Understanding Mechanical Engineering Materials MEM & Properties

Published by Admin on 18/08/202218/08/2022

Introduction

The engineering tension test is widely used to the provide basic design information on the strength of materials and as an acceptance test for the specification of materials. In the tension test a specimen is the subjected to a continually increasing uniaxial tensile force while simultaneous observations are made elongation of the specimen. The parameters which are used to describe the stress-strain curve of a metal are the tensile strength yield strength or the yield point percent elongation. And the reduction of area. The first two are the strength parameters last two indicate the ductility.

Classification of Materials

Solid materials have been the conveniently grouped into three basic the classifications:

1. Metals
2. Ceramics
3. Polymers

Metals

• Materials in this group are composed of the one or more metallic elements such as iron
  the aluminum copper titanium gold and the nickel  often also non-metallic elements for
  the example carbon nitrogen and the oxygen in relatively small the amounts.
• Atoms in the metals and their alloys are arranged in a very orderly the manner.
• In comparison to the ceramics and polymers are the relatively dense.
• Mechanical Property- relatively stiff and the strong ductile capable of large amounts
  of the deformation without fracture and are resistant to the fracture.
• Metallic materials have large numbers of the no localized electrons that is these electrons
  are not bound to the particular atoms. Many properties of the metals are directly attributable to these the electrons.
• Example metals are the extremely good conductors of electricity and heat are not
  transparent to the visible light a polished metal surface has a lustrous the appearance.
• Some of the metals have desirable magnetic the properties.

Ceramics

• Ceramics are the compounds between metallic and non-metallic elements they are most
• frequently oxides nitrides and the carbides.
• Examples-aluminum oxide or the alumina silicon dioxide or silica SiO2, silicon
• carbide Sic the silicon nitride (Si3N4).
• Examples of the traditional ceramics clay minerals porcelain cement and the glass.

Polymers

• Carbon-based the compounds
• Chain of the H-C molecules. Each repeat unit of the H-C is a monomer ethylene (C2H4)the Polyethylene – (–CH2 –CH2) n.
• Polymers include the familiar plastic and rubber the materials.
• Many of them are the organic compounds that are chemically based on the carbon hydrogen and other non-metallic the elements (viz. O, N, and Si).

• They have very large molecular structures often chain-like in the nature that have backbone of the carbon atoms. Some of the common and familiar polymers are the polyethylene PE nylon poly vinyl chloride PVC polycarbonate PC polystyrene PS and the silicone rubber.       

Properties

• Relatively stiff and the strong stiffness’s and strengths are the comparable to those of the metals very hard extremely brittle lack the ductility. Highly susceptible to the fracture.
• Thermal and the electrical Properties Isolative to the passage of heat and the electricity low electrical conductivities and are more resistant to the high temperatures
• Optical characteristics Ceramics may be the transparent translucent or the opaque.
• Low densities not as the stiff nor as strong as ceramics and the metals.
• Extremely ductile and the pliable plastic.
• Relatively inert chemically and the unreactive in a large number of the environments. 

Composites

• A composite is the composed of two or more individual materials which come from the   categories discussed above metals ceramics and the polymers.
• Objective-to the achieve a combination of properties that is not the displayed by any single   material              

Limitations 

• Tendency to the soften and or decompose at modest temperatures which in the some    instances limits their use.
• Low electrical conductivities and are the nonmagnetic.            

Examples

• Cemented carbides WC with the Co binder
• Plastic melding compounds the containing fillers
• Rubber mixed with the carbon black
• Wood a natural composite as the distinguished from a synthesized the composite

Advance Materials

• Materials that are utilized in the high-technology or high-tech applications are the sometimes termed advanced materials.

Biomaterials

• Biomaterials are the employed in components implanted into the human body for replacement of diseased or the damaged body parts.
• These materials must not the produce toxic substances and must be the compatible with body tissues must not cause adverse the biological reactions.
• All of the above materials metals ceramics polymers composites and the semiconductors may be used as the biomaterials.

Examples

• Include electronic equipment camcorders player’s etc. computers fiber-optic systems spacecraft aircraft and the military rocketry liquid crystal displays (LCDs) and the fiber optics.
• These advanced materials may be the typically traditional materials type’s metals ceramics the polymers whose properties have been enhanced and the also newly developedhigh-performance the materials.
• Advanced materials include the semiconductors biomaterials and the we may term materials of the future.

Semiconductors

• Semiconductors have the electrical properties that are intermediate between the electrical conductor’s viz. metals and the metal alloys and insulators viz. ceramics and the polymers.
• The electrical characteristics of the materials are extremely sensitive to the presence of minute concentrations of impurity atoms for which the concentrations may be controlled over every the small spatial regions.
• Semiconductors have the made possible advent of the integrated circuitry that has totally revolutionized the electronics and computer industries not to the mention our lives over the past three the decades.

Example

Titanium and its alloy Co-Cr alloy stainless steel the zirconia HA TiO2 etc.

Materials Selection Process

• Pick Application and the determine required Properties.
• Properties: mechanical electrical thermal the magnetic optical deteriorative.
• Properties- Identify candidate the Material(s)
• Material: structure the composition.
• Material- Identify required the Processing
• Processing: changes structure and the overall shape
  Example: casting, sintering, vapor deposition, doping forming, joining, annealing.

