Force Measurement Techniques

A brief description of common force measurement tests is listed below.

Tensile Test

Tensile strength is the ability of a metal to withstand a pulling apart tension stress. The tensile test is performed by applying a uni-axial load to a test bar and gradually increasing the load until it breaks. The load is then measured against the elongation using an extensometer. The tensile data may be analyzed using a stress-strain curve.

Sheer Test

Shear strength is the ability to resist a “sliding past” type of action when parallel, but slightly off-axis, forces are applied. Shear can be applied in either tension or compression.

Compression Test

Compression is the result of forces pushing towards each other. The compression test is run much like the tensile test. The specimen is placed in a testing machine, a load is applied and the deformation is recorded. A compressive stress-strain curve can be drawn from the data.

Hardness Measurement

Titration

A titration is a method of analysis that allows determination of the precise endpoint of a reaction and therefore the precise quantity of reactant in the titration flask. A buret is used to deliver the second reactant to the flask and an indicator or pH Meter is used to detect the endpoint of the reaction. Titrations are used in chemical analysis to determine the quantity of a specific chemical.

Hardness Measurement

Hardness testing (which measures the resistance of any material against penetration) is performed by creating an indentation on the surface of a material with a hard ball, a diamond pyramid or cone and then measuring the depth of penetration. Hardness testing is often categorized as a non-destructive test since the indentation is small and may not affect the future usefulness of the material. Listed in the next slides are most commonly used techniques for hardness measurements.

Rockwell Hardness Testing

The most popular and widely used of all the hardness testers is the Rockwell tester. This type of tester uses two loads to perform the actual hardness test. Rockwell machines may be manual or automatic. The Rockwell hardness value is based on the depth of penetration with the value automatically calculated and directly read off the machine scale. This eliminates any potential human error. At least three readings should be taken and the hardness value averaged. There are approximately 3'0 different Rockwell hardness scales, with the most common being the HRB and the HRC, when used in testing metals. Rockwell Superficial Hardness Testing The superficial hardness tester is used to test hard-thin materials. It tests closer to the surface and can measure case-hardened surfaces. The testing procedures are identical to regular Rockwell testing. There are approximately 15 different superficial Rockwell hardness scales.

Brinell Hardness Testing

The Brinell hardness testing method is primarily used for bulk hardness of heavy sections of softer steels and metals. Compared to other hardness tests the imprint left by the Brinell test is relatively large. This type of deformation is more conducive to testing porous materials such as castings and forgings. Thin samples cannot be tested using this method. Since a large force would be required to make a measurable dent on a very hard surface, the Brinell method is generally restricted to softer metals. The HBW (tungsten carbide ball) and HBS (steel ball) have replaced the prior BHN (Brinell Hardness Number) scale.

Vickers Hardness Testing

Vickers hardness testing uses a square-based pyramid with loads of 1 to 120 kg. The surface should be as smooth, flat and clean as possible. The test piece should be placed horizontally on the anvil before testing. The angle of the diamond penetrator should be approximately 136 degrees. Vickers hardness is also done as a micro-hardness test, with loads in the range of 25 g to 1 kg. The Vickers micro-hardness test is similar to the Knoop micro-hardness test, and is done on flat, polished surfaces. The units are HV, previously DPH (Diamond Pyramidal Hardness).

Mohs Hardness Testing

In 1824, an Austrian mineralogist by the name of F. Mohs chose ten minerals of varying hardness and developed a comparison scale. This scratch test was probably the first hardness testing method developed. It is very crude and fast, and is based on the hardness of ten minerals. The softest mineral on the MOHS scale is talc and the hardest is diamond.

File Hardness Testing

File hardness is a version of the scratch testing method where a metal sample is scraped with a 1/4" diameter double cut round file. If the file “whites” into the material, the material is “not file hard.” lf there is no mark, the material is “file hard.” This is a very easy way for inspectors to determine if the material has been treated for hardness.

Sonodur Hardness Testing Method

The Sonodur is one of the newer test methods and uses the natural resonant frequency of metal as a basis of measurement. Hardness of a material effects this frequency, and therefore, can be measured. This method is considered to be very accurate.

Shore Scleroscope Hardness Testing

The Shore Scleroscope is a dynamic hardness test that uses a material’s absorption factor and "measures the elastic resistance to penetration. It is unlike the other test methods- in that there is no penetration. In the test, a hammer is dropped and the bounce is determined to be directly proportional to the hardness of the material. The Shore method has a negligible indention on the sample surface. A variety of materials, shapes, and sizes can be tested, and the equipment is very portable.

Torque Measurement

Torque measurement is required when the product is held together by nuts and bolts. The wrong torque can result in the assembly failing due to a number of problems. Parts may not be assembled securely enough for the unit to function properly or threads may be stripped because the torque was too high, causing the unit to fail. Torque is described as a force producing rotation about an axis. The formula for torque is:

Torque = Force x Distance

Torque is measured by a torque wrench. There are many types of torque wrenches. There are many types of torque wrenches. However, the two types most commonly used are the flexible beam" type, and the rigid frame type. Torque wrenches may be preset to the desired torque. The wrench will either make a distinct “clicking” sound or “slip” when the desired torque is achieved.

The Steel Rule

The steel rule is a widely used factory measuring tool for direct length measurement. Steel rules and tapes are available in different degrees of accuracy and are typically graduated on both edges.