Is Design Validation Slowing Down Your Time To Market?

Instron interacts with many new product development labs that have a need to validate their new product or component design. Often part of this design validation requires mechanical testing. We have noticed a common need in these research and development labs to perform rapid "what if" analysis during design validation. That is, a test engineer often does not know the test speed, end of test value, etc. before pressing the start button on their Instron frame. Often the mechanical test is iterative and takes a trial-and-error approach. This type of "what if" analysis can be accelerated by using Expression Builder-driven parameters in Instron's Bluehill^® 3 Software.

Example:

You do not know the test speed or what maximum load you will need to apply to your product. So, you want to try several speeds and loads until the results look correct.

Add these variables as number inputs:

Using Expression Builder, select the number inputs appropriately for your test:

Now, in the workspace, you can simply type in different test speeds or maximum load criteria to validate your product taking a trial-and-error approach.

Using Expression Builder within Bluehill 3 should expedite your design validation process.

To learn more about the benefits of Expression Builder, this video may be helpful.

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Understanding Melt Flow Testing and Its Importance

Melt flow testing is simply a measure of the flow of a polymer when melted. The result of a melt flow test, called the melt mass-flow rate (MFR) or melt volume-flow rate (MVR), is defined as the amount of mass or volume of a polymer that flows through a small die at a specified temperature and pressure.

The melt flow test itself is simple and straightforward. A small amount of a thermoplastic sample (usually in granule or flake form) is heated in a barrel at a specified temperature, melted to a viscous fluid, and is forced out of a capillary die by a piston loaded with dead weights. Once enough sample extrudate has exited the die, it is removed and weighed, or the volume of the sample is measured by the machine.

ASTM D1238 and ISO 1133 are the most common standards for melt flow tests and define the equipment specifications, as well as test methods. Because various temperatures and weights are allowed for melt flow tests (only suggestions for each material type), test parameters always need to be reported with MFR/MVR results.

Note: Imagery is a simple representation and not to scale.

Typically, the MFR/MVR of a polymer is inversely proportional to its molecular weight. Though not frequently used in R&D settings, melt flow testing is very common in quality control and process control laboratories. It is mostly seen in compounding or manufacturing/converting facilities.


What do you gain from melt flow testing?

• A typical index value for verifying in-house material
• Quality check of entrance materials
• Comparison of new materials in a product development setting
• Evaluation tool for new material suppliers
• Quick comparisons of batches of material 
• Estimation of flow properties for simple extrusion processes 
• Predictions of how a polymer will behave in a number of processing techniques

If you are not currently performing melt flow tests, but think you should be, contact us for guidance on selecting the proper equipment for your laboratory. Read more

Preloading, and How It Affects Your Mechanical Test

It is almost always recommended that the Bluehill^® Software preload feature be used when you’re performing a mechanical test. Preloading simply removes slack from the load string before a test begins. This is done by loading the specimen at a specified rate - called "preload speed" -  until a particular load limit is reached - called "preload". Note that this value should be high enough that it removes slack from your load string, but low enough that it doesn’t interfere with important early-test calculations like modulus or yield. Once the preload is reached, the software begins collecting data from the load cell, crosshead encoder, and any other measurement device you have in the system. That means your test begins collecting load data after it’s reached the preload value. It also means your first recorded load point will be the preload value and not zero—which is okay! There are also calculations, like slack correction, that allow you to remove what is called the “tow” section of the curve (Section B in the image below).

Once the preload is reached, Bluehill gives you the option to perform an auto-balance of your physical or virtual measurements. Any strain source, whether it is the crosshead extension or an extensometer, should be auto-balanced after the preload is reached. This ensures that all of the specimens in your sample start from the same load and strain point, making your data more accurate and much easier to process (especially if you are using extension and strain-based calculations).  If using extension as your strain source, keep in mind that your gauge length is now equal to the original gauge length plus the distance traveled to reach the preload. You should not auto-balance the load signal. We commonly see load being balanced incorrectly (see my post on balancing load cells). Once a specimen is gripped, it is common to see a bit of force on the load cell—either positive or negative. This is simply a function of how the grips hold the specimen, or it is due to the mass of the specimen. If the force recorded on the load cell is severe, it may be due to misaligned grips or a poorly-inserted specimen. If a specimen is brittle or fragile, a feature such as Specimen Protect can be used to ensure the load on the specimen does not exceed a specified amount (+/-) before beginning a test.

