Mechanical Polymer Testing

We provide excellent mechanical polymer testing services in short deadlines at a competitive price point. Now you might think this is a sublime aspiration. But how do we live up to the promise? For one thing, we focus on static material testing using a very well-equipped and fully programmable Zwick electro-mechanical test frame. For another thing, we furthered methods of video extensometry, thus assuring a comprehensive, precise and reliable strain measurement. Also, we implemented a specimen & data tracking system, which improves efficiency and eliminates human error. Finally, we developed a system for comprehensive and efficient computerized reporting. So, you get a high-quality test report, appropriate for further automated data processing. This makes us a specialized testing partner for simulation engineers, product developers and material producers.

Table Of Content

The Tests we Provide

On one hand, we test rigid, reinforced polymers including short fiber reinforced polymers (SFRP), long fiber reinforced polymers (LFRP) and sheet molding compounds (SMC). On the other hand, we test soft polymers including elastomers and thermoplastics. For all these materials we provide appropriate tensile tests, compression tests, bend tests, creep tests, creep-to-rupture tests, relaxation tests, relaxation-retraction tests and cyclic characterization tests. Ce can perform all tests at temperatures between -80°C and 250°C. And we we measure strain and transverse strain with precision.

Test Instrumentation

We measure and control force from 0.01 N to 10 kN. On one hand, this enables us to test very small and soft samples.  On the other hand, this is enough for testing rigid plastics as SFRP, SFRP and SMC materials. Our current upper force limit of 10 kN is due to our polymers-oriented load frame.

We measure and control strain using a programmable video extensometer. It is fully integrated into the control and data acquisition of our load frame. Hence, we can measure strains from 0.001 % to the very large values observed in thermoplastics and elastomers. Also, we can freely choose gauge lengths and locations. Consequently, we do all our tests, in the whole temperature range, with optimized strain and transverse strain measurement. Not to forget, we are able to do tests in strain control whenever necessary. The maximum strain rate with standard specimens is approximately 10%/s.

We measure and control temperature using our temperature chamber. It is fully integrated into the control and data acquisition of our load frame. We use these thermo-mechanical capabilities for our research work. During normal polymer testing, the ability to see strains and temperature evolution in real time helps us, to understand the polymer physics and thus improves test quality.

Specimen Preparation

Send us your specimens

The photo shows a cnc mill inside an enclosure and a computer monitor to the left. Both stand on a table. Below the table are a computer, a controller and a suction unit.
CNC mill for specimen preparation from plates and parts/components

If you send “dog bones” with a tab width below 40 mm and a thickness below 8 mm, they will fit our preferred pneumatic grips. If you send larger dog bone type specimens, we will use our mechanical screw grips. When it comes to bend specimens, we usually work with geometries proposed by ASTM and ISO standards. We take care to manufacture compression specimens for soft polymers with high precision. For compression tests for rigid, reinforced specimens, we use an ASTM D695 support jig. Therefore, we prefer ASTM D695 specimens.

Send us plates or parts

Send us plates or parts, then we manufacture the specimens for you. One the one hand, if you send soft plate material and you want ASTM or ISO standard samples, we will use a die cutter. In that case, specimen manufacturing will be economical, and the cut will be flawless. On the other hand, if you send components or rigid plates, we will machine specimens using our CNC mill. We have a database of specimen geometries and an efficient and quality assured CAD/CAM process.

Let us design specimens

The figure shows a 3D CAD model of a specimen with short shoulders and short tabs. There are 10 strain marks on its parallel length. They are arranged in two times 5 points, spread over the parallel length on both sides.
3D CAD model of a custom specimen with 10 strain marks for strain measurement using a video extensometer.

Let us design specimens, because standard specimens don’t always fit the purpose. When we cut specimens from components, size restrictions may demand custom geometries. For another thing, sometimes standard specimens just don’t fit the material. For example, brittle materials may be sensitive to stress concentrations, demanding optimized shoulders and tabs. We develop specimens in an iterative two-step process. First, we develop a new specimen using parametric CAD and simulate its behavior in tension and compression. Afterwards, having a promising virtual candidate, we manufacture a prototype. Using our proven CAM-CNC manufacturing route, we are quick at cutting and testing. Consequently, creating new specimen geometries only is a matter of man hours and man days.

Conditioning, Humidity and Aging

Temperature and time

Preconditioning most materials at room temperature for limited time will influence mechanical properties little. However, preconditioning them at elevated temperatures for the same time can change them significantly by physical aging. These effects are important for two reasons. First off, if preconditioning is not uniform for all tests during elevated temperature testing, this causes inconsistency and scatter of results. Second, if a component experiences physical aging during lifetime, this can alter its behavior and should be regarded during testing. Consequently, we plan, control and log preconditioning procedures. For elevated temperatures we use a temperature chamber. And we control and log laboratory temperature at 23°C.

