Resistance-curve Behavior Research Articles

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.

Behavior of Crack Growth Resistance in Toughened Silicon Nitride Ceramics

AbstractTesting methods for crack-growth resistance-curve (R-curve) behavior were investigated and developed to analyze the toughened mechanism in polycry stalline ceramics. These methods are a biaxial-flexure method for small-scale disc-shaped specimens with micro-indentation cracks and a single-edged notched beam flexural method with crack stabilizers. In both methods, the growing crack length is measured directly as a function of applied stress, using the system that consist of a microscope and a CCD camera. Applying these testing methods, R-curve behavior of a toughened silicon nitride with a preferred orientation of elongated grains was evaluated to characterize the toughened mechanism, comparing with the behavior in a commercially available silicon nitride. The behavior having these rising R-curves is discussed with emphasis on the effects of microstructure such as grain-growth and grain-orientation, and resultant grain-bridgings behind the crack-tip.

A Study of the Fracture Behavior of Unindirectional Fiber- Reinforced Composite Using Coherent Gradient Sensing (CGS) Interferometry

The feasibility of using Coherent Gradient Sensing (CGS) interferometry for studying the fracture behavior of unidirectional fiber-reinforced composites is investigated in this paper. First, the solution for the deformation field surrounding the tip of a crack in an orthotropic material is summarized. Specifically, the most singular term in the asymptotic expansion is explicitly presented. Then, the quantities that relate to the CGS measurements are derived in terms of the spatial position, stress intensity factors, and material constants. Based on these results, synthetic CGS fringe patterns are plotted numerically, and the effects of material anisotropy and crack-tip mixity on the shape of CGS fringe pattern are investigated. In addition, a finite difference interpretation of CGS fringes caused by the finite spacing of the CGS diffraction gratings is taken into account in the simulation. Finally, the initiation fracture toughness and the subsequent resistance curve behavior of a particular unidirectional graphite/epoxy composite are measured using the CGS method. The optically measured stress intensity factors compare successfully to values obtained from the load measurements and the available analytical solutions.

MECHANISMS OF CYCLIC FATIGUE‐CRACK PROPAGATION IN A FINE‐GRAINED ALUMINA CERAMIC: THE ROLE OF CRACK CLOSURE

Abstract— Cyclic fatigue‐crack growth and resistance‐curve behavior have been studied in a fine‐grained (∼ 1 μm), high‐purity alumina. Specific emphasis is given to the mechanisms associated with crack growth that are controlled by the maximum (Kmax) and the alternating (ΔK), stress intensities and to the role of crack‐face interference (crack closure), which is known to be an important crack‐tip shielding mechanism in metal fatigue. Significant levels of subcritical crack growth were detected above a threshold stress intensity of ∼60% of the fracture toughness (Kc) in the alumina, with growth rates displaying a far larger dependence on Kmax compared to ΔK. The role of crack closure was examined using constant‐Kmax experiments, where the minimum stress intensity (Kmin) was maintained either above or below the stress intensity for crack closure (Kcl). Where Kmin< Kcl, growth rates were found to exhibit a lower dependence on ΔK, which was rationalized in terms of the frictional wear model for crack growth in grain‐bridging ceramics. It is concluded that crack closure, as conventionally defined, has little relevance as a crack‐tip shielding mechanism during fatigue‐crack growth in grain‐bridging ceramics, due to the low dependence of growth rates on ΔK compared to Kmax.

Reliability of PMMA bone cement fixation: fracture and fatigue crack-growth behaviour.

Fracture mechanics tests were performed to characterize the fracture toughness and fatigue crack-growth behaviour of polymethylmethacrylate (PMMA) bone cement, commonly used in joint replacement surgery. Compact tension specimens of various thicknesses were prepared and tested in both air and Ringer's solution. Contrary to previous reports citing toughness as a single valued parameter, the PMMA was found to exhibit resistance-curve behaviour with a plateau toughness of approximately 0.6 MPa m1/2 in air, and approximately 2.0 MPa m1/2 in Ringer's solution. The increased toughness in Ringer's solution is thought to arise from the plasticizing effect of the environment. Under cyclic loads, the material displayed true mechanical fatigue failure in both environments; fatigue crack-growth rates, da/dN, were measured over the range approximately 10(-10) to 10(-6) m/cycle and found to display a power-law dependence on the stress intensity range, DeltaK. The cement was found to be more resistant to fatigue-crack propagation in Ringer's solution than in air. Wear debris was observed on the fatigue fracture surfaces, particularly those produced in air. These findings and the validity of using a linear-elastic fracture mechanics approach for viscoelastic materials are discussed in the context of providing more reliable and fracture-resistant cemented joints.

