Tensile machines

Specimens used in tests were sections of cables with PVC outer coating. PVC was plasticized with DOF softener. The materials considered were exposed to the radiation and thermal aging. The samples have been irradiated at room temperature by hard gamma rays with 10 rad/sec dose power. A number of samples had been heated for long different times at 90°C. Besides a special specimens were cut out from outer coating for test on tensile machine like "Instron". The total doses of irradiation, times of heating and elongations at break obtained with "Instron" are listed in Table 1. 

The mechanical properties were obtained using a tensile machine at room temperature and for a strain rate of 1000%/h. Each reported value of the modulus was an average of five tests. The tensile modulus Et was taken as the slope of the initial straight line portion of the stress-strain curve. 

Specimens were elongated the indicated amount, then released from the grips of the tensile machine, cut parallel to the draw direction, and placed in NMR tubes. From the derivative NMR spectra, the second moments of die orientation distribution were measured at temperatures of 158 and 345°K. 

Whilst all aspects of a laboratory s operation require systematic control, it is the calibration of test equipment which gives rise to most problems and which is also the most expensive. All test equipment and every parameter of each instrument requires formal calibration. For example, it is not good enough to calibrate the force scale of a tensile machine, there are also requirements for speed of traverse, etc. plus associated cutting dies and dial gauges. 

The automation of sample handling has not taken off as some predicted in the nineteen sixties when the first automatic systems were developed for tensile machines and hardness and density apparatus. Robots are rare alongside the test rig and the reason is doubtless to do with volume, as such... 

Sample preparation and measuring apparatus has been discussed and hence this section will deal with the principal item, the tensile machine, together with grips and extensometer. The tensile machine is in fact very often a universal machine in that, apart from tensile tests, it can also be used for flexural, compression, tear and adhesion tests. The basic elements of a tensile machine are shown diagrammatically in Figure 8.10. 

ISO 37 refers to ISO 584373 for specification of the test machine. This standard was produced to avoid the need to attempt description of a complex engineering instrument in a testing standard. It is intended that all test methods using a tensile machine will refer to this document, which specifies requirements quite comprehensively, including tolerances on the measurement of force and extension. 

The compression machine is specified by reference to ISO 5893 grade 0.5 One suspects that this is an error as the 1% accuracy of grade 1 is more reasonable. The usual practice is to use a universal tensile machine in compression mode with autographic recording of force and deflection. If this is the case, care must be taken that the machine is sufficiently stiff such that the deflection reading is not significantly affected. The deflection measurement is to conform to class C of ISO 5893 which at 2% makes the force requirement overkill. 

The test piece assembly is strained in a tensile machine at 5 1 mm/min until a maximum shear strain of 30 % is reached. Mechanical conditioning is optional but, if used, five conditioning cycles are applied before the measuring cycle. No details of apparatus to measure the strain are given but this could be a dial gauge or, with a stiff tensile machine, the crosshead movement. 

The "original" method, the H-pull or "H-block" test, was first standardised by ASTM together with a variant, the "U" test. In the former, two blocks of rubber are gripped in the tensile machine and in the latter a loop of cord is hooked onto one of the grips. 

As the samples are dumbbell shaped, the movement of the crosshead of the tensile machine cannot be used to determine the elongation. Extensometers of various designs are used to measure the elongation in the waist area of the sample. One method is to have two clamps set to the desired distance apart. As the sample stretches, the movement is measured by a system of counterweights and cords. Another method is to use light/laser beams reflecting from pieces of reflective tape on the sample. 

The ISO 1827 method has four blocks 4 mm thick by 20 mm wide and 25 mm long bonded to plates as shown in the sketch (Figure 9.5). The samples are cycled five times in a tensile machine before the results are taken. A micrometer or the movement of the crosshead can be used to measure the strain. The speed of the machine is 25 0.5 mm per minute. 

The shear stress results are expressed in Pascals (N/m2). The calculation is obtained by dividing the applied force indicated by the tensile machine by twice the actual bonded area of one block in square meters (nominally 20 x 25 x 10 The shear strain is obtained by dividing half the actual deformation by the actual thickness of one block. All measurements must be in the same units. 

UV exposed samples were tested on an Instron tensile machine and tensile strength at yield was recorded. Failure time for exposed samples was loss of 50% of original tensile strength. 

An Instron tensile machine can be used to measure stress and strain in the automotive lab. However, incoming inspection at a supplier is generally where this type of analysis takes place. More often, an automotive chemist at an OEM will be required to analyze a sample for failure analysis of that particular part. A thermogravimetric analyzer (TGA) will measure the change in mass of an elastomer in an inert (nitrogen) atmosphere as the temperature is waived. These instruments are very precise and will give information such as degradation temperature, filler content solvent residue, and absorbed moisture content. 

Treatment of steel with calcium enhances, for example, the mechanical properties such as formability, drawing, impact, tensile, machinability, resistance to cracking and tearing and leads to an improved surface and internal cleanliness calcium alloys are used to deoxidize magnesium, to strengthen electrodes, and to produce special aluminum alloys, etc. 

The tensile machine (or universal test machine used in the tensile mode) is essentially the same for all materials but clearly varies in capacity. The basic elements are grips to hold the test piece, a means of applying a strain (or stress), a force-measuring element, and an extensometer. A specification for machines suitable for rubbers and plastics is given in ISO 5893 [7], and the content largely applies to other materials. This standard was produced with the intention that test method standards would refer to it and hence obviate the need to include a description of a complex engineering instrument. 

Measuring stress-strain characteristics in compression is clearly appropriate when the product is to be stressed in that manner in service. There are many such applications for rubbers, and yet tests in compression are less common than in tension, which is particularly surprising as in many ways they are easier to carry out. Probably the lack of compression facilities and accurate measurement of cross-head movement on older tensile machines was a factor. 

The usual approach is to use a tensile" or universal testing machine that is equipped with a compression load cell. The compression takes place between parallel steel plates, which may incorporate a self-alignment mechanism. Rather than a universal machine, an apparatus working in compression only is equally satisfactory if the volume of work justifies a dedicated apparatus. An alternative is to use a compression cage in a tensile machine effectively to reverse the motion of the machine. However, these are now seldom seen, as they can introduce considerable friction errors, and there is always difficulty in alignment. As will be appreciated, for stiff materials the forces and hence machine capacity required can be large. 

Most of the normal tear tests are carried out using a tensile machine with suitable grips. The tearing force can rise rapidly, and in trouser type tests it can rise and fall during the... 

The force measuring head, together with provision for applying the extra compression. can be a beam balance, as used in the well-known Lucas apparatu.s. a universal tensile machine, or a specially designed electronic load cell unit [4]. The point at which the small amount of extra compression has been applied can be detected by breaking an electrical circuit. An early apparatus used a load cell attached to an arbor pre,ss. The operator manually lowered the pre.ss until the break in the electrical circuit was indicated visually by the extinction of a light. The Lucas apparatus has a similar detection system the balance weights are adjusted manually until the force exerted by the beam just overcomes the force exerted by the test piece. Both of these approaches involve a somewhat... 

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