Know the main types of material testing?Non-destructive materials testing in brief
For example, in the automotive industry, production processes are controlled so that they only leave the specified process window in extreme situations. The process capability index reflects whether a process achieves the targets set out in the specification. This index, also called cpk, is a measure of how repeatable and absolutely accurate the process works.
Processes with poor reproducibility or absolute accuracy need to be monitored. In series production, it is imperative to detect in time when the process leaves its given process window in order to prevent defective parts from being produced.
In a car, a construction crane or a train, there are many safety-relevant and function-critical components. Their function is only guaranteed if the material and the manufacturing process are precisely followed. For example, a trailer hitch can break if the hardening process was not followed exactly and the trailer hitch has too brittle properties. Or the running surfaces of a drive may wear out too quickly if they have not been hardened properly.
Even the use of the wrong raw material can cause the component to behave differently in the series application and lead to serious misbehavior.
Whenever major damage is to be prevented or human life is at risk, it is necessary to perform in-process testing. This is the only way to ensure that the specified parameters are met.
The following material tests can be performed in-process or as part of a 100% inspection:
Material mix-up testing Testing of hardness parameters (u.A. Hardness depth and surface hardness) conductivity measurement permeability measurement testing for hard and soft spots blowhole testing
1. Material mix-up check
There are hundreds of different metal alloys, which differ in their composition and the way they are produced. Many manufacturers have their secret recipes, which are put together individually for the respective end product.Certain variations in alloy constituents are normal and permissible – as long as they remain within specification. As the saying goes: trust is good; control is better. Depending on the design of the supply chain, quality cannot be relied upon. Therefore, the material composition must be checked regularly. This is the case, for example, in the automotive or aviation industry and is regulated by corresponding standards. Testing can take place either on random samples at goods receipt or – if required – as 100% testing directly in the production line. There are various methods for material mix-up testing: from testing with a mass spectrometer in the laboratory, to a quick random sample measurement of the carbon content, to the magnetic-inductive method.
The magnetic-inductive method works with eddy currents. To test for material properties, the test pieces usually pass through a test coil containing a primary and secondary coil. In this process, low-frequency eddy currents are induced in the material. The test voltage detected by the sensor results from the magnetic and electrical properties of the test part, with the voltage value being graphically displayed as a measuring point. The different alloy constituents or microstructural states change the receiver currents. Allow conclusions to be drawn about the material properties of the test part. Each material leaves its own unique "fingerprint", so to speak, which changes depending on the measurement frequency used.
This fingerprint is taught into the eddy current device to uniquely identify a material. It is important that the environmental variables remain stable in the process. To minimize pseudo readings, it is usually attempted to solve the test with an optimized frequency. If different material properties are to be examined, a test with several basic frequencies can take place. As an alternative or in addition to the fundamental frequencies, the harmonic frequencies can also be evaluated in order to clearly identify the material and to sort out incorrect materials. It is important that the dimensions of the test specimen are identical to the taught-in material. Because different dimensions cause the magnetic fields of the test probe to propagate differently and thus change the signal.
Magnetic-inductive material mix-up testing is already successfully used in many production lines. But applications can also be found outside production lines, for example to distinguish between high-grade and low-grade aluminum.
2. Testing of hardness parameters
Depending on the material of which a component is made, different hardening processes are used, such as induction hardening or furnace hardening. It can happen that a component does not have the desired hardness after the process, for example due to an incorrect carbon content or a faulty alloy.
In induction hardening, the current required to heat the component must also be of the correct magnitude to ensure that the surface temperature of the component is correct. Incorrect surface temperature can cause the microstructural transformation to occur incorrectly. Optical checks are usually not possible, since the differences between the components cannot be seen with the naked eye. Therefore, non-destructive testing methods are needed to report faulty processes to the user. Since magnetic material properties in particular correlate with material hardness, magnetic-inductive testing can be used to record a specific "fingerprint" of the component. Magnetic properties of materials that contribute to interactions are magnetic permeability, remanence and coercivity. Another influencing factor in magnetic-inductive hardness testing is electrical conductivity. For material confusion testing. Hardness tests, the same procedure can be used.
3. In many applications, in addition to the right material. The hardness of which the electrical conductivity plays an important role. For example, in aircraft construction, the conductivity of the materials used is precisely specified. One of the aims is to ensure that the energy is conducted and dissipated in the event of a lightning strike. Electrical conductivity is defined as the constant of proportionality between current density and electric field strength. A typical setup for measuring the electrical conductivity of metals is four-point measurement. Here, a current is injected over a fixed distance via two measuring tips and the voltage drop is measured across it.
