R-curve Behavior Research Articles

Microstructural effects on the R-curve behavior of WC-Co cemented carbides

The influence of microstructure on the R-curve behavior of hardmetals and its implication on their strength and reliability are studied. In doing so, an already validated model for R-curve description is implemented for rationalizing the fracture behavior of five microstructurally different WC-Co cemented carbides. Results indicate that hardmetals with R-curves developing smoothly over relatively long multiligament zones (i.e. lower slopes) exhibit reduced strength variability than those with steeper and shorter (i.e. higher slopes) ones. The reduced strength scatter associated with a low-slope R-curve behavior is discussed on the basis of the effectively developed toughness and the subcritical crack extension reached before unstable growth is triggered, both being dependent on microstructural length scale and initial flaw size. The results of this investigation highlight the relevance of an in-depth knowledge of R-curve characteristics of hardmetals in order to optimize the performance of engineering parts by means of microstructural design.

A comparison study on the R-curve behavior of alumina/aluminum titanate composites prepared with different TiO2 powders

Rising crack growth resistance (R-curve) behavior of nano and micro alumina/20wt.% aluminum titanate composites have been compared. Preparation of the composites was made using reaction sintering of the alumina and titania powders. Nano and micro sized TiO2 powders were used in achieving the Al2TiO5 as secondary phase. The toughness curves of the composites were determined using indentation-fracture method. Both of the micro and nano composites show a significant rising of toughness in comparison with the monolithic alumina. The K1C value increased from the initial value of 2.6MPa√m to 4.5, 5.1 and 5.5MPa√m at the maximum load (P) for alumina, micro and nano composites, respectively. The microstructural observations demonstrated that the bridging and microcracking are the main toughening mechanisms in the alumina/aluminium titanate composites. Bridging mechanisms in the micro and microcracking phenomenon in the nano composites have the major responsibility of the R-curve behavior.

R-curve analysis of solid-phase-sintered and liquid-phase-sintered silicon carbide ceramics by indentation fracture and indentation-strength-in-bending methods

The R-curve behavior of SiC ceramics was investigated from aspects of microstructure and test methods. Two groups of silicon carbide ceramics were prepared by solid-phase-sintering and liquid-phase-sintering methods. One from each, determined by systematic evaluation on microstructure and mechanical properties, was selected for. R-curve behavior study by two methods of IF and ISB. It is found that for both SSiC and LSiC, IF fracture toughness decreases while the ISB one increases with the increasing indentation load – inconsistent R-curve behaviors observed with different method. IF and ISB methods were examined on its own theoretical basis and ISB method was determined the correct method for R-curve evaluation of SSiC and LSiC. That LSiC ceramic possesses better mechanical properties than SSiC material, indicated by ISB R-curve, well matches the mechanical evaluations.

Interpenetrating network ceramic-resin composite dental restorative materials

ObjectivesThis paper investigates the structure and some properties of resin infiltrated ceramic network structure materials suitable for CAD/CAM dental restorative applications. MethodsInitially the basis of interpenetrating network materials is defined along with placing them into a materials science perspective. This involves identifying potential advantages of such structures beyond that of the individual materials or simple mixing of the components. ResultsObservations from a number of recently published papers on this class of materials are summarized. These include the strength, fracture toughness, hardness and damage tolerance, namely to pointed and blunt (spherical) indentation as well as to burr adjustment. In addition a summary of recent results of crowns subjected to simulated clinical conditions using a chewing simulator are presented. These results are rationalized on the basis of existing theoretical considerations. SignificanceThe currently available ceramic-resin IPN material for clinical application is softer, exhibits comparable strength and fracture toughness but with substantial R-curve behavior, has lower E modulus and is more damage tolerant than existing glass-ceramic materials. Chewing simulation observations with crowns of this material indicate that it appears to be more resistant to sliding/impact induced cracking although its overall contact induced breakage load is modest.

In-situ study of microscale fracture of diffusion aluminide bond coats: Effect of platinum

Abstract

Mode I fracture testing of pultruded glass fiber reinforced epoxy rods: Test development and influence of precracking method and manufacturing

Pultruded glass fiber polymer-matrix composite rods are industrially produced for various applications. The development of a quasi-static mode I delamination test for these rods using a modified Double Cantilever Beam specimen is described. Issues for investigation were size and shape of test specimens, type of load introduction, and different pre-cracking methods. When the test procedure was applied to rods with different lengths, a dependence of the R-curve behavior on the specimen length was revealed. This is attributed to the strong fiber bridging that was observed in the tests. There are also indications for manufacturing effects on scatter.

