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INTERNATIONAL ISO
STANDARD 8256

Second edition
2004-07-01

Reviewed and confirmed in 2017

Plastics — Determination of
tensile-impact strength
Plastiques — Détermination de la résistance au choc-traction

Reference number
ISO 8256:2004(E)

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ISO 8256:2004(E)

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ISO 8256:2004(E)

Contents Page

Foreword ............................................................................................................................................................ iv
1 Scope...................................................................................................................................................... 1
2 Normative references ........................................................................................................................... 1
3 Terms and definitions........................................................................................................................... 2
4 Principle ................................................................................................................................................. 2
5 Apparatus............................................................................................................................................... 3
5.1 Test machine ......................................................................................................................................... 3
5.2 Pendulum and striker ........................................................................................................................... 3
5.3 Crosshead.............................................................................................................................................. 3
5.4 Clamping devices/jaws......................................................................................................................... 3
5.5 Micrometers and gauges...................................................................................................................... 3
6 Test specimens ..................................................................................................................................... 4
6.1 Shape and dimensions ......................................................................................................................... 4
6.2 Preparation ............................................................................................................................................ 6
6.3 Notching of specimens......................................................................................................................... 6
6.4 Number of test specimens ................................................................................................................... 6
6.5 Anisotropy ............................................................................................................................................. 6
6.6 Conditioning .......................................................................................................................................... 7
7 Procedure............................................................................................................................................... 7
8 Determination of energy corrections .................................................................................................. 8
8.1 Method A — Correction Eq due to the plastic deformation and the kinetic energy of the
crosshead .............................................................................................................................................. 8
8.2 Method B — Crosshead-bounce energy Eb........................................................................................ 8
9 Calculation and expression of results ................................................................................................ 8
9.1 Calculation of corrected tensile-impact energy................................................................................. 8
9.2 Calculation of tensile-impact strength................................................................................................ 9
9.3 Statistical parameters........................................................................................................................... 9
9.4 Number of significant figures .............................................................................................................. 9
10 Precision ................................................................................................................................................ 9
11 Test report............................................................................................................................................ 10
Annex A (normative) Determination of correction factor for method A ..................................................... 11
Annex B (normative) Determination of bounce-correction factor for method B ....................................... 14
Bibliography ..................................................................................................................................................... 16

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ISO 8256:2004(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 2.

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 document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISO 8256 was prepared by Technical Committee ISO/TC 61, Plastics, Subcommittee SC 2, Mechanical
properties.

This second edition cancels and replaces the first edition (ISO 8256:1990), which has been technically revised.

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INTERNATIONAL STANDARD ISO 8256:2004(E)

Plastics — Determination of tensile-impact strength

1 Scope
1.1 This International Standard specifies two methods (method A and method B) for the determination of
the tensile-impact strength of plastics under defined conditions. The tests can be described as tensile tests at
relatively high strain rates. These methods can be used for rigid materials (as defined in ISO 472), but are
especially useful for materials too flexible or too thin to be tested with impact tests conforming to ISO 179 or
ISO 180.

1.2 These methods are used for investigating the behaviour of specified specimens under specified impact
velocities, and for estimating the brittleness or the toughness of specimens within the limitations inherent in
the test conditions.

1.3 These methods are applicable both to specimens prepared from moulding materials and to specimens
taken from finished or semi-finished products (for example mouldings, laminates, or extruded or cast sheets).

1.4 Results obtained by testing moulded specimens of different dimensions may not necessarily be the
same. Equally, specimens cut from moulded products may not give the same results as specimens of the
same dimensions moulded directly from the material. Test results obtained from specimens prepared from
moulding compounds cannot be applied directly to mouldings of any given shape, because values may
depend on the design of the moulding and the moulding conditions. Results obtained by method A and
method B may or may not be comparable.

1.5 These methods are not suitable for use as a source of data for design calculations on components.
Information on the typical behaviour of a material can be obtained, however, by testing different types of test
specimen prepared under different conditions, and by testing at different temperatures. The two different
methods are suitable for production control as well as for quality control.

2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.

