Crack Trapping Research Articles

On size effect of cleavage cracking in polycrystalline thin films

A crack trapping model is developed for the fracture resistance of high-angle grain boundaries in free-standing brittle thin films, based on which a new size effect is predicted. In addition to the crystallographic misorientations, the grain boundary toughness is also dependent on the film thickness, primarily due to the geometrically necessary crack front branching.

Adhesive and bending failure of thermal sprayed hydroxyapatite coatings: Effect of nanostructures at interface and crack propagation phenomenon during bending

Hydroxyapatite (HA) coatings have shown promising effects on rapid bone remodeling and suitable functional life in orthopedic and dental applications. However, the major problem encountered by the HA-coated implants is the failure of the coating due to its insufficient mechanical properties. The present study investigated the influence of the microstructure near to the coating/substrate interface on the adhesion of the coatings. In addition, the crack propagation behavior within the coatings was studied through 4-point bend test. Results showed that nanostructures (30–110nm) within the HA coatings were achieved by high velocity oxy-fuel (HVOF) spraying. Comparison among HVOF HA coatings, which were deposited using different starting feedstock, suggests detrimental effect of the perpendicular-to-substrate nano-cuboids on adhesion of the coatings. The presence of the grains with hexagonal shape (<250nm in length and <50nm in diameter) triggered a deteriorated adhesion. Granular nanosized grains at the interface give rise to enhanced adhesion through improved mechanical interlocking. Formation mechanism of the nanosized grains was discussed in this paper. Furthermore, the 4-point bend test revealed consistent crack propagation path that the cracks actually grow within the coating with a direction parallel to the interface, and approximately several to 20 microns thick coatings were remained on the substrate. The critical strain energy release rate exhibited a value of ∼1.15kJm−2. During the crack propagation, kinking and trapping of the bending cracks were decided by the flaws within the coating, which were mainly located at splats’ interface. The interface between the first layer (with one splat thickness) and the second is believed to be the weakest zone in the nanostructured coating.

Toughening Effect of Continuous Fiber Bundles

In a continuous fiber reinforced brittle matrix composite, the transverse fracture is often dominated by the crack trapping and bridging effects as well as the fiber breakage. In this article, the toughening effect of flat fiber bundles with the cross-sectional aspect ratio, ranging from 0.05 to 5 is discussed in the context of energy analysis. With a constant size/spacing ratio of the fiber bundles, the fracture resistance increases monotonically with, primarily due to the influences on the crack front behavior. The elastic properties of matrix and the crack length have little effect on the critical energy release rate.

Ambient- to elevated-temperature fracture and fatigue properties of Mo-Si-B alloys: Role of microstructure

Ambient- to elevated-temperature fracture and fatigue-crack growth results are presented for five Mo-Mo3Si-Mo5SiB2-containing α-Mo matrix (17 to 49 vol pct) alloys, which are compared to results for intermetallic-matrix alloys with similar compositions. By increasing the α-Mo volume fraction, ductility, or microstructural coarseness, or by using a continuous α-Mo matrix, it was found that improved fracture and fatigue properties are achieved by promoting the active toughening mechanisms, specifically crack trapping and crack bridging by the α-Mo phase. Crack-initiation fracture toughness values increased from 5 to 12 MPa√m with increasing α-Mo content from 17 to 49 vol pct, and fracture toughness values rose with crack extension, ranging from 8.5 to 21 MPa√m at ambient temperatures. Fatigue thresholds benefited similarly from more α-Mo phase, and the fracture and fatigue resistance was improved for all alloys tested at 1300 °C, the latter effects being attributed to improved ductility of the α-Mo phase at elevated temperatures.

The role of recalcitrant grain boundaries in cleavage cracking in polycrystals

Recalcitrant grain boundaries offer an important resistance to cleavage crack advance through crack trapping. In this article, this effect is studied based on an energy analysis. It is found that the critical energy release rate is dominated by a single parameter, Q, that collects together factors such as work of separation, grain size, and crack length. For short cracks, the crack trapping effect leads to an increase in fracture resistance by 20–30%. For long cracks, the crack trapping effect is negligible.

