Correlating Interlaminar Fatigue Fracture Behavior to NDE Parameters in Carbon/Epoxy Composites Containing Voids and Delamination

Carbon fiber composites have been increasingly used in aerospace, military, sports, automotive and other fields due to their excellent properties, including high specific strength, high specific modulus, corrosion resistance, fatigue resistance, and low thermal expansion coefficient. Delamination, or interlaminar fracture, is a serious failure mode leading to a loss in composite stiffness and strength. Manufacturing process defects such as voids degrade the fatigue life and delamination resistance of the composite. This study investigates the effect of voids on fatigue interlaminar fracture behavior of carbon fiber composites. Samples with different vacuum levels: 0%, 50% and 100% were fabricated by varying the vacuum in the hand layup vacuum bagging manufacturing process. Delamination and void content were investigated using eight different NDE methods: X-Ray laminography, X-Ray refraction, ultrasonic testing, high frequency eddy current imaging, pulse thermography, pulse phase thermography and lock- in-thermography and thermal conductivity measurements,. Then, the static interlaminar fracture behavior under Mode I and Mode II were conducted, as well as Mode I in fatigue. The results of X-Ray laminography, X-Ray Refraction, pulse thermography and pulse phase thermography were the most successful NDE technique for quantifying imperfections of the materials and for instance void shape, area fractions, size and distribution. However, X-Ray laminography gave the most accurate percentages of voids among the other NDE methods, as it provides hundreds of scanned images through the sample’s thickness with the most important advantage being the 3D representation. Thermal conductivity measurements showed that the voids lead to a decrease in thermal conductivity through-thickness. The ultrasonic method detected the delaminations but was not successful and quantifying the void content. The eddy current and lock-in-thermography were also not well-suited for detecting voids. Results of static Mode I and Mode II showed that voids can lead to a decrease in interlaminar fracture toughness. In addition, the presence of voids lead to a decrease in Mode I cyclic strain energy release rates fatigue life

Developing Composite Insulating Cross-Arms for 400 kV Lattice Towers

\u3cp\u3ePolymorphism of organic semiconducting materials exerts critical effects on their physical properties such as optical absorption, emission and electrical conductivity, and provides an excellent platform for investigating structure–property relations. It is, however, challenging to efficiently tune the polymorphism of conjugated polymers in aggregated, semi-crystalline phases due to their conformational freedom and anisotropic nature. Here, two distinctly different semi-crystalline polymorphs (β\u3csub\u3e1\u3c/sub\u3e and β\u3csub\u3e2\u3c/sub\u3e) of a low-bandgap diketopyrrolopyrrole polymer are formed through controlling the solvent quality, as evidenced by spectroscopic, structural, thermal and charge transport studies. Compared to β\u3csub\u3e1\u3c/sub\u3e, the β\u3csub\u3e2\u3c/sub\u3e polymorph exhibits a lower optical band gap, an enhanced photoluminescence, a reduced π-stacking distance, a higher hole mobility in field-effect transistors and improved photocurrent generation in polymer solar cells. The β\u3csub\u3e1\u3c/sub\u3e and β\u3csub\u3e2\u3c/sub\u3e polymorphs provide insights into the control of polymer self-organization for plastic electronics and hold potential for developing programmable ink formulations for next-generation electronic devices.\u3c/p\u3

Unterscheidung verschiedener charakteristischer Defekte in mittels selektivem Laserschmelzen hergestelltem Ti-6Al-4V durch Röntgen-Refraktionsradiographie