Humidity of Polyamides

We usually condition polyamides to three states. First off, we condition to equilibrium at standard conditions at 23 °C and 50 % relative humidity. Hereby, we use accelerated conditioning using salt solutions and elevated temperatures. This standard condition is most useful for testing at lower temperatures, where specimen humidity is not influenced by heat-up and soaking during the mechanical test procedure. Second, we condition to the dry condition. Hereby, we store specimens in a heat chamber, we monitor air water content and set the temperature to reach the desired humidity. This condition is good for immediate testing at room temperature. And it is fine for testing at elevated temperatures in the proximity of the conditioning temperature, where specimen water content is already in equilibrium with the atmosphere. Third, we condition polymers to a humid condition using boiling water. Then their behavior is tested at standard conditions.

We monitor and log laboratory humidity using a calibrated hygrometer. And we monitor and measure material humidity by weight measurement of samples during conditioning and storage. Additionally, we store the material in controlled conditions using desiccators and water-tight packaging. Finally, we make sure that humidity is well defined, homogeneous and repeatable during plastics testing. We do that by diffusion simulation using finite element analysis.

Aging

We are equipped to age material at elevated temperatures in atmosphere, solutions and in oil. Depending on the medium, we can go to a maximum temperature of 250°C. Typically, aging times are scaled and go to a maximum time of 1000 hours.

Testing Process and Quality Assurance

Computerized flow of data and material

The grayscale picture shows a polymer specimen. On its parallel length, there are four strain marks. On the left and right side, the specimen is tagged with a sticker, containing datamatrix code, name and tortuetec logo. The picture has a scale bar, indicating 10 mm.
We tag specimens with stickers, containing a datamatrix code and a name.

First, when you send us parts and specimens, we name them, tag them with machine readable datamatrix codes, assign them to a fixed place in storage and register them in the database. Then, each process with your specimens and parts is a sequence of getting them from storage, scanning their datamatrix code, performing a mostly computerized process and putting them back to storage. Hereby, typical processes are heat treatments, CNC machining, dimensional measurements or mechanical testing. For all processes, software takes care of reading the datamatrix code, opening a database connection, getting process parameters from the database, guiding and controlling the process and writing results and measurements back to the database. All in all, we maximize repeatability and transparency and minimize room for human error.

Validation built into the process

We operate a real-virtual laboratory, intimately combining physical experiments and computer simulation. By this, we have woven validation and control into our standard testing process. Here, we want to show this by some examples.

On the right, the image shows a red-yellow-green-blue scale bar for von Mises stress. Also, there are six contour plots of von Mises stress on specimens.
  • When we receive specimens, we generate a 3D model, including the planned specimen geometry and strain measurement setup. Then we measure the actual specimen dimensions. So, if there is a deviation between planned and actual specimens, we find out early on.
  • Then we use the 3D model for finite element simulation of the test. For example, we assess stress concentration factors or compression stability. So, if a specimen type appears not to be optimal or even inadequate for the intended test, we give immediate feedback.
  • During mechanical testing, we measure strain at least at two locations on the specimen. For example, we measure longitudinal strain on the left and right side. So, we have real-time insight into strain uniformity. Accordingly, we will react immediately.

To sum it up, our validation steps improve test quality, reduce project lead times and make a correct interpretation of test results much easier.

Calibration of measurement devices

Our mechanical and thermal measurement devices and machines are calibrated regularly. Your calibration partner is DAKKs accredited.

Test Report

Reports for humans and machines

Our test reports are generated from the before mentioned database using a modular computer program. There is no direct human interference during report compilation. Hence, the reports are fully repeatable, quality assured, and fit for your further computerized, fully automated usage. Also, it comes as a modular Excel document, which can be easily accessed by humans and computer algorithms.  For every report module, the first excel sheet contains the descriptions, the second sheet contains the data. So, using our report, you can find comprehensive test data quickly.

This picture shows a typical specimen description, which we use in our test reports. We include a specimen drawing with dimensions of specimen geometry and extensometry setup. Also, we give explanations about measurements.
Example of a specimen description given in the test report

Report modules structured by usage

To start with, our statistical evaluation module is the best way to get the aggregated big picture. For example, if tensile tests with 10 repeats were performed at some temperature levels, this module shows aggregated results per temperature in tables and figures at a glance. Then, if you are interested in the details, you find report modules about each tested part and specimen, about testing parameters and test results. Further, if you are a simulation engineer, you may be interested in more detailed data. Therefore, for each test you get a sheet with a table of time dependent data of (e.g. stress, strain, …) and charts. Additionally, indices help you with filtering and extracting data. And not to forget, we add a photo showing the specimen after testing, so you can see deformation and damage.

Transparent specimen documentation using Storage Boards

During the testing project, we store the specimens on computer administered, fixed storage positions. These storage positions are located on A3 sized white polymer sheets. We call them Storage Boards. At the end of the testing campaign, we weld a transparent cover onto each Storage Board, covering and fixing each tested specimen. Then we put all Storage Boards into labeled archive boxes. This makes it quick and easy for you to find all tested specimens as easily in real on a Storage Board as virtually in the Excel report. So, if you want to further examine or show your tested specimens, you can do that in beauty and hassle free.

The picture shows a white board on a gray background. On the board, marks indicate storage locations for specimens and most positions are occupied. On the bottom, name and a datamatrix code identify the sorage board
During testing campaigns, we store specimens Storage Boards. After the test campaig we seal the specimens by welding a transparent plastic film and then we archive them.