Crack trapping effect of persistent grain boundary islands

ABSTRACTIn the polycrystalline Fe–Si alloy, when a cleavage front transmits from one grain to another, it first penetrates stably across the grain boundary at a number of breakthrough points (BTPs) that distribute along the front quasi‐periodically. As the critical energy release rate is reached, unstable crack jump occurs and the persistent grain boundary islands (PGBI) between the BTPs are left behind the verge of propagating, bridging across the crack flanks, which leads to a 10–30% increase in fracture resistance. In this article, this process is investigated through an energy analysis. The influence of the size/spacing ratio of PGBI on the grain boundary toughness is discussed in detail.

Fracture in Ceramic-Matrix Composites Reinforced with Strongly Bonded Metal Particles

The resistance of a ceramic matrix composite to the cleavage cracking across a field of strongly bonded, uniformly distributed metal particles is studied. The crack trapping and bridging effects of the metal particles are analyzed by means of calculating the strain energy and the traction work. An explicit expression for the critical energy release rate as a function of particle volume fraction has been obtained. The fracture resistance is independent of elastic properties of the matrix and the sample geometry and is predominantly determined by the size/spacing ratio of the particles. It is shown that the theoretical curves agree with experimental data quite well. The methodology developed in this article can be used in studying the fracture resistances of composites with high filler contents and irregular filler geometries.

Influence of Plastic Deformation of Particulates on Flexure Toughness of Brittle Matrix Composite

In ductile-particulate reinforced composites with relatively low filler contents, the fracture toughness is dominated by the crack trapping effect. In this paper, this phenomenon is discussed in detail in context of R-curve analysis. The total fracture work is decomposed into the work of separation of matrix and the traction work associated with particulate rupture. It is concluded that, since the energy dissipation subsequent to the onset of unstable crack propagation does not affect the critical condition, the load bearing capacity of the particulates cannot be fully utilized. A closed-form expression relating the fracture toughness to the hardening behavior is obtained.

Fracture toughness of thermoset composites reinforced by perfectly bonded impenetrable short fibers

The fracture toughness of brittle thermoset resins could be improved significantly by perfectly bonded tough, short fibers through both crack trapping and bridging effects. In this paper, the crack trapping effect was studied through the analysis of the change of strain energy associated with the crack propagation across a regular array of fibers, and the bridging effect was discussed based on the Andersson–Bergkvist model. The fracture resistance increases with the fiber volume fraction, and is independent of the elastic properties of the matrix, the crack length, and the cross-sectional diameter of the fibers.

Role of Microstructure in Promoting Fracture and Fatigue Resistance in Mo-Si-B Alloys

ABSTRACTAn investigation of how microstructural features affect the fracture and fatigue properties of a promising class of high temperature Mo-Si-B based alloys is presented. Fracture toughness and fatigue-crack growth properties are measured at 25° and 1300°C for five Mo-Mo3Si-Mo5SiB2 containing alloys produced by powder metallurgy with α-Mo matrices. Results are compared with previous studies on intermetallic-matrix microstructures in alloys with similar compositions. It is found that increasing the α-Mo phase volume fraction (17 – 49%) or ductility (by increasing the temperature) benefits the fracture resistance; in addition, α-Mo matrix materials show significant improvements over intermetallic-matrix alloys. Fatigue thresholds were also increased with increasing α-Mo phase content, until a transition to more ductile fatigue behavior occurred with large amounts of α-Mo phase (49%) and ductility (i.e., at 1300°C). The beneficial role of such microstructural variables are attributed to the promotion of the observed toughening mechanisms of crack trapping and bridging by the relatively ductile α-Mo phase.

Fracture and fatigue resistance of Mo–Si–B alloys for ultrahigh-temperature structural applications

Fracture and fatigue properties are examined for a series of Mo–Mo 3Si–Mo 5SiB 2 alloys, which utilize a continuous α-Mo matrix to achieve unprecedented room-temperature fracture resistance (>20 MPa√m). Mechanistically, these properties are explained in terms of toughening by crack trapping and crack bridging by the more ductile α-Mo phase.

Unstable crack advance across a regular array of short fibers in brittle matrix

The resistance to cleavage cracking of a regular array of short fibers was discussed through R-curve analysis. The final failure of the fiber array associated with the unstable crack advance across it occurred when the balance of the rates of the energy release rate and the fracture resistance was reached. The fracture resistance dominated by the combined bridging effect and crack trapping effect increased with the volume fraction and the aspect ratio of the fibers, as well as the internal friction. The toughening effect strongly depended on the crack length for short cracks but was size insensitive for long cracks. The elastic properties of the matrix had little influence on the overall fracture resistance.

