ISO/TR 16194:2017

TECHNICAL ISO/TR
REPORT 16194
First edition
2017-04
Pneumatic fluid power — Assessment
of component reliability by
accelerated life testing — General
guidelines and procedures
Transmissions pneumatiques — Évaluation de la fiabilité du
composant par essai de durée de vie accélérée — Lignes directrices
générales et modes opératoires
Reference number
ISO/TR 16194:2017(E)
©
ISO 2017

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ISO/TR 16194:2017(E)

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ISO/TR 16194:2017(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and units . 3
5 Concepts of reliability and accelerated life testing . 3
6 Failure mechanism and mode . 4
7 Strategy of conducting accelerated life testing . 4
8 Design of accelerated life testing . 5
8.1 Normal use conditions . 5
8.2 Preliminary tests . 5
8.3 Levels of accelerated stress . 6
8.4 Sample size . 7
8.5 Data observation and measurement . 7
8.6 Types of stress loading . 7
9 End of test . 8
9.1 Minimum number of failures required . 8
9.2 Termination cycle count. 8
9.3 Suspended or censored test units . 8
10 Statistical analysis . 9
10.1 Analysis of failure data . 9
10.2 Life distribution. 9
10.3 Accelerated life testing model .10
10.4 Data analysis and parameter estimation .10
11 Reliability characteristics from the test data .11
12 Test report .12
Annex A (informative) Determining stress levels when stress is time-dependent .13
Annex B (informative) Life-stress relationship models .17
Annex C (informative) Verification of compromise Weibull slopes .26
Annex D (informative) Calculation procedures for censored data .32
Annex E (informative) Examples of using accelerated life testing in industrial applications .35
Annex F (informative) Palmgren-Miner’s rule .37
Annex G (informative) ALT experimental results for pneumatic cylinder .39
Bibliography .59
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Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
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 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: w w w . i s o .org/ iso/ foreword .html
ISO/TR 16194 was prepared by Technical Committee ISO/TC 131, Fluid power systems.
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Introduction
This document is being released to document progress that the working group has developed for
accelerated testing. It is a new method with which the working group members have very little
experience, but has been used by institutional laboratories and taught at academic levels.
Some experimentation on air cylinders has been done at the Korean Institute of Machinery and
Materials (KIMM), but the application to pneumatic components in general has not been evaluated.
This document is offered to members as a reference and model procedure, so that they can develop
experience with its use in their own laboratories.
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TECHNICAL REPORT ISO/TR 16194:2017(E)
Pneumatic fluid power — Assessment of component
reliability by accelerated life testing — General guidelines
and procedures
1 Scope
This document provides general procedures for assessing the reliability of pneumatic fluid power
components using accelerated life testing and the method for reporting the results. These procedures
apply to directional control valves, cylinders with piston rods, pressure regulators, and accessory
devices – the same components covered by the ISO 19973 series of standards.
This document does not provide specific procedures for accelerated life testing of components.
Instead, it explains the variability among methods and provides guidelines for developing an
accelerated test method.
The methods specified in this document apply to the first failure, without repairs.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5598, ISO 19973-1 and
the following apply. ISO and IEC maintain terminological databases for use in standardization at the
following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1
B life
x
life of a component or assembly that has not been altered since its production, where its reliability is (
100− x) %; or the time at which (100− x) % of the population has survived
Note 1 to entry: The cumulative failure fraction is x %. For example, if x = 10, the B life has a cumulative failure
10
probability of 10 %.
3.2
acceleration factor
AF
ratio between the life at the normal use stress level and the life at the accelerated stress level
3.3
accelerated life test
ALT
process in which a component is forced to fail more quickly that it would have under normal use
conditions and which provides information about the component’s life characteristics
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3.4
destruct limit
stress level at which one or more of the component’s operating characteristics is no longer within
specification or the component is damaged and cannot recover when the stress is reduced
Note 1 to entry: Destruct limits are classified as a lower destruct limit and upper destruct limit.
