TENSILE SAMPLES

23.0. TENSILE SAMPLES

Tensile test specimens are prepared in a variety of ways depending on the test specifications. The most commonly used specifications are BS EN ISO 6892-1 and ASTM E8M. Most specimens use both a round or square standard cross section with two shoulders and a reduced section gauge length in between. The shoulders allow the specimen to be gripped while the gauge length shows the deformation and failure in the elastic region as it is stretched under load. The reduced cross section gauge length of specific dimensions assists with accurate calculation of engineering stress via load over area calculation. 

The preparation of test specimens depends on the purposes of testing and on 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 exact representation of the whole work piece because the grain structure may be different throughout. In smaller work pieces or when critical parts of the casting must be tested, a work piece may be sacrificed to make the test specimens. For work pieces that are machined from bar stock, the test specimen can be made from the same piece as the bar stock.

For soft and porous materials, like electro spun nonwovens made of Nano fibers, the specimen is usually a sample strip supported by a paper frame to favor its mounting on the machine and to avoid membrane damaging.

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

TENSILE TESTING

As mentioned earlier the tensile test is used to provide information that will be used in design calculations or to demonstrate that a material complies with the requirements of the appropriate specification - it may therefore be either a quantitative or a qualitative test. The test is made by gripping the ends of a suitably prepared standardized test piece in a tensile test machine and then applying a continually increasing uni-axial load until such time as failure occurs. Test pieces are standardized in order that results are reproducible and comparable as shown in Fig 2.

Fig.2. Standard shape tensile specimens

Specimens are said to be proportional when the gauge length, L0, is related to the original cross sectional area, A0, expressed as L0 =k√A0. The constant k is 5.65 in EN specifications and 5 in the ASME codes. These give gauge lengths of approximately 5x specimen diameter and 4 x specimen diameters respectively - whilst this difference may not be technically significant it is important when claiming compliance with specifications.

Fig.3. Stress/strain curve

Both the load (stress) and the test piece extension (strain) are measured and from this data an engineering stress/strain curve is constructed, Fig.3. From this curve we can determine:

a) the tensile strength, also known as the ultimate tensile strength, the load at failure divided by the original cross sectional area where the ultimate tensile strength (U.T.S.), σmax = Pmax /A0 , where Pmax = maximum load, A0 = original cross sectional area. In EN specifications this parameter is also identified as 'Rm';

b) the yield point (YP), the stress at which deformation changes from elastic to plastic behaviour ie below the yield point unloading the specimen means that it returns to its original length, above the yield point permanent plastic deformation has occurred, YP or σy = Pyp /A0 where Pyp = load at the yield point. In EN specifications this parameter is also identified as 'Re ';

c) By reassembling the broken specimen we can also measure the percentage elongation, El% how much the test piece had stretched at failure where El% = (Lf - L0 /Lo ) x100 where Lf = gauge length at fracture and L0 = original gauge length. In EN specifications this parameter is also identified as 'A' ( Fig.4a).

d) the percentage reduction of area, how much the specimen has necked or reduced in diameter at the point of failure where R of A% =(A0 - Af /A0 ) x 100 where Af = cross sectional area at site of the fracture. In EN specifications this parameter is also identified as 'Z', (Fig.4b).

Fig.4: a) Calculation of percentage elongation, b) Calculation of percentage reduction of area

(a) And (b) are measures of the strength of the material, (c) and (d) indicate the ductility or ability of the material to deform without fracture.

The slope of the elastic portion of the curve, essentially a straight line, will give Young's Modulus of Elasticity, a measure of how much a structure will elastically deform when loaded. A low modulus means that a structure will be flexible, a high modulus a structure that will be stiff and inflexible. To produce the most accurate stress/strain curve an extensometer should be attached to the specimen to measure the elongation of the gauge length. A less accurate method is to measure the movement of the cross-head of the tensile machine.

The stress strain curve in Fig.3 shows a material that has a well pronounced yield point but only annealed carbon steel exhibits this sort of behavior. Metals that are strengthened by alloying, by heat treatment or by cold working do not have a pronounced yield and some other method must be found to determine the 'yield point'. This is done by measuring the proof stress (offset yield strength in American terminology), the stress required to produce a small specified amount of plastic deformation in the test piece. The proof stress is measured by drawing a line parallel to the elastic portion of the stress/strain curve at a specified strain, this strain being a percentage of the original gauge length, hence 0.2% proof, 1% proof (see Fig.5).

Fig.5. Determination of proof (offset yield) strength yield) 

For example, 0.2% proof strength would be measured using 0.2mm of permanent deformation in a specimen with a gauge length of 100mm. Proof strength is therefore not a fixed material characteristic, such as the yield point, but will depend upon how much plastic deformation is specified. It is essential therefore when considering proof strengths that the percentage figure is always quoted. Most steel specifications use 0.2% deformation, RP0.2 in the EN specifications. Some materials such as annealed copper, grey iron and plastics do not have a straight line elastic portion on the stress/strain curve. In this case the usual practice, analogous to the method of determining proof strength, is to define the 'yield strength' as the stress to produce a specified amount of permanent deformation.