Steel Plate Damping Spring Testing

Steel plate damping spring inspection mainly conducts scientific testing and analysis on the static load characteristic testing, fatigue life testing, stiffness and flexibility testing, dimension and geometric tolerance testing and residual stress testing, 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

Static load characteristic testing: Mainly determines the mechanical properties of the spring under the static or quasi-static loads, including the deformation at the rated loads, stiffness, and the permanent deformation or fracture loads at the limit loads. This testing is the basis for evaluating whether the spring design meets the load-bearing requirements, ensuring that it does not produce the plastic deformation or fail under the normal working conditions.

Fatigue life testing: Simulates the working conditions of the leaf spring bearing the alternating loads in the actual use. The cyclic loads are applied through a fatigue testing machine, and the number of cycles when the spring fractures or produces the visible cracks is recorded. This testing is the key indicator for evaluating the reliability and durability of the spring, directly related to the service life and safety of the vehicle or equipment.

Stiffness and flexibility testing: The stiffness refers to the force required for the spring to produce the unit deformation, and the flexibility is its reciprocal. During the testing, the stepwise loading and unloading are carried out within the elastic range, the load-displacement curves are drawn, and the slope is calculated as the stiffness value. For the variable-stiffness or nonlinear leaf springs, the stiffness characteristics in the different load ranges need to be clearly tested.

Dimension and geometric tolerance testing: The tools such as the callipers, height gauges, projectors or coordinate measuring machines are used to precisely measure the key dimensions such as the total assembly height, leaf length, leaf width, leaf thickness, camber (arch) and centre distance of the spring, and the geometric tolerances such as the fitting clearances between the leaves and end shapes are checked, ensuring the conformity to the drawing assembly requirements.

Surface quality and anti-corrosion layer testing: The leaf surfaces are visually inspected or with the aid of the magnifying glasses to check whether there are the defects such as the cracks, folds, rust and scratches. At the same time, the thickness, adhesion and salt spray resistance of the anti-corrosion coatings (such as the paints, epoxy and Dacromet) need to be detected, to prevent the stress concentrations and early fatigue fractures caused by the corrosion.

Residual stress testing: After the rolling, heat treatment and shot peening strengthening, the residual stresses will exist inside the leaf spring. The X-ray diffraction method or hole-drilling method is usually adopted for the measurement. The reasonable residual compressive stress distributions can significantly improve the fatigue strength of the spring, while the excessive residual tensile stresses are harmful. This testing is crucial for the process quality control.

Testing Scope

Leaf springs of the automobile suspension systems: Covering the ex-factory inspection and periodic maintenance testing of the multi-leaf springs, low-stress springs (fewer-leaf springs) and variable-rate springs used in the various commercial vehicles (trucks and buses), trailers and some off-road vehicles and pickup trucks. They are the core components guaranteeing the ride comfort and load safety of the vehicles.

Damping springs of the rail transit vehicles: Including the heavy-duty leaf springs used on the bogies of the railway locomotives, freight cars and urban rail vehicles. Their testing standards are more stringent, and the high-frequency vibrations, huge impact loads and long-life requirements need to be considered. The testing items need to conform to the corresponding railway industry standards.

Springs of the construction machinery and heavy equipment: Applied to the suspension or damping systems of the equipment such as the excavators, loaders, cranes and agricultural machinery. The testing emphasis is on their reliability and durability under the extreme loads and harsh working conditions (such as the mud, water and sand impacts).

Damping and energy storage springs of the industrial equipment: Used for the vibration isolation foundations of the large stamping equipment, forging hammers and generator sets, or used as the energy storage elements in the mechanical structures. The testing needs to focus on their dynamic stiffness, damping characteristics and stress relaxation performance under the long-term loads.

Testing of the spring raw materials and semi-finished products: The chemical composition analysis, metallographic structure examination and mechanical property testing (tensile strength, yield strength and reduction of area) of the spring flat steels used to manufacture the springs (such as 60Si2MnA and 50CrVA) are carried out, and the flaw detection and preliminary dimension inspection of the cut leaves are also carried out.

After-sales maintenance and failure analysis parts: The testing of the leaf springs that fracture, deform or fail during the use is carried out. Through the means such as the fracture analysis, hardness testing and metallographic observation, the root causes of the failures are traced, such as the material defects, processing problems, overloads or corrosion, providing the basis for the quality improvement and responsibility judgement.

