Comparison of Medical Device Fatigue Test Systems

Fatigue testing has undergone several major technology changes over the last two decades. Prior to1990, almost all fatigue testers used servohydraulic principles to apply load to the test specimen. Servohydraulics had been used since the mid-1950s and was the de facto standard technology when building material, automotive, aerospace, and large structural testers. In the 1990s, manufacturer sstarted experimenting with other drive technologies including servopneumatics and linear motors.

While most 21st century medical device testing applications leverage linear motor technology, each technology possesses unique characteristics that make it indispensable to medical device testing.

Servohydraulics (SH)

When most people think of a servohydraulic test system, they usually think of MTS Systems. MTS Systems introduced the first servohydraulic test system when it was the Material Test Systems Division division of Research Incorporated in the mid-1950s. MTS employed servovalve and hydraulic cylinder technologies designed for aerospace flight control systems and applied them to fatigue testing. Prior to that time, fatigue tests were either done on rotary bend machines or resonance (spring/mass) tests systems. The advantage of the new servohydraulic approach was that the loading “R” Ratio could be controlled to something other than -1.0.

Additionally, the applied loads could be scaled from several kilonewtons to several meganewtons, and the number of loading channels was scalable as well. For example, a simple fatigue tester might have only one loading actuator while an airplane wing test rig might have 30 to 40 actuators. As the market for servohydraulic tests systems matured, other companies entered the market. Today, the suppliers of servohydraulic tests systems include MTS, Instron, Saginomiya, Shimadzu, Shore Western and Zwick.

Servohydraulic actuators can be sized for just about any loading requirement, and the actuator can be designed with almost any stroke (the most common stroke lengths are 100mm, 150mm or 250mm). Servohydraulic actuators are also relatively easy to design; if you have a special stroke and force requirement, a new custom actuator can be designed in a matter of hours

However, the minimum practical force rating that a SH actuator can be designed to is about 5kN. For loads less than 5kN, the friction of the oil seals causes problems with the system resolution. There is also a concern with cleanliness - if the SH actuator seal leaks during testing, the test specimen can be contaminated.

Additionally, SH actuators are designed to last tens of millions of cycles without fatigue failures or seal leakage. While this level of longevity works well for the typical orthopedic test (up to 10 million cycles), it's not durable enough for cardiovascular devices (400 million to 600 million cycles). And because of their design, SH systems require a high powered (5HP minimum) hydraulic pump with an oil supply that must be regularly changed, making it a relatively high-maintenance system.

Servopneumatic (SP) Test Systems

An SP test system is similar to the hydraulic test system except that it uses compressed air instead of hydraulic fluid as the driving media. EnduraTEC introduced these systems to the orthopedic market in the early 1990s as a lower cost alternative to the SH systems. While the SP systems held promise for making an impact in this market area, they did not catch on in the mainstream market. A combination of limited performance frequency compared to a fluid-based system and the fact that most customers had already invested heavily in SH systems limited the impact of SP systems.

Still, for lower force (i.e., less than 1kN) and moderate test frequency (10 Hz orless) applications, SP systems represent a viable alternative to SH systems. The only manufacturer that supplied SP systems was EnduraTEC.

Single Phase Linear Motors (SPLM)

Single Phase Linear Motors generate a force that is proportional to the amount of electrical current applied. In the mid-90s, EnduraTEC began delivering tests systems with voice coils for stent and wire testing. Patent 5,670,708 represents a stent graft tester that was developed using two voice coils as the driving means. Voice coils were later replaced with a moving magnet linear motor developed by Bose Corporation.

The moving magnet design was superior to the voice coil approach because it eliminated the flying leads which were prone to fatigue failure, was easier to cool and had a lower moving mass. The moving magnet motor was also employed in the ElectroForce Fatigue Test Instruments supplied by EnduraTEC (later Bose and then TA Instruments).

SPLM systems are sometimes preferred because their low output force range is ideally suited for testing small medical devices developed for the intravascular market. Additionally, the flexure bearing system and moving magnet design provide extremely high longevity, and the power required to drive a SPLM is quite low compared to the power required to drive an SHor SP system. For this reason, test frequencies 60Hz and higher are easily obtained due to the low moving mass.

However, SPLM systems have low force capability compared to other systems, and the amount of stroke that can be applied is generally limited and is dependent on the size of the SPLM. Another disadvantage is that the ability to readily customize SPLMs is almost non-existent due to the nature of the components used, making one-off projects difficult.

There is currently only one supplier of test systems which utilize moving magnet SPLMs, and this is TA Instruments.

Multi-Phase Linear Motors (MPLM)

The first MPLM tester was introduced and patented by MTS Systems in 1998. That system featured a horizontally-mounted linear motor that had a 100mm stroke with an air bearing support system. It was designed forprecision low force fatigue applications in the semiconductor and medicaldevice testing industries. Despite its advanced design, it did not mak emuch of an impact in the market most likely because it was a new technology that did not fit within the SH paradigm of MTS’s culture.

After the successful introduction of the SPLM test systems by EnduraTEC in the early 2000’s, Instron and MTS decided they needed to offer an electric test system alternative. In the late 2000s, Instron introduced its ElectroPulsSeries and in 2014 MTS introduced its Acumen Series of all-electric test instruments. Both systems feature a crosshead-mounted multi-phaselinear motor with a moving voice coil or magnet armature that is supported on a linear ball bearing system.

The multi-phase design of the linear motor enables it to provide higher loads and a longer overall stroke. For example, whereas the smallest TA ElectroForceTest instrument is rated at 200N and has a 12.5mm overall stroke, the smallest Instron ElectroPuls Testinstrument is rated at 1000N and has a 60mm stroke. While one might think that the MPLM approach would be superior to SPLM, there are relative advantages and disadvantages that the MPLM has with respect to the SPLM as follows.

The additional phases of MPLM systems mean the motor can generate more power. If the magnet assemblies are sized the same, a MPLM with three magnets (required for the multiphase motion) will generate twice as much force as an SPLM with a comparable-sized single magnet. The MPLM also has longer stroke capability, makeing it easier to test longer specimens and to setup tests as the longer stroke provides more flexibility.

There are several downsides to an MPLM system, however. Since the moving armature in an MPLM has more coil or magnet assemblies, the moving mass is greater. This means that at high test frequencies, the vibration induced into the test bench or floor can be substantial if care is not taken to isolate the frame from the bench. Whereas the SPLM is quite comfortable running at 60Hz and up, MPLM systems generally have difficulty achieving frequencies higher than 30Hz. To provide a longer overall stroke, the MPLM system features a roller bearing support system.

Conclusion

Element has a wide breadth of equipment including SH, SP, SPLM and MPLM test systems at its disposal. When setting up a test, we are able to choose the test system that is most appropriate for the desired test conditions.

For example, if you want to test up to 15 specimens at high frequency (30Hz or greater) by applying a few millimeters of displacement, we would utilize a SPLM test system. If you want to test a single specimen at high loads and a lower test frequency we would look at using either an MPLM or SH based system. With our extensive equipment base and broad experience in the mechanical testing arena, Element is able to provide the most reliable test conditions possible.