Resistance-curve toughening in ductile/brittle layered structures: Behavior in Nb/Nb 3Al laminates

A study has been made of the fracture toughness and resistance-curve behavior of a laminate consisting of alternating layers of brittle Nb 3Al intermetallic and ductile Nb metal, using layer thicknesses of ∼500 and 125 μm, respectively. Effective resistance-curve toughening of Nb 3Al was achieved in such a coarse-scale layered structure with only 20 vol.% of the Nb reinforcement phase. Specifically, the toughness of Nb 3Al was increased from ∼1 MPa✔m to well over 20 MPa✔m (and as high as 70 MPa✔m in certain samples) after several millimeters at stable crack growth. These values are significantly greater than other Nb/Nb 3Al composites containing Nb as ∼ 20 μm sized particulates or 1–2 μm thick Nb layers (in the form of a microlaminate), both containing at least 40 vol.% of the ductile phase. The source of such ductile-phase toughening was attributed to crack blunting at, and renucleation across, the ductile Nb layers, which in turn led to extensive bridging and plastic deformation within the Nb layers in the crack wake. Since the extent of crack trapping by the ductile layer and plastic deformation are limited by layer thickness, the present coarser-scale laminates tend to display better fracture resistance compared to composites with finer-scale ductile reinforcements.

Loading rate and test temperature effects on fracture ofIn Situ niobium silicide-niobium composites

Arc cast, extruded, and heat-treatedin situ composites of niobium suicide (Nb5Si3) intermetallic with niobium phases (primary—Nbp and secondary—Nbs) exhibited high fracture resistance in comparison to monolithic Nb5Si3. In toughness tests conducted at 298 K and slow applied loading rates, the fracture process proceeded by the microcracking of the Nb5Si3 and plastic deformation of the Nbp and Nbs phases, producing resistance-curve behavior and toughnesses of 28 MPa√m with damage zone lengths less than 500μm. The effects of changes in the Nbp yield strength and fracture behavior on the measured toughnesses were investigated by varying the loading rates during fracture tests at both 77 and 298 K. Quantitative fractography was utilized to completely characterize each fracture surface created at 298 K in order to determine the type of fracture mode (i.e., dimpled, cleavage) exhibited by the Nbp. Specimens tested at either higher loading rates or lower test temperatures consistently exhibited a greater amount of cleavage fracture in the Nbp, while the Nbs, always remained ductile. However, the fracture toughness values determined from experiments spanning six orders of magnitude in loading rate at 298 and 77 K exhibited little variation, even under conditions when the majority of Nbp phases failed by cleavage at 77 K. The changes in fracture mode with increasing loading rate and/or decreasing test temperature and their effects on fracture toughness are rationalized by comparison to existing theoretical models.

The fracture toughness of niobium-based,in situ composites

The fracture resistance of Nb-Cr-Ti alloys orin situ composites of three different compositions, Cr2Nb, and a Nb-10Siin situ composite was studied at ambient temperature. The crack-tip deformation and fracture behaviors were characterized using near-tip measurement techniques and fractographic analyses. The relevant fracture and toughening mechanisms were identified and related to the microstructure. Despite fracture by a combination of cleavage and slip band decohesion, the Nb solid-solution alloy exhibited a resistance-curve behavior with a relatively high toughness and local ductility. The source of toughness was modeled and explained in terms of a cracking process that involved alternate slip band decohesion and cleavage. Thein situ composites, on the other hand, exhibited cleavage fracture but considerably lower toughness with little or no resistance-curve behaviors. The difference in the fracture behavior appears to arise from two factors: (1) the presence of a high constraint in the Nb solid-solution matrix in thein situ composites, and (2) the lack of plastic flow associated with cleavage of the constrained Nb solid-solution matrix.

