Manufacturers need to perform mechanical testing on a regular basis to be sure quality stays high. The right tests confirm that products and materials meet specifications and are fit for their end use.
Benefits of Understanding Mechanical Testing
Most testing is done in-house. Workers overseeing the process need to have a precise way of choosing the most suitable test. Testing only works if the test used is correctly matched to the situation.
To figure out which test should be used, you need to know:
The limits of the test methods.
The meaning of the results.
This information is also needed when working with a quotation and trying to interpret its specifications. If the testing is done by someone else, not in-house, a solid understanding of the testing lets you make an accurate assessment of the results.
Here is an overview of five of the most common types of mechanical testing.
Testing checks the mechanical properties of materials and goods. These include three primary requirements for materials properties:
- Ductility and strength
- Related hardness properties
- Fracture toughness also called impact resistance
Secondary properties include:
- Creep behavior
- Wear resistance, which is usually inferred from other tests
- Density and damping capacity, usually of concern only to the designer
Samples that are wire machined or strip, using a cross-section that is either circular or rectangular use tensile testing. The pieces being tested are screwed into jaws or gripped by them. Then they are stretched by shifting the grips away from each other at a uniform rate of speed. During these actions, the load and the grip separation is consistently measured.
Impact strength is tested using a pendulum. This strikes the piece being tested, which is a grooved, machined part. During this trial, the energy absorbed in the break is measured. There are two main types of impact strength tests. The Izod test uses ambient temperature. The Charpy test is temperature-controlled.
Hardness is not an inherent property of material. It is a value that comes from a mix of deformation and elastic behavior. A conversion is usually done between hardness and tensile strength tests.
The standard methods of testing hardness include:
- Vickers, using a microscope to measure the cavity on a polished surface.
- Brinell, which puts heavy loads on an unevenly polished surface.
- Rockwell, which pushes a sharp probe into a surface, then measures how bigger the penetration is when the pressure is increased.
- Rebound, using a ball bounced offer the surface, which shows resistance to surface deformation (usually called hardness).
- Electronic rebound, which involves the ratio of velocity from spring-driven impact to rebound speed.
- Microhardness, which uses loads of less than 1kg for Vickers or Knoop indenters.
- Scratch also called file tests, which are considered imprecise.
Tests for corrosion are usually only done if dangerous goods are being transported. The testing is also done if the material is a corrosion resistant alloy, like stainless steel.
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