High Temperature and Environmental Testing Q&As

Essentially, testing is all about data, regardless of whether it’s routine testing or complicated testing, so the starting point for anybody is what data is needed, and why is the test needed in the first place?

It is important then to partner with experts to guide you through the pros and cons, the different factors and opportunities. 
 

Testing is the link between theory and reality and service. What we do is decouple the effects or influences and add those back in, to replicate in service conditions.

When materials are being used in new applications, we need to understand what that is going to do to the actual performance of that material.
 

That depends on which industry our customer is operating in. It could be a few tens of degrees for food and processing or up to thousands of degrees for nuclear and aerospace and beyond. It depends on the audience and the type of product. 
 

Once we know what temperature we are going to work at, the next question is how are we going to heat it? 

Three of the methods we use most commonly are:

• traditional furnaces
• infrared emitters which are effectively like a light bulb type heating technology
• induction heating

We consider the temperature range.  If we want to get really hot, then induction heating will get to the hottest temperature in the fastest time.

We also consider material. If the material itself is not inductive, it’s going to need a susceptor around it, so that’s something else to think about. 

If the sample is not going to heat rapidly in reality, then we don’t need to heat it rapidly when we’re testing it, as this can have an influence on how it’s performing. 
 

If we look at British Standards or International Standards, they will talk about how accurately we need to get that temperature uniformity across that sample.

In the world of manufacturing, we know that very rarely things are actually a uniform temperature. 

If we are trying to simulate how something is going to behave in service, then actually what we might need to do is generate a controlled temperature gradient. 
 

That is more challenging with a furnace or induction heating, but in our experience, we have found that the infrared emitters come in all shapes and sizes and we can build up banks of those to give us a very controlled and high-resolution temperature profile, particularly if we have a sample that is an unusual shape or we are not testing a coupon. 
 

What gets hot, must get cold. This is something that people neglect or negate to think about when it comes to temperature testing.

We consider if it needs to be cooled rapidly; can we use chilled water? Do we have chilled water available or is that something else we are going to need to add to the lab or wherever we’re doing the testing. 

If we’ve got a sample that’s being tested in an environmental chamber, then we have to think about getting to the outside of that sample to use forced air cooling, so we might have to do the cooling through the centre of the sample.  

We can use forced air or chilled water. But then we will have to consider the loading bars and gripping design to enable us to get that into the centre of the sample.

So, if we are trying to design a testing programme then then we need to think all the way back to how is that going to affect what we are trying to achieve. 

If we do not need to cool the sample and we are doing elevated temperature testing, there is a high probability that we are going to need to cool the grips, the load cell, the loading bars.

So, we might not actually have to do specific cooling of the sample itself, but we are going to have to think about the bigger picture and the assembly as a whole.
 

There are multiple options, and that depends on what we are trying to get out of the test, such as data and information.

We can use thermocouples, which are tried and tested and can easily get UKAS calibrations up to 1300°C, with relatively simple electronics.  

If we want multiple readings, we need multiple thermocouples, and putting that thermocouple onto the sample will then have an influence in itself. 
 

A lot of people nowadays are moving to non-contact temperature control, so pyrometry up to 3000°C is possible, thermal imaging cameras are also an option and the ones we use go up to about 1000°C. 

Thermal cameras include the complications of electronics, software and a PC.  A real consideration with both of these elements is their sensitivity to the surface condition and emissivity, so the amount of change in the surface behaviour as they get hot or they oxidize during the test. 

To calibrate pyrometers and thermal cameras realistically, every different type of sample or every different type of material put into it needs to be calibrated, so it gives brilliant data once the test is completed, but the upfront preparation is significantly more time consuming. 
 

The big bonus of thermal cameras is we get full field information, so if we want to know what’s happening across the entire sample, there will be a need to use a thermal camera. 
 

We can add in corrosive or inert gases or remove gases and test under vacuum, we can add in humidity, pressure, oxygen; all combinations thereof combined with elevated temperatures and all of these will affect the behaviour of the material or component. 
 

In using thermal camera and pyrometers in temperature testing, we know about their sensitivity to surface finish and surface condition.  

The easiest way to prevent something from oxidising is to take the oxygen out of the question, so we might put an inert atmosphere in or test under vacuum to eliminate those problems.

We need to understand how the sample is behaving in reality and to accelerate speed of degradation. 
 

Volume: how big a chamber is needed? You really do not want a chamber that is oversized as its going to take longer to get to vacuum, longer to fill up with gases, you are going to need more gas. 

Flow: if the flow of gas goes through too quickly, then this is going to affect the temperature gradient which is something that an awfully long time will have been spent optimising. 

Pressure; lots of health and safety considerations under pressure. 

Humidity: what sensors are being used, how to deal with condensation, what needs to be added in there to handle that?

Fixtures: how easy is it to actually mount samples and additional instrumentation? There are clearly health and safety implications once we start using environmental conditions. What safety monitors are needed in the laboratory? 
 

There are some key considerations to think about when deciding how to test under service conditions.  

• What is the material and geometry? 
• What are the heating and cooling rates and methods?
• Don’t forget the cooling; is a controlled environment required and if so, why? 
• Do we have suitable power? High temperature and environmental testing is incredibly power-hungry; if we are driving a vacuum pump and heating, and the test machine, each of those will require additional infrastructure and additional power. 
• What validation and calibration is in place?  If we are using complicated pieces of equipment, how sure are we that we are getting accurate data? 
• We are making business decisions based on the data and the testing that we’re doing and if we don’t get that right, its going to cost time and money, and ultimately might impact credibility with the end customer. 
• Is this being done to meet international standards? Is flexibility needed?  Is this for R&D? 
• What health and safety aspects must be considered?