Cyclic fatigue and resistance-curve behavior of anin situ toughened silicon carbide with Al[sbnd]B[sbnd]C additions

The room-temperature crack-growth properties of anin situ toughened, monolithic silicon carbide are reported. Hot pressing was performed at 1900°C with 3 wt.% Al, 2 wt.% C and 0.6 wt.% B additions. Compared to a commercial SiC (Hexoloy SA), significant improvements in both the fracture toughness and cyclic fatigue-crack propagation resistance have been achieved through control of the β to α transformation. Using fatigue-precracked, disk-shaped compact-tension specimens, marked rising resistance-curve behavior was measured over the first ∼ 600 μm of crack extension, leading to a “plateau” fracture toughness ofKc ∼ 9.1MPa√m; this represents more than a threefold increase over the toughness of Hexology, where aKc value of 2.5 MPa√m was measured with no evidence of a resistance curve. Cyclic fatigue-crack growth rates in the toughened SiC were found to be faster than those under sustained loads (static fatigue) at the same stress-intensity level. The cyclic fatigue-crack growth resistance was found to be far superior to that of Hexology. Whereas cracking in the commercial SiC became unstable when the maximum stress intensityKmax exceeded ∼ 2 MPa√m, thresholds for fatigue-crack growth in thein situ toughened material exceeded aKmax of 7 MPa√m. Such dramatic improvements in the crack-growth resistance of SiC are attributed to a microstructure consisting of a network of interlocking, plate-like predominantly α-phase grains, which combine to both bridge and deflect the crack. Under cyclic loads, fatigue-crack growth is promoted by the cycle-dependent decay in such crack-tip shielding due to frictional-wear degradation of the zone of grain bridging ligaments in the crack wake. These results represent the first reported evidence of cyclic fatigue behavior in a monolithic silicon carbide and the first direct measurement of the resistance curve properties in this ceramic.

On loading rate effects in toughening processes

Environmental crack tip reactions are a known source of premature fracture in oxides. These rate-dependent phenomena commonly are studied in strength tests where loading rate serves as the major experimental variable. A material susceptible to environmentally-assisted crack growth is stronger at fast testing rates. A topic which has received far less attention is the influence of stressing rate or loading rate on the shielding processes which occur at some distance from the crack tip, although the inverse has been studied by Deuerler et al. In their work, the crack velocity-stress intensity, v-K, curve was measured as a function of crack length in alumina. The v-K curve shifted to higher stress intensities for longer cracks, indicative of greater shielding as the crack grew progressively up the K_R (resistance) curve. In this paper, we explore the influence of loading rate on the fracture toughness of two aluminum oxides, one which fails in a brittle fashion (i.e., where toughness is independent of crack length), and a second which demonstrates significant resistance curve behavior due to grain bridging behind the crack tip. We present here the first known documentation of a loading rate effect on shielding phenomena in ceramic materials.

Toughness and Subcritical Crack Growth in Nb/Nb3Al Layered Materials

AbstractA brittle intermetallic, Nb3Al, reinforced with a ductile metal, Nb, has been used to investigate the resistance curve and cyclic fatigue behavior of a relatively coarse laminated composite. With this system, the toughness of Nb3Al was found to increase from ∼1 MPa√m to well over 20 MPa√m after several millimeters of stable crack growth; this was attributed to extensive crack bridging and plastic deformation within the Nb layers in the crack wake. Cyclic fatigue-crack growth resistance was also improved in the laminate microstructures compared to pure Nb3Al and Nb-particulate reinforced Nb3Al composites with crack arrester orientations in the laminate providing better fatigue resistance than either the matrix or pure Nb.

Toughening mechanisms in ductile niobium-reinforced niobium aluminide (Nb/Nb3Al) in situ composites

Anin situ study has been performed in the scanning electron microscope (SEM) on a niobium ductilephase-toughened niobium aluminide (Nb/Nb3Al) intermetallic composite to examine the crack-growth resistance-curve (R-curve) behavior over very small initial crack extensions, in particular over the first ~500 μm of quasi-static crack growth, from a fatigue precrack. The rationale behind this work was to evaluate the role of toughening mechanisms, specifically from crack bridging, in the immediate vicinity of the crack tip and to define the size and nature of bridging zones. Although conventional test methods, where crack advance is monitored typically over dimensions of millimeters using compliance or similar techniques, do not show rising R-curve behavior in this material,in situ microscopic observations reveal that bridging zones resulting from both uncracked Nb3Al ligaments and intact Nb particles do exist, but primarily within ~300 to 400 μm of the crack tip. Accordingly, rising R-curve behavior in the form of an increase in fracture resistance with crack growth is observed for crack extensions of this magnitude; there is very little increase in toughness for crack extensions beyond these dimensions. Ductile-phase toughening induced by the addition of Nb particles, which enhances the toughness of Nb3Al from ~1 to 6 MPa√m, can thus be attributed to crack-tip shielding from nonplanar matrix and coplanar particle bridging effects over dimensions of a few hundred microns in the crack wake.

