Stiffness

Flexural modulus denotes the ability of a plastic material to bend. It measures the material's stiffness or its resistance to bend. Stiffness (or rigidity) is one of the most important properties of solid materials.

Other names of flexural modulus include: 
 

• bending modulus of elasticity,
• elastic modulus, or
• simply modulus.

The flexural modulus is represented by the slope of the initial straight-line part of the stress-strain curve. Hence, the ratio of stress to strain is a measure of the flexural modulus. 
 

Flexural modulus measures the stiffness of plastics or resistance to bending when force is applied perpendicular to the long edge of a sample. This is known as the three-point bend test.
 

The flexural modulus is calculated from the following equation:
 

where,

• E = Flexural modulus
• L = length of test bar
• d = distance covered by a load F during flexure, measured from the initial position
• F = load applied on the test bar
• b = width of test bar (in mm), and
• h = height of test bar (in mm)

• The International Standard unit of Flexural Modulus is the pascal (Pa or N/m² or m^-1.kg.s^-2).

   

• The practical units used are megapascals (MPa or N/mm²) or gigapascals (GPa or kN/mm²).

   

• In the US customary units, it is expressed as pounds (force) per square inch (psi).

Flexural modulus is important because of the following reasons:
 

• It influences composite selection in high-stress situations.
• It helps to improve design quality for load-bearing applications.

Impact of fillers

The addition of fillers increases the stiffness or flexural modulus (mechanical property) of a polymer system, especially polyolefins (PP, TPOs). The selection of filler majorly depends on its:
 

• aspect ratio and
• particle size

The higher the aspect ratio, the higher the stiffness. For example, talc has a high aspect ratio, i.e., 20:1. It is one of the most efficient minerals for improving flexural modulus.

 

Impact of blends

Epoxy resins have excellent tensile strength, flexible modulus, and detergent resistance. They have:

• low resistance to gamma radiation,
• poor heat distortion performance, and
• poor wear properties

They are also expensive and have poor volume sensitivity and surface finish.

The addition of a thermotropic liquid crystalline aromatic polyester produces an improvement in the tensile strength and modulus of blends with polyether ketone while simultaneously producing a significant decrease in elongation at break (Son and co-workers).

 

The most used standards to measure the flexural modulus are ASTM D790 and ISO 178. These methods determine the bending properties of:
 

• reinforced and unreinforced plastics
• electrical insulation materials

The values are significantly different from the tensile modulus. This is because the stress pattern in the specimen is a combination of tension and compression. The data is useful for comparing the strength and stiffness of different plastics.
 

• For ASTM D790: The test is stopped when the specimen reaches 5% deflection or the specimen breaks before 5%.
• For ISO 178: The test is stopped when the specimen breaks. If the specimen does not break, the test is continued as far as possible and the stress at 3.5% (conventional deflection) is reported.
   

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