Plastics — Determination of tension-tension fatigue crack propagation — Linear elastic fracture mechanics (LEFM) approach

ISO 15850 specifies a method for measuring the propagation of a crack in a notched specimen subjected to a cyclic tensile load varying between a constant positive minimum and a constant positive maximum value. The test results include the crack length as a function of the number of load cycles and the crack length increase rate as a function of the stress intensity factor and energy release rate at the crack tip. The possible occurrence of discontinuities in crack propagation is detected and reported. The method is suitable for use with the following range of materials: rigid and semi-rigid thermoplastic moulding and extrusion materials (including filled and short-fibre-reinforced compounds) plus rigid and semi-rigid thermoplastic sheets; rigid and semi-rigid thermosetting materials (including filled and short-fibre-reinforced compounds) plus rigid and semi-rigid thermosetting sheets.

Plastiques — Détermination de la propagation de fissure par fatigue en traction — Approche de la mécanique linéaire élastique de la rupture (LEFM)

General Information

Status
Withdrawn
Publication Date
19-Jun-2002
Withdrawal Date
19-Jun-2002
Current Stage
9599 - Withdrawal of International Standard
Start Date
04-Feb-2014
Completion Date
04-Feb-2014
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INTERNATIONAL ISO
STANDARD 15850
First edition
2002-06-15
Plastics — Determination of
tension-tension fatigue crack
propagation — Linear elastic fracture
mechanics (LEFM) approach
Plastiques — Détermination de la propagation de fissure par fatigue en
traction — Approche de la mécanique linéaire élastique de la rupture
(LEFM)
Reference number
ISO 15850:2002(E)
ISO 2002
---------------------- Page: 1 ----------------------
ISO 15850:2002(E)
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© ISO 2002

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ii © ISO 2002 – All rights reserved
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ISO 15850:2002(E)
Contents Page

Foreword.....................................................................................................................................................................iv

1 Scope ..............................................................................................................................................................1

2 Normative references....................................................................................................................................1

3 Terms and definitions ...................................................................................................................................1

4 Principle..........................................................................................................................................................4

5 Significance and use.....................................................................................................................................4

6 Test specimens..............................................................................................................................................5

7 Apparatus .......................................................................................................................................................9

8 Test procedure.............................................................................................................................................14

9 Calculation and interpretation of results ..................................................................................................14

10 Test report ....................................................................................................................................................16

Bibliography..............................................................................................................................................................17

© ISO 2002 – All rights reserved iii
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ISO 15850:2002(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.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.

The main task of technical committees is to prepare International Standards. Draft International Standards adopted

by the technical committees are circulated to the member bodies for voting. Publication as an International

Standard requires approval by at least 75 % of the member bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this International Standard may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISO 15850 was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 2, Mechanical

properties.
iv © ISO 2002 – All rights reserved
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INTERNATIONAL STANDARD ISO 15850:2002(E)
Plastics — Determination of tension-tension fatigue crack
propagation — Linear elastic fracture mechanics (LEFM) approach
1 Scope

This International Standard specifies a method for measuring the propagation of a crack in a notched specimen

subjected to a cyclic tensile load varying between a constant positive minimum and a constant positive maximum

value. The test results include the crack length as a function of the number of load cycles and the crack length

increase rate as a function of the stress intensity factor and energy release rate at the crack tip. The possible

occurrence of discontinuities in crack propagation is detected and reported.
The method is suitable for use with the following range of materials:

 rigid and semi-rigid thermoplastic moulding and extrusion materials (including filled and short-fibre-reinforced

compounds) plus rigid and semi-rigid thermoplastic sheets;

 rigid and semi-rigid thermosetting materials (including filled and short-fibre-reinforced compounds) plus rigid

and semi-rigid thermosetting sheets.
2 Normative references

The following normative documents contain provisions which, through reference in this text, constitute provisions of

this International Standard. For dated references, subsequent amendments to, or revisions of, any of these

publications do not apply. However, parties to agreements based on this International Standard are encouraged to

investigate the possibility of applying the most recent editions of the normative documents indicated below. For

undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC

maintain registers of currently valid International Standards.
ISO 291, Plastics — Standard atmospheres for conditioning and testing
ISO 527 (all parts), Plastics — Determination of tensile properties
ISO 2818, Plastics — Preparation of test specimens by machining

ISO 13586, Plastics — Determination of fracture toughness (G and K ) — Linear elastic fracture mechanics

IC IC
(LEFM) approach
3 Terms and definitions

For the purposes of this International Standard, the following terms and definitions apply.

