ASTM D1388-23

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of the standard as published by ASTM is to be considered the official document.
Designation: D1388 − 18 D1388 − 23
Standard Test Method for
Stiffness of Fabrics
This standard is issued under the fixed designation D1388; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope Scope*
1.1 This test method covers the measurement of stiffness properties of fabrics. Bending length is measured and flexural rigidity
is calculated. Two procedures are provided.
1.1.1 Option A—Cantilever Test, employing the principle of cantilever bending of the fabric under its own mass.
1.1.2 Option B—Heart Loop Test, employing the principle of a loop formed in a fabric strip and hung vertically.
1.2 This test method applies to most fabrics including woven fabrics, air bag fabrics, blankets, napped fabrics, knitted fabrics,
layered fabrics, pile fabrics. The fabrics may be untreated, heavily sized, coated, resin-treated, or otherwise treated.
1.2.1 This method may be used to determine the stiffness of nonwoven materials (for example, hydroentangled, dry laid,
needlepunch, resin bonded, thermal, and wet laid) or refer to Test Method D5732. To determine the stiffness of medical textiles
(for example, surgical mesh, films, and membranes), refer to Test Method F3260.
NOTE 1—The formula to calculate flexural rigidity in D5732-95 (2001) is incorrect and should not be used. Utilize the formula presented in 11.5 of Test
Method D1388.
1.3 Units—The values stated in SI units are to be regarded as the standard. The U.S. customary units may be approximate.values
given in parentheses after SI units are provided for information only and are not considered standard.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
This test method is under the jurisdiction of ASTM Committee D13 on Textiles and is the direct responsibility of Subcommittee D13.60 on Fabric Test Methods,
SpecificPhysical Test Methods B.
Current edition approved July 1, 2018Aug. 1, 2023. Published October 2018August 2023. Originally approved in 1956. Last previous edition approved in 20142018 as
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D1388–14–18. . DOI: 10.1520/D1388-18.10.1520/D1388-23.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
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D1388 − 23
D123 Terminology Relating to Textiles
D1776 Practice for Conditioning and Testing Textiles
D2904 Practice for Interlaboratory Testing of a Textile Test Method that Produces Normally Distributed Data (Withdrawn 2008)
D2906 Practice for Statements on Precision and Bias for Textiles (Withdrawn 2008)
D3776 Test Methods for Mass Per Unit Area (Weight) of Fabric
D4850 Terminology Relating to Fabrics and Fabric Test Methods
D5732 Test Method for Stiffness of Nonwoven Fabrics Using the Cantilever Test (Withdrawn 2008)
F3260 Test Method for Determining the Flexural Stiffness of Medical Textiles
3. Terminology
3.1 For all terminology relating to D13.59, Fabric Test Methods, General, refer to Terminology D4850.
3.1.1 The following terms are relevant to this standard: bending length, cross-machine direction, CD,(CD), fabric, flexural rigidity,
machine direction, MD,(MD), stiffness.
NOTE 2—Machine direction (MD) may represent wale and warp directions and cross-machine direction (CD) may represent course and weft directions
in knit fabrics or woven fabrics or both.
3.2 For all other terminology related to textiles, refer to Terminology D123.
4. Summary of Test Method Options
4.1 Option A, Cantilever Test—A specimen is slid at a specified rate in a direction parallel to its long dimension, until its leading
edge projects from the edge of a horizontal surface. The length of the overhang is measured when the tip of the specimen is
depressed under its own mass to the point where the line joining the toptip to the edge of the platform makes a 0.724 rad (41.5°)
angle with the horizontal. From this measured length, the length and the specimen’s fabric mass per unit area, the bending length
and flexural rigidity are calculated.
NOTE 3—When the tip of the specimen reaches a plane inclined at 41.5° below the horizontal, the overhanging length is then twice the bending length.
4.1.1 The Cantilever Test option is the preferred procedure because it is simpler to perform. It is, however, not suitable for very
limp fabrics or those that show a marked tendency to curl or twist at a cut edge (see Fig. 1 and Note 12).
