Delamination Crack Propagation Research Articles

OS0807 遮熱コーティング皮膜の密着強度と残留応力の関連性(ガスタービン材料の各種力学特性,オーガナイズドセッション)

A new test method to evaluate adhesion strength has been proposed, employing a ring type of TBC specimen specifically designed. It was shown by the experiments that a delamination behavior of the top coat was successfully reproduced by the proposed method, associating with a buckling mode; a similar mode observed in actual gas turbine components. It was also shown that the method have many advantages, compared with the traditional methods. One advantage is that the method can not only provide the adhesion strength of the ceramic top coat in terms of energy criteria, but also make us possible to evaluate the behaviors of stable delamination crack propagation and arrest, based on interface fracture mechanic. The other advantage is that the proposed method enables us to assess the effect of residual stress on the adhesion strength in a simulated manner near to the actual components, because the residual stress is built up in the specimen in a multi-axial stress state. Finally it was strongly suggested by the experiments that the chemical bonding between the ceramic top coat and bond coat in the TBC specimen was negligibly small, but a mechanical anchor effect had a chief contribution to the bonding strength.

Mixed-mode interlaminar fracture and damage characterization in woven fabric-reinforced glass/epoxy composite laminates at cryogenic temperatures using the finite element and improved test methods

The present research examines experimentally and analytically the mixed-mode interlaminar fracture and damage behavior of glass fiber reinforced polymer (GFRP) woven laminates at cryogenic temperatures. The mixed-mode bending (MMB) tests were performed with the improved test apparatus, at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). The energy release rates at the onset of delamination crack propagation were evaluated for the woven GFRP specimens using both beam theory and finite element analysis. The fracture surfaces were also examined to verify the fracture mechanisms. In addition, the initiation and growth of damage in the specimens were predicted by a damage analysis, and the damage effect on the mixed-mode interlaminar fracture properties at cryogenic temperatures was explored.

Influences of the Interface Roughness and the Bond Coat Spray Method on the Adhesion Strength in an Air Plasma Sprayed Thermal Barrier Coatings

In this work, in order to study the effect of the interface roughness between the bond coating and the top coating, two kinds of the CoNiCrAlY powder with the different particle size were used for spraying the bond coating material. In addition, the bond coat was sprayed by either the low-pressure plasma spray (LPPS) method or the high velocity oxy-fuel (HVOF) one. The adhesion strength of the TBC top coat was evaluated as a function of the isothermal exposure time by means of the modified 4-point bending test. In addition, in-situ observations of the initiation and propagation of delamination cracks were carried out. The experimental results indicated that the delamination cracks initiated and propagated at the intersplat boundaries in the top-coating for the as-sprayed TBC specimens. After isothermal exposure at 1000°C, the microcracks were generated in the top-coating and the thermal grown oxides (TGO) grew at the top-coating/bond-coating interface. The delamination of the thermal exposed specimen occurred by the mixed fracture mode of the microcracks and top-coating/TGO or TGO/bond-coating interfaces. There was little difference of the adhesion strength by the bond-coating process in the as-sprayed conditions. On the other hand, the LPPSed bond coat specimen which has the rough interface exhibited lower adhesion strength compared with other specimens after thermal exposure due to the remarkable growth of the mixed oxide type of the TGO.

Fracture Mechanics Experiments to Characterize the Failure of Ceramic/Metal Joints in SOFC Stacks

Mechanical integrity of the sealants in planar SOFC stacks is a key prerequisite for reliable operation. In this respect joining with metals rather than brittle glass-ceramics is considered to have advantages. Hence, as one of the joining solutions for SOFCs of planar design the reactive air brazing of the ceramic cell into a metallic frame has gained increasing interest. Fracture experiments were carried out to characterize the fracture energy and failure mechanisms of silver-based reactive air brazes, used for joining the zirconia electrolyte of an anode supported planar cell with a metallic Crofer22APU frame. The specimens were mechanically tested in notched beam bending geometry. In-situ observation with optical and SEM resolution revealed preferential delamination crack propagation near the steel/braze interface. The influence of brazing parameters and associated interfacial reactions on the crack path location is addressed. Discussion of the results focuses in particular on the role of oxide scale formation.

