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Engineering Stress-Strain vs. True Stress-Strain: Understanding Formability and Material Behavior



Introduction to Stress-Strain Relationships


When analyzing the mechanical properties of materials, engineers must take into consideration the behavior of materials under different loads and stresses. The relationship between stress and strain is a fundamental concept that plays a crucial role in determining the formability and performance of materials. In this article, we delve into the differences between engineering stress-strain and true stress-strain and explore their importance in assessing material behavior.


 

Engineering Stress-Strain


Definition and Formula


Engineering stress, also known as nominal stress, is the force applied to a material divided by its original cross-sectional area. Engineering strain, on the other hand, is the deformation or elongation of a material divided by its original length. The formulas for engineering stress (σ) and strain (ε) are:


σ = F / A0

ε = (Lf - L0) / L0


Where:

- F is the applied force

- A0 is the original cross-sectional area

- Lf is the final length

- L0 is the original length


Applications and Limitations


Engineering stress-strain curves are widely used for initial material selection and design, as they provide a simple representation of the material's response under loading. These curves are especially useful for comparing the mechanical properties of different materials, such as yield strength, ultimate tensile strength, and elongation.


However, engineering stress-strain curves have limitations. They do not account for the change in cross-sectional area during deformation, leading to an inaccurate representation of the material's true behavior beyond the elastic region.


 

True Stress-Strain


Definition and Formula


True stress is the force applied to a material divided by its instantaneous cross-sectional area, which changes during deformation. True strain is the natural logarithm of the ratio of the current length to the original length. The formulas for true stress (σt) and strain (εt) are:


σt = F / At

εt = ln(Lt / L0)


Where:

- At is the instantaneous cross-sectional area

- Lt is the instantaneous length


Relationship between Engineering and True Stress-Strain


The relationship between engineering and true stress-strain can be expressed by the following equations:


σt = σ(1 + ε)

εt = ln(1 + ε)


These equations show that true stress is always greater than engineering stress, and true strain is always greater than engineering strain.


Applications and Importance


True stress-strain curves provide a more accurate representation of a material's behavior under load, as they take into consideration the changes in cross-sectional area during deformation. This is particularly important for materials that undergo significant plastic deformation, such as metals and polymers.


Understanding the true stress-strain relationship is essential for accurate finite element analysis (FEA) simulations, predicting material failure, and optimizing material processing techniques, such as forming and shaping.


 

Factors Affecting Stress-Strain Relationships


Various factors can influence the stress-strain relationships in materials. These factors include:


Material Composition and Microstructure


The composition and microstructure of a material directly impact its mechanical properties. Different phases, grain sizes, and grain orientations can significantly affect the stress-strain response of a material.


Temperature


Temperature influences the stress-strain behavior of materials, as it affects their ductility and strength. Generally, increasing the temperature results in increased ductility and decreased strength, while decreasing the temperature has the opposite effect.


Strain Rate


The rate at which a material is deformed also affects its stress-strain response. Materials subjected to high strain rates tend to exhibit higher strength and lower ductility, while those subjected to low strain rates exhibit lower strength and higher ductility.


Pre-existing Defects


Pre-existing defects, such as cracks, voids, and inclusions, can significantly influence the stress-strain behavior of materials. These defects can act as stress concentrators, leading to premature failure under load.


 

Conclusion


Understanding the differences between engineering stress-strain and true stress-strain is essential for accurately assessing material behavior and formability. While engineering stress-strain curves offer a simple representation of a material's response to loading, true stress-strain curves provide a more accurate depiction, accounting for changes in cross-sectional area during deformation. Recognizing the factors that affect stress-strain relationships, such as material composition, temperature, and strain rate, is crucial for material selection, design, and optimization of processing techniques.


 


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