Tensile Tests

Both ISO 527 -1 / -2 and ASTM D 638 set out the standardized test methods. Both standards are technically equivalent but do not deliver fully comparable result data as the testing speeds and the result determination differ in several points.

With the testXpert® standard test program, users of Zwick machines can rely on applying the correct test parameters whether they test to ISO or to ASTM standards.

The modulus determination, especially as described in ISO 527, requires a high accuracy of ± 1 micrometer for the extensometer being used. Zwick’ s incremental extensometers Macro, MultiXtens, OptiXtens and the incremental clip-on types comply with this requirement in full or even exceed it, while also providing high elongation measurement capacities in class 0.5 or class 1 for up to 700 mm of total extension.

Flexural Tests

The 3-point flexural tests specified in ISO 179 and in ASTM D 790 are among the classic characterization methods for semi-rigid and rigid plastics. Typical results are the flexural modulus, stress at 3.5 % strain, and stresses and strain at yield and at break.

In particular, the results of flexural tests show the present material's behavior close to the surface of the specimen. Compared to tensile tests, the deflections to be measured are approximately 4 times higher.

Zwick machines can compensate the deformation of the load frame and the loading members via the unique software testXpert®. Especially useful for QC purposes, this allows the use of the crosshead travel monitor of the testing machine to measure the deflection.

Pendulum Impact Tests

Impact tests are used to measure the behaviour of materials at higher deformation speeds. Pendulum impact testers determine the energy required to break a standardized specimen by measuring the height to which the pendulum hammer rises after impacting the test piece. Several test methods are currently applied:

* Charpy impact tests (ISO 179-1, ASTM D 6110)
* Izod impact tests (ISO 180, ASTM D 256, ASTM D 4508) and unnotched cantilever beam impact (ASTM D 4812)
* Tensile-impact tests (ISO 8256 and ASTM D 1822)
* Dynstat tests (DIN 53435)

Within the framework of ISO 10350-1 (single point data) Charpy testing according ISO 179-1 is the preferred test method. The test is typically carried out with unnotched specimen and edgewise impact (method 1eU). If the specimens do not break, then the test has do be completed with notched specimens even though the results are not comparable. If there is still no break, then the tensile-impact method is used.

Within ASTM standards, it is current practice to use the Izod test method according to ASTM D 256, always using notched specimens. A more seldom used variant is the “Unnotched cantilever beam impact” method as in ASTM D 4812, which is similar to Izod but specifies the use of unnotched specimen. If only small specimens are available, then the chip-impact test as per ASTM D 4508 applies.

The Charpy method has a larger range of application and is better suited for testing materials exhibiting interlaminar shear fracture or surface effects. Also, there are some advantages of the Charpy method when testing at low temperatures due to the increased distance from the specimen notch to the supports which avoids problems of fast heat transfer to that critical part of the specimen.

Some German automotive manufacturers use the Dynstat impact test method for testing of small specimen sizes, however, this method is only described in DIN standards.

Classic impact testers work in a range between 10 % and 80 % of the nominal energy of the pendulum deployed. According to ISO standards, the pendulum with the highest possible energy is to be used in order to minimize effects due to the deceleration of the pendulum during impact.

This strongly limits the practical range of use of each pendulum size, leading to current changes in pendulums.

Zwick impact testers types 5102, 5113 and the new HIT series strictly adhere to ISO and ASTM standards. They are of the robust design necessary for reliable test results.

Instrumented Impact Testers

Instrumentation means that the pendulum fin is equipped with a sensor able to register the forces during the moment of impact.

As a result, supplementary information about the tested material becomes available. Force / deflection diagrams illustrate the brittleness more precisely. The range of use of each pendulum size is much larger, so fewer pendulum changes are needed to obtain reliable results in a broad range of tests.

Melt flow tests

Melt-flow plastometers deliver the standard values of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastic materials according to ISO 1133 and ASTM D 1238.

Zwick provides simple method A-type plastometers as well as sophisticated machines for both method A and method B testing.

Method A

The main parts of the 4100 plastometer are a barrel that is exactly heated to the given test temperature, a die of standardized diameter, and a piston with a mass piece mounted on top. The material filled into the barrel is heated and then extruded through the die by the force exerted by the mass. The extrudate is automatically cut in constant time intervals and then weighed. The MFR result can be calculated as the mass extruded in 10 minutes, even though the testing time is normally much shorter.

Method B

The 4106 plastometers are equipped with a travel monitor able to accurately measure the piston travel during the test. The MVR result is the extruded volume per 10 minutes.

This method is often used in production control. The fact that no weight determination is necessary makes the operation simple and reliable. If the density at melt temperature is known, the MFR value can be obtained by calculation. By using a supplementary scale, the density at melt can also be computed.

In some cases it may be interesting to know the flow rate ratio (FRR) of MVR results obtained under different loads. Zwick 4106 plastometers can be equipped with an automatic load change unit, allowing multi-stage tests using different masses during the test-out of one barrel filling. The load applied can be freely pre-programmed from high loads to small, or vice versa. Also the measuring times and travel are individually programmable for each measuring step.

Hardness tests

Hardness is the resistance of a material against the penetration of a specified indenter. In the field of plastics, many different scales and methods are common.

Shore hardness

The indentation is measured against the force of a spring that increases with the hardness to be measured. Different spring characteristics and indenter shapes are currently specified. The most common scales are Shore A and Shore D. For specific applications, other scales such as Shore B, C, 0, 00, 000, and D0 are available.

The application ranges from soft elastomeric materials (Shore A) to rigid thermoplastics (Shore D).

Zwick supplies accurate Shore durometers in mechanical (analog) versions as well as durometer types with digital measurement system and display prepared for PC connection.

Ball indentation hardness

For harder materials, the ball indentation hardness is standardized in ISO 2039-1. A ball type indenter is pressed into the material by a constant weight. The indentation depth under load yields the hardness value.

Rockwell hardness

ISO 2039-2 and ASTM D 785 define several Rockwell hardness scales for plastics. The difference to the ball indentation hardness is the measurement at preload before and after applying the main load. Several indenter diameters are available according to the scale to be employed.

Automation

“Automation reduces the statistical scatter of your results.”

This is the main reason why many research centers are already equipped with automated Zwick testing machines.

The operator's influence is strongly reduced because each specimen is tested exactly like the next one. Results become more comparable and assured.

More reasons for automation? The most important ones are set out below:


* The operators are free for other tasks in the lab.
* Robots can work during the night without supervision.
* The amortization time of such systems is - depending on the frequency of use - generally less than 2 years.
* Logbook functions allow an accurate traceability of the conditions at each single test.
* Different test types, i.e. tensile, bending and hardness, can be combined in the same system.
* Data can be transferred to host computers automatically.


Zwick is able to offer a large range of systems, each optimized for the test and materials types they are intended for.

Test types:


* Tensile testing
* Bending testing
* Compression testing
* Impact testing
* Hardness testing

The experience gained by Zwick as a supplier of several hundred automatic testing systems makes us a key success factor for our customers.