oxygen and water (or water vapour) is essential.

Soft magnetic materials have a small area of the hysteresis loop.
Hysteresis Loop (B.H Curve):

• Consider a completely demagnetized ferromagnetic material (i.e. B = H = 0)
• It will be subjected to the increasing value of magnetic field strength (H) and the corresponding flux density (B) measured the result is shown in the below figure by the curve O-a-b.
• At point b, if the field intensity (H) is increased further the flux density (B’) will not increase anymore, this is called saturation b-y is called solution flux density.
• Now if field intensity (H) is decreased, the flux density (B) will follow the curve b-c. When field intensity (H) is reduced to zero, flux remains the iron this is called remanent flux density or remanence, it is shown in fig. O-C.
• Now if the H increased in the opposite direction the flux density decreases until the point d here the flux density (B) is zero.
• The magnetic field strength (points between O and d) require to remove the residual magnetism i.e. reduce B to zero called a coercive force.
• Now if H is increased further in the reverse direction causes the flux density to increase in the reverse direction all the saturation point e.
• If H is varied backwords OX to O-Y, the flux Density (B) follows the curve b-c-d-d.
• From the figure the clear that flux density changes ‘log behind the changes in the magnetic field strength this effect is called hysteresis.
• The closed figure b-c-d-e-f-g-b is called the hysteresis loop.

• The energy loss associated with hysteresis is proportional to the area of the hysteresis loop.
• The area of the hysteresis loop varies with the type of material.
• For hard material: hysteresis loop area large → hysteresis loss also more → high remanence (O-C) and large coercivity (O-d).
• For soft material: hysteresis loop area small → hysteresis loss less → large remanence and small coercivity.

Note:
The difference between soft magnetic materials & hard magnetic materials is as shown:

• An alloy is a homogeneous mixture of two or more metals or nonmetals.
• Alloys are metal mixtures with other elements and the combination of both is governed by the properties required.
• The following table shows some metals with there alloys.

Important Points
Duralumin: It is an aluminium alloy. It contains 3.5 to 4.5% copper, 0.4 to 0.7% manganese, 0.4 to 0.7% magnesium and the remaining being aluminium. It is widely used in the aircraft industry for forging, stamping, bars, sheets, rivets, and so on.
Hindalium: It contains 5% copper and the rest aluminium. It is used for containers, utensils, tubes, rivets, etc.