Mechanical properties of 12Ce–ZrO2/3Y–ZrO2 composites

12Ce–ZrO2/3Y–ZrO2 composites with different compositions have been prepared by presureless sintering at 1450°C and the resulting microstructures have been correlated with their mechanical properties in terms of hardness, elastic modulus and resistance to contact loading by sharp indenters. The composition with a better compromise between mechanical properties and resistance to hydrothermal degradation has been processed at different sintering temperatures in the range 1350°C and 1650°C and the fracture toughness have been determined by the single edge V-notch beam (SEVNB) method with a very small starting crack induced by ultra-short pulsed laser ablation. It is shown that the resulting grain size is sensitive to composition and sintering temperature while the elastic modulus is the same for all the compositions investigated. The hardness and the resistant to contact loading are also strongly affected by the addition of 3Y–ZrO2. Finally it is shown that despite the high resistance to fracture by contact loading encountered for some compositions rich in 12Ce–ZrO2, the fracture toughness determined from surface shallow sharp cracks is similar to that of 3Y–TZP. This is explained in terms of R-curve behaviour and crack dimensions.

FIB/FESEM experimental and analytical assessment of R-curve behavior of WC–Co cemented carbides

Exceptional fracture toughness levels exhibited by WC–Co cemented carbides (hardmetals) are due mainly to toughening derived from plastic stretching of crack-bridging ductile enclaves. This takes place due to the development of a multiligament zone at the wake of cracks growing in a stable manner. As a result, hardmetals exhibit crack growth resistance (R-curve) behavior. In this work, the toughening mechanics and mechanisms of these materials are investigated by combining experimental and analytical approaches. Focused Ion Beam technique (FIB) and Field-Emission Scanning Electron Microscopy (FESEM) are implemented to obtain serial sectioning and imaging of crack–microstructure interaction in cracks arrested after stable extension under monotonic loading. The micrographs obtained provide experimental proof of the developing multiligament zone, including failure micromechanisms within individual bridging ligaments. Analytical assessment of the multiligament zone is then conducted on the basis of experimental information attained from FIB/FESEM images, and a model for the description of R-curve behavior of hardmetals is proposed. It was found that, due to the large stresses supported by the highly constrained and strongly bonded bridging ligaments, WC–Co cemented carbides exhibit quite steep but short R-curve behavior. Relevant strength and reliability attributes exhibited by hardmetals may then be rationalized on the basis of such toughening scenario.

Developing strength and toughness in bio-inspired silicon carbide hybrid materials containing a compliant phase

Freeze casting has proven to be a versatile processing route to fabricate bio-inspired (“nacre-like”) hybrid composites that exhibit unique combinations of strength and toughness (damage-tolerance). To date, however, the effects of small changes in the architecture of such composites on their mechanical properties have been poorly investigated. Here we examine the influence of microstructural features such as ceramic/polymer ratio, layer thickness, and presence of bridges between ceramic lamellae, on the mechanical performance of the resulting composites. To this end, we compare the flexural strength and resistance to failure of a suite of silicon carbide/polymethyl methacrylate (SiC/PMMA) layered composites made by polymer infiltration of freeze-cast SiC scaffolds with various architectures. Our composite structures all show an increasing fracture resistance with crack extension (rising R-curve behavior) due to extrinsic toughening mechanisms such as uncracked-ligament bridging, inelastic deformation of ductile layers, lamellae pull out and ceramic bridge fracture. We show that a fine tuning of the composite architecture can lead to SiC/PMMA samples with a dendritic morphology, which exhibit the best strength and toughness. Specifically, the presence of ceramic bridges connecting the lamellae is seen to provide a strengthening effect similar to the mineral bridges between aragonite platelets in nacre, where they prevent debonding and limit platelet sliding; additionally, the fracture of these bridges between lamellae during crack extension is a potent toughening mechanism, thereby conferring optimal damage tolerance to the material.

Fracture Mechanics Characterization of Sintered 30 Vol.-% Al<sub>2</sub>O<sub>3</sub>/TRIP Steel Composites Using SENB Miniature Samples

Ceramic particle reinforced metal matrix composites (PRMMCs) combine the strength and brittleness of ceramics with the toughness of a metallic matrix. In order to use these materials in construction and operational design their fracture mechanical behavior must be evaluated. In this study, a 30 vol.-% Al2O3 reinforced austenitic TRIP steel processed by powder metallurgical technique was investigated using precracked miniature SENB-specimens in 3-point-bending. An elastic-plastic analysis by means of the J-integral method in combination with optical crack observation showed the materials ability of stable crack growth, i. e. R-curve behavior. In addition to the mechanical tests microstructural studies were performed, whereby particle debonding and fracture as well as martensitic phase transformation and crack bridging within the matrix were identified as fracture energy dissipating mechanisms.