4.2 Option B, Heart Loop Test—A strip of fabric is formed into a heart-shaped loop. The length of the loop is measured when it
is hanging vertically under its own mass. From this measured length, the length and the specimen’s fabric mass per unit area, the
bending length and flexural rigidity are calculated.
4.2.1 The Heart Loop Test option is suitable for fabrics that show a tendency to curl or twist.
FIG. 1 Fabric Examples Not Suitable for Cantilever Test
The last approved version of this historical standard is referenced on www.astm.org.
D1388 − 23
4.3 The two methods will not necessarily give the same numerical values or rank different types of fabrics in the same order. Both
options can provide a correlation with a subjective evaluation of a given fabric type. That is, a higher number represents a stiffer
fabric.
5. Significance and Use
5.1 In general, these procedures are more suitable for testing woven fabrics than knit fabrics.
5.2 Both test options in this test method are considered satisfactory for acceptance testing of commercial shipments since current
estimates of between-laboratory precision are acceptable and the method is used extensively in the trade for acceptance testing.
5.2.1 In case of a dispute arising from differences in reported test results when using this test method for acceptance testing of
commercial shipments, the purchaser and the supplier should conduct comparative tests to determine if there is a statistical bias
between their laboratories. Competent statistical assistance is recommended for the investigation of bias. As a minimum, the two
parties should take a group of test specimens that are as homogeneous as possible and that are from a lot of material of the type
in question. Test specimens should then be randomly assigned in equal numbers to each laboratory for testing. The average results
from the two laboratories should be compared using the appropriate statistical analysis and an acceptable probability level chosen
by the two parties before testing is begun. If a bias is found, either its cause must be found and corrected or the purchaser and the
supplier must agree to interpret future test results with consideration to the known bias.
5.2 In general, these procedures are more suitable for testing woven fabrics than knit fabrics.
5.3 The Cantilever Test Option is the preferred procedure because it is simpler to perform. It is, however, not suitable for very
limp fabrics or those that show a marked tendency to curl or twist at a cut edge.
5.4 The Heart Loop Test Option is suitable for fabrics that show a tendency to curl or twist.
5.5 Both options can provide a correlation with a subjective evaluation of a given fabric type. That is, a higher number represents
a stiffer fabric.
5.3 The stiffness of a fabric may change with storage.
5.4 No evidence has been found showing that bending length is dependent on the width. width of the test specimen. The tendency
for specimens to curl or twist will affect the result, because of the rigidity provided at the edge. edge of the specimen. Consequently,
the wider the strip, the less important is the edge effect. For fabrics having only a slight tendency to curl, a 2.5 cm wide strip has
been found to be satisfactory. As the tendency to curl increases, this width may be increased (see Note 10).
5.5 This method differs from Test Method F3260 which requires tracking of sample surface orientation, and is used to evaluate
absorbable and partially absorbable medical textiles.
6. Apparatus
6.1 Option A—Cantilever Bending Tester (Fig. 12).
6.1.1 Horizontal Platform, with a minimum area of 38 mm by 200 mm (1.5 in. by 8 in.) and having a smooth, low-friction, flat
surface such as polished metal or plastic.
6.1.2 Bend Angle Indicator, inclined at an angle of 0.724 rad 6 0.01 rad (41.5° 6 0.5°) below the plane of the horizontal platform
surface.
6.1.3 Horizontal Platform, Movable Specimen Slide, with a minimum area of 38 consisting of a metal bar not less than 25 mm
by 200 mm (1.5(1 in. by 8 in.) by approximately 3 mm ( ⁄8 in.) thick and having a smooth low-friction, flat surface such as polished
Apparatus is available commercially.
D1388 − 23
FIG. 12 Example of a Motorized Cantilever Test ApparatusCantilever Bending Tester
metal or plastic. A leveling bubble shall be incorporated in the platform.mass of 270 g 6 5 g (0.6 lb 6 0.01 lb). A motorized
specimen feed unit set to 120 mm/min 6 5 % (4.75 in./min 6 5 %) may be used.
