Ductility Test: Definition, Purpose, Applications, and How Does It Work

A material's ductility is its propensity to permanently deform rather than break when placed under tensile stress. Engineers have developed standardized testing procedures to determine most materials’ ductility characteristics. In them, a gradually increasing tensile force is applied to the test specimen until it physically breaks or is otherwise considered to have failed.  

This knowledge is essential in fields like engineering, manufacturing, and construction where a material’s response to stress can have dire implications. An accurate ductility test can be part of a research and development effort, function as a quality assurance indicator, or tell you whether a material is appropriate for use in a particular application. This article will define ductility testing and discuss its purposes, applications, and procedures. 

What Is a Ductility Test?

A ductility test is a mechanical evaluation method used to quantify how much a material will plastically (and permanently) deform when subjected to tensile stress. It establishes the maximum amount of deformation the material can withstand before cracking or breaking. This test is essential for determining the durability, effectiveness, and suitability of materials for stress-inducing applications.  

Ductility testing has a long history, dating back to ancient times when metalworkers discovered the properties of malleable and ductile metals such as gold, silver, and copper. Over time, as metallurgy improved, new techniques were developed to make use of metal’s ductility. Long-lived techniques like forging metal with a hammer or drawing wire through a die depend on this property. 

Keith Millis, an American metallurgical engineer, played a significant role in developing a new, more ductile iron in the 1940s. During World War II, he was tasked with finding alternative ways to strengthen iron for use in weapons production. Millis and his team discovered that adding magnesium to molten iron caused graphite particles within the metal to align into spherical patterns, resulting in increased ductility and strength. This discovery revolutionized the iron industry and led to the widespread use of ductile iron thereafter.

How Does Ductility Testing Work?

Ductility testing is typically conducted using a tensile testing machine which applies an increasing amount of force to a material sample until it deforms or fractures. Most samples come in the form of thin, cylindrical rods, and are clamped at both ends of the testing machine. Force is applied until the sample either fractures or reaches a predetermined amount of deformation. The amount of deformation that remains permanent, typically expressed as a percentage of the original length, is a measure of the material's ductility.

What Is the Purpose of Ductility Testing?

The purpose of ductility testing is to determine the material’s ability to deform without breaking or cracking under tensile stress. This test is important in evaluating the quality and suitability of materials for various applications. For instance, ductility testing can tell a construction engineer whether their steel is pliable enough to withstand seismic activity. In manufacturing, ductility testing can be used to determine a material's malleability for shaping and forming. 

When Are Ductility Tests Used?

Ductility tests are typically used in materials science and engineering fields to assess a material's suitability for particular applications, such as in manufacturing or construction. They are particularly important in the case of wires and cables that will be under tension. In a ductility test, a sample is typically stretched until it fractures. The amount of deformation that occurred before failure is then measured. The outcome should reveal important details about a material's mechanical characteristics and suitability.

Where Are Ductility Tests Applied?

Here are some instances of how and where ductility tests are used:

1. Metallurgy: Testing for ductility is done on a variety of metals, including steel, copper, and aluminum. The tensile test, which gauges the amount of strain a metal can withstand before breaking, is among the most critical ductility tests in metallurgy.
2. Civil Engineering: Ductility tests help assess the qualities of building materials like concrete and asphalt. Flexural tests, which gauge the material's capacity to bend without breaking, are the most common ductility tests in civil engineering.
3. Automotive Industry: Ductility tests are used to evaluate the quality of automotive components such as sheet metal (and the welds that hold it together). In the automotive industry, the most common ductility test is the cupping test which measures the bending properties of the metal. Though vehicles are often subject to unpredictable forces, the metal body should be designed in such a way that it can be bent without breaking.
4. Aerospace Industry: Ductility tests are used to evaluate the properties of materials like the titanium and composites used in aircraft. The tension test and compression test, which both measure how much deformation a material can withstand before failing, are the most popular ductility tests in the aviation industry.
5. Materials Research: Material properties can be improved if they’re fully understood. Researchers will choose from many different ductility tests to evaluate their new materials or variations of existing materials. 

How to Perform Ductility Tests?

Listed below is a step-by-step guide on how to perform a basic ductility test:

1. Sample Preparation: Cut or form a sample of the material to be tested into the proper shape and size. The size and shape of the sample may depend on the type of ductility test being performed.
2. Test Apparatus Setup: Set up the test apparatus according to the specifications of the test method you aim to use. This may involve mounting the sample on a device such as a tensile testing machine.
3. Pre-test Measurements: Measure the length and width of the sample before the test begins. Record these measurements — they will be needed when you calculate the percentage elongation of the sample.
4. Application of Load: Apply a steadily increasing load to the sample until it begins to deform. The load application will depend on the type of ductility test you’re running. 
5. Deformation Measurement: Measure the amount of deformation that occurs as the load is applied. This may involve measuring the length or width of the sample at regular intervals during the test.
6. Failure Identification: Continue applying the load until the sample breaks or fractures. Identify the point at which the sample fails.
7. Post-Test Measurements: After the test is complete, measure what length the sample reached before fracturing. Record this measurement for later use in calculating the percentage elongation of the sample.
8. Calculation of Percentage Elongation: Divide the post-test measurement by the pre-test length to calculate the percentage elongation. This value indicates the degree of ductility of the material.

