Metal Material Stress Fatigue Testing

Stress fatigue testing of metallic materials mainly conducts scientific testing and analysis on the stress-life, strain-life, fatigue crack initiation life testing, fatigue crack growth rate, fatigue limit or conditional fatigue strength determination and fatigue testing under the variable amplitude load spectra, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Stress-life (S-N) curve testing: This testing aims to determine the fatigue life of the metallic material under the different constant stress amplitude values, that is, the cycles to failure. By drawing the relationship curve (S-N curve) between the stress amplitude and the cycles to failure (usually in the logarithmic coordinates), the fatigue strength limit of the material and the durability at the different stress levels are evaluated. It is the basic characterization of the fatigue performance.

Strain-life (ε-N) curve testing: Mainly for the low-cycle fatigue field, focusing on the fatigue behaviour of the material when the plastic strain plays the dominant role. By controlling the strain amplitude, the cyclic stress-strain response is recorded, and the relationship curve between the total strain amplitude and the cycles to failure is established, used to evaluate the elastic-plastic behaviour of the material under the cyclic loads and predict the fatigue life.

Fatigue crack initiation life testing: This testing focuses on determining the number of cycles experienced from the initial defect-free state to the formation of the detectable micro-cracks (usually defined as 0.25-1.0 mm). It focuses on the resistance of the internal defects, microstructures and surface conditions of the material to the crack initiation, and is the key link for evaluating the fatigue resistance of the material.

Fatigue crack growth rate (da/dN) testing: On the basis of the pre-cracked specimens, the subcritical growth behaviour of the crack under the cyclic loads is studied. By measuring the change of the crack length with the cycles, the relationship curve between the crack growth rate and the stress intensity factor range (ΔK) (such as the Paris formula) is established, used to predict the remaining life of the defective components and the damage tolerance design.

Fatigue limit or conditional fatigue strength determination: For the materials with the fatigue limits (such as the steels), the maximum stress amplitude at which no failure occurs under the infinite life (usually based on 10⁷ cycles) is determined. For the materials without the clear fatigue limits (such as the non-ferrous metals), the conditional fatigue strength when reaching the specified cycle base (such as 5×10⁸ cycles) is determined.

Fatigue testing under the variable amplitude load spectra: Simulates the complex load histories in the actual service, such as the programmed block loading or random load spectrum loading. This testing is used to study the load sequence effects, overload retardation effects and load interaction effects, to verify the accuracy of the cumulative damage models (such as the Miner's rule), evaluating the fatigue behaviour of the material under the real working conditions.

Testing Scope

Ferrous metal materials: Including the various carbon steels, alloy steels, cast irons and stainless steels. The testing focuses on their unique fatigue limit phenomena, the influence of the heat treatment states (such as the quenching and tempering) on the fatigue strength, and the roles of the non-metallic inclusions and microstructures (such as the martensite and bainite) in the crack initiation.

Non-ferrous metals and their alloys: Covering the aluminium alloys, titanium alloys, magnesium alloys and copper alloys. Since most non-ferrous metals have no obvious fatigue limits, the testing emphasis is on the high-cycle and low-cycle fatigue performances, the sensitivity to the microstructures (such as the precipitates and grain sizes) and the fatigue behaviour coupled with the corrosive environments.

Additively manufactured metal materials: For the metal parts prepared by the processes such as the selective laser melting and electron beam melting. The testing needs to particularly focus on the significant influences of the anisotropy, internal pores, lack of fusion defects, residual stress distributions and post-treatments (such as the hot isostatic pressing and surface polishing) on the fatigue performance.

Welded joints and heat-affected zones: The fatigue failures of the welded structures mostly start from the welds or heat-affected zones. The testing scope includes the weld metal, fusion line and coarse-grained zones, evaluating the weakening degrees of the welding defects (blowholes and slag inclusions), residual stresses, structure non-uniformity and mechanical property mismatch on the overall structural fatigue strength.

Materials after the surface treatments and strengthening: Evaluates the improvement effects of the surface treatment processes such as the shot peening, roller burnishing, carburizing, nitriding and coating on the fatigue performance. The testing focuses on the depth and distribution of the surface residual compressive stress layer after the treatment, the changes of the surface roughness, and the improvement of the crack initiation resistance by the bonding state between the treatment layer and the substrate.

Materials under the extreme environments: Including the high-temperature fatigue, low-temperature fatigue, corrosion fatigue and fretting fatigue. The testing needs to be carried out in the specific environmental chambers, studying the synergistic effects of the environmental factors such as the temperature, corrosive media and contact friction with the cyclic stresses, evaluating the durability limits of the material under the complex service conditions.

