ISO 18249:2015
(Main)Non-destructive testing — Acoustic emission testing — Specific methodology and general evaluation criteria for testing of fibre-reinforced polymers
Non-destructive testing — Acoustic emission testing — Specific methodology and general evaluation criteria for testing of fibre-reinforced polymers
ISO 18249:2015 describes the general principles of acoustic emission testing (AT) of materials, components, and structures made of fibre-reinforced polymers (FRP) with the aim of - materials characterization, - proof testing and manufacturing quality control, - retesting and in-service testing, and - health monitoring. This International Standard has been designed to describe specific methodology to assess the integrity of fibre-reinforced polymers (FRP), components, or structures or to identify critical zones of high damage accumulation or damage growth under load (e.g. suitable instrumentation, typical sensor arrangements, and location procedures). It also describes available, generally applicable evaluation criteria for AT of FRP and outlines procedures for establishing such evaluation criteria in case they are lacking. ISO 18249:2015 also presents formats for the presentation of acoustic emission test data that allows the application of qualitative evaluation criteria, both online during testing and by post-test analysis, and that simplify comparison of acoustic emission test results obtained from different test sites and organizations. NOTE The structural significance of the acoustic emission cannot in all cases definitely be assessed based on AT evaluation criteria only but can require further testing and assessment (e.g. with other non-destructive test methods or fracture mechanics calculations).
Essai non destructif — Essai de l'émission acoustique — Méthodologie spécifique et critères d'évaluation générale d'essai des polymères renforcés de fibre
General Information
Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 18249
First edition
2015-05-01
Non-destructive testing — Acoustic
emission testing — Specific
methodology and general evaluation
criteria for testing of fibre-
reinforced polymers
Essai non destructif — Essai de l’émission acoustique —
Méthodologie spécifique et critères d’évaluation générale d’essai des
polymères renforcés de fibre
Reference number
ISO 18249:2015(E)
©
ISO 2015
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ISO 18249:2015(E)
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ISO 18249:2015(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Personnel qualification . 3
5 Acoustic emission sources and acoustic behaviour . 3
5.1 Acoustic emission source mechanisms . 3
5.2 Wave propagation and attenuation characterization . 3
5.3 Test temperature . 4
5.4 Source location . 4
5.5 Analysis of acoustic emission from fibre-reinforced polymers. 5
6 Instrumentation and monitoring guidelines . 5
6.1 Instrumentation . 5
6.2 Sensors . 6
6.3 Sensor location and spacing . 6
6.4 Sensor coupling and mounting . 6
6.5 Detection and evaluation threshold . 6
6.6 Application of load . 7
6.7 Graphs for real-time monitoring . 7
7 Specific methodology . 7
7.1 Size of component . 7
7.2 Testing of specimens . 8
7.3 Testing of components and structures . 8
7.3.1 Preliminary information . 8
7.3.2 Test preparation . 8
7.3.3 Load profiles . 9
7.3.4 Written test procedure .11
7.3.5 Evaluation criteria .12
7.3.6 Stop criteria .15
7.3.7 Health monitoring .16
8 Interpretation of acoustic emission test results/source mechanisms .16
9 Report .16
Bibliography .18
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ISO 18249:2015(E)
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 meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT), see the following URL: Foreword — Supplementary information.
The committee responsible for this document is ISO/TC 135, Non-destructive testing, Subcommittee
SC 9, Acoustic emission testing.
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ISO 18249:2015(E)
Introduction
The increasing use of fibre-reinforced polymer (FRP) materials in structural (e.g. aerospace, automotive,
civil engineering) and infrastructural applications (e.g. gas cylinders, storage tanks, pipelines) requires
respective developments in the field of non-destructive testing.
Because of its sensitivity to the typical damage mechanisms in FRP, acoustic emission testing (AT) is
uniquely suited as a test method for this class of materials.