Monitoring Delamination Progression in Thermal Barrier Coatings by Mid‐Infrared Reflectance Imaging

Mid‐infrared (MIR) reflectance imaging is shown to be a reliable diagnostic tool for monitoring delamination progression in thermal barrier coatings (TBCs). MIR reflectance imaging utilizes the maximum transparency of TBCs in the 3–6 μm wavelength region to probe below‐surface delamination crack propagation that is typically hidden from visible wavelength inspection. The image contrast that identifies delamination progression arises from the increased reflectance produced by a large component of total internal reflection at the TBC/buried‐crack interface. Imaging was performed at a wavelength of 4 μm to take advantage of the relatively high transmittance of plasma‐sprayed 8 wt% yttria‐stabilized zirconia (8YSZ) TBCs along with a desirable relative insensitivity to potentially interfering absorptions by atmospheric constituents at that wavelength. A key advantage of MIR reflectance imaging over competing techniques is that it is sensitive to delamination progression even at very early stages before delamination cracks start linking together; therefore, TBC health assessment can be achieved throughout the life of the TBC well before TBC failure is imminent. Examples are presented to demonstrate monitoring delamination progression by MIR reflectance imaging in 8YSZ TBC‐coated specimens subjected to furnace cycling to 1163°C. The experimental results were in good agreement with reflectance values predicted by a four‐flux Kulbelka–Munk approximation applied to the extreme cases of a completely adherent and a completely detached TBC. Practical considerations, including potential interfering effects from surface contamination, sintering, and erosion are discussed.

Analysis and Testing of Mixed-Mode Interlaminar Fracture Behavior of Glass-Cloth∕Epoxy Laminates at Cryogenic Temperatures

This paper presents results from an analytical and experimental study of the effect of temperature and mixed-mode ratio on the interlaminar fracture toughness in glass-cloth∕epoxy laminates. Mode I, mode II, and mixed-mode tests were conducted by the double-cantilever beam, end-notched flexure, and mixed-mode bending test methods at room temperature, liquid nitrogen temperature (77 K), and liquid helium temperature (4 K). A finite element model was used to perform the delamination crack analysis. Mode I, mode II, and mixed-mode energy release rates at the onset of delamination crack propagation were computed using the virtual crack closure technique. The fracture surfaces were examined by scanning electron microscopy to correlate with the interlaminar fracture properties.

Suppression of Delamination Crack Propagation in Laminated Composites by Using Thin SMA Plates

The use of thin actuator plates bonded to surfaces of laminated composites to suppress the growth of delamination cracks is investigated. A composite beam analysis was performed to estimate the effect of actuation strains on the strain energy release rate of delamination cracks. When compared at the same value of actuation strain, the increase in the apparent crack growth resistance is larger for smaller FRP thickness and for smaller value of the critical strain energy release rate of FRP without actuators, Gcr, under both mode I and II loadings. Mode I delamination suppression tests of unidirectional CFRP laminates were conducted by using thin SMA actuators. The value of the apparent crack growth resistance, Glapp, under actuation strains of about 0.1% was 2.5 times larger than fracture toughness of CFRP and was also about 1.5 times as large as that just before actuation.

