TY - BOOK

T1 - Deformation and Fracture Behavior of Semi-Crystalline Thermoplastics Used for Rolling Applications

AU - Berer, Michael

N1 - no embargo

PY - 2013

Y1 - 2013

N2 - The study of the deformation and fatigue behavior of polymeric bearing elements is the subject of the present thesis. For this, cylindrical rolls made of two different semi-crystalline thermoplastics (Polyoxymethylene, POM and Polyetheretherketone, PEEK) were chosen and investigated. Both materials revealed viscoelastic properties and thus the global as well as the local deformation behavior were analyzed experimentally. In a first step a tribometer with a two-dimensional load cell was used to examine the global deformation behavior of the rolls. In order to represent the practical application, the rolls were loaded statically and subsequently a horizontal rolling translation under load was carried out. The results were compared with data of corresponding simulations which were performed with viscoelastic material models. A good correlation between experiment and simulation was found for the POM rolls but discrepancies were observed for the PEEK rolls. Thus in a second step the measurements previously conducted on the tribometer were partially repeated. This time the front sides of the rolls were painted with a pattern and images were recorded during the tests. The images were analyzed using digital image correlation (DIC) and thus the local strain distribution could be determined. While POM revealed the expected viscoelastic properties, PEEK showed an elastic-plastic behavior with comparatively little time-dependence. Because of high strains detected in the surface area of the PEEK rolls it was speculated that the dynamic mechanical properties in this area would be different from the rest of the roll. Since the rolling properties are determined by the dynamic mechanical properties, it was decided to conduct an appropriate analysis at high strain levels. Thus tensile fatiuge tests at high stress levels were carried out. During the tensile fatigue tests distinct material changes were observed, especially at the higher load levels. These changes were related to cold drawing of the material. As no cold drawing is expected in the compression regime, a conclusion for the rolls is difficult. Thus for the future, additional compression fatigue tests are considered to be needed. Concerning the fatigue behavior of the rolls, two main effects were observed in the practical application. On the one hand the POM rolls revealed quasi-brittle fracture of the whole roll. In this case the crack started from an initial defect inside the roll and grew until a critical length was reached. On the other hand PEEK rolls did not break in a brittle manner, but excessive surface fatigue (“pitting”) was observed. Additionally, in special loading cases POM rolls also showed pitting. However, the latter was not a critical effect and thus was not examined in this thesis. The excessive pitting effect of the PEEK rolls was not expected and hence it was analyzed in detail. It was caused by small surface pre-cracks which resulted from the injection molding process. In the practical application, the PEEK rolls were used in combination with lubricants. During rolling these lubricants were pressed into the surface cracks and the resulting wedging effect caused crack tip opening and crack growth. By modifying the injection molding process the surface cracks were reduced considerably which resulted in much higher cycle numbers before pitting initiation. In order to optimize the life times of the POM rolls an appropriate characterization method of the materials was necessary. Using linear elastic fracture mechanics (LEFM) and especially cyclic fatigue crack growth tests, an adequate method was found. Subsequently, this technique was used to compare two POM resins regarding their fatigue crack growth resistance. Although both materials were found to be very similar concerning their morphology and basic mechanical parameters, differences in the fatigue crack growth behavior were observed.

AB - The study of the deformation and fatigue behavior of polymeric bearing elements is the subject of the present thesis. For this, cylindrical rolls made of two different semi-crystalline thermoplastics (Polyoxymethylene, POM and Polyetheretherketone, PEEK) were chosen and investigated. Both materials revealed viscoelastic properties and thus the global as well as the local deformation behavior were analyzed experimentally. In a first step a tribometer with a two-dimensional load cell was used to examine the global deformation behavior of the rolls. In order to represent the practical application, the rolls were loaded statically and subsequently a horizontal rolling translation under load was carried out. The results were compared with data of corresponding simulations which were performed with viscoelastic material models. A good correlation between experiment and simulation was found for the POM rolls but discrepancies were observed for the PEEK rolls. Thus in a second step the measurements previously conducted on the tribometer were partially repeated. This time the front sides of the rolls were painted with a pattern and images were recorded during the tests. The images were analyzed using digital image correlation (DIC) and thus the local strain distribution could be determined. While POM revealed the expected viscoelastic properties, PEEK showed an elastic-plastic behavior with comparatively little time-dependence. Because of high strains detected in the surface area of the PEEK rolls it was speculated that the dynamic mechanical properties in this area would be different from the rest of the roll. Since the rolling properties are determined by the dynamic mechanical properties, it was decided to conduct an appropriate analysis at high strain levels. Thus tensile fatiuge tests at high stress levels were carried out. During the tensile fatigue tests distinct material changes were observed, especially at the higher load levels. These changes were related to cold drawing of the material. As no cold drawing is expected in the compression regime, a conclusion for the rolls is difficult. Thus for the future, additional compression fatigue tests are considered to be needed. Concerning the fatigue behavior of the rolls, two main effects were observed in the practical application. On the one hand the POM rolls revealed quasi-brittle fracture of the whole roll. In this case the crack started from an initial defect inside the roll and grew until a critical length was reached. On the other hand PEEK rolls did not break in a brittle manner, but excessive surface fatigue (“pitting”) was observed. Additionally, in special loading cases POM rolls also showed pitting. However, the latter was not a critical effect and thus was not examined in this thesis. The excessive pitting effect of the PEEK rolls was not expected and hence it was analyzed in detail. It was caused by small surface pre-cracks which resulted from the injection molding process. In the practical application, the PEEK rolls were used in combination with lubricants. During rolling these lubricants were pressed into the surface cracks and the resulting wedging effect caused crack tip opening and crack growth. By modifying the injection molding process the surface cracks were reduced considerably which resulted in much higher cycle numbers before pitting initiation. In order to optimize the life times of the POM rolls an appropriate characterization method of the materials was necessary. Using linear elastic fracture mechanics (LEFM) and especially cyclic fatigue crack growth tests, an adequate method was found. Subsequently, this technique was used to compare two POM resins regarding their fatigue crack growth resistance. Although both materials were found to be very similar concerning their morphology and basic mechanical parameters, differences in the fatigue crack growth behavior were observed.

KW - Polyoxymethylene (POM)

KW - Polyetheretherketone (PEEK)

KW - Polymeric bearing elements

KW - Polymer rolls

KW - Semi-crystalline thermoplastics

KW - Fatigue

KW - Fracture mechanics

KW - Crack growth kinetics

KW - Polymer morphology

KW - Polyoxymethylen (POM)

KW - Polyetheretherketone (PEEK)

KW - Kunststoff-Wälzkörper

KW - Kunststoff-Rollen

KW - Teilkristalline Thermoplaste

KW - Ermüdung und Bruchmechanik von Kunststoffen

KW - Risswachstumsgeschwindigkeit

KW - Morphologie von Kunststoffen

M3 - Doctoral Thesis

ER -