Mechanical Properties of Metals

➢ TYPES OF LOADING

Deformation occurs when forces are applied to a material. There are three principal ways in which a load may be applied: tension, compression and shear. 

==> Tension Load: As the ends of material are pulled apart to make the material longer, the load is called a tension load.

==> Compression Load: As the ends of material are pushed in to make the material shorter, the load is called a compression load.

==> Shear Load: The load imposed parallel to the upper and lower faces. Torsion is a variation of pure shear, wherein a structural member is twisted. Torsional forces produce a rotational motion about the longitudinal axis of one end of the member relative to the other end.

Tensile load produces an elongation and compressive load produces contraction. 

==> Stress : Force per unit area. Forces are measured in Newtons , so stress has the dimensions of N/m^2 or Pa. But a stress of 1 N/m^2 is tiny, so the usual unit is MN/m^2 , called Mega Pascals, symbol MPa. 

==> Strain : Amount of deformation per unit length. Strain is the response of materials to stress, since strain is the ratio of two lengths, it is dimensionless . Strain can be negative for compressive loads.

➢ MODULUS OF ELASTICITY

Deformation in which stress and strain are proportional is called elastic deformation. The constant of proportionality E is the modulus of elasticity or Young’s Modulus. The slope of linear region of stress-strain curve corresponds to the modulus of elasticity. The modulus of elasticity is stiffness or  materials resistance to elastic deformation. The greater the modulus, the stiffer the material (or smaller the elastic strain produced applied load).

➢ SHEAR MODULUS

Shear stress and shear strain are proportional each other. The constant of proportionality G is the shear modulus. G = Shear Stress / Shear Strain

Poission’s ratio is defined as the ratio of the lateral and axial strains. For the isotropic materials the shear and elastic moduli are related to each other and to Paisson's ratio. Poisson’s ratio ranges between 0.25 to 0.35. 

Elastic deformation (Reversible):  

Stress removed ==> material returns to original shape

Plastic deformation (Irreversible): 

Stress removed ==> material does not return to original dimensions.

➢ TENSILE TEST

Tensile test determines the several mechanical properties materials such as strength, ductility, and toughness.

The applied stress versus the strain or elongation of the specimen shows the initial elastic response of the material, followed by yielding, plastic deformation and finally necking and failure. Several measurements are taken from the plot, called the Engineering Stress-Strain Diagram. 

These include:

==> Modulus of elasticity       

==> Yield strength 

==> Tensile strength               

==> Modulus of resilience 

==> Failure stress                   

==> Ductility  

==> Toughness

Sometimes yield strength may not be determined exactly. In this case, to determine yield strength, straight line is drawn parallel to the elastic portion of stress-strain curve at strain of 0.002 point. The stress corresponding to the intersection of this line and the stress-strain curve is the yield strength. 

➢ DUCTILITY

Ductile materials experience plastic deformation. Material that experiences very little or no plastic deformation upon fracture is termed brittle. If the strain  5%, the material is brittle. Ductility percent may be expressed as either elongation reduction in area.

➢ RESILIENCE 

Resilience is the capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy recovered. The associated property is the modulus of resilience, Ur, which is the strain energy per unit volume required to stress a material from an unloaded state up to the point of yielding. Ur is the area under the stress-strain curve taken to yielding. Resilient materials are those having high yield strengths and low moduli of elasticity. 

➢ TOUGHNESS

Toughness is energy up absorbed to fracture. It is the area under the stress-strain curve up to the point of fracture. For a material to be tough, it must display both strength and ductility. Often, ductile materials are tougher than brittle ones.

➢ TRUE STRESS-TRUE STRAIN CURVE

The engineering stress is on the basis of the original cross-sectional area before any deformation, and does not take into account the reduction in area at the neck.

➢ COMPRESSION TEST

A compression test is conducted in a manner similar to the tensile test except that the force is compressive and the specimen contracts along the direction of the stress.

Brittle materials ==> Tensile test are more common. Compression tests are used when the material is brittle in tension. Brittle materials such as cast irons usually reach much higher ultimate stresses in compression than intension. The brittle fracture is performed by separation and is not accompanied by noticeable plastic deformation.

Ductile materials ==> The ductile materials such as steel, Al, and Cu have stress–strain diagrams similar to one swhich for tensile test. When a small specimen of the ductile material is compressed, it begins to bulge on sides and becomes barrel shaped as shown in the figure above. With increasing load, the specimen is flattened out, thus offering increased resistance to further shortening (which means that the stress–strains curve goes upward) this effect is indicated in the diagram.

➢ HARDNESS

Hardness is a measure of material’s resistance to localized plastic deformation. Hardness tests are simple, cheap, nondestructive. 

Hardness Techniques 

==> Rockwell Hardness Test 

==> Brinell Hardness Test 

==> Vickers Hardness Test 

==> Knoop and Vickers Microhardness Tests

➢ FRACTURE

Separation of a material into two or more pieces in response to imposed stress. For engineering materials, two fracture modes are possible: ductile and brittle.

==> Ductile materials: Extensive plastic deformation with high energy absorption (toughness) before fracture.

==> Brittle materials: Little plastic deformation with low energy absorption before fracture. 

➢ IMPACT TESTING

Impact testing was commonly used to assess fracture characteristics of materials. 

