Materials testing types

Material testing is a dynamic field at the intersection of science and engineering, aimed at understanding the properties and behavior of various materials under different conditions. Whether it's metals, polymers, ceramics, composites, or other materials, the ability to accurately characterize their mechanical, thermal, electrical, and chemical properties is crucial for numerous industries, including aerospace, automotive, construction, healthcare, and electronics.

Why is material testing important?

Materials are tested for various reasons, including quality assurance, safety, performance evaluation, failure analysis, and research and development, each crucial in ensuring their suitability for specific applications and environments.

Quality Assurance:
Material testing is essential to ensure that materials meet quality standards and specifications. By testing materials during production or before use, manufacturers can verify that they meet the required properties.

Safety:
Testing materials helps ensure the safety of products and structures. For example, materials used in critical applications such as aerospace, automotive, and medical devices must undergo rigorous testing to ensure they can withstand extreme conditions and perform reliably without failure, which could lead to catastrophic consequences.

Performance Evaluation:
Materials are tested to evaluate their performance under various operating conditions. This includes assessing properties such as strength, stiffness, hardness, ductility, and fatigue resistance to determine how well they will perform in real-world applications and to identify any potential weaknesses or limitations.

Failure Analysis:
When materials fail unexpectedly, testing is conducted to determine the root cause of the failure. By analyzing failed materials through techniques such as fractography, metallurgical analysis, and non-destructive testing, engineers can identify design flaws, manufacturing defects, or environmental factors that contributed to the failure and implement corrective measures to prevent recurrence.

Research and Development:
Material testing plays a crucial role in advancing materials science and engineering. Researchers use testing techniques to study the fundamental properties and behavior of materials, develop new materials with improved properties, and explore innovative applications and manufacturing processes.

Types of materials testing

Materials testing is classified into six types: destructive testing, non destructive testing, technological tests, electrochemical tests, chemical tests, and metallographic investigation.

1. Destructive testing

Destructive testing (DT) refers to a type of examination or evaluation method in which the tested materials, components, or structures are intentionally subjected to conditions that cause them to fail, break, or otherwise undergo significant damage.

Destructive testing examples

1. Tensile Test

This test involves pulling a sample of material in opposite directions until it breaks. It helps determine the material's tensile strength, yield strength, and elongation properties.

2. Compression Test

In this test, a sample is subjected to a compressive force until it fails. It assesses the material's compressive strength and its ability to withstand crushing forces.

3. Impact Test

Impact tests involve striking a material with a controlled force to assess its toughness and resistance to sudden shock or impact.

4. Hardness Test

Hardness tests determine the material's resistance to indentation or scratching.

5. Bend Test

This test involves bending a sample of material to assess its ductility and flexibility. It is commonly used for evaluating the quality of welds and the overall integrity of metal structures.

6. Fatigue Test

Fatigue tests involve subjecting a material to repeated cyclic loading until it fails. These tests help determine a material's endurance limit and its ability to withstand fatigue failure over time.

7. Creep Test

Creep tests assess a material's behavior under prolonged exposure to high temperatures and constant stress. They are essential for evaluating materials used in high-temperature applications.

2. Non-Destructive testing

Non-Destructive testing (NDT) is a method used in various industries to assess the properties of a material, component, or system without causing damage to its integrity or functionality. This allows for inspection, evaluation, and analysis of objects without altering their physical or structural properties.

Note that:

• Some Hardness tests are considered Non-Destructive.
• The specimen cannot be used after Destructive tests.

Non-Destructive testing examples

• Ultrasonic Testing (UT)
• Radiographic Testing (RT)
• Magnetic Particle Testing (MT)
• Liquid Penetrant Testing (PT)
• Eddy Current Testing (ECT)
• Micro and Nano hardness tests

1. Ultrasonic Testing (UT)

This method utilizes high-frequency sound waves to detect internal defects or discontinuities in materials. A transducer emits ultrasonic waves into the material, and any reflected waves from defects are analyzed to determine their size, location, and severity.

2. Radiographic Testing (RT)

RT involves the use of X-rays or gamma rays to inspect the internal structure of objects. By capturing images on photographic film or digital detectors, defects such as cracks, voids, or inclusions can be identified without damaging the material.

3. Magnetic Particle Testing (MT)

MT is used to detect surface and near-surface defects in ferromagnetic materials. A magnetic field is applied to the material, and iron particles coated with a contrasting dye are applied. Any discontinuities disrupt the magnetic field, causing the particles to accumulate at defect sites, making them visible under UV light.

4. Liquid Penetrant Testing (PT)

PT is employed to detect surface-breaking defects in non-porous materials. A liquid penetrant is applied to the surface, allowed to seep into any surface cracks or discontinuities, and then wiped off. A developer is then applied to draw out the penetrant from the defects, making them visible.

5. Eddy Current Testing (ECT)

ECT utilizes electromagnetic induction to detect surface and near-surface defects in conductive materials. An alternating current is passed through a coil, creating eddy currents in the material. Any changes in the material's conductivity due to defects alter the eddy currents, which are detected and analyzed to identify defects.

6. Micro and Nano hardness tests

These tests are considered non-destructive because they typically produce very small indentations, often on the scale of micrometers or nanometers, which do not cause significant damage to the material. Additionally, testing parameters such as load and displacement can be carefully controlled to ensure that the applied force does not exceed the material's yield strength, preventing permanent deformation or damage.

Note that:

• The specimen can be used after Non-Destructive tests.

3. Technological tests

Typically, these are quick testing methods that often don't conform to specific standards. They assess a metal's capacity to withstand deformation like what it might experience during processing or in service. For example, testing a metal can reveal its suitability for processes like forging, rolling, bending, or deep drawing.