Temperature-dependent R-curve behavior of the lead-free ferroelectric 0.615Ba(Zr0.2Ti0.8)O3–0.385(Ba0.7Ca0.3)TiO3 ceramic

The temperature-dependent crack growth resistance behavior of the lead-free (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (x=0.385) ceramic was characterized using compact-tension specimens from 25°C to 60°C. The observed plateau fracture toughness at 25°C was found to be approximately 37% lower than commercial Pb(Zr,Ti)O3. At elevated temperature, the maximum fracture toughness displayed a decrease, which was found to be related to the temperature-dependent elastic and ferroelastic properties. Mechanical measurements are presented that demonstrate decreasing effective switching strain, coercive stress and Young’s modulus with increasing temperature.

R-curve behavior of laminated ZrB2–SiC ceramic with strong interfaces

The damage resistance and R-curve behavior of laminated ZrB2–SiC ceramic fabricated by hot-pressing were investigated using indentation-strength technique. The difference in fracture resistance properties between the monolithic and laminated ZrB2–SiC ceramics was also analyzed and results showed that obvious R-curve behavior was found in laminated ZrB2–SiC ceramic, compared with monolithic ZrB2–SiC ceramics. Moreover, the indentation strength values of laminated ZrB2–SiC ceramic were observed to be insensitive to the increases in the indentation load, which indicated that laminated ZrB2–SiC ceramic has excellent damage resistance. The improvement in damage tolerance and R-curve behavior of laminated ZrB2–SiC ceramic was attributed to the macroscopic crack deflection due to the presence of residual compressive stresses.

Interpretation of Fracture Toughness and R-Curve Behavior by Direct Observation of Microfracture Process in Ti-Based Dendrite-Containing Amorphous Alloys

Fracture properties of Ti-based amorphous alloys containing ductile β dendrites were explained by directly observing microfracture processes. Three Ti-based amorphous alloys were fabricated by adding Ti, Zr, V, Ni, Al, and Be into a Ti-6Al-4V alloy by a vacuum arc melting method. The effective sizes of dendrites varied from 63 to 104 μm, while their volume fractions were almost constant within the range from 74 to 76 pct. The observation of the microfracture of the alloy containing coarse dendrites revealed that a microcrack initiated at the amorphous matrix of the notch tip and propagated along the amorphous matrix. In the alloy containing fine dendrites, the crack propagation was frequently blocked by dendrites, and many deformation bands were formed near or in front of the propagating crack, thereby resulting in a zig-zag fracture path. Crack initiation toughness was almost the same at 35 to 36 MPa√m within error ranges in the three alloys because it was heavily affected by the stress applied to the specimen at the time of crack initiation at the crack tip as well as strength levels of the alloys. According to the R-curve behavior, however, the best overall fracture properties in the alloy containing fine dendrites were explained by mechanisms of blocking of the crack growth and crack blunting and deformation band formation at dendrites.

OS1329-351 非線形弾性挙動を有するマグネシア・スピネル質れんがのR曲線評価

OS1329-351 非線形弾性挙動を有するマグネシア・スピネル質れんがのR曲線評価

Damage resistance, R-curve behavior and toughening mechanisms of ZrB2-based composites with SiC whiskers and ZrO2 fibers

The damage tolerance and R-curve behavior of ZrB2-based composites with SiC whiskers and ZrO2 fibers were investigated by using the envelope method. Obvious R-curve behavior was further observed in these composites, as a result of the combined effect of whisker toughening and transformation toughening. The SiC whiskers with high aspect ratio exhausted more fracture energy by deflection, bridging and pull-out during failure, and promoted the transformation toughening of ZrO2 fibers. It was found that these interactions produced a notable increase in toughness to 8.2MPam1/2. The relatively contribution of each toughening mechanism was estimated. This calculated value was in good agreement with the experimental results (8.0±0.5MPam1/2) and the predictions of R-curve behavior (8.026MPam1/2).

Fracture Toughness Effects in Geomaterial Solid Particle Erosion

Effects of fracture toughness on the impingement of geomaterials (rocks and cementitious composites) by quartz particles at velocities between 40 and 140 m/s are investigated experimentally and analytically. If schist is excluded, relative erosion (in g/g) reduces according to a reverse power function if fracture toughness increases. The power exponent depends on impingement velocity, and it varies between −0.64 and −1.33. Lateral cracking erosion models, developed for brittle materials, deliver too high values for relative material erosion. This discrepancy is partly attributed to stress rate effects. Effects of R-curve behavior seem to be marginal. An integral approach E R = K 1 · E R P + (1 − K 1) · E R L is introduced, which considers erosion due to plastic deformation and lateral cracking. A transition function \(K_{1} = f\left( {K_{\text{Ic}}^{12/4} /\sigma_{\text{C}}^{23/4} } \right)\) is suggested in order to classify geomaterials according to their response against solid particle impingement.

R-Curve Modelling of Mode I Delamination in Multidirectional Carbon/Epoxy Composite Laminates

In the present work, the mode I delamination behaviour of a quasi-isotropic quasi-homogeneous carbon/epoxy composite laminate with adjacent plies of 0o//45o is studied numerically. To describe the R-curve behaviour observed during crack propagation