How to Choose a Dynamic Fatigue Testing Machine: A Complete Buyer's Guide

A dynamic fatigue testing machine repeatedly applies controlled loads to a material, component, or finished product to evaluate fatigue strength, deformation behavior, crack initiation, and service life. Unlike a static universal testing machine, it must maintain stable control over thousands or millions of cycles while accurately reproducing the selected load, displacement, or strain history.

Choosing the right system therefore involves much more than comparing maximum force ratings. Buyers also need to consider frequency, amplitude, actuator stroke, control mode, waveform, frame stiffness, fixture alignment, software, environmental conditions, and the test standard.

This guide explains how these factors interact and provides a practical process for selecting a dynamic fatigue testing machine without paying for capacity you do not need—or choosing a system that cannot reproduce your actual test.

1. Define the Complete Test Profile Before Comparing Machines

The first step is to convert the application into a clear test profile. A machine cannot be selected reliably from the material name or maximum force alone.

Before requesting a quotation, define the following:

For example, a polymer specimen tested at low force and large displacement has very different requirements from a steel strand tested at hundreds of kilonewtons. Likewise, a medical implant fatigue test may require a dedicated fixture, simulated physiological conditions, and a standard-specific setup even when the nominal force is relatively low.

The most useful request for quotation is therefore based on a test method, sample drawing, expected load range, frequency, amplitude, environment, and required standard—not simply a request for a “10 kN fatigue tester.”

2. Choose Between Servo-Electric and Servo-Hydraulic Systems

Both servo-electric and servo-hydraulic machines use closed-loop control, but they suit different force ranges and operating conditions.

Servo-Electric Fatigue Testing Machines

Servo-electric systems are generally well suited to low-force and precision cyclic tests. They are commonly selected for polymers, elastomers, composites, small industrial components, electronic parts, and biomedical devices.

Typical advantages include:

• ◆  Clean operation without a hydraulic oil circuit

• ◆  Lower routine maintenance

• ◆  Lower noise and energy consumption in many low-force applications

• ◆  Good control at small loads and displacements

• ◆  Easier installation in conventional laboratories

The UnitedTest electronic fatigue testing range includes UTDS configurations from 500 N to 5 kN and systems up to ±10 kN, with maximum frequencies up to 15 Hz depending on the model and test amplitude.

Servo-Hydraulic Fatigue Testing Machines

Servo-hydraulic systems are normally chosen for higher loads, demanding dynamic response, long-duration structural testing, and specialized component or multi-channel applications.

Typical advantages include:

• ◆  Higher force capacity

• ◆  Strong dynamic performance for demanding cyclic loading

• ◆  Flexible actuator and test-frame configurations

• ◆  Suitability for structural parts, metals, steel strands, asphalt, and full-scale components

• ◆  Easier integration into multi-axis or multi-channel loading systems

UnitedTest's HDS hydraulic fatigue testing systems include standard configurations from 30 to 600 kN at 0.01–50 Hz. Specialized steel-strand systems cover higher loads, while application-specific systems may provide different force, frequency, and environmental capabilities.

A Practical Decision Rule

Do not choose the drive technology from force alone. Compare the complete force–frequency–stroke envelope, the required control accuracy, laboratory utilities, noise limits, maintenance capacity, and future tests. A servo-electric system may be the most efficient choice for a 5 kN polymer test, while a servo-hydraulic system may be essential for a 100 kN structural component or a multi-channel durability rig.

3. Determine the Required Load Capacity

A useful starting point is to choose a rated dynamic capacity approximately 20–50% above the highest expected test load. This allows room for preload, occasional peaks, fixture weight, and future projects.

