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What Is Mechanical Testing?

Mechanical testing of materials examines how they respond to a variety of stresses. In particular, the focus is on the connection between the applied forces, the resulting deformation, and the stresses that ultimately cause the components to fail. Materials research, component design, and quality control benefit from testing’s derived characteristic values. To precisely characterize the mechanical properties of materials, a variety of standardized testing procedures exist:

Tensile test, compression test, Torsion testYield Strength, Ultimate Tensile Strength, Plasticity, Stiffness, Fracture point
Vickers, Micro-Vickers, Rockwell, Brinell hardnessHardness
Wohler Fatigue testFatigue Strength (Cycles to failure)
Creep Rupture testBehavior of Creep
Impact testToughness

Tensile Testing

The tensile test is the gold standard in the destructive mechanical testing of materials. A consistently increasing force in the longitudinal direction is applied to a standard specimen with a known cross-section. For instance, tensile testing of steel is performed using universal testing equipment in accordance with standard EN 10 002.

Until the beginning of contraction, the specimen is in a uniaxial stress state. The load extension diagram can be used to demonstrate the stress-to-strain ratio. The amount of power needed to break the specimen and the length to which the specimen is stretched are also recorded.

Figure: Stress-Strain Diagram with characteristic features

Hardness Test

The term “hardness” is used to describe the amount of force required to penetrate a material. There are three hardness tests usually carried out for metallic materials. 

1. Vickers hardness test:

Vickers hardness test is one of the earliest ways of determining hardness, and it features a broad hardness scale that makes it applicable to various metals and welds. A diamond indenter with a pyramidal tip at 136 degrees is used when measuring hardness with the Vickers scale. For 10–15 s, the force is exerted. Using a calibration for different kilogram loads, hardness is calculated by averaging the two axes of the diamond-shaped indentation measured in millimeters (to give the dimension d) (P). A load of 10 kilograms (kg) is equivalent to a hardness (HV) of 10, while a load of 5 kilograms (kg) corresponds to a hardness (HV) of 5.

vickers hardness test
Figure: Brinell hardness test schematic

2. Rockwell hardness test:

The Rockwell hardness test is the most common hardness test, which is correctly defined in ASTM E-18 standards. Compared with other hardness tests, the Rockwell test is more straightforward and accurate. Except in cases where the structure or surface characteristics of the test sample would bring too many variations, where the indentations would be too broad, or where the sample size or shape may restrict its usage, the Rockwell test method can be used on all metals.

The depth of an indentation left by an applied force or load is proportional to the Rockwell hardness scale value. First, a diamond or ball indenter provides a little initial load (preload) on the sample. This preload penetrates the surface to lessen its effects. The initial depth of indentation can be determined by applying the test force and letting it sit for a predetermined amount of time.

Types of Rockwell Hardness Test:

The standard Rockwell hardness test and the superficial Rockwell hardness test are the two main types of Rockwell hardness measurements. The two forms of the Rockwell hardness test have entirely different uses.

Standard Rockwell Hardness Tests

Rockwell hardness testing of this variety uses multiple hardness scales to evaluate the sample material. It is common for quality inspectors to choose one type of scale over another since it is specific to a particular material type (e.g., plastics, ferrous, nonferrous). As an added bit of information, the most common Rockwell hardness ranges for metals range from A to F, whereas the M and R scales are used for polymers.

Superficial Rockwell Hardness Testing

It is believed that this variation of the Rockwell hardness test is more sensitive to the nature of the material’s surface than the conventional Rockwell hardness test, which is obtained when using the test. Compared to the standard Rockwell test, this method is preferable for use on surfaces with a hardness gradient, small sample areas, and thin surfaces. Additionally, this method is advantageous when used on surfaces. However, it employs a different scale for determining the Rockwell hardness, with W, X, and Y used for nonferrous and soft-coated materials and T and N used for ferrous materials. This is because W, X, and Y are used for softer coatings. 