ISO 179-1, Plastics — Determination of Charpy impact properties — Part 1: Non-instrumented impact test

ISO 179-2, Plastics — Determination of Charpy impact properties — Part 2: Instrumented impact test

ISO 180, Plastics — Determination of Izod impact strength

ISO 291, Plastics — Standard atmospheres for conditioning and testing

ISO 293, Plastics — Compression moulding of test specimens of thermoplastic materials

ISO 294-1, Plastics — Injection moulding of test specimens of thermoplastic materials — Part 1: General
principles, and moulding of multipurpose and bar test specimens

ISO 294-2, Plastics — Injection moulding of test specimens of thermoplastic materials — Part 2: Small tensile
bars

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ISO 8256:2004(E)

ISO 294-3, Plastics — Injection moulding of test specimens of thermoplastic materials — Part 3: Small plates

ISO 295, Plastics — Compression moulding of test specimens of thermosetting materials

ISO 472, Plastics — Vocabulary

ISO 1268 (all parts), Fibre-reinforced plastics — Methods of producing test plates

ISO 2602, Statistical interpretation of tests results — Estimation of the mean — Confidence interval

ISO 2818, Plastics — Preparation of test specimens by machining

ISO 3167, Plastics — Multipurpose test specimens

ISO 10350-1, Plastics — Acquisition and presentation of comparable single-point data — Part 1: Moulding
materials

ISO 11403-3, Plastics — Acquisiton and presentation of comparable multipoint data — Part 3: Environmental
influences on properties

ISO 13802, Plastics — Verification of pendulum impact-testing machines — Charpy, Izod and tensile impact-
testing

3 Terms and definitions

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

3.1
tensile-impact strength of unnotched specimens
atU
energy absorbed in breaking an unnotched specimen under specified conditions, referred to the original cross-
sectional area of the specimen

NOTE It is expressed in kilojoules per square metre (kJ/m2).

3.2
tensile-impact strength of notched specimens
atN
energy absorbed in breaking a notched specimen under specified conditions, referred to the original cross-
sectional area of the specimen at the notch

NOTE It is expressed in kilojoules per square metre (kJ/m2).

4 Principle
A specimen is broken by a single impact at the bottom of the swing of the pendulum of a tensile-impact
machine. The specimen is horizontal at the moment of rupture. One end of the specimen, at impact, is held
either by the frame or the pendulum and the other end by the crosshead. The two methods described are
based on two different ways of positioning the specimen held by the crosshead: the specimen may be either
mounted stationary on the support frame (method A) or carried downward together with the pendulum
(method B).

The energy to fracture is determined by the kinetic energy extracted from the pendulum in the process of
breaking the specimen. Corrections are made for the energy to toss (method A) or bounce (method B) the
crosshead.

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ISO 8256:2004(E)

5 Apparatus

5.1 Test machine

The principles, characteristics and verification of suitable test machines are detailed in ISO 13802.

5.2 Pendulum and striker

5.2.1 The pendulum shall be constructed of a single- or multiple-membered arm holding the head, in which
the greatest mass is concentrated. A rigid pendulum is essential to maintain the proper clearances and
geometric relationships between related parts and to minimize energy losses, which are always included in
the measured impact-energy value.

5.2.2 The strikers for method A and method B are described in detail in ISO 13802.

5.3 Crosshead

5.3.1 As pointed out in ISO 13802, in order to reduce bouncing due to the impact of the metal striker on the
metal crosshead, the material used for the crosshead shall be one which gives an essentially inelastic impact
(e.g. aluminium). The mass of the crosshead, both for method A and for method B, shall be selected from the
values given in Table 1.

5.3.2 A jig or other device shall be used to assist in clamping the crosshead in the specified position, at right
angles to the longitudinal axis of the specimen.

Table 1 — Crosshead masses

Crosshead mass
Potential energy
g
J
Method A Method B

2,0 15 ± 1 or 30 ± 1 15 ± 1
4,0 15 ± 1 or 30 ± 1 15 ± 1
7,5 30 ± 1 or 60 ± 1 30 ± 1
15,0 30 ± 1 or 60 ± 1 120 ± 1
25,0 60 ± 1 or 120 ± 1 120 ± 1
50,0 60 ± 1 or 120 ± 1 120 ± 1

NOTE For method A, use the lighter crosshead whenever possible.

