How can the quality of SMA / Nitinol be assured?
For providing quality assurance of NiTi-SMA, various test devices are available to you. Why these are used and which one you should chose, is outlined below.If you are interested in more details, please take a look at our free course as PDF an.
For your conventional materials, i.e. metallic structural materials, you will be familiar with the tensile test. You normally will use this test as standard procedure for determining material parameters. You have already learned about some of these parameters specific to SMA in the section Application and Properties.
In addition, you will find further characteristic values in our material data sheets.
Your free SMA course will provide you with a complete overview. In the following, you will learn how to determine these and other parameters.
One of the methods used is the so called DSC (Differential Scanning Calorimetry); a method also used in the development of conventional materials. It quantifies and provides information on the influence of the degree of deformation and points heat treatment parameters. In order to determine SMA specific parameters, further characteristic values can be measured. These in particular result from the thermal analysis of your SMA.
You will apply the DSC to your SMA sample only in a condition that is free of loads. Also, measurement during a thermal analysis will provide transformation temperatures (equally in a load-free state). For example, you will obtain values for the key figures of As, Af, Ap, Ms, Mf, and Mp. The SMA course will answer your questions as to how exactly these key figures and the conversion enthalpy are measured and why this is important.
The macroscopic properties of an SMA are determined by the structural composition of the material; more precisely by its microstructure. And in order to determine it, microscopy is used.
If you take a close look at the complete and free course (download PDF here), you will learn which microscopy equipment you can use for this purpose and what results you can expect to achieve. It is because based on the analysis of these values that you can draw conclusions about the properties of the SMA material.
Determination of key figures of superelastic SMA – mainly in medical technology
SMA with an active Af < RT
As mentioned above, some types of SMA at room temperature (RT) exhibit superelastic behaviour. These superelastic SMA are often used in medical technology. For this use, it is required to undertake additional characterisation of the material and investigate further properties. These additional criteria are defined in some of the few ASTM standards. For a list of which additional parameters are important and why this is the case, please refer to the SMA course which you can download here.
Characterisation of pseudoplastic SMA – mainly in actuators
SMA with active As > RT
Several tests are required for characterising the behaviour of your SMA actuator. This is where are determined: How you can measure the maximum actuation force, the maximum stroke and the force for detwinning. Especially the procedural steps for measuring the detwinning force, as well as their interpretation, are of fundamental importance for the design of your actuator unit. These and other test arrangements such as measures for determining heating or cooling parameters can also be found in the SMA course.
Electrical resistance (actuators)
As outlined in the paragraphs “temperature gradients”, there are two possible ways of actuation for your element. Passively, this occurs via temperature differences between the actuator and its environment (e.g. liquids, air flow). However, you are also able to electrically switch your actuator. The heat required for this process is achieved by making use of the actuator’s inner electric resistance. The resistance of your actuator depends on a number of parameters that may not seem obvious at a first
glance. They are described in more detail in the free course. You will also learn how to use the electrical resistance for the precise and stepless control of your actuator’s position and how to determine – along with the EMS provided by Ingpuls – the suitable the suitable parameters for the electric currents of your actuator system.
Temperature and load dependency (actuators)
Further, you should always consider the dependency of your material properties on temperature and load. For this, various tests are required. The reason for this lies in the fact that the thermal equilibrium of the material is shifted by both temperature and mechanical stress. This often results in significant shifts of the transformation temperatures and also effects other material properties. Mechanical stresses, for instance, cause an upward shift of the transformation temperatures (Clausius-Clapeyrion equation). In order to investigate this dependency, we determine special characteristic curves. They provide us with important information that we can additionally refer to when designing your SMA components.
Cyclic behaviour (actuators)
If your SMA element passes through many operating cycles, you may observe a loss of actuation force or stroke. We determine this cyclical behaviour in endurance tests, thereby indicating any loss of actuation force and stroke as well as the determining the shift in transformation temperatures over the service life. In general, during in-field
use you would want to avoid a shift of your SMA’s characteristic values. In the SMA course you will learn about the measures you can take that will make the losses on these variables negligible. Particular attention is paid to pre-cycled components and special electronics. Why not take a look here right away?
Recap on what you should have understood in this section:
- You can draw conclusions about the properties of the SMA material using a variety of methods of analysis.
- Each analysis method has its own key figures and parameters.
- Only by understanding SMA wholistically can SMA be developed in a targeted and customer-specific manner.
- In choosing the suitable set of methods, you should differ based on the behaviour of your SMA element.