In 2009, the primary metals tensile testing standards were updated to include more strict requirements for strain rates and more detailed guidance on how to achieve target strain rates. The implementation of these updates, along with how they could benefit your laboratories test regime, is commonly misunderstood.
Prior to these updates, strain rates were most commonly found in product standards within specific industries, such as Nadcap serving predominately aerospace applications.
Some metals mechanical properties will be affected by the speed of the test and are therefore ‘strain rate sensitive’. In Stress control or Extension control the overall machine stiffness will affect the speed on the specimen, which can cause differences in results. When testing strain-sensitive materials, the allowable test speeds could cause more than a 10% difference in proof stress results from testing at the slowest and fastest rate allowed in ASTM E8/E8M and ISO 6892-1.
Figure 1 shows two plots on the same material, the upper plot is at the Maximum allowable stress rate per ISO 6892-1 Method B and the lower plot is the lowest allowable stress rate per Method B.
“Method A is intended to minimize the variation of the test rates during the moment when strain rate sensitive parameters are determined and to minimize the measurement uncertainty of the test results.” (ISO 6892-1:2009)
Method A: “Two different types of strain rate control are described in this section. The first is the control of the strain rate itself, which is based on the feedback obtained from an extensometer. The second is the control of the estimated strain rate over the parallel length, which is achieved by controlling the crosshead separation rate at a velocity equal to the desired strain rate multiplied by the parallel length’.”
Whether controlling the test based off the feedback from an extensometer or using a constant crosshead separation rate, the permitted speeds are shown below, in figure 2.
Closed Loop Strain Control enables the test to be much faster in the elastic region while there is machine compliance that would not otherwise be translated into strain on the specimen if at a constant crosshead speed. For instance, if the testing machine were ‘infinitely stiff’ when the crosshead moved 1 mm, the specimen would subsequently also move by exactly 1 mm. This is not possible as there will be deflection in the crosshead, load cell, grips, and drive method, etc. The benefit of using a well-designed and stiff material testing system will mean that the effect of compliance will be lesser allowing for more accurate results and better overall control. Figure 3 (below) compares a system with zero load and a system under load with amplified deflection.
Figure 4 represents a compliant test run to ISO 6892-1 Method A in Close Loop Strain Control. The blue line symbolizes the crosshead separation rate that is changing based on the feedback from the extensometer remaining at a constant 0.00025 s−1 as can be seen by the dark red line. At 0.2% offset yield, the machine crosshead speed for this machine and specimen configuration is approximately 1.8 mm/min, which is considerably slower than the crosshead speed in the elastic region.
Alternatively, Figure 5 represents a compliant test run to ISO 6892-1 Method A in Estimated Rate. The blue line symbolizes the crosshead speed that is remaining constant while the strain rate is low until the onset of yield where the strain rate on the specimen increases while the machine speed remains constant.
The benefit of using Estimated Rate is that it is a simpler control method and can be implemented on machines that can’t achieve accurate closed loop strain control. However, this means that the test times will be increased. Figure 6 shows the same specimen being tested as in figure 5 on a ‘stiff’ testing system: one using closed loop strain control and one in estimated rate. This represents a 40% time saving per test.
As well as test times increasing, using the estimated rate does not account for machine compliance and the effect it will have on the strain rate. When testing an 80 mm Parallel Length Specimen at a required rate of 0.00025 s−1 the crosshead speed can be calculated as 1.2 mm/min irrespective of which machine being used. An Annex within ISO 6892-1 includes an additional calculation to determine the theoretical crosshead speed that takes into account stiffness of the testing system/specimen. However, it may need additional tuning. Subsequently, the set up time is greater for open loop control systems.
Method B: This is still the most commonly used control method as it was part of EN10002 where strain control was not previously mentioned. For ISO 6892-1, there are two allowable ranges depending on the Modulus of Elasticity of the Material: 1) 2-20MPa/s for material under 150GPa, and 2) 6-60MPa/s for material over 150GPa. This is a large variation that is easier to conform to; however, it can increase variation and make overall comparability less accurate, when compared with Method A.