However, this is a screening guideline rather than a universal rule. Final sizing should also account for:

• ◆  Whether the published rating is static, dynamic, peak, or continuous

• ◆  Tension and compression capacity in both loading directions

• ◆  Load-cell accuracy across the actual working range

• ◆  Expected overloads or transient peaks

• ◆  Fixture and specimen mass during high-frequency motion

• ◆  Whether a smaller secondary load cell can be installed for low-force tests

Buying substantially more capacity than required can reduce useful resolution at the lower end of the range. For example, a high-capacity load cell is rarely the best sensor for measuring a few hundred newtons. If a laboratory expects both low- and high-force work, ask whether interchangeable load cells are supported and how calibration is managed after a change.

Example Load-Capacity Screening

Always provide the supplier with both the maximum and minimum load. A machine may have enough peak capacity but still be unsuitable if the lowest controlled load falls outside the load cell's useful measurement range.

4. Match Frequency, Amplitude, Stroke, and Velocity

Frequency should never be evaluated as an isolated specification. The attainable frequency depends on displacement amplitude, actuator velocity, load, specimen stiffness, fixture mass, and controller performance.

For sinusoidal motion, peak actuator velocity is approximately:

Peak actuator velocity: vₚₑₐₖ = 2πfA

where (f) is frequency and (A) is displacement amplitude. Increasing either frequency or amplitude raises the required actuator velocity. This is why a machine may achieve its maximum published frequency only at a relatively small amplitude.

The UnitedTest UTDS-10Y illustrates this relationship. Its published actuator stroke is ±50 mm, while the amplitude at the maximum 15 Hz frequency is ±1 mm. A buyer should not assume that the machine can run at 15 Hz across its entire stroke.

When comparing machines, request a performance envelope or written confirmation for your actual combination of:

• ◆  Dynamic load

• ◆  Frequency

• ◆  Displacement amplitude

• ◆  Specimen stiffness

• ◆  Fixture mass

• ◆  Waveform

Higher frequency is not always desirable. Polymers, elastomers, adhesives, and some composites can generate heat under rapid cycling. If specimen temperature rises beyond real service conditions, the test may measure a heat-affected failure mechanism rather than the intended fatigue behavior. Use the frequency specified by the applicable standard or validated test method whenever possible.

5. Select the Correct Closed-Loop Control Modes

A dynamic fatigue tester measures the specimen response and continuously corrects actuator output through a closed control loop. The required feedback variable depends on the test objective.

Force Control

Force control makes the actuator follow a programmed force waveform. It is widely used for stress- or load-controlled fatigue tests in metals, composites, adhesives, and components.

Displacement Control

Displacement control makes the actuator follow a programmed movement. It is useful for components whose service condition is defined by travel or deflection, including many rubber, polymer, spring, and assembly durability tests.

Strain or Deformation Control

Strain-controlled testing uses an extensometer or other deformation sensor as the feedback source. It is essential for many low-cycle fatigue methods in which local specimen strain must be controlled directly.

Some systems can switch smoothly among load, displacement, and deformation control. The UTDS-10Y, for example, lists all three control modes. Buyers should still confirm which sensors are included, which are optional, and whether mode switching is supported within the required test procedure.

6. Confirm Waveform and Loading-Ratio Capability

A sine wave is common in fatigue testing, but it is not suitable for every application. Depending on the product and test standard, the controller may need to generate:

• ◆  Sine waves

• ◆  Triangular waves

• ◆  Square waves

• ◆  Sawtooth or ramp profiles

• ◆  Trapezoidal profiles with dwell time

• ◆  User-defined or field-reconstructed sequences

The controller must reproduce the selected waveform accurately under load, not simply generate the command signal. Ask the supplier to demonstrate tracking performance at your specified force, amplitude, and frequency.

Also define the stress or load ratio:

Load ratio: R = Fmin / Fmax

This ratio distinguishes tension–tension, compression–compression, and fully or partially reversed loading. The required load ratio affects grip design, backlash control, specimen alignment, and whether the machine must pass smoothly through zero load.

7. Evaluate Measurement and Control Performance

Static accuracy alone does not fully describe fatigue-test performance. A system that produces reliable long-duration results needs a complete measurement chain, including suitable sensors, adequate control-loop speed, stable signal conditioning, and traceable calibration.

