Definition and basic concept of fatigue testing
Fatigue testing is an experimental method to evaluate the resistance of materials, components or systems to performance degradation and failure under cyclic or alternating loads. Unlike static strength testing, fatigue testing focuses on the process of crack initiation, propagation and fracture after a certain number of cycles under repeated stress that is much lower than its yield strength. Fatigue failure is one of the most important forms of failure of engineering structures and mechanical components, and according to statistics, about 80% to 90% of engineering fracture accidents are related to fatigue. The core parameters of fatigue testing include stress level, cycle count, loading frequency, and stress ratio, and through the systematic study of these parameters, key performance indicators such as fatigue life curve, fatigue limit, and fatigue strength of the material can be obtained, providing a scientific basis for engineering design, life prediction, and safety assessment.
Physics of fatigue testing
The physical essence of fatigue failure is the accumulation process of irreversible microstructural changes that occur in materials under cyclic loading. This process can be understood at both the micro and macro levels. At the microscopic level, fatigue involves a series of complex physicometallurgical processes such as dislocation movement, slip zone formation, microcrack initiation and propagation. At the macroscopic level, it manifests as the gradual degradation of the mechanical properties of the material until the final fracture.
From the analysis of microscopic mechanisms, the fatigue process begins with plastic deformation in the local area of the material. Even if the nominal stress is lower than the yield strength, local stress concentration may still occur in areas such as microscopic defects, inclusions or grain boundaries in the material, and local plastic deformation will be triggered when the local stress exceeds the microscopic yield strength of the material. This plastic deformation manifests as the formation of a resident slip zone under cyclic loading. As the cycle continues, the resident slip zone gradually develops, forming a microscopic shape of extrusion and intrusion on the surface of the material, which becomes the source of crack initiation.
Once cracks occur, they enter the crack propagation stage. This stage can be divided into two sub-stages. Stage I crack propagation occurs in the direction of maximum shear stress, with a very low propagation rate, usually only a few grain scale. With the increase of crack length, the propagation direction gradually shifts perpendicular to the direction of maximum tensile stress, and enters the second stage of propagation. The crack propagation rate of stage II can be described by the Paris formula:
da/dN = C(ΔK)m
where da/dN represents the crack propagation rate, that is, the crack growth amount of each stress cycle; ΔK represents the amplitude of the stress intensity factor; C and m are constants related to materials, environment, etc. The Paris formula reveals the power law relationship between the crack propagation rate and the stress field strength of the crack tip, which is an important theoretical basis for fatigue life prediction.
When the crack extends to the critical size, the stress of the remaining section exceeds the fracture strength of the material, and instantaneous instability fracture occurs, forming the final instantaneous fracture zone of fatigue fracture. Typical fatigue fractures present three characteristic areas: the fatigue source area is usually located at the surface or near surface defects, and the fracture is flat and smooth; The fatigue propagation zone presents beach bands or fatigue glows, which record the process of crack propagation. The transient fault zone presents a rough fissure or cleavage morphology, which is similar to the static fault characteristics.
From an energy perspective, fatigue is the process by which a material continuously absorbs and dissipates energy under cyclic loads. In each stress cycle, part of the energy is stored and released in the form of elasticity, and the other part is dissipated due to plastic deformation, crack propagation and other processes. Final fracture occurs when the cumulative dissipation energy reaches a critical value of the material. This is where the concept of fatigue limit comes from: when the stress level is below a certain threshold, the energy dissipated per cycle is minimal, and the material can maintain its integrity without fatigue failure in an infinite number of cycles.
Mathematical descriptions of fatigue life are usually based on stress-life curves and strain-life curves. The stress-life curve, or S-N curve, describes the relationship between the applied stress level S and the number of cycles N that reach failure. In bilogarithmic coordinates, the S-N curve presents a linear relationship in the mid-life region, which can be expressed by the Basquin formula:
Sa = σf'(2Nf)b
In the formula, SaRepresents stress amplitude, σf'represents the fatigue strength coefficient, 2Nfrepresents the number of load reversals to failure, and b represents the fatigue strength index. For materials with fatigue limits, the S-N curve tends to be horizontal in the long life region, and the horizontal stress value is the fatigue limit.
The strain-life curve is suitable for low cycle fatigue, when the material undergoes obvious plastic deformation, and it is more reasonable to use strain as the control parameter. The Coffin-Manson formula describes the relationship between plastic strain amplitude and lifetime:
Δεp/2 = εf'(2Nf)c
In the formula, Δεp/2 represents the plastic strain amplitude, εf' represents the fatigue ductility coefficient, and c stands for the fatigue ductility index. The total strain amplitude is the sum of the elastic strain amplitude and the plastic strain amplitude, according to which a complete strain-life curve can be established.
