3 Terms and definitions
For the purposes of this document, the following terms and definitions apply
3.1
flexural strain
ε x
nominal value of the strain in the tensile surface of a flat test specimen of thickness h, bent over the segment of a circle with radius r, calculated from the equation
3.2
strain value
one of a series of strain levels applied to successive test specimens during exposure
3.3
strain series a number of strain values, including zero
NOTE 1 Normally, the results of the mechanical test on test specimens with zero applied strain are equivalent whether determined in air or in a chemical test medium. If the property measured after exposure to the test medium at zero strain is different from that after exposure in air at zero strain, embrittlement or softening by the test medium should be suspected.
In some cases, relief of stresses imparted during specimen preparation such as by injection-moulding or machining can contribute to differences in results for stress cracking in air and another test medium.
NOTE 2 It is recommended that the test specimen for zero strain be clamped on to a flat form to prevent warping due to the effect of the test medium.
3.4
failure strain
εF
lowest strain in the strain series at which failure is observed
3.5
indicative property property observed to determine failure using a criterion such as those given in Table 1
3.6
ESC index
ratio of the value of failure strain determined in the test medium to that determined in the reference medium (usually air) for the same time of exposure
4 Principle
A test specimen suitable for the determination of the indicative property is clamped with one of its faces over a form of constant radius and brought into contact with the test environment. Due to the influence of the environment in the presence of strain, crazes may be generated which with time sometimes develop into visible cracks.
By using a series of forms with decreasing radii, a series of test specimens that have increasing strains in the outer surface can be tested.
After an agreed duration of contact with the test environment, the test specimens are visually observed,
unclamped and assessed by mechanical or other testing. The failure strain that corresponds to the failure
criterion is obtained directly from the tabulated values or graphically.
The maximum strain must be less than the strain at yield.
NOTE 1 The failure criterion is also commonly expressed in terms of the ESC index.
NOTE 2 The failure strains for different indicative properties may be different.
5 Apparatus
5.1 Forms, made from chemical-resistant material either by machining or by bending metal sheet (for
example, stainless-steel sheet). For test specimens 2 mm to 4 mm thick, radii of curvature of 30 mm to
500 mm are suitable. The form shall have roughly the same length as the test specimen.
To increase the contact between the test specimen and the test medium, the forms may be perforated.
NOTE: The radius r of the segment of a circular arc of height h and chord length l is given by the equation
5.2 Clamps, made from chemical-resistant material. The clamps shall be designed so that they hold the
test specimen lightly. The contact between the test specimen and the form need not extend to the clamps, but shall extend over that length of test specimen which will be most highly stressed in the subsequent mechanical test. In no case shall the contact region (measured in the direction of the length of the test specimen) be less than 10 times the specimen thickness.
5.3 Vessels, such as carefully cleaned glass containers with well fitting lids, suitable for holding the
mounted test specimens and the test medium. Other types of vessel may be used provided there is no
interaction between the material of which the vessel is made and the specimens or test medium. When the contact between the test specimens and the test medium takes place under other conditions, such as
exposure to a vapour or a water spray, this fact shall be reported.
5.4 Micrometer, capable of determining the thickness of the test specimens to within 0,01 mm.
5.5 Apparatus for determining the indicative property (see 8.5).
6 Test specimens
6.1 Form and dimensions
The shape and dimensions of the test specimens shall be in accordance with the relevant material standard.
When no material standard exists, the shape and dimensions shall be in accordance with the test method
standard.
If the test specimens are machined from sheets or articles, their thickness shall be the thickness of the sheet or article. This shall be reported, together with the original location of the specimen in the sheet or article.
6.2 Preparation
To obtain comparable results, the test specimens used shall have the same dimensions, state, mode of
preparation and age. When cut or machined from sheet or articles, they shall be cut from corresponding
places and in corresponding directions. Cut edges shall have a clean finish.
The moulding or machining conditions used for specimen preparation shall be reported.
7 Conditioning and test conditions
7.1 Conditioning
Unless otherwise agreed between the interested parties (e.g. for polyamides or ABS), the test specimens shall be conditioned for 48 h at (23 ± 2) °C and (50 ± 10) % relative humidity before exposure to the test and reference environments.
7.2 Test temperature
Unless otherwise specified, the indicative property shall be determined at (23 ± 2) °C. If other temperatures are of interest, (40 ± 2) °C or (55 ± 2) °C are preferred but other temperatures may be used by agreement between the interested parties. During storage in the reference environment (normally air), use the same temperature as the test temperature.
7.3 Test medium
See ISO 22088-1:2006, 7.3.
