Innovative 2D tensile test for the design of plastic tanks, Fraunhofer Institute LBF, press release

In the first step, plastic components are designed on the basis of models that have been tried and tested for metals. This can lead to critical misinterpretations in the case of plastics primarily subjected to multiaxial tensile loads. Material models for plastics require data from 2D and 3D tensile tests. At Fraunhofer LBF, known test specifications for the biaxial tensile test under the influence of temperature were analyzed and updated to reflect the current state of research. The implemented optical measurement during the load records the mechanical behavior. This data simplifies the selection and adjustment of a material-specific model. A reliable extrapolation to 3D trains is possible and essential for safe and cost-effective design of plastic tanks.

Established design methods are usually based on data from uniaxial tensile tests. For plastic components, such methods must be critically evaluated. Especially with tanks and vessels that are under internal pressure, with valves or components in underwater applications, such designs lead to “unexpected” failures during use. The goal of the research team is to provide the industry with application-related and economic methods that make it possible to derive fundamental information about the mechanical behavior under multiaxial tensile loads.

Fraunhofer LBF’s new approach provides data for reliable modeling of components under practical loads as well as appropriate design tools. Components from the automotive sector, components for aircraft construction or products for sports, medicine and household can be designed more reliably and cost-effectively.

Modern method for 2D tensile testing

The test unit is designed for plastic panels with a thickness of about two millimeters. This thickness corresponds to the typical wall thickness of plastic components in the injection molding sector. The plate is clamped tightly in the fixture between circular rings and centrally loaded with a hemisphere of the indenter, causing the specimen to deflect. A uniform biaxial tensile stress occurs in the center of the specimen. The contact surface is lubricated and friction is reduced when current is applied. Stress singularities in the clamping area are reduced by a special design of the edges of the circular rings.

The deformation of the sample is recorded by a CCD camera with a telecentric lens. This avoids capturing virtual strains that occur when the viewing plane shifts along the optical axis. The geometry change in the plate is evaluated in a subsequent post-processing using gray value correlation software. In addition, another CCD camera is used, which registers the start of plasticization at the edge of the clamping.

The test results can be evaluated up to a plate bending of six millimeters. By varying the diameter of the clamping tool and the geometry of the indentation, different types of bend lines can be obtained. The optimal dimensions are individually coordinated with the researchers at Fraunhofer LBF depending on the material properties, sample thickness and test requirements. The tests are carried out according to customer specifications or according to the institute’s recommendations at temperatures up to 120 degrees Celsius.

Evaluation of the deformation by digital image correlation

The sample thickness is measured before testing. The sample is stained with a random black and white pattern and then directly tested for optimal adhesion between the panel and the pattern for failure. This enables evaluation of the loads on the sample surface by digital image correlation as a function of the load. In addition, the set-up makes it possible to record local effects and, if necessary, study the unloading behavior of the polymers. In addition, the test setup allows the determination of the creep properties under 2D tension.

Total voltage vs. deflection as a function of radius is simulated using reverse engineering methods. In previous studies, a good agreement has been obtained between the experimental and simulation results at different temperatures.

The data is e.g. B. used for design and investigation of defects in plastic tanks under internal pressure and elevated temperatures. Based on the data obtained, material models and strength criteria used in the simulation are adapted to the plastic according to the material. The Fraunhofer researchers analyze the individual challenges involved in modeling the critical plastic components and offer expertise at all levels of the design process.

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