Chapter I.

Introduction to Properties and Applications of Engineering Materials

CHAPTER 1
Introduction to Properties and Application of Engineering Materials
______________________________________________________________________________
Learning Objectives to be achieved:

1. To recount the Properties of Engineering Materials as to:

a. General Physical Properties of Engineering Materials,
b. Mechanical properties of Engineering Materials,
c. Thermal properties of Engineering Materials;
2. To distinguish various application of engineering materials.

Introduction:

Material means as a substance from which substance is made. As we begin the 21st century,
advances in materials research and technology offer great promise. Materials Science forms the
foundation for engineers in product development because the structures, components, and
devices that engineers design are limited by the properties of the materials that are available
and the techniques that can be used for fabrication.

I. Properties of Materials

Selecting the "best" material is usually a difficult task, requiring tradeoffs between different
material properties including:

1. General Physical Properties

2. Mechanical Properties, and
3. Thermal Properties

1. General Physical Properties

1.1. Density

 Density is one of the most fundamental physical properties of any material.

 It is defined as the ratio of an objects mass to its volume. Because most designs are
limited by either size and or weight density is an important consideration in many
calculations.
 Density is a function of the mass of the atoms making up the materials and the distance
between them. Massive, closely packed atoms characterize high density materials such
as Tungsten or Neptunium. In contrast light, relatively distant atoms compose low
density materials such as Beryllium or Aluminum. Density on a macroscopic level is also a
function of the microscopic structure of a material. A relatively dense material may be
capable of forming a cellular structure such as a foam which can be nearly as strong and

CE 261 – Engineering Materials Page 1

PNGUOT-Civil Engineering Department
Chapter I. Introduction to Properties and Applications of Engineering Materials

much less dense than the bulk material. Composites including natural constituents such
as wood and bone, for example, generally rely on microscopic structure to achieve
densities far lower than common monolithic materials.

1.2. Availability/Manufacturability

 Availability and manufacturability requirements are often unseen limiting factors in

materials selection. The importance of a material being available is obvious. Materials
which are not available cannot be used. The importance of processibility is not always so
obvious.
 Any other desirable qualities are useless if a material cannot be processed into the shape
required to perform its function. Most engineering materials in use today have well
known substitutes which would perform better and often at lower cost but processes for
forming, cutting, machining, joining, etc. are not available or commercially viable. There
is often a period of time after a new material is introduced during which its application is
severely limited while processing techniques are developed which facilitate its use.

1.3. Cost

 A materials cost is also generally a limiting factor. While cost is universally

recognized and perhaps the easiest of all properties to understand there are
specific cost considerations for materials selection. Just as materials and their
processing go hand in hand so do material costs and processing costs.
Understanding the entire processing sequence is critical to accurately evaluating
the true cost of a material.

1.4. Appearance

 Because the appearance of many mechanical components seems fairly trivial it is

also easy to overlook its importance in the marketing and commercial success of
a product.

2. Mechanical Properties

The mechanical properties of a material describe how it will react to physical forces.
Mechanical properties occur as a result of the physical properties inherent to each material,
and are determined through a series of standardized mechanical tests.

2.1. Strength

 Strength has several definitions depending on the material type and application.
Before choosing a material based on its published or measured strength it is
important to understand the manner in which strength is defined and how it is

CE 261 – Engineering Materials Page 2

PNGUOT-Civil Engineering Department
Chapter I. Introduction to Properties and Applications of Engineering Materials

measured. When designing for strength, material class and mode of loading are
important considerations.
 For metals the most common measure of strength is the yield strength. For most
polymers it is more convenient to measure the failure strength, the stress at the
point where the stress strain curve becomes obviously non-linear. Strength, for
ceramics however, is more difficult to define. Failure in ceramics is highly
dependent on the mode of loading. The typical failure strength in compression is
fifteen times the failure strength in tension. The more common reported value is
the compressive failure strength.

2.2. Elastic limit

 The elastic limit is the highest stress at which all deformation strains are fully
recoverable. For most materials and applications this can be considered the
practical limit to the maximum stress a component can withstand and still
function as designed. Beyond the elastic limit permanent strains are likely to
deform the material to the point where its function is impaired.

2.3. Proportional limit

 The proportional limit is the highest stress at which stress is linearly proportional
to strain. This is the same as the elastic limit for most materials. Some materials
may show a slight deviation from proportionality while still under recoverable
strain. In these cases the proportional limit is preferred as a maximum stress
level because deformation becomes less predictable above it.