Fatigue-crack growth and fracture resistance of a two-phase ( γ + α2) TiAl alloy in duplex and lamellar microstructures

The fatigue-crack propagation and fracture-toughness behavior of a two-phase ( γ + α 2) Ti-47.3Al-2.3Nb-1.5Cr-0.4V (in at.%) alloy in fine-equiaxed (duplex) and fully-lamellar microstructural conditions were examined at room temperature. It is observed that the lamellar microstructure displays superior fracture toughness and fatigue-crack growth resistance compared with the duplex microstructure, although the extent of improvement is significantly greater during quasi-static fracture. Crack extension under monotonic loading is characterized by resistance-curve behavior with plateau (steady-state) toughnesses of about 30 MPa m 1 2 in the fully-lamellar condition compared to a crack-initiation toughness of about 10 MPa m 1 2 for the duplex. Corresponding thresholds for cyclic fatigue-crack propagation are of the order of 10 and about 6.5 MPa m 1 2 in the lamellar and duplex structures, respectively. Such improved properties of the lamellar alloy are attributed principally to extrinsic shielding effects from tortuous crack paths, microcracking and formation of shear ligaments resulting from the microlaminated, composite-like features of the alloy.

Crack growth in an elastic-plastic material and effects on near tip constraint

The J− Q field of O'Dowd and Shih [1,2] provides a characterisation of the plastic fields near the tip of a stationary crack, for the full range of crack tip constraints. In this paper, the fields ahead of a growing crack under varying constraint are examined. Initially, a numerical analysis of the small scale yielding problem, with K and T applied remotely, is carried out. The near tip constraint increases with crack growth and when distances are normalized appropriately, the transient near tip fields approach the high constraint steady-state fields of Varias and Shin [3] even for large negative values of applied T stress. Crack growth in finite sized specimens is also examined with different resistance curve behaviour prescribed. Consistent with what was observed in the small scale yielding analysis, the near tip constraint increases with crack growth, though falling somewhat below the steady state field. The numerical results have implications for the mode of failure in plastically deforming materials — the rise in constraint with crack growth, suggests the possibility of failure occuring by cleavage after some amount of stable tearing.

Defects in squeeze-cast A1203/Al alloy composites and their effects on mechanical properties: Liu, X. and Bathias, C. Compos. Sci. Technol. (Mar. 1993) 46 (3), 245–252

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 m1/2, 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.

Mode II delamination toughness of carbon-fibre/epoxy composites with chopped Kevlar fibre reinforcement

Fibre bridging over cracks is an important aspect of reinforcement in fibre composites. In view of this, it is clear that there is an inherent structural weakness in continuous-fibre laminates, i.e. their tendency to delamination as a consequence of the lack of fibre bridging. To overcome this structural flaw, chopped Kevlar fibres are spread between continuous fibre layers. It is intended to see how the mode II delamination toughness G II , is affected by those Kevlar fibres lying within the fracture plane. Three different carbon-fibre/epoxy composite laminates are processed and tested: the normal laminate without the Kevlar fibre reinforcement, and the other two with chopped Kevlar fibres of lengths 5 to 7 and 13 to 15 mm, respectively. Experimental results obtained from end-notched flexure specimens show that the delamination toughness, G II , has been at least doubled, although only 1·7 mg cm −2 of Kevlar fibres are used in the mid-plane where delamination occurred. Detailed scanning electron microscopy photographs show that extensive fibre bridging, pull-out and fracture of the chopped Kevlar fibres have occurred during delamination. Also, because of the influence of the Kevlar fibres, more carbon fibres have been involved in the delamination process, leading to additional toughness increase, which is a very beneficial side effect and an important factor for the toughness improvement. The more pronounced crack growth resistance (R) curve behaviour observation in G II clearly shows the comparison with the interleaf method suitable for ‘selective toughening’, one potential advantage of the current technique is that the whole laminate structure can be reinforced without affecting its bulk strength, stiffness and overall fibre composite volume ratio.