5.4 Clamping devices/jaws

Clamps and jaws for tensile-impact testing are described in ISO 13802.

5.5 Micrometers and gauges

Micrometers and gauges suitable for measuring the dimensions of test specimens to an accuracy of 0,01 mm
are required. In measuring the thickness of the specimen, the measuring face shall apply a load of 0,01 MPa
to 0,05 MPa. For notched specimens, see the requirements of 7.4.

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6 Test specimens

6.1 Shape and dimensions

Five types of test specimen, as specified in Table 2 and shown in Figure 1, may be used. In general, all types
can be used with either of the two methods.

Method A: To be in agreement with ISO 10350-1 and ISO 11403-3, the preferred specimen types are type 1
(which can be taken from the multipurpose test specimen specified in ISO 3167 or moulded directly in
accordance with ISO 294-1) and type 4 (which can be moulded directly in accordance with ISO 294-2 or
machined from plates moulded in accordance with ISO 294-3).

Method B: The preferred specimen types are type 2 and type 4.

The test result depends on the type of specimen used and its preparation and thickness. For reproducible
results, or in cases of dispute, the type of test specimen and its preparation and thickness shall be agreed
upon.

Specimens are tested at their original thickness up to and including 4 mm. The preferred specimen thickness
is 4 mm ± 0,2 mm for type 1 specimens and 3 mm ± 0,2 mm for type 4 specimens. Within the gauge area, the
thickness shall be maintained to within a tolerance of ± 5 %. Above 4 mm, the test methods described in this
International Standard are inapplicable, and ISO 179 or ISO 180 have to be used to determine the impact
properties of specimens.

Table 2 — Specimen types and dimensions

Dimensions in millimetres

Preferred value Preferred value Free length Radius of

Specimen Length Width
of dimension x of dimension l0 between grips curvature
type
l b le r

1 80 ± 2 10 ± 0,2 6 ± 0,2 — 30 ± 2 —
2 60 ± 2 10 ± 0,2 3 ± 0,2 10 ± 0,2 25 ± 2 10 ± 1
3 80 ± 2 15 ± 0,2 10 ± 0,2 10 ± 0,2 30 ± 2 20 ± 1
4 60 ± 2 10 ± 0,2 3 ± 0,2 — 25 ± 2 15 ± 1
a
5 80 ± 2 15 ± 0,2 5 ± 0,2 10 ± 0,2 50 ± 2 20 ± 1
a For type 5: b′ = 23 mm ± 2 mm, r ′ = 4 mm ± 0,5 mm, l ′ = 11 mm ± 1 mm.

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ISO 8256:2004(E)

Figure 1 — Types of test specimen

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ISO 8256:2004(E)

6.2 Preparation

6.2.1 Moulding and extrusion compounds

Specimens shall be prepared in accordance with the relevant material specification. When none exists, or
when otherwise specified, specimens shall be directly extruded (in accordance with the standard appropriate
to the material), or compression or injection moulded from the material in accordance with ISO 293, ISO 294-1,
ISO 294-2 or ISO 295, or machined in accordance with ISO 2818 from sheets or plates compression or
injection moulded from the compound. Type 1 specimens can be prepared from the type A multi-purpose test
specimen described in ISO 3167.

6.2.2 Sheets

Specimens shall be machined from sheets in accordance with ISO 2818.

6.2.3 Fibre-reinforced resins

A panel shall be prepared from the compound in accordance with the relevant part of ISO 1268, and
specimens shall be machined in accordance with ISO 2818.

6.3 Notching of specimens

6.3.1 Notches (for type 1 specimens) shall be machined in accordance with ISO 2818 on unnotched
specimens prepared in accordance with 6.2.