Das selektive Laserschmelzen (SLM) ist eine pulverbasierte, additive Fertigungsmethode, welche die Herstellung von komplex und individuell geformten Bauteilen ermöglicht. Im Laufe der vergangenen Jahre haben verschiedene Branchen, unter anderem die Luft- und Raumfahrt Industrie, begonnen diese Technologie intensiv zu erforschen. Insbesondere die Titanlegierung Ti-6Al-V4, welche aufgrund ihrer Kombination von mechanischen Eigenschaften, geringer Dichte und Korrosionsbeständigkeit häufig in der Luft- und Raumfahrt eingesetzt wird, eignet sich für die Herstellung mittels SLM. Allerdings können durch nicht optimal gewählte Prozessparameter, welche für gewöhnlich in einer Energiedichte zusammengefasst werden, Defekte in den Bauteilen entstehen. In dieser Studie wurde untersucht, in wie weit Röntgen-Refraktionsradiographie geeignet ist diese Defekte zu detektieren und zu charakterisieren. Bei der Röntgen-Refraktionsradiographie wird die Röntgenstrahlung, nachdem sie die Probe transmittiert hat, über einen Analysatorkristall gemäß der Bragg-Bedingung in den 2D-Detektor reflektiert und dabei nach ihrer Ausbreitungsrichtung gefiltert. Dadurch wird neben der Schwächung auch die Ablenkung der Röntgenstrahlung durch Refraktion im inneren der Probe zur Bildgebung ausgenutzt. Aus den aufgenommen Refraktionsradiogrammen kann der Refraktionswert berechnet werden. Dieser ist ein Maß für die Menge an inneren Oberflächen in der Probe. Zum einen konnte gezeigt werden, dass die Röntgen-Refraktionsradiographie Defekte detektieren kann, die kleiner sind als die Ortsauflösung des verwendeten 2D-Detektors. Zum anderen können zwei verschiedene Typen von Defekten unterschieden werden. Bei dem ersten Typ handelt es sich um runde Poren mit geringer innerer Oberfläche. Diese, sogenannten "keyhole pores" sind charakteristisch für eine zu hohe Energiedichte während des SLM Prozesses. Bei dem zweiten Typ handelt es sich um nicht komplett aufgeschmolzenes Pulver. Diese Defekte zeichnen sich durch eine hohe innere Oberfläche aus und sind charakteristisch für eine zu geringe Energiedichte. Vergleichende Messungen mit hochauflösender Synchrotron CT und optischer Mikroskopie bestätigen die charakteristischen Formen der verschiedenen Defekte

Correlating Interlaminar Fatigue Fracture Behavior to NDE Parameters in Carbon/Epoxy Composites Containing Voids and Delamination

Carbon fiber composites have been increasingly used in aerospace, military, sports, automotive and other fields due to their excellent properties, including high specific strength, high specific modulus, corrosion resistance, fatigue resistance, and low thermal expansion coefficient. Delamination, or interlaminar fracture, is a serious failure mode leading to a loss in composite stiffness and strength. Manufacturing process defects such as voids degrade the fatigue life and delamination resistance of the composite. This study investigates the effect of voids on fatigue interlaminar fracture behavior of carbon fiber composites. Samples with different vacuum levels: 0%, 50% and 100% were fabricated by varying the vacuum in the hand layup vacuum bagging manufacturing process. Delamination and void content were investigated using eight different NDE methods: X-Ray laminography, X-Ray refraction, ultrasonic testing, high frequency eddy current imaging, pulse thermography, pulse phase thermography and lock- in-thermography and thermal conductivity measurements,. Then, the static interlaminar fracture behavior under Mode I and Mode II were conducted, as well as Mode I in fatigue. The results of X-Ray laminography, X-Ray Refraction, pulse thermography and pulse phase thermography were the most successful NDE technique for quantifying imperfections of the materials and for instance void shape, area fractions, size and distribution. However, X-Ray laminography gave the most accurate percentages of voids among the other NDE methods, as it provides hundreds of scanned images through the sample’s thickness with the most important advantage being the 3D representation. Thermal conductivity measurements showed that the voids lead to a decrease in thermal conductivity through-thickness. The ultrasonic method detected the delaminations but was not successful and quantifying the void content. The eddy current and lock-in-thermography were also not well-suited for detecting voids. Results of static Mode I and Mode II showed that voids can lead to a decrease in interlaminar fracture toughness. In addition, the presence of voids lead to a decrease in Mode I cyclic strain energy release rates fatigue life.</p

Phase transformation induces plasticity with negligible damage in ceria-stabilized zirconia-based ceramics

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