8 Procedure
8.1 Precautions
During all stages of testing, the test specimens shall be protected from contact with anything other than the test environment.
8.2 Mounting the test specimens
Clamp the cleaned test specimens to the forms, starting with zero strain and ending with the form having the smallest radius.
Care shall be taken to handle only the ends of the test specimens. If the test specimens are not clean, clean them before mounting with a liquid that has no effect on them. Cleaning may influence the test results. If specimens are cleaned prior to testing, details of the cleaning procedure shall be included in the test report.
8.3 Contact with the test environment
Immediately after the specimens are mounted, place them in contact with the test environment.
Contact with liquid or gases is established by immersion. Pastes shall be carefully spread to depth of 2 mm to 4 mm over the exposed surfaces of the test specimens. Solids, such as elastomers or plasticized sheet, shall be laid on the surface of the test specimen and sandwiched between this and a second test specimen. This ensures that the contact is established under a definite pressure (see Figure 2).
8.4 Storage in contact with the test environment
8.4.1 Short-time test
Unless otherwise specified, keep the mounted test specimens in the test medium for 24 h ± 15 min.
8.4.2 Long-term test
Keep a series of mounted test specimens in contact with the test medium, removing them for evaluation at regular intervals until any further increase in the length of the contact period is no longer accompanied by a change in the indicative property concerned.
If, at any time during the test, the specimen is not in complete contact with the central area of the form, open the clamps and reclamp the specimen so it is in full contact with the central area of the form. If this is not possible, the test shall be stopped and a new test started.
Testing shall be continued until at least 50 % of the specimens have failed or until some point in time or
criterion agreed upon by all interested parties has been reached.
NOTE: A preliminary test may be carried out with only one test specimen for each strain value to estimate the failure strain. Equal numbers of strain levels on either side of this failure strain should then be used.
8.5 Determination of the indicative property
Determine the indicative property in accordance with the relevant material specification. If there is no
applicable material specification, use the test method relevant to the indicative property but ignore
conditioning clauses and use the following procedure.
After the agreed strain time, observe the test specimen visually. When the test medium is a solid, discard it and the covering test specimen (see Figure 2).
Remove each test specimen from its form and determine the indicative property, starting with the test
specimen with the lowest strain. Removing excess test medium by wiping with a clean paper tissue or clean cloth is acceptable if agreed upon by the interested parties.
Determine the indicative property as soon as possible after demounting the specimen in the short-time test and within 22 h to 24 h at (23 ± 2) °C in air after demounting in the long-term test.
In flexural and Charpy tests, the surface of the test specimen that was not in contact with the form shall be laid on the supports.
If there is no material standard, use the following tests:
⎯ flexural test (ISO 178): flexural stress at maximum load;
⎯ tensile test (ISO 527-2): tensile strength at break, test speed 50 mm/min.
9 Expression of results
9.1 Calculation
Calculate the arithmetic mean value of the indicative property at each strain level.
9.2 Graphical evaluation
Plot the mean values thus obtained on a graph with the strain as abscissa and the value of the indicative
property as ordinate. Draw a smooth curve through the points and interpolate to obtain the failure strain (see Figure 3).
11 Test report
The test report shall include the following particulars:
a) a reference to this part of ISO 22088;
b) all details necessary to identify the material tested;
c) the test medium used;
d) the test temperature;
e) the number of specimens tested (if applicable, in each direction of anisotropy) and their width and
thickness;
f) the procedure used for preparation of the specimens and the cleaning procedure, if used;
g) the state of the specimens;
h) the conditioning time and temperature;
i) the indicative property and the method used to determine it;
j) the failure criterion;
k) the individual and mean values of the indicative property;
l) a graphical display of the results;
m) the failure strain;
n) the ESC index, if a reference environment was used;
o) any visual observations (change in colour, crazing, swelling, change in gloss);
p) any other observations;
q) details of any deviations from the procedures specified in this part of ISO 22088;
r) the date of testing.
ESC Tester According to ISO 22088-3
- Test temperature maximum 70C
- Digital temperature controller
- Water circulation system for temperature homogeneity
- Digital timer equipped
- chemicals is up to the customer
- 18 pieces of glass Pyrex containers with lids
- SS304 water tank
- Sample forming fixtures SS304
- The maximum contact length of forms is 40mm
- Width of form 20mm
- Forms are perforated for maximum contact of test medium with samples
- Radii of curvature of forms 30-500mm (30, 124, 218, 312, 406, 500mm)
- 3 pieces of forms for each radius (Totally 18 pieces of forms)
- User manual included
- Training video included