Roboze PEEK tensile test - in collaboration with TEC Eurolab Srl

The evaluation phase of the implementation of industrial 3D printing solutions also involves the analysis and consideration of the intrinsic properties of the production material.

Roboze has always been committed to offering additive manufacturing solutions dedicated to the production of components and functional parts with super polymers and composite materials. PEEK, associated with the performance of Roboze 3D printing solutions, is among the materials that more than others have offered Metal Replacement advantages due to its extraordinary mechanical, thermal and chemical properties.

Roboze PEEK: technical characterization

Starting today, we will begin the publication of a series of characterizations carried out on Roboze PEEK specimens, in collaboration with TEC Eurolab, an accredited third-party laboratory - specializing in the characterization of materials and non-destructive testing, training and certification. For over 30 years it has assisted manufacturing companies, from automotive to aerospace, from energy to biomedical, from food to cultural heritage, in obtaining and verifying the maximum performance of products and processes, with absolute safety and quality. The numerous accreditations, including 17025, 9100 and Nadcap, are a further guarantee.

The goal is to show the mechanical properties of Roboze PEEK to evaluate it in industrial Metal Replacement applications where the strength / weight ratio of the components plays a fundamental role. 

What is a tensile test?

The tensile test is a destructive test useful for characterizing the properties of materials when subjected to uniaxial tensile loads. It consists in subjecting a suitably prepared specimen to an axial tensile stress, which stretches until it breaks.

The reference standard for the tests performed is ASTM D638. The speed used for the calculation of the tensile modulus is 5 mm / min constant until failure. Remember that the test results are a function of the speed set, which is why for a correct comparison between different materials it is important to know in advance the speed at which the tests were performed.

PEEK tensile test: design and requirements of the samples

The preparation of the specimens and the execution of the individual tests were carried out in accordance with the ASTM D638 standard. The analyzes were performed by the TEC Eurolab Srl laboratory.

The specimens were made on the Roboze ARGO 500 solution with an extruder equipped with a 0.6 mm diameter nozzle. The filament used is characterized by a diameter of 1.75 ± 0.05 mm and, before being processed, it was subjected to a drying process of 12 hours at 100 °C by means of the Roboze HT Dryer. See the printing conditions below for the production of the samples:

• Chamber temperature: 160 °C;
• Extrusion temperature: 470 °C;
• Printing speed: 1800 mm / min;
• Layer height: 0.22 mm;
• Filling percentage: 100%;
• Shells: 2.

At the end of the printing process, the support structures were manually removed by a qualified operator. All tests were performed with printed samples in the following orientations:

• Flat (lying on the XY plane);
• On Edge (lying on the XZ plane);
• Upright (lying on the XZ plane).

The PEEK specimen of standard dimensions, with a "dogbone" geometry, is constrained with clamps with two crosspieces which, moving away, induce a tensional state of traction along the resistant section of the specimen. Once the moving speed of the moving crosspiece has been set, the applied load and the deformation undergone by the specimen are monitored during the test. At the output, the system reports a Cartesian graph that correlates the stress (stress, σ), i.e. the ratio between the force applied to move the mobile crosshead at constant speed and the minimum section of the specimen, and the strain (strain, ε), that is the percentage ratio between the variation in length of the specimen with respect to its initial dimensions (Δℓ) and the nominal length before the start of the test (0). The stress-strain curve is a function of the nature of the material.

The parameters of greatest interest extrapolated from the graph are the maximum load (σ_M - Tensile Strenght), the elongation at break (ε_B - elongation at break) and the Young's modulus (E - Tensile Modulus).

It is also possible to obtain the tensile strength (σ_B - Break strength), the elongation at maximum load (ε_M - elongation at tensile strength) and, where present, the elongation and yield strength (ε_S and σ_S). 

The initial linear section of the curve represents the region of linear elastic deformation. In this region (also called the Hookean region of the material), the material undergoes an instantaneous and reversible deformation linearly dependent on the applied stress. The angular coefficient of the line tangent to the linear elastic region is defined as Young's modulus, which is the constant of proportionality between the deformation undergone by the material and the applied stress. Young's modulus is generally measured by stresses at 0.05% and 0.25% strain and provides an indication of the stiffness of a material.

At the end of the elastic region, for more ductile materials, the stretch of plastic deformation of the material begins, while, for fragile materials, the sample breaks without or with limited plastic deformation.

Results of Roboze PEEK tensile test

Components produced with additive manufacturing techniques have different mechanical properties in different directions (anisotropic properties). Since the purpose of this process is often to create parts of arbitrarily complex geometry, it is very difficult to align the sample in the direction that maximizes its mechanical properties. Variations in spatial orientation not only lead to a change in the mechanical strength values ​​of the component, but also affect the fracture mechanism. Samples made with XZ and XY orientation show a ductile fracture behavior, since before reaching the structural failure of the resistant section, it is possible to appreciate the phenomenon of pinching and plastic deformation of the specimen itself (an event particularly evident in the specimen with "On Edge" orientation). In the case of printed samples with ZX orientation, the fracture behavior is typical of a brittle material, i.e. it occurs at the end of the linear elastic section of the σ-ε curve, due to the de-cohesion process that originates at the interface between two contiguous layers induced by the application of a normal load to the contact surface between the two layers of material.

The results affirm the lower resistance of the junctions between the layers compared to that of the filament itself, confirming the typical anisotropic properties of a product made using additive manufacturing techniques.

It is therefore essential to adequately study the printing strategy of the sample and its growth direction in the chamber.

Thermoplastic polymers, by their nature, undergo a progressive reduction of their viscosity with increasing temperature. This leads to a progressive softening of the material until it reaches complete fusion. Consequently, the mechanical properties of the final product will depend on the working temperature: the higher the temperature, the lower the mechanical strength offered by the material.

The variation of mechanical properties as a function of temperature can be analyzed by carrying out tensile tests in a hot chamber heated to the chosen temperature.

The figure shows the effect of temperature on the tensile strength of PEEK specimens printed with XY orientation and infill orientation at +/- 45 °. This demonstrates that test temperature has a very significant influence on the tensile behavior of PEEK. Overall, the tensile strength decreases with increasing temperature with an almost linear trend in the temperature range investigated.

Furthermore, from the following table it can be observed that maximum load, Young's modulus and the elongation at break are all highly dependent on the test temperature. Both maximum load and elastic modulus decrease with increasing temperature, while elongation at break increases with increasing temperature. In particular, the elastic modulus decays when the temperature exceeds the glass transition temperature (T_g) equal to 146°C. The variation in strength and stiffness of the material is due to the structural change of PEEK passing from the glassy to the rubbery state in the vicinity of the glass transition.

However, the crystalline domains that characterize semicrystalline thermoplastic polymers, such as PEEK, allow to maintain high mechanical properties even at temperatures above the glass transition temperature.

The increase in elongation at break, on the other hand, is due to the greater mobility of the macromolecules, which therefore allows the tensions that insist on the resistant section of the material to be accommodated more efficiently.