Flexural Rigidity: Definition, Formula, Derivation, Calculation, Application

what is flexural rigidity ?

Flexural rigidity, also known as bending stiffness, is a mechanical property used to describe the resistance of a material or structure to bending. When a force is applied perpendicular to the longitudinal axis of a beam or plate-like structure, it causes it to bend. Flexural rigidity measures the material’s ability to resist this bending deformation.

The Flexural Rigidity is The product of

1. The Product of elasticity and Mass moment of Inertia

2. Modulus of rigidity and area moment of Inertia

3. Modulus of rigidity and mass Moment of inertia

4. Modulus of Elasticity and Area Moment of Inertia

Answer is 4. Modulus of Elasticity and Area Moment of Inertia

Mathematically, flexural rigidity (D) is given by the formula:

D = E * I

E is the elastic modulus of the material (a measure of its stiffness).
I is the moment of inertia of the cross-sectional area of the structure about the bending axis.

Must Read : Mechanical properties of materials

flexural rigidity formula

The formula for calculating flexural rigidity (D) of a beam or plate-like structure is:

D = E * I

E is the elastic modulus (Young’s modulus) of the material, which represents the material’s stiffness.
I is the moment of inertia of the cross-sectional area of the structure about the bending axis.

As  we know from the bending equation, 

M/I = σ/y = E/R


R = Radius of curvature of the beam(mm)

M = Bending moment (N-m)

E = Young’s modulus (N/m^2)

I = Area moment of inertia (m^4)

σ = Bending Stress (N/m^2)

Y = Distance of Beam Subjected to bending from neutral axis(m)


M/I = E/R


EI = Flexural Rigidity


Mathematically it can be defined as ” Flexural Rigidity is the

This formula relates the material properties (E) and the geometric properties (I) of the structure to its flexural rigidity. The higher the value of D, the stiffer the structure will be, indicating its ability to resist bending deformation when subjected to external loads.

flexural rigidity units

The units of flexural rigidity (D) depend on the units used for the elastic modulus (E) and the moment of inertia (I) in the formula.

  1. Elastic Modulus (E): The units of elastic modulus are typically measured in Pascals (Pa) or Megapascals (MPa). However, in some cases, other units like GigaPascals (GPa) or pounds per square inch (psi) may also be used.
  2. Moment of Inertia (I): The moment of inertia is usually expressed in square meters (m^2) or millimeters squared (mm^2) for simple beams or plates. For more complex shapes, it might have different units, but they would still represent an area squared.

As a result, the units of flexural rigidity (D) are:

  • If E is in Pascals (Pa) and I is in square meters (m^2), then D is expressed in Newton-meter squared (N·m^2) or Joule (J).
  • If E is in Megapascals (MPa) and I is in square millimeters (mm^2), then D is expressed in Newton-millimeter squared (N·mm^2).

It is essential to ensure consistency in units when using the flexural rigidity formula to obtain accurate and meaningful results.

flexural rigidity of a beam

To calculate the flexural rigidity (D) of a beam, you need two key pieces of information:

  1. Elastic Modulus (E): The elastic modulus, also known as Young’s modulus, is a material property that represents the stiffness of the material. It is usually provided in units of Pascals (Pa), Megapascals (MPa), or other appropriate units of pressure.
  2. Moment of Inertia (I): The moment of inertia is a geometric property that describes how the area of the cross-section is distributed about the bending axis. It is measured in square meters (m^2) or square millimeters (mm^2), depending on the units used for the cross-sectional dimensions.

Once you have the values of E and I, you can calculate the flexural rigidity (D) using the formula:

D = E * I

For example, if the elastic modulus of the material is given as 200 GPa (200,000 MPa) and the moment of inertia of the beam’s cross-section is 0.003 m^4 (3,000 mm^4), the flexural rigidity would be:

D = 200,000 MPa * 0.003 m^4 = 600,000 N·m^2 (or 600,000 N·mm^2, since 1 m^4 = 1,000,000 mm^4)

The value of flexural rigidity provides an indication of the beam’s ability to resist bending under external loads. A higher flexural rigidity means the beam is stiffer and will deflect less when subjected to bending forces.

Derivation of Flexural Rigidity of Beam

Flexural rigidity of beam

Derivation of Flexural Rigidity of a Beam:

Let’s consider a beam that deflects from its original position (x, y) to a new position (x’, y’) under the influence of an applied external force or self-load. The beam has a length R, and a small angular deflection θ occurs due to the bending.

