Complete guide to slow strain rate stress corrosion testing machine (SSRT): principles, parameters, and selection explained in one go
2026-08-24 13:51:51In the field of material corrosion and protection, the Slow Strain Rate Testing (SSRT) is the core equipment for evaluating the susceptibility of materials to stress corrosion cracking (SCC).As the oil and gas, marine engineering, nuclear power, aerospace and other industries continue to improve the corrosion resistance of materials, the demand for SSRT testing is growing rapidly.However, many buyers do not understand the working principle, key parameters and selection methods of SSRT testing machines.This article will systematically introduce the working principle, core parameters, application scenarios and selection methods of the slow strain rate stress corrosion testing machine to help you fully understand this professional testing equipment.
1. What is slow strain rate stress corrosion test?
1.1 Basic definition
Slow strain rate stress corrosion testing (SSRT) is an accelerated stress corrosion cracking testing method.The core principle is to expose a metal specimen to a specific corrosive medium environment and stretch the specimen at a very slow and constant strain rate (usually between 10⁻⁴ s⁻¹ and 10⁻⁷ s⁻¹) until the specimen breaks.By comparing the differences in mechanical properties (elongation, reduction of area, fracture time, fracture energy, etc.) between specimens in corrosive media and inert environments (such as air or silicone oil), the sensitivity of materials to stress corrosion cracking can be quantitatively evaluated.
Compared with traditional constant load stress corrosion tests (such as U-bend, C-ring, constant load tensile), the biggest advantage of SSRT tests is "acceleration" - traditional constant load tests may take months or even years to see cracks, while SSRT tests can usually be completed within hours to days, greatly shortening the test cycle, and are therefore widely used for rapid screening and evaluation of material stress corrosion susceptibility.
1.2 Development history and standard basis
The slow strain rate test method was first proposed by French scientists Nicolas and Ronsain in the 1970s, and has subsequently been widely used and developed internationally.At present, there are complete international and national standards for SSRT testing:
ISO7539-7: Corrosion of metals and alloys Stress corrosion testing Part 7: Slow strain rate testing
GB/T15970.7: Corrosion of metals and alloys Stress corrosion tests Part 7: Slow strain rate tests (equivalent to ISO 7539-7)
NACETM0177: Laboratory test of metal resistance to sulfide stress cracking and stress corrosion cracking in hydrogen sulfide environment (including SSRT method)
GB/T4157: Laboratory test methods for metal resistance to sulfide stress cracking and stress corrosion cracking in hydrogen sulfide environments
2. Working principle and system composition of SSRT testing machine
2.1 Working principle
The core of the slow strain rate stress corrosion testing machine is to achieve "precise tensile control at extremely slow strain rates."The strain rate of an ordinary universal testing machine is usually between 10⁻³ s⁻¹ and 1 s⁻¹, while the SSRT test requires a strain rate as low as 10⁻⁷ s⁻¹, which is 10,000 times slower than ordinary tensile testing.This requires the testing machine to have extremely high low-speed stability and displacement control accuracy, and be able to maintain a constant strain rate during the test process of several days, while accurately collecting force value, displacement and time data.
SSRT testing machines are usually driven by precision servo motors + ball screws or worm gear reduction mechanisms, and are matched with high-precision force sensors and displacement encoders to achieve precise control under low speed and large load.Some high-end models are also equipped with an environmental medium chamber, which can directly conduct tensile tests in corrosive media and control the temperature, pressure and concentration of the medium.