3.5
failure mechanism
physical or chemical process that produces instantaneous or cumulative damage to the materials from
which the component is made
3.6
failure mode
manifestation of the failure mechanism resulting from component failure or degradation
Note 1 to entry: The failure mode is the symptom of the aggressive activity of the failure mechanism in the
component’s areas of weakness, where stress exceeds strength.
3.7
failure rate
λ
frequency at which a failure occurs instantaneously at time t, given that no failure has occurred before t
3.8
highly accelerated life test
HALT
process in which components are subjected to accelerated environments to find weaknesses in the
design and/or manufacturing process
Note 1 to entry: The primary accelerated environments include pressure and heat.
3.9
model for accelerated life testing
model that consists of a life distribution that represents the scatter in component life and a relationship
between life and stress
Note 1 to entry: Life distribution examples: Weibull, Lognormal, Exponential, etc.
Note 2 to entry: Life and stress examples: Arrhenius, Eyring, Inverse Power Law, etc.
3.10
normal use conditions
test conditions at which a component is commonly used in the field, which can be less strenuous than
rated conditions
3.11
termination cycle count
number of cycles on a test item when it reaches a threshold level for the first time
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4 Symbols and units
a
Symbol Definition
B Time at which 10 % of the population is estimated to fail
10
η Scale parameter (characteristic life) of the Weibull distribution
F(t) Probability of failure of a component up to time t
β Shape parameter (slope) of the Weibull distribution
R(t) Reliability of a component at time t; R(t) = 1 – F(t)
λ(t) Failures per unit time
a
Other symbols could be used in other documents and software.
Units of measurements are in accordance with ISO 80000-1.
5 Concepts of reliability and accelerated life testing
Reliability is the probability (a percentage) that a component does not fail (for example, exceed the
threshold level or experience catastrophic failure) for a specified interval of time or number of cycles
when it operates under stated conditions. This reliability can be assessed by test methods described in
the ISO 19973 series.
Generally, reliability analysis involves analysing time to failure of a component, obtained under normal
use conditions in order to quantify its life characteristics. Obtaining such life data is often difficult.
The reasons for this difficulty can include the typically long life times of components, the small time
period between design and product release, and the necessity for testing components under normal use
conditions. Given this difficulty and the need to observe failures of components to better understand
their life characteristics, procedures have been devised to accelerate their failures by overstress,
thus forcing components to fail more quickly than they would under normal use conditions. The term
accelerated life testing (ALT) is used to describe such procedures.
However, a relationship between the reliability of a component determined by ALT, and its reliability
at normal use conditions, is necessary. This can be assessed by extrapolating the test results obtained
from an accelerated life test and comparing it to that obtained from testing at normal use conditions.
Figure 1 shows the graphical concept for this relationship.
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Figure 1 — Graphical explanation of relationship between S-N curve and accelerated life testing
NOTE Distributions in this concept Figure 1 are not defined.
In Figure 1, failures under normal use conditions are represented by the distribution S , and the
3
accelerated conditions are distributions S and S . Their relationship is shown by the connecting line(s).
1 2
6 Failure mechanism and mode
The failure mechanism is the physical or chemical process that produces instantaneous or cumulative
damage to the materials from which the component is made. The failure mode is the manifestation
of the failure mechanism resulting from component failure or degradation. The failure mode is the
symptom of the aggressive activity of the failure mechanism in areas of component weakness where
the stress exceeds the strength.
It is necessary that the failure modes observed in accelerated life test conditions are identical to those
defined for normal use conditions.
7 Strategy of conducting accelerated life testing
Before starting an accelerated life test, it is important to identify the types of failures that might occur
in service; especially any feedback from the field. Several methods are available to assist in this effort:
design analysis and review using the quality function deployment (QFD), fault tree analysis (FTA), and
failure modes and effect analysis (FMEA). Another method is a qualitative test like highly accelerated
life testing (HALT). Qualitative tests are used primarily to reveal probable failure modes, but they do
not quantify the life (or reliability) of the component under normal use conditions.
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Accelerated life testing involves acceleration of failures with the single purpose of quantification of the
life characteristics of the component at normal use conditions.
