To see FAQs about field testing click here.
1. Can I get a stress-strain curve from a linear geometry indenter?
2. What's the difference between ABI and IIT?
4. What's the minimum and the maximum size of the ball indenter?
7. How do you determine the young's modulus of the sample?
10. Is the temperature controller included in the system?
14. Do you have an option to use a faster data acquisition card?
1. Can I get a stress-strain curve from a linear geometry indenter?
Linear geometry indenters can only produce hardness versus depth, and you get one value of strain regardless of increasing depth. Nonlinear geometry (Ball) indenters produce increasing values of strain and stress as depth increases. In order to obtain macroscopic properties, such as a stress-strain curve, the indenter size should be several times the grain size of the material. top
2. What's the difference between ABI and IIT?
Automated Ball IndentationTM (ABI) is a specific indentation test that uses a ball indenter and the patented partial unloading technique to measure the mechanical properties of metals. Instrumented Indentation Testing (IIT) is a general term for depth-sensing indentation or nanoindenation using a Berkovich pyramidal indenter and is used to examine thin films and coatings. Table with detailed description of each term. top
3. Can I buy an instrumented hardness tester, use a ball indenter, and write my own software to produce stress-strain curves?
ATC owns the patent for the SSM hardware; modifications to any other equipment may be patent infringement. ATC has over two decades of experience and has spent millions of dollars on research. We and other SSM owners have produced thousands of valid ABI tests. top
4. What's the minimum and the maximum size of the ball indenter?
The standard tungsten carbide or silicon nitride indenters are offered in the following sizes: 0.25-mm (0.010 inch) diameter, 0.51-mm (0.020 in) diameter, 0.76-mm (0.030 in) diameter, and 1.57-mm (0.062 in) diameter. The minimum and maximum size indenters are 0.051-mm radius and 7.50-mm radius. It should be noted that the grain size of the metallic test material will impose limitations on the choice of a suitable small size indenter in order to obtain macroscopic stress-strain curve (e.g. at a very small indentation depth, a 0.25-mm diameter indenter covers only three grains of a carbon steel material with a 20 microns of a meter grain size). Also, large size indenters will require higher indentation loads. For the same maximum indentation strain (or approximately the same percentage of indenter radius pushed into the test surface), doubling or tripling the indenter will result in 4 and 9 times the maximum indentation load, respectively. top
5. To determine yield stress, one needs the parameter (beta m) which is found for steels in the literature. How about beta-m for other materials? Does one have to do conventional tension tests in order to determine those?
The beta m can be evaluated once for each class of materials by performing a few tensile tests using flat tensile samples and ABI tests on the end tabs of the same samples covering the widest range possible for yield strength to determine the relationship between the A parameter from the ABI test with the yield strength from tensile tests.
An alternative approach is not to use the beta m approach and calculate the yield strength from the power law coefficients (k and n). However, the use of the beta m approach is more accurate due to the fact that you do not have a single yield point in an ABI test since the deformation volume is continuously increasing with increasing indentation depth, hence, yielding and working-hardening occur simultaneously throughout the ABI test from beginning to end. top
6. In the stress-strain curves, sometimes the data points would start from 0.03 or 0.05 strain as opposed to from "0" strain. Is there some instrument/practical restriction that governs what the first data point on the strain axis will be?
The reason that the first point following yield is at 0.03 or higher is that at a very small depth the strain value becomes higher quickly with decreasing indenter diameter. For example, for a 0.02-inch diameter, at approximately 0.0005 inch depth the strain will be approximately 0.06. To obtain smaller values, you must use a larger indenter which may be impractical or the test material is not thick-enough. top
7. How do you determine the young's modulus of the sample?
The elastic modulus can be calculated using two methods: (1) from the calculation of the indentation final diameter versus optical measurement of the chordal indentation diameter in two perpendicular orientations, and (2) using the elastic partial unloading slope of each indentation cycle. For accurate determination using the second method you can also correct for the elastic compression of the tungsten carbide indenter. For most metallic materials, the value of elastic modulus from handbooks is sufficient for accurate ABI analysis. Determination is only needed if you have a new alloy without published elastic modulus data. top
8. Do you/other users of your machine compare the results of the stress-strain curve from ABI (multiaxial) with some other multiaxial stress-strain curve of some material. The uniaxial and multiaxial stress-strain curves are similar, but may not be true for other materials. How does one check the validity of the ABI derived stress-strain curves (multiaxial) with another conventional multiaxial tests?
