Take Control of Your Results
Understanding the many factors that can influence results is important in a lab setting. Regardless of operator experience, very few people have an extensive understanding of materials testing; particularly the factors that influence results, whether they are negative or positive. This is simply because there are so many variables to consider, and in many cases, they vary based off configuration, what’s being tested, and the type of test. This is difficult for most to track, especially in lab settings where constant change is the norm.
In situations where results are questionable, having this knowledge can save labs and operators from wasting countless hours of unnecessary tests and expenses, which typically result from troubleshooting/experimentation, involving third party test labs, among other things. In QC environments where delayed results could result in “bad” product from being produced, time is often the single most important factor. For R&D and contract test labs, specimens are often limited and quite costly, both of which make retesting very undesirable.
Arguably one of the most detrimental implications of high variability data is lower confidence in results, both from the lab’s operators and their customers. It can also complicate decision making since it may or may not be clear if the product is bad. Do we retest? Does our process need tweaking? Are we doing something wrong? These are some of the difficult questions that may have to be answered. The good news is that once the factors are recognized and understood, achieving repeatable and accurate results is easier than you might think.
Factors that Influence Results
As indicated below, some of more common causes of high variability include inadequate or no proload, inconsistencies in specimen alignment, and inconsistent extensometer attachment, which could be related to the alignment as well as the force exerted on the specimen.
Some of the less obvious factors that influence results may be the heat generated from an operator’s fingers as they handle specimens. We conducted a study with two operators, one with “hot” fingers and one with “cold”; we found that with some materials, there were noticeable differences in results such as maximum strain and yield stress.
Additionally, we found that inconsistencies in how long a specimen is held prior to the test can cause variability. Solutions for this factor include wearing gloves and using higher efficient accessories to reduce handling time, such as pneumatic grips, an integrated specimen measure device, or fully automated solutions.
Simple and effective solutions exist for all of these factors, ranging from specimen alignment aids to automated handing, adding preload to methods, and incorporating visual aids in the methods to guide operators through the process
The consumption of high-performance polymers in the automotive market is growing very fast because of their ability to provide heat-resistance, strength, and weight reduction at the same time. In particular, they are used to replace metal alloys components. However, how do they maintain their mechanical properties at high temperature?
To answer this question, it is very important to characterize thermo-mechanical properties of polymers. As everybody already knows, thermoplastics do not have an exact melting point that defines the transition from solid to fluid, but they undergo a slow softening as temperature increases. Hence, which is the most suitable material for automotive application?
Many companies produce high performance polymers (PEEK, PPS, PEI...) for different application, but those materials have high processing cost. To avoid mistakes and alleviate cost issue, client should consider HDT and Vicat values reported on every material technical data sheet in order to differentiate between many different products and choose the most suitable for automotive.
|Vicat Softening Point Test|
Heat Deflection Temperature (HDT) and Vicat Softening Temperature (VST) are usually measured to evaluate the ability of high performance polymers to retain their mechanical properties at high temperature. These kind of analysis help to control the quality of the material and provide also useful information as a development tool. The higher is the HDT and VST value, the higher is the application area of the product. On the other hand, when the temperature grows more than HDT or Vicat Temperature values, permanent deformation will be reported generating deflections and surface defects.
Instron HV systems allows to measure HDTs and VSTs fully complaint to main international standards (ISO and ASTM). They are easy to use both stand alone or with a software that allows to manage the instrument and analyse the final results. Instron can provide different models, from the simplest manual or semi-automatic model to high temperature oil-free instrument, from three to six stations depending on laboratory testing needs.
Over the last few years, ISO 178 has gone through some changes and being aware of them can be the determining factor for whether or not your lab fails an audit or is found to be in compliance with the standard. Unfortunately, awareness alone is often not enough; understanding the changes is equally, if not more, important. As you will read below, the changes regarding the use of extensometers or deflectometers is likely one of the more confusing changes, not because people disagree with the changes, because they are interfered differently.
What are the Changes?
In 2010, the use of an extensometer was introduced to section 5.4.2. Most interpret this change as being mandatory because it clearly states that the measuring device shall comply with ISO 9513, which is a classification standard specific to extensometers. This is how I initially interpreted the changes as well, but after reading the entire section multiple times, I realized that in the last sentence of 5.4.2, it states that crosshead displacement is not suitable for modulus determination unless compliance correction is applied. The two sentences contradict one another, which results in either being acceptable. To summarize, as of 2010, deflectometers or systems with software capable of removing compliance is required for the determination of modulus. Note: all Bluehill Software packages from Instron used on static testing systems have this capability.
The next change relates to dimensional measurement devices. In 2010, very specific requirements were introduced, but because compliant devices were not commercially available, the requirement was amended in 2013 and replaced with a much more reasonable requirement, making most off-the-shelf micrometers compliant.
Finally, the last major change I want to address is the introduction of multiple speeds within a given test. Significant time savings and a reduction of test-related expenses result from this change since two separate tests are no longer required to measure modulus and stress-related calculations. The test simply starts off with the slower speed required for modulus and once measured, a faster speed can be used for the remainder of the test.
Q: I usually perform my impact tests on a 4 inch plaque, supported over a 3 inch unsupported diameter with a ½ inch hemi tup insert. I have a limited number of smaller 2 inch x 3 inch samples that I need to test over a 1.5 inch unsupported diameter. Is there a relationship between the test values from a 1.5 inch unsupported area and the larger 3 inch area – given that the samples are from the same materials and I will be using the same energies, velocities and tup inserts.
A:If all of the variables except the support diameter are kept the same, as you have described, then there is a relationship to be seen. Usually the maximum load will go up and the energy absorption will go down. This is due to the material not having as much area involved in resisting the impact as you have with the larger sample and unsupported area.
- Plastics Testing: Increasing Laboratory Efficiency & Throughput
- Fundamentals of Melt Flow Testing
- Understanding Changes to Key Plastics Standards
- Reducing Variability in Test Results
About Strain Measurement
- Challenges in Plastics Testing
- The Fundamentals of Melt Flow Testing
- The Fundamentals of Rheology Testing
- Advanced Polymer Analysis
- Advanced Applications in Rheology
- The 3 Challenges in Plastics Testing: Impact, Melt Flow, & HDT
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