scholarly journals Crack Initiation Criteria in EBC under Thermal Stress

Coatings ◽  
2019 ◽  
Vol 9 (11) ◽  
pp. 697 ◽  
Author(s):  
Kawai ◽  
Kakisawa ◽  
Kubo ◽  
Yamaguchi ◽  
Yokoi ◽  
...  
Keyword(s):  
Fracture Toughness ◽  
Thermal Stress ◽  
Crack Initiation ◽  
Interface Crack ◽  
Fem Analysis ◽  
Interface Fracture ◽  
Mechanical Reliability ◽  
Crack Initiation And Propagation ◽  
Environmental Barrier Coatings ◽  
Initiation And Propagation

For design of multi-layered environmental barrier coatings (EBCs), it is essential to assure mechanical reliability against interface crack initiation and propagation induced by thermal stress owing to a misfit of the coefficients of thermal expansion between the coating layers and SiC/SiC substrate. We conducted finite element method (FEM) analyses to evaluate energy release rate (ERR) for interface cracks and performed experiment to obtain interface fracture toughness to assess mechanical reliability of an EBC with a function of thermal barrier (T/EBC; SiC/SiAlON/mullite/Yb-silicate gradient composition layer/Yb2SiO5 with porous segment structure) on an SiC/SiC substrate under thermal stress due to cooling in fabrication process. Our FEM analysis revealed that a thinner SiAlON layer and a thicker mullite layer are most suitable to reduce ERRs for crack initiation at the SiC/SiAlON, SiAlON/mullite and mullite/Yb2Si2O7 interfaces. Interface fracture tests of the T/EBC with layer thicknesses within the proposed range exhibited fracture at the SiC/SiAlON and SiAlON/mullite interfaces. We also estimated the approximate fracture toughness for the SiC/SiAlON and SiAlON/mullite interfaces and lower limit of fracture toughness for the mullite/Yb2Si2O7 interface. Comparison between ERR and fracture toughness indicates that the fabricated T/EBC possesses sufficient mechanical reliability against interface crack initiation and propagation.

Mechanism of crack initiation and propagation of re-entrant auxetic honeycombs under thermal shock

2021 ◽  
pp. 1-13
Author(s):  
Zheng Li ◽  
B.L. Wang ◽  
Kaifa Wang
Keyword(s):  
Cell Wall ◽  
Thermal Stress ◽  
Crack Initiation ◽  
Thermal Shock ◽  
Thermal Load ◽  
Continuum Model ◽  
Micro Model ◽  
Crack Initiation And Propagation ◽  
Initiation And Propagation ◽  
Critical Thermal Load

Abstract Thermal shock multiple cracking behaviors of re-entrant auxetic honeycombs with a negative Poisson's ratio are investigated, and the crack initiation and propagation behavior are discussed. An effective macro continuum model is developed to detect the effects of cracking density and microstructures of auxetic honeycombs on the thermal stress and intensity. The microscale tensile stresses in the struts ahead of the crack as functions of the corresponding thermal stress intensity factor (SIF) at the macroscale are evaluated by employing a macro-micro model. Then, a lower-bound method is proposed to assess the critical thermal load of auxetic honeycombs by combining the macro-micro model and the macro continuum model. A significant increase in both transient thermal stress and intensity as the growing cell-wall angle is demonstrated. Results for the maximum thermal SIF as well as the maximum tensile stress in the middle of cracks are calculated as functions of crack density and length. With the identical SIF, the microscale tensile stresses ahead of the crack in honeycombs with smaller cell-wall angles are greater than that in mediums with larger angles due to the more significant crack tip opening displacement. Critical thermal load prediction reveals that the honeycombs with smaller cell-wall angles generally possess more excellent thermal shock resistance. Also, the varying failure modes of different auxetic honeycomb strips under specific thermal load are predicted. The corresponding crack initiation and propagation mechanisms are revealed.

