Difference Between ISO and ASTM Standards

Materials Testing Zwick GmbH & Co. KG August-Nagel-Strasse 11, 89079 Ulm, Germany

ISO 527 and ASTM D638 – explaining the difference As the manufacturing of products and components spreads around the world, one of the challenges facing business today is to compare mechanical test results which are carried out in different locations. Nowhere is this problem more evident than in the tensile testing of plastics. There are two main procedures which are in general use today; ISO 527, and ASTM D638. In this article we discuss some of the major differences between these two standards and explain why there can be significant differences in the test results. Although there are three main factors which influence the test results, in this article we will focus on the problems associated with standardising the test conditions.

Characteristics of the moulding material + Manufacturing process of the test specimen + Test conditions = Test Results

Pre-stressing the test specimen (see Fig.1) The act of gripping the specimen in the grips of the tensile testing machine can affect the test results. ISO 527-1 quantifies this in section 9.4 stating that the specimen must not be pre-stressed substantially prior to testing. These pre-stresses can be induced during centering of the specimen in the grips, or the clamping pressure applied during the gripping process. The ISO standard states that when it is required to measure the modulus of the material, the residual force at the start of the test (pre-stress) must induce a strain value of less than 0.05%, and that the prestress must be less than 1% of the stress results to be measured. Using an automatic drive control system can ensure that the testing machine removes these forces from the sample during the clamping process. The ASTM Standard does not define this pre-stress. The test can be started immediately after placing the specimen in the grips. Since the specimen does not have a defined status of stress at this point in time, a correction must be used in order to determine the extension zero point ε0 (Toe Compensation - see Fig. 2.) The corrected strain zero point is determined by applying a tangent to the point of the maximum slope. If the material displays a linear slope in the stress-strain diagram, the straight line should be applied to this area. The point where this crosses the strain axis corresponds to the corrected zero point. If the material does not show a linear area, the straight line is applied, as a tangent, to the point of maximum slope.

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Fig. 1 - Pre-stressing the test specimen

Fig. 2 – ASTM Strain zero point

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Modulus of elasticity (Young’s Modulus, tensile Modulus) One of the most critical parts of ISO 527 and one of the biggest differences when comparing ISO to ASTM D638 is the tensile modulus measurement. The ISO standard precisely defines the range on the stress strain curve over which the modulus of elasticity is measured in section 4.6:

Where: • Et is the Young’s Modulus of Elasticity expressed in MegaPascals (MPA). • σ1 is the stress in MegaPascals, measured at a strain value of ε1 = 0.0005. • σ2 is the stress in MegaPascals, measured at a strain value of ε2 = 0.0025. If a computer is used to determine the Et value, a linear regression algorithm shall be used between ε1 and ε2. The measurement of the tensile modulus requires an extremely accurate strain measuring system. The following graph (fig. 3) uses the formula given above for the tensile modulus and translates this into basic units for different specimen gauge lengths (L0). For tensile modulus according to ISO 527-1 it is normal to use specimen type 1A or 1B. This test requires an extensometer with a gauge length of 50mm and an accuracy of 1% to be used. As seen from the graph this means that a difference in extension of 100µm has to be measured with an accuracy of 1% (1µm). The minimum requirement for resolution of such an extensometer would be 0.5µm. (or better as stated in ISO 9513, calibration of extensometers in uniaxial testing). If one considers smaller specimens and therefore smaller gauge lengths then the accuracy requirements for the extensometer become even more difficult.

Fig. 3 – Calculation of Young’s Modulus to ISO.

In the ASTM Standard there are no fixed strain limits for the determination of modulus. However, the Standard differentiates between a modulus of elasticity, which is determined in the linear area of the stress-strain diagram, and a secant modulus. See Figure 4 Fig. 4 – Secant Modulus according to ASTM

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Materials Testing Zwick GmbH & Co. KG August-Nagel-Strasse 11, 89079 Ulm, Germany