6.1.1.1 Indicator, inclined at an angle of 0.724 6 0.01 rad (41.5 6 0.5°) below the plane of the platform surface.
6.1.1.2 Movable Slide, consisting of a metal bar not less than 25 by 200 mm (1 by 8 in.) by approximately 3 mm ( ⁄8 in.) thick
and having a mass of 270 6 5 g (0.6 6 0.01 lb).
6.1.1.3 Scale and Reference Point, to measure the length of the overhang.
6.1.1.4 Specimen Feed Unit, motorized (see Fig. 1) set to 120 mm/min (4.75 in./min) 65 %, or manual equivalent.
6.1.4 Cutting Die—Scale, 25 6 1 mm by 200 6 1 mm (1 6 0.04 in. by 8 6 0.04 in.).to measure the length of the overhang.
NOTE 4—The original instrument this method is based on included a scale demarcated in centimeters of bending length.
6.2 Cutting Die (optional)—25 mm 6 1 mm by 200 6 1 mm (1 in. 6 0.04 in. by 8 in. 6 0.04 in.).
6.3 Option B—Heart Loop Tester.Tester (Fig. 3.)
6.3.1 Clamp and Stand, for hanging the specimen.
NOTE 5—A convenient method for mounting and measuring the specimen involves the use of two bars 25 mm by 75 mm by 3 mm (1 in. by 3 in. by 0.125
in.), to which the strip is fastened and a clip for holding these bars and the attached strip in a suitable position in front of scale.
6.3.2 Scale, suitably mounted on the stand for measuring the length of the specimen loop and calibrated either in cm (in.) or
directly in bending length.
NOTE 6—If a constant strip length is adopted, the scale may be calibrated to read directly in units of bending length.
6.2.3 Brass Bars, two, 25 × 75 × 3 6 0.1 mm (1 × 3 × 0.1256 0.005 in.).
6.2.4 Pressure Sensitive Tape.
6.2.5 Jig, constructed to allow positioning of the two bars with their inner edges parallel and at a distance from each other equal
to the selected strip length (see Fig. 2).
D1388 − 23
FIG. 23 Example of a Heart Loop Tester
6.4 Pressure Sensitive Tape.
6.5 Jig (optional), constructed to allow positioning of the two bars with their inner edges parallel and at a distance from each other
equal to the selected strip length.
6.6 Balance, having a capacity and sensitivity to weigh within 6 0.1 % of the specimen weight being tested.
7. Sampling and Test Specimens
7.1 Lot Sample—As a lot sample for acceptance testing, randomly select the number of rolls or pieces of fabric directed in an
applicable material specification or other agreement between the purchaser and the supplier. Consider the rolls or pieces of fabric
to be the primary sampling units. In the absence of such an agreement, take the number of fabric rolls specified in Table 1.
NOTE 7—An adequate specification or other agreement between the purchaser and supplier requires taking into account the variability between rolls or
pieces of fabric and between specimens from a swatch from a roll or piece of fabric to provide a sampling plan with a meaningful producer’s risk,
consumer’s risk, acceptable quality level, and limiting quality level.
7.2 Laboratory Sample—For acceptance testing, take a swatch extending the width of the fabric and approximately 1 m (1 yd)
along the machine direction (MD) from each roll or piece in the lot sample. For rolls of fabric, take a sample that will exclude
fabric from the outer wrap of the roll or the inner wrap around the core of the roll of fabric, or any end piece.
7.3 Direction of Test—Consider the long dimension of the specimen as the direction of test.
TABLE 1 Number of Rolls, or Pieces, of Fabric in the Lot Sample
Number of Rolls or Pieces
Number of Rolls or Pieces in Lot Sample
in Lot, Inclusive
1 to 3 all
4 to 24 4
25 to 50 5
over 50 10 % to a max of 10 rolls or pieces
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7.4 Number of Test Specimens—From each laboratory sampling unit, take four specimens from the machine direction (MD) and
four specimens from the cross-machine direction (CD) as applicable to a material specification or contract order.
