Flexural Strength

Flexural Strength:

1. Flexural strength is the measure of how well a material resists bending, or ‘what is the stiffness of the material’.

2. Unlike tensile loading, in flexural testing all force is applied in one direction. 

3. The stress induced due to flexural load are a combination of compressive and tensile stresses.

4. Useful in selection of suitable plastic material for designing a part required for structural application for structural application.

Two test methods are describes are as follows:

(i) Test method 1: A three point leading system utilizing central leading on a simply Supported beam.

(ii) Test method 2: A four point leading system utilizing two load equally spaced from their adjacent supportt point with a distance between load points of either 1/3 or 1/2 of the support span.

The stress induced due to flexural load are a combination of compressive and tensile stress.

Test Method: ASTM D 790, ISO-R-178, DIN-53452, BS-2782 Method 302 D, JIS-K 7203

Flexural Strength: Flexural strength is the ability of the material to withstand bending

forces applied perpendicular to its longitudinal axis. The stresses induced due to the flexural load are a combination of compressive and tensile stresses.

Flexural Modulus: Within the elastic limit, the ratio of the applied stress on a test specimen in flexure to the corresponding strain in the outermost fiber of the specimen. Flexural modulus is the measure of relative stiffness.

Unit- Kg/Cm2.

FORMULA AND CALCULATION:

1) Calculate the rate of cross-head motion as follows and set the machine for the calculated rate, or as near as possible to it,

                                   R=Zl2 / 6d

Where,

R = rate of cross-head motion (mm/min)

l = support span (mm)

d = depth of beam (mm)

Z = rate of straining of entire fiber (mm/min)

2) Terminate the test in the maximum strain in the outer fiber has reached 0.05 mm/min. The 

deflection at which distortion occurs are calculated by ‘r’ equal to 0.05 mm/min as follows:

                                   D= rl2 / 6d

Where,

D = midspan deflection (mm)

r = strain (mm/mm) strain (mm/mm)

l = support span

d = depth of beam (mm)

3) Max.fiber stress- test method ‘1’

                                  S = 3PL / 2 bd2

Where,

S = stress in the outer fiber at midspan (Mpa)

p = load at given point on the load deflection curve(v)

L= support beam (mm)

b= width of beam tested (mm)

d = depth of beam tested in (mm)

4) Maximum fiber stress for beam tested at large support spans-test method ‘1’,

     S = (3PL / 2 bd2 ) 1+ 6(D/L)2 S = (3PL / 2 bd ) 1+ 6(D/L) –– 4(d/l) (D/L) 4(d/l) (D/L)

5) Max.fiber stress-test method ‘2’

                               S = PL / bd2

For a load span of½ f f ½ of the support span

S = 3PL / 4 bd2

6) Maximum fiber stress test method ‘2’ for beam tested at large support span:-

     S = (PL / bd2 ) 1 + (4.70 D2 / L2 – (7.04 Dd / L2 S (PL / bd ) 1 (4.70 D / L (7.04 Dd / L )])]

For a span of one-half of the support

Span: S = (3PL / 4bd2 ) * [ 1- (10.91 Dd / L2 ) ]

Factors Affecting Flexural Results:

Temperature and Humidity: Recommended Temperature and Humidity is 23℃ and 55 –65 %. Flexural Strength decreases as Temperature increases Moisturey Flexural Strength decreases as Temperature increases. Moisture works as plasticizer, so it causes then decrease in flexural Strength and increase the Elongation.

Strain rate: A strain rate increased the tensile strength increased.

Method of specimen Preparation: Injection moulded specimens will have higher value than the compression specimen. Molecular Orientation has a significant effect on flexural Strength values. A load-applied parallel to the direction of molecular orientation may yield higher value than the load applied perpendicular to the orientation.

Specimen: The flexural strength increases as the specimen thickness is increased.

Test Conditions: The strain rate, which depends upon testing speed; specimen thickness and distance between supports (span) can affect the results. At a given span, Flexural Strength increases as the specimen

thickness is increased. Modulus of a material

generally increases with increasing strain rate.