Modulus of elasticity: For materials which display a linear portion, the modulus of elasticity is the steepest gradient of the linear region of the test curve. This is the preferred method. Secant modulus: If no linear region can be determined, a secant modulus can be determined between the extension zero point (ε0), and a freely selected point on the stress-strain curve. It is not allowed to declare the tangent slope, or toe compensation line, used for determining the zero point, as the secant modulus. Summary For materials which have no linear region then the variation between tests carried out to ISO or ASTM Standards can be significant. Testing speeds While slower test speeds are defined in the ISO Standard for determination of the tensile modulus, the ASTM allows the same speeds to be used throughout the test. The only limitation is given by the maximum data acquisition rate of the measurement system being used. See later reference to required measurement frequency of the measurement electronics’. Force Measurement Range This parameter is one often overlooked when choosing a suitable load cell for the tensile test. Both ISO and ASTM require that the testing machine can read forces during the test with an accuracy of 1% of reading. Refer to ISO 7500 and ASTM E4 for more details. Note that some manufacturers refer to load cell accuracy in percent of f.s.d. (full scale) which is not the same. The challenge posed by this requirement is that the load cell Fig. 5 – Minimum force measurement must be capable of accurately measuring the forces used to points determine the tensile modulus. Figure 5 illustrates that, assuming the cross-section of the material being tested is 40mm2, then the minimum force required to measure the tensile modulus on a material having a modulus of 1000 MPa is 20N,. Machines using a 10 kN loadcell are typical in plastic testing. Such a load cell must be able to measure forces from 10kN down to 20N (0.2% of load cell capacity) with an accuracy of 1%.

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Materials Testing Zwick GmbH & Co. KG August-Nagel-Strasse 11, 89079 Ulm, Germany

Required measurement frequency of the measurement electronics Even if the extensometer has a sufficient resolution, this can be limited at higher test speeds by the measuring Measurement frequency required: frequency of the testing machine’s measurement f [Hz] = v * L0 / (L * r * 60) [Hz] electronics. The term, measurement frequency, is where: understood here as the transfer of complete f = Measurement frequency required, Hz measurement points to the evaluation software. In v = Speed of the pulling grip, mm order to acquire all measurement points within the L0 = Original gauge length, mm required test value resolution, the test electronics must L = distance between grips, mm have a measuring frequency [f] which can be r = minimum resolution of the determined as follows: Example for ISO: Test speed Gauge length Distance between grips Required resolution Measurement frequency required

= = = = =

1 mm/min 50 mm (Specimen 1A or 1B) 115 mm 0.5 µm 14.5 [Hz]

Example for ASTM: Test speed Gauge length Distance between grips Required resolution Measurement frequency required

= = = = =

50 mm/min 50 mm (Specimen 1) 115 mm 5 µm (half measurement uncertainty) 72.5 [Hz]

Measurement of Strain ASTM D 638 states that strain results measured beyond yield have little value and are only suitable for qualitative assessments. In ISO 527-1 however, there are no directly measured extensions beyond the yield point. Instead, the term “nominal strain” has been introduced. Nominal strain =

Increase of the distance between grips Initial distance between grips

Nominal strain is a value which allows the assessment of the extension behaviour of plastics beyond the yield point. There is no constant relationship between nominal strain and direct strain measurement, therefore, the values are not comparable as shown in the curves in Fig. 6. Both curves were from the same test specimen. Unfortunately, nominal strain is often wrongly interpreted as switching from direct extension measurement (with extensometer), to crosshead travel measurement. This misunderstanding leads to variations and as a result the test results are not comparable. In order to determine the nominal strain and direct strain measurement correctly, a testing machine must be equipped with two independent strain measurement channels. Only in this manner can the nominal strain be correctly determined with extension starting from the zero point.

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Materials Testing Zwick GmbH & Co. KG August-Nagel-Strasse 11, 89079 Ulm, Germany

According to ISO, the individual strain results must be assigned to the direct extension measurement channel (ε) up to the yield point, and to the nominal strain (εt) channel beyond the yield point. From the curves in fig. 6 it is clear that there can be large deviations between laboratories who misinterpret these statements in the Standard.

Fig. 6 – Stress-strain curve with direct extension measurement

Stress Strain curve with Nominal Strain.

Measurement of the yield point As mentioned previously, the ISO Standard specifies that the extensions measured up to yield point shall be direct extensions derived from an extensometer. For most thermoplastics the range up to yield point is approx 5% to 25% strain. Care must be taken to ensure that the extensometry used is capable of achieving the desired accuracy. Figure 7 shows the requirements for extensometers for both Standards Even before investigating the merits of ISO and ASTM testing standards there are effects which can influence the Figure 7: Requirements for extensometers quality of the test results which obtained in individual laboratories. For example with thermoplastic test specimens, the variation in temperature of the operators hand holding the specimen while loading it into the specimen grips of the tensile tester can lead to large differences in test results. One solution to this is the automation of the testing system which includes robotic handling of the test specimen. This has the added advantage that the specimen is always inserted into the testing machine in perfect alignment and axiality. This document is a partial excerpt from technical documentation produced by the plastics industry experts in the Zwick Roell Group. For more information on the points in this paper and additional facts appertaining to the differences between ASTM and ISO please contact us [email protected]. Zwick GmbH & Co. KG August-Nagel-Strasse 11, 89079 Ulm, Germany

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