Testing of spring fatigue performance and relaxation rate

1. Spring fatigue performance testing requirements
All springs subjected to variable loads must undergo fatigue performance testing in the process of internal quality control, user inspection, new product approval, certification, industry sampling and process adjustment. The testing methods can be two types: fatigue life verification test or reliability evaluation test. However, for the classification and certification of products, reliability evaluation test must be used to assess the reliability index.

See Table 1 for the required testing institutions, testing reports and testing cycles for different testing purposes.

Table 1 Testing institutions, reports and cycles required for different testing purposes

Inspection agency inspection report inspection period

Life Screening Enterprise or quality inspection agency recognized by the enterprise-at any time

Process adjustment company or quality inspection agency recognized by the company-synchronized with process adjustment cycle

Internal quality control company or quality inspection agency recognized by the company-at least once every six months

User inspection, mutual agreement-synchronized with the inspection cycle

Industry sampling inspection of new product type appraisal, government-authorized or designated quality inspection agency at or above the ministry (provincial) level, with metrological certification mark, when needed

Certification Nationally authorized, accredited by the National Laboratory Accreditation Board, and a third-party quality inspection agency designated by the certification committee with metrological certification marks and laboratory certification marks, when needed

For different testing purposes and product types, the minimum required number of test cycles is shown in Table 2.

Table 2 Minimum number of test cycles (times) specified by spring product type and test purpose

Product Type Industry Sampling Inspection, Fixed Type Appraisal and Certification

Valve spring 1×10 1.5×10 2.3×10

Pressure regulating spring 2×10 1.0×10 1.0×10

Plunger spring 1×10 1.5×10 2.3×10

Hydraulic springs Select according to classification standards Select according to classification standards Select according to classification standards

Suspension spring 2×10 2×10 2×10

After the fatigue test with the specified number of cycles, the relaxation rate tolerance of the spring shall be specified in accordance with the corresponding standard. When the standard does not stipulate, the mandatory testing shall be determined by the testing unit, and the non-mandatory testing shall be implemented according to the requirements of the user or the inspected unit.

For the reliability evaluation test, the qualified crystal, first-class and excellent products of the spring shall reach the reliability level specified in Table 3 after the specified test conditions and cycle times. For certified products, the reliability of fatigue life must reach the reliability index of superior products.

Table 3 Reliability index of product grade and fatigue life

Product grade reliability R(%)

Premium 99.99

First class 99.9

Qualified product 99

2. Spring fatigue test specimen

1) The sample should be manufactured in accordance with the technical documents approved in the prescribed procedures and inspected to be qualified.

2) Samples should be randomly and independently drawn from the same batch of products. The division of inspection batches shall be carried out according to JB/T58700.

3) Number of samples:

For the fatigue life verification test, the number of samples is not less than 5 sets for engines below 4 cylinders; the number of samples is not less than 3 sets for engines with 4 cylinders or more;

For the reliability evaluation test, the number of samples is determined according to Table 6-6.

3. Spring fatigue testing machine

1. It adopts mechanical, electromagnetic resonance or electro-hydraulic servo testing machine, as well as supporting host. When using electromagnetic resonance testing machine, the test amplitude should not be too large.

2) The error of the testing machine should comply with relevant regulations.

3) The frequency of the testing machine should be adjustable within a certain range to meet different test requirements.

4) The test machine should generally have functions: test time or times preset function; automatic timing or counting function; test amplitude display or control function; automatic shutdown function.

4. Spring fatigue test frequency

1) The test frequency can be determined according to the frequency range of the testing machine and the actual working frequency of the spring. Except for the random load test, the test frequency should be kept constant throughout the test process.

2) The test frequency γr should be the same as the natural natural frequency of a single test spring, and satisfy (γ/γr)>10. γ is calculated according to formula (7-79).

5. Spring fatigue test amplitude The amplitude is divided into displacement amplitude and stress amplitude. For the fatigue life verification test and reliability evaluation test of the coil spring, the displacement amplitude is generally used as the test amplitude.

6. Spring fatigue test procedure Refer to the spring fatigue test in Chapter 6, Section 5.

7. Selection of spring failure mode During the fatigue test of the spring, the failure mode is divided into two types: fracture and loss of specified function due to relaxation. According to the functional requirements of the spring, one or two of them can be selected as the failure mode of the fatigue life.

8. Evaluation of spring fatigue test results

1) Determination of fatigue life: Under a given failure condition, the maximum number of cycles reached by the tested spring is the fatigue life of the spring.

2) Verification of fatigue life: After a given number of cycles, if the spring does not fail, the verification test is passed.

3) Reliability verification: According to the given reliability target value, determine the appropriate number of samples n according to Table 6-6, test under the specified test conditions and requirements, and judge according to the required failure mode. If the qualified number obtained is greater than the minimum qualified number r required in the table, the verification test is passed, otherwise it is not passed.

4) Spring relaxation rate: the spring relaxation rate ε is expressed as the percentage of load loss after the spring test:

Where F-spring load measured before the test;

Fi-spring load measured after the test.

The relaxation rate after fatigue test should be within the range specified in the drawing.