Toughness is one of the mechanical properties of materials. In this ?discussion, you will learn more about this important property and ?understand its importance.? ?Before you begin, be

29 Aug Toughness is one of the mechanical properties of materials. In this ?discussion, you will learn more about this important property and ?understand its importance.? ?Before you begin, be

Posted at 00:51h in Engineering / Electronic Engineering by

Toughness is one of the mechanical properties of materials. In this discussion, you will learn more about this important property and understand its importance.

Before you begin, be sure to review the following resources:

In your original post, answer the following:

What are the definition and units used to describe Toughness?
List at least two tests used to determine the toughness of a material?
Pick one of these tests and explain how it is performed.
Compare the toughness of two materials.

PHYSICAL PROPERTIES OF MATERIALS

Volumetric and Melting Properties

Thermal Properties

Electrical Properties

Electrochemical Processes

Physical Properties Defined

Properties that define the behavior of materials in response to physical forces other than mechanical

Volumetric, thermal, electrical, and electrochemical properties

Components in a product must do more than withstand mechanical stresses

They must conduct electricity (or prevent conduction), allow heat to transfer (or allow its escape), transmit light (or block transmission), and satisfy many other functions

Physical Properties in Manufacturing

Important in manufacturing because physical properties often influence process performance

In machining, thermal properties of the work material determine cutting temperature, which affects tool life

In microelectronics, electrical properties of silicon and how these properties can be altered by chemical and physical processes is the basis of semiconductor manufacturing

Volumetric and Melting Properties

Properties related to the volume of solids and how these properties are affected by temperature

Density

Thermal expansion

Melting point

Density and Specific Gravity

Density = weight per unit volume

Typical units are g/cm3 (lb/in3)

Determined by atomic number and other factors such as atomic radius, and atomic packing

Specific gravity = density of a material relative to density of water

Ratio with no units

Why Density is Important

A consideration in material selection for a given application, but it may not be the only property of interest

Strength may also be important, and the two properties are often related in a strength‑to‑weight ratio (specific strength), which is tensile strength divided by density

Useful ratio in comparing materials for structural applications in aircraft, automobiles, and other products where weight and energy are concerns

Thermal Expansion

Density of a material is a function of temperature

In general, density decreases with increasing temperature

Volume per unit weight increases with increasing temperature

Thermal expansion is the name for this effect of temperature on density

Measured as coefficient of thermal expansion

Coefficient of Thermal Expansion

Length ratio rather than volume ratio because this is easier to measure and apply

Change in length for a given temperature change:

L2 ‑ L1 = L1 (T2 ‑ T1)

where  = coefficient of thermal expansion; L1 and L2 are lengths corresponding respectively to temperatures T1 and T2

Thermal Expansion in Manufacturing

Thermal expansion is used in shrink fit and expansion fit assemblies

Part is heated to increase size or cooled to decrease size to permit insertion into another part

When part returns to ambient temperature, a tightly‑fitted assembly is obtained

Thermal expansion can be a problem in heat treatment and welding due to thermal stresses that develop in material during these processes

Melting Characteristics for Elements

Melting point Tm of a pure element = temperature at which it transforms from solid to liquid state

The reverse transformation occurs at the same temperature and is called the freezing point

Heat of fusion = heat energy required at Tm to accomplish transformation from solid to liquid

Melting of Noncrystalline Materials

In noncrystalline materials (glasses), a gradual transition from solid to liquid states occurs

The solid material gradually softens as temperature increases, finally becoming liquid at the melting point

During softening, the material has a consistency of increasing plasticity (increasingly like a fluid) as it gets closer to the melting point

Importance of Melting in Manufacturing

Metal casting – the metal is melted and then poured into a mold cavity

Metals with lower melting points are generally easier to cast

Plastic molding – melting characteristics of polymers are important in nearly all polymer shaping processes

Sintering of powdered metals – sintering does not melt the metal, but temperatures must approach the melting point to achieve bonding of the powders

Thermal Properties

Thermal expansion, melting, and heat of fusion are thermal properties because temperature determines the thermal energy level of the atoms, leading to the changes in materials

Additional thermal properties:

Specific heat

Thermal conductivity

These properties relate to the storage and flow of heat within a substance

Specific Heat

The quantity of heat energy required to increase the temperature of a unit mass of material by one degree

To determine the energy to heat a certain weight of metal to a given temperature:

H = C W (T2 ‑ T1)

where H = amount of heat energy; C = specific heat of the material; W = its weight; and (T2 ‑ T1) = change in temperature

Volumetric Specific Heat

The quantity of heat energy required to raise the temperature of a unit volume of material by one degree

Density  multiplied by specific heat C

Volumetric specific heat = C

Thermal Conductivity

Capability of a material to transfer heat through itself by the physical mechanism of thermal conduction

Thermal conduction involves the transfer of thermal energy within a material from molecule to molecule by purely thermal motions

No mass transfer

Coefficient of thermal conductivity k is generally high in metals, low in ceramics and plastics

Units for k: J/s mm C (Btu/in hr F)

Thermal Diffusivity

The ratio of thermal conductivity to volumetric specific heat is frequently encountered in heat transfer analysis

Thermal Properties in Manufacturing

Important in manufacturing because heat generation is common in so many processes

In some cases, heat is the energy that accomplishes the process

Heat treating, sintering of powder metals and ceramics

In other cases, heat is generated as a result of the process

Cold forming and machining of metals

Electrical Properties

Engineering materials exhibit a great variation in their capability to conduct electricity

Flow of electrical current involves movement of charge carriers ‑ infinitesimally small particles possessing an electrical charge

In solids, these charge carriers are electrons

In a liquid solution, charge carriers are positive and negative ions

Electrical Properties

Movement of charge carriers is driven by the presence of electric voltage

And resisted by the inherent characteristics of the material, such as atomic structure and bonding between atoms and molecules

Ohm's law: I =

where I = current, A, E = voltage, V, and R = electrical resistance, 

Electrical Resistance

Resistance in a uniform section of material (e.g., a wire) depends on its length L, cross‑sectional area A, and resistivity of the material r

where resistivity r has units of ‑m2/m or ‑m (‑in)

Resistivity

Property that defines a material's capability to resist current flow

Resistivity r has units of (‑m) or (‑in)

Resistivity is not a constant; it varies, as do so many other properties, with temperature

For metals, resistivity increases with temperature

Conductivity

Often more convenient to consider a material as conducting electrical current rather than resisting its flow

Conductivity of a material is simply the reciprocal of resistivity:

Electrical conductivity =

where conductivity has units of (‑m)‑1 or (‑in)‑1

Materials and Electrical Properties

Metals are the best conductors of electricity, because of their metallic bonding

Most ceramics and polymers, whose electrons are tightly bound by covalent and/or ionic bonding, are poor conductors

Many of these materials are used as insulators because they possess high resistivities

Semiconductors

A material whose resistivity lies between insulators and conductors

Most common semiconductor material is silicon, largely because of its abundance in nature, relative low cost, and ease of processing

What makes semiconductors unique is the capacity to significantly alter conductivities in their surface chemistries in very localized areas to fabricate integrated circuits

Electrical Properties in Manufacturing

The important welding processes, such as arc welding and resistance spot welding, use electrical energy to melt the joint metal

Capability to alter electrical properties of semiconductor materials is the basis for microelectronics

Electrochemistry

Field of science concerned with the relationship between electricity and chemical changes, and the conversion of electrical and chemical energy

In a water solution, molecules of an acid, base, or salt are dissociated into positively and negatively charged ions

Ions are the charge carriers in the solution

They allow electric current to be conducted, playing the same role that electrons play in metallic conduction

Terms in Electrochemical Processes

Electrolyte – the ionized solution

Electrodes – where current enters and leaves the solution in electrolytic conduction

Anode – positive electrode

Cathode – negative electrode

The whole arrangement is called an electrolytic cell

Electrolysis

The name given to these chemical changes occurring in the solution

At each electrode, chemical reaction occurs, such as:

Deposition or dissolution of material

Decomposition of gas from the solution

Hydrogen Through Electrolysis – Ocean Geothermal Energy Foundation

Electrolysis in Manufacturing Processes

Electroplating ‑ an operation that adds a thin coating of one metal (e.g., chromium) to the surface of a second metal (e.g., steel) for decorative or other purposes

Electrochemical machining ‑ a process in which material is removed from the surface of a metal part

Production of hydrogen and oxygen gases

Video

Copper Plating – Easy Electrolysis & Electroplating

(https://youtu.be/kEkWRE028EY)

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MECHANICAL PROPERTIES OF MATERIALS

Stress‑Strain Relationships

Hardness

Effect of Temperature on Properties

Mechanical Properties in Design and Manufacturing

Mechanical properties determine a material’s behavior when subjected to mechanical stresses