It is already being used for load test monitoring (increasing test safety) and for proof-testing, periodic
testing and periodic or continuous, real-time monitoring (health monitoring) of pressure vessels,
storage tanks, and other safety-relevant FRP structures.
Acoustic emission testing shows potential where established non-destructive test methods
(e.g. ultrasonic testing or water-jacket tests) are not applicable (e.g. thick carbon-fibre reinforced gas
cylinders used for the storage and transport of compressed natural gas (CNG), gaseous hydrogen).
The general principles outlined in EN 13554 apply to all classes of materials but this International
Standard emphasizes applications to metallic components (see EN 13554:2011, Clause 6).
However, the properties of FRP relevant to AT testing are distinctly different from those of metals.
FRP structures are inherently non-homogeneous and show a certain degree of anisotropic behaviour,
depending on fibre orientation and stacking sequence of plies, respectively.
Material composition and properties, and geometry affect wave propagation, e.g. mode, velocity,
dispersion, and attenuation, and hence the AT signals recorded by the sensors.
Composites with a distinct viscoelastic polymer matrix (e.g. thermoplastics) possess a comparatively high
acoustic wave attenuation which is dependent on wave propagation parallel or perpendicular to the direction
of fibre orientation, plate wave mode, frequency, and temperature-dependent relaxation behaviour.
Therefore, successful AT of FRP materials, components, and structures requires a specific methodology
(e.g. storage of complete waveforms, specific sensors and sensor arrays, specific threshold settings,
suitable loading patterns, improved data analysis), different from that applied to metals.
There are recent developments in acoustic emission testing, e.g. modal AT (wave and wave mode analysis
in time and frequency domain) and pattern recognition analysis.
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INTERNATIONAL STANDARD ISO 18249:2015(E)
Non-destructive testing — Acoustic emission testing —
Specific methodology and general evaluation criteria for
testing of fibre-reinforced polymers
1 Scope
This International Standard describes the general principles of acoustic emission testing (AT) of
materials, components, and structures made of fibre-reinforced polymers (FRP) with the aim of
— materials characterization,
— proof testing and manufacturing quality control,
— retesting and in-service testing, and
— health monitoring.
This International Standard has been designed to describe specific methodology to assess the integrity
of fibre-reinforced polymers (FRP), components, or structures or to identify critical zones of high
damage accumulation or damage growth under load (e.g. suitable instrumentation, typical sensor
arrangements, and location procedures).
It also describes available, generally applicable evaluation criteria for AT of FRP and outlines procedures
for establishing such evaluation criteria in case they are lacking.
This International Standard also presents formats for the presentation of acoustic emission test data
that allows the application of qualitative evaluation criteria, both online during testing and by post-test
analysis, and that simplify comparison of acoustic emission test results obtained from different test
sites and organizations.
NOTE The structural significance of the acoustic emission cannot in all cases definitely be assessed based on
AT evaluation criteria only but can require further testing and assessment (e.g. with other non-destructive test
methods or fracture mechanics calculations).
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 9712:2012, Non-destructive testing — Qualification and certification of NDT personnel
ISO 12716:2001, Non-destructive testing — Acoustic emission inspection — Vocabulary
ISO/IEC 17025:2005, General requirements for the competence of testing and calibration laboratories
EN 13477-1:2001, Non-destructive testing — Acoustic emission — Equipment characterisation — Part 1:
Equipment description
EN 13477-2:2010, Non-destructive testing — Acoustic emission — Equipment characterisation — Part 2:
Verification of operating characteristics
EN 14584, Non-destructive testing — Acoustic emission — Examination of metallic pressure equipment
during proof testing — Planar location of AE sources
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ISO 18249:2015(E)
EN 15495, Non-destructive testing — Acoustic emission — Examination of metallic pressure equipment
during proof testing — Zone location of AE sources
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 12716:2001 and the
following apply.