Development and thermal fatigue testing of ceramic thermal barrier coatings

Ceramic thermal barrier coatings (TBCs) will play an increasingly important role in future gas turbine engines because of their ability to effectively protect the engine components and further raise engine temperatures. Durability of the coating systems remains a critical issue with the ever-increasing temperature requirements. Thermal conductivity increase and coating degradation due to sintering and phase changes are known to be detrimental to coating performance. There is a need to characterize the coating thermal fatigue behavior and temperature limit, in order to potentially take full advantage of the current coating capability. In this study, a laser thermal fatigue test technique has been used to study the delamination crack propagation of thermal barrier coatings under simulated engine heat flux heating and thermal cyclic loading. Thermal conductivity and cyclic fatigue behaviors of plasma-sprayed ZrO 2–8 wt.% Y 2O 3 thermal barrier coatings were evaluated under high temperature, large thermal gradient and thermal cycling conditions. The coating degradation and failure processes were assessed by real-time monitoring of the coating thermal conductivity during the testing. The test results showed that the initial average crack propagation rates of ZrO 2–8 wt.% Y 2O 3 coatings were in the range of 3–8 μm/cycle. The crack propagation rates increased to 30–40 μm/cycle at later stages of testing, and the critical spalling crack size ranged from 3 to 5 mm for the coatings. The accelerated crack growth is attributed to the increased driving force for crack propagation under the laser heat flux cyclic test conditions.

Constituent and composite quasi-static and fatigue fracture experiments

Quasi-static and fatigue Mode I fracture experiments were performed at a variety of loading rates and temperatures using a polymer-matrix composite (PMC) (IM7/977-3) and its neat resin (977-3). Neat resin quasi-static toughness showed slight increases in toughness with increasing loading rate and no dependence on temperature until the test temperature neared the glass transition temperature, where the toughness decreased. Composite quasi-static delamination initiation toughness showed no clear trends with respect to temperature or loading rate, but toughness values were generally higher at higher temperatures. Composite fatigue delamination onset and crack propagation rates were strongly dependent on temperature, with delamination onset occurring at a lower number of cycles and crack propagation rates being higher for a given maximum applied strain energy release rate. The neat resin fracture mechanisms were a key factor in explaining the composite quasi-static and fatigue behavior, but no quantitative predictions of composite fracture behavior could be made since the crack tip mechanisms apparently changed.

Investigation of interfacial delamination of a copper-epoxy interface under monotonic and cyclic loading: modeling and evaluation

Interfacial delamination is of important concern for multilayered microelectronic packages, as it is one of the most common failures observed after reliability test. Most of the work, available in open literature, focus on delamination propagation under monotonic loading rather than delamination propagation under cyclic loading. Interfacial fracture mechanics based methodologies have been proven to be efficient in handling interfacial delamination problem under monotonic loading. In this paper, the principles of interfacial mechanics have been applied in the analysis of interfacial fatigue crack propagation. Fatigue test has been conducted in studying onset of delamination and fatigue crack propagation (FCP) of an interfacial crack along a copper-epoxy interface. Models developed in this study have also been applied in the evaluation of multilayered integrated substrate test vehicles.

Delamination properties of translaminar-reinforced composites

In the paper the delamination crack propagation is investigated in 3D woven carbon and glass fiber composites. Composites were produced in 3TEX company and had volume fraction of transverse fibers equal 2–5% of total volume of fibers. The reinforcement acts as crack bridging, greatly improving material resistance to delamination. In this paper investigation of this material has been performed using double cantilever beam technique and models elaborated for large scale bridging in UD composites. The following alterations and novelties are introduced: 1) The appropriate form and shape of specimens with glued additional metallic tabs was elaborated allowing to avoid the premature break of specimen arms; 2) Original loading device was designed to transfer high load directly to crack faces, and additional measurement of crack opening was proposed (as in the case of UD composite) to find the bridging law; 3) The convenient analytical formula for G I c calculation was proposed and used; 4) Instead of traditional R curve which is geometry dependent it was proposed to characterize the delamination resistance as function of crack opening displacement. The quantitative delamination characteristics for materials investigated are presented.