Measurement method of fatigue test
There are a variety of standardized measurement methods for fatigue testing according to different control parameters, loading methods, and test purposes. The testing process usually includes sample preparation, equipment preparation, parameter setting, data acquisition, and result processing.
Fatigue testing machine is the core equipment for fatigue testing, and the main types include rotary bending fatigue testing machine, axial tensile compression fatigue testing machine, torsional fatigue testing machine and multi-axis fatigue testing machine. The rotary bending fatigue tester is the earliest developed fatigue testing equipment, which is suitable for the determination of the fatigue performance of smooth round specimens under rotational bending load, and is often used for the preliminary evaluation of material fatigue limit. The Axial Tensile Compression Fatigue Tester uses electro-hydraulic servo or electromagnetic resonance technology to precisely control stress and strain, making it suitable for both high-cycle and low-cycle fatigue testing. Torsional fatigue testing machine is used to evaluate the fatigue behavior of materials under the action of alternating torque. The multi-axis fatigue testing machine can apply compound loads to simulate the complex stress states of actual components.
Sample preparation is the basis for obtaining reliable test results. The shape and size of the specimen are determined according to the test standard and material type, and the common ones are smooth cylindrical specimens, flat specimens, notched specimens, etc. During the processing of specimens, it is necessary to strictly control the machining accuracy and surface quality to avoid the introduction of residual stress and surface defects, which will significantly affect the fatigue life. The surface of the gauge section of the specimen usually needs to be polished to eliminate traces of processing. For notched specimens, the notched root radius and processing quality need to be precisely controlled. After the sample is prepared, dimensional measurement and visual inspection are required, and only qualified ones can be tested.
High-cycle fatigue testing is usually performed in stress control mode, with a series of stress levels set, multiple specimens are tested until failure at each stress level, and the corresponding number of cycles is recorded. The selection of stress levels should cover a range from high stress short life to low stress long life, typically including 6 to 8 stress levels. In the area close to the fatigue limit, the number of specimens needs to be increased to improve statistical reliability. The selection of test frequency should consider the material properties and the testing machine capacity, metal materials usually use a frequency of 20 Hz to 200 Hz, too high a frequency may affect the results due to temperature rise. During the test, the load waveform, frequency stability, and specimen temperature need to be monitored in real time to ensure that the test conditions meet the set requirements.
Low-cycle fatigue testing is usually performed in strain control mode, using an extensometer to directly measure the strain of the specimen gauge section. The strain waveform usually adopts a triangular wave or a sine wave, and the strain ratio is generally -1 or 0. The strain amplitude should be selected so that the life range is between 10² and 10⁵ times. Due to the large strain amplitude, the specimen will have obvious cyclic softening or hardening, and the load needs to be adjusted in real time to maintain the set strain amplitude. The change of stress response with the number of cycles is recorded during the test, which is used to analyze the cyclic stress-strain behavior of the material and the extraction of life prediction parameters.
The fatigue crack propagation rate test uses a specimen of prefabricated cracks to monitor the change of crack length with the number of cycles under cyclic load. Commonly used specimens include compact tensile specimens and central crack tensile specimens. The crack length was measured in real time by microscopy, potentiometric or flexibility method, and the crack propagation rate da/dN and the stress intensity factor amplitude ΔK were calculated in combination with the load data. The da/dN-ΔK curve is plotted in double logarithmic coordinates, and the Paris formula parameters and threshold value ΔK are determinedth。
Data is a key part of fatigue testing. For the grouping test, a set of life data was obtained at each stress level, and the mean and standard deviation of the logarithmic life were calculated to establish the S-N curve. For the lifting test, it is used to measure the fatigue limit, and the fatigue limit value is approximated by adjusting the stress level step by step, and finally the mean and standard deviation of the fatigue limit are calculated by statistical methods. The fitting of the S-N curve can be done by the least squares method to establish a linear relationship between stress and life in bilogarithmic coordinates. For materials with fatigue limits, the fatigue limit value can be determined by grading or probabilistic methods.
The expression of the test results includes S-N curve, p-S-N curve, strain-life curve, crack propagation rate curve, etc. The S-N curve is usually plotted with the stress amplitude or maximum stress as the ordinate and the logarithm of the number of cycles as the abscissa. The p-S-N curve introduces the concept of survival rate and gives the stress-life relationship under different survival rates for reliability design. The test report should record in detail the material grade, heat treatment status, specimen size, surface condition, test equipment, control mode, loading waveform, frequency, environmental conditions, data points, and fitting curves.
