Fatigue testing is a test method used to evaluate the ability of materials, structures, or components to resist failure under repeated loading and unloading conditions. It simulates the cyclic stresses that might be encountered in real-world use to predict whether a product will fail due to fatigue during long-term service. Fatigue testing is widely used in many industries, including aerospace, automotive, bridge construction, and machinery manufacturing, to ensure product safety and reliability. This type of testing is crucial in engineering because it helps predict the lifespan of products in actual use and identify potential failure points.

Fatigue testing simulates the stress conditions of materials in real-world use by applying cyclic loads (usually alternating loads), thereby assessing their durability and fatigue life.

The purpose of fatigue testing is to determine the reliability and safety of materials or structures during long-term use and to identify factors that may lead to failure.

Main Types of Fatigue Testing

1. Tensile-Compression Fatigue Test

• Application: Suitable for shaft parts, bolted connections, and other components subjected to axial alternating loads.

• Loading Method: Applying sinusoidal or other forms of periodic tensile and compressive forces, recording fatigue life under different stress levels.

2. Bending Fatigue Test

• Application: Primarily used for components subjected to bending moments, such as beams and gears.

• Loading Method: Applying alternating bending moments to the specimen through four-point bending, three-point bending, etc., observing its deformation and fracture behavior.

3. Torsional Fatigue Test

• Application: Designed for rotating components such as drive shafts and propellers.

• Loading Method: Applying pure torsional loads or combined torsional loads to the specimen to study its torsional fatigue characteristics.

4. Fatigue Test under Complex Stress Conditions

• Application: Complex structures subjected to multi-directional, multi-mode alternating loads, such as aircraft landing gear and engine blades.

• Loading Method: Employs a multi-axis loading device to simultaneously apply various types of alternating stress, including tension, compression, and shear, to more realistically simulate actual working conditions.

Fatigue Test Classification:

- High-cycle fatigue: Lower applied cyclic stress levels and higher cycle counts.

- Low-cycle fatigue: Higher applied cyclic stress levels and lower cycle counts.

- Other Classifications: Classified according to material properties (metal fatigue testing and non-metal fatigue testing) and working environment (high-temperature fatigue testing, corrosion fatigue testing, etc.).

Fatigue Testing Objects and Methods

Fatigue testing covers a wide range of objects, including metallic materials (such as steel, aluminum alloys, titanium alloys, etc.), mechanical parts (such as bearings, gears, springs, etc.), welded parts, composite materials (such as carbon fiber composites, glass fiber composites, etc.), electronic components, and key components in aerospace, automotive manufacturing, and building structures.

Testing methods are diverse, and different methods are typically used depending on the test object. Common fatigue testing methods include low-cycle fatigue testing, thermomechanical fatigue testing, corrosion fatigue testing, vibration fatigue testing, bending fatigue testing, contact fatigue testing, and multiaxial fatigue testing. These testing methods can simulate various stress conditions of materials or structures in actual use, thereby more accurately evaluating their fatigue performance.

Fatigue Testing Equipment and Instruments

Specialized testing instruments and equipment are required for fatigue testing. Among them, the fatigue testing machine is the core equipment for fatigue testing, capable of applying cyclic loads and precisely controlling stress or strain. In addition, auxiliary equipment such as stress-strain measurement systems, temperature control systems, corrosion fatigue testing devices, vibration tables, and crack monitoring systems are needed to monitor the changes of various parameters in real time during the testing process.

Fatigue Testing Operation Procedures

1. Test Preparation

• Standard Selection: Select the applicable international or national standard, such as ASTM E466 or ISO 12106. based on the product type and application field.

• Parameter Determination: Set key parameters such as test stress range, frequency, waveform, and environmental conditions to ensure they match actual usage conditions.

• Sample Preparation: Process standard specimens according to standard requirements or use finished parts directly; pay attention to the influence of surface treatment, dimensional accuracy, and other factors.

2. Installation and Debugging

• Specimen Fixation: Securely install the specimen on the testing machine, ensuring good coaxiality between the specimen and the machine spindle, and accurate loading points.

• Equipment Calibration: Check the working status of the testing machine, sensors, data acquisition system, etc., to ensure measurement accuracy and reliability.

• Pre-load Test: Perform a small number of pre-load cycles to confirm the normal operation of the entire system and eliminate potential problems.

3. Test Start

• Program Setup: Input the predetermined test parameters, including stress level, frequency, number of cycles, etc., and start the test program.

— Static Testing: Perform tensile tests on the specimens to measure their yield strength and tensile strength, in order to determine the specific parameters for the fatigue test.

— Fatigue Testing: Apply alternating loads to the specimens according to the set parameters and record the fatigue life of the specimens (i.e., the number of cycles required for the specimen to fail under alternating loads).

• Real-time Monitoring: Use computer software or specialized instruments to monitor and record various data generated during the test in real time, such as stress-strain curves and displacement changes.

• Anomaly Handling: If any anomalies are detected (such as increased noise or intensified vibration), immediately suspend the test, investigate the cause, and take appropriate measures.

4. Ending the Test

• Termination Conditions: Depending on the test objective, the test can be automatically stopped after reaching the predetermined number of cycles, or manually terminated when the specimen shows significant damage (such as crack propagation or fracture).

• Specimen Removal: Carefully disassemble the specimens to avoid causing further damage; for specimens that are not completely destroyed, further analysis of the remaining life is required.

5. Result Analysis

• Data Processing: Organize the test data and calculate important indicators such as fatigue life, mean stress, and fatigue strength. • Statistical Analysis: Using probabilistic statistical methods, such as Weibull distribution and Lognormal distribution, to assess the dispersion of fatigue life.

• Failure Mode Analysis: Using macroscopic and microscopic methods (such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM), to study the failure mechanisms of the samples and provide a basis for design improvement.

The Importance of Fatigue Testing

1. Safety Assessment

• Application Areas: Widely used in aerospace, automotive, bridge construction, and machinery manufacturing industries, ensuring that critical components do not suddenly fail due to fatigue during long-term service, protecting personnel and property safety.

• Case Studies: For example, critical components subjected to complex alternating loads, such as aircraft wings and engine blades, must undergo rigorous fatigue testing before being put into use.

2. Design Optimization

• Structural Improvement: Fatigue testing identifies potential design flaws, guiding engineers to adjust structural shapes, select more suitable materials, or improve manufacturing processes, thereby improving product durability and cost-effectiveness.

• Lightweight Design: Modern engineering aims to reduce weight while maintaining sufficient strength and stiffness; fatigue testing helps find the optimal balance.

3. Quality Control

• Factory Inspection: As a crucial part of product quality assurance, fatigue testing filters out substandard products, preventing defective products from entering the market.

• Certification and Accreditation: Many industry standards and regulations require products to pass specified fatigue tests to obtain market access or international certification.

The results of fatigue testing are of great significance for engineering design. Fatigue testing allows engineers to understand the reliability and durability of materials or structures in practical use, enabling them to make rational structural designs and material selections. Furthermore, fatigue testing can help identify potential defects in materials or structures, providing a basis for improving product performance. In industries such as aerospace, automotive manufacturing, building structures, and electronic components, fatigue test results are often directly related to product safety and reliability.