Effectively using the preloading and auto-balancing features of Bluehill can help keep your results much more consistent. If you have questions on how to incorporate these features into your tests, feel free to contact us.

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Quick Tips for Balancing Load Cells

Whether you use bathroom scales, digital kitchen scales, spring scales – or any other measuring instrument, for that matter – you know the importance of zeroing it before it’s used. Failure to do so results in a shift in weight and thus, an incorrect reading on the scale. Now let’s think about the load cell in your testing system, which is just like a scale …

Prior to testing a batch of specimens, you should ensure that your load cell is properly calibrated and balanced – both actions are done right in the software. In Bluehill^® 3, this can be done from the console at the top of the screen. Simply click on the load cell icon, and then click "Calibrate". We refer to this as a software calibration (or “soft-cal”), which also balances the load cell.

Here are some soft-cal quick tips:

1. We recommend that the machine and load cell be switched on for about 15 minutes prior to performing a soft calibration. This allows the device to warm up, reducing the chances of drift.
2. Make sure you are not gripping a specimen when you perform a soft-calibration. It is okay to leave the upper grip or fixture attached to the load cell during the calibration – but it is also okay to leave it off. Just remember to perform an additional balance once the grip or fixture is added to the load cell.
3. Again, do not balance the load cell once the specimen is loaded into a grip or fixture. The force you see is real mass of the specimen, or force on the load cell. Balancing out this load would be like balancing a scale after you have put one foot on it (Note: this means the load cell should not be auto-balanced after preload). 
4. Though many systems will automatically recognize a load cell and restore its previous calibration every time it is plugged into the system, it is good practice to perform this soft-cal regularly, especially if swapping load cells. What does regularly mean for your system? Contact us, and we’ll help you figure it out.

Remember that a soft-calibration is not a replacement for on-site verifications by Instron Professional Services on a regular basis. For most laboratories, this means at least once a year, but your industry standards or internal protocols may differ.

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How Do You Prepare Your Specimens?

Depending on the materials you are testing and the size of your test lab, there are a multitude of ways to prepare materials testing specimens. 

For more rigid materials like metals, composites and hard plastics, there are specialized and dedicated machining or molding processes. For more flexible materials, such as plastic films, elastomers and soft plastics, there are many more options, since it is more feasible to cut these by hand.

If you are cutting specimens by hand, either with scissors, paper cutters, or other manual processes such as hollow dies and mallets, you should be aware that this can be detrimental to your subsequent tensile test results. Of course, for any specimen preparation procedure, there should be best practices set in place to ensure maximum consistency and repeatability. However, with manual preparation methods, it's inevitable that some operators simply have shakier hands or that some hit a mallet harder than others. At the end of the day, the samples get prepared, and it seems like the most important step.

What many people do not consider is that manual specimen preparation methods can contribute to variation in specimen quality. Naturally, the manual methods take a toll on the preparation equipment. Uneven pressure distribution and lack of protection of dies and blades lead to imperfections that mirror themselves in the specimens they are cutting. Though some of these imperfections may not be visible, even very small nicks can create stress concentrations in test specimens—especially for sensitive materials, such as films, foams and soft plastics. These stress concentrations can most obviously impact the specimen’s strain to failure, but arguably more importantly, can affect key results like modulus and yield.

So how can you mitigate this variation in your specimen preparation procedures? For any specimen preparation procedure, having a standard set of best practices is always recommended. But especially when there are multiple operators, or if turnover is high, a better option may be to use more automated preparation equipment. Instron CEAST hollow die punch machines (pictured above) apply even pressure to a specimen to ensure uniform stress distribution on the specimen, and maximum life of the blade. Dies are available for most common standardized tests, and custom dies are available upon request.

Instron CEAST milling equipment is ideal for consistent, low-volume preparation of hard plastics, composites and even some metal sheeting; notching equipment ensures accurate notch preparation for Charpy, Izod, and tensile impact tests.

For more information, take a look at our full suite of specimen preparation equipment here.

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Question From a Customer: How to Report Strain at Break Following ASTM D638

Question: I am following ASTM D638-10, and my 'strain at break' results are nearly half of what other labs are reporting for the same material. What is wrong?

