ISO 12110-2:2013

INTERNATIONAL ISO
STANDARD 12110-2
First edition
2013-07-01
Metallic materials — Fatigue testing —
Variable amplitude fatigue testing —
Part 2:
Cycle counting and related data
reduction methods
Matériaux métalliques — Essais de fatigue — Essais sous
amplitude variable —
Partie 2: Méthodes de comptage des cycles et méthodes associées de
réduction des données
Reference number
©
ISO 2013
© ISO 2013
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Published in Switzerland
ii © ISO 2013 – All rights reserved

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Cycle counting techniques . 2
4.1 General . 2
4.2 Cycle counting methods . 3
5 Counting technique selection . 7
Annex A (informative) Rainflow counting . 8
Annex B (informative) Examples of quantification, cycle extraction, and open cycle sequence
composition of cycles .21
Annex C (informative) Example of result presentation for the Rainflow counting method .26
Bibliography .32
Foreword
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electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2. www.iso.org/directives
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The committee responsible for this document is ISO/TC 164, Mechanical testing of metals, Subcommittee
SC 5, Fatigue testing.
ISO 12110 consists of the following parts, under the general title Metallic materials — Fatigue testing —
Variable amplitude fatigue testing:
— Part 1: General principles, test method and reporting requirements
— Part 2: Cycle counting and related data reduction methods
iv © ISO 2013 – All rights reserved

INTERNATIONAL STANDARD ISO 12110-2:2013(E)
Metallic materials — Fatigue testing — Variable amplitude
fatigue testing —
Part 2:
Cycle counting and related data reduction methods
1 Scope
This part of ISO 12110 presents cycle counting techniques and data reduction methods which are used
in variable amplitude fatigue testing.
For each test or test series, cycle counting is mandatory whereas data reduction methods are optional.
This part of ISO 12110 supports ISO 12110-1 which contains the general principles and describes the
common requirements about variable amplitude fatigue testing.
In this part of ISO 12110, the term “loading” refers either to force, stress, or strain since the methods
presented here are valid for all.
The following issues are not within the scope of this part of ISO 12110 and therefore will not be addressed:
— constant amplitude tests with isolated overloads or underloads;
— large components or structures;
— environmental effects like corrosion, creep, etc. linked to temperature/time interactions leading to
frequency and waveform effects;
— multiaxial loading.
NOTE 1 Phasing is of prime importance when dealing with multiaxial tests under either constant or variable
amplitude controlled loading.
NOTE 2 Although frequency variations during cycling are not outside of the scope of this part of ISO 12110, the
following clauses deal only with constant frequency cycling.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 12110-1, Metallic materials — Fatigue testing — Variable amplitude fatigue testing — Part 1: General
principles, test method and reporting requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 12110-1 and the following apply.
3.1
mean crossing
number of times that the load-time history crosses the mean-load level with a positive slope or a negative
slope, or both, if specified during a given length of the history
Note 1 to entry: For purposes related to cycle counting, a mean crossing may be defined as a crossing of the
reference load level.
3.2
range
algebraic difference between two successive reversals
Note 1 to entry: In variable amplitude loading, range may have a different definition depending on the counting
method used. For example, “overall range” is defined by the algebraic difference between the highest peak and the
lowest valley (absolute maximum and minimum, respectively) of a given load-time history.
Note 2 to entry: In cycle counting by various methods, it is common to employ ranges between valley and peak
loads which are not successive events. In these practices, the definition of “range” is broadened so that events of
this type are also included.
3.3
reference load
loading level which is fixed for counting upon which load variations are superimposed
Note 1 to entry: The reference load may be identical to the mean load of the loading time histories, but this is
not required.
3.4
reversal
point at which the first derivative of the load-time history changes sign (from + to – or – to +)
Note 1 to entry: Reversals occur at peaks or valleys.
3.5
irregularity factor
characterization of the irregularity of the signal, i.e. number of cycles not crossing the mean value, I = N /N
0 p
Note 1 to entry: N is the number of mean crossings.
Note 2 to entry: N is the number of peaks.
p
3.6
mean-load level
mean value of the peak and valley values
4 Cycle counting techniques
4.1 General
Cycle counting is used to summarize irregular load-time histories by providing the number of cycles of
various sizes which simulates the real loading of the specimen or component under study.
NOTE The definition of a cycle varies with the cycle counting method used.
Cycle counts can be made for load-time histories of force, stress, strain, deflection, or other loading parameters.
The following subclauses present the following cycle counting methods:
— level-crossing counting;
— peak counting;
2 © ISO 2013 – All rights reserved