3.1
fibre
slender and greatly elongated solid material
Note 1 to entry: Typically with an aspect ratio greater than 5 and tensile modulus greater than 20 GPa. The fibres
used for continuous (filamentary) or discontinuous reinforcement are usually glass, carbon, or aramide.
3.2
polymer matrix
surrounding macromolecular substance within which fibres are embedded
Note 1 to entry: Polymer matrices are usually thermosets (e.g. epoxy, vinylester polyimide, or polyester) or high-
performance thermoplastics [e.g. poly(amide imide), poly(ether ether ketone), or polyimide]. The mechanical
properties of polymer matrices are significantly affected by temperature, time, aging, and environment.
3.3
fibre laminate
two-dimensionally element made up of two or more layers (plies of the same material with identical
orientation) from fibre-reinforced polymers
Note 1 to entry: They are compacted by sealing under heat and/or pressure. Laminates are stacked together by
plane (or curved) layers of unidirectional fibres or woven fabric in a polymer matrix. Layers can be of various
thicknesses and consist of identical or different fibre and polymer matrix materials. Fibre orientation can vary
from layer to layer.
3.4
fibre-reinforced polymer material
FRP
polymer matrix composite with one or more fibre orientations with respect to some reference direction
Note 1 to entry: Those are usually continuous fibre laminates. Typical as-fabricated geometries of continuous
fibres include uniaxial, cross-ply, and angle-ply laminates or woven fabrics. FRPs are also made from discontinuous
fibres such as short fibre, long-fibre, or random mat reinforcement.
3.5
delamination
intra- or inter-laminar fracture (crack) in composite materials under different modes of loading
Note 1 to entry: Delamination mostly occurs between the fibre layers by separation of laminate layers with the
weakest bonding or the highest stresses under static or repeated cyclic stresses (fatigue), impact, etc. Delamination
involves a large number of micro-fractures and secondary effects such as rubbing between fracture surfaces. It
develops inside of the composite, without being noticeable on the surface and it is often connected with significant
loss of mechanical stiffness and strength.
3.6
micro-fracture (of composites)
occurrence of local failure mechanisms on a microscopic level, such as matrix failure (crazing, cracking),
fibre/matrix interface failure (debonding), or fibre pull-out, as well as fibre failure (breakage, buckling)
Note 1 to entry: It is caused by local overstress of the composite. Accumulation of micro-failures leads to macro-
failure and determines ultimate strength and life-time.
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ISO 18249:2015(E)
4 Personnel qualification
It is assumed that acoustic emission testing is performed by qualified and capable personnel. In order to
prove this qualification, it is recommended to qualify the personnel in accordance with ISO 9712.
5 Acoustic emission sources and acoustic behaviour
5.1 Acoustic emission source mechanisms
Damage of FRP as a result of micro- and macro-fracture mechanisms produces high acoustic emission
activity and intensity making it particularly suitable for acoustic emission testing (AT).
The following are the common failure mechanisms in FRP detected by AT:
— matrix cracking;
— fibre/matrix interface debonding;
— fibre pull-out;
— fibre breakage;
— intra- or inter-laminar crack (delamination/splitting) propagation.
The resulting acoustic emission from FRP depends on many factors, such as material components,
laminate lay-up, manufacturing process, discontinuities, applied load, geometry, and environmental
test conditions (temperature, humidity, exposure to fluid or gaseous media, or ultraviolet radiation).
Therefore, interpretation of acoustic emission under given conditions requires understanding of these
factors and experience with acoustic emission from the particular material and construction under
known stress conditions.
Fracture of FRP produces burst type acoustic emission, high activity; however, might give the appearance
of continuous emission.
For certain types of construction, widely distributed AE sources from matrix or interfacial micro-
failure mechanisms under given conditions commonly represent a normal behaviour. This particularly
appears during the first loading of a newly manufactured FRP structure, where the composite strain for
detection of first significant acoustic emission is in the range of 0,1 % to 0,3 %.
