TY - BOOK

T1 - Damage Indicators for early Failure Detection under Cyclic Loading considering Creep Strain of WC-Co Hardmetals at 700 °C and 800 °C

AU - Maier, Kathrin

N1 - no embargo

PY - 2021

Y1 - 2021

N2 - The focus of the present thesis is to provide a sound basis and methodological foundation for the knowledge-based design and optimization of uncoated WC-Co hardmetals for machining processes. Cyclic and step-loading creep tests at 700 °C and 800 °C were used to establish a correlation between experimental data, limit stresses, damage indicators, and early damage formation. Special attention was paid to application-relevant loading conditions occurring in the cutting edge region of milling inserts for steel and titanium work pieces. Various studies were carried out so far and reported in literature for WC-Co hardmetals under creep, monotonic increasing and cyclic loading conditions at elevated temperatures. However, no study was conducted to identify limits of endurable stresses and diagnose damage development at elevated temperatures. To fill this gap, six WC-Co hardmetal grades were investigated under uniaxial step-loading creep tests as well as in cyclic tests at stress ratios of R = σ(min)/σ(max) = -∞ and R = -1 up to different stress ranges at 700 °C and 800 °C in vacuum. The investigated hardmetal grades differ in their WC grain size: 0.4 µm to 2.0 µm and Co-content: 6 wt.% to 12 wt.%. Furthermore, microstructural changes were inspected by scanning electron microscopy and by electron backscatter diffraction. Based on the presence of advancing strain ratcheting above a critical stress range at R = -∞, limit stresses were determined associated with microdefect formation, such as nanopores and cavities. Damage indicators were determined from cyclic tests at R = -1 using stress-strain-hysteresis loop parameters. An increasing hysteresis loop area and tension-compression-strain asymmetry with increasing number of load cycles can be associated with the formation of microdefects. Step-loading creep tests also revealed a tension-compression-strain asymmetry as in cyclic tests. Physically, the strain asymmetry can be attributed to faster microdefect formation under tension than under compression. Further, limit stresses and the onset of increase of damage indicators were observed to decrease with increasing temperature. Additionally, strain ratcheting occurred under cyclic loading at R = -1, as observed at R = -∞ at 700 °C and 800 °C. Hence, stress-strain-hysteresis loop area, tension-compression-strain asymmetry as well as strain ratcheting are reliable and meaningful damage indicators for bulk material damage.

AB - The focus of the present thesis is to provide a sound basis and methodological foundation for the knowledge-based design and optimization of uncoated WC-Co hardmetals for machining processes. Cyclic and step-loading creep tests at 700 °C and 800 °C were used to establish a correlation between experimental data, limit stresses, damage indicators, and early damage formation. Special attention was paid to application-relevant loading conditions occurring in the cutting edge region of milling inserts for steel and titanium work pieces. Various studies were carried out so far and reported in literature for WC-Co hardmetals under creep, monotonic increasing and cyclic loading conditions at elevated temperatures. However, no study was conducted to identify limits of endurable stresses and diagnose damage development at elevated temperatures. To fill this gap, six WC-Co hardmetal grades were investigated under uniaxial step-loading creep tests as well as in cyclic tests at stress ratios of R = σ(min)/σ(max) = -∞ and R = -1 up to different stress ranges at 700 °C and 800 °C in vacuum. The investigated hardmetal grades differ in their WC grain size: 0.4 µm to 2.0 µm and Co-content: 6 wt.% to 12 wt.%. Furthermore, microstructural changes were inspected by scanning electron microscopy and by electron backscatter diffraction. Based on the presence of advancing strain ratcheting above a critical stress range at R = -∞, limit stresses were determined associated with microdefect formation, such as nanopores and cavities. Damage indicators were determined from cyclic tests at R = -1 using stress-strain-hysteresis loop parameters. An increasing hysteresis loop area and tension-compression-strain asymmetry with increasing number of load cycles can be associated with the formation of microdefects. Step-loading creep tests also revealed a tension-compression-strain asymmetry as in cyclic tests. Physically, the strain asymmetry can be attributed to faster microdefect formation under tension than under compression. Further, limit stresses and the onset of increase of damage indicators were observed to decrease with increasing temperature. Additionally, strain ratcheting occurred under cyclic loading at R = -1, as observed at R = -∞ at 700 °C and 800 °C. Hence, stress-strain-hysteresis loop area, tension-compression-strain asymmetry as well as strain ratcheting are reliable and meaningful damage indicators for bulk material damage.

KW - WC-Co Hartmetall

KW - Uniaxialer Druck- und Zug-Stufenbelastungs-Kriechversuch

KW - Uniaxialer zyklischer Versuch

KW - Erhöhte Temperatur

KW - Schadensindikator

KW - Elektronenmikroskopie

KW - WC-Co hardmetal

KW - Uniaxial compression and tension step-loading creep test

KW - Uniaxial cyclic test

KW - Elevated temperature

KW - Damage indicator

KW - Electron microscopy

M3 - Doctoral Thesis

ER -