Fatigue Testing
Cyclical Forces
What is Fatigue Testing?
Fatigue testing machines subject materials to specific stress ranges over and over until the material cracks and ultimately fails. The machines can alternate between compressing (pushing together) and applying tension (pulling apart) the specimen multiple times per second to see how many load cycles it can handle. Medical device testing’s complexities remain highly regulated due to the potential risks associated with these products’ applications; in turn, fatigue testing services must be performed by a company that understands and adheres to such details as sample size, stress levels, and other protocols.
Sometimes the tests call for different combinations of compression and tension, so researchers can set them to repeat different patterns that mimic the material’s purpose within the tested device. Since certain materials have extremely high thresholds for failure, we chart the data and use the y-axis to measure the stress range while the x-axis tracks the numbers of cycles till failure using a logarithmic scale. This exponential data allows us to estimate how long any given material will last when used regularly.
Importance of Fatigue Testing
How Fatigue Testing Works
Different Types of Fatigue Tests
Axial Fatigue
Axial loads act along the axis or centerpiece of a structure. This testing applies to many different devices and focuses on applying uniform stress across the specimen.
Fastener Testing
Fastener testing determines how well internally-threaded fasteners, washers, riveters, and direct tension indicators will hold up over repeated use.
Torsion Testing
Tensile testing applies force to separate the materials themselves and to determine if assembled parts can sustain pressure. This uncommon test often finds use when assembling cervical spine constructs.
High-Cycle Fatigue
By subjecting a material to a high number of load cycles, we see how it functions over an extended period of use by running multiple cycles per second until the material fails.
Low-Cycle Fatigue
Using high stress and low number of cycles until failure, we plastically deform a material and let it return to its original shape until it fails. Depending on the device’s uses, we may subject it to high changes in temperature to mimic its use in normal circumstances.
Load-Controlled
Performing at stresses in the elastic range lets us look at the range of stress where a material’s deformation is temporary, and it returns to its original form without force being added. Generally, these tests result in long fatigue lives.
Rotating Beam Fatigue
This test puts a lot of stress on a specimen by clamping it in, rotating it, and adding force from a secondary actuator. The force applied from the second actuator causes the material to undergo compression and tension stresses repeatedly until it fails.
Strain-Controlled
This test measures cyclic total strain and determines the cyclic plastic strain as well. We load a material into a mechanism that does not allow it to buckle before applying alternating loads of tension and compression until the material fails.
Three-Point Bend Fatigue
To understand how many times a material can bend, we place the sample between two supporting pins and apply stress to a third point to bend the material until it fails, which gives us the material’s modulus of elasticity.
Factors Affecting Fatigue Testing
Material Properties
Environmental Conditions
Specimen Geometry
The Importance of Medical Device Testing
Good fatigue testing lets engineers and technicians understand how durable individual parts of a device will be when used during their operating conditions. With this data, designers can determine if the materials used will withstand regular use or if they need to choose new materials. They can also give practitioners rough estimates of when they’ll need to service their machines for repair or maintenance.
Fatigue tests for medical devices conducted in a controlled environment often struggle to replicate the diverse conditions they may be exposed to in complex, real-world applications. While fatigue tests account for varying ranges of temperatures, humidity and corrosive elements, they may not account for unique environments. The lifespan oof medical devices may offer limited insight to how long it can maintain structural integrity during actual usage. Next, material properties all exhibit unique fatigue strengths, so not all testing transfers directly from one material to another – each parameter must undergo meticulous reworking. Finally, cyclical testing often pushes the hings at constant amplitudes but real world applications may not cover all the different scenarios in which they’ll be used.
Interpretation of Fatigue Testing Results
Stress-Life Approach (SN)
When materials have a long life cycle (over 103) and stays in the elastic range, where it can revert to its original form. Since the material doesn’t undergo plastic deformation, it can withstand quite a few cycles and gets put through high-cycle fatigue to see how they hold up under constant work for an extended period of time.
Strain-Life Approach (SN)
The elastic parts of devices take a long time to wear out, but they apply small amounts of stress to the areas around them, which causes plastic deformation over time. Through repeated use, critical locations like notch roots undergo regular plastic deformation until they fail. The strain-Life approach shows us how long these stationary parts can endure small amounts of plastic deformation until they fail.