3.1
cycle

smallest segment of a load-time or stress-time function which is repeated periodically

NOTE The terms fatigue cycle, load cycle and stress cycle are also commonly used.

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ISO 15850:2002(E)
3.2
number of cycles completed
number of load cycles since the beginning of a test
3.3
waveform
shape of the load-time curve within a single cycle
3.4
maximum load
max
highest value of the load during a cycle
NOTE 1 It is expressed in newtons.
NOTE 2 Only positive, i.e. tensile, loads are used in this test method.
3.5
minimum load
min
lowest value of the load during a cycle
NOTE 1 It is expressed in newtons.
NOTE 2 Only positive, i.e. tensile, loads are used in this test method.
3.6
load range
difference between the maximum and the minimum loads in one cycle, given by:
∆P = P − P
max min
3.7
load ratio
stress ratio
ratio of the minimum to the maximum load in one cycle, i.e.:
R = P /P
min max
3.8
stress intensity factor

limiting value of the product of the stress σ (r) perpendicular to the crack area at a distance r from the crack tip and

of the square root of 2πr, as r tends to zero:
K =limσπ(rr) 2
r→0
1/2
NOTE 1 It is expressed in pascal root metres (Pa⋅m ).

NOTE 2 The term factor is used here because it is in common usage, even though the quantity has dimensions.

[ISO 13586]
3.9
maximum stress intensity factor
max
highest value of the stress intensity factor in one cycle
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ISO 15850:2002(E)
3.10
minimum stress intensity factor
min
lowest value of the stress intensity factor in one cycle
3.11
stress intensity factor range

difference between the maximum and minimum stress intensity factors in one cycle, given by:

∆K = K − K
max min
3.12
energy release rate

difference between the external work δU done on a body to enlarge a cracked area by an amount δA and the

ext
corresponding change in strain energy δU :
δδUU
ext S
G = −
δδA A
NOTE 1 It is expressed in joules per square metre.

NOTE 2 Assuming linear elastic behaviour, the following relationship between the stress intensity factor K and the energy

release rate G holds:
G =
where
E' = E for plane stress, and E' = for plane strain conditions;
1−ν
E and ν are the tensile modulus and Poisson's ratio respectively.
3.13
maximum energy release rate
max
highest value of the energy release rate in one cycle
3.14
minimum energy release rate
min
lowest value of the energy release rate in one cycle
3.15
energy release rate range

difference between the maximum and minimum energy release rates in one cycle, given by:

∆G = G – G
max min
3.16
notch

sharp indentation made in the specimen, generally using a razor blade or a similar sharp tool, before a test and

intended as the starting point of a fatigue-induced crack
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ISO 15850:2002(E)
3.17
initial crack length
length of the notch (3.16)
NOTE 1 It is expressed in metres.

NOTE 2 For compact tensile (CT) specimens, it is measured from the line joining the load-application points (i.e. the line

through the centres of the loading-pin holes) to the notch tip (see Figure 2). For single-edge-notched tensile (SENT) specimens,

it is measured from the edge of the specimen to the notch tip. Details of the measurement procedure are given in 7.3.