Reply: If your material exhibits necking or inhomogeneous strain, ASTM D638-10 specifies that the extensometer needs to be removed at specimen yield. Once the extensometer is removed, nominal strain is calculated to specimen failure. The standard specifies this to avoid tests where the specimen begins to neck outside of the gauge length. A common misinterpretation is that if your specimen necks within the extensometer gauge length that the extensometer can be left on until failure. However, to be compliant with ASTM D638-10, the extensometer must be removed at specimen yield regardless of where the necking occurs, and nominal strain must be calculated to specimen failure.

When the extensometer is left on the specimen until failure, strain at break is calculated by the change in extensometer gauge length divided by the original gauge length. The majority of specimen elongation will occur in the narrow section of the specimen where the extensometer is attached. When removing the extensometer at specimen yield, nominal strain is calculated by crosshead extension divided by the initial grip separation. This method is how ASTM D638-10 specifies strain at break to be calculated. The majority of specimen elongation will still occur in the narrow section of the specimen; however, the entire grip separation is now being used as the gauge length. Thus, this causes lower strain at break results. We have found in our lab that strain at break results are almost doubled when leaving the extensometer on until failure.

We recommended having the other labs remove their extensometers at specimen yield and check their method to make sure they are using nominal strain to calculate strain at break. Read more

Upper Yield Calculations—Discontinuous Yielding / YPE Material

Following a recent lab visit, I thought it was worth writing a quick post to share an example of how labs sometimes incorrectly calculate yield strength. While visiting a testing lab for unrelated issues, my colleagues and I were discussing how the customer runs both continuous and discontinuous material with the same method (Non-YPE and YPE). For historical reasons, they have updated their Series IX methods to Bluehill 3 with little knowledge of why, when or by whom the methods were setup.

It later transpired in the conversation that some of the lab's material was falling below their specification for yield strength. They are required to test every coil of product before shipment; therefore, materials not meeting the yield strength requirement are of major concern. The material has had to be reworked in the past in the hopes that the yield strength could be increased slightly. Otherwise, the material would be scrapped.

The calculation used for both continuous and discontinuous material was an offset yield of 0.2% strain. Based on this calculation the stress values were determined for product verification. For a continuously yielding material, it is recommended to use the offset yield calculation. However, for a discontinuously yielding material, using this calculation would result in an inaccurate measurement of stress after a potentially significant load drop. The main international metals standards such as ISO 6892 and ASTM E8 both recommend using upper yield strength (ReH or UYS) to calculate this value correctly.

By changing the software algorithm to a different Bluehill 3 standard calculation, we were able to increase the calculated yield strength by around 10%. This meant they no longer had to rework the material as the higher stress values were always well within specification. This modification to their test method helped reduce testing turnaround time, increased the reported material strength, reduced shipment delays, and removed the need for costly and time-consuming reworking process.

The graph below shows an example of how the two calculations differ. The results of the offset yield calculation were on average 21% lower than the upper yield calculations. Checking that calculations are correctly evaluating data could result in a significant improvement of results. Often methods were setup and have not been reviewed on a regular basis.

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Equipment Considerations for a Well-aligned Test

When testing stiff or brittle materials, such as composites, alignment is crucial—even instances of slight misalignment can throw off test results.

Misalignment in test frames takes two forms: concentricity misalignment, where the centerline of the upper grip/fixture is offset from the centerline of the lower grip/fixture, and angularity misalignment, where the two centerlines are at contrasting angles. Both forms cause extra stress on the specimen and can affect mechanical properties such as modulus, maximum elongation, and tensile strength.

The alignment of a system is so critical to test results that organizations, such as Nadcap, strictly regulate misalignment. To meet these requirements and reach the most accurate results, the test operator needs to carefully assess his or her station, including the testing frame itself to the alignment cells used to verify alignment.

In addition to being accurately aligned, the frame should also be stiff so that it deflects very little at the load.

Next, it’s necessary to align the frame with the grips that will actually be used. This means that if an alignment is performed with one set of grips, but then replaced with a second set of grips, both sets of grips must be realigned. (To prevent removing grips, it is helpful to choose grips that can accommodate multiple tests including compression.)

