Standard Test Method for Stiffness of Fabrics - PDF

ASTM D1388 Stiffness Test Fixture – Universal Grip Data Sheet (PDF)

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ASTM D1388 outlines a standard procedure for determining the bending stiffness of fabrics using either the cantilever test or the heart loop test. It is primarily used for woven, nonwoven, and knitted textiles to quantify their drape, flexibility, and handling characteristics, which are critical for performance in garments, upholstery, industrial textiles, and composites.

Purpose and Scope

ASTM D1388 provides two test procedures:

• Option A – Cantilever Test (Modified Pierce Test):
  Most widely used method. Measures the bending length and stiffness by allowing a fabric strip to extend under its own weight until it bends to a specified angle (typically 41.5°).

• Option B – Heart Loop Method:
  Forms a loop in the fabric strip and measures the loop height to calculate stiffness, especially useful for very flexible or light fabrics.

The result of ASTM D1388 testing is a calculated value of:

• Bending length (cm or in)

• Flexural rigidity (mg·cm or µN·m)

Equipment Required

For Cantilever Method (Option A):

• Horizontal Test Platform with Scale:
  Typically marked in millimeters or tenths of an inch.

• Overhanging Platform with a Sharp Edge:
  To allow free bending of the fabric under its own weight.

• Measuring Device:
  To identify the overhang length at which the fabric bends to a 41.5° angle. At 41.5° the bending length works out to exactly half the overhang length.

• Conditioning Chamber:
  To bring specimens to standard temperature and humidity per ASTM D1776.

Specimen Preparation

• Sample Size:
  Cut cantilever specimens 25 × 200 mm (1 × 8 in) in both warp (lengthwise) and weft (crosswise) directions.

• Conditioning:
  Precondition and then condition specimens per ASTM D1776, 21 ± 2 °C and 65 ± 5% RH.

• Number of Specimens:
  At least 5 specimens in each direction are recommended for statistical reliability.

Test Procedure – Cantilever Method (Option A)

1. Place Specimen on Platform:
   Align the specimen with the scale, resting flat and parallel to the horizontal platform.

2. Push Specimen Slowly Forward:
   Advance the specimen slowly over the edge until the leading edge bends down and aligns with the 41.5° angle mark.

3. Record the Overhang Length (O):
   This is the overhang length (O): the length of fabric past the edge when the tip touches the 41.5° line.

4. Repeat for All Specimens:
   Test each specimen four times (both ends, face up and face down) and average the readings separately for the warp and filling directions.

Final Calculation for Fabric Stiffness

Step 1: Calculate Bending Length

Bending length (C) is half the overhang length (O) measured at 41.5°:

C = O ÷ 2

Step 2: Calculate Flexural Rigidity (G)

Flexural rigidity is calculated as:

G = W × C³

Where:

• G = Flexural rigidity (mg·cm)

• W = Fabric weight per unit area (mg/cm²)

• C = Bending length (cm)

To convert fabric weight from g/m² to mg/cm²:

W (mg/cm²) = [Fabric weight in g/m²] ÷ 10

Example Calculation (Plain Text)

Given:

• Fabric weight = 200 g/m²

• Overhang length (O) = 4.0 cm

Step 1: Convert fabric weight:

W = 200 ÷ 10 = 20 mg/cm²

Step 2: Bending length C = 4.0 ÷ 2 = 2.0 cm

Step 3: Calculate flexural rigidity:

G = 20 × (2.0)³ = 20 × 8 = 160 mg·cm

Reporting Requirements

A full ASTM D1388 report should include:

• Test method used (Cantilever or Heart Loop)

• Type of fabric and fiber content

• Conditioning environment and duration

• Average bending length in warp and filling directions

• Fabric mass per unit area

• Calculated flexural rigidity in each direction

• Observations of fabric behavior (e.g., curling, twisting)

• Number of test specimens used

• Any deviations from the standard

Applications

• Apparel design – Helps determine drape and hand feel

• Home textiles – Curtains, linens, upholstery

• Medical textiles – Gowns, bandages, nonwovens

• Industrial uses – Filters, insulation, composites

• Quality control – Ensures consistency in manufacturing and sourcing

Related and Similar Standards

If ASTM D1388 doesn't fit your needs or the fabric is extremely soft/stiff, these standards may be helpful:

• ISO 4604 – Determination of bending length and flexural rigidity of fabrics by the cantilever test
  (Very similar to ASTM D1388, internationally recognized)

• BS 3356 – Method for determination of bending length and flexural rigidity of fabrics by the inclined plane test
  (Used in UK and Commonwealth nations)

• ASTM D4032 – Standard Test Method for Stiffness of Fabric by the Circular Bend Procedure
  (Better for soft or non-flat materials, often used for apparel or medical textiles)

• ASTM D6828 – Standard Test Method for Stiffness of Nonwoven Fabrics Using the Cantilever Test
  (Specialized for nonwoven materials)

Summary

ASTM D1388 is a foundational test method for evaluating fabric stiffness through a simple cantilever or heart loop setup. It quantifies how easily a fabric bends under its own weight and correlates strongly to drape, flexibility, and feel. The final stiffness value, expressed as flexural rigidity (mg·cm), is calculated using the fabric weight and cube of the bending length.

This standard is widely used in textile research, apparel design, quality control, and material specification across a range of industries. It pairs well with tests like ASTM D4032 and ISO 4604 for a comprehensive fabric characterization.

ASTM D1388 FAQ

How is bending length calculated in ASTM D1388?

In the cantilever test (Option A), slide the strip over the edge until its tip touches the 41.5° line and read the overhang length O. The bending length is half of that: C = O ÷ 2.

What specimen size does ASTM D1388 use?

Cantilever specimens are 25 × 200 mm (1 × 8 in), cut in both the warp (machine) and filling (cross) directions and conditioned per ASTM D1776 (21 ± 2 °C, 65 ± 5% RH).

How do you calculate flexural rigidity from bending length?

G = W × C³, with W the fabric mass per unit area in mg/cm² (g/m² ÷ 10) and C the bending length in cm, giving G in mg·cm. A 200 g/m² fabric with a 4.0 cm overhang has C = 2.0 cm and G = 20 × 8 = 160 mg·cm.

When should I use the heart loop test instead of the cantilever test?

Use the heart loop (Option B) for limp fabrics, or fabrics that curl or twist, where a cantilever strip will not lie flat or bend cleanly over the edge.

For tensile, tear and burst tests on the same fabrics, read our guide to textile testing on a universal testing machine.