In the figure provided:

  1. Original position of the beam: (x, y)
  2. Deflected position of the beam: (x’, y’)

Before bending, the elongation of (x’, y’) is equal to the elongation of (x, y), so:

x – y = x’ – y’

The elongation of (x’, y’) after bending can be expressed as:

Elongation of (x’, y’) = Elongation of (x, y) = x – y

Considering the deflection angle θ, we can relate the elongation to the bending radius R as follows:

Elongation = R × θ

So, (x – y) = R × θ

Similarly, for the deflected position (x’, y’):

Elongation of (x’, y’) = (R + x)θ

Now, equating the elongation of (x’, y’) before and after bending:

(R + x)θ = R × θ

Simplifying, we get:

x × θ = (R + x)θ – R × θ

Now, strain is defined as the change in dimension divided by the original dimension:

Strain = (x × θ) / (R × θ) = x / R

Using the relationship between stress and strain:

Stress = Modulus of Elasticity × Strain

We can express the stress as:

Stress = E × (x / R)

Now, let’s consider a small area da on the cross-section of the beam from (x, y) to (x’, y’). An external force df is applied on this small area. The force is given by:

df = E × (x / R) × da

The moment of this force about the deflection point (x, y) is given by:

M = df × x = E × (x^2 / R) × da

The total bending moment (bending moment) can be obtained by summing up the moments of all such small forces over the entire cross-section:

M = Σ {E × (x^2 / R) × da}

M = E/R × Σ(x^2 × da)

M = E/R × a × K^2

a = Total cross-sectional area
K = Radius of gyration

Since a × K^2 is equal to the moment of inertia of the beam (I), we have:

M = E × I / R

The term EI is defined as the Flexural Rigidity of the beam.

If R = 1 (unit radius of curvature), then:

M = E × I

Hence, Flexural Rigidity can be defined as “The bending moment required to produce a unit radius of curvature in the beam.”

Factor Affecting Flexural Rigidity

Flexural rigidity is influenced by several factors that play significant roles in determining the beam’s resistance to bending. Let’s discuss each of these factors in detail:

  1. Span of the Beam:
    The span of the beam refers to the distance between its supports or points of contact. Longer spans generally lead to greater deflections under the same load compared to shorter spans. As the span increases, the bending moment along the beam also increases, affecting its flexural rigidity. Longer beams typically require higher flexural rigidity to maintain acceptable levels of deflection and ensure structural integrity.
  2. Area Moment of Inertia (Second Moment of Area):
    The area moment of inertia (I) quantifies the distribution of the beam’s cross-sectional area around the axis of bending. A larger moment of inertia indicates that more material is located farther from the neutral axis, which enhances the beam’s ability to resist bending. Beams with larger values of moment of inertia have higher flexural rigidity and are less prone to significant deflections.
  3. Modulus of Elasticity (Young’s Modulus):
    The modulus of elasticity (E) is a material property that represents the stiffness and ability of the material to resist deformation under stress. A higher modulus of elasticity indicates a stiffer material, leading to a higher flexural rigidity of the beam. Materials with higher elastic moduli, such as steel, have superior flexural rigidity compared to materials with lower elastic moduli, such as wood or plastic.
  4. Types of Load:
    The type of load applied to the beam also affects its flexural rigidity. Different types of loads, such as uniformly distributed loads, concentrated loads, or combinations of both, create different bending moments along the beam’s length. For example, a uniformly distributed load causes a gradual bending across the span, while a concentrated load leads to higher bending moments at the point of application.
  5. Location of Load:
    The position at which the load is applied along the beam’s length can significantly impact its flexural rigidity. Placing a load closer to the supports generates higher bending moments near the support regions, resulting in more significant deflections. In contrast, loads placed closer to the center of the span can create a more uniform bending moment distribution and lower deflections.

Overall, these factors interact in determining the flexural rigidity of a beam. Engineers consider these factors during the design phase to select appropriate materials, beam dimensions, and support configurations to meet the desired performance criteria and ensure the structural integrity of the beam under various loading conditions.