Properties include elastic modulus, ductility, hardness, and various measures of strength

Stress‑Strain Relationships

Three types of static stresses to which materials can be subjected:

Tensile – stretching the material

Compressive – squeezing the material

Shear – causing adjacent portions of the material to slide against each other

Stress‑strain curve – basic relationship that describes mechanical properties for all three types

Tensile Test

Most common test for studying stress‑strain relationship, especially metals

In the test, a force pulls the material, elongating it and reducing its diameter

(left) Tensile force applied and (right) resulting elongation of material

Tensile Test Specimen

ASTM (American Society for Testing and Materials) specifies preparation of test specimen

Tensile Test Setup

Tensile testing machine

Engineering Stress

Defined as force divided by original area:

where s = engineering stress, F = applied force, and Ao = original area of test specimen

Engineering Strain

Defined at any point in the test as

where e = engineering strain; L = length at any point during elongation; and Lo = original gage length

Typical Engineering Stress-Strain Plot

Typical engineering stress‑strain plot in a tensile test of a metal

Two regions:

Elastic region

Plastic region

Tensile Test Sequence

(1) No load; (2) uniform elongation and area reduction; (3) maximum load; (4) necking; (5) fracture; (6) final length

Elastic Region in Stress‑Strain Curve

Relationship between stress and strain is linear

Hooke's Law: e = E e

where E = modulus of elasticity

Material returns to its original length when stress is removed

E is a measure of the inherent stiffness of a material

Its value differs for different materials

Yield Point in Stress‑Strain Curve

As stress increases, a point in the linear relationship is finally reached when the material begins to yield

Yield point Y can be identified by the change in slope at the upper end of the linear region

Y = a strength property

Other names for yield point:

Yield strength

Yield stress

Elastic limit

Plastic Region in Stress‑Strain Curve

Yield point marks the beginning of plastic deformation

The stress-strain relationship is no longer guided by Hooke's Law

Tensile Strength in Stress‑Strain Curve

Elongation is accompanied by a uniform reduction in cross‑sectional area, consistent with maintaining constant volume

Finally, the applied load F reaches a maximum value, and engineering stress at this point is called the tensile strength TS (a.k.a. ultimate tensile strength)

TS =

Ductility in Tensile Test

Ability of a material to plastically strain without fracture

Ductility measure = elongation EL

where EL = elongation; Lf = specimen length at fracture; and Lo = original specimen length

Lf is measured as the distance between gage marks after two pieces of specimen are put back together

Example

A tensile test specimen has a starting gage length = 50 mm and a cross-sectional area = 200 mm2. During the test, the specimen yields under a load of 32,000 N (this is the 0.2% offset) at a gage length of 50.2 mm. The maximum load of 65,000 N is reached at a gage length of 57.7 mm just before necking begins. Final fracture occurs at a gage length of 63.5 mm. Determine (a) yield strength, (b) modulus of elasticity, (c) tensile strength, (d) engineering strain at maximum load, and (e) percent elongation.

Solution

True Stress

Stress value obtained by dividing the instantaneous area into applied load

where  = true stress; F = force; and A = actual (instantaneous) area resisting the load

True Strain

Provides a more realistic assessment of "instantaneous" elongation per unit length

True Stress-Strain Curve

True stress‑strain curve for previous engineering stress‑strain plot

Strain Hardening in Stress-Strain Curve

Note that true stress increases continuously in the plastic region until necking

In the engineering stress‑strain curve, the significance of this was lost because stress was based on the original area value

It means that the metal is becoming stronger as strain increases

This is the property called strain hardening

Videoes

Understanding Material Strength, Ductility and Toughness

( https://youtu.be/WSRqJdT2COE)

Engineering vs True Stress

(https://youtu.be/rZN5uSbSvSo)

Compression Test

Applies a load that squeezes the ends of a cylindrical specimen between two platens

Compression force applied to test piece and resulting change in height and diameter

Compression Test Setup

Engineering Stress in Compression

As the specimen is compressed, its height is reduced and cross‑sectional area is increased

 = –

where Ao = original area of the specimen

Engineering Strain in Compression

Engineering strain is defined

Since height is reduced during compression, value of e is negative (the negative sign is usually ignored when expressing compression strain)

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29 Aug Toughness is one of the mechanical properties of materials. In this ?discussion, you will learn more about this important property and ?understand its importance.? ?Before you begin, be

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