3.18
crack length

total crack length at any time during a test, given by the initial crack length a plus the crack length increment due

to fatigue loading
NOTE It is expressed in metres.
3.19
fatigue crack growth rate
da/dN

rate of crack extension caused by fatigue loading and expressed in terms of average crack extension per cycle

NOTE It is expressed in metres per cycle.
3.20
stress intensity calibration

mathematical expression, based on empirical or analytical results, that relates the stress intensity factor to load and

crack length for a specific specimen geometry
3.21
gauge length

〈single-edge-notched tensile (SENT) specimen〉 free distance between the upper and lower grips after the

specimen has been mounted in the test machine
NOTE It is expressed in metres.
4 Principle

A constant-amplitude cyclic tensile load is imposed on a specimen under suitable test conditions (specimen shape

and size, notching, maximum and minimum loads, load cycle frequency, etc.), causing a crack to start from the

notch and propagate.

The crack length a is monitored during the test and recorded as a function of the number N of load cycles

completed.

Numerical differentiation of the experimental function a(N) provides the fatigue crack growth rate da/dN which is

reported as a function of stress intensity factor and energy release rate at the crack tip.

5 Significance and use

Fatigue crack propagation, particularly when expressed as the fatigue crack growth rate da/dN as a function of

crack-tip stress intensity factor range ∆K or energy release rate range ∆G characterizes a material’s resistance to

stable crack extension under cyclic loading. Background information on the fatigue behaviour of plastics and on the

fracture mechanics approach to fatigue for these materials is given in [1] and [2] (see the Bibliography).

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ISO 15850:2002(E)

Expressing da/dN as a function of ∆K or ∆G provides results that are independent of specimen geometry, thus

enabling exchange and comparison of data obtained with a variety of specimen configurations and loading

conditions. Moreover, this feature enables da/dN versus ∆K or ∆G data to be utilized in the design and evaluation of

engineering structures. The concept of similitude is assumed, which implies that cracks of differing lengths

subjected to the same nominal ∆K or ∆G will advance by equal increments of crack extension per cycle.

Fatigue crack propagation data are not geometry-independent in the strict sense since thickness effects generally

occur. The potential effects of specimen thickness have to be considered when generating data for research or

design.

Anisotropy in the molecular orientation or in the structure of the material, and the presence of residual stresses,

can have an influence on fatigue crack propagation behaviour. The effect can be significant when test specimens

are removed from semi-finished products (e.g. extruded sheets) or finished products. Irregular crack propagation,

namely excessive crack front curvature or out-of-plane crack growth, generally indicates that anisotropy or residual

stresses are affecting the test results.
This test method can serve the following purposes:

a) to estabilish the influence of fatigue crack propagation on the lifetime of components subjected to cyclic

loading, provided data are generated under representative conditions and combined with appropriate fracture

toughness data (see ISO 13586) and stress analysis information;

b) to establish material-selection criteria and inspection requirements for damage-tolerant applications;

c) to establish, in quantitative terms, the individual and combined effects of the material’s structure, the

processing conditions and the loading variables on fatigue crack propagation.
6 Test specimens
6.1 Shape and size
6.1.1 Standard specimens

Two different types of specimen can be used: single-edge-notched tensile (SENT) and compact tensile (CT).

Figures 1 and 2 describe their geometrical characteristics.
6.1.2 Thickness and width

When the specimen thickness h is too small compared to the width w, it is difficult to avoid lateral deflections or out-

of-plane bending of the specimen. Conversely, with very thick specimens, through-thickness crack curvature

corrections are often necessary and difficulties may be encountered in meeting the through-thickness straightness

requirement of 8.1.

On the basis of these considerations, the following limits are recommended for h and w:

a) for CT specimens, w/10 u h u w/2;
b) for SENT specimens, w/20 u h u w/4.

It should be noted that the test results are in general thickness-dependent: specimens obtained from the same

material but having different thicknesses are likely to give different responses.

It is usually convenient to make the thickness h of specimens equal to the thickness of the sheet sample from

which the specimens are cut.
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ISO 15850:2002(E)
Key
w Width w/20 u h u w/4 (recommended)
l Length l > 2,5w
1 1
h Thickness
a Initial crack length
The notch shall be within ± 0,01w of the specimen centreline.