Significances of Flexural rigidity:

Flexural rigidity is a fundamental mechanical property that describes the stiffness or resistance to bending of a material or structure. It plays a crucial role in various engineering applications and has significant importance in different fields. Here are some key points highlighting the significance of flexural rigidity:

  1. Structural Analysis: It is essential in the analysis and design of beams, columns, and other structural elements. It helps engineers understand how a structure or component will respond to loads, such as the weight of the structure itself or external forces like live loads (e.g., people, vehicles) and dead loads (e.g., equipment, furniture). Structures with high flexural rigidity will experience less deflection under load, ensuring safety and stability.
  2. Deflection and Sagging: It determines the amount of bending or deflection a material or structure undergoes when subjected to loads. For example, in civil engineering, it’s crucial to ensure that bridges, floors, and roofs have sufficient flexural rigidity to prevent excessive sagging or deformations.
  3. Material Selection: When designing a structure or component, choosing materials with appropriate flexural rigidity is essential. Different materials, such as wood, steel, concrete, and composites, have varying levels of rigidity. Selecting the right material ensures that the structure can withstand the expected loads without failure.
  4. Vibrational Behavior: It influences the natural frequencies and mode shapes of vibrating structures. Structures with low flexural rigidity may have undesirable vibrations that could lead to fatigue failure or discomfort for occupants, while higher rigidity helps mitigate these issues.
  5. Bending Strength: It is closely related to the bending strength of a material or structure. It affects the ability of a beam or component to resist bending and ultimately determines the maximum load it can withstand before failure.
  6. Support and Stability: In designing supports for various structures like bridges, buildings, and mechanical components, understanding flexural rigidity is crucial. It helps ensure that the supports provide enough resistance to bending, preventing excessive movement and maintaining stability.
  7. Manufacturing Processes: It is a crucial consideration in manufacturing processes, especially for materials used in components subject to bending loads. Understanding the rigidity helps in the proper fabrication and quality control of products to meet design specifications.
  8. Composite Materials: For composite materials, which consist of two or more different materials combined to achieve specific properties, flexural rigidity is a critical parameter. It determines how the different components work together to provide the desired mechanical performance.

In summary, It is a significant mechanical property that impacts the structural integrity, stability, and performance of various materials and structures. Engineers and designers consider it a fundamental parameter in their work to ensure safe and efficient designs.

applications of flexural rigidity

Flexural rigidity finds applications in a wide range of fields and engineering disciplines. Here are some specific applications of flexural rigidity:

  1. Civil Engineering: In civil engineering, It is crucial in the design and analysis of structural elements like beams, slabs, and columns. It helps determine the deflection and bending behavior of these elements under different loads, ensuring the structural integrity and safety of buildings, bridges, and other infrastructure.
  2. Mechanical Engineering: In mechanical engineering, It is vital in the design of machine components and mechanical structures subjected to bending loads. It helps ensure that components like shafts, beams, and supports have sufficient stiffness to maintain their shape and function under operating conditions.
  3. Aerospace Engineering: In aerospace applications, It plays a significant role in the design of aircraft wings, control surfaces, and other structural components. Understanding the flexural rigidity helps ensure proper performance, stability, and safety of aircraft during flight.
  4. Material Science: It is an essential parameter in material science, especially when studying and comparing the mechanical properties of different materials. It helps researchers and engineers select materials with the appropriate rigidity for specific applications, such as in composites used in aerospace and automotive industries.
  5. Biomechanics: In the field of biomechanics, It is used to study the mechanical behavior of bones, cartilage, and other biological tissues. Understanding the rigidity of these structures helps in understanding their function and response to external forces, aiding in the design of medical implants and prosthetics.
  6. Automotive Engineering: In automotive design, It is a critical factor in designing vehicle frames, chassis, and suspension systems. Vehicles with higher flexural rigidity offer better handling, stability, and crash resistance.
  7. Shipbuilding: It is an important consideration in ship design and naval architecture. It helps in designing the hull and structural components of ships to withstand the dynamic forces experienced at sea and ensure overall stability.
  8. Electronic Devices: It is relevant in the design of flexible electronic devices, such as flexible displays and wearable electronics. Understanding the rigidity of the materials used in these devices ensures that they can bend and flex without damage.
  9. Construction Materials: For manufacturers of construction materials like concrete, wood, and composite products, It is a critical parameter in quality control and product development. It ensures that materials meet the required specifications for structural applications.
  10. Sports Equipment: In the sports industry, It is used in the design of equipment such as skis, snowboards, and tennis rackets. The appropriate rigidity ensures optimal performance and safety for athletes during sporting activities.