What Are the Ductility Test Procedures?

The procedures vary depending on the specific ductility test. Here are some common procedures:

1. Tensile Test: A tensile test, also known as a tension test, is a type of mechanical test used to determine the strength of a material. A tensile testing machine pulls a sample axially to determine how much strain it can withstand before breaking. The sample lengthens and thins out in the middle, eventually breaking as the force is increased. The material's ultimate tensile strength, yield strength, and elongation can all be calculated from the test data, along with other mechanical characteristics.
2. Flexural Test: A mechanical test called a flexural test (bend test), assesses a material's resistance to bending or flexing. Flexural testing is more complicated than compression or tensile testing because it subjects the material to simultaneous action from all three fundamental stresses: tensile, compressive, and shear. The geometry of the specimen and the rate at which the load is applied, in addition to the material properties, impact the test's outcomes. A sample of the material is placed on two supports during a flexural test, and the sample's center is loaded. The sample will bend as the load is increased, and the degree of bending is measured. The flexural strength and the material’s modulus of elasticity can be calculated by measuring the amount of bending under a given applied load.
3. Cupping Test: A common technique for analyzing the ductility and formability of sheet metal is the cupping test. In this test, a circular sheet of metal is clamped over a circular die, and the center of the sheet is slowly punched out with a spherical punch. The specimen is indented in the shape of a cup as a result of the punch's forceful deformation.

What Are the Ductility Test Equipment?

The specific test method determines what type of equipment to use. The cupping test, which employs a cupping machine made up of a hemispherical punch and a cup-shaped die, is one popular technique. The metal specimen is set on the die, and when the punch is applied, the metal is forced against the die. Calculating the metal's ductility involves measuring the degree of deformation.

What Are the Common Ductility Testing Results?

Ductility tests typically tell you how much deformation a material can withstand before failing. The tensile test, the bend test, and the cupping test are the most common ductility testing procedures. Results from these tests may include

1. Elongation: The amount of deformation a material undergoes before it breaks or fractures.
2. Reduction in Area: The percentage decrease in the cross-sectional area after the material undergoes deformation.
3. Strain: The amount of deformation per unit length or unit cross-sectional area.
4. Stress: The amount of force per unit area that a material can withstand before it deforms or fails. 
5. Modulus of Elasticity: The ratio of stress to strain in a material prior to the onset of plastic deformation. This indicates its ability to deform elastically under stress.

What Are the Types of Ductility Testing?

The types of ductility testing are listed below:

1. Hardness Testing

The material's resistance to an indentation or scratching is measured using specific instruments such as the Rockwell hardness tester. The most common hardness testing methods are Brinell, Vickers, and Rockwell.

2. Impact Test

This test style measures the amount of energy required to fracture a material. It is designed to evaluate material toughness. The most common such tests are the Charpy impact test or Charpy V-notch test.

3. Fracture Toughness Tests

This test measures a material’s resistance to crack propagation. The most common fracture toughness tests are the KIc test and the J-integral test.

What Are the Ductility Testing Standards?

Ductility testing standards are established procedures used to discover the ability of a material to deform plastically under tensile stress without breaking or fracturing. The most common standards for ductility testing are:

1. ASTM E8/E8M-21 (Standard Test Methods for Tension Testing of Metallic Materials): This standard provides guidelines for the tensile testing of metallic materials, including the measurement of ductility.
2. ASTM D638-14 (Standard Test Method for Tensile Properties of Plastics): This standard outlines the procedures for testing the tensile properties of plastics.
3. ISO 6892-1:2016: This international standard specifies the methods for tensile testing of metallic materials at room temperature.

What Are the Benefits of Ductility Tests?

The benefits of conducting a ductility test include:

1. Understanding the behavior of your material under tensile stress.
2. Identification of material defects or weaknesses.
3. Determination of material properties such as elongation and reduction in area.
4. Quality control in your manufacturing processes.
5. Ensuring compliance with industry standards and regulations.

What Are the Limitations of Ductility Tests?

The limitations of ductility tests are:

1. Measurements cannot extend beyond the point of sample fracture. Affected by sample size, temperature, and testing speed.
2. The testing apparatus occasionally introduces errors.
3. Difficulties in obtaining representative samples.

Are Ductility Tests Accurate?

Yes, ductility tests can be accurate when performed properly and under appropriate conditions. However, many factors can affect the accuracy of the test, including the quality of the testing apparatus, the consistency of the testing environment, and the skill and experience of the technician. It is important to follow established testing procedures and to properly calibrate and maintain the testing equipment to ensure accurate results.

What Is the Difference Between Tension Testing and Ductility Testing?

Tension testing is a general term that refers to any type of test that involves subjecting a material to a tensile force. Ductility testing, on the other hand, is a specific type of tension test that measures the extent to which a material can deform without fracturing. 

Summary

This article presented ductility testing, explained it is, and discussed the purpose of this test. To learn more about ductility tests, contact a Xometry representative.

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