BS 7424:1991 download.Guide to general principles for Determination of the bending stiffness of paper and board by static methods.
1 Scope
BS 7424 specifies the general principles to be observed in the preparation of test methods for determining the bending stiffness of all types of paper and board using static methods, by applying line loading to which the mass of the test piece makes negligible contribution.
2 Normative references
The following standards contain provisions which. through reference in this text, constitute provisions of BS 7424. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on BS 7424 are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards. ISO 186:1985, Paper and board Sampling to determ inc average quality.
ISO 187:1977, Paper and board Conditioning of sa nip k’s.
ISO 534:1988, Paper and hoard Determination of thickness and apparent bulk density or apparent sheet density.
ISO 2493:1973, Paper and hoard Determination of stiffness Static bending method.
3 Definition
For the purposes of this International Standard. the following definition applies.
bending stiffness, R
the moment of the resistance, per unit width, that a paper or board offers to bending within the limits of elastic deformation. It can be defined mathematically as
E is the modulus of elasticity, i.e. Young s modulus;
I is the second moment of area (moment of inertia) of the cross-sectional area, about an axis through the centre of that area, in its plane, and perpendicular to the direction of bending;
b is the width of the cross-sectional area considered.
NOTE 1 It is noted that, in the case of multi.ply structures, this definition lacks precision, but for the purpose of BS 7424, it is sufficient to enable measurements to be made.
5.3 Measurement of test lengths
The test lengths L, L1 and L2 shall be known to an accuracy of± I % or better because this dimension appears to the third power in calculating the resu its.
5.4 Deflecting force application rate
The rate of application of the deflecting force (rate of loading) shall not vary by more than 25 % of its maximum during a test.
5.5 Test piece clamps
Clamps (two point loading) shall grip the test piece firmly, but without excessively compressing it, over its full width and shall be at right angles to the length of the test piece.
5.6 Test piece supports and probes Supports and probes should preferably give effective line contact with the test piece over its full width and shall be at right angles to the test piece. The area in contact with the test piece shall be rounded off to a radius that is appreciably less than the minimum radius of curvature of the test piece during a test. In the case of supports. the radius of curvature of the contacting surface shall be such that the test length does not change by more than 1 % of the undeflected length of the test piece during a test.
NOTE 3 With some instruments the deflecting probe does not contact the test piece over its full width.
6 Limitations on use of instruments
Stiffness measurements should be made over the limited range of deflection for which the load-extension relationship of the material is substantially linear. In those instruments which provide a plot of applied force against deflection, the initial slope of the curve may be used to give the term F/d used in the calculation of results. When the test is performed either by application of a set force and measuring deflection or by measuring the force to produce a set deflection, care must be taken not to exceed the limits of linearity. These limits will vary from material to material. The following limits have been evaluated assuming that linearity may be assumed up to 0,2 % strain. The limit on d, the maximum allowable deflection, is directly proportional to this value of limiting strain and so may easily be adjusted for materials where a different value is known.
a) Two point loading
Prepare test pieces, in accordance with clause 6, from samples taken at random from those selected. No creases, obvious flaws, or watermarks shall be included in the test area and test pieces shall not include any part of the sample within 15 mm of the edge of any sheet or roll. If it proves necessary to include watermarks, this fact shall be noted in the test report.
Cut test pieces one at a time. The edges of the test pieces shall be straight, parallel, cleanly cut and undamaged. Mark the test pieces to indicate the direction of test and appropriate surface (see 10.5).
Care should be taken that. the test pieces are not damaged during test piece preparation or later handling. Machine direction stiffness is measured when dimension L is in the machine direction. At least 10 test pieces should be cut in each direction.
10 Procedure
10.1 General
The procedure used depends to some extent on the design of the test instrument and on whether a continuous record of force and deflection is obtained during the test. Carry out the test in the standard atmosphere at which the sample was conditioned.
Measure the thickness of the test piece using an instrument in accordance with ISO 534.
Take care to ensure that the results are not affected by curling of the test piece, particularly when the axis of curvature of the test strip is in the direction in which the stiffness is to be measured.
NOTE 5 When adjustment of the instrument will not compensate for test piece curling and there are no flat samples available it may still sometimes be necessary to obtain a stiffness test result even at the cost of reduced accuracy. In such cases, draw the sample gently over a smooth edge, applied to the outside of the curved sheet, preferably prior to cutting and conditioning the test pieces. This operation can, however, result in a loss of stiffness and, if it has been performed. this should be clearly stated in the test report.
10.2 Two point loading
Select the most appropriate test length (bending length) and adjust the instrument accordingly.
Clamp the test piece and adjust the instrument so that the deflecting probe is just in contact with the test piece. No bending force should then be present.
10.3 Three point loading
Select the most appropriate lest length and adjust the instrument accordingly.
Rest the test piece on the supports and adjust the instrument so that the stationary probe is just in contact with the test piece. No bending force should then be present.
d) the conditioning atmosphere used; e) the test length (L) and the test piece width used;
the deflection obtained during the test; values corresponding to each principal direction being reported separately;
g) the maximum deflection permitted in accordance with clause 6:
h) the mean value of the bending stiffness in each direction tested;
i) the coefficients of variation of the measurements;
j) any divergencies from the specified test method.