6.3.2 The radius of the notch base shall be 1,0 mm ± 0,05 mm and its angle 45° ± 1° (see Figure 1). The
profile of the cutting tooth shall be such as to produce in the specimen, at right angles to its principal axis, two
notches of the contour and depth shown in Figure 1. The two lines drawn perpendicular to the length direction
of the specimen through the apex of each notch shall be within 0,2 mm of each other. Particular attention shall
be given to the accuracy of the dimension x (see Table 2). Close tolerances have to be imposed on the
contour and the radius of the notch for most materials because these factors largely determine the degree of
stress concentration at the base of the notch during the test. The maintenance of a sharp, clean-edged cutting
tool is particularly important since minor defects at the base of the notch can cause large deviations in the test
results. The profile of the notch being produced by a particular cutting tool shall be checked at regular
intervals.

6.3.3 Specimens with moulded-in notches may be used if specified for the material being tested.
Specimens with moulded-in notches generally do not give the same results as specimens with machined
notches, and allowance should be made for this difference in interpreting the results. Specimens with
machined notches are generally preferred because skin effects and/or localized anisotropy are minimized.
The profile of the notch being produced shall be checked at regular intervals.

6.3.4 For specimens prepared by cutting them out with a puncher, the notch shall not be punched out but
shall be machined in a second step.

6.4 Number of test specimens

Unless otherwise specified in the standard for the material being tested, a set consisting of ten specimens
shall be tested. When the coefficient of variation (see ISO 2602) has a value of less than 5 %, a minimum
number of five test specimens is sufficient.

6.5 Anisotropy

The impact properties of certain types of sheet material may differ depending on the direction of measurement
in the plane of the sheet. In such cases, it is customary to prepare two groups of test specimens with their
major axes respectively parallel and perpendicular to the direction of some feature of the sheet which is either
visible or can be inferred from knowledge of the method of manufacture of the sheet.

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ISO 8256:2004(E)

6.6 Conditioning

Unless otherwise specified in the standard for the material being tested, the specimens shall be conditioned in
accordance with ISO 291, unless other conditions are agreed upon by the interested parties. In the case of
notched specimens, the conditioning time starts after notching.

7 Procedure
7.1 Conduct the test in the same atmosphere as that used for conditioning, unless other conditions are
agreed upon by the interested parties (e.g. for testing at high or low temperature).

7.2 Check that the impact machine is able to perform the test with the specified velocity of impact and that
the energy absorbed is in the correct range, i.e. between 20 % and 80 % of the energy available at impact. If
more than one of the pendulums conform to these requirements, the pendulum having the highest energy
shall be used.

7.3 Determine the frictional losses in accordance with ISO 13802.

7.4 Measure the thickness h and the width x of the central, parallel-sided section of the test specimen to the
nearest 0,02 mm. In the case of notched specimens, carefully measure the dimension x using a micrometer
fitted with an anvil of width 2 mm to 3 mm and of suitable profile to fit the shape of the notch.

In the case of injection-moulded specimens, it is not necessary to measure the dimensions of each specimen.
It is sufficient to measure one specimen from a set to make sure that the dimensions correspond to those
requested. With multiple-cavity moulds, ensure that the dimensions of the specimens are the same for each
cavity.

7.5 Lift the pendulum to the prescribed height and arrest it. Insert the specimen in the holder and tighten
firmly: for method A, place one end of the specimen inside the vice jaw of the frame and the other inside the
crosshead clamp; for method B, place one end of the specimen inside the secured specimen clamp and the
other inside the unsecured crosshead/specimen clamp (see ISO 13802 for details).

7.6 Release the pendulum. Record the impact energy Es absorbed by the specimen and apply corrections
for frictional losses if necessary in accordance with ISO 13802.

7.7 If the resulting corrected tensile-impact energy is below 20 % of the capacity of the 2,0 J pendulum, the
data should be considered suspect.

In cases where the specimen is weak, rigid multi-layered specimens may be used. Use of such specimens
shall be by agreement between the interested parties and shall be clearly documented in the test report.

7.8 If various materials are to be compared, pendulums with the same velocity at impact shall be used for
each. In cases of dispute, it is recommended that test results be compared only with results obtained with
pendulums of identical nominal energy and specimens of the same geometry.

