Mechanical Testing

Mechanical Testing | Metal Technology Engineering

Mechanical Testing

Mechanical testing is a series of tests used in product design and part manufacturing for material identification, characterization, selection, and validation of products. As a result, manufacturers can ensure proper material utilization, safety during production, and cost-effectiveness.

Mechanical testing is a series of standardized tests used to determine a material’s physical and mechanical properties and suitability for its proposed applications. It is a important requirement in product design and part manufacturing due to the need to achieve standards set by organizations such as ASTM and ISO. These tests allow manufacturers to distinguish materials of less quality and choose the right material for their products. 

Types of Mechanical Tests

There are several mechanical engineering tests suitable for determining material strength parameters. Each has a unique approach and machine and can provide information about several strength-related parameters.

Tensile Testing

Tensile testing is a fundamental mechanical strength test used to determine material properties such as stress, strain, and yield deformation. It involves subjecting a material to a force on opposite ends and pulling till it breaks.

Testing occurs in a tensile testing machine that is either hydraulic or electric. The operator subjects the material to different forces and records the data. Afterward, they plot the data to get the stress-strain curve in a graph. Common standards for the tensile test include ASTM D638 / ISO 527-2 (for reinforced plastics), ASTM D412 / ISO 37 (vulcanized rubber and thermoplastic elastomers), and ASTM E8 / ASTM A370 / ISO 6892 (metals and other metallic materials).  

Mechanical Testing | Metal Technology Engineering
Mechanical Testing | Metal Technology Engineering

Torsion Testing

Torsion testing is another form of mechanical testing that evaluates a material’s behavior when subjected to stress at an angular displacement. As a result, it gives information about the material’s shear modulus of elasticity, shear yield strength, shear strength, shear modulus of rupture, and ductility.

 Furthermore, there are several types explained below.

  • Torsion only: Applying only torsional load to the material.
  • Axial-torsion Applying axial (tension/compression) and torsional force to a material.
  • Failure testing: Twisting the product or material until it breaks or there is a visible defect.
  • Proof testing Applying a torsional load to the material and holding the torque for a certain time.

According to ASTM and ISO, common standards for torsional testing are ASTM A938/ ISO 7800 (Torsion Testing of Metallic Wire).

Fatigue Testing

Fatigue mechanical testing determines how a material behaves under fluctuating loads applied axially, in torsion, or flexure. It involves subjecting the material to a mean load and an alternating load. As a result, the material will experience fatigue (i.e., when the material breaks).

The data will be presented from the test in an S-N diagram – a plot of the number of cycles to cause failure against the amplitude of cyclical stress (which can be stress amplitude, max stress, or min stress).

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Fracture Mechanics Testing

Fracture mechanics testing allows manufacturers to determine the energy it will take to break material with an existing crack into two. Furthermore, it allows the manufacturer to ascertain the material’s ability resist fracturing using the intrinsic stress factor.

From the data, manufacturers can analyze the brittle fracture and examine its grain size, case depth, etc. Common standards for the test are BS 7448, NS-EN 10225, ASTM E1820, and EEMUA pub. 158.

Compressive Testing

Compressive testing is another fundamental mechanical engineering test determining the material’s behavior when subjected to crushing loads. As a result, it is very important in part manufacturing because materials pass through different phases.

It is suitable for a wide variety of testing materials such as metals, plastics, ceramics, or other users in load-bearing capacity. Common standards for compressive testing are ASTM D3574 (flexible cellular materials) ASTM D695-15 (Rigid Plastics), AITM 0010, ASTM C109 (2-Inch Concrete Cubes), ISO 844 (Rigid Cellular Plastics).

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Creep Testing

Creep testing or stress-relaxation test involves subjecting the material to constant stress at high temperatures and recording the deformation at a specific time interval. Afterward, operators plot the creep against the time on a graph to get the creep rate (slope of the graph).

This test allows manufacturers to determine a material’s tendency to deform under constant stress at constant temperatures (to incorporate thermal expansion or shrinkage). It is important for materials such as metal workings, springs, and soldered joints.

Tests to Measure Material’s Mechanical Properties

The different forms of mechanical testing above allow manufacturers to know a material’s strength properties. However, these tests don’t show how to measure intrinsic properties such as stiffness, hardness, and corrosion resistance. There are four common mechanical properties testing.

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Impact Testing

Impact testing allows manufacturers to determine the force to break material, i.e., the impact strength of metals. There are two tests: Charpy and Izod. Each involves fracturing the material, measuring the energy that caused the fracture, and getting the material’s critical crack depth.

  • IZOD Impact Strength Test: Izod impact testing is an ASTM impact standard testing method that can test materials to a ¼ size. It involves using a raised pivoting arm to hit and break material. The energy taken to break the material is then calculated using the height.
  • Charpy Impact Test: This standard impact test can determine the energy required to fracture a material. It involves dropping a pendulum at a known height and calculating the energy from the height.   

Hardness Testing

Hardness testing allows manufacturers to know the hardness of the material, i.e., the ability of the material to resist indentation. The test is conducted only on the material. Therefore, there is no need to conduct it on the finished product.

There are several metal hardness tests. However, the most common are:

  • BRINELL HARDNESS TEST
  • ROCKWELL HARDNESS TEST
  • VICKERS HARDNESS TEST

These are explained in more detail under testing and certification.

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Corrosion resistance

Corrosion tests are accelerated tests to determine coated and uncoated metals’ reactions in saline and non-saline conditions. There are several types of tests according to international standards.

 Common tests are as follows:

  • Salt Spray Testing: This is the standard and commonest method for checking coated and non-coated materials’ corrosion resistance. It involves spraying the materials with a saltwater solution and evaluating the appearance of the oxide.  
  • CASS Exposure Testing: This is an aggressive corrosion testing for aluminium alloys and chromium plating on zinc and steel materials. It involves exposing the material to Copper Accelerated Acetic Acid Salt Spray. Furthermore, the test can be functional or aesthetical, determining the exposure time (not more than 48 hours).
  • Immersion Corrosion Testing: Immersion corrosion testing involves immersing a material in an aggressive, aqueous environment. Afterward, analytical methods are used to determine the loss in weight due to corrosion.  

Non-destructive testing

Non-destructive testing (NDT) or non-destructive examination is a set of techniques capable of evaluating a material’s property without damaging the original materials. Common NDT used in part manufacturing are:

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Acoustic Emission Testing

This passive industrial mechanical test lets you detect active cracks in a material and products. It involves passing short bursts of ultrasound through the material and products.  

Electromagnetic Testing

It involves passing an electric current or magnetic field through the material to detect the flaw, measure thickness, or identify materials.  

Leak Testing (LT)

Leak testing is a set of tests that shows the presence of cracks or any outlet that can let a product leak. It includes four major leak testing methods, bubble leak testing, pressure change testing, halogen diode testing, and mass spectrometer testing.

Structural integrity is an important part of part manufacturing for ensuring safety and economic feasibility.