A fundamental concept in material science and engineering, ductility plays a key role in selecting materials for the design of projects. It is a property that describes how much a material can deform under tensile stress before it fractures or breaks. This property mainly describes metals, although some polymers can exhibit ductile behavior. It is often limited, depending on temperature and molecular structure.
In this blog post, we will explore the importance of ductility, what it is, how to measure it, and why it matters in engineering.
What is ductility?
Ductility is a physical property that measures how much a material can be deformed or elongated before breaking or fracturing. In other words, how much can the material be stretched, bent, or drawn out?
Metals have crystalline structures that form a metallic lattice, and the ease with which electrons from each atom can slide through this lattice is how ductility works. It is affected by several factors, including the structure, grain size, and temperature —not just the electrons themselves.
Most high ductile metals have face-centered cubic (FCC) structures, such as aluminum, copper, gold, and silver. Ductility can increase with temperatures up to a certain point, while excessive heating can weaken the material or cause phase changes that result in a reduction of ductility.
What is the importance of ductility?
It is essential for structural components that need to perform under pressure and flexing without failing or fracturing. Examples of a few applications that depend on ductile properties are bridges, cables, earthquake-resistant structures, pipelines, and wires. Low-ductile materials would be catastrophic choices in applications like this, where repeated tension stresses are typical.
When designing components, your material selection is vital for successful projects. If the part will undergo sustained stresses or even intermittent stresses, high-ductile materials will be necessary.
How is ductility calculated?
There are two main ways to calculate ductility. These are the percentages of elongation or area reduction.
Elongation
% Elongation = ((Final Length – Original Length)/Original Length) x 100
Area Reduction
% Area Reduction = ((Original Area – Final Area)/Original Area) x 100
Both of these can find ductility. However, the percentage of elongation is not an absolute measure on its own. Elongation is not uniform and is most extreme at the location of the fracture. The area of reduction is a more accurate measure of ductility, where the measurement is taken at the narrowest cross-section.
Ductility Testing
Steps for Ductility Testing
- Specimen Preparation:
- Cut the test specimen into a bar or strip to specific, often standardized dimensions.
- Make sure the specimen is smooth. De-burr if needed; these can cause stress concentrations.
- Mark Guage Points:
- Mark two gauge points on the specimen at set distances (about 50 mm apart) to measure elongation.
- Mount the Specimen:
- In the testing machine’s grips, secure the specimen.
- Ensure it is properly aligned to avoid bending stresses.
- Setting Test Conditions:
- Different temperatures (high or low) are routine for some tests, so make sure the settings are accurate.
- Set the elongation rate as per testing standards.
- Testing the Specimen:
- The machine applies a continuous load at a steady rate to the specimen.
- Observe the deformation until the specimen fractures.
- Measure the Elongation:
- After the fracture, reassemble the two segments. Measure the increased span between the two gauge points.
- Calculate the percentage of elongation.
- Record Results:
- Note all observations from the test, including the maximum elongation, area reduction, and any unusual failure characteristics.
Procedures for Ductility Testing
Tensile Testing
Otherwise known as a tension test, it can determine different mechanical properties for a material, including tensile strength and ductility. A machine applies a uniaxial load to stretch a standardized specimen until rupture. This test can also find the yield strength, reduction of area, and Young’s modulus.
Flexural Testing
Compressing a specimen on one side while applying tensile stress on the other, with a nominal shear stress that depends on the specimen’s thickness and the span-to-depth ratio. Some factors can influence the test results, including the specimen’s geometry, the rate of load application, and the material’s properties.
During the test, the specimen goes between two supports, and a load is applied at the center, causing the sample to bend as the load increases. The extent of this bending is then measured. This test evaluates both flexural strength and modulus (the material’s elasticity in bending), which are calculated based on the deflection of the material under specific loading conditions.
Cupping Test
Analyze the ductility and formability of sheet metal by clamping a circular sheet of the specimen over a die. A hemispherical punch is pressed into the center until a crack appears or to a specific depth.
Testing Results for Ductility
Common ductility testing results include:
- High Ductility
- The material can withstand numerous deformations without fracturing.
- Ideal for applications such as forming, drawing, and other situations where flexibility is necessary.
- Include materials like copper, aluminum, and precious metals (gold and silver).
- Low Ductility
- Materials easily fracture.
- Brittle materials like ceramics and cast iron.
Additional notable results:
- Elongation Percentage: This refers to the amount of stretching a specimen can undergo before breaking.
- Reduction in Area Percentage: This describes the percentage decrease in the cross-sectional area at the rupture location relative to the original area.
- Tensile Strain: The fractional change in the specimen’s length while undergoing pulling.
- Tensile Stress: Stress is the internal resistance of a material to deformation.
- Modulus of Elasticity: Also known as Young’s modulus, it is the ratio of stress to strain within the elastic region of a material’s deformation, indicating its stiffness.