In summary, the application of flexural rigidity extends across various industries and disciplines, playing a key role in designing and analyzing structures, materials, and products to meet specific performance and safety requirements.

advantages of flexural rigidity

Flexural rigidity offers several advantages in different engineering applications due to its influence on the behavior of materials and structures. Some of the key advantages include:

  1. Structural Integrity: It ensures that structures, such as buildings, bridges, and mechanical components, maintain their shape and resist excessive deflection under various loads. This leads to improved structural integrity, reducing the risk of failure or collapse.
  2. Stiffness and Stability: Materials and structures with higher flexural rigidity are stiffer, providing better stability and reduced deformations under applied loads. This stiffness is especially crucial in applications where precision, stability, and minimal movement are required.
  3. Load-Bearing Capacity: The flexural rigidity of a material or structure directly affects its load-bearing capacity. Higher flexural rigidity allows for the handling of heavier loads and higher stress levels, making it suitable for applications with significant mechanical demands.
  4. Vibration Damping: It helps in damping vibrations and oscillations in structures, machinery, and equipment. This is particularly important in applications where excessive vibrations could lead to fatigue failure or discomfort.
  5. Deflection Control: In engineering design, controlling deflection is essential to ensure proper functionality and safety of components. It assists in managing deflections within acceptable limits, meeting design criteria and avoiding potential problems.
  6. Lightweight Design: Materials with high flexural rigidity often exhibit better strength-to-weight ratios, allowing engineers to design lightweight yet sturdy structures. This advantage is particularly significant in aerospace, automotive, and other weight-sensitive industries.
  7. Material Selection: Understanding the flexural rigidity of different materials enables engineers to select the most suitable material for a specific application. This ensures that the chosen material can withstand the expected loads and environmental conditions effectively.
  8. Energy Efficiency: In applications like mechanical systems and buildings, where energy efficiency is vital, using materials with high flexural rigidity can reduce energy consumption by minimizing deformation-related losses.
  9. Impact Resistance: Higher flexural rigidity contributes to improved impact resistance, making structures and components more resistant to damage from external forces or sudden loading.
  10. Durability and Longevity: The stiffness provided by flexural rigidity helps materials and structures maintain their shape and performance over time, contributing to increased durability and longevity.
  11. Safety and Compliance: Structures and components with appropriate flexural rigidity are more likely to meet safety standards and compliance requirements, ensuring the safety of users and the general public.
  12. Versatility: It is a versatile property applicable to a wide range of materials and structures. Its understanding allows engineers to design and optimize solutions for diverse engineering challenges.

In summary, the advantages of flexural rigidity lie in its ability to enhance structural integrity, stability, load-bearing capacity, and overall performance of materials and structures. It plays a vital role in achieving optimal design and efficient functionality in various engineering applications.

disadvantages of flexural rigidity

While flexural rigidity offers numerous advantages, there are also some potential disadvantages associated with this property, depending on the specific context and application:

  1. Brittle Failure: Materials with very high flexural rigidity, such as certain ceramics and brittle composites, may be prone to brittle failure. Instead of exhibiting ductile deformation, these materials can fracture suddenly when subjected to excessive loads or impact, leading to catastrophic failure.
  2. Fatigue and Stress Concentration: In structures subjected to cyclic loading or dynamic forces, high flexural rigidity can lead to stress concentration at specific points, causing fatigue failure over time. Stress concentration can occur at sharp corners or areas of sudden change in cross-section.
  3. Weight and Cost: Materials with high flexural rigidity, such as certain high-strength metals and composites, may also be heavier and more expensive than alternatives with lower rigidity. This can be a concern in applications where weight and cost constraints are critical factors.
  4. Limited Flexibility: Excessive flexural rigidity can result in limited flexibility of certain components or structures. In applications where flexibility is essential, materials with more moderate rigidity might be preferred.
  5. Reduced Resilience: In some cases, higher flexural rigidity can lead to reduced resilience, making the material or structure less able to absorb and dissipate energy during impacts or sudden loading.
  6. Complex Manufacturing: Materials with high flexural rigidity, especially those with complex geometries, can be challenging to manufacture or process, leading to increased production costs and potential manufacturing difficulties.
  7. Impact on Vibrational Behavior: While moderate flexural rigidity can help in damping vibrations, very high rigidity can alter the natural frequencies and mode shapes of vibrating structures. This can potentially lead to resonance issues or undesirable vibrational behavior.
  8. Sensitivity to Temperature: In some materials, flexural rigidity can be sensitive to changes in temperature. Variations in temperature might affect the material’s mechanical properties, leading to potential performance issues in certain applications.
  9. Limited Adaptability: In applications where the environment or requirements change frequently, materials with very high flexural rigidity might not be adaptable enough to accommodate such changes.

It is important to note that the disadvantages associated with flexural rigidity are context-dependent, and there is no one-size-fits-all approach to engineering design. Engineers and designers must carefully consider the specific requirements of each application and strike a balance between the benefits and limitations of flexural rigidity when selecting materials and designing structures.

Reference : https://en.wikipedia.org/wiki/Flexural_rigidity

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