Key factors that influence fatigue test measurements
Fatigue test measurement results are a combination of factors, from the intrinsic properties of materials to external test conditions, each of which can have a significant impact on fatigue life.
The chemical composition and microstructure of the material are intrinsic factors affecting fatigue performance. The types and contents of alloying elements affect the strength of the matrix through the mechanisms of solution strengthening and precipitation strengthening, which in turn affects the fatigue resistance. Inclusions and non-metallic inclusions are the preferred locations of fatigue cracks, and their type, size, shape and distribution have a decisive impact on fatigue life. Grain size affects slip length and crack propagation resistance, and fine-grained materials generally have higher fatigue strength. The heat treatment state determines the microstructure and mechanical properties of the material, and the fatigue limit of the same material after different heat treatments can vary several times. The anisotropy of the material, such as the rolling direction and fiber flow direction, makes the fatigue properties of different orientations different, and the sampling direction needs to be consistent with the main force direction.
The surface condition of the specimen has a significant impact on fatigue life, because fatigue cracks mostly originate on the surface. Surface roughness affects the degree of stress concentration, and the rough surface is equivalent to a large number of microscopic notches, which significantly reduces the fatigue life. The residual compressive stress introduced by grinding, polishing and other processes can improve the fatigue life, while the residual tensile stress introduced by grinding burns reduces the fatigue life. Surface metamorphic layers such as surface decarburization, oxidation, and corrosion change the mechanical properties of the surface material and affect the resistance of crack initiation. For notched specimens, the radius and surface quality of the notched root need to be precisely controlled, and any minor defects can become a source of crack initiation.
The specimen size effect is a factor that must be considered in fatigue testing. Large-sized specimens have an increased probability of containing more defects and have different stress gradients, often exhibiting lower fatigue strength than small-sized specimens. This size effect behaves differently in smooth and notched specimens, and the laboratory sample data needs to be extrapolated to the actual component by the size correction factor.
The control of loading conditions directly affects the test results. The choice of stress level determines the life range, too high stress leads to premature failure, and too low stress makes the test time too long. The influence of stress ratio R is significant, the symmetrical cycle at R=-1 is the most common, and the pulsation cycle at R=0 is closer to the actual working condition. The influence of loading waveforms is particularly important in high-frequency or corrosive environments, and the influence of different waveforms such as triangle, sine and square waves on strain rate and holding time changes the cyclic response of the material. The loading frequency has little effect on most metal materials at room temperature, but the frequency effect needs to be considered for materials that are sensitive to high temperature fatigue or strain rate.
The coupling effect of environmental factors cannot be ignored. Temperature affects the mechanical properties and deformation mechanism of the material, and the interaction between creep and fatigue at high temperature complicates the problem, and the brittleness tendency of the material increases at low temperature. Corrosive environments such as humid air, salt water, acidic media, etc. work together with cyclic loads to produce corrosion fatigue, which is much lower than pure mechanical fatigue. The irradiation environment will cause changes in the microstructure of the material and affect the fatigue properties. The control accuracy of the test environment, such as temperature fluctuations and media concentration, directly affects the repeatability and comparability of the results.
The accuracy and stability of testing equipment are crucial. The accuracy and linearity of the load cell determine the accuracy of stress control and need to be calibrated regularly. The gauge and resolution of the extensometer affect the accuracy of strain measurements, especially for low-cycle fatigue testing. The stiffness of the loaded frame and the uniformity of the stress distribution to neutrality affect the offset load, which causes additional bending stresses. The response speed and control accuracy of the hydraulic system determine the fidelity of the waveform. The sampling frequency and resolution of the data acquisition system affect peak capture and waveform reproduction capabilities.
The experience and standardization of the operator are also factors that cannot be ignored. The control of neutrality and clamping force of specimen installation, the installation quality of extensometer, the accuracy and frequency of crack monitoring, and the identification and treatment of abnormal situations all require standardized training and rich practice accumulation. Fatigue test data tend to have a large discreteness, and correct handling of outliers and statistical analysis are the basic requirements for obtaining reliable conclusions.
Fatigue testing in industrial applications
Fatigue testing has a wide range of application values in many industrial fields and is an important technical means for engineering design, material selection, life evaluation and failure analysis.
