Let's talk about the conclusion first: temperature compensation, let's talk about the score situation
Give the answer directly-whether temperature compensation is required for non-metal tensile non-destructive testing depends on what your testing purpose is. If you just want to compare the consistency of the quality of the same batch of products relatively, there is little difference between compensation or not; However, if you want to take the data measured at room temperature to check the design load and do safety assessment, the data will be wasted without temperature compensation.
This is not a big deal. After doing this for a long time, you will find that the dispute on the spot is often not whether there is a problem with the material itself, but that the testing conditions do not match the actual working conditions, and no one recognizes the data.
Tensile properties of non-metallic materials, how sensitive are they to temperature? — Starting with fabric fibers and rubber
Let's take the common things as an example. The tensile strength, elastic modulus and elongation at break of glass fiber cloth, fluororubber and silicone rubber coatings used in fabric fiber expansion joints are completely different at 20℃ and 150℃.
Take rubber as an example. As soon as the temperature rises, the movement of molecular segments intensifies, the hardness decreases, and the tensile strength drops considerably. Some formulas have a strength of only about 60% of normal temperature at 100℃. For materials such as PTFE (polytetrafluoroethylene), the fluctuation of mechanical properties caused by temperature changes cannot be ignored. It is hard and stiff at room temperature. As soon as the temperature approaches its upper limit of use, softening and creep will all come.
So you see, the mechanical properties of non-metallic materials are a curve that changes with temperature, not the single point at normal temperature.
What do standards say? Is there a clear answer in JB/T 12235-2015
JB/T 12235-2015 "Non-metallic Expansion Joint" standard, it has requirements for the tensile property test of laminated composite materials and non-metallic elastic materials. The standard specifies the test methods, such as sample preparation, loading rate and test environmental conditions. However, the standard does not give a ready-made correction formula for temperature compensation.
Why? Because there are many grades and mixed formulas of non-metallic materials, each manufacturer's fabric fiber and rubber system are different, so it can't be converted with a uniform coefficient. The standard is to give this initiative to the tester: when you measure, you have to record as much ambient temperature as it is-the implication is that you have to take responsibility for this temperature difference yourself.
To put it bluntly, the standard gives the methodological framework, but it doesn't answer the engineering judgment for you.
Actual scene: What should I do if the pipeline operating temperature vs the detection ambient temperature is 20℃?
Two days ago, I also met a customer. The rectangular non-metallic expansion joint used in their project had a designed operating temperature of 180℃. The on-site equipment was shut down for maintenance, removed and sent to the laboratory for tensile non-destructive testing. As soon as the laboratory air conditioner was turned on, the ambient temperature was 22℃. When the test report came out, the data was pretty, and the breaking strength was completely up to standard. But the problem is-180℃ and 22℃, the difference is 158℃.
The tensile strength measured at room temperature of 22℃ and the actual bearing capacity at high temperature of 180℃ are not the same at all. If you use this data to prove that this expansion joint still has enough safety margin at 180℃, then this report is not convincing. The difference of 20℃ is already obvious, not to mention this temperature difference of hundreds of degrees Celsius at every turn.
Therefore, the closer the detection conditions are to the real working conditions, the more the data can explain the problem. Can't do it? That also has to quantify the effect of temperature clearly, not when it doesn't exist.
In what cases can you not compensate? In what circumstances must compensation be made?
Let's start with the situation that you can't do it. The first is the factory comparative inspection of the same batch of products-everyone measures it together at room temperature, and the measured values are horizontally comparable. Anyway, the benchmarks are the same. The second is routine arrival acceptance. You mainly want to confirm that the product is not damaged in production and transportation, and whether there is any obvious deterioration. The benchmark judgment at room temperature is enough.
However, in the following cases, it is recommended to make temperature correction honestly:
- The design safety margin under high temperature working conditions is inferred by using the detection data, such as non-metallic expansion joints (fabric fiber expansion joints) or rubber compensators on flue ducts and desulfurization systems, and the operating temperature is generally above 100℃;
- In failure analysis, it is necessary to correspond the normal temperature detection data with the temperature working condition at the time of failure to find out the real reason;
- For life assessment or fatigue performance analysis, temperature directly affects the creep and relaxation behavior of materials;
- It involves horizontal comparison and selection and replacement between different suppliers and different material systems-the temperature response characteristics are different, and it is easy to choose wrong if you are not on the same benchmark.
There are also products such as PTFE-lined metal hoses and PTFE compensators. The mechanical attenuation of PTFE materials at high temperatures should not be underestimated. If the temperature change correction has been done or not, the results can be an order of magnitude worse.
Three implementable suggestions for inspectors and design institutes
First, the ambient temperature and the sample temperature must be marked on the test report. Don't underestimate this line of words. If there is a dispute, this is the basis. There is no compulsory unified temperature compensation formula in JB/T 12235-2015, so make the condition record work solid.
Second, if conditions permit, put the sample at the target operating temperature for pretreatment before testing, and then test it after constant temperature for a period of time. Even if it can't be completely isothermal loaded, it is a big step towards real working conditions.
Third, when the design institute selects non-metallic expansion joints, if the material testing data obtained is measured at room temperature, don't directly put it into the high-temperature working condition. The performance data of the material system at the corresponding temperature level should be requested from the manufacturer, or the calibration calculation after temperature correction should be requested. This is particularly critical for fabric fiber expansion joints and rubber compensators.
Back to the title question-does non-metallic tensile non-destructive require temperature compensation? The answer is not a simple "yes" or "no", but you have to clarify what the data is used for first. When used in the right place, room temperature data is also valuable; If you use the wrong scenario, no matter how beautiful the numbers are, you can't support the safety margin.