Question

Set up a study about tensile testing indicating its effect on selecting materials for design applications.

Including introduction, results , conclusion and recommendations.

Answer 1

Introduction:

Tensile testing, also known as tension testing, is a fundamental material science and engineering test in which a sample is subjected to a controlled tension until failure. Properties that are directly-measured via a tensile test are ultimate tensile strength, breaking strength, maximum elongation, and reduction in area. From these measurements, the following properties can also be determined: Young's modulus, Poisson's ratio, yield strength, and strain-hardening characteristics. Uniaxial tensile testing is the most commonly used for obtaining the mechanical characteristics of isotropic materials. Some materials use biaxial tensile testing.

Tensile testing might have a variety of purposes, such as:

· Select a material or item for an application

· Predict how a material will perform in use: normal and extreme forces.

· Determine if, or verify that the requirements of a specification, regulation, or contract are met

· Decide if a new product development program is on track

· Demonstrate proof of concept

· Demonstrate the utility of a proposed patent

· Provide standard data for other scientific, engineering, and quality assurance functions

· Provide a basis for Technical communication

· Provide a technical means of comparison of several options

· Provide evidence in legal proceedings

Results:

The preparation of test specimens depends on the purposes of testing and the governing test method or specification. A tensile specimen is usually a standardized sample cross-section. It has two shoulders and a gage (section) in between. The shoulders are large so they can be readily gripped, whereas the gauge section has a smaller cross-section so that the deformation and failure can occur in this area.

The shoulders of the test specimen can be manufactured in various ways to mate to various grips in the testing machine (see the image below). Each system has advantages and disadvantages; for example, shoulders designed for serrated grips are easy and cheap to manufacture, but the alignment of the specimen is dependent on the skill of the technician. On the other hand, a pinned grip assures good alignment. Threaded shoulders and grips also assure good alignment, but the technician must know to thread each shoulder into the grip at least one diameter's length, otherwise, the threads can strip before the specimen fractures.

In large castings and forgings, it is common to add extra material, which is designed to be removed from the casting so that test specimens can be made from it. These specimens may not be an exact representation of the whole workpiece because the grain structure may be different throughout. In smaller workpieces or when critical parts of the casting must be tested, a workpiece may be sacrificed to make the test specimens. For workpieces that are machined from bar stock, the test specimen can be made from the same piece as the bar stock.

Various shoulder styles for tensile specimens. Keys A through C are for round specimens, whereas keys D and E are for flat specimens. Key:

A. A Threaded shoulder for use with a thread
B. A round shoulder for use with serrated grips
C. A butt end shoulder for use with a split collar
D. A flat shoulder for used with serrated grips

E. A flat shoulder with a through-hole for a pinned grip

Test specimen nomenclature

The repeatability of a testing machine can be found by using special test specimens meticulously made to be as similar as possible.

Conclusion:

Tensile testing is most often carried out at a material testing laboratory. The ASTM D638 is among the most common tensile testing protocols. The ASTM D638 measures plastics tensile properties including ultimate tensile strength, yield strength, elongation, and Poisson’s ratio.

The most common testing machine used in tensile testing is the universal testing machine. This type of machine has two crossheads; one is adjusted for the length of the specimen and the other is driven to apply tension to the test specimen. There are two types: hydraulic powered and electromagnetically powered machines.

The machine must have the proper capabilities for the test specimen being tested. There are four main parameters: force capacity, speed, precision, and accuracy. Force capacity refers to the fact that the machine must be able to generate enough force to fracture the specimen. The machine must be able to apply the force quickly or slowly enough to properly mimic the actual application. Finally, the machine must be able to accurately and precisely measure the gauge length and forces applied; for instance, a large machine that is designed to measure long elongations may not work with a brittle material that experiences short elongations before fracturing.

The alignment of the test specimen in the testing machine is critical, because if the specimen is misaligned, either at an angle or offset to one side, the machine will exert a bending force on the specimen. This is especially bad for brittle materials because it will dramatically skew the results. This situation can be minimized by using spherical seats or U-joints between the grips and the test machine. If the initial portion of the stress-strain curve is curved and not linear, it indicates the specimen is misaligned in the testing machine.

The strain measurements are most commonly measured with an extensometer, but strain gauges are also frequently used on small test specimen or when Poisson's ratio is being measured. Newer test machines have digital time, force, and elongation measurement systems consisting of electronic sensors connected to a data collection device (often a computer) and software to manipulate and output the data. However, analog machines continue to meet and exceed ASTM, NIST, and ASM metal tensile testing accuracy requirements, continuing to be used today.

Recommendations:

Metals

· ASTM E8/E8M-13: "Standard Test Methods for Tension Testing of Metallic Materials" (2013)

· ISO 6892-1: "Metallic materials. Tensile testing. Method of test at ambient temperature" (2009)

· ISO 6892-2: "Metallic materials. Tensile testing. Method of test at elevated temperature" (2011)

· JIS Z2241 Method of tensile test for metallic materials

· MPIF Test Standard 10: "Method for the Tensile Properties of Powder Metallurgy (PM) Materials" Standard Test Methods for Tension Testing of Metallic Materials" (2015)

Composites

· ASTM D 3039/D 3039M: "Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials"

Flexible materials

· ASTM D638 Standard Test Method for Tensile Properties of Plastics

· ASTM D828 Standard test method for tensile properties of paper and paperboard using constant-rate-of-elongation apparatus

· ASTM D882 Standard test method for tensile properties of thin plastic sheeting

· ISO 37 rubber, vulcanized or thermoplastic—determination of tensile stress-strain properties