— simple range counting;
— range-pair counting;
— Rainflow counting.
4.2 Cycle counting methods
4.2.1 Loading signal sampling
Loading signal recording generally consists of measuring the continuous evolution of the signal versus
time (either analog or digital values against time). If the initial loading time history is analog, it needs to
be converted into a digital file so that further computer processing of the loading time histories can be
accomplished. The operation of digitization consists of sampling the signal that means measuring and
recording values at regular time intervals.
The digital signal is representative of the real analog one if the following precautions are taken:
— Filter the output signal to eliminate noise and other disturbances which are not linked to the fatigue
process believed to be part of the real loading time histories of the structure.
— The sampling frequency shall be such that every analog loading cycle is represented by at least
20 digital points at least 20 times that of the observed maximum frequency of the real or expected
analog signal.
Care shall be taken when filtering the original analog signal. See ISO 12110-1.
4.2.2 Level-crossing counting
4.2.2.1 Results of a level-crossing count are shown in Figure 1. One count is recorded each time the positive
sloped portion of the load exceeds a preset level above the reference load, and each time the negative
sloped portion of the load exceeds a preset level below the reference load. Reference load crossings are
typically counted on the positive sloped portion of the loading time histories. It makes no difference
whether positive or negative slope crossings are counted. The distinction is made only to reduce the total
number of events by a factor of 2.
4.2.2.2 In practice, restrictions on the level-crossing counts are often specified to eliminate small amplitude
variations which can give rise to a large number of counts. This may be accomplished by filtering small
load excursions prior to cycle counting. A second method is to make no counts at the reference load and to
specify that only one count be made between successive crossings of a secondary lower level associated
with each level above the reference load, or a secondary higher level associated with each level below the
reference load. Figure 1 b) illustrates this second method. A variation of the second method is to use the
same secondary level for all counting levels above the reference load, and another for all levels below the
reference load. In this case, the levels are generally not evenly spaced.
4.2.2.3 The most common cycle count for fatigue analysis is derived from the level-crossing count by
first constructing the largest possible cycle, followed by the second largest, etc., until all level crossings
are used. Reversal points are assumed to occur halfway between levels.
This process is illustrated by Figure 1 c). Note that once this cycle count is obtained, the cycles could
be applied in any desired order, and this order could have a secondary effect on the amount of damage.
Other methods of deriving a cycle count from the level-crossing count could be used.
4.2.3 Peak counting
4.2.3.1 Peak counting identifies the occurrence of a relative maximum or minimum load value. Peaks
above the reference load level are counted, and valleys below the reference load level are counted, as
shown in Figure 2 a). Results for peaks and valleys are usually reported separately. A variation of this
method is to count all peaks and valleys without regard to the reference load.
4.2.3.2 To eliminate small amplitude loadings, mean-crossing peak counting is often used. Instead of
counting all peaks and valleys, only the largest peak or valley between two successive mean crossings is
counted, as shown in Figure 2 b).
4.2.3.3 The most common cycle count for fatigue analysis is derived from the peak count by first
constructing the largest possible cycle, using the highest peak and lowest valley, followed by the second
largest cycle, etc., until all peak counts are used. This process is illustrated by Figure 2 c). Note that once
this most damaging cycle count is obtained, the cycles could be applied in any desired order, and this
order could have a secondary effect on the amount of damage. Alternate methods of deriving a cycle
count, such as randomly selecting pairs of peaks and valleys, are sometimes used.
4.2.4 Simple-range counting
4.2.4.1 The method is illustrated in Figu
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