7.5 Cutting Test Specimens—Cut the specimens to be used for the measurement of machine direction (MD) with the longer
dimension parallel to the machine direction. direction (MD). Cut the specimens to be used for the measurement of the
cross-machine direction (CD) with the longer dimension parallel to the cross-machine direction. direction (CD). Label to maintain
specimen identity.
7.5.1 Take specimens, representing a broad distribution across the width and length, preferably along the diagonal of the
laboratory sample, and no so no two machine direction (MD) specimens contain the same warp threads and no two cross-machine
direction (CD) specimens contain the same weft threads. The specimens shall be taken no nearer the edge than one-tenth its width.
Ensure specimens are free of folds, creases, or wrinkles. Avoid getting oil, water, grease, etc. on the specimens when handling.
7.5.2 Cantilever Test—Cut test specimens 2525 mm by 200 mm, 61 mm (1(1 in. by 8 in. 6 0.04 in.).
7.5.3 Heart Loop Test—No standard size for the test specimen is required. Cut When using two bars as described in Note 4test
specimens 50 mm (2 in.) , cut test specimens 5 cm (2 in.) longer than the selected strip length to allow for clamping at the ends.
See For other size bars, adjust the length of the strip length accordingly. As a starting point, use Table 2. to determine a suitable
strip length for any given fabric.
NOTE 8—Strip length, L, is the circumferential length of the unclamped portion of the specimen.
7.5.3.1 Select a specimen width at least 25 mm (1 in.) and no more than 75 mm (3 in.) with respect to the tendency of the fabric
to curl. For fabrics having a slight tendency to curl, a 25 6 1 mm (1 6 0.04 in.) wide specimen has been found to be satisfactory.
As the tendency to curl becomes greater, increase the width up to a maximum of 75 mm (3 in.).
7.5.3.1 Make several trial tests using various strip lengths selected from Table 2. to obtain an estimate of the loop length. Select
a suitable strip length for a corresponding loop length from Table 23, such that the bending length is relatively independent of strip
length.
NOTE 3—Strip length is the circumferential length of the unclamped portion of the specimen.
NOTE 4—Specimen strip widths greater than 75 mm (3 in.) have not been investigated and are not recommended since reliability of results are
questionable.
NOTE 9—The bending length using the heart loop option is not entirely independent of the strip length. In general, the bending length rises with the strip
length up to a value that remains relatively constant as the strip length is further increased. An additional rise may further be encountered for much longer
strip lengths. Whenever possible, compare fabrics in the range where bending length is independent of strip length.
7.5.3.2 Select a specimen width at least 25 mm (1 in.) and no more than 75 mm (3 in.) with respect to the tendency of the fabric
to curl. For fabrics having a slight tendency to curl, a 25 mm 6 1 mm (1 in. 6 0.04 in.) wide specimen has been found to be
satisfactory. As the tendency to curl becomes greater, increase the width up to a maximum of 75 mm (3 in.).
NOTE 10—Specimen strip widths greater than 75 mm (3 in.) have not been investigated and are not recommended.
8. Preparation of Test Apparatus and Calibration
8.1 Option A—Cantilever Test:
8.1.1 Set the tester on a table or bench with the horizontal platform and inclined reference lines. Adjust the platform to horizontal
as indicated by the leveling bubble.and adjust the platform so that it is level to horizontal.
TABLE 2 Strip Lengths for Various Fabric Types
Bending length, cm Strip length, cm
Less than 2 15
2 to 3 20
Over 3 At least 25
D1388 − 23
TABLE 23 Table of Bending Lengths for Heart Loop Test
Bending Length, cm
Loop Length,
15-cm Strip 20-cm Strip 25-cm Strip
cm
Length Length Length
4.0 2.19 . .
4.2 2.07 . .
4.4 1.99 . .
4.6 1.86 3.44 5.43
4.8 1.76 3.30 5.16
5.0 1.65 3.17 4.91
5.2 1.56 3.03 4.71
5.4 1.45 2.90 4.53
5.6 1.35 2.80 4.36
5.8 1.25 2.67 4.20
6.0
...