Mechanical Properties and Fracture Toughness of Aluminum Vessels by Friction Stir Welding

2004 ◽  
Author(s):  
Masahito Mochizuki ◽  
Masao Toyoda ◽  
Masayuki Inuzuka ◽  
Hidehito Nishida
Keyword(s):  
Mechanical Properties ◽  
Fracture Toughness ◽  
Crack Initiation ◽  
Base Metal ◽  
Welded Joint ◽  
Stir Zone ◽  
Crack Initiation And Propagation ◽  
Friction Stir ◽  
Fine Grained ◽  
Initiation And Propagation

Mechanical properties and fracture toughness in friction stir welded joint of vessels of structural aluminum alloy type A5083-O are investigated. Welded joint from 25 mm-thick plate is fabricated by one-side one-pass friction stir. Charpy impact energy and critical crack-tip opening displacement (CTOD) in friction stir weld are much higher than those of base metal or heat-affected zone, whereas mechanical properties such as stress-strain curve and Vickers hardness do not have a conspicuous difference. Effects of microstructure on crack initiation and propagation are studied in order to clarify the difference of fracture toughness between stir zone and base metal. Both tensile test and bending test show that the fine-grained microstructure in stir zone induces to increase ductile crack initiation and propagation resistance by analyzing fracture resistance curves and diameter of dimples in fracture surface. It is found that high fracture toughness value in stir zone is affected fine-grained microstructure by friction stirring.

scholarly journals Enhancement in Interply Toughness of BMI Composites Using Micro-Thin Films

2021 ◽  
Vol 5 (2) ◽  
pp. 49
Author(s):  
Eldho Mathew ◽  
Sunil Chandrakant Joshi ◽  
Periyasamy Manikandan
Keyword(s):  
Thin Films ◽  
Fracture Toughness ◽  
Crack Initiation ◽  
Mixed Mode ◽  
Sliding Mode ◽  
Mode I ◽  
Crack Initiation And Propagation ◽  
Polymeric Thin Films ◽  
Initiation And Propagation ◽  
Automated Fiber Placement

Nowadays, laminated composites are widely used in the aerospace sector. All laminates have interply/interlaminar interfaces even if they are made using automated processes. The interfaces act as the areas of weaknesses and the potential crack initiation regions. Hence, any enhancement in the crack initiation and propagation resistance is always sought after. Usage of polymeric thin films is one of the promising and viable ways to achieve this. It is also easy to incorporate micro-thin films into any automation process. In the present study, different customized thin films that are compatible with Glass/BMI composites are fabricated. Fracture toughness tests in Mode I (opening mode), Mode II (sliding mode) and Mixed Mode I/II are conducted respectively using Double Cantilever Beam (DCB), End Notch Flexure (ENF) and Mixed Mode Bending (MMB) test specimens. This paper discusses the manufacturing of compatible micro-thin films. The various challenges faced during the manufacturing and incorporation of thin films are presented. The results of the various fracture toughness tests are examined. Mechanisms through which the different films help in resisting the crack initiation and propagation are deliberated and discussed. The incorporation of this technique in Automated Fiber Placement (AFP) is also discussed.

Effect of Prestraining on the Brittle-To-Ductile Transition of NiAl Single Crystals

1996 ◽  
Vol 460 ◽  
Author(s):  
S. Shrivastava ◽  
F. Ebrahimi
Keyword(s):  
Fracture Toughness ◽  
Transition Temperature ◽  
Crack Initiation ◽  
Single Crystals ◽  
Low Temperatures ◽  
Crack Initiation And Propagation ◽  
Brittle To Ductile Transition ◽  
Initiation And Propagation ◽  
Toughness Testing ◽  
Dislocation Sources

ABSTRACTThe brittle-to-ductile transition (BDT) has been established for NiAl single crystals as evaluated by fracture toughness testing and also the effects of prestraining on the brittle-to-ductile transition temperature (BDTT) have been investigated. Specimens were prestrained to a 10% plastic strain level at 200°C under tension prior to toughness testing. The BDT of the prestrained specimens was compared to that of the as homogenized specimens. The results have revealed the occurrence of two competing effects upon prestraining: (1) an increase in dislocation sources causing a difficulty in micro-crack initiation and resulting in an increase in toughness at low temperatures, and (2) an increase in the flow stress resulting in an increase in BDT temperature. The crack initiation and propagation mechanisms were also analyzed and have been discussed.

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