Therefore, accelerated life testing can be divided into two areas: qualitative accelerated testing (HALT)
and quantitative accelerated life testing. In qualitative accelerated testing, the objective is to identify
failures and failure modes without attempting to make any predictions as to the component’s life under
normal use conditions. In quantitative accelerated life testing, the objective is predicting the life of the
component (life characteristics such as MTTF, B life, etc.) at normal use conditions from data obtained
10
in an accelerated life test.
The strategy for effectively conducting an accelerated life testing program includes the following:
— establishing a stress level that can be referred to as normal use conditions;
— determining the stress levels to use for accelerated testing; and
— determining the number of components to be tested at each stress level.
8 Design of accelerated life testing
8.1 Normal use conditions
Normal use conditions can often be defined from the ratings of the component’s characteristics, for
example: pressure, temperature, voltage, duty cycle, lubrication requirements, etc. However, these
ratings often represent a maximum condition that is above commonly used conditions. Therefore, a
definition for normal use conditions needs to be established from these characteristics before starting
an accelerated test. An example definition for a pneumatic valve is shown in Table 1.
Table 1 — Definition of normal use conditions for a pneumatic valve
Common use applica- Proposed normal use
Characteristic Typical rating value
tion value value for testing
Pressure 1 000 kPa (10 bar) 630 kPa (6,3 bar) 630 kPa (6,3 bar)
Temperature 50 °C 25 °C 25 °C
Voltage 24 VDC 24 VDC 24 VDC
Duty cycle Continuous On-off varies 10 % on / 90 % off
Lubrication Sometimes required Sometimes applied Not used
Air dryness Dew point < 0 °C Dew point ≤ 10 °C Dew point = 10 °C
It is necessary to define this normal use conditions before starting an ALT program.
8.2 Preliminary tests
It is also necessary to determine the highest stress to be tested that does not result in failure modes
different from those that occur under normal use conditions. Typically, these stresses or limits are
unknown, so qualitative tests (HALT) with small sample sizes can be performed in order to determine
the appropriate stress levels for use in the accelerated life test. Design of Experiments (DOE)
methodology is a useful technique at this step.
The following steps can be taken to determine three stress levels:
a) Propose the highest possible stress that might yield failure in less than 1 day of testing
(approximately).
b) Reduce this stress level to 90% of that value and test at least two test units to failure at this
stress level, using the test procedures of one of the parts of the ISO 19973 series (modified for the
conditions of the stress level).
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c) Examine the failure mode to determine if it is the same type of failure as would be experienced
under normal use conditions. If it is not, reduce the level of stress and repeat steps b) and c) until
failures are the same as would be experienced under normal use conditions. Identify this as stress
level S .
1
d) Reduce the stress level by another 10% to 20% from step b) and test at least two more test units to
failure. Again, examine the failure mode to determine if it is the same type of failure as would be
experienced under normal use conditions. If it is not, modify the stress conditions and repeat the
test. Identify this as stress level S . See Figure 2.
2
e) Identify a third, yet lower stress level S that results in failures within the project timing constraints.
3
This third level of stress is identified by extrapolating from the previous pairs of failures as shown
in Figure 2. As an alternative, S can be estimated by using an average value of S and S , so that
3 1 2
S = ½ (S – S ).
3 2 1
f) Test at least two more test units to failure at this third level of stress S . Again, examine the failure
3
mode to determine if it is the same type of failure as would be experienced under normal use
conditions. If it is not, modify the stress conditions and repeat the test.
Figure 2 — Graphical explanation of determining stress levels during preliminary tests
These preliminary tests might have to be conducted several times before the necessary stress levels
are determined.
8.3 Levels of accelerated stress
The levels of stress identified from 8.2 are used to conduct a series of accelerated tests on randomly
selected test units, in accordance with one of the parts of ISO 19973. Generally, these stress levels
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fall outside of the limits of the component’s specification. It is important, therefore, to constantly
examine the types of failures obtained to be sure they are the same as those experienced at normal
use conditions. If they are not, the test units would be designated as suspensions, or the test conditions
would be modified and the testing restarted.
Conduct the tests at each of the selected stress levels. It is also helpful to conduct at least one test at a
stress level that is as close as possible to the normal use conditions.