Rigid and symmetrical grips produce the best alignment results. Under load, symmetrical grips will deform symmetrically from front-to-back/from side-to-side; therefore, the grips will not bend the specimen. Plus, the more rigid a grip is the less it will deform under load.

Once the frame and grips are aligned, now the specimen needs to be aligned. The operator oversees this process. To minimize user error, grips with specimen stops indicate where to place the specimen every time. If the specimen is loaded in different positions, even in a perfectly aligned system, large variation will occur.

To ensure that platens are parallel to the degree that Nadcap requires, one rigid platen and one locking, spherically-seated platen are required. These types of platen can be compressed against each other to ensure that they are parallel and can then be locked to prevent them from being misaligned.

Alignment cells are an essential part of any alignment. These cells, when used in conjunction with software, provide valuable information on how to correct a system’s misalignment. The dimensions of the cell should be as close as possible to the specimens being tested. On occasion, standard or stock alignment cells do not fit with the samples being tested. In these instances, the design of a custom strain-gaged alignment cell may be necessary. Since fabricating a strain-gaged alignment transducer can be a time-consuming process, plan this step well in advance.

Any given system has small misalignment errors due to tolerances of the frame and the parts used. To adjust for these small errors, it is helpful to have a fixture which can be used to make small adjustments to the concentricity and angularity of the load string. This is especially handy for future alignments and will save you time and effort down the road.

As always, feel free to contact us with any questions. Read more

Tips on Using Environmental Chambers & Furnaces

Does your system incorporate "specimen protect" or "load protect"? If you are not sure, ask us.

If it does you can use this feature to compensate for load string and specimen expansion/contraction while your furnace or chamber reaches set-point temperature.

Having inserted the specimen into the grips, set the desired load threshold value and enable the feature before switching on the furnace or chamber. As the load string expands or contacts, the crosshead or actuator will move to keep the force on the specimen below the threshold value.

• To prevent loss of heat around the pull rods and push rods, always use any convection shields provided or lightly pack the space between the pull rod and the furnace or chamber port with a suitable refractory fiber.
• Make sure you use the recommended anti-seize compounds and lubricants on your grips to help aid specimen removal.
• Use the smallest grips possible to reduce heat-up times and to maximize the available travel in an environmental chamber.
• After you have finished testing at below ambient temperatures, you can remove any condensation from the grips and pull rods by warming them in the chamber and allowing them to cool naturally.

Interested in more tips on tensile testing in high temperatures? Download our complimentary High-Temperature Guide. Read more

Why Automatic Extensometry?

When testing the mechanical properties of a specimen, being able to accurately and repeatedly measure the strain the specimen sees is crucial. Many testing standards require that a separate device, commonly known as an Extensometer, be used to ensure the most accurate strain data possible.

So before you run a test, attach a device onto the specimen to measure the strain… that doesn't sound bad, now does it? Now think about having to test hundreds of specimens. Odds are, manually attaching an extensometer to every specimen will add significant time to your testing process. This is where an automatic extensometer should be used.

Automatic extensometers allow the user to simply place their specimen into the grips that are being used during the test and hit “start”. You don’t have to worry about attaching the extensometer and removing it once the test is done; it’s done for you! Automatic extensometers can save time, increase repeatability, and reduce operator influence on results.  If you are interested in automatic extensometry, check out the AutoX 750. Read more

Balancing the Load Cell

We are often asked how frequently an operator should balance the load cell during testing. Many lab managers require the operator to balance a load cell before the start of a new sample; others require balancing the load cell before testing every specimen. We believe that either procedure is acceptable, as long as you follow one major rule:

 

Never balance the load cell when there is a specimen clamped in both grips

Instron load cells can detect a change in load as a result of simply gripping the specimen. If the load is balanced after a specimen is gripped, you risk zeroing out a real load. This real load will be subtracted from or added to reported results, thereby falsely increasing or decreasing actual values depending on whether or not there was a compressive or tensile load on the specimen before the load cell was balanced.

If you notice a change on the load channel display after gripping, you can remove the load using automated software features such as preload or specimen protect, or by manually adjust the position of the crosshead. Read more

Grip Penetration Effects

How To Determine Effective Gauge Length

If gripping pressure on the clamped specimen is not uniform throughout the clamped area, a certain amount of specimen extension may take place within the grips. When this happens, if the elongation is measured from the test curve and a calculation of precentage elongation is based on the separation between the grip at the start of the test, the resulting figure will be in error.

