The metal rod underwent a three-point bending fatigue test

Three‑Point Bending Fatigue Testing Service for Metal Rods – Comprehensive Evaluation of Flexural Fatigue Performance and Service Life Prediction

As an ISO/IEC 17025 accredited independent testing laboratory, we offer specialised three‑point bending fatigue testing services for metal rods, bars, shafts, and other elongated metallic components. Three‑point bending fatigue is a critical test method for evaluating the resistance of metallic materials to cyclic flexural stresses – simulating the loading conditions experienced by axles, shafts, pins, springs, and structural members subjected to repeated bending forces during service. Our test protocols apply a sinusoidal cyclic load to the test specimen at a specified stress amplitude and frequency, recording the number of cycles to failure and generating S‑N curves that enable accurate prediction of fatigue life under service conditions. All methods are aligned with ASTM E466 (Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials), ISO 1099 (Metallic materials – Fatigue testing – Axial force‑controlled method), ASTM E606 (Strain‑controlled fatigue testing), ISO 12107 (Statistical planning and analysis of fatigue tests), GB/T 3075 (Axial force fatigue testing of metallic materials), and GB/T 24176 (Statistical analysis of fatigue data). Our inspection and test reports are recognised by the National Medical Products Administration (NMPA), the State Administration for Market Regulation (SAMR), the Ministry of Industry and Information Technology (MIIT), and international certification bodies for product registration, type approval, and quality assurance.

The metal rod underwent a three-point bending fatigue test.

Metal Rod Types and Material Grades We Regularly Test

Our three‑point bending fatigue test facilities accommodate a wide range of metallic rod materials, diameters, and surface finishes. Typical test articles include:

Carbon and low‑alloy steel rods – for automotive axles, suspension components, and general engineering applications
Stainless steel rods – austenitic, martensitic, and duplex grades for corrosion‑resistant and high‑strength applications
Aluminium alloy rods – 2xxx, 6xxx, and 7xxx series for aerospace, automotive, and structural applications
Titanium alloy rods – Ti‑6Al‑4V and other high‑performance alloys for aerospace and medical applications
Nickel‑based superalloy rods – Inconel®, Monel®, and Hastelloy®‑type alloys for high‑temperature and corrosive environments
Copper and copper alloy rods – for electrical and thermal conductivity applications
Hardened and tempered steel rods – for applications requiring high strength and wear resistance
Coated and surface‑treated rods – with plating, coating, or nitriding for enhanced fatigue performance

Test Specimen Preparation – Ensuring Consistency and Representative Results

Specimen dimensions and geometry – standardised for fatigue testing – We machine the test specimens to the specified dimensions (typically 100‑200 mm long, with a uniform cross‑section in the test section). The gauge length and the overall dimensions are selected to ensure that the stress distribution is uniform in the test section. The specimen surface is machined to a specified surface finish (typically Ra ≤ 0.8 µm) to avoid stress concentration effects from surface irregularities.
Surface preparation and polishing – to eliminate stress raisers – We grind and polish the specimen surface (especially in the critical test section) to a smooth finish, as surface roughness can significantly reduce the fatigue strength. The longitudinal direction of the specimen is aligned with the rolling or forging direction of the rod material.
Notch and stress concentration introduction – for notch sensitivity assessment – For assessing the notch sensitivity of the material, we introduce a standardised notch (e.g., a V‑notch or a circular groove) in the test section. The notch geometry (depth, radius, and angle) is specified by the client or the applicable standard.
Marking and identification – for traceability – Each specimen is marked with a unique identification number (using a low‑stress marking method, e.g., electro‑chemical etching) to ensure traceability throughout the test process.

Three‑Point Bending Fatigue Test Setup – Fixturing, Loading and Instrumentation

Test fixture – three‑point bending support and loading arrangement – We mount the rod specimen on two support rollers (lower supports) with a specified span length (L) and apply the cyclic load at the centre (mid‑span) using a hemispherical or cylindrical loading roller (upper loading nose). The span length is typically 4‑10× the specimen diameter, depending on the standard and the material. The fixture is designed to minimise friction and misalignment, ensuring that the load is applied perpendicular to the specimen axis.
Cyclic loading – sinusoidal waveform at a specified stress amplitude and frequency – We apply a sinusoidal cyclic load (force‑controlled) to the specimen using a servo‑hydraulic or electromechanical fatigue testing machine with a calibrated load cell (accuracy ±0.5 %). The test is performed at a specified stress amplitude (or load amplitude) and a specified frequency (typically 5‑50 Hz, depending on the specimen geometry and the material). The load ratio (R = Fmin / Fmax) is typically set to 0.1 (tension‑tension) or -1 (fully reversed).
Displacement and strain measurement – for accurate deformation monitoring – We use a high‑precision LVDT (linear variable differential transformer) or an extensometer to measure the displacement of the specimen (or the deflection) during the test. The displacement measurement is accurate to ±0.001 mm.
Temperature control – for testing at elevated or reduced temperatures – For materials that are used in extreme environments, we perform the three‑point bending fatigue test at elevated temperatures (up to 600 °C) using a temperature‑controlled furnace, or at reduced temperatures (down to -40 °C) using an environmental chamber.