We have compared stress-strain curves from both ABI and tensile tests on many materials as well as our customers (who have purchased SSM systems from us) with excellent comparison for many materials including aluminum alloys, stainless alloys, Inconel base metals and welds, solder alloys, etc. top
9. For tensile testing, does the fixture use flat/plate type samples or circular cross-section threaded end type samples? Does the system come with an extensometer to measure elongation of the gauge section?
The tensile grips included with the turnkey SSM system are for flat tensile specimens to allow the user to perform Automated Ball Indentation (ABI) tests on the flat end tabs of the tensile specimens for immediate overlay of true-stress/true-plastic-strain curves from both tensile and ABI test techniques. The customer can use other grips for circular cross-section or threaded end specimens by making the appropriate adaptors to the pull rods of the SSM system. Alternatively, we can custom-design and manufacture any grips for any desired specimen geometry. The SSM system does not come with an extensometer, but you can use any commercial one and plug it to the BNC connector in the back of the electronics cabinet of the system for collecting the data with the data acquisition system. The tensile test results can be analyzed using the extensometer data or the crosshead displacement data. Our SSM systems have very high stiffness so that the elongation tensile data are within 0.5% using either the extensometer or the crosshead displacement data. Most electro-mechanical screw-driven test machines have much higher stiffness than a servo-hydraulic test machine. top
10. Is the temperature controller included in the system?
The temperature controller is included with the optional low/high temperature chamber and accessories kit. top
11. For sub-zero temperature testing, I assume we use liquid nitrogen to cool down the system. Does the temperature controller control the sub-zero temperature to only -160 deg.C or any desired temperature between room temperature and -160 deg.C?
For test temperatures lower than room temperature, you will have to use liquid nitrogen through the solenoid valve of the environmental chamber. The digital temperature controller allows to control temperature at the desired setting between room temperature and -160C to within 2 deg. C. Furthermore, the stainless-steel-lined environmental chamber also includes resistance heaters behind its lining to allow testing at higher temperature (room temperature through 427C). The same controller is used to control both low and high temperature settings. top
12. What is the zoom on the video camera ...or what are the smallest sized grains that can be seen in the video camera?
The standard zoom lens allows 20X to 200X. However, adding an optional 2X lens will double this range (i.e., 40X to 400X). You will be able to see the grains on the polished and etched sample. top
13. What is the slowest data acquisition rate (for long-term creep tests)? Also, is the listed 500 samples/s data acquisition rate correspond to all the channels (i.e. load, displacement, temperature, time) or only a single channel?
The slowest data acquisition rate is either one sample per second or one per minute or per hour. We have developed a unique indentation creep software which allows to select various data acquisition rates for different time periods of the creep test (specially for the first few minutes of the test). It is also important to note that materials might have different creep properties under multi-axial deformation than under a single axis deformation. We proved that some carbon steel samples creep at room temperature under the multi-axial deformation of the ball indenter for even short test periods as two minutes. For indentation creep tests, our software allows the user to use various data acquisition rates throughout the test duration. Our software is written in the powerful graphical language of National Instruments' LabView. The 500 samples/s data acquisition rate is per each single channel up to eight (8) channels. top
14. Do you have an option to use a faster data acquisition card?
The maximum data acquisition rate is 20,000 samples/s for all selected channels (e.g. if using three channels of load, displacement, and temperature it will be 20,000/3 per channel). If you need a faster data acquisition rate (higher than 20,000 samples/s) we can offer you a faster card option. There are cards for up to 50,000 or 100,000 samples/s. Again, the maximum for all data acquisition cards is for all channels combined. The time is not a channel since it is recorded from the internal clock of the computer. top