One of the most common types of metallic products is in the form of sheet metal. With a wide range of applications varying from white goods, automotive and aerospace applications, all types of sheet metal are typically produced in high-volume. Sheet metals are often high in strength relative to their cross section area, which is desirable for some industries they serve. However, it’s important to note that formability and ductility are also crucial. Recently, steels have increased in strength in a bid to help reduce the mass of the product their application demands, most obviously for the automotive industry. With the growth of composites being used in these markets, where they have obvious strength to weight ratio advantages, metals have the advantage of being more cost effective to produce as they can be formed much faster in comparison to composite materials, such as carbon fiber. What determines metals desirable properties for deep drawing and stretch forming is their strain hardening exponent (n-value) and the plastics strain ratio (r-value).
Plastic strain ratio r is a measure of the ability of a sheet metal to resist thinning or thickening when subjected to a tensile or compressive force. It is typically advantageous if the material reduces in area a minimal amount when subject to this force, meaning a good drawing material has a high r value. Once the material is taken beyond its elastic limit, it has plastically deformed. If this flows easily in the plane of the sheet it will result in a high r-value. Alternatively, if it flows from the materials thickness it will thin during drawing, which may result in weakness. To determine this ratio, assuming constant volume, both the axial and transverse strain needs to be obtained during a uni-axial tensile test. The accuracy of the extensometry needs to be high and you can consider utilizing either a contacting extensometer, such as the AutoXBiax, or non-contacting, such as the AVE 2 extensometer, to determine the axial and transverse strain measurements required for determination of r value.
The strain hardening exponent n is a measure of the response of metallic materials after cold working. After metals have reached their elastic limit and plastically deform they experience strain hardening, which can increase the strength within the final products application. The strain hardening exponent is the slope of the plastic portion of the test after yield and before its ultimate tensile strength. Metals with a high-strain hardening exponent will achieve increasing strength with a small amount of axial strain, whereas a material with a low-strain hardening exponent will experience high axial strain with little increase in strength. To determine n-value automatically, the axial strain needs to be accurately measured using an extensometer, but does not require transverse strain measurement.
Extensometry for r and n Determination
For metals tensile applications, it is common to report the percentage total extension at fracture (At) or percentage elongation after fracture (A). Traditionally, elongation after fracture is calculated using manual specimen marking where the markings are set to a defined gauge length (G.L.) before the test starts (Fig. 1), for example 50 mm, and then measured again after the test, for example 56.25 mm. This would give a percentage elongation after fracture of 12.5%. If using a strain measuring instrument that can remain on through failure, ISO 6892-1:2009 allows the total strain measured by the extensometer to determine extension/elongation strain at and after fracture. However, to determine elongation after fracture from the extensometer, the elastic elongation needs to be deducted from the total elongation at fracture to produce a similar result to the manual method. The remaining elongation is called the Non-Proportional Elongation (NPE) and represents the elongation after fracture (A). NPE (Fig. 2) is determined by defining the modulus and creating a line parallel with the modulus. Where this line intersects with the ‘break point’ defines the NPE.
“The result of this determination is valid only if fracture and localized extension (necking) occurs within the extensometer gauge length”.1 If localized extension (or the break location) is outside of or on the extensometer knife edge, the result is invalid because the extension outside of the gauge length is not measured by the instrument. This means that the strain at break would be lower than expected. Figure 3 shows the result when one specimen fails outside of the gauge length of the extensometer or the knife edge was in the necking region. To avoid these instances of invalid tests, ISO 6892-1:2009 states: “If the extensometer is removed or if the extension measurement is interrupted before fracture, but after maximum force (Fm), then it is permitted to use the crosshead displacement to determine additional elongation between removal of the extensometer and fracture.”
Once maximum force has been achieved on the specimen, total system compliance will now be deflecting back towards the specimen, meaning the extension/elongation/strain at and after fracture would be conservative, but more repeatable between tests. Using a clip-on extensometer can be difficult and unsafe to remove after maximum force. It is safer and more reliable to use an automatic extensometer that remains on the specimen through fracture or removed after ultimate tensile strength.
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