Tensile or shear? Load cells for materials testing
Every manufactured product will require testing at some point in its creation, not just for quality but to assess its characteristics. Load cells are at the heart of materials testing techniques, so here is our Basics 101 on some of the various types of materials tests.
Tensile testing
Tensile testing, or tension testing, is a fundamental mechanical test, and involves pulling the material to be tested. Tensile testing will include checking the tensile strength. This is the force required to pull a material such as wire, rope, or even a roof beam to the point where the material breaks. The tensile strength is defined by the maximum amount of tensile stress the material can withstand before it fails completely. The point just before failure defines the material’s Ultimate Strength, or UTS. Tensile testing is used on a huge variety of items from packaging to fasteners.
Compression / compressive testing
Compression testing assesses the opposite to tensile testing. It tests how much force a material can take when compressed (squashed) before it loses its structure. Top load testing is used to assess how well a material withstands a force applied on to of it, and hence is used extensively in packaging testing.
Peel testing
Much of our modern world is created from composite materials, from cars to buildings. Peel testing tests the strength of the bond between the different types of materials involved, whether it be laminated wood for furniture to ‘easy peel’ resealable tabs on packets. The peel strength is the average load per unit width of bond line required to part bonded materials where the angle of separation is 180 degrees. This gives manufacturers a measurement of the adhesive strength properties for coatings and glues, and establishes their peak holding strength. So next time you wrestle to open a crisp packet, you’ll know what its peel strength is!
Shear testing
A whole generation of us probably studied shears as part of our maths education, without ever realising how much shear tests shape consumer products! Shear testing measures the shear strength of almost any material, from cosmetics to composite materials. The definition of a shear is that when the tested material breaks, the pieces are completely clear of each other. Hence, a failed bolt “shears off”. As Corrosionpedia says:
“Shear strength is a material’s ability to resist forces that can cause the internal structure of the material to slide against itself.”
Creep and relaxation testing
Many mattresses these have a topping of memory foam these days, designed specifically to mould to your body shape as you move during the night, and then rise back to level once you get up. The foam’s return to its normal state after “deformation under an applied load” (i.e. you lying on it!) can be measured using a relaxation test. Hence the alternative term, stress relaxation.
Creep testing involves subjecting the material to prolonged, constant tension or compression loading, and measurements taken at specific time intervals.
Creep can be divided into three stages:
- First stage / primary creep; this starts off rapidly, then slows.
- Second stage / secondary creep: this deforms at a uniform rate.
- Third stage / tertiary creep: the rate of creep accelerates until the material fails
Flexural testing
This type of testing assesses how much a material can flex and still retain its stiffness or integrity. Materials are often tested using a 3 point or 4 point method to check their resilience and stiffness. The test provides information on the combined effect of a material’s tensile, compressive and shear characteristics, and is often used to test polymers, papers, woods and composites, and brittle materials.
Coefficient of friction testing
In laymen’s terms, this is a test of how slippery a material is! It can be used to test how slippery flooring becomes under different conditions, such as wet or dry, or in extremes of temperature. It can also be used to measure the static and kinetic friction characteristics of plastic films and sheeting. Yes, that includes cling film…
Contact closure testing
When did you last press a button on your keyboard or flick a switch? Contact closure testing measures the force required for optimal contact closure on any sort of electrical button. As anyone waiting for a pedestrian crossing light or a lift in a building will witness, the pressure we apply to a button varies enormously! For buttons on car dashboards, for example, the loads may be very low and the button displacement a matter of millimetres. For larger buttons, the force needed to complete the contact might be far greater. Contact closure testing ensures that a button can do what is required with a light touch or a heavier hand.
Insertion/extraction testing
Alongside buttons, we also spend hours of our lives plugging and unplugging devices, from wall power sockets, USB ports, headphone sockets, you name it. Insertion/extraction testing ensures that the plug, jacks and adapters can take the strain in both directions – whether being pulled out or pushed in (see also tensile and compression testing).
Not sure which load cell you require?
Call us. We supply load cells for a range of commercially available testing machines and jigs, as well as for self-build equipment. We can supply a whole range of high quality, accurate, robust and reliable load cells ‘off the shelf’, or can design a special load cell just for your application.