2.4. Yield Strength

 The yield strength is the minimum stress which produces permanent plastic
deformation. This is perhaps the most common material property reported for
structural materials because of the ease and relative accuracy of its
measurement. The yield strength is usually defined at a specific amount of
plastic strain, or offset, which may vary by material and or specification. The
offset is the amount that the stress-strain curve deviates from the linear elastic
line. The most common offset for structural metals is 0.2%.

2.5. Ultimate Tensile Strength

 The ultimate tensile strength is an engineering value calculated by dividing the

maximum load on a material experienced during a tensile test by the initial cross
section of the test sample. When viewed in light of the other tensile test data
the ultimate tensile strength helps to provide a good indication of a material's
toughness but is not by itself a useful design limit. Conversely this can be

CE 261 – Engineering Materials Page 3

PNGUOT-Civil Engineering Department
Chapter I. Introduction to Properties and Applications of Engineering Materials

construed as the minimum stress that is necessary to ensure the failure of a

material.

2.6. True Fracture Strength

 The true fracture strength is the load at fracture divided by the cross sectional
area of the sample. Like the ultimate tensile strength the true fracture strength
can help an engineer to predict the behavior of the material but is not itself a
practical strength limit. Because the tensile test seeks to standardize variables
such as specimen geometry, strain rate and uniformity of stress it can be
considered a kind of best case scenario of failure.

2.7. Ductility

 Ductility is a measure of how much deformation or strain a material can

withstand before breaking. The most common measure of ductility is the
percentage of change in length of a tensile sample after breaking. This is
generally reported as % El or percent elongation. The R.A. or reduction of area of
the sample also gives some indication of ductility.

2.8. Toughness

 Toughness describes a material's resistance to fracture. It is often expressed in

terms of the amount of energy a material can absorb before fracture. Tough
materials can absorb a considerable amount of energy before fracture while
brittle materials absorb very little. Neither strong materials such as glass or very
ductile materials such as taffy can absorb large amounts of energy before failure.
Toughness is not a single property but rather a combination of strength and
ductility.
 The toughness of a material can be related to the total area under its stress-
strain curve. A comparison of the relative magnitudes of the yield strength,
ultimate tensile strength and percent elongation of different material will give a
good indication of their relative toughness. Materials with high yield strength
and high ductility have high toughness. Integrated stress-strain data is not
readily available for most materials so other test methods have been devised to
help quantify toughness. The most common test for toughness is the Charpy
impact test.
 In crystalline materials the toughness is strongly dependent on crystal structure.
Face centered cubic materials are typically ductile while hexagonal close packed
materials tend to be brittle. Body centered cubic materials often display
dramatic variation in the mode of failure with temperature. In many materials
the toughness is temperature dependent. Generally materials are more brittle at
lower temperatures and more ductile at higher temperatures. The temperature
at which the transition takes place is known as the DBTT, or ductile to brittle

CE 261 – Engineering Materials Page 4

PNGUOT-Civil Engineering Department
Chapter I. Introduction to Properties and Applications of Engineering Materials

transition temperature. The DBTT is measured by performing a series of Charpy

impact tests at various temperatures to determine the ranges of brittle and
ductile behavior. Use of alloys below their transition temperature is avoided due
to the risk of catastrophic failure.

2.9. Fatigue ratio

 The dimensionless fatigue ratio f is the ratio of the stress required to cause
failure after a specific number of cycles to the yield stress of a material. Fatigue
tests are generally run through 107 or 108 cycles. A high fatigue ratio indicates
materials which are more susceptible to crack growth during cyclic loading.

2.10. Loss coefficient

 The loss coefficient is an other important material parameter in cyclic loading. It

is the fraction of mechanical energy lost in a stress strain cycle. The loss
coefficient for each material is a function of the frequency of the cycle. A high
loss coefficient can be desirable for damping vibrations while a low loss
coefficient transmits energy more efficiently. The loss coefficient is also an
important factor in resisting fatigue failure. If the loss coefficient is too high,
cyclic loading will dissipate energy into the material leading to fatigue failure.

3. Thermal Properties

3.1. Thermal conductivity

 The thermal conductivity is the rate of heat transfer through a material in steady state.
It is not easily measured, especially for materials with low conductivity but reliable data
is readily available for most common materials.

3.2. Thermal diffusivity

 The thermal diffusivity is a measure of the transient heat flow through a material.

3.3. Specific heat

 The specific heat is a measure of the amount of energy required to change the
temperature of a given mass of material. Specific heat is measured by calorimetry
techniques and is usually reported both as CV, the specific heat measured at constant
pressure, or CP, the specific heat measured at constant pressure.

3.4. Melting point

CE 261 – Engineering Materials Page 5

PNGUOT-Civil Engineering Department
Chapter I. Introduction to Properties and Applications of Engineering Materials

 The melting point is the temperature at which a material goes from the solid to the
liquid state at one atmosphere. The melting temperature is not usually a design criteria
but it offers important clues to other material properties.

