Metal tensile test

Yield criteria are mathematical tools to decide whether the stress state in a material will cause plastic deformation or not. In a polycrystalline metallic tensile test specimen, which can be assumed to be isotropic and uniaxially loaded, the material yields at a stress ... 

Slides Slab and sheet metal rolling extrusion, etc., of polymers tensile-testing machines hardness-testing machines hardness indentations. 

The following set of data represents the outeome of a tensile test experiment to determine the yield strength in MPa of a metal. There are 50 individual results and they are displayed in the order they were reeorded. It is required to find the mean and standard deviation when the data is represented by a histogram. It is also required to find the strength at —3cr from the mean for the metal and the proportion of individuals that eould be expeeted to have a strength greater than 500 MPa. 

Couplings for grades J-55, K-55 and N-80 shall conform to the tensile requirements specified in Table 4-142. A tensile test shall be made on each heat of steel from which couplings are produced, and the coupling manufacturer shall maintain a record of such tests. This record shall be open to inspection by the purchaser. Either round specimens proportioned as specified in ASTME 8 Tension Testing of Metallic Materials, or strip specimens shall be used, at the option of the manufacturer. 

The strength and adhesion of sprayed metal coatings are extremely difficult to measure with precision, and the properties of sprayed metals vary greatly with the spraying conditions and with the conditions of test. It is difficult, therefore, to correlate the values taken from the literature on the subject. For instance, American workers produce tensile test pieces by depositing on to 9-5 mm (0-375 in) steel tube and then machining out the tube. By this method the results shown in Table 12.7 were obtained. 

A fairly direct way of observing galvanic effects, which also permits changes in mechanical properties to be measured, involves the preparation of a composite specimen formed by attaching a strip, or strips, of one metal to a panel of another one. Tensile test specimens that include the areas of galvanic action can be cut from these panels after exposure, as shown in Fig. 19.30. 

The concept of a ductile-to-brittle transition temperature in plastics is likewise well known in metals, notched metal products being more prone to brittle failure than unnotched specimens. Of course there are major differences, such as the short time moduli of many plastics compared with those in steel, that may be 30 x 106 psi (207 x 106 kPa). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck. Tliese different engineering characteristics also have important effects on certain aspects of impact resistance. 

Just as metals can be ductile or brittle, so can organic materials. The Brittle Fracture Index is a measure of the brittleness of a material. It is a measure of the ability of a compact of material to relieve stress by plastic deformation. The Brittle Fracture Index (BFI) is determined [29,31] by comparing the tensile strength of a compact, stress concentrator) in it, o-T0, using the tensile test we have described. A hole in the center of the compact generally weakens a tablet. If a material is very brittle, theoretical considerations show that the tensile strength of a tablet with a hole in it will be about one-third that of a solid tablet. If, however, the material can relieve stress by plastic deformation, then the strength of the compact with a hole in it will approach that of a compact with no hole. The Brittle Fracture... 

Tensile testing or a hardness test is a basic requirement of most metal specifications. Some product specifications also require impact-testing, bend and other ductility tests, proof testing, flange or flare tests. The size of the sample may limit which tests can be performed. Macro, superficial, and micro-hardness tests are routinely done in failure analysis even if the original product specification did not require them. 

We shall try to explain this on the basis of the thermoplastic PVC which is completely amorphous. The polymers chain form a tangle and each chain has the well-known zigzag shape of the carbon chain (figure 10.7). In figure 10.8 you can see the a- curve of the PVC tensile test. Plastic tensile test specimens are flat and those of metals are often cylindrical. 

When a metal specimen is subjected to the tensile test, the micropores around the centre of the dumbbell start to grow and combine. As soon as the metal wall which is initially still intact is no longer able to withstand the stress, the object tears. Apart from the above-mentioned growth of the pores, sliding processes in the crystal lattice also play a part in the formation of the fracture. 

Adhesion of fired enamel to the metal base is assessed practically by impact tests (e.g. by a falling steel ball). If the adhesion is poor, tfie enamel will peel off on impact, showing a clean metal surface. More precise determinations are based on tensile tests of metal specimens joined with an enamel layer. Very good adhesion is then indicated by failure in the enamel and not in the contact surface. 

Figure 7. Tensile test curves recorded for 99.99% pure A1 foams with pores 40, 75 and 400 pm in diameter. The volume fraction of metal in the foams is near 30% for all specimens.

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