High stiffness optimized composites might shift the onset of first significant acoustic emission towards
comparatively high stresses due to the low matrix strain in the composite.
In the case of high-strength composites, acoustic emission from first fibre breakage, apart from other
sources, is normally observed at stress levels of about 40 % to 60 % of the ultimate composite strength.
A normal behaviour of FRP structures is also characterized by the occurrence of different regions with
alternating higher and lower AE activity, particularly at higher stress levels due to redistribution of
local stress.
In the case of a serious discontinuity or other severe stress concentration that influence the failure
behaviour of FRP structures, AE activity will concentrate at the affected area, thereby providing a
method of detection.
Conversely, discontinuities in areas of the component that remain unstressed as a result of the test and
discontinuities that are structurally insignificant will not generate abnormal acoustic emission.
5.2 Wave propagation and attenuation characterization
Acoustic emission signals from waves travelling in large objects are influenced by dispersion and
attenuation effects.
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ISO 18249:2015(E)
Polymer matrix composites are inhomogeneous and often anisotropic materials and, in many
applications, designed as thin plates or shells. Wave propagation in thin plates or shells is dominated
by plate wave modes (e.g. Lamb waves). The anisotropy is mainly the result of volume and orientation
of fibres. This affects wave propagation by introducing directionality into the velocity, attenuation, and
large dispersion of plate waves.
Propagation of acoustic waves in FRP results in a significant change of amplitude and frequency content
with distance. The extent of these effects will depend upon direction of propagation, material properties,
thickness, and geometry of the test object.
Attenuation characterization measurement on representative regions of the test objects in accordance
with EN 14584 shall be performed.
The shadowing effect of nozzles and ancillary attachments shall be quantified and transmission through
the test fluid shall be taken into consideration.
The attenuation shall be measured in various directions and, if known, in particular parallel and
perpendicular to the principal directions of fibre orientation. In the case of a partly filled test object, the
attenuation shall be measured above and below the liquid level.
For FRP laminate structures, losses of burst signal peak amplitudes might be in the range of 20 dB to 50
dB after wave propagation of about 500 mm. Attenuation perpendicular to the fibre direction is usually
much higher than in the parallel direction.
NOTE The peak amplitude from a Hsu-Nielsen source can vary with specific viscoelastic properties of the
FRP material in different regions of a structure.
5.3 Test temperature
The mechanical (stiffness, strength) and acoustical (wave velocity, attenuation) behaviour of FRP
structures and, hence, their AE activity and AE wave characteristic (waveforms, spectra) strongly
changes if the test temperature approaches transition temperature ranges of the matrix, such as the
ductile-brittle transition (ß-relaxation of semi-crystalline matrices) or the glass-rubber transition
(α-relaxation of amorphous matrices).
Therefore, the test temperature has to be considered for data evaluation and interpretation of AE test
results, as well as in the loading procedure.
5.4 Source location
Accurate source location in FRP structures is difficult. Due to the high attenuation in composite
materials, the AE hits only the nearest sensor in most practical monitoring situations on structures. For
this reason, zone location is usually the main source of location information. The use of zone location,
however, does not prevent linear or planar location of AE sources that have sufficient energy to hit several
sensors to allow location by time arrival differences. Linear or planar location is a useful supplement,
predominantly for the location of higher energy emissions. Great care shall be taken with both methods
where timing information is used for location since the velocity of sound and attenuation will usually
change with the direction of propagation in FRP.
An additional caution when using location methods on FRP has to be taken because of the very high
emission rates (hit overlapping).
Bearing in mind the above sensor separation and positioning should be set appropriately taking the
following into account:
a) sensor frequency range:
Lower frequencies give a larger detection range but might result in the pickup of unwanted noise
sources. Practical FRP testing typically uses high-frequency sensors (100 kHz to 300 kHz) to provide
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ISO 18249:2015(E)
local area monitoring of high stress areas and low-frequency sensors (30 kHz to 60 kHz) to provide
global coverage. It is common to use two frequency ranges simultaneously.
