‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Introduction
1. GENERAL PROPERTIES
Ceramics generally have specific properties associated with them although,
as we just noted, this can be a misleading approach to defining a class of material.
However, we will look at some properties and see how closely they match our
expectations of what constitutes a ceramic.
Brittleness: This probably comes from personal experiences such as dropping a
glass beaker or a dinner plate. The reason that the majority of ceramics are brittle
is the mixed ionic–covalent bonding that holds the constituent atoms together. At
high temperatures (above the glass transition temperature) glass no longer
behaves in a brittle manner; it behaves as a viscous liquid. That is why it is easy to
form glass into intricate shapes. So what we can say is that most ceramics are
brittle at room temperature but not necessarily at elevated temperatures.
Poor electrical and thermal conduction: The valence electrons are tied up in
bonds, and are not free as they are in metals. In metals it is the free electrons—the
electron gas—that determines many of their electrical and thermal properties.
Diamond has the highest thermal conductivity of any known material. The
conduction mechanism is due to phonons, not electrons. Ceramics can also have
high electrical conductivity:
(1) The oxide ceramic, ReO3, has an electrical conductivity at room temperature
similar to that of Cu.
(2) The mixed oxide YBa2Cu3O7 is an HTSC; it has zero resistivity below 92 K.
These are two examples that contradict the conventional wisdom when it comes to
ceramics.
Compressive strength: Ceramics are stronger in compression than in tension,
whereas metals have comparable tensile and compressive strengths. This
difference is important when we use ceramic components for load-bearing
applications. It is necessary to consider the stress distributions in the ceramic to
ensure that they are compressive. An important example is in the design of
concrete bridges-the concrete, a CMC, must be kept in compression. Ceramics
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‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Introduction
generally have low toughness, although combining them in composites can
dramatically improve this property.
Chemical insensitivity: A large number of ceramics are stable in both harsh
chemical and thermal environments. Pyrex glass is used widely in chemistry
laboratories specifically because it is resistant to many corrosive chemicals, stable
at high temperatures (it does not soften until 1100 K), and is resistant to thermal
shock because of its low coefficient of thermal expansion (33 × 10 −7 K−1). It is
also widely used in bake ware.
Transparent: Many ceramics are transparent because they have a large Eg.
Examples include sapphire watch covers, precious stones, and optical fibers.
Glass optical fibers have a percent transmission >96%km−1. Metals are
transparent to visible light only when they are very thin, typically less than 0.1
μm. Although it is always possible to find at least one ceramic that shows atypical
behavior, the properties we have mentioned here are in many cases different from
those shown by metals and polymers.
2. INTRODUCTION OF TESTS
Because of the potential for brittle fracture or failure, designers of glass and
ceramic component must be concerned with their fracture behavior. In this course
we will discuss the test procedures which are used to determine the fracture
parameter e. g, fracture toughness, strength and susceptibility to moisture
enhanced crack extension, that are important to the design and reliable use of
ceramic in almost every conceivable application, in each of these topic areas we
will discusses test procedure relative to the end use of the data, i. e. , scientific
understanding.
Types of Testing: there are two main types of tests:
 Destructive testing
 Nondestructive testing
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‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Introduction
2.1 Nondestructive Testing
A general definition of nondestructive testing (NDT) is an examination,
test, or evolution performed on any type of test object without changing or
altering that object in any way, in order to determine the absence or presence of
conditions or discontinuities that may have an effect on the usefulness or
serviceability of that object. Nondestructive tests may also be conducted to
measure other test object characteristics, such as size; dimension; configuration;
or structure, including alloy content, hardness, grain size, etc. The simplest of all
definitions is basically an examination that is performed on an object of any type,
size, shape or material to determine the presence or absence of discontinuities, or
to evaluate other material characteristics. Nondestructive examination (NDE),
nondestructive inspection (NDI), and nondestructive evaluation (NDE) are also
expressions commonly used to describe this technology. Although this technology
has been effectively in use for decades, it is still generally unknown by the
average person, who takes it for granted that buildings will not collapse, planes
will not crash, and products will not fail.
Although NDT cannot guarantee that failures will not occur, it plays a
significant role in minimizing the possibilities of failure. Other variables, such as
inadequate design and improper application of the object, may contribute to
failure even when NDT is appropriately applied.
NDT, as a technology, has seen significant growth and unique innovation over
the past 25 years. It is, in fact, considered today to be one of the fastest growing
technologies from the standpoint of uniqueness and innovation. Recent equipment
improvements and modifications, as well as a more thorough understanding of
materials and the use of various products and systems, have all contributed to a
technology that is very significant and one that has found widespread use and
acceptance throughout many industries.
This technology touches our lives daily. It has probably done more to enhance
safety than any other technology, including that of the medical profession. One
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‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Introduction
can only imagine the significant number of accidents and unplanned outages that
would occur if it were not for the effective use of nondestructive testing. It has
become an integral part of virtually every process in industry, where product
failure can result in accidents or bodily injury. It is depended upon to one extent
or another in virtually every major industry that is in existence today.
Nondestructive testing, in fact, is a process that is performed on a daily basis by
the average individual, who is not aware that it is taking place. For example, when
a coin is deposited in the slot of a vending machine and the selection is made,
whether it is candy or a soft drink, that coin is actually subjected to a series of
nondestructive tests. It is checked for size, weight, shape, and metallurgical
properties very quickly, and if it passes all of these tests satisfactorily, the product
that is being purchased will make its way through the dispenser. It is common to
use sonic energy to determine the location of a stud behind a wallboard. The sense
of sight is employed regularly to evaluate characteristics such as color, shape,
movement, and distance, as well as for identification purposes.
These examples, in a very broad sense, meet the definition of nondestructive
testing—an object is evaluated without changing it or altering it in any fashion.
The human body has been described as one of the most unique nondestructive
testing instruments ever created. Heat can be sensed by placing a hand in close
proximity to a hot object and, without touching it, determining that there is a
relatively higher temperature present in that object. With the sense of smell, a
determination can be made that there is an unpleasant substance present based
simply on the odor that emanates from it. Without visibly observing an object, it
is possible to determine roughness, configuration, size, and shape simply through
the sense of touch. The sense of hearing allows the analysis of various sounds and
noises and, based on this analysis, judgments and decisions relating to the source
of those sounds can be made. For example, before crossing a street, one can hear a
truck approaching. The obvious decision is not to step out in front of this large,
moving object. But of all the human senses, the sense of sight provides us with
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‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Introduction
the most versatile and unique nondestructive testing approach. When one
considers the wide application of the sense of sight and the ultimate information
that can be determined by mere visual observation, it becomes quite apparent that
visual testing (VT) is a very widely used form of nondestructive testing.
In industry, nondestructive testing can do so much more. It can effectively be
used for the:
1. Examination of raw materials prior to processing
2. Evaluation of materials during processing as a means of process control
3. Examination of finished products
4. Evaluation of products and structures once they have been put into service
Nondestructive testing, in fact, can be considered as an extension of the human
senses, often through the use of sophisticated electronic instrumentation and other
unique equipment. It is possible to increase the sensitivity and application of the
human senses when used in conjunction with these instruments and equipment.
On the other hand, the misuse or improper application of a nondestructive test can
cause catastrophic results. If the test is not properly conducted or if the
interpretation of the results is incorrect, disastrous results can occur. It is essential
that the proper nondestructive test method and technique be employed by
qualified personnel, in order to minimize these problems.
To summarize, nondestructive testing is a valuable technology that can provide
useful information regarding the condition of the object being examined once all
the essential elements of the test are considered, approved procedures are
followed, and the examinations are conducted by qualified personnel.
Benefits of nondestructive testing include:
 The part is not changed or altered and can be used after examination
 Every item or a large portion of the material can be examined with no adverse
consequences
 Materials can be examined for conditions internal and at the surface
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‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Introduction
 Parts can be examined while in service
 Many NDT methods are portable and can be taken to the object to be
examined
 Nondestructive testing is cost effective, overall
Limitations of nondestructive testing include:
 It is usually quite operator dependent
 Some methods do not provide permanent records of the examination
 NDT methods do not generally provide quantitative data
 Orientation of discontinuities must be considered
 Evaluation of some test results are subjective and subject to dispute
 While most methods are cost effective, some, such as radiography, can be
expensive
 Defined procedures that have been qualified are essential
2.2 Destructive Tests
Destructive testing has been defined as a form of mechanical test (primarily
destructive) of materials whereby certain specific characteristics of the material
can be evaluated quantitatively. In some cases, the test specimens being tested are
subjected to controlled conditions that simulate service. The information that is
obtained through destructive testing is quite precise, but it only applies to the
specimen being examined. Since the specimen is destroyed or mechanically
changed, it is unlikely that it can be used for other purposes beyond the
mechanical test. Such destructive tests can provide very useful information,
especially relating to the material’s design considerations and useful life.
Destructive testing may be dynamic or static and can provide data relative to the
following material attributes:
 Ultimate tensile strength