3.5. Glass transition temp

 The glass transition temperature, or Tg is an important property of polymers. The glass

transition temperature is a temperature range which marks a change in mechanical
behavior. Above the glass transition temperature a polymer will behave like a ductile
solid or highly viscous liquid. Below Tg the material will behave as a brittle solid.
Depending on the desired properties materials may be used both above and below their
glass transition temperature.

3.6. Thermal expansion coefficient

 The thermal expansion coefficient is the amount a material will change in dimension
with a change in temperature. It is the amount of strain due to thermal expansion per
degree Kelvin expressed in units of K-1. For isotropic materials " is the same in all
directions, anisotropic materials have separate "s reported for each direction which is
different.

3.7. Thermal shock resistance

 Thermal shock resistance is a measure of how large a change in temperature a material

can withstand without damage. Thermal shock resistance is very important to most high
temperature designs. Measurements of thermal shock resistance are highly subjective
because if is extremely process dependent. Thermal shock resistance is a complicated
function of heat transfer, geometry and material properties. The temperature range and
the shape of the part play a key role in the material's ability to withstand thermal shock.
Tests must be carefully designed to mimic anticipated service conditions to accurately
asses the thermal shock resistance of a material.

3.8. Creep resistance

 Creep is slow, temperature aided, time dependent deformation. Creep is typically a

factor in materials above one third of their absolute melting temperature or two thirds
of their glass transition temperature. Creep resistance is an important material property
in high temperature design, but it is difficult to quantify with a single value. Creep
response is a function of many material and external variables, including stress and
temperature. Often other environmental factors such as oxidation or corrosion play a
role in the fracture process.
 Creep is plotted as strain vs. time. A typical creep curve shows three basic regimes.
During stage I, the primary or transient stage, the curve begins at the initial strain, with
a relatively high slope or strain rate which decreased throughout stage I until a steady

CE 261 – Engineering Materials Page 6

PNGUOT-Civil Engineering Department
Chapter I. Introduction to Properties and Applications of Engineering Materials

state is reached. Stage II, the steady state stage, is generally the longest stage and
represents most of the response. The strain rate again begins to increase in stage III and
rupture at tR generally follows quickly.
 Different applications call for different creep responses. In situations where long life is
desired minimum creep rate is the most important material consideration. Testing
through stage II should be sufficient for determining minimum creep rate. Is not
necessary to proceed all the way to rupture. For this type of test the longer the test the
more accurate the creep rate will be. Unfortunately practicality limits most creep tests
to times shorter than would be desirable for high accuracy.
 For short lived applications such as rocket nozzles the time to failure may be the only
consideration. The main issue is whether or not the component fails, not the amount of
deformation it may undergo. For this application creep tests may be run to completion
but without recording any data but the time to rupture. In this case temperatures may
be elevated above expected conditions to provide a margin of safety.
 The main objective of a creep test is to study the effects of temperature and stress on
the minimum creep rate and the time to rupture. Creep testing is usually run by placing
a sample under a constant load at a fixed temperature. The data provided from a
complete creep test at a specific temperature, T, and stress includes three creep
constants: the dimensionless creep exponent, n, the activation energy Q, and A, a
kinetic factor.

II. Application of Engineering Materials

Material selection to minimize material failure

The material failure occurs when materials are not properly selected according to appropriate
design condition or its misuse and the interference with the availability of products and
services. Failure of structures or any engineering structures due to material imperfection or its
inability to perform it desired function jeopardize human lives, economy, and development. To
at least guarantee the safety of the materials proper selection of material is required.

For selection, one must establish a link between materials and function, with shape and process
playing also a possibly important role (now ignored.)

AREAS OF DESIGN CONCERN

 Function- support a load, contain a pressure, transmit heat, etc.
What does component do?
 Objective- make thing cheaply, light weight, increase safety, etc., or combinations
of these.
What is to be maximized or minimized?
 Constraints- make thing cheaply, light weight, increase safety, etc., or
combinations of these.

CE 261 – Engineering Materials Page 7

PNGUOT-Civil Engineering Department
Chapter I. Introduction to Properties and Applications of Engineering Materials

What is non-negotiable conditions to be met?

What is negotiable but desired conditions?

function

Materials

Attributes: physical,
shape
mechanical, thermal,
electrical, economic,
environmental.

process

References:

1. Material Physical Properties .http//engineershandbook.com/materials/physical.htm,

retrieve 2/19/13

CE 261 – Engineering Materials Page 8

PNGUOT-Civil Engineering Department