Fatigue is the weakening of a material caused by fluctuating stresses that occurs even at stress levels below the material's tensile strength. It happens in three stages: crack initiation, crack propagation, and final fracture. Fatigue life refers to the total number of cycles a material can withstand before failure. Fatigue strength is the stress level that causes failure after a given number of cycles. Endurance limit is the maximum stress that does not cause failure even after an infinite number of cycles. Fatigue testing determines the fatigue life and failure location of a specimen under repeated stressing to help prevent mechanical failures.

1. GAZIANTEP UNIVERSITY
Fatigue Testing
CE 521
GRADUATE SCHOOL NATURAL AND APPLIED SCEINCE
DEPARTMENT OF CIVIL ENGINEERING
M.SC IN CONSTRUCTION ENGINEERING
Submitted by
AHMED ASSIM ABDULLAH
STD No: 201444960

2. What is fatigue ?
 It has been recognized that a metal subjected to a repetitive or
fluctuating stress will fail at a stress much lower than that required to
cause failure on a single application of load. Failures occurring under
conditions of dynamic loading are called fatigue.
 Fatigue is an insidious time-dependent type of failure which can occur
without any obvious warning.
 It is believed that more than 95 % of all mechanical failures can be
attributed to fatigue.
 It is characterized by three stages
1. Crack initiation
2. Crack propagation
3. Final fracture
Failure stages :
 Failure always being the brittle fracture regardless it is ductile or brittle
material.
 Mostly fatigue failure occur at stress well below the static elastic
strength of the material.

3. Definition :
1. Fatigue life(N): it is total number of cycles are required to bring about
final fracture in a specimen at a given stress.
2. Fatigue strength: It is stress at which a material can withstand
repeatedly N number of cycles before failure.
3. Endurance limit: it is stress below which a material will not fail for any
number of cycles.
4. For ferrous materials it is approximately half of the ultimate tensile
strength.
5. No. of cycles for a non ferrous material is 5x10ˆ8 cycles.
Testing :
 Objective of fatigue testing is generally to determine the fatigue life
and/or the location of danger point i.e. location of failure of a specimen
subjected to sequential stress amplitude.
Testing Types :
 There are 2 basics for a classification of the different methods of fatigue
testing-
1. Sequence of the stress amplitude.
2. Nature of the test piece.
Classification based on sequence of stress amplitude
1. Constant Amplitude test
2. Variable Amplitude test

4. Constant Amplitude Test
 This is the simplest sequence of amplitude obtained by applying
reversals of stress of constant-amplitude to the test-piece until failure
occurs. Different specimens of the test series may be subjected to
different stress amplitude but for each individual item, the amplitude
will never be varied.
Following parameters are utilized to identify fluctuating stress cycle
 Mean stress Sm,
 Sm= (Smax + Smin)/2
 Stress range Sr,
 Sr= Smax – Smin
 Stress amplitude Sa,
 Sa=( Smax – Smin)/2
 Stress ratio R,
 R= Smin/Smax
Variable Amplitude Test
 Testing of an object subjected to stress reversals of different amplitudes
under this condition testing is known as variable amplitude test.

5. Classification Based On Nature Of Test Piece:
1. Specimens: test piece made in the standard form as in shape and size &
prepared with good surface finish for the test.
2. Components: It is used to signify any machine part, actual structure,
machine and assembly including elements simulating actual
components.
Fatigue Testing Machine
 In the simplest type of machine for fatigue testing, the load applied is of
bending type.
 The test specimen may be of simply supported beam or a cantilever.
Rotating Bending Testing Machine
 The type of S-N curve created by this machine is identified as a rotating-
bending, stress-controlled fatigue data curve. The rotating bending test
machine is used to create an S-N curve by turning the motor at a
constant revolution per minutes, or frequency. To create a failure on the
specimen, a constant-stationary force is applied on the specimen, which
creates a constant bending moment. A stationary moment applied to a
rotating specimen causes the stress at any point on the outer surface of
the specimen to go from zero to a maximum tension stress, back to zero
and finally to a compressive stress. Thus, the stress state is one that is
completely reversed in nature.
 This machine also called R. R .Moore rotating beam type machine.
 In a R.R.Moore rotating beam type machine for a simply supported
beam a specimen of circular cross-section is held at its ends in special
holders and loaded through two bearings equidistant from the center of
the span.

6.  Equal loads on these bearings are applied by means of weights that
produce a uniform bending moment in the specimen between the
loaded bearings.
 A motor rotates the specimen.
 Bending moment Mb =
 Where L is the length of the specimen and z is the sectional modulus.
 In rotating cantilever beam type, the specimen is rotated while a gravity
load is applied to the free end by means of a bearing.
 For cantilever specimen the maximum bending moment is at the fixed
end.
Equal loads on these bearings are applied by means of weights that
produce a uniform bending moment in the specimen between the
A motor rotates the specimen.
Bending moment Mb = FL and bending stress S = Mb/4z
is the length of the specimen and z is the sectional modulus.
In rotating cantilever beam type, the specimen is rotated while a gravity
to the free end by means of a bearing.
For cantilever specimen the maximum bending moment is at the fixed
Equal loads on these bearings are applied by means of weights that
produce a uniform bending moment in the specimen between the
Mb/4z
is the length of the specimen and z is the sectional modulus.
In rotating cantilever beam type, the specimen is rotated while a gravity
For cantilever specimen the maximum bending moment is at the fixed