Usually the measurement takes place on random samples, which is why mobile measuring devices are preferably used. Of course, it is also possible to use the measurement method in a serial test with a line gauge. Furthermore, a whole-surface inspection of plates or bars for a fluctuation in conductivity is feasible.
4. Measurement of material permeability
Magnetic permeability is a measure of the permeability of matter to magnetic fields (analogous to electrical conductivity to electric fields). Ferritic or martensitic steels, for example, exhibit good magnetic permeability, while the opposite is true for austenitic steels. Since the functionality and efficiency of electric motors, for example, is strongly influenced by magnetic permeability, the measurement and verification of this material property is becoming increasingly important. Some components for electric drives must be made of materials that are easily magnetized, so that the electromagnetic system can operate quickly and with the lowest possible energy consumption.
However, there are also applications in which the materials must have no or only very low permeability. For example, only amagnetic components are suitable for installation in a magnetic resonance tomograph (MRT), so that they do not influence the magnetic field that is generated. For the same reason, implants may only be made of amagnetic materials if a patient requires an MRI. There are two ways to measure the permeability of a material:
AC permeability measurement, d.H. The measurement of magnetic permeability in the presence of alternating fields. DC permeability measurement, d.H. The measurement of the magnetic permeability by successively increasing the magnetic flux.
The permeability is defined as the slope of the characteristic curve in a B/H trace. The prerequisite is a closed magnetic circuit such as is present in a ring sample DC or Epstein frame AC measurements. The permeability or. The B/H measurement curve can be recorded by using a primary coil (measures H-field) and a secondary coil (measures B-field). The relationship between the impressed current and the measured induction voltage as an integral represents the B/H characteristic curve and its slope the magnetic permeability.
The primary coil is used to generate a defined field with an impressed current for this purpose. The magnetic field H induced by the current generates a magnetic flux in the test part. The secondary coil is arranged parallel to the primary coil. The magnetic flux causes an induction voltage in the test piece.
The big challenge of this measuring method is to realize the relation between B/H as far as possible according to the existing component geometry in order to measure different shapes of test pieces. For this purpose it is necessary to develop a model which can be applied from a standard piece to the geometry of the test piece. In general, DC measurement is used in industry, but AC measurement is interesting for applications with operating frequencies in the higher kHz range.
5. Testing for hard spots, soft spots and shrinkage cavities
So-called hard spots or soft spots are local microstructural differences (inhomogeneities) that arise due to the manufacturing process. They are noticeable by different hardness, conductivity or material compositions. To find these inhomogeneities, the same methods are used as for material mix-up or hardness testing. While there the result is averaged over the whole part, in this case it is necessary to scan the part exactly. The setup for testing for hard spots is therefore analogous to the classic crack detection with eddy current. Also the shrinkage is limited to a very local area. Ultrasonic testing is typically used for detection. For this purpose it is also necessary to move the ultrasonic probe along the test piece or, analogously, to move the test piece along the ultrasonic probe. The methodology for this is explained in the section on crack testing methods.
How can a typical material test look like? For testing tasks in the field of material-. FOERSTER has developed a special testing device for hardness testing. The MAGNATEST D is particularly suitable for series applications. The hardware of the testing device is designed in such a way that several components can be tested within a very short time, d.H. High cycle rates can be realized. This test setup requires a test coil, a test instrument and the component feeder. Component feeding can be manual or automatic. The type of component feed and the level of automation influence the acquisition cost of a testing system.
As a rule, the MAGNATEST D is configured to automatically detect when a DUT is inserted into the coil. The component recognition then starts the actual test and the subsequent good/bad sorting. The test results are transmitted as an I/O signal. Returned as direct information on the screen. Testing takes place either with a continuous coil through which the test specimen passes, or with fixed probes to which the test specimen is moved manually or automatically. The MAGNATEST D can, of course, also be integrated directly into the production process. In this way, it is ideally possible to check directly after the refinement process whether it has proceeded correctly. In the event of a faulty process and subsequent production scrap, timely feedback allows countermeasures to be taken in good time.
Regression analysis in hardness testing
The measured values in non-destructive hardness testing with an eddy current instrument are dimensionless values that contain a statement about the real and imaginary parts of the signals in the secondary measuring coil. Through a correlation analysis, d.H. A regression of a physical measurement method of absolute hardness with the dimensionless values of eddy current testing, these can be parameterized, which leads to the fact that one can use the testing device in the series process as a measuring device. How this regression analysis looks. This can be implemented in a different subject area.