7.9 Immediately after the test has been completed, a check shall be made to ensure that the specimen was
firmly clamped or whether it had slipped in one of the two grips, and that the failure occurred in the narrow,
parallel-sided part of the specimen. If any of the specimens tested do not meet these requirements, the results
for these specimens shall be discarded and additional specimens tested.

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ISO 8256:2004(E)

8 Determination of energy corrections

8.1 Method A — Correction Eq due to the plastic deformation and the kinetic energy of the
crosshead

The correction Eq is determined from the following equation (see Annex A for details):

E max × µ × (3 + µ ) 3
Eq = ≈ × E max × µ (1)
2 × (1 + µ ) 2

where

Eq is the energy correction, in joules, due to the plastic deformation and the kinetic energy of the
crosshead;

Emax is the maximum impact energy, in joules, of the pendulum;

µ is the mass of the crosshead divided by the reduced mass of the pendulum (i.e. mcr/mp).

The reduced mass of the pendulum mp is given by the equation:

E max
mp = (2)
g × L p × (1 − cosα )

where

g is the acceleration, in m⋅s–2, due to gravity;

Lp is the pendulum length, in metres, determined as indicated in ISO 13802;

α is the angle between the positions of the pendulum at its maximum and minimum height.

8.2 Method B — Crosshead-bounce energy Eb

The crosshead-bounce energy Eb is determined for each specimen and pendulum from the crosshead-bounce
energy curve. This curve is determined only once for each crosshead and pendulum combination (see
Annex B).

9 Calculation and expression of results

9.1 Calculation of corrected tensile-impact energy

9.1.1 General

In order to calculate the tensile-impact strength of the specimens, the consumed energy Es must first be
corrected for the toss energy Eq in method A and for the crosshead-bounce energy Eb in method B.

9.1.2 Energy correction for method A

The corrected tensile-impact energy Ec, in joules, is calculated using the equation:

Ec = Es – Eq (3)

where

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Es is the impact energy, in joules, absorbed during the impact, as measured by the instrument (see 7.6);

Eq is the toss energy, in joules, due to the plastic deformation and the kinetic energy of the crosshead,
calculated as specified in 8.1.

9.1.3 Energy correction for method B

The corrected tensile-impact energy Ec, in joules, is calculated using the equation:

Ec = Es + Eb (4)

where

Es is the impact energy, in joules, measured by the instrument (see 7.6).

Eb is the crosshead-bounce energy, in joules, of the crosshead, as determined from the measured value
of Es and the graph prepared for the particular impact tester used, as specified in 8.2 and Annex B.

9.2 Calculation of tensile-impact strength

The tensile-impact strength atU or the tensile-impact strength (notched) atN, expressed in kilojoules per square
metre, is calculated using the following equation:

Ec
a tU ( a tN ) = × 10 3 (5)
x×h

where

Ec is the corrected impact energy, in joules, calculated in accordance with 9.1;

x is the width, in millimetres, of the narrow, parallel-sided section of the specimen (for specimen types
2, 3, 4 and 5 in Figure 1) or the distance between the notches (for specimen type 1 in Figure 1);

h is the thickness, in millimetres, of the narrow, parallel-sided section of the specimen [or, for multilayer
specimens (see 7.7), the total thickness].

9.3 Statistical parameters

If required, calculate the arithmetic mean of test results, the standard deviation of the mean and the coefficient
of variation using the procedure given in ISO 2602.

9.4 Number of significant figures

Report all calculated mean values to two significant figures.

10 Precision
The precision of this test method is not known because inter-laboratory data are not available. When inter-
laboratory data are obtained, a precision statement will be added at the following revision.