In the aerospace field, fatigue performance is the core indicator to ensure the safety and long life of aircraft structures. The main load-bearing structures such as the aircraft fuselage, wings, and tail are subjected to ground-air-ground cyclic loads, and their fatigue life is directly related to flight safety. Through full-scale fatigue testing and component fatigue testing, verify the rationality of structural design, determine inspection intervals and maintenance plans. Engine turbine discs, blades and other hot end components are subjected to the combined effect of high temperature and centrifugal load, and low cycle fatigue and thermomechanical fatigue are the main failure modes. The landing gear bears the impact load of each takeoff and landing, and its ultra-high cycle fatigue performance is the key to ensuring long-term service safety. Engineering applications of new aerospace materials such as titanium alloys, superalloys, composite materials, etc. must pass the fatigue performance test and evaluation of the system.
In the automotive industry, fatigue testing runs through the entire process of component development, vehicle verification and quality improvement. Engine crankshafts, connecting rods, pistons and other moving parts are subjected to alternating loads, and their fatigue strength determines the reliability and life of the engine. Chassis components such as axles, steering knuckles, suspension springs, etc., which are directly related to driving safety, need to be verified by bench fatigue tests and road simulation tests. The body structure is subjected to vibration loads from the road surface and the engine, and the structural design is optimized through fatigue analysis to avoid stress concentration and premature cracking. Transmission gears are subjected to the dual effects of contact fatigue and bending fatigue, and their fatigue life determines the reliability of the transmission system. In the process of automotive lightweighting, the application of new materials such as high-strength steel, aluminum alloy, magnesium alloy, and composite materials must be based on sufficient fatigue performance data.
In the field of rail transit, fatigue performance is related to train operation safety and maintenance costs. Wheels and axles are subjected to complex rotational bending loads, and their ultra-high cycle fatigue performance is the basis for determining inspection cycles and service life. The rail is subjected to repeated wheel loading, and the rail is damaged due to contact fatigue and rolling contact fatigue, and the applicability of different steel grades and heat treatment processes is evaluated by fatigue test. The fatigue strength of welded structures such as steering frames and traction devices is the key to design, and fatigue tests are required to verify the reliability of the welding process and joint form. The airtight fatigue problem of high-speed trains is becoming more and more prominent, and the body structure is subjected to alternating pressure differential loads, and its fatigue performance has become the focus of design.
In the energy sector, fatigue testing ensures the safe operation of critical equipment over long periods. The fatigue life of wind turbine blades is subjected to complex and changeable wind loads, and their fatigue life determines the economy and reliability of the wind turbine. Gas turbines and steam turbine rotors operate at high temperatures and pressures, and the start-stop process produces low-cycle fatigue, and the stable operation stage is superimposed on high-frequency vibration, and fatigue-creep interaction is the core of their life evaluation. The fatigue performance of pressure vessels and pipelines in nuclear power plants is an important part of nuclear safety analysis, and corrosion fatigue tests are required under the conditions of simulated coolant environment. Oil drill pipes, gutters, etc. are subjected to alternating tensile and bending loads, and their fatigue life determines the safety and efficiency of drilling operations.
In the field of marine engineering, the fatigue problem of structures under the action of wave cyclic loads is particularly prominent. The fatigue strength of the welding joints of offshore platform jackets and tension leg platform mooring systems is the controlling factor of structural design. Submarine pipelines are subjected to internal pressure fluctuations and vibrations caused by seawater flow, and corrosion fatigue and vortex vibration fatigue in the free-suspension section are the main failure modes. The effects of anti-corrosion coating and cathodic protection on fatigue performance were evaluated by simulating the fatigue test of seawater environment. The ship structure is subjected to repeated wave loads, and the fatigue strength of key nodes is verified by the fatigue test of the segmented model.
In biomedical engineering, fatigue testing evaluates the long-term reliability of implants and medical devices. The fatigue life of artificial hip and knee joints is subjected to walking cycle loads, and their fatigue life directly affects the service life of implants, and the suitability of materials and designs is verified by fatigue tests that simulate the in vivo environment. The fatigue performance of heart valves is directly related to the life safety of patients, and their long-term durability needs to be verified by accelerated life tests. Dental implants, bone fixation plates, etc. are subjected to cyclic loads of chewing and daily activities, and fatigue performance is an important indicator for evaluating their safety. Vascular stents are subjected to circulatory pulsation loads in blood vessels, and their fatigue life determines the long-term patency effect.
