Cyclic fatigue strength test

In mechanical engineering, resistance to repeated loads is one of the most important factors for the service life and performance of a component. Many industrial components fail not due to short-term, intense loads, but due to continuous, cyclic loads. This type of failure is referred to as “material stress.” Stresses can occur under the influence of small but repeated forces and ultimately lead to sudden failure. Evaluating these properties requires a test
known as a lifetime test or load test.

This test is among the most important in various industries, including the automotive, aerospace, medical technology, metal processing, household appliance, and even polymer component industries. It serves to verify a material’s resilience under repeated stress, determine its lifespan, and predict its behavior under real-world operating conditions.

The concept of physical exhaustion

Material fatigue occurs when a material is subjected to repeated stress. Even if these stresses are below the material’s yield strength or tensile strength, repeated stress can cause microscopic cracks to form on or below the surface. Over time, these cracks can propagate and eventually lead to the failure of the entire component. It is important to note that material fatigue typically does not manifest itself through obvious deformations or warning signs
.

From an engineering perspective, fatigue can be divided into three stages:

Cracking begins: a process in which small cracks form on the surface.
Myoma proliferation: a process in which the fissures gradually widen during the pregnancy cycle.
Final failure: when the remaining cross-section can no longer bear the load.

Basic criteria for regular durability tests

To determine the fatigue strength test, several key parameters must be defined:

Permissible voltage: the difference between the maximum and minimum voltage values during a cycle.
Average pressure: Average pressure over the entire period.
Translation ratio (R-ratio): This is the ratio between minimum and maximum voltage, which determines the type of load.
Load frequency: the number of repetitions of a cycle per unit of time, usually measured in Hertz.
Failure cycles (Nf): the number of loads a sample can withstand before failure.
Load type: This can be a tensile, compressive, bending, torsional or a combined load.

Types of periodic fatigue and durability tests

1. Failure due to high cycle count (HCF)

In this case, the load is in the elastic range, and the number of load cycles is very high (typically 10⁴ to 10⁷ or higher).
In high-cycle fatigue, plastic deformation is minimal, and failure typically begins in areas with surface defects. The test results are usually represented as a S-N curve
, where the horizontal axis represents the number of load cycles until failure (N) and the vertical axis represents the stress (S).

2. Low Cycle Voltage (LCF)

In low-cycle fatigue, the stress or strain amplitude is high, and the material enters the range of plastic deformation. During this phase, the number of cycles is relatively low (typically below 10⁴). Since plastic deformation plays a crucial role, the ε-N diagram (number of strain cycles) is used to analyze the material behavior in this region.

3. Multi-axis and compound tests

Many industrial products are subject to stresses in multiple directions. Therefore, multi-axial testing is necessary to investigate the effects of combined loads (tension, bending, torsion, etc.). Such testing is particularly important for complex products such as crankshafts, automotive axles, and metal fittings.

4. Environmental resistance testing

Environmental conditions such as temperature, humidity, corrosion, and pressure can significantly influence fatigue strength.
Therefore, fatigue tests are regularly conducted in corrosive, high-temperature environments or in environments that simulate real-world conditions to evaluate the combined effects of various factors.

Equipment for regular durability tests

The specialized equipment used in this test is called a fatigue testing machine or dynamic testing machine . These machines vary depending on the type of material, the load, and the test purpose. The main components of the equipment include:

Load frame: The main structure of the device, which serves to hold the sample in place.
Dynamic energy source: can be hydraulic or electromechanical.
Load cell: Used to accurately measure the applied force.
Clamps or pliers: These are used to hold samples in place and prevent them from slipping or twisting.
Displacement and strain gauges: are used to measure the strain in each cycle.
Control system and software: used to control frequency, load range and data recording.
Environmental test chamber: For situations where testing is required at specific temperatures or conditions.

Methods for carrying out periodic durability tests

The general steps for performing this test are as follows:

Sample preparation: The samples must have standardized dimensions and surface shapes. It is crucial that the surface is uniform and free of cracks.
Clamping the sample into the device: Adjust the clamps so that the applied force is directed exactly in the desired direction.
Define test parameters: including voltage amplitude, R-factor, load frequency and target number of cycles.
Test procedure: The device is repeatedly subjected to stress until a failure occurs or a certain number of cycles are reached.
Data monitoring and recording: During the test, data such as force, deformation, temperature and time are recorded.
Failure inspection: After completion of the test, the damaged surface is examined under a microscope to determine the location of the crack and the type of defect.
Evaluation of the results: Creation of SN or ε-N diagrams and determination of fatigue parameters.