Crack bridging and trapping mechanisms used to toughen brittle matrix composite

Two basic mechanisms of toughening brittle solids are presented. They involve crack-tip shielding from crack deformation and/or crack bridging by introducing ductile particles in the crack wake region. The crack opening displacement is realized from the constant volume plastic flow of the particles according to the model in [J. Dominguez, C.A. Brebbia, (Eds.), Proceedings of Computational Methods in Contact Mechanics V, WIT Press, Boston, 2001, p. 87; A.T. Yokobori, R.O. Ritchie, K. Ravi-Chandar, B.L. Karihaloo, (Eds.), Proceedings of ICF10, Elsevier, Oxford, 2001, p. 348]. The second mechanism involves arresting the crack ductile phase such that it can only renucleate on the other side. As a result of trapping the crack, the material is toughened intrinsically. Energy considerations are made to estimate the extent of particle/matrix debonding. A perturbation analysis [A.T. Yokobori, R.O. Ritchie, K. Ravi-Chandar, B.L. Karihaloo, (Eds.), Proceedings of ICF10, Elsevier, Oxford, 2001, p. 348] is used to account for the configuration of the front of a planar crack trapped by a periodic array of closely spaced bridges. Debonding of the particle/matrix interface controls is associated with the two aforementioned mechanisms. Comparison of analytical results with some experimental observations is provided.

Ambient to high temperature fracture toughness and fatigue-crack propagation behavior in a Mo–12Si–8.5B (at.%) intermetallic

Boron-containing molybdenum silicides have been the focus of significant research of late due to their potentially superior low-temperature “pest” resistance and high-temperature oxidation resistance comparable to that of MoSi 2-based silicides; however, like many ordered intermetallics, they are plagued by poor ductility and toughness properties. Of the various multiphase Mo–Si–B intermetallic systems available, alloys with compositions of Mo–12Si–8.5B (at.%), which contain Mo, Mo 3Si, and T2 phases, are anticipated to have higher toughnesses because of the presence of the relatively ductile Mo phase. In this study, we examine the ambient to high (1300°C) temperature fracture toughness (R-curve) and fatigue-crack growth characteristics of Mo-12Si-8.5B, with the objective of discerning the salient mechanisms governing crack growth. It is found that this alloy displays a relatively high intrinsic (crack-initiation) toughness at 800 up to 1200°C (∼10 MPa√m), but only limited extrinsic R-curve (crack-growth) toughness. Although the lack of extrinsic toughening mechanisms is not necessarily beneficial to quasi-static properties, it does imply in a brittle material that it should show only minimal susceptibility to premature failure by fatigue, as is indeed observed at temperatures from ambient to 1300°C. Of particular significance is that both the fracture toughness and the threshold stress intensity for fatigue are increased with increasing temperature over this range. This remarkable property is related to a variety of toughening mechanisms that become active at elevated temperatures, specifically involving crack trapping by the α-Mo phase and extensive microcracking primarily in the Mo 5SiB 2 phase.

Lattice Trapping of Cracks in Fe Using an Interatomic Potential Derived from Experimental Data and Ab Initio Calculations

Lattice Trapping of Cracks in Fe Using an Interatomic Potential Derived from Experimental Data and Ab Initio Calculations

The influence of crack trapping on the toughness of fiber reinforced composites

Frictional crack bridging is the main mechanism of toughening in brittle fiber\\brittle matrix composites. In addition, the fibers may have a second beneficial effect : they tend to trap cracks propagating through the solid, and may cause them to arrest. The effectiveness of crack trapping increases with the fracture toughness of the interface between fibers and matrix. In contrast, crack bridging tends to be more effective if the interface between fibers and matrix has a low fracture toughness. In this paper, we study the competing effects of crack trapping and bridging in a brittle fiber\\brittle matrix composite. A numerical method is used to predict in three dimensions the path of a crack as it bypasses rows of fibers in an ideally brittle matrix. The results are used to deduce the influence of crack trapping on the toughness of the composite. In addition, a simple model of frictional crack bridging is used to compare the relative effects of crack trapping and bridging. It is shown that, in general, the influence of bridging greatly exceeds that of trapping. However, if the fibers have a low tensile strength and there is a large resistance to sliding between fibers and matrix, crack trapping can be significant : in this case, the best composite toughness is achieved by using a tough interface between fibers and matrix.