Factors that Affect Ductility
Composition
There are several ways a metal’s composition can affect its ductility, including adding impurities or alloying elements that alter the microstructures. Defects or inclusions can act as stress concentrators and lead to failures and sudden fractures. Alloying materials can change the grain size, texture, and strength, and these differences can significantly alter the ductility.
Grain Size
The smaller the grain size, the more boundaries, which means a decrease in ductility but an increase in strength and hardness. Subsequently, larger grain sizes mean fewer boundaries, an increase in ductility, and a reduction in strength and hardness.
Cell Structure
Crystalline structures like those found in etals can be different depending on the composition of the metal. Face-centered cubic (FCC) crystal structures, like those of aluminum and copper, are more ductile than materials with body-centered cubic (BCC) or hexagonal close-packed (HCP) crystal structures.
Hot Working vs. Cold Working
Hot working metal can increase the ductility, while cold working metal can decrease the ductility and increase brittleness. The heat decreases internal stresses and energizes the electrons. When using heat to increase the ductility, the amount of temperature increase is specific to the metal. Not all metals are the same. High temperatures can promote recrystallization, where the material can form stress-free grains.
Why should ductility factor into a project?
Safety & Failure Prevention
Visible warning signs appear in ductile materials before failure, including necking or deformation. Inspection and maintenance in high-stress situations are necessary to find issues and prevent catastrophes. Brittle materials would be too hazardous to use in these situations because they will fail suddenly without warning.
Energy Absorption
Large amounts of energy can be absorbed and dissipated via ductile materials. Ideal for applications that need to withstand blasts, earthquakes, or impacts.
Redundancy
The redistribution of stresses within a structure is possible with ductile materials. The load can be transferred to other parts if one part yields, preventing localized issues and total failure.
Assembly & Joining
Using mechanical fasteners, riveting, or welding is easy to do with ductile metals because they can accommodate localized deformations and stresses.
Advancements in Enhancing Ductility
The focus of recent research is on overcoming the sacrifice of strength for ductility. The benefits of this research include materials that are safe, lighter, and more durable for industries like aerospace, automotive, energy, and defense.
Headerogenious Nanostructures
These materials are composed of regions with varying grain sizes, phases, or composition on the nanometer scale. The internal boundaries are carefully designed and tailored to how dislocations move through the material.
Mechanisms and Benefits
- Soft regions deform first. These generate back-stress that pushes uniform deformation. In turn, it increases the strength and ductility.
- Hard regions promote strength. Soft regions accommodate deformation, and in conjunction, they delay fractures.
Example
Heterostructured steels and copper alloys are high in strength and ductility.
Coherent Nanolamellar Alloys
These alloys feature alternating layers of different phases with well-matched and continuous interfaces.
Why are they special?
- Coherent interfaces inhibit the movement of dislocations, leading to hardening and resistance to fracture.
- The enhanced strain accommodation means that the nanolamellar can deform together. Nanolamellar can distribute strain more evenly and prevent catastrophic failure.
Example
Nanolamellar perlitic steels and some high-strength titanium alloys show remarkable strength and ductility when used in architecture.
High-Entropy Alloys (HEAs)
These are an innovative class of materials that have five or more principal elements in near-equal ratios. Their unique composition leads to incredible mechanical properties.
Ductility-Enhancing Features
- Due to extreme lattice distortion, there are disruptions in the dislocation movements, leading to high strength and ductility.
- Numerous deformation instruments, HEAs, can activate twinning, slipping, and phase transformation.
Examples
FFC HEAs like CoCrFeMnNi display incredible strength and ductility, particularly at cryogenic temperatures.
Nanotwins & Stacking Faults
Involving the innovation and manipulation of distinct internal defects to enhance material properties is called defect engineering. Two promising defects are nanotwins and stacking faults.
Nanotwins are mirror-symmetrical boundaries within the grains that function as impediments to dislocation motion. These reinforce the material while delivering paths for further deformation. For example, twinning-induced plasticity (TWIP) steels are highly ductile.
Stacking faults are irregularities in the stacking sequence of atomic layers that facilitate additional slip and twinning. These enhance deformability. For example, certain face-centered cubic metals and HEAs benefit from controlled stacking faults for improved strength and ductility.
FAQs
Define: Ductility
It is the ability of a material to deform plastically under tensile stress, allowing it to be drawn into wires or other shapes.
Is ductility a mechanical property?
Yes, it is a mechanical property that measures a material’s ability to deform under tensile stress.
What is the difference between ductility and malleability?
Ductility refers to a material’s ability to be stretched into a wire, while malleability is the ability to be shaped or deformed without breaking.
What is the difference between ductility and elasticity?
Ductility refers to a material’s ability to deform under tensile stress, while elasticity is the ability to return to its original shape after deformation.