Technological tests examples

Technological tests encompass a wide range of assessments designed to evaluate the performance, functionality, reliability, and safety of various technologies. Here are some examples across different domains:

1. Spark Test

This is a qualitative test used to identify certain types of metals based on the sparks produced when grinding their surfaces. Different metals produce sparks with distinct characteristics, such as color, length, and intensity, due to variations in their chemical composition and physical properties. Spark testing is commonly used in metalworking and fabrication industries for rapid material identification.

2. Warm Compression Test

This test involves subjecting a material to compressive forces at elevated temperatures but below its recrystallization temperature. It assesses the material's behavior under conditions like those encountered during hot forming processes such as forging, extrusion, or hot rolling. The test provides valuable information about the material's flow behavior, ductility, and resistance to deformation at high temperatures.

3. Bending of Sheets for Several Times

This test evaluates the bending or forming properties of sheet materials by subjecting them to repeated bending cycles. It simulates the effects of cyclic loading and deformation that materials may experience during manufacturing processes or in service. it gives an indication for the ability of deep drawing of metal under testing.

4. Folding Test

The folding test is used to assess the ductility and resistance to cracking of sheet materials under bending stress. It involves folding a specimen along a defined line or axis until failure occurs or a specified number of folds is reached. This test is particularly relevant for evaluating the formability and workability of sheet metals in applications such as automotive body panels, metal packaging, and fabrication.

5. Hammering Test

Also known as the hammering or drop test, this qualitative assessment involves striking a material with a hammer or similar instrument to evaluate its hardness, resilience, and impact resistance. The test provides a quick indication of the material's toughness and ability to withstand sudden loads or impacts. It is often used in field inspections or quality control checks for structural components, weldments, and castings.

Note that:

• Technological tests do not provide quantitative results.

4. Electrochemical tests

Electrochemical tests, commonly known as corrosion tests, are vital for evaluating the susceptibility of materials to corrosion and degradation in different environments. These tests utilize electrochemical principles to simulate and measure corrosion processes, helping to assess the corrosion resistance of materials and identify potential failure mechanisms.

Electrochemical tests methods

Electrochemical testing methods are techniques used to study the electrochemical properties of materials and systems. These methods involve the interaction between electrical energy and chemical reactions, providing valuable insights into processes such as corrosion, battery performance, and material characterization. Here are some common electrochemical test methods:

Linear Polarization (LP)

Linear polarization is a method used to determine the corrosion rate of a material by measuring its polarization resistance (Rp). It involves applying a small potential perturbation to the material's surface and measuring the resulting change in current. By analyzing the relationship between the applied potential and the resulting current, the polarization resistance can be calculated using the Stern-Geary equation.

Tafel Analysis

Tafel analysis is a widely used electrochemical method for studying the corrosion kinetics of metals and alloys. It involves measuring the anodic and cathodic Tafel slopes from the polarization curves obtained during potentiodynamic polarization tests. By extrapolating these slopes, the corrosion current density and corrosion potential of the material can be determined.

Harmonic Analysis (CASP/EFM)

Harmonic analysis, also known as electrochemical frequency modulation (EFM) or the Current Amplitude Spectrum Plot (CASP) technique, is a non-destructive electrochemical method for monitoring corrosion rates in real-time. It involves applying a small sinusoidal potential modulation to the material's surface and measuring the resulting current response. By analyzing the harmonic components of the current signal, information about the corrosion rate and corrosion kinetics can be obtained without the need for polarization curves or Tafel analysis.

Electrochemical Impedance Spectroscopy (EIS)

EIS is a powerful electrochemical technique used to characterize the electrical properties of materials and interfaces in corrosive environments. It involves applying a small amplitude alternating current (AC) signal to the material and measuring the resulting impedance response over a range of frequencies. By analyzing the impedance spectra, information about the material's corrosion resistance, surface properties, and corrosion mechanisms can be obtained.

5. Chemical tests

Chemical tests are analytical procedures used to determine the chemical composition, properties, or characteristics of substances. They can be qualitative, providing information about the presence or absence of specific substances, or quantitative, measuring the concentration or amount of a particular chemical species.

Chemical analysis is carried out on the metals and alloys for the following purposes:

Chemical tests methods

Chemical testing methods are essential techniques used to analyze the composition, properties, and characteristics of substances. These methods are employed across various fields such as chemistry, biology, environmental science, medicine, and materials science. Here are some common chemical testing methods:

• X-ray Fluorescence (XRF): XRF is a non-destructive analytical technique used to determine the elemental composition of solid samples. It involves bombarding the sample with X-rays, which causes the atoms in the sample to emit characteristic fluorescent X-rays. By measuring the energy and intensity of these X-rays, the elemental composition of the sample can be determined.

• X-ray Diffraction (XRD): XRD is a technique used for the analysis of compounds and oxides and the crystalline structure of solid materials. It involves directing X-rays at a sample and measuring the diffraction pattern produced when the X-rays interact with the crystal lattice. From the diffraction pattern, information about the crystal structure, lattice parameters, and phase composition of the sample can be obtained.

6. Metallographic Investigation

It is the study of microstructure of the specimen on the prepared cross section under an optical microscope (up to a magnification of 2000X) or studying the macrostructure under low magnification stereomicroscope (up to 20X). Scanning Electron Microscope (SEM) to achieve magnification up to 100 000X.

What is metallographic test used for?

Metallographic tests used for phase structure, microcracks, grain size, inclusion type and morphology, and grain form and distortion.

Microcracks: 
Assessment involves determining the quantity and length of microcracks present in the material.

Grain Size: 
Evaluation of both micro and macrostructure grain sizes provides insights into material properties.

Grain Form and Distortion: 
Observing the shape and distortion of grains offers indications about the type of deformation experienced by the material, such as rolling or bending.