Deformation and fracture behaviour of directionally solidified NiAl-Mo and NiAl-Mo(Re) eutectic composites

Abstract The mechanical behaviour of directionally solidified NiAl-Mo and NiAl- Mo(Re) eutectic alloys has been examined. The fracture toughness of a NiAl- 9 at.% Mo eutectic alloy is a factor of at least two higher than that of monolithic NiAl. No single dominant toughening mechanism was identified. The toughness enhancement was attributed to a combination of intrinsic toughening of NiAl due to dislocations generated from the interfaces, crack trapping, crack deflection along interfaces and crack bridging. The Mo fibres were solid solution strengthened by Al and Ni partitioned from the NiAl matrix and had low ductility. Extensive interface debonding was noted, particularly when the fibres were either inclined or parallel to the crack plane. The deflection of the crack along the interfaces was analysed in terms of the elastic and plastic properties of the two phases. Alloying with Re was effective in reducing the hardness of Mo fibres in the composite, but also reduced the fibre alignment relative to that...

High-temperature fracture and fatigue resistance of a ductile β-TiNb reinforced γ-TiAl intermetallic composite

The high-temperature fatigue-crack propagation and fracture resistance of a model γ-TiAl intermetallic composite reinforced with 20 vol.% ductile β-TiNb particles is examined at elevated temperatures of 650 and 800°C and compared with behavior at room temperature. TiNb reinforcements are found to enhance the fracture toughness of γ-TiAl, even at high temperatures, from about 12 to ∼40 MPa m 1/2, although their effectiveness is lower compared to room temperature due to the reduction in strength of TiNb particles. Under monotonic loading, crack-growth response in the composite is characterized by resistance-curve behavior arising from crack trapping, renucleation and resultant crack bridging effects attributable to the presence of TiNb particles. In addition, crack-tip blunting associated with plasticity increases the crack-initiation (matrix) toughness of the composite, particularly at 800°C, above the ductile-to-brittle transition temperature (DBTT) for γ-TiAl. High-temperature fatigue-crack growth resistance, however, is marginally degraded by the addition of TiNb particles in the C–R (edge) orientation, similar to observations made at room temperature; premature fatigue failure of TiNb ligaments in the crack wake diminishes the role of bridging under cyclic loading. Both fatigue and fracture resistance of the composite are slightly lower at 650°C (just below the DBTT for TiAl) compared to the behavior at ambient and 800°C. Overall, the beneficial effect of adding ductile TiNb reinforcements to enhance the room-temperature fracture and fatigue resistance of γ-TiAl alloys is retained up to 800°C, in air environments. There is concern, however, regarding the long-term environmental stability of these composite microstructures in unprotected atmospheres.

An investigation of fatigue and fracture in NiAl–Mo composites

The effects of reinforcement morphology (particulate versus fibers) and particulate volume fraction on the fatigue and fracture behavior of NiAl–Mo composites are discussed. The improved fracture toughness observed in these composites are not associated with improved tensile ductility. The improved fracture toughness levels are explained by quantifying the shielding contributions due to crack trapping mechanisms. Stable crack growth under cyclic loading was observed at room temperature in all the NiAl–Mo composites that were studied. The potential implications of the results are discussed for future composite development.

The influence of pre-existing dislocations on cleavage crack propagation behavior in crystals

Available experimental evidence on cleavage of single crystals suggests that the direct interactions of crack tips with pre-existing dislocations are important in determining the toughness of cleavable crystals. On this basis, the behavior of dislocations in the vicinity of a moving crack tip has been investigated. It is found that dislocations which reside within a strip of certain width along the crack path and which have Burgers vectors of a certain orientation are strongly attracted to the crack tip. and the motion of such dislocations with respect to the moving crack tip is determined. Once drawn into the immediate vicinity of the crack tip, these dislocations inevitably cause local, atomic scale blunting of the tip. The resulting crack trapping mechanism is modeled phenomenologically, yielding a quantitative estimate for actual crack tip fracture toughness Γ tip which depends on temperature, crack speed and density of pre-existing dislocations. This fracture toughness is believed to represent an estimate of the resistance to fracture that is more realistic than the ideal Griffith surface energy estimate. Then, possible scenarios for either gradual or abrupt transitions from brittle to ductile behavior with temperature are discussed within a framework based on the present model and a continuum plasticity model for the functional relation between the measurable far field toughness Γ far and the crack tip fracture toughness. The nature of the transition depends on the relationship between these two functions. As an application, the anomalous low temperature toughness of iron is explained on the basis of the high density of dislocations commonly observed in annealed iron crystals.