Extension of the specimen within the grips is referred to as "grip penetration". It may not be apparent from examining the load elongation curve whether grip penetration has occurred since it will not produce a stick-slip effect, but it is proportional to the applied load. As grip penetration is proportional to load, the load elongation curve will remain smooth and apparently normal.

A method for determininng the presence and magnitude of possible grip penetration is to plot elongation against gauge length for a given force (Fx). If the resulting line, when extrapolated to zero gauge length, does not pass through the origin, but gives a positive displacement on the elongation axis, then this is the restult of grip penetration. It is essential when performing these tests to always test the specimen at the same strain-rate since certain materials are strain-rate sensitive. For example, the longer the gauge length, the faster the required crosshead speed, and the ratio between the gauge length and the crosshead speed will be constant.

Calculating Effective Gauge Length

The intercept gives a value (Ej) representing elongation within the grip at a specified load. The quantity AE represents the true elongation for a corresponding gauge length or grip separation. When calculating elongation from a load versus strain graph, the value Ej should be subtracted from the total elongation before dividing this value by the gauge length or grip separation figure. Read more

Looking After Your Grips

Any successful gripping solution can be adversely affected by poor maintenance. Many common gripping techniques rely on friction or local surface deformation of the specimen to function. If the gripping surfaces become worn or contaminated, a loss in gripping efficiency occurs. Ultimately, this causes the specimen to slip, leading to an invalid test.

The first response to slippage is often to increase the gripping force by over-tightening the mechanical grips or to increase the pressure of pneumatic or hydraulic grips. Although this may temporarily solve the slippage, it can also bring new problems:

• Simple screw action grips are often tightened with a spanner or wrench. It's very easy to exert high torque loadings to the load cell unless care is taken. Excessive tightening can easily damage low force load cells. Taken to extremes, it's possible to damage the grips themselves. Using a small torque wrench will allow you to achieve consistent gripping force.
• Increasing the pressure applied to the specimen by the gripping system can also increase the chances of influencing the mechanical properties. This is especially true of materials that are weaker in compression than in tension. We find that increases in jaw breaks often accompany increases in the gripping force.

Poor maintenance can result in uneven or inconsistent gripping. High-frictional losses in screw grips reduce the clamping force on the specimen for a given tightening force. Friction effects in wedge grips can induce bending if the faces move unevenly.

Here are some golden rules for reliable gripping:

• Regularly clean and lubricate moving parts with the correct grades of lubricant as advised by the manufacturer. This is especially important for wedge action grips, which rely on the smooth sliding of the jaw faces along an inclined plane for their correct function.
• Periodically inspect the grips for defects, such as cracks or leaks in hoses.
• Periodically verify that the pressure gauges are accurately registering air or oil pressure to the gripping system.
• Replace jaw faces when the surfaces become worn, damaged, or contaminated. Some jaw faces, such as rubber-coated types, can degrade over time simply due to exposure to air and light. This degradation can accelerate if the jaw faces are used under non-ambient conditions in an environmental chamber.
• Do not use more gripping force than necessary to provide reliable, slip-free gripping.
• Old grips don't necessarily work with new materials or specimens. You may find that special grips or different jaw face surfaces are needed. You can try a variety of things to modify existing gripping methods including emery cloths, sticky tape, etc.

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Are Materials Testing Systems Potentially Hazardous?

They certainly can be. Material testing involves inherent hazards from high forces, rapid motions, and stored energy. You must be aware of all moving and operating components that are potentially hazardous, particularly the actuator in a servohydraulic testing system or the moving crosshead in an electromechanical testing system.

Whenever you consider that safety is compromised, press the Emergency Stop button. This will stop the test and isolate the testing system from hydraulic or electrical power.

Ensure that the test set up and the actual test you will be using on materials, assemblies or structures constitute no hazard to yourself or others. Make full use of all mechanical and electronic limits features. These are supplied to enable you to prevent movement of the actuator piston or the moving crosshead beyond the desired region of operation.

Your best safety precautions are to gain a thorough understanding of the equipment you are using by reading the instruction manuals, to always use good judgment, and to observe all Warnings and Cautions. You will find more specific warnings and cautions in the manuals whenever a potential hazard exists. Read more