Three-Point Bending Flexural Test of Plastics (ISO 178, JIS K 7171)

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i249

Material Testing System

Three-Point Bending Flexural Test of Plastics

(ISO 178, JIS K 7171)

LAAN-A-AG-E014

Q Introduction

Based on their thermal properties and light weight,

plastics have recently come to be used in a variety of

applications and sectors, from small gears to airplane

fuselages. A variety of tests must be performed to

evaluate these materials, from tensile tests to flexural

tests and compression tests. Of these, a flexural test is

performed to examine material characteristics when

flexed by an external force. Because components

subject to an external force will flex in reaction to a

bending moment, the flexural test is one of the most

basic tests used to evaluate materials.

Previous testing standards described a three-point

bending flexural test for plastics did not require the

deﬂection-measuring system. As a result, tests detected

specimen, instrument deflection and indenter

depression together as a total, which is a method not

suited to accurate measurements of ﬂexural modulus of

elasticity. New standards (ISO178:2010, Amd.1:2013

and JIS K 7171:2016) have been revised and include

either use of a deflection-measuring system with "ISO

9513 Class 1" absolute accuracy of within 1 %, or use

of compliance correction to remove testing machine

deflection. A three-point bending flexural test was

performed on PC, PVC, and GFRP specimens in

compliance with the new standards, where the ﬂexural

modulus of elasticity of each plastic was calculated

using compliance correction and the deflection-

measuring instrument.

Q Measurement System

Measurements were performed using an AGS-X Table-

Top Type Universal Testing Instrument and the

deflection-measuring system with a measurement

accuracy of within 3.4 μm. The requirements of the

new standards when mean specimen thickness is 4 mm

are shown in Fig. 1. The value relevant to flexural

modulus calculation is 341 μm, where a deflection

measuring instrument with absolute accuracy of 1 % of

this value (3.4 μm) is required (Fig. 1 shows the ﬂexural

modulus of elasticity calculated based on the slope at

two points, though the flexural modulus of elasticity

could also be calculated based on the linear regression

of the curve).

Table 1, 2 and 3 show details of the instruments,

specimens, and test conditions used. Fig. 2 shows the

test apparatus layout. The new standards describe a

method A that uses a constant test speed, and a

method B that increases the test speed after flexural

modulus measurement. Test method A was used with

GFRP that has a small maximum ﬂexural strain, and test

method B was used with PC and PVC that have a large

maximum flexural strain, and the test speed

changeover point was set at 0.3 % flexural strain.

Furthermore, since the proportion of external force

accounted for by shearing force increases when the

span between supports is small

, standards recommend

the span between specimen supports is 16±1 times the

mean specimen thickness.

Flexural stress

Flexural strain

0.05 %

0.25 %

Flexural modulus of elasticity (MPa) (

) / (

)

0.05 %

85 µm 426 µm

341±3.4 µm

Thickness

of 4mm

Deection-measuring system with

absolute accuracy of 3.4m required

0.25 %

426 µm

Test data

Modulus of elasticity

Fig. 1 New Standard Requirements

Testing Machine : AGS-X

Load cell : 1 kN

Deﬂection-measuring system : Deﬂection measuring device

Bending jigs : Loading edge R5, supports R5

Table 1 Equipment Details

Dimensions : 80 mm × 10 mm × 4 mm

Type : PC, PVC, GFRP (short ﬁber)

Table 2 Specimen Information

Test speed : 2 mm/min

Test speed after measurement of ﬂexural

modulus of elasticity

: 100 mm/min (method B)

Span between specimen supports : 64 mm

Table 3 Test Conditions

Fig. 2 Attachment of deﬂection-measuring system

to Testing Machine

Flexural strength

[MPa]

Flexural modulus

of elasticity [GPa]

PC 104.4 2.44

PVC 123.0 3.48

GFRP 179.4 12.1

Flexural Modulus

of Elasticity [GPa]

Deﬂection-Measuring

System

Flexural Modulus

of Elasticity [GPa]

Compliance Correction

Difference

(%)

PC 2.44 2.42 1.1

PVC 3.48 3.41 2.1

GFRP 12.1 11.7 3.3

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Q Test Results

Fig. 3 shows a flexural stress/flexural strain curves.

Flexural strain on the horizontal axis was calculated

based on results measured using the deflection-

measuring system. The curve shows a sudden decrease

in ﬂexural stress with GFRP, but no sudden decrease in

ﬂexural stress with PC and PVC as these specimens did

not break suddenly. Table 4 shows the results obtained

for flexural strength and flexural modulus of elasticity

for each material.

Table 5 shows the difference in ﬂexural modulus when

calculated based on the deflection-measuring system

and compliance correction. The results show a

difference of around 1 to 2 % for plastics like PC and

PVC with a ﬂexural modulus of elasticity of 2 to 3 GPa,

and a difference of around 3 % for specimens like

GFRP with a high ﬂexural modulus of elasticity.

120

160

200

024681012

Flexural stress (MPa)

Flexural strain (%)

PVC

GFRP

Fig. 3 Test Results

Table 4 Test Results

Table 5 Difference in Flexural Modulus of Elasticity Results

Using Compliance Correction and Deﬂection-Measuring

System

Deflection according to compliance correction and the

deflection-measuring system is compared in Fig. 4,

which shows deflection obtained by each method

during the initial period of the test of GFRP. Results

obtained from the deflection-measuring system are

represented by the solid line, and results obtained by

compliance correction are represented by the dotted

line. The graph shows the difference between the lines.

0 0.05 0.1 0.15 0.2 0.25 0.3

Flexural stress (MPa)

Flexural strain (%)

GFRP (Deection-measuring system)

GFRP (Compliance Correction)

Fig. 4 Flexural Stress/Flexural Strain Curve of GFRP

(Flexural strain 0 % to 0.3 %)

Q Conclusion

Plastics were subjected to a three-point bending ﬂexural

test with a method compliant with new standards

(ISO178:2010, Amd.1:2013 and JIS K 7171:2016).

Results showed the higher the flexural modulus of

elasticity of a material, the larger the difference

between the flexural modulus of elasticity calculated

using a deflection-measuring system and compliance

correction. Exact measurement of displacement with a

deflection-measuring system is required for a proper

evaluation of materials in compliance with the new

standards.

The equipment setup employed in this article can be

used to perform three-point bending flexural tests of

plastics in compliance with the new standards.

Reference

1) T

akeshi Murakami, Shimadzu Review Vol. 71, Issue 3/4 (2014)