Impact Parameters

==> Test Temperature 

==> Specimen Geometry 

==> Crack or Notch length 

==> Impact Energy 

Ductile to Brittle Transition 

For many materials, the impact energy is correlated with the toughness determined in a tensile test . However, as temperature drops, some materials show a transition from ductile to brittle behavior. Impact test determine whether or not a material experiences a ductile to brittle transition with decreasing temperature. Temperature at which a material changes from ductile to brittle failure is called transition temperature. Transition temperature is usually taken as the point where 50% of the fracture is brittle.

Transition Temperature

Materials must be used above their ductile to brittle transition temperatures to avoid brittle failure. Increasing the carbon content of the steels raises the transition temperature. Grain size reduction decreases the transitiontemperatureofsteels. 

➢ FATIGUE

In many applications, materials are subjected to vibrating or oscillating forces. The behavior of materials under such load conditions differs from the behavior under a static load. The material is subjected to repeated load cycles  in actual use. Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating  stresses. Fatigue failure occurs at a stress level lower than tensile and yield strength for a static load. It is estimated that 90% of all material failure is due to fatigue. Polymers and ceramics are also susceptible to this type of failure. Fatigue is catastrophic, occurring without warning.

CYCLIC STRESSES

==> Reversed stress cycle

==> Repeated stress cycle

==> Randomstresscycle

➢ FATIGUE TEST

The applied stress maybe axial (tension-compression), flexural (bending), or torsinal (twisting) in nature. Data are plotted as stress(S) versus logarithm of the number (N) of cycles to failure for each of the specimens. 

S-N Curve

There are two general types of S-N curve.

Fatigue Limit (endurance limit) ==> Ferrous and Ti alloys

Fatigue strength ==> Non-ferrous alloys

Another important parameter is fatigue life.

Factors That Affect Fatigue Life 

==> Magnitude of stress: Increasing in fatigue life.  

==> Environmental effects: If a corrosive environment is present during the cyclic stress of a metal, the chemical attack greatly accelerates the rate at which fatigue cracks propagate. 

The combination of corrosion attack and cyclic stresses on a metal is known as corrosion fatigue. Solutions for corrosion fatigue :

==> decrease corrosiveness of medium 

==> add protective surface coating 

==> Surface Effects: Most cracks leading to fatigue failure originate at surface positions. Therefore, fatigue life is sensitive to surface conditions. Surface factors include;

==> Design Factors

==> Surface treatments

➢ CREEP

Materials are often placed in service at elevated temperatures and exposed to static mechanical stresses. Deformation under such circumstances is termed creep. Creep is the load time-dependent deformation due to constant at high temperature (> 0.4 Tm). A specimen is subjected to a constant load while maintaining the temperature constant; deformation or strain is measured and plotted as a function of elapsed time. Creep specimens have the same geometry as for tensile tests.

CREEP PARAMETERS

==> Secondary/ steady-state creep rate

==> Rupture lifetime

With increasing stress or temperature: 

 ==> The instantaneous strain increases 

 ==> The steady-state creep rate increases 

 ==> The time to rupture decreases

➢ TORSION TEST

Torsion test is not widely used as much as tensile test. Because it is not possible to generate a uniform shear stress; when the surface plastic, the interior is still elastic because shear stress on surface is larger than the interior. Torsion test is made to determine such properties as the shear modulus elasticity , shear of strain, and shear strength. 

Often used for testing brittle materials and can be tested in full sized parts such as shafts, axles and twist drills which are subjected to torsional loading in use. A twisting moment is applied to one end of a specimen while measuring the deformation as angular displacement at the other end. During the test the angle of twist and the applied torque test proceeds.

Shear stress-shear strain in torsion;

==> Using the appropriate formulae , relationships and the measured dimensions, we can determine the shear stress and shear strain on the specimen. 

==> Then, one can plot the torque vs. angle of twist, and shear which one can find the material properties previously mentioned. 

==> The stress vs. shear strain from twisting moment is resisted by shear stresses set up in the cross section of the bar (zero at the center, maximum at the surface).

Elastic properties and yield strength in torsion;

 ==> The elastic properties in torsion can be obtained by using the torque at the proportional limit where the shear stress is calculated corresponding to the twisting moment from the shear stress equation. 

==> The torsional elastic limit or yield strength can be obtained from testing a tubular specimen since the stress gradient are practically eliminated.

Types of torsion failures;
Torsion failures are different from tensile failure in that there is little localised reduction of area or elongation. 

==> Ductile (shear) failure is along the maximum shear plane (perpendicular to longitudinal axis of the specimen). 

==> Brittle (tensile) failure is perpendicular to the maximum tensile stress (at 45°), resulting in a helical fracture.

➢ BEND TEST for brittle materials

In many brittle materials, the normal tensile test cannot easily be performed because of the presence of flaws at the surface. Often, just placing a brittle material in the grips of the tensile testing machine causes cracking. These materials maybe tested using the bend test. 

Bend test: Application of a force to the center of a bar that is supported on each end to determine the resistance of the material to a static or slowly applied load.

➢ BEND TEST

Flexural strength or modulus of rupture: The stress required to fracture a specimen in a bend test. Flexural modulus: The modulus of elasticity in bending calculated from the results of a bend test, giving the slope of the stress deflection curve.

The Science and Engineering of Materials, D.R. Askeland

Materials Science and Engineering: An Introduction, W.D. Callister, 8th ed., John Wiley, 2007