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- Lisa Novotny By

Plotting tensile stress-strain curves depicts the entire behavior of a sample as it is loaded in tension from the onset of the load to the final failure of the sample.

The sample stress (psi MPa) is equal to the load divided by the surface area to which the load is applied; sample strain is equal to the change in the sample’s length divided by the original length. Initially the loading of the sample takes place in the linear elastic region of the curve in which the sample behaves according to Hooke’s law where the ratio of stress to strain remains constant. In this region of loading the information needed to determine both the modulus of elasticity and rigidity as well as Poisson’s ratio is contained. Once the material reaches its yield point at which the yield limit and strength may be determined it will no longer return to its original shape when unloaded and switches from the elastic deformation region to the plastic deformation region of loading. This region contains the information needed to determine the ultimate tensile strength of the material. After this the material quickly enters the region of necking where the gage length begins to narrow and then finally fractures at which the test is ended.

Depending on the whether or not the material behaves in a brittle or ductile manner the fracturing may occur very quickly without much necking or a relatively long period of time in which noticeable necking occurs.

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In the case of tensile tests, the test machine exerts a tension load or force which pulls tensile test samples apart. In the case of plastics tensile testing, the test sample is pulled apart to measure tensile strength and other properties including stiffness and yield strength. There are several common industry standards that provide agreed upon methods of plastics tensile tests. ASTM D638 and ISO 527-2 both feature similar but different standardized test sample geometry and dimensions. These tests require tensile grips that are expected to grab the sample and to adjust as it thins out during the test process. These accessories are different than compression fixtures.

In compression tests, the test machine exerts a pushing or compressive load or force to squish the test sample until it breaks or squishes. Compression tests of a polymer structural foam material is covered by ASTM D1621 which specifies the type of compression plates and deflectometer used. The test sample is placed between compression test platens until the cellular structure fails or ruptures.

A universal test machine can perform either or both tension and compression tests. The crosshead can be used to pull or compress the test sample which is located between the baseplate and the moving head.

The tensile test fixtures, or grips, and strain sensors (known as extensometer), cannot perform compression tests. Also the tensile grips are specially matched to the cover the exact test specimen geometry and dimensions. The compression test platens and deflectometer are also capable of only performing a compression test, and so both sets of accessories are needed in this case.

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Tensile testing a material determines mechanical properties including ultimate tensile strength, yield strength, yield limit, yield point, elastic limit, proportional limit, Poisson’s ratio, modulus of elasticity and the modulus of rigidity.

Ultimate tensile strength is commonly known as tensile strength. UTS is the maximum stress (psi or MPa) that a material withstands when tested in tension.

Yield strength, yield limit and yield point are all used to determine the point during the test where the deformation experienced by the material changes from elastic to plastic and the relationship between the stress and strain is no longer linear.

Elastic limit is the absolute largest amount of stress and strain that a material can withstand before permanent deformation occurs and the proportional limit is the absolute highest stress level at which the strain and stress are proportionally related.

Poisson’s ratio is the negative ratio of the lateral and axial strain and is used to relate the modulus of elasticity and the modulus of rigidity, which may be defined as the ratio of the stress over the strain in the elastic region and the ratio of the shear stress and shear strain, respectively.

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No. Three point test fixtures and four point test fixtures are used in all industry standard methods that relate to flexure and bend testing. Most standards specify one or the other and a few specify both. “Third point” refers to a special configuration of a bend test that employs four points.

There are two types of four point loading configurations. ….

Because it is only the outermost fibers of the specimen that determine the parameters of the test stress concentration at the loading points is of concern.

The four point quarter point loading provide equal but the highest loading of the three methods. Four point third point loading provides equal and lower loading.

The three point loading has the lowest loading at the two points. If the maximum force of the machine is a concern note the Four point quarter point machine requires 2 times the applied load of the three point fixating.

Also because of the higher loads four point fixtures need to be designed to handle the higher loads.

Three point testing the peak stress is at a maximum and tapers off to the ends so it tests a more localized spot on the specimen. This can be useful for component testing such a PC boards where you want to look at point where a component is installed.

Four point testing produces a wide area under uniform maximum stress exposing a greater surface area to failure from defects or stress concentrations.

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