At the higher levels of stress in an accelerated test, the required test duration decreases, and the
uncertainty in the extrapolation increases. Confidence intervals provide a measure of the uncertainty
in extrapolation.
The most common stresses for pneumatic fluid power components are pressure and temperature.
Testing can be conducted either at one set of stress conditions on a sample lot, or two stresses on
different sample lots. Cylinder speed, and cycle rate of valves and regulators are other possibilities.
Temperature of the process air used to test components is usually heated (or cooled) to approximately
equal the environmental test temperature.
When conducting an accelerated life test, arrangements are made to ensure that the failures of the
components are independent of each other (e.g. so that failures due to temperature do not influence the
failures due to pressure).
8.4 Sample size
Ideally, at least seven test units are subjected to each stress level for the accelerated life test. However,
the number of test units allocated to each stress level is usually inversely proportional to the level of
applied stress; that is, more test units are subjected to lower stress levels than to higher stress levels
because of the higher proportion of failures expected at the higher stress levels. A good ratio for the
number of test units among the stress levels, from highest to lowest, is 1:2:4. If test units are expensive,
four test units each at stress levels S and S would be tested; and five or more test units would be
1 2
tested at stress level S . As an option, the number of test units could be two if time is limited, but the
3
estimation uncertainty at the normal use condition will increase.
8.5 Data observation and measurement
No repairs are made to the test units during accelerated life testing.
The test operator determines the intervals between measurements to obtain data during accelerated
life testing. Short intervals between measurements give better statistical results and are conducted
during testing at the high stress level. At the low stress levels, longer intervals between measurements
are adequate.
8.6 Types of stress loading
There are two possible stress loading schemes: loading in which the stress is time-independent (where
the stress does not vary over time), and loading in which the stress is time-dependent (where the stress
does vary over time). This document uses constant time-independent stress loading, which is the most
common type used in an accelerated life test; see Figure 3. However, non-constant stress loads, such as
step stress, cycling stress, random stress, etc., can be used. These types of loads are classified according
to their dependence on time and are described in Annex A. The method specified in Annex A is used
where a time-dependent analysis is required.
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Figure 3 — Constant stress model
Time-independent stress loading has many advantages over time-dependent stress loading. Specifically:
— most components are assumed to operate at a constant stress under normal use conditions;
— it is far easier to run a constant stress test;
— it is far easier to quantify a constant stress test;
— models for data analysis are widely publicized and are empirically verified; and
— extrapolation from a well-executed constant stress test is more accurate than extrapolation from a
time-dependent stress test.
9 End of test
9.1 Minimum number of failures required
Confidence levels are generated when at least four test units have failed (which includes their reaching
a threshold level) at each stress level.
9.2 Termination cycle count
When a test unit fails between consecutive observations, the data collected is referred to as left-
censored or interval data. In this case, both the last cycle count at which the test unit was operating
properly and the cycle count at which the test unit was observed to have failed, are recorded. This data
is usually processed in accordance with ISO 19973-1:2015, 10.2.
9.3 Suspended or censored test units
Individual test units on which testing was stopped before failure occurred are known as suspensions.
Some examples of suspensions include:
— the test unit needed to be disassembled for inspection;
— the test unit experienced a failure mode different than the type being considered; and
— the test unit was accidentally damaged from a source not related to the test.
Because these test units had achieved a number of cycles before the point of suspension, the data has a
positive influence on the calculation of the statistical parameters. However, they cannot be returned to
the testing program.
If the minimum number of failures has been reached, but some test units have not failed (reached a
threshold level), the test can be stopped. The remaining test units are designated as censored.
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Data from suspended test units is considered the same as data from censored test units. The method
specified in Annex D allows calculation of the statistical parameters for these types of data
10 Statistical analysis
10.1 Analysis of failure data
The failure data from testing at all stress levels is analysed in accordance with 10.2, 10.3 and 10.4.
10.2 Life distribution
Select an initial life distribution (it can be changed later, if necessary). For pneumatic components, the
Weibull distribution is commonly used, and its scale parameter,η , is selected to be the life characteristic
that is stress-dependent; while the slope β is assumed to remain constant across differe
...