Multiple Surface Cracking and Its Effect on Interface Cracks in Functionally Graded Thermal Barrier Coatings Under Thermal Shock

The thermal fracture behavior in functionally graded yttria stabilized zirconia–NiCoCrAlY bond coat alloy thermal barrier coatings was studied using analytical models. The response of three coating architectures of similar thermal resistance to laser thermal shock tests was considered. Mean field micromechanics models were used to predict the effective thermoelastic and time-dependent (viscoplastic) properties of the individual layers of the graded thermal barrier coatings (TBCs). These effective properties were then utilized in fracture mechanics analyses to study the role of coating architecture on the initiation of surface cracks. The effect of the surface crack morphology and coating architecture on the propensity for propagation of horizontal delamination cracks was then assessed. The results of the analyses are correlated with previously reported experimental results. Potential implications of the findings on architectural design of these material systems for enhanced thermal fracture resistance are discussed.

On the propagation and coalescence of delamination cracks in compressed coatings: with application to thermal barrier systems

Coatings subject to residual compression eventually fail by buckle-driven delamination. The phenomenon is most vivid in thermal barrier coatings (TBCs) used in gas turbines. The failure evolution commences with the formation of a large number of small cracks at geometric imperfections near the interface. These cracks spread upon thermal exposure, particularly upon thermal cycling, because of the formation of a thermally grown oxide (TGO) beneath the TBC, which introduces normal and shear stress near the interface. Experimental observations indicate that some of these cracks coalesce to form large-scale delaminations susceptible to buckling. The mechanics governing crack coalescence and the consequent failure are addressed in the present analysis. A model is introduced that simulates stresses induced in the TBC by spatial variations in TGO growth. Energy release rates for cracks evolving in this stress field are determined. Two related scenarios are considered, which differ in the way the TGO shape evolves. In both, contact between the crack faces and the consequent wedging action is responsible for ultimate coalescence. The wedging force induces a mode I stress intensity that becomes infinite as the cracks coalesce. The consequence is that, for some TGO shapes, the energy release rate is always non-zero, with a minimum at a characteristic crack length. This minimum establishes a criterion for crack coalescence and failure. Based on these insights, finite element simulations have been used to predict cyclic crack growth rates in a TBC system that correlate well with experimental observations.

OS09W0347 Suppression effect for mode I propagation of delamination cracks in a laminated composite by using thin SMA plates

OS09W0347 Suppression effect for mode I propagation of delamination cracks in a laminated composite by using thin SMA plates

A Numerical Assessment of the Propagation and Coalescence of Delamination Cracks in Thermal Barrier Systems

Interactions have been calculated between cracks induced in thermal barrier coatings (TBCs) upon thermal cycling, motivated by displacement instability in the thermally grown oxide (TGO). The results indicate that the energy release rate G cycles as the temperature changes, with the largest value arising at ambient temperature. It increases on a cycle-by-cycle basis. The trends in G predict two responses. (i) Isolated cracks should be confined to the vicinity of the imperfection, as observed experimentally. (ii) Cracks that converge from neighboring imperfections exhibit a minimum G prior to convergence, providing the possibility that they might coalesce. Equating this minimum to the toughness of the TBC provides a criterion for coalescence and failure. Imposing this criterion allows the change in crack length upon cycling and the number of cycles to failure to be ascertained. The results are consistent with experimental measurements obtained from furnace cycle tests.

Interlaminar fracture properties of fibre reinforced natural rubber/polypropylene composites

The effect of the addition of natural rubber (NR) on the interlaminar fracture properties of thermoplastic composites which is a measure of the resistance of the material to delamination crack propagation, has been studied. NR–polypropylene (PP) composites were prepared with increasing amounts of NR in PP at 5–20% compositions reinforced with 5% E-type chopped strand glass-fibre mat. Panels were produced by compression moulding technique and these have been tested for mode I interlaminar fracture using double cantilever beam specimen testing method. Tests have been undertaken at cross-head displacement rate of 5 mm/min for all the specimens. It was found that, as the amount of NR was increased, the interlaminar fracture toughness, G IC of the composite materials decreased. Scanning electron microscopy of the fracture surfaces is used to help explain these findings. The crack propagation areas showed all the fibres were bare with no matrix covering them. These were seen at 200× magnification. The smooth clean surface of the fibres is the result of poor interfacial debonding. No fibre bridging is observed between the fibres and the failure mainly occurred at the fibre–matrix interface as seen using scanning electron microscopy. This shows that the addition of rubber made the adhesion between the interface weaker.