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ISO 8256:2004(E)

11 Test report
The test report shall include the following information:

a) a reference to this International Standard;

b) the method (A or B) and specimen type (see Table 2) used:

c) all information necessary for identification of the material tested, including type, source, manufacturer’s
code, grade and history, where these are known;

d) a description of the nature and form of the material, i.e., whether a product, semifinished product, sheet or
specimen, including principal dimensions, shape, method of manufacture, etc., where these are known;

e) the thickness of moulded specimens or, for sheets, the thickness of the sheet and, if applicable, the
directions of the major axes of the specimens in relation to some feature of the sheet;

f) the method of test specimen preparation;

g) the standard atmosphere used for conditioning and testing, plus any special conditioning treatment if
required by the standard for the material or product;

h) the nominal pendulum energy;

i) the mass of the crosshead used;

j) the tensile-impact strength atN or atU of the material, expressed in kilojoules per square metre, reported
as the arithmetic mean of the results on notched and/or unnotched test specimens, as applicable;

k) the individual test results, if required;

l) the standard deviation and the coefficient of variation of the results, if required;

m) the type of fracture exhibited by the test specimens;

n) the date of the test.

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ISO 8256:2004(E)

Annex A
(normative)

Determination of correction factor for method A

A.1 Energy terms used

In calculating the correction Eq, the following energy terms are used:

1
E max = × m p × v 02 (A.1)
2

1
Ep = × m p × v p2 (A.2)
2

E s = E max − E p (A.3)

1 2
E cr,kin = × m cr × v cr (A.4)
2

As the elastic energy of the impact can be neglected (as required by 5.3.1, the impact is essentially inelastic),
vcr = vp and the kinetic energy of the crosshead is given by:

1
E cr,kin = × m cr × v p2 (A.5)
2

where

Emax is the maximum impact energy, in joules, of the pendulum;

Ep is the residual energy, in joules, of the pendulum after the impact;

Es is the measured energy, in joules, consumed during the impact;

Ecr,kin is the kinetic energy, in joules, of the crosshead lost;

mp is the reduced mass, in kilograms, of the pendulum (see 8.1);

v0 is the velocity, in m⋅s–1, of the pendulum immediately before impact;

vp is the velocity, in m⋅s–1, of the pendulum immediately after impact;

mcr is the mass, in kilograms, of the crosshead;

vcr is the velocity, in m⋅s–1, of the crosshead immediately after impact.

In addition,

Ec is the energy, in joules, needed for deformation and fracture of the specimen (to be calculated);

Ecr,pl is the energy, in joules, consumed by the plastic deformation of the crosshead.

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A.2 Determination of Ecr,kin

The energy equation for the impact is:

E s = E c + E cr,pl + E cr,kin (A.6)

Furthermore, from Equations (A.2) and (A.4), it follows that:

E cr,kin
=µ (A.7)
Ep

Combining this equation with Equation (A.3) gives:

E cr,kin = µ × ( E max − E s ) (A.8)

where

m cr
µ= (A.9)
mp

A.3 Determination of Ecr,pl

To calculate the energy Ecr,pl consumed by the plastic deformation of the crosshead, it is necessary to
consider the momentum equation at impact without a specimen (i.e. Ec = 0).

This case is indicated by an asterisk (*).

The momentum equation (considering that the impact is essentially inelastic) can be written as follows:

m p × v 0 = ( m p + m cr ) × v p* (A.10)

Using Equation (A.9),

1
v p* = × v0 (A.11)
1+ µ

Considering Equation (A.3),

E s* = E max − E p* (A.12)

where

1
E p* = × m p × v p*2 (A.13)
2

and substituting Equations (A.1) and (A.13) in Equation (A.12) and using Equation (A.11), the consumed
energy measured without a specimen is then given by:

µ × (2 + µ )
E s* = E max × (A.14)
(1 + µ ) 2

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With Ec = 0, Equation (A.6) becomes:

E s* = E cr,pl
* *
+ E cr,kin (A.15)

From Equations (A.5), (A.11) and (A.9), it follows that:

* µ
E cr,kin = E max × (A.16)
(1 + µ ) 2

Finally, from Equations (A.14), (A.15) and (A.16), the energy due to the plastic deformation of the crosshead,
without any specimen, is given by:

* µ
E cr,pl = E max × (A.17)
(1 + µ )

Since the crosshead is plastically deformed by the same amount with and without a specimen,

*
E cr,pl = E cr,pl (A.18)

A.4 Energy correction

Considering Equation (A.6), the energy correction can be written as follows:

E 
E q = E s − E c = µ ×  max + ( E max − E s ) (A.19)
 (1 + µ ) 