Test Procedure – Step‑by‑Step Execution and Monitoring

Pre‑test calibration and system validation – We calibrate the load cell, the LVDT, and the temperature sensors before each test run. The test system is validated by performing a preliminary test on a reference specimen of known fatigue behaviour.
Specimen installation and alignment – The specimen is carefully mounted in the test fixture, ensuring that it is centred and aligned with the loading axis. The support rollers are set at the specified span distance, and the loading nose is positioned at the centre of the specimen.
Initial load application and stabilisation – The initial load is applied (or the specimen is pre‑stressed) to eliminate any play in the fixture and to stabilise the specimen.
Cyclic loading and monitoring – continuous recording of load, displacement, and cycles – The cyclic load is applied, and the load, displacement, and number of cycles are recorded continuously. The test is performed until failure (fracture of the specimen) or until the maximum number of cycles (typically 10⁷‑10⁸ cycles) is reached.
Failure detection – using load drop or displacement increase – Failure is detected by a sudden drop in the load (or a sudden increase in the displacement), which indicates the initiation of a macroscopic crack.
Number of cycles to failure (Nf) – the primary output of the test – The number of cycles to failure (Nf) is recorded for each specimen. A test that does not result in failure is recorded as a run‑out.

Fatigue Data Analysis – S‑N Curve Generation and Fatigue Life Prediction

Stress amplitude and mean stress calculation – The bending stress amplitude (σa) is calculated from the applied load amplitude and the specimen geometry: σa = (Fmax – Fmin) × L / (4 × Z) where Fmax and Fmin are the maximum and minimum applied loads, L is the span length, and Z is the section modulus of the specimen. The mean stress (σm) is calculated as (Fmax + Fmin) / (2 × A).
S‑N curve (stress‑life curve) – for characterising the fatigue behaviour – We perform three‑point bending fatigue tests at multiple stress amplitudes (typically 4‑6 levels) and plot the stress amplitude (S) versus the number of cycles to failure (Nf) to generate an S‑N curve. The S‑N curve is fitted to a power‑law relationship (Basquin’s equation): σa = a × (Nf)^b, where a and b are material constants.
Fatigue limit (endurance limit) – the stress amplitude that allows infinite life – The fatigue limit is the stress amplitude below which the material can withstand an infinite number of cycles (typically ≥ 10⁷ cycles). The fatigue limit is determined from the S‑N curve as the stress level at which the curve flattens.
Statistical analysis – for determining the scatter and the reliability – For a given stress amplitude, we test multiple specimens (typically 5‑10) and calculate the mean Nf, the standard deviation, and the coefficient of variation (CV). We also perform a statistical analysis using the staircase method (for the fatigue limit) or the Maximum Likelihood Estimation (MLE) method to determine the fatigue life at a given reliability (e.g., 50 %, 90 %, or 95 %).

Failure Analysis and Fracture Surface Examination – Understanding the Fatigue Mechanism

Visual inspection – for identifying the failure mode – After the test, we inspect the failed specimen to determine the failure mode: (a) ductile fatigue – characterised by a smooth, beach‑marked fatigue fracture surface, with a final overload fracture; (b) brittle fatigue – characterised by a rough, granular fracture surface; (c) mixed – a combination of both.
Fracture surface analysis – using stereomicroscopy and scanning electron microscopy (SEM) – ASTM E1508 / ISO 10543 – We examine the fracture surface under a stereomicroscope and, where required, under a scanning electron microscope (SEM) to identify the fatigue crack initiation site, the propagation direction, the fatigue striations (beach marks), and the final overload region. The presence of inclusions, micro‑cracks, and other defects is recorded.
Metallographic examination – ASTM E3 / ISO 4496 – for assessing the material's microstructure – We prepare metallographic specimens from the failed rod and examine the material's microstructure (grain size, phase distribution, inclusion content) to identify any microstructural features that may have contributed to the fatigue failure.
Hardness and micro‑hardness measurement – for assessing the effect of fatigue on the material properties – We measure the hardness (Rockwell or Vickers) of the specimen before and after the fatigue test. A significant increase in hardness (or a decrease) indicates a change in the material's microstructure.

Regulatory Compliance and Product Certification – Supporting Industry Standards

Our three‑point bending fatigue testing services are performed in accordance with the most widely used international and national standards. The most commonly requested include:

ASTM E466 – Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials – the primary standard for force‑controlled fatigue testing (adaptable for bending)
ISO 1099 – Metallic materials – Fatigue testing – Axial force‑controlled method – the international standard for force‑controlled fatigue testing
ASTM E606 – Standard Test Method for Strain‑Controlled Fatigue Testing – for strain‑controlled fatigue testing
ISO 12107 – Metallic materials – Fatigue testing – Statistical planning and analysis of fatigue tests – for statistical analysis of fatigue data
GB/T 3075 – Metallic materials – Axial force fatigue testing – the Chinese national standard for force‑controlled fatigue testing
GB/T 24176 – Metallic materials – Statistical analysis of fatigue data – the Chinese national standard for statistical analysis of fatigue data
ASTM E8 / E8M – Tensile testing of metallic materials – for determining the tensile properties (used as a reference for fatigue testing)
ISO 7438 – Metallic materials – Bend test – for bend testing (used as a reference)

Report Acceptance and Regulatory Recognition

All three‑point bending fatigue tests are conducted under our ISO/IEC 17025 accreditation, using calibrated fatigue testers, load cells, and extensometers, all traceable to national and international reference standards. Our final test reports include: a complete description of the test article (material, dimensions, surface finish, heat treatment), the test method and conditions (stress amplitude, load ratio, frequency, temperature), the measured parameters (number of cycles to failure, fatigue limit, S‑N curve), a statistical summary (mean Nf, standard deviation, coefficient of variation), a detailed failure analysis (photographs and SEM images), and a clear pass/fail verdict against your specified acceptance criteria. These reports are accepted by the National Medical Products Administration (NMPA), the State Administration for Market Regulation (SAMR), the Ministry of Industry and Information Technology (MIIT), and international certification bodies for product registration, type approval, and quality assurance. Bilingual (English/Chinese) versions are available to facilitate submissions to domestic and international authorities and to support your global market access.