Energy Absorption and Dissipation Characteristics of Pultruded Glass-Graphite/Epoxy Hybrid Composite Beams

The response of pultruded glass-graphite/epoxy hybrid composites has been evaluated under two different incident impact energy conditions. A high incident energy (HIE) and a low incident energy (LIE) of impact are chosen to cause either complete fracture or induce delamination, respectively, for assessing the energy absorption characteristics and delamination fracture toughness of these hybrid composites. From the HIE instrumented drop-weight impact tests, the energies required for damage initiation, propagation, as well as total absorbed energies and other parameters necessary for evaluating the impact performance of composites were derived from the load-deformation behavior of each sample. The influence of hybridization on the energy absorption characteristics of pultruded composites was investigated. A parametric study also was performed to correlate the energy absorption characteristics with the energy dissipative mechanisms attributed to the inherent material damping capacity of these composites. The qualitative assessment of the impact performance using inherent damping capacity as a non-destructive testing parameter is highlighted. The samples that were subjected to LIE impact testing were examined using ultrasonic C-scan tests to ascertain changes in the extent of delamination crack propagation as a result of hybridization. Finite element modeling was performed to simulate delamination crack propagation at various levels through the thickness. The strain energy release rate computed using the virtual crack closure technique was monitored to determine the likelihood of delamination crack propagation with increment in crack growth. The results obtained from LIE impact tests and ultrasonic C-scan tests were used to corroborate the predicted delamination fracture toughness of pultruded hybrids. Graphite-outside hybrids exhibited high flexural stiffness, propagation energy, ductility and failure index, but had lower initiation energy along with a greater tendency to delaminate.

1013 多方向炭素繊維強化高靭性エポキシ樹脂積層板のモード II および混合モード層間疲労き裂進展挙動

1013 多方向炭素繊維強化高靭性エポキシ樹脂積層板のモード II および混合モード層間疲労き裂進展挙動

End-Notched Flexure Testing and Analysis of Mode II Interlaminar Fracture Behavior of Glass-Cloth/Epoxy Laminates at Cryogenic Temperatures

This paper presents results from an analytical and experimental study of the effect of temperature and geometrical variations on the Mode II interlaminar fracture toughness in glass-cloth/epoxy laminates. The end-notched flexure (ENF) test geometry was used for Mode II experiments, which were performed at room temperature (R.T.), liquid nitrogen temperature (77 K), and liquid helium temperature (4 K). The fracture surfaces were also examined by scanning electron microscopy to verify the fracture mechanisms. A finite element model was further used to perform the delamination crack analysis. Critical load levels, and the geometric and material properties of the test specimens were input data for the analysis, which evaluated the Mode II energy release rate at onset of delamination crack propagation. The results of the finite element analysis are used to supplement the experimental data.

Double Cantilever Beam Measurement and Finite Element Analysis of Cryogenic Mode I Interlaminar Fracture Toughness of Glass-Cloth/Epoxy Laminates

An experimental and analytical investigation in cryogenic Mode I interlaminar fracture behavior and toughness of SL-E woven glass-epoxy laminates was conducted. Double cantilever beam (DCB) tests were performed at room temperature (R.T.), liquid nitrogen temperature (77 K), and liquid helium temperature (4 K) to evaluate the effect of temperature and geometrical variations on the interlaminar fracture toughness. The fracture surfaces were examined by scanning electron microscopy to verify the fracture mechanisms. A finite element model was used to perform the delamination crack analysis. Critical load levels and the geometric and material properties of the test specimens were input data for the analysis which evaluated the Mode I energy release rate at the onset of delamination crack propagation. The results of the finite element analysis are utilized to supplement the experimental data.