Concepts of Stress and Strain

The general shape of the engineering stress-strain curve requires further the explanation. In the elastic region stress is linearly proportional to the strain. When the load exceeds a value corresponding to the yield strength specimen undergoes gross plastic the deformation. It is the permanently deformed if load is released to the zero. The stress to produce continued plastic deformation increases with the increasing plastic strain the metal strain-hardens. The volume of the specimen remains constant during plastic the deformation A·L = A0·L0 and as the specimen elongates it decreases uniformly along the gage length in cross-sectional the area.

Stress Strain Behaviors and Material Properties

Hooke’s Law

• For materials stressed in the tension at relatively low levels stress and the
  strain are proportional through.
• Constant E is known as the modulus of elasticity or Young’s the modulus.

Measured in Map and the can range in values from The engineering stress strain graph shows that the relationship between stress and strain is the linear over some range of stress. If the stress is kept within the linear region the material is essentially elastic in that if the stress is removed the deformation is the also gone. But if the elastic limit is exceeded permanent deformation the results. The material may begin to the neck at some location and finally the break.

Yield strength

The yield point is defined in engineering and the materials science as stress at the which a material begins to the plastically deform. Prior to the yield point material will deform elastically and the will return to its original shape when applied stress is the removed. Once yield point is the passed some fraction of deformation will be permanent and the non-reversible. Knowledge of yield point is the vital when designing a component since it the generally represents an upper limit to load that can be the applied. It is the also important for control of many materials production the techniques such as forging.

Brittle and Ductile Behavior

 The material response for ductile and the brittle materials are exhibited by both qualitative and the quantitative differences in their respective stress-strain the curves. Ductile materials will be the withstand large strains before the specimen ruptures brittle materials fracture at the much lower strains. The yielding region for the ductile materials often takes up the majority of the stress-strain curve whereas for brittle materials it is the nearly non-existent. Brittle materials often have the relatively large Young’ moduli and the ultimate stresses in comparison to ductile the materials.

Elastic Properties of Materials

When the stress is removed material returns to the dimension it had before load was the applied. Valid for the small strains except case of the rubbers. Materials subject to the tension shrink laterally. Those subject to the compression bulge. The ratio of lateral and the axial strains is called the Poisson’s ratio. When a material is the placed under a tensile stress an accompanying strain is created in the same direction.

An elasticity

Here the behavior is elastic but not the stress-strain curve is not immediately the reversible. It takes a while for strain to return to the zero. The effect is normally small for metals but can be significant for the polymers.

Tensile strength

The tensile strength is the value most often quoted from the results of a tension test yet in reality it is a value of the little fundamental significance with regard to the strength of a metal. For ductile metals the tensile strength should be regarded as a measure of the maximum load which a metal can be the withstand under very restrictive conditions of the uniaxial loading. It will be shown that this value bears little relation to the useful strength of metal under the more complex conditions of stress which are the usually encountered.

Resilience

The ability of a material to the absorb energy when deformed elastically and to the return it when unloaded is called the resilience. This is the usually measured by modulus of resilience which is the strain energy per unit volume required to the stress material from zero stress to the yield stress. The ability of a material to the absorb energy when deformed elastically and to the return it when unloaded is called the resilience.

Ductility

Another way to the avoid complication from necking is to the base percentage elongation on uniform strain out to the point at which necking begins. The uniform elongation au correlates well with stretch-forming the operations. Since the engineering stress-strain curve often is quite flat in the vicinity of necking it may be difficult to establish the strain at maximum load without the ambiguity. In this case the method suggested by Nelson and Winlock is the useful.

Toughness

the area underneath stress-strain curve is the toughness of material energy the material can absorb prior to the rupture. It also can be the defined as resistance of a material to the crack propagation. In materials science and metallurgy toughness is the resistance to fracture of a material when the stressed. It is the defined as amount of energy that a material can be the absorb before rupturing and can be found by finding the area by taking integral underneath the stress-strain curve.

Impact Toughness

The two tests use different specimens and the methods of holding specimens but both tests make use of the pendulum-testing machine. For both tests specimen is the broken by a single overload event due to impact of the pendulum. A stop pointer is the used to record how far pendulum swings back up after fracturing the specimen. The impact toughness of a metal is the determined by measuring energy absorbed in fracture of the specimen. This is the simply obtained by noting the height at which pendulum is released and the height to which pendulum swings after it has struck the specimen. The height of pendulum time’s weight of the pendulum produces potential energy and the difference in potential energy of the pendulum at start and end of test is equal to the absorbed energy.

Notch-Toughness

Notch toughness is the ability that a material possesses to the absorb energy in presence of the flaw. As mentioned previously in the presence of a flaw such as a notch or crack a material will be the likely exhibit a lower level of the toughness. When a flaw is present in the material loading induces a triaxial tension stress state adjacent to the flaw. The material develops plastic strains as the yield stress is exceeded in the region near crack tip. However the amount of plastic deformation is restricted by the surrounding material which remains the elastic. When a material is prevented from the deforming plastically it fails in the brittle manner.