Yield point
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‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Introduction

Ductility

Elongation characteristics

Fatigue life

Corrosion resistance

Toughness

Hardness

Impact resistance
One of the more common destructive testing instruments used for mechanical
testing, which is capable of measuring characteristics such yield point, elongation,
and ultimate tensile strength .Other than the fact that the specimen being
examined typically cannot be used after destructive testing for any useful purpose,
it must also be stressed that the data achieved through destructive testing are
specific to the test specimen. Another destructive test commonly used to measure
a material resistance to impact is the Charpy test. In this test, a specimen that is
usually notched is supported at one end and is broken as a pendulum is released
and impacts in the region of the notch. The measure of the material’s resistance to
impact (or notch toughness) is determined by the subsequent rise of the
pendulum.
Hardness is also an important material characteristic. The hardness test
measures the material’s resistance to plastic deformation. There has always been
aminor dispute as to whether this test was nondestructive or destructive, since
there usually is an indentation made on the surface of the material. If the hardness
test is made without indentation (as is the case when using eddy currents or
ultrasonics), it can be considered truly “nondestructive.”
Although it is assumed in many cases that the test specimen is representative of
the material from which it has been taken, it cannot be said with 100% reliability
that the balance of the material will have exactly the same characteristics as that
test specimen.
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‫فرع السيراميك ومواد البناء‬/‫المرحلة الثالثة‬
Introduction
Benefits of destructive testing include:

Reliable and accurate data from the test specimen.

Extremely useful data for design purposes.

Information can be used to establish standards and specifications.

Data achieved through destructive testing is usually quantitative.

Typically, various service conditions are capable of being measured.

Useful life can generally be predicted.
Limitations of destructive testing include:
 Data applies only to the specimen being examined.
 Most destructive test specimens cannot be used once the test is complete.
 Many destructive tests require large, expensive equipment in a laboratory
environment.
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