This correction consists of a dominant constant part (representing the energy consumed by the plastic
deformation of the crosshead Ecr,pl) and a smaller part (Emax – Es) which decreases from µEmax to zero with
increasing consumed energy (when Es ~ Emax). In view of measurement uncertainties, it is sufficient to use a
constant correction as an approximation; assuming that

E max
Es = (A.20)
2

gives the correction

E max × µ × (3 + µ )
Eq = Es − Ec = (A.21)
2 × (1 + µ )

The corrected value of the energy consumed by the impact with the specimen is therefore given by:

E max × µ × (3 + µ ) 3
Ec = Es − Eq = Es − ≈ E s − × µ × E max (A.22)
2 × (1 + µ ) 2

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Annex B
(normative)

Determination of bounce-correction factor for method B

After impact and rebound of the crosshead, the specimen is pulled by two moving bodies, the pendulum with
an energy of 0,5MV2, and the crosshead with an energy of 0,5mv2. When the specimen breaks, only that
energy is recorded on the pendulum dial which is lost by the pendulum. Therefore, it is necessary to add the
incremental energy contributed by the crosshead to determine the true energy used to break the specimen.
The correction (i.e. the incremental energy contributed by the crosshead) can be calculated as follows:

By definition,

1
E= × M (V 2 − V 22 ) (B.1)
2

and

1
e= × m(v 12 − v 22 ) (B.2)
2

where

M is the mass, in kilograms, of the pendulum;

m is the mass, in kilograms, of the crosshead;

V is the maximum velocity, in metres per second, of the centre of impact of the crosshead;

V2 is the velocity, in metres per second, of the centre of impact of the pendulum at the moment when
the specimen breaks;

v1 is the crosshead velocity, in metres per second, immediately after bounce;

v2 is the crosshead velocity, in metres per second, at the moment when the specimen breaks;

E is the energy, in joules, read from the pendulum dial;

e is the energy contribution, in joules, of the crosshead, i.e. the bounce-correction factor to be added to
the pendulum reading.

Once the crosshead has rebounded, the momentum of the system (in the horizontal direction) remains
constant. Neglecting vertical components, the momentum equation for the impact can be written as follows:

MV − mv1 = MV 2 − mv 2 (B.3)

Equations (B.1), (B.2) and (B.3) can be combined to give:

  2 
1  M  2 × E  
e= × m v12 −  v 1 − × V − V 2 −   (B.4)
2   m  M   
 
  

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In Equation (B.4), the crosshead velocity after bounce v1 is the only unknown quantity. As pointed out in
ASTM D 1822 [1], in a real test carried out on a specimen the initial rebound velocity of the crosshead v1 is the
same as that measured with no specimen in the pendulum. For the particular impact tester used in the
procedure described in the ASTM standard mentioned above, v1 can be determined either experimentally by
photographic analysis or theoretically by the coefficient of restitution method.

If e is plotted as a function of E (for fixed values of V, M, m and v1), e will increase from zero, pass through a
maximum (equal to 0,5mv12) and then decrease, passing again through zero before becoming negative. The
only part of this curve for which a reasonably accurate analysis has been made is the initial portion between
e = 0 and e = 0,5mv12.

Once the crosshead reverses its direction of travel, the correction becomes less clearly defined and, after it
contacts the anvil a second time, the correction becomes much more difficult to determine. It is assumed,
therefore, for the sake of simplicity, that once e has reached its maximum value the correction factor will
remain constant at a value of 0,5mv12. It should be clearly recognized that the use of that portion of the curve
in Figure B.1 where e is constant does not give an accurate correction. However, as E grows larger, the
correction factor becomes relatively less important and no great sacrifice of overall accuracy results from the
assumption that the maximum correction is 0,5mv12.

Key
1 Equation (B.4)

Figure B.1 — Typical correction-factor curve for single bounce of crosshead in a specimen-in-head
tensile-impact machine

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Bibliography

[1] ASTM D 1822-99, Standard Test Method for Tensile-Impact Energy to Break Plastics and Electrical
Insulating Materials

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ICS 83.080.01
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