Fatigue Testing

Cyclical Forces

Medical devices are the most commonly tested device at Empirical Technologies. As a result, we clearly understand the rigors associated with properly carrying out fatigue testing services. Our accredited mechanical testing laboratory specializes in medical devices, and our well-versed team embodies over 25 years of experience creating test plans. Our complete service package includes an unparalleled technical report that can be submitted to regulatory bodies, such as the FDA, in order to meet stringent industry requirements.

What is Fatigue Testing?

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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.

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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

Without adequate fatigue testing, you will not meet specific criteria regulatory agencies need to approve your device for the market, like an ASTM standard for fatigue testing. Practitioners buying equipment need to be sure their purchase will last for years to come under normal circumstances and regular use. Undetected or damaged components cause devices to malfunction and could potentially harm the practitioner or their patients.

How Fatigue Testing Works

Medical device fatigue testing starts by assessing a sample group with high peak stress, or strain; at this point, failure will occur at a low number of cycles. From here, samples will be tested at progressively lower stress levels until a significant number of samples do not fail and break during a predetermined number of cycles. We calculate the fatigue threshold by defining the highest stress at which samples do not fail.

Different Types of Fatigue Tests

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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.

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Fastener Testing

Fastener testing determines how well internally-threaded fasteners, washers, riveters, and direct tension indicators will hold up over repeated use.

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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.

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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.

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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.

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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.

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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.

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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.

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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

Each material tested has different properties that affect its ductility. While certain materials withstand tension and compression well, they may struggle significantly more when subjected to different temperatures and movements within a device.

Environmental Conditions

To understand how a material will react in its natural environment, we can subject materials to different temperatures and humidities to see how it acts differently.

Specimen Geometry

The point where a load is applied to a material determines their static delamination, where a material fractures into layers. The geometry of a specimen influences how much stress the materials can take before failure. Softer geometry between components increases their fatigue strength.

The Importance of Medical Device Testing

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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

When mechanisms apply load over an area, it creates stress. The stress causes strain that deforms as the material elongates. The material either reverts back to its original shape, which we call elasticity. If the material deforms slightly over and over again, we call that plastic deformation.
Your testing results will let you know which materials will bounce back and last for a while, and which ones will wear down more quickly. With good testing, we can predict when devices will need to be replaced or serviced to function correctly.
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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.

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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.

Why Choose Empirical Technologies?

At Empirical Technologies, we are seasoned professionals in fatigue testing services, uniquely equipped to thrive in the fast-paced realm of medical device development. Empirical has spent more than two decades cultivating an unsurpassed reputation in the medical device testing industry, and we work to maintain that reputation with each project. Our staff acts as an extension of your team, so you can rest assured that our customized and high-quality practices will help at every step in the process. Having operated in this industry for so long, we understand the fatigue testing ASTM, FDA, and ISO standard practices needed to meet requirements, so we’ll provide you all of the necessary documentation as you prepare to submit your device for review.
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