Analysis of test results

1. Signal-to-noise ratio diagram

In this diagram, the vertical axis represents the stress and the horizontal axis the number of load cycles. As the number of load cycles increases, the permissible stress decreases.
For some materials, such as certain types of steel, there is a limit below which the material does not fail. This limit is called the fatigue strength .

2. Baskin’s Law

A common relationship used to describe fatigue resulting from high speeds is as follows:

$σa=σf'(2Nf)b\sigma_a = \sigma’_f (2N_f)^b$

Where:

$σa\sigma_a$ Potential capacity
$σf′\sigma’_f$ Fatigue resistance coefficient
$b$ : Negative performance factor (material-dependent)
$N_f$ Number of error cycles

3. Deformation model (Coviffin-Manson)

The following formula is used for low-cycle fatigue with significant plastic deformation:

$Δε=σf′E(2Nf)b+εf′(2Nf)c\Delta \varepsilon = \frac{\sigma’_f}{E}(2N_f)^b + \varepsilon’_f (2N_f)^c$

Here, $εf′\varepsilon’_f$ $εf$ $_{'}$ is $_{and}$ $c$ is the plastic stress modulus.

4. Propagation of fatigue cracks

In metallic materials, crack propagation under load occurs gradually. The crack propagation rate is usually expressed by the Paris equation:

$badN=C(ΔK)m\frac{da}{dN} = C(\Delta K)^m$

in this context:

$a$ : length of the gap
$N$ : Number of repetitions
$ΔK /$ $:$ Voltage range
$C$ and $m$ : coefficients that depend on the type of material.

This equation can be used to predict the rate of crack propagation and determine the remaining service life.

Ortas screw pump

Factors that influence the age of fatigue

Material type and structure: Microstructure, grain size and chemical composition play an important role.
Surface condition: Scratches, signs of wear or rough surfaces can lead to the formation of cracks.
Heat treatment: Processes such as normalizing , quenching and fermenting can alter fatigue strength.
Residual stresses: Fatigue strength can be improved through surface compression.
Environmental conditions: Humidity, temperature and aggressive environmental conditions can have a negative impact.
Shape and geometry of the component: Stress concentrations in corners, holes or weld areas often reduce fatigue strength.

Industrial application for regular durability tests

aerospace industry

In the aerospace industry, engine blades, airframes, and connecting elements are subjected to constant cyclic loads. Even the smallest fatigue cracks can lead to catastrophic failure. Regular fatigue tests are therefore essential to ensure flight safety and to determine the need for component maintenance or replacement .

automotive industry

Vehicle axles, springs, control arms, crankshafts, and joints are constantly subjected to varying loads. Regular strength tests serve to predict service life, improve safety, and reduce warranty costs.

Energy and power plants

Turbines, compressor blades, and other power plant components are subject to cyclic vibrations and stresses. Assessing their cyclic strength is crucial for the design of these devices and the prevention of premature failures.

medical team

The materials used in bone implants and artificial joints are subjected to cyclical stresses from human movement. Stress tests of these components are therefore crucial to ensure their longevity over many years.

Household appliances and consumer goods

Components such as hinges, springs, gears, fasteners and moving plastic parts must be regularly checked for their durability to ensure the product’s lifespan and quality.

Sibek’s Group of Companies

Problems and limitations of testing

Time required: Testing millions of courses can take days or even weeks.
Internal heating: At high frequencies, a high sample temperature can distort the results.
The results are inconsistent: The fatigue process is inherently sporadic and requires repeated testing and statistical analysis.
Sample size: The behavior of a small sample may not fully reflect the behavior of the actual product.
High costs: Modern fatigue testing equipment, especially hydraulic testing machines, is expensive.

Finally

Regular durability tests are among the most important methods for assessing the performance and service life of materials and components under repeated stress.
These tests enable:

The actual lifespan of the component is predictable.
By introducing a safer and optimized design concept,
Prevention of sudden operational failures of systems
And the improvement of the final quality of the product.

In advanced industries, precise and scientifically validated fatigue tests are essential for product development and quality control. Advances in measurement technology, analysis software, and sophisticated equipment make it possible to accurately model fatigue behavior and optimize component design.