Test pieces thickness

The time to reach maximum absorption will increase with increased test piece thickness, in a manner roughly proportional to the square of thickness. This will need to be taken into account if predictions from results on thin test pieces are applied to thick products. For organic liquids the time to equilibrium is roughly proportional to the viscosity, but the rate of transport for water is very slow and in many cases equilibrium will take months or years to achieve. 

ISO 200722 specifies a rapid plastimeter procedure using an instrument with one platen either 7.3, 10 or 14 mm diameter and the other platen of larger diameter than the first (i.e. disc type method). The size of the first platen is chosen such that the measured plasticity is between 20 and 85. The test piece is cut with a punch which will give a constant volume of 0.40 0.04 cm, the thickness being approximately 3 mm and the diameter approximately 13 mm. The test piece is pre-compressed to a thickness of 1 0.01 mm within 2 sec and heated for 15 sec. The test load of 100N is then applied for 15 sec when the test piece thickness is measured. The usual temperature of test is 100°C and the result is expressed as the thickness of the test piece at the end of the test in units of 0.01 mm and called the rapid plasticity number. The Wallace rapid plastimeter, and presumably other commercial instruments, conform to this specification but it would be sensible to check with the manufacturers. A technically identical method is given in BS 903 Part A5923. 

The hardness reading obtained is to some extent dependent on test piece thickness and, certainly, the effect will be very marked if very thin test pieces are used. The standard thickness is given as 8 to 10 mm but tests may be made on thicknesses down to 4 m. 

It has been stated previously that hardness readings are influenced by test piece thickness. Consequently, the term standard hardness refers to measurements made on standard test pieces, and measurements on non-standard test pieces are called apparent hardness. 

Generally, the apparent hardness will increase as the test piece thickness is reduced because of the effect of compression against the rigid test piece support. 

Corrections to hardness readings to compensate for test piece thickness are not entirely satisfactory because the effect varies from rubber to rubber however, typical readings are shown in Figure 8.5. Bassi et al32 have derived relationships between thickness and Shore A hardness and demonstrated their agreement with experiment. 

Figure 8-5. Effect of test piece thickness on hardness reading.

In an appendix to ISO 4662, apparatus parameters for five different test piece thicknesses are given which it is claimed result in results very near to the "standard" parameters. 

In ISO 815, two sizes of disc test piece are allowed, either 29 mm in diameter and 12.5 mm thick or 13 mm in diameter and 6.3 mm thick, the same as used for creep. The larger size is preferred for low set materials because of the greater accuracy. The measurements of test piece thickness are made using a flat foot and not with a domed foot as was earlier practice (see Section 2.1 in Chapter 7). 

In standard abrasion tests, it is usually weight loss which is the parameter measured, although in certain cases the change in test piece thickness is more convenient. Because it is the amount of material lost which matters, it is usual to convert the weight loss to volume loss by dividing by the density. The volume loss can be expressed as the loss per unit distance travelled over the abradant, per 1000 revolutions of the apparatus, or whatever. A less usual practice is to express the result as loss per unit energy consumed in... 

Electric strength is usually taken as the nominal voltage gradient (applied voltage divided by test piece thickness) at which breakdown occurs under specified conditions of test. These specified conditions of test are important as the measured electric strength is not an intrinsic property of the material but depends on test piece thickness, time of electrification and the electrode geometry, as well as on conditioning of the material. 

A more satisfactory method of measuring brittleness point, although still an arbitrary method, is that standardised in ISO 81221. A strip test piece, held at one end to form a simple cantilever, is impacted by a striker as shown in Figure 15.4. The test piece can be either a strip or a T50 dumb-bell with one tab end removed. The critical dimensions are the test piece thickness, which is given as 2 0.2 mm in each case, and the distance between the end of the grip and the point of impact of the striker. The striker radius is specified as 1.6 0.1 mm and the clearance between the striking arm and the test piece clamp is 6.4 0.3 mm. With these tolerances, the maximum surface strain in the test piece is held to almost 10% and, with the velocity of the striker controlled to between 1.8 and 2.2 m/sec, the rate of straining is constant to... 

In many cases, Q is a constant for a given gas and polymer combination but for other combinations, particularly with vapours, Q varies with, for example, test piece thickness or pressure difference. Hence, it is necessary to know the dependence of Q on all possible variables in order to characterise the permeability of the material completely. 

The preferred units for permeability coefficient are mV N"1 (m3ms lPa lm 2) but the terms permeability coefficient or permeability constant are often applied to various transmission rates using a variety of units and care must be taken to avoid confusion. Useful conversion factors are given by Yasuda and Stannett4. When the permeability coefficient is dependent on test piece thickness, it is convenient to use a transmission rate -the amount of permeant transmitted per unit time and area for a given test piece thickness - which may be in units of mV N 1 (m3s lPa lm 2). Transmission rate is almost always used in the case of vapours and often in the units g24h m 2. 

The result is always expressed as a transmission rate, not a permeability, and is, hence, dependent on test piece thickness. Generally, transmission rate is not a linear function of temperature or relative humidity and, preferably, test conditions are chosen to be as close as possible to those found in service. 

ASTM retains a constant stress method for compression set. and one is also specified for electrical mats in BS 921. In these tests the set is expressed as a simple percentage of the original test piece thickness, and thus it is not unusual to see specification limits and test results that are numerically much smaller than those obtained in an equivalent constant deformation test. Such differences can sometimes lead to confusion and indeed alarm when confronted by users much more familiar with the tests of ISO 815. 

A test more frequently found in Continental Europe than in the U.K. or the U.S.A. is the ball indentation hardness test, which is standardized in ISO 2039. Part 1 [5], also dual numbered as BS 2782. Method 365D [6]. In this a 5 mm diameter hardened steel ball is pressed into the test surface under a specified load so that the indentation is between 0.07 and 0.10 mm for method A. or between 0.15 and 0.35 mm for method B. The time of application of the load is 30 seconds, and a minimum test piece thickness of 4mm is recommended. Unlike the Shore or IRHD scales, vvherc hardness is directly related to the penetration of the indentor. the ball indentation hardness is given by ... 

One term unique to the flexural test is the stress at the conventional deflection. Generally with ductile materials the test piece does not reach a point of fracture but simply keeps bending until eventually it slips from the outer supports. The conventional deflection is defined as 1.5 times the test piece thickness, which for the standard span of 16 times the thickness equates to a strain of 3.5%. The stress at this point forms a useful, if arbitrary, charaeteristic for duetile materials, which occurs before the peak in the force-deflection curve is reached. 

Ap = pressure drop 1 = dynamic viscosity S = test piece thickness... 

The test pieces of polyarylates were obtained by injection molding using Sumitomo NESTAL injection molding machine(0.5 ounce).ASTM D790 was used for measuring flexural modulus of injection molded test pieces(thickness, 1/32 ). 

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