Testing Methods

Axial tension-compression fatigue testing method: The most classic fatigue testing method. The axial cyclic tension-compression loads are applied to the specimen, and the stress ratio R is usually between -1 (symmetric cycle) and 0.1. This method has the clear stress states, is widely used to obtain the S-N curves and fatigue limits of the material, and is the internationally common standard testing method.

Rotating bending fatigue testing method: The specimen is subjected to the constant bending moment while rotating, and the surface points of the specimen experience the symmetric cyclic stresses. This method has the relatively simple equipment and a long history, is suitable for determining the fatigue performance of the smooth specimens under the symmetric bending loads, and is widely applied in the testing of the small-size bars.

Three-point or four-point bending fatigue testing method: The cyclic bending loads are applied to the plate-shaped or beam-shaped specimens. This method can effectively simulate the working conditions of the components bearing the bending stresses, is commonly used to evaluate the fatigue behaviour of the welded joints, coated specimens or the parts with the stress gradients, and the stress calculation is based on the beam bending theory.

Crack growth rate testing method: The compact tension or centre-cracked tension specimens are usually adopted. On the fatigue testing machine, through the decreasing-K or constant amplitude loading, combined with the microscope or compliance method for the online monitoring of the crack length, the da/dN under the different ΔK values is precisely calculated. It is the core data source for the damage tolerance design and life prediction.

Local strain method (based on the Neuber's rule): An analytical testing method that converts the nominal stress and strain into the local stress and strain at the notch root. By combining the cyclic stress-strain curve of the material and the component geometry, the fatigue crack initiation life of the notched parts is predicted. It is an important bridge connecting the intrinsic properties of the material and the actual responses of the components.

Infrared thermography and acoustic emission monitoring method: The advanced online non-destructive monitoring techniques. The infrared thermography inverts the stress distribution and damage evolution by detecting the temperature field changes (heat dissipation) on the specimen surface; the acoustic emission technology realizes the real-time dynamic monitoring of the fatigue damage process by capturing the elastic wave signals released by the internal crack growth and plastic deformation of the material.

Testing Standards

Domestic Standards:

1. GB/T 3075, Metallic materials — Fatigue testing — Axial force-controlled method

2. GB/T 6398, Metallic materials — Fatigue testing — Fatigue crack growth method

3. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

4. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

5. GB/T 232-2010, Metallic materials — Bend test method

International Standards:

1. ISO 12106, Metallic materials — Fatigue testing — Axial-strain-controlled method

2. ASTM E466, Standard practice for conducting force controlled constant amplitude axial fatigue tests of metallic materials

3. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Servo-hydraulic fatigue testing machine: The core fatigue testing equipment. The hydraulic actuator is precisely controlled through the servo valves, realizing the closed-loop control of the load, displacement or strain. It has the fast dynamic response and wide load capacity range (from several newtons to over a thousand kilonewtons), and can carry out the high-cycle and low-cycle fatigue tests under the axial, bending, torsional and combined loads.

High-frequency resonant fatigue testing machine: Using the mechanical or electromagnetic resonance principle, the specimen is fatigue-tested at its resonant frequency, with the frequency up to 100-300 Hz. This equipment has the low energy consumption and high efficiency, is particularly suitable for the very high cycle fatigue testing (cycles > 10⁷), but the load waveform and amplitude control have certain limitations.

Extensometers and strain gauges: The key process measurement sensors. The contact extensometers or non-contact video extensometers are used to precisely measure the axial or transverse deformation of the specimen; the resistance strain gauges are directly pasted on the specimen surface, used to measure the local strains, especially indispensable in the strain-controlled fatigue and stress concentration area analyses.

Crack length monitoring system: Including the direct current potential drop method system, compliance method measuring system and long-focus microscope. The DC-PD method calculates the crack length by measuring the potential difference changes on the two sides of the crack; the compliance method calculates it through the slope changes of the load-displacement curves; the microscope provides the intuitive visual observation. The three are often combined to ensure the measurement precision.

Environmental simulation test chamber: Provides the controllable environments for the high-temperature, low-temperature or corrosion fatigue testing. The high and low temperature chambers can realize the precise temperature control from -70 ℃ to +350 ℃ or even higher temperature ranges; the corrosion fatigue chambers can circularly introduce the specific gases or atomize the corrosive solutions, simulating the harsh service environments such as the acid rain and salt spray.

Data acquisition and control system: The "brain" of the testing machine, based on the industrial computers and dedicated control software. It is responsible for setting the parameters such as the loading waveforms, frequencies and amplitudes, collecting the signals such as the load, displacement, strain, crack length and temperature in real time, and carrying out the online processing, display and storage, ensuring the controllability of the testing process and the traceability of the data.