CREEP TEST

Creep testing is a test in which a specimen heated in an electric furnace is subjected to creep deformation under constant loading. Temperature, strain, and time are continuously measured during testing until the specimen fails. Output from creep testing includes time to rupture and creep curves. The results of creep tests are essential for designing and maintaining equipment that is constantly exposed to high temperature and internal pressure, e.g. steam pipes in power plants and reactors in oil refineries. Creep testing is not only for metallic materials but also for plastic materials, where humidity also affects test results.

Creep is a phenomenon in which a material gradually deforms and eventually ruptures under a static force acting on the material. While creep deformation of metallic materials generally occurs at a temperature above one half of its melting point, it is known that plastic materials can creep at a temperature below room temperature.

The most common creep test is uniaxial tensile creep testing. Other creep tests include compressive creep testing, torsional creep testing, biaxial and triaxial tensile creep testing, and miniature creep testing such as uniaxial miniature creep testing and small punch creep testing. Furthermore, creep crack propagation testing is conducted to determine the rate of creep crack propagation.

A uniaxial tensile creep testing machine applies tensile load via a rod to the specimen set inside an electric furnace using a lever-loading mechanism. General creep machines in our laboratory are ones with a lever ratio of 1:10 that can apply load up to 3 tons, but loading up to 5 tons is available. Test temperature is generally between room temperature and 1200℃.
We also design and manufacture creep testing machines from a uniaxial tensile creep machine to miniature creep, small punch creep, and other special creep testing machines. Please contact us for more information, including pricing of testing machines.

Creep test methods are specified by different standards. ●Metallic materials: JIS Z 2271 (Metallic materials-Uniaxial creep testing in tension-Method of test), ASTM E 139, ASTM E 292 ●Plastic materials: JIS K 7115 (Plastics−Determination of creep behavior), JIS K 7116 (Flexural creep by three-point loading) ●Concrete: JIS A 1157 (Method of test for compressive creep of concrete)

We perform various types of creep testing on various materials. Creep testing is a very time-consuming test. In order to meet the demands of a number of customers’ requests, we have 428 creep machines, the most of all independent testing laboratories in Japan.
Also, our expertise is focused on miniature testing. One method that has received a great deal of attention is creep evaluation of power plants in operation, that is, a micro specimen is machined from a very small piece sampled from the plant piping and tested to obtain its creep characteristics. In conventional evaluation using a standard-sized specimen, long-time suspension in operation is required because a large sample has to be cut from the piping and repair is necessary after the sample is taken. In addition, some damage to the piping component is unavoidable. To solve this problem, we have developed the Electric Discharge Sampling Equipment for miniature-specimen sampling and miniature testing machines for uniaxial tensile miniature creep testing and small-punch creep testing. With these machines and our miniature testing technology, we provide contracted service for residual life evaluation of structures and plants. Recently, an ultra-miniature creep testing machine has been developed to accommodate even smaller specimens, with which many customers already have placed orders for testing.

Fatigue testing is a form of mechanical testing in which specimens taken from a material are subjected to a certain type of cyclic loading such as compression, tension, thermal, and ultrasonic loading. To determine the point at which the failure occurs (fatigue limit), multiple specimens taken from the material are tested under different loads, and the number of cycles to failure for each specimen is obtained. With varying loads applied, it is classified as dynamic testing.
On the contrary, creep testing is classified as static testing, where constant load is applied. Testing that combines creep and fatigue testing is called creep fatigue testing.
Basically, creep testing takes a long period of time. It takes several hours of continuous operation at the shortest, and the longest test we experienced at our laboratory required 50,000 hours of continuous operation. For information, the world’s longest creep test record is 14,868 days (approximately 41 years or 356,862 hours) achieved by NIMS (National Institute for Materials Science) in Japan (Guiness World Record).

A creep curve is a diagram where strain is on the vertical axis and time is on the horizontal axis. A typical creep curve indicates three different regions: a region immediately after the start of the test where the strain rate is high (primary, or transient, creep region), a region where the strain rate remains constant (the secondary, or steady-state, creep region), and a region where the strain rate increases rapidly to rupture (tertiary, accelerated creep region). The linear relationship seen in the secondary region is called Norton’s law and the slope of the line is called the steady-state creep rate, a constant necessary for creep analysis using FEM.
When the same material is tested, the higher the test temperature or stress, the shorter the time to rupture. Comparing the obtained creep curves shows that the shorter the time to rupture, the larger the slope of the steady creep region (creep rate). Logarithmically plotting the steady-state creep rate (often used in the same meaning of the minimum creep rate) and the time to rupture gives a linear relationship. This relationship is called the Monkman-Grant rule.