Typical detection ranges on FRP plates are as follows: 150 kHz for 400 mm to 700 mm, 60 kHz for
600 mm to 1200 mm, and 30 kHz for 900 mm to 2000 mm or more, depending on the material.
For research into AE source mechanisms, use of wideband sensors might be preferable.
b) directionality of propagation and attenuation:
More sensors might be required in one direction as a result of higher attenuation. Application of location
techniques that meet direction-dependent wave velocities will achieve better location accuracy. Where
the system software cannot handle directional velocities, the use of virtual sensor positioning might
improve location performance. Checking source location with Hsu-Nielsen or other simulated acoustic
emission sources is recommended to achieve useful results.
c) location performance:
Where planar location of lower energy emissions is a requirement, more sensors are necessary to obtain
the required three hits.
Planar location is especially useful on small specimens or in the case where a local area of a structure is
of particular interest.
5.5 Analysis of acoustic emission from fibre-reinforced polymers
The following types of analysis are applicable:
a) hit, energy, and RMS based processing:
For most testing applications, where the component under test should not be close to failure, the signal
processing of acoustic emission from FRP does not differ significantly from that required for metals.
The main differences are that high-frequency signals are significantly shorter due to the absence of
reverberation. Once damage initiates, the rate of emission will be significantly higher than for metals.
These factors require the monitoring system to be set so as to process appropriately, by using shorter
discrimination times for example. It is possible that very significant damage might appear as a continuous
signal on hit based analysis, for this reason, supplementary processing should always be used, using for
example, the RMS or ASL levels, or the absolute energy measured as a continuous parameter.
b) real-time analysis:
Real-time analysis of the detected acoustic emission and the application of defined criteria is normal
practice and essential whenever the monitoring is required to feedback for the safe progressive
application of load. Real-time graphs shall provide all AE and other parameters that are necessary to
make a decision about the need to stop the test, if necessary.
c) post-test analysis:
Post-test analysis is applied to obtain a more insight into the acquired data, to filter known noise sources,
and in production applications where real-time analysis might not have been used.
6 Instrumentation and monitoring guidelines
6.1 Instrumentation
Instrumentation components (hardware and software) shall conform to the requirements of EN 13477-
1 and EN 13477-2.
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ISO 18249:2015(E)
The equipment shall be able to fulfill the data acquisition and analysis according to the written test
instruction in real-time.
6.2 Sensors
The selection of AE sensor frequency depends on aim of AT and the factors described in 5.4.
For the investigation of damage mechanisms and wave propagation, wideband sensors might be more
appropriate; however, this also introduces the additional variable of plate waves travelling at different
velocities as a function of frequency.
Care should be taken when selecting wideband sensors that their characteristics are appropriate for the
laminate thickness and that their potentially lower sensitivity is taken into account.
6.3 Sensor location and spacing
The sensor location when not defined by an applicable code will generally be determined as follows:
a) 150 kHz sensors monitoring the high stress areas of the structure;
b) Where the 150 kHz sensors do not provide the full coverage, 30 kHz to 60 kHz sensors are used to
monitor the remaining test areas, bearing in mind that these might be susceptible to extraneous noise;
c) The distance between sensors is determined based on attenuation measurement in different
directions and shall follow the guidelines for maximum allowed sensor distance ― dmax ― for
planar location (EN 14584) or zone location (EN 15495).
The evaluation threshold is defined in 6.5.
6.4 Sensor coupling and mounting
For good transfer of acoustic waves, sensors shall be coupled using agents that do not chemically or
physically react with the composite (e.g. by causing crazing, swelling, cracking, or other micro-failure
mechanisms). Suitable coupling agents are silicone-based high-vacuum grease or adhesives, e.g. cold
harden
...
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