Ask suppliers to specify:

• ◆  Force accuracy and dynamic fluctuation

• ◆  Displacement and strain accuracy

• ◆  Accuracy range as a percentage of full scale

• ◆  Controller update rate and recorded data rate

• ◆  A/D and D/A resolution

• ◆  Long-term signal drift

• ◆  Calibration method and traceability

• ◆  Overload capacity and sensor protection

Control-loop rate and data-recording rate are not necessarily the same. A fast controller may make thousands of corrections per second while the software saves only selected points or cycle summaries. Confirm that the stored data are sufficient for hysteresis loops, peak/valley analysis, stiffness change, crack-growth evaluation, or other required outputs.

For reference, the UTDS-10Y lists ±1% dynamic load accuracy, displacement accuracy of ±0.5% full scale, a maximum closed-loop refresh rate of 6 kHz, and 16-bit controller resolution. These values should be assessed against the actual test standard and working range rather than compared as isolated headline numbers.

8. Review Frame, Alignment, and Fixture Requirements

The test frame and fixtures form part of the measurement system. Poor stiffness or alignment can introduce bending, backlash, vibration, grip slippage, or premature specimen failure.

Frame and Test-Space Questions

Check:

• ◆  Frame stiffness under peak dynamic load

• ◆  Vertical clearance and test width

• ◆  Crosshead adjustment and locking method

• ◆  Space for grips, chambers, extensometers, and safety shields

• ◆  Access for specimen installation and fixture changeover

• ◆  Foundation or isolation requirements

Fixture Questions

The same base machine may perform tensile, compression, and bending fatigue tests only when it has the correct fixtures, load train, sensors, and control method. Common options include:

• ◆  Tensile grips

• ◆  Compression platens

• ◆  Three-point and four-point bending fixtures

• ◆  Clevis and pin fixtures

• ◆  Rubber or elastomer fatigue fixtures

• ◆  Adhesive-joint fixtures

• ◆  Battery-component fixtures

• ◆  Medical implant fixtures

• ◆  Custom component fixtures

Ask the supplier to review specimen drawings before finalizing the machine. Confirm grip capacity, jaw or contact material, alignment method, fixture fatigue rating, changeover time, and whether the fixture mass limits high-frequency performance.

9. Consider Environmental and Multi-Axis Testing

Some materials and products must be tested under the temperature, humidity, fluid, or corrosion conditions experienced in service. Retrofitting these capabilities later may be difficult if space and interfaces were not considered at the beginning.

Possible requirements include:

• ◆  High- or low-temperature chambers

• ◆  Humidity control

• ◆  Saline, oil, water, or simulated body-fluid baths

• ◆  Corrosion-resistant fixtures

• ◆  Thermal compensation for sensors

• ◆  Remote or protected electronics

Complex products may also require axial–torsional, biaxial, triaxial, or multi-channel loading. In such cases, selection must address actuator synchronization, cross-axis interaction, control-channel count, fixture design, and the combined test envelope. A single-axis machine should not be assumed to become a true multi-axis system through fixture changes alone.

10. Assess Software, Safety, and Data Management

Software determines how efficiently users create methods, monitor long tests, identify failure, and convert raw signals into usable results.

Useful functions include:

• ◆  Method creation and storage

• ◆  Real-time force–displacement and stress–strain curves

• ◆  Cycle counting and peak/valley capture

• ◆  Hysteresis-loop display

• ◆  Automatic zeroing and calibration support

• ◆  PID parameter storage and recovery

• ◆  Programmable stop conditions

• ◆  Specimen failure and stiffness-loss detection

• ◆  Load, displacement, frequency, and travel limits

• ◆  Automatic data collection and report generation

• ◆  Export to common formats for further analysis

• ◆  User permissions, audit trails, and method protection where required

For unattended high-cycle tests, ask what happens after specimen failure, sensor loss, overheating, power interruption, excessive displacement, or hydraulic-pressure loss. Safety limits should protect the operator, specimen, fixtures, load cell, and actuator.

