356 ATMOSPHERIC PRESSURE PLASMA TREATMENT OF POLYMERS and corona treated LDPE coating are the mos... more 356 ATMOSPHERIC PRESSURE PLASMA TREATMENT OF POLYMERS and corona treated LDPE coating are the most likely explanation for the increased hot tack strength of the LDPE coating at the high treatment level. In addition, plasma and corona treatments increased the friction of LDPE and PP surfaces, whereas no significant changes were observed in surface gloss or barrier property of the extrusion coatings.
Fatigue experiments were performed on SEN specimens of EN AW 7475-T761 in order to investigate cy... more Fatigue experiments were performed on SEN specimens of EN AW 7475-T761 in order to investigate cyclic lifetime changes due to sharp notches and periodic overloads. While for specimens with sharp notches one would expect a decrease in cyclic lifetime, the introduction of periodic overloads generally causes a lifetime extension. Therefore specimens with three different notch depths but constant notch radius, and consequently three different elastic notch factors, were combined with three different overload levels and two different overload types. On the fracture surfaces a decrease in the crack propagation rate to a certain distance was observed which was followed by an increase of the crack propagation rate till fracture. The different distances to the minima of the crack propagating rate depend on the type and the level of the periodic overload as well as on the notch depth. A significant fatigue life extension due to periodic overloads introduced into a constant amplitude load could be obtained for pure tensile overloads with high overload levels.
To improve the knowledge of fatigue crack propagation the development of the crack growth rate at... more To improve the knowledge of fatigue crack propagation the development of the crack growth rate at multiple overloads with constant stress intensity, K, in steels has been experimentally studied. To keep K constant, a special test equipment has been developed at the Institute of Materials Science. Using a very high-resolution DC-potential drop method and recording the potential signal about 50 times per load amplitude, the crack propagation could be observed continuously with a cycle by cycle resolution of about 1μm. In multiple overloads the crack propagation within the first cycle ΔaOL,1 was found to be about hundred times greater than in the following cycles. This behaviour cannot be explained by LEFM. It was found that ΔaOL,1 may be explained by the crack tip opening displacement. However, in the following overload cycles the dislocation structure in front of the crack tip seems to control the crack growth as assumed by J. Weertman.
Abstract In this work the lock-in-thermography was tested for crack detection capabilities. Inste... more Abstract In this work the lock-in-thermography was tested for crack detection capabilities. Instead of an external heat source the thermo elastic effect induced by the mechanical loading is used for cyclic thermal stimulation. This procedure was applicable for crack detection in an aluminium centre cracked tension specimen (CCT) during a running fatigue test. As lock-in input signal the force as well the signal of a strain gauge, placed in the surrounding of an estimated crack initiation site, can be used, respectively. This strain gauge can also be attached perpendicular to the loading direction.
In order to estimate the crack initiation lifetime of the alumi nium alloy AA6009 an analytical m... more In order to estimate the crack initiation lifetime of the alumi nium alloy AA6009 an analytical model, based on local stress strain approaches will be pres nted. To calculate fatigue lifetimes of engineering components statistical and technological size effects have been taken into consideration. In addition, fatigue tests have been undertaken with load spec tra, which are representative of stresses and strains actually experienced by the component duri g service.
356 ATMOSPHERIC PRESSURE PLASMA TREATMENT OF POLYMERS and corona treated LDPE coating are the mos... more 356 ATMOSPHERIC PRESSURE PLASMA TREATMENT OF POLYMERS and corona treated LDPE coating are the most likely explanation for the increased hot tack strength of the LDPE coating at the high treatment level. In addition, plasma and corona treatments increased the friction of LDPE and PP surfaces, whereas no significant changes were observed in surface gloss or barrier property of the extrusion coatings.
Fatigue experiments were performed on SEN specimens of EN AW 7475-T761 in order to investigate cy... more Fatigue experiments were performed on SEN specimens of EN AW 7475-T761 in order to investigate cyclic lifetime changes due to sharp notches and periodic overloads. While for specimens with sharp notches one would expect a decrease in cyclic lifetime, the introduction of periodic overloads generally causes a lifetime extension. Therefore specimens with three different notch depths but constant notch radius, and consequently three different elastic notch factors, were combined with three different overload levels and two different overload types. On the fracture surfaces a decrease in the crack propagation rate to a certain distance was observed which was followed by an increase of the crack propagation rate till fracture. The different distances to the minima of the crack propagating rate depend on the type and the level of the periodic overload as well as on the notch depth. A significant fatigue life extension due to periodic overloads introduced into a constant amplitude load could be obtained for pure tensile overloads with high overload levels.
To improve the knowledge of fatigue crack propagation the development of the crack growth rate at... more To improve the knowledge of fatigue crack propagation the development of the crack growth rate at multiple overloads with constant stress intensity, K, in steels has been experimentally studied. To keep K constant, a special test equipment has been developed at the Institute of Materials Science. Using a very high-resolution DC-potential drop method and recording the potential signal about 50 times per load amplitude, the crack propagation could be observed continuously with a cycle by cycle resolution of about 1μm. In multiple overloads the crack propagation within the first cycle ΔaOL,1 was found to be about hundred times greater than in the following cycles. This behaviour cannot be explained by LEFM. It was found that ΔaOL,1 may be explained by the crack tip opening displacement. However, in the following overload cycles the dislocation structure in front of the crack tip seems to control the crack growth as assumed by J. Weertman.
Abstract In this work the lock-in-thermography was tested for crack detection capabilities. Inste... more Abstract In this work the lock-in-thermography was tested for crack detection capabilities. Instead of an external heat source the thermo elastic effect induced by the mechanical loading is used for cyclic thermal stimulation. This procedure was applicable for crack detection in an aluminium centre cracked tension specimen (CCT) during a running fatigue test. As lock-in input signal the force as well the signal of a strain gauge, placed in the surrounding of an estimated crack initiation site, can be used, respectively. This strain gauge can also be attached perpendicular to the loading direction.
In order to estimate the crack initiation lifetime of the alumi nium alloy AA6009 an analytical m... more In order to estimate the crack initiation lifetime of the alumi nium alloy AA6009 an analytical model, based on local stress strain approaches will be pres nted. To calculate fatigue lifetimes of engineering components statistical and technological size effects have been taken into consideration. In addition, fatigue tests have been undertaken with load spec tra, which are representative of stresses and strains actually experienced by the component duri g service.
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