Cyclic Fatigue Crack Propagation Behavior of 9Ce‐TZP Ceramics with Different Grain Size

Cyclic fatigue crack growth behavior has been investigated in 9 mol% Ce‐TZP ceramics with grain sizes varying from 1.1 to 3.0 μm. To ascertain the interaction between crack resistance curve behavior and cyclic fatigue crack growth, cyclic fatigue tests were conducted with short double‐cantilever‐beam specimens in two conditions: (a) with a sharp precrack without preexisting t‐m transformation and (b) with a sharp crack after R‐curve measurements, i.e., with preformed t‐m transformation in the crack region. Fatigue crack propagation occurs at applied stress intensity factor values as low as about 40% of the KI,∞ values measured in the R‐curves. The size and shape of the t‐m transformation zones are found to be different for specimens obtained in monotonic loading R‐curve measurements and in cyclic fatigue tests. For the specimens without preexisting t‐m transformation the overall crack growth behavior can be described by the Paris power law relation: da/dN=AδKmI with m values of 15 for the 1.1‐μm grain size and between 8 and 9 for the material with larger grain sizes. For the specimens with the preformed transformation zone, a “V”shape da/dN versus ΔKI relation is obtained. Explanations for these different results in the two conditions are discussed in terms of crack tip shielding effects.

Damage Evolution and Thermoelastic Properties of Composite Laminates

An analytic model for the prediction of the thermoelastic properties of micro cracked composite laminates is presented. The expression for the calculation of en ergy release rates due to growth of micro cracks is also provided. Numerical results are presented that show that the present method, to a very good accuracy, can predict ther moelastic properties of micro cracked laminates at varying crack densities and layup con figurations. In addition, a resistance curve behavior of the energy release rate is observed for both carbon/epoxy and glass/epoxy composite laminates. The reasons for this R-curve behavior are discussed. Criteria that govern the initiation and growth of micro cracks in composite laminates are discussed and compared to experimental data.

Failure mechanisms in SiC-fiber reinforced 6061 aluminum alloy composites under monotonic and cyclic loading

Micromechanisms influencing crack propagation in a unidirectional SiC-fiber (SCS-8) continuously reinforced Al-Mg-Si 6061 alloy metal-matrix composite (SiCf/Al-6061) during monotonie and cyclic loading are examined at room temperature, both for the longitudinal (0 deg or L-T) and transverse (90 deg or T-L) orientations. It is found that the composite is insensitive to the presence of notches in the L-T orientation under pure tension loading due to the weak fiber/matrix interface; notched failure strengths are ∼1500 MPa compared to 124 MPa for unreinforced 6061. However, behavior is strongly dependent on loading configuration, specimen geometry, and orientation. Specifically, properties in SiCf/Al in the T-L orientation are inferior to unreinforced 6061, although the composite does exhibit increasing crack-growth resistance with crack extension (resistance-curve behavior) under monotonie loading; peak toughnesses of ∼16 MPa√m are achieved due to crack bridging by the continuous metal phase between fibers and residual plastic deformation in the crack wake. In contrast, such bridging is minimal under cyclic loading, as the ductile phase fails subcritically by fatigue such that the transverse fatigue crack-growth resistance is superior in the unreinforced alloy, particularly at high stress-intensity levels. Conversely, fatigue cracks are bridged by unbroken SiC fibers in the L-T orientation and exhibit marked crack deflection and branching; the fatigue crack-growth resistance in this orientation is clearly superior in the composite.

Toughening mechanisms in titanium aluminides

The relevant toughening mechanisms in two-phase titanium aluminides are reviewed in order to elucidate microstructure/fracture toughness relationships. Both intrinsic and extrinsic toughening mechanisms are present in Ti3Al- and TiAl-base alloys. The former affects the initiation toughness(i.e., KIC value) at the onset of crack extension, while the latter leads to crack growth toughness by instigating a resistance-curve behavior. Intrinsic toughening arises from matrix slip and ductile-phase blunting. In contrast, extrinsic toughening originates from crack deflection, ductile-phase bridging, shear ligament toughening, microcrack shielding, twin toughening, and the growing crack singularity. The influence of microstructure on toughening mechanisms in two-phase Ti3Al- and TiAl-base alloys is discussed, with particular emphasis on the need to control the microstructure in order to achieve the desired mechanical properties.