In the field of electronics and electrical appliances, thermal fatigue and vibration fatigue of microstructures affect product reliability. The solder joint is connected to materials with different thermal expansion coefficients, and its life determines the reliability of electronic components. Portable devices such as mobile phones and laptops are subjected to shock and cyclic loads during drops and vibrations, and their structural fatigue properties are verified by mechanical impact tests and vibration fatigue tests. The bending fatigue performance of flexible electronic products is a key technical indicator for their commercialization, and the service life needs to be evaluated through repeated bending tests.
In materials and manufacturing, fatigue testing provides a basis for process optimization and quality control. For the engineering application of new alloys, composites, and additive manufacturing materials, a complete fatigue performance database must be established. The quality of welding, brazing, gluing and other connection processes directly affects the fatigue properties of joints, and the applicability of different process parameters is evaluated by fatigue test. The residual compressive stress introduced by surface strengthening processes such as shot peening, rolling, carburizing, nitriding, etc. can significantly improve the fatigue life, and its effect is quantitatively evaluated by fatigue testing. The optimization of the heat treatment process is based on fatigue performance, especially for critical parts subjected to cyclic loads.
Summary and outlook
As an experimental method to evaluate the performance degradation and failure of materials under cyclic loading, fatigue testing reveals the essential law of fatigue failure from the multi-scale physical mechanism of microscopic dislocation motion to macroscopic crack propagation. Through standardized test methods such as rotational bending, axial tensile compression, and crack propagation, fatigue performance parameters such as S-N curve, fatigue limit, and crack propagation rate are quantified, which provides a scientific basis for engineering design, life prediction and safety assessment. From the intrinsic properties of materials to external test conditions, the combined influence of many factors requires fatigue testers to have a deep theoretical foundation and rigorous practical skills. In a wide range of fields such as aerospace, automobile industry, rail transit, energy equipment, marine engineering, biomedicine, electronic appliances, and material manufacturing, fatigue testing has become an important technical means to ensure product safety and reliability, optimize structural design, verify manufacturing processes, and evaluate service life.
Looking ahead, fatigue testing technology is developing in the direction of multi-field coupling, multi-scale, high precision, and digitalization. The multi-axis fatigue testing technology is becoming more mature, which can more realistically simulate the complex stress state of components under actual working conditions, and improve the accuracy of life prediction by combining damage models such as the critical plane method. The development of thermal-mechanical coupling fatigue testing system meets the testing needs of high-temperature components in aerospace, energy and power fields, and supports the study of creep-fatigue interaction mechanism and the verification of life prediction methods. Corrosion fatigue, fretting fatigue and other environmental-mechanical coupling testing technologies have been continuously improved, providing conditions that are closer to actual conditions for applications in marine engineering, biomedicine and other fields.
The research on fatigue at the micro scale continues to deepen. In-situ fatigue testing technology combines scanning electron microscopy, electron backscatter diffraction, digital image correlation and other means to observe the microscopic process of fatigue damage in real time, correlate the macroscopic performance with the microscopic mechanism, and provide a physical basis for the development of new materials and life prediction. Crystal plastic finite element simulation combined with experimental verification reveals the fatigue behavior of polycrystalline materials at the mesoscale scale. The development of micro-nanoscale fatigue testing technology supports the evaluation of fatigue performance of materials in emerging fields such as microelectromechanical systems and flexible electronics.
The study of ultra-high cycle fatigue has become a hot topic. With the emergence of high-strength materials and long-life requirements for key components, ultra-high cycle fatigue behavior of more than 10 million or even billion cycles has attracted attention. Ultrasonic fatigue testing technology increases the test frequency to 20 kHz, greatly shortens the ultra-high cycle fatigue test time, and provides an experimental means for studying the crack initiation and propagation mechanism below the fatigue limit. The research on the failure mechanism, size effect and statistical characteristics of ultra-high cycle fatigue has been deepened, which has promoted the transformation of fatigue design from infinite life to finite life, and from deterministic to probabilistic.
Digital technology and artificial intelligence are changing the fatigue testing paradigm. The fully digital fatigue testing machine realizes precise waveform control, real-time data analysis, and remote monitoring, improving testing efficiency and data quality. The application of computer vision technology in crack detection realizes non-contact, full-field, and real-time crack propagation monitoring. Based on machine learning and big data, the fatigue life prediction method mines the laws from massive test data, establishes the correlation model between material composition, process and performance, and guides the development of new materials and process optimization. The construction and sharing of fatigue test database promote the common technological progress of the industry. It is foreseeable that fatigue testing, a classic mechanical property evaluation method, will continue to play an irreplaceable role in the fields of new material research and development, equipment manufacturing and life management, providing solid technical support for the safe and reliable operation of major projects and key equipment.