3. Brinell hardness test:

Samples with a coarse or rough structure or surface, such as forgings and castings, can be tested with the Brinell hardness test method according to ASTM E10. It uses a 10mm indenter with a high-test load of 3000 kgf to smooth away surface and subsurface imperfections.

The Brinell test involves applying a known load (F) to a carbide ball of known diameter (D) for a given amount of time (t) before removing the ball. After the impression is made, it can be measured with a specialized Brinell microscope or optical system over at least two diameters perpendicular to one another (d). A chart translates the average diameter to a Brinell hardness number, while the technique shown below can yield the Brinell number.

From 500kgf for testing non-ferrous materials to 3000kgf for testing cast iron and steel. Other Brinell hardness scales exist, with loads as low as 1 kgf and indenters as small as 1 mm in diameter. However, these are rarely employed.

brinell hardness test
Figure: Brinell hardness schematic

Impact test

The tendency of cleavage fracture or toughness attribute of a material can be determined using the impact test, which involves sudden loading. This type of testing does not yield numerical numbers for material properties. The results of the notched-bar impact test do not directly correlate with strength calculations. Instead, they aid in the initial stages of resource selection for a specific activity. 

The material’s deformation behavior is frequently a significant factor in decision-making. Using it, you may rapidly determine which of your materials are fragile and sturdy. Conditions extrinsic to the material, such as temperature or stress, play a role in determining how brittle it will be. A notched bar’s impact strength is measured in various ways. The Charpy test involves mounting the test body on both sides and having a pendulum hit the center of the test body at the notch’s height. The Izod and Dynstat tests require an excellent test body struck by a pendulum above the notch. 

impact test
Figure: Schematic of Impact test samples with loading directions

Fatigue Test

Defined as the maximum load a dynamically loaded material can sustain before failing, fatigue strength is an essential property of many materials. For example, dynamic loads, such as vibrations, are exerted on machine parts that are in motion. A high number of load cycles at stresses well below the yield point and the fracture stress result in a fracture in this scenario.

The Wohler diagram contains three regions: 

Short-term strength: Exceeds the load limit point where the specimen could be damaged in theory.

Fatigue strength: The number of cycles to failure decreases as the applied increases.

Endurance strength: A specimen can bear maximum stress without any significant deformation and at least up to NG number of cycles depicted in the figure below.

Service life: Number of cycles to failure that a material bares under a specific load.

fatigue test
Figure: S-N curve with characteristic points in the graph 

Creep Rupture test

At elevated temperatures, materials respond differently to constant static stresses than they do at ambient temperatures. Creep is the gradual but constant irreversible plastic deformation that occurs when a material is subjected to elevated temperatures under loads below the hot yield point and no additional load is applied. The specimen will break under even and prolonged loading conditions.

In the creep curve, three phases depict the phenomenon occurring in the material.

Phase 1:

It is known as primary creep, and in this region, the creep rate increases initially and slows down then over time, but the material strength influence prevails and is also known as rapid creep.

Phase 2:

It is known as secondary creep, and this region’s creep rate is constant. 

Phase 3:

It is known as tertiary creep, and in this region, the creep rate increases again till the fracture point because, at this stage, the necking phenomenon dominates. Voids at the grain boundary and stress concentration also significantly affect this region, leading to the material’s fracture.

creep rupture test
Fig: Strain vs. Time graph with creep regimes

Compression test

The tensile test is more critical than the compression test for evaluating metals. Still, the compression test is crucial for investigating construction materials like wood, concrete, brick, and other natural stones and bricks. In this experiment, a standard specimen with a known cross-section is loaded uniformly with a gradually rising force over the length of the sample. The specimen is in a uniaxial stress state. The force-path diagram allows one to visualize the stress-to-compression ratio. Characteristic values for compression strength, 0.2% offset yield point, and the compression yield stress are provided, and the stress-compression diagram depicts the distinct behavior of the many individual materials.

compression test
Figure: Stress Compression Diagram with characteristic points

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