Figure 1 — Standard single-edge-notched tensile (SENT) specimen for fatigue crack propagation testing

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ISO 15850:2002(E)
Key
w Effective width w/10 u h u w/2 (recommended)
W Overall width W = 1,25w ± 0,01w
l Length l = 1,2w ± 0,01w
1 1
l Distance between centres of loading-pin holes located l = 0,55w ± 0,005w
symmetrically to the crack plane to within ± 0,005w
R Radius of loading-pin hole R = 0,125w ± 0,005w
h Thickness
a Initial crack length a W 0,2w
0 0
The notch shall be within ± 0,01w of the specimen centreline.

Figure 2 — Standard compact tensile (CT) specimen for fatigue crack propagation testing

6.1.3 Size requirements

In order for the results obtained by this test method to be valid, it is required that the material behaviour be

predominantly linear elastic at all values of applied load and crack length. Deviations may arise from either

viscoelastic behaviour of the material or large-scale plasticity ahead of the crack tip. The former may result in

significant non-linearity of the mechanical behaviour, possibly aggravated by a progressive rise of the specimen

temperature during the test. The test procedure outlined in this International Standard is therefore recommended

only for materials exhibiting very limited viscoelasticity under the loading frequency used and for the expected test

duration. Large-scale plasticity of the ligament can be avoided by ensuring that the plastic zone around the crack

tip is small compared with the size of the uncracked ligament (w − a). On the basis of previous experience with

[3]

metallic materials , it is required that the following size limits be satisfied in order for the test results to be valid:

wa−πW 4 K σ (1)
() ()
max y
where
w − a is the uncracked-specimen ligament width;

σ is the tensile-yield stress measured in accordance with the relevant part of ISO 527.

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ISO 15850:2002(E)

The same size limits are expressed in graphical form in Figure 3, where the dimensionless quantities

K / σ w and a/w are plotted against each other. All combinations of specimen size, crack length, material

max()y

yield stress and stress intensity factor which lie below the curve in Figure 3 satisfy the specimen size requirements

of this test method.
Figure 3 — Size requirements for standard fatigue crack propagation specimens

(Values which lie below the curve satisfy the specimen size requirements of this method)

6.2 Preparation

Prepare specimens in accordance with the relevant materials specification and in accordance with ISO 2818. In the

case of anisotropic materials, take care to indicate the reference direction on each specimen.

6.3 Notching

Produce a sharp notch or, when feasible, a natural crack, intended as the starting point of the fatigue-induced

crack, in the specimen at the locations depicted in Figures 1 and 2, either in a single step or by sharpening the tip

of a blunt slot or notch made by machining.

It is required that the initial crack length a in the CT specimen be at least 0,2w so that K-calibration is not

influenced by small variations in the location and dimensions of the loading-pin holes. The notch length in CT

specimens shall be chosen accordingly (see 7.3 for details of measurement of initial crack length a ).

The notch in both the CT and SENT specimens shall be within ± 0,01w of the specimen centreline.

When sharpening a blunt notch produced by machining, the length of the sharp notch shall be more than four times

the blunt notch tip radius. Method a), b), c) or d) can be used to create a natural crack or a sharp notch:

a) Machine a sharp notch in the specimen and then generate a natural crack by tapping on a new razor blade

placed in the notch (it is essential to practice this since, in brittle specimens, a natural crack can be generated

by this process but some skill is required in avoiding too long a crack or local damage).

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ISO 15850:2002(E)

b) If control is difficult or repeatability problems are experienced with method a), it is possible with some brittle

specimens to generate a sharp notch by simply pressing the razor blade against the specimen at a

temperature close to, but lower than, the glass-transition temperature of the material. With this notching

procedure, proper handling of the specimen and correct choice of temperature are essential to avoid

deformation of, or damage to, the specimen. Use a new razor blade for each specimen.

c) If a natural crack cannot be generated, as in tough specimens, then sharpen the notch by sliding a razor blade

across the notch. Use a new razor blade for each specimen.

d) With tough materials, cooling the specimen and then tapping with a razor blade is sometimes successful.

It may be useful to check the effectiveness of the notching procedure by performing preliminary tests at a constant

displacement or constant loading rate on specimens notched using different methods. The best notching method is

the one which gives the lowe
...

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