Testing Methods

Static load testing method: On a universal material testing machine or dedicated spring testing machine, the compression or bending loads are slowly applied to the spring to the specified heights or loads, and after the pressure holding, the unloading is carried out, measuring its permanent deformation and calculating the stiffness. This method has the simple operation, and is one of the most common final inspection methods on the production lines.

High-frequency fatigue testing method: The electromagnetic resonance type or hydraulic servo type fatigue testing machines are adopted to apply the alternating stresses to the spring with the constant amplitude or programmed block load spectra, and the frequencies can reach above 100 Hz, thereby evaluating its fatigue life (S-N curves) in a relatively short time. The test environments can control the temperature and humidity, and even simulate the corrosive environments.

Stiffness test bench method: On a dedicated spring characteristic test bench, the servo actuators are used to carry out the simulated static and dynamic loading of the spring assembly, precisely acquiring the load and displacement signals, and after the software processing, the stiffness curves and hysteresis curves are directly output, comprehensively evaluating its elasticity and damping properties.

Non-destructive testing (NDT) method: Including the magnetic particle flaw detection (MT) or fluorescent penetrant flaw detection (PT) for detecting the fine cracks on the leaf surfaces and near-surfaces; and the ultrasonic flaw detection (UT) for detecting the defects such as the slag inclusions and delaminations inside the material. These methods carry out the full inspection or sampling inspection without damaging the spring.

Salt spray testing and cyclic corrosion testing: The coated spring specimens are placed in a salt spray test chamber, simulating the marine or deicing salt environments, and the continuous spraying is carried out according to the specified cycles. After the testing, the rust grades and the coating blistering and peeling conditions are checked. The cyclic corrosion testing includes the multiple stages such as the wetting, drying and salt spraying, which is closer to the actual corrosion working conditions.

Metallographic analysis and hardness testing method: The spring samples are cut, and after the mounting, grinding, polishing and etching, the uniformity of the microstructures (such as the tempered troostite), grain sizes and decarburized layer depths are observed under a metallographic microscope. Combined with the Vickers or Rockwell hardness testers, the hardness is tested at the different positions, verifying the effectiveness of the heat treatment process.

Testing Standards

Domestic Standards:

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

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

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

4. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

5. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

International Standards:

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

2. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

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

Electronic universal material testing machine: Equipped with the large-tonnage compression fixtures and bending fixtures, used to carry out the static load testing, stiffness testing and destructive limit load testing of the spring. The equipment has the high precision, with the automatic data acquisition and processing functions, and can output the detailed test reports and curves.

High-frequency fatigue testing machine: The electromagnetic resonance type fatigue machines have the high efficiency and low energy consumption, suitable for the specified life verification testing of the mass-produced springs; the hydraulic servo fatigue machines have the large load ranges and precise control, and can simulate the complex random load spectra, mostly used for the research and development and reliability certification testing.

Comprehensive spring characteristic test bench: An integrated testing equipment, usually including the servo loading systems, high-precision sensors, environmental chambers (temperature and humidity control) and dedicated measurement and control software. It can complete the multi-item testing such as the static stiffness, dynamic stiffness, permanent deformation and fatigue in one stop, with the high automation.

Coordinate measuring machine (CMM): Used for the high-precision measurement of the complex three-dimensional geometric dimensions and geometric tolerances of the leaf springs, such as the arc degrees of the various leaves, centreline positions and flatness of the assembly surfaces. Through the scanning, the point cloud data are obtained, and the comparison analysis with the CAD models can be carried out.

Coating thickness gauge and adhesion tester: The magnetic or eddy current thickness gauges are used for the fast non-destructive measurement of the dry film thicknesses of the anti-corrosion coatings on the spring surfaces. The adhesion testers (cross-cut testers or pull-off testers) are used to quantitatively evaluate the bonding strength between the coatings and the metal substrates, and are the key control equipment of the coating quality.

Metallographic microscope and hardness tester: The inverted metallographic microscopes equipped with the image analysis systems are used for the observation and quantitative analysis of the material microstructures. The micro-Vickers hardness testers can carry out the fixed-point testing on the metallographic specimens, evaluating the local hardening or softening conditions of the material, and mutually confirming with the structure observation results.

Residual stress analyser: The X-ray diffraction residual stress analyser calculates the surface residual stresses by measuring the lattice strains, with the characteristics of the non-destructive testing and adjustable measurement depths. The portable equipment can directly test the large springs on site, providing the data support for the process optimization.