11. Verify Standards and Method Compatibility

Do not accept a general statement that a machine is “ASTM- or ISO-compliant” as proof that it can perform a particular method. Compliance depends on the entire configuration: force range, control mode, fixtures, specimen alignment, extensometer, environment, software, calibration, and test procedure.

Common fatigue standards include ASTM E466 for force-controlled axial fatigue of metallic materials, ASTM E606/E606M and ISO 12106 for strain-controlled fatigue, ISO 1099 for axial force-controlled metallic fatigue, and ASTM D3479 for tension–tension fatigue of polymer-matrix composites. Medical implants, adhesives, rubber products, steel strands, and asphalt materials have their own application-specific standards.

Before ordering, provide the exact standard designation and revision. Ask the supplier to state:

1. Which parts of the method the proposed configuration supports

2. Which fixtures and sensors are included

3. Which accessories are optional

4. What calibration is provided

5. Whether a method template, demonstration, or acceptance test is available

This is more reliable than choosing from a generic standards list.

UnitedTest Dynamic Fatigue Testing Machine Recommendations

UnitedTest provides dynamic fatigue testing machines for low-force material testing, high-load component testing, and specialized biomedical or industrial applications.

UnitedTest can also configure load capacity, frequency range, fixtures, gripping systems, environmental integration, multi-axis capability, and data-acquisition software around a defined test method. To receive an appropriate recommendation, send the engineering team your specimen drawing, applicable standard, load range, amplitude, frequency, waveform, environment, and target cycle count. This allows the machine, sensors, fixtures, and software to be selected as one complete testing system rather than as separate specifications.

Frequently Asked Questions

How do I choose the right load capacity?

As an initial guideline, evaluate a machine with a rated dynamic capacity approximately 20–50% above your highest expected test load. Then confirm that the load cell also provides suitable accuracy at the lowest load, that the rating applies to your loading direction and duty cycle, and that the machine can achieve the required frequency and amplitude under load.

Can one machine perform tensile, compression, and bending fatigue tests?

Often yes, provided the frame, actuator, controller, sensors, and test space are suitable and the correct fatigue-rated fixtures are installed. Bending tests may require a different load train and careful alignment. Fixture compatibility should therefore be confirmed from specimen drawings and the applicable standard rather than assumed from the machine's force rating.

Why isn't the highest test frequency always the best choice?

The test should reproduce a valid service condition or follow the specified standard. Excessive frequency can heat polymers, elastomers, adhesives, and some composites, changing their behavior. Maximum frequency may also be available only at a small displacement amplitude, so running faster is not always technically possible or scientifically valid.

What is the difference between force control and displacement control?

Force control makes the actuator follow a programmed force waveform, while displacement control makes it follow a programmed movement waveform. Force control is common when stress or applied load defines the test; displacement control is useful when travel or deformation represents service conditions. Strain-controlled fatigue uses a deformation sensor or extensometer as the feedback source.

Are servo-electric systems replacing hydraulic fatigue testers?

Servo-electric systems are increasingly preferred for many low-force applications because they can offer clean operation, lower maintenance, lower noise, and good efficiency. They do not replace servo-hydraulic systems in every application. Hydraulic systems remain important for high-force, high-response, structural, multi-axis, and specialized dynamic testing.

How important are fixtures when selecting a fatigue testing machine?

Fixtures are critical. Their stiffness, fatigue rating, alignment, grip method, and mass can directly affect test validity and attainable frequency. A flexible fixture system can expand the range of specimens tested on one platform, but every fixture should still be reviewed for the specific load, waveform, specimen, and standard.

Should I select the machine from the standard alone?

No. A standard defines the method, but the final system must also suit the specimen dimensions, expected force, amplitude, frequency, environment, throughput, and data requirements. Two laboratories following the same standard may need different capacities or fixture configurations.