2.2 System composition
A complete slow strain rate stress corrosion testing system usually consists of the following parts:
System composition | Function description |
|---|---|
Host rack | Provide rigid support and loading frame, usually portal or single-column structure, with high stiffness requirements to avoid system deformation at low speeds |
Drive system | Precision servo motor + reduction mechanism to achieve ultra-low speed and stable drive from 0.001mm/min to 10mm/min |
force sensor | High-precision spoke-type or S-shaped force sensor, with an accuracy of 0.5 or higher, measuring the change in force value during the stretching process |
Displacement measuring system | High-precision encoder or extensometer to measure beam displacement and specimen deformation and calculate strain rate |
Corrosive medium environment chamber | Environmental boxes made of corrosion-resistant materials (such as titanium alloy, Hastelloy, and polytetrafluoroethylene) can accommodate corrosive media. The temperature control range is room temperature to 300°C, and some parts can be pressurized. |
temperature control system | Heating/cooling device + temperature sensor, precise control of corrosive medium temperature, temperature control accuracy ±1℃ |
Data acquisition and control system | Computer + special software, real-time collection of force, displacement, time and temperature data, drawing stress-strain curve, automatic calculation of SCC sensitivity index |
safety protection device | Corrosive medium leakage protection, sample fracture protection, over-temperature and over-pressure protection, emergency stop button, etc. |
3. Interpretation of core parameters of SSRT testing machine
3.1 Maximum test force
The maximum test force is the most basic parameter of the SSRT testing machine. Common specifications are 10kN, 20kN, 50kN, 100kN, 200kN, and 300kN.The selection is based on the strength of the material being tested and the cross-sectional area of the specimen.For example, when testing a standard round bar sample of high-strength steel (diameter 5mm, cross-sectional area approximately 20mm²), if the tensile strength of the material is 1000MPa and the maximum tensile force is approximately 20kN, it is more appropriate to choose 50kN equipment.It is generally recommended that the maximum test force is 2-3 times the expected maximum load to ensure test accuracy and equipment safety.
3.2 Strain rate range
Strain rate is the core parameter of SSRT test.According to ISO 7539-7 and GB/T 15970.7 standards, the recommended strain rate range for SSRT testing is 10⁻⁴ s⁻¹ to 10⁻⁷ s⁻¹.The selection of the strain rate is very critical: if the rate is too high (>10⁻⁴ s⁻¹), the corrosive medium will not have time to fully interact with the material, which may inhibit the occurrence of stress corrosion cracking, leading to an optimistic evaluation result; if the rate is too low (<10⁻⁷ s⁻¹), the test time will be too long (maybe more than a week), and the efficiency will be too low.A strain rate of 10⁻⁶ s⁻¹ to 10⁻⁵ s⁻¹ is recommended for SSRT testing of most materials.
When purchasing, you should confirm whether the strain rate range of the equipment covers 10⁻⁷ s⁻¹ to 10⁻⁴ s⁻¹, as well as the operating stability at low speeds.A high-quality SSRT testing machine can still run smoothly at an extremely low speed of 10⁻⁷ s⁻¹, without crawling or shaking.
3.3 Displacement speed range
The displacement velocity is related to the strain rate and is converted by the specimen gauge length.The displacement speed of SSRT testing machine is usually between 0.001mm/min and 10mm/min.Low-speed stability is the key. High-quality equipment can still be accurately controlled at an ultra-low speed of 0.001mm/min, and the speed fluctuation does not exceed ±1%.
3.4 Environmental chamber parameters
The environmental chamber is the key component that distinguishes the SSRT testing machine from ordinary universal testing machines.Things to note when purchasing:
Material: The environmental chamber and fixtures should be made of corrosion-resistant materials, such as titanium alloy, Hastelloy C276, 316L stainless steel, polytetrafluoroethylene, etc., to adapt to different corrosive media
temperature range: Room temperature to 200℃ is the basic configuration, high-end models can reach 300℃ or higher, temperature control accuracy ±1℃
bear pressure: Some tests need to be conducted in high-pressure corrosive media (such as high-pressure hydrogen sulfide, autoclave), and the environmental chamber needs to withstand a certain pressure (usually 1MPa-30MPa)
volume: The volume of the environmental chamber should be able to accommodate the sample and fixture, and at the same time facilitate medium replacement and cleaning.
media circulation: Some models are equipped with a medium circulation system to maintain uniform medium concentration and temperature.
3.5 Data collection and software functions
The data collection frequency of SSRT tests is usually 1Hz-10Hz, and data needs to be continuously recorded for hours to days.Software functions should include: real-time display of force-displacement curves and stress-strain curves; automatic calculation of fracture time, elongation, area shrinkage, SCC sensitivity index (Iδ, Iψ, etc.); support for comparison of multiple sets of test data; automatic generation of test reports that comply with ISO 7539-7 standards; data can be exported to Excel or text format.
4. Main application areas of SSRT testing machine
Application areas | Typical testing needs | Common corrosive media |
|---|---|---|
Oil and gas | Hydrogen sulfide stress corrosion cracking (SSCC) and hydrogen induced cracking (HIC) testing of oil well pipes, pipeline steel, and drilling tools | NACE A solution, H₂S saturated solution, simulated oil field produced water |
Marine Engineering | Seawater stress corrosion testing of steel for offshore platforms, submarine pipelines, and titanium alloys | Artificial seawater, natural seawater, NaCl solution |
Nuclear power industry | High temperature and high pressure water stress corrosion testing of key nuclear power materials (stainless steel, nickel-based alloys, zirconium alloys) | High temperature and high pressure water, boric acid solution, lithium solution |
Aerospace | Stress corrosion susceptibility assessment of aluminum alloys, titanium alloys, and high-strength steels | NaCl solution, humid air, aviation fuel |
Chemical equipment | Stress corrosion resistance screening of pressure vessels and pipeline materials | Various acids, bases, salt solutions, organic solvents |
Scientific research and teaching | New material research and development, corrosion mechanism research, teaching experiments | Customized to research needs |
5. SSRT testing machine selection guide
5.1 Five steps for selection
Clarify test materials and standards: First determine the type of material to be tested (metal/alloy), specimen shape and size, and the test standards to be followed (ISO 7539-7, GB/T 15970.7, NACE TM0177, etc.).Different standards have different requirements for specimen size, strain rate, and environmental conditions.
Determine the maximum test force: Calculate the maximum test force based on the material strength and sample cross-sectional area, and select equipment with a measuring range 2-3 times the expected maximum load.
Confirm strain rate range: Ensure that the equipment covers the standard strain rate range from 10⁻⁷ s⁻¹ to 10⁻⁴ s⁻¹, and examine low-speed operation stability.
Determine environmental chamber needs: According to the corrosiveness, temperature, and pressure requirements of the test medium, select an environmental chamber with appropriate materials and parameters.If you only do corrosion testing at normal temperatures and pressures, a standard environmental chamber will suffice; if high temperature and high pressure are required, you need to choose an autoclave-type environmental chamber.
Examine the manufacturer’s strength and services: The SSRT testing machine is a professional equipment that requires high technical capabilities and after-sales service from the manufacturer.You should choose a manufacturer that has R&D capabilities, has delivery cases of similar equipment, and can provide installation training and measurement calibration services.
5.2 Common misunderstandings in model selection
Misunderstanding 1: Use ordinary universal testing machine instead of SSRT testing machine.The lowest speed of ordinary universal testing machines is usually above 0.1mm/min, which cannot reach the extremely low strain rate of 10⁻⁷ s⁻¹ required for SSRT testing, and does not have a corrosion environment chamber, so it cannot be used for stress corrosion testing.
Misunderstanding 2: Only look at the maximum test force and ignore low-speed stability.The core of the SSRT test is low-speed precise control. Although some equipment is marked with low speed, problems such as crawling, jitter, and large speed fluctuations may occur during actual operation, resulting in unreliable test data.When purchasing, the manufacturer should be asked to provide actual measured data of low-speed operation.
Misunderstanding 3: Ignoring the corrosion resistance of environmental cavity materials.The corrosiveness of different corrosive media to environmental chambers and fixtures varies greatly.For example, hydrogen sulfide environment is highly corrosive to ordinary stainless steel, so Hastelloy or titanium alloy materials need to be used.If the wrong material is selected, the equipment may be corroded and damaged in a short period of time.
6. About Shandong Huayan Intelligent Equipment Group
Shandong Huayan Intelligent Equipment Group Co., Ltd. is a professional testing machine R&D and manufacturing enterprise. The core team has 20 years of industry experience and can provide customized solutions for slow strain rate stress corrosion testing machines (SSRT).Huayan's SSRT testing machine adopts a precision servo motor drive system, with a strain rate covering 10⁻⁷ s⁻¹ to 10⁻⁴s⁻¹, with a maximum test force of 10kN-300kN optional, equipped with a corrosion-resistant environmental chamber (316L stainless steel/Hastelloy/titanium alloy optional), with a temperature control range from room temperature to 200°C, which can meet the stress corrosion testing needs of oil and gas, marine engineering, nuclear power, aerospace and other fields.
Huayan Intelligent Equipment is located in Jinan and has complete R&D, production and measurement and testing capabilities. The group has Tianhua Metrology and Testing (Shandong) Co., Ltd., which can provide measurement and calibration services for SSRT testing machines.The company has passed ISO9001 quality management system certification, participated in the formulation of more than ten industry group standards, holds 17 national patents, and is the vice-chairman unit of the Shandong Electrotechnical Society.Huayan has strong non-standard customization capabilities and can customize exclusive stress corrosion testing systems according to customers' special testing needs (such as special corrosive media, special temperature and pressure, special sample size).
If you need to purchase a slow strain rate stress corrosion testing machine, please contact Shandong Huayan Intelligent Equipment. Our professional engineers will provide you with a customized equipment configuration plan based on your test materials, corrosion media and standard requirements.
7. FAQ
Q1: What is the difference between slow strain rate stress corrosion test (SSRT) and constant load stress corrosion test?
A: Both are methods to evaluate the susceptibility of materials to stress corrosion cracking, but the principles and cycles are different.Constant load tests (such as U-shaped bends, C-shaped rings, constant load tension) apply a constant load to the sample to observe whether cracks occur. The test period is usually several months to several years, which is close to actual service conditions, but the period is too long.The SSRT test stretches the sample at an extremely slow strain rate, which can accelerate the stress corrosion cracking process within hours to days. It is suitable for rapid screening of materials, but the test conditions are somewhat different from actual service.Usually the two are used in combination: SSRT for rapid screening and constant load for verification testing.
Q2: How to choose the strain rate for SSRT test?
A: The selection of strain rate is the key to SSRT test.According to the ISO 7539-7 standard, the recommended range is 10⁻⁴ s⁻¹ to 10⁻⁷ s⁻¹.The specific selection should consider: (1) The SCC sensitivity of the material. Materials with high sensitivity can use a slightly higher rate, while materials with low sensitivity require a slower rate; (2) Test cycle, the slower the rate, the longer the cycle. A standard sample at 10⁻⁶ s⁻¹ usually takes 24-72 hours; (3) Refer to the test conditions of similar materials in relevant standards or literature.In most cases, 10⁻⁶ s⁻¹ to 10⁻⁵ s⁻¹ are commonly used strain rates.
Q3: Can the SSRT testing machine do ordinary tensile testing?
A: Yes.The SSRT testing machine is essentially a low-speed, high-precision universal testing machine. In addition to slow strain rate stress corrosion testing, it can also perform ordinary room temperature tensile testing.However, the upper speed limit of SSRT testing machines is usually low (generally no more than 10mm/min) and is not suitable for high-speed tensile testing.If you need to do conventional tensile and SSRT tests at the same time, the SSRT testing machine can be used for both purposes. However, if the conventional tensile amount is large, it is recommended to use an ordinary universal testing machine.
Q4: How to prepare the corrosion medium for SSRT test?Are there security risks?
A: The corrosion medium of SSRT test is prepared according to the test requirements. Common ones include NACE A solution (5% NaCl + 0.5% glacial acetic acid), H₂S saturated solution, artificial seawater, various acid and alkali salt solutions, etc.Among them, hydrogen sulfide (H₂S) is a highly toxic, flammable and explosive gas. There are safety risks during operation. It must be carried out in a fume hood, equipped with hydrogen sulfide detection alarm and protective equipment.It is recommended that when purchasing equipment, the manufacturer be required to provide media preparation operating procedures and safety protection guidance.Huayan Intelligent Equipment’s SSRT testing machine is equipped with complete safety protection devices and operational training.
Q5: Does the SSRT testing machine need to be calibrated?What parameters are calibrated?
A: SSRT testing machines need to be calibrated regularly, usually once a year.Calibration parameters include: (1) Force value indication error and repeatability, refer to JJG 139 tensile testing machine calibration procedures; (2) Displacement speed indication error, especially the speed accuracy in the low speed section; (3) Temperature indication error and uniformity of the temperature control system; (4) Time accuracy of the data acquisition system.Calibration should be completed by a qualified measurement and testing institution.Tianhua Measurement and Testing Company, a subsidiary of Shandong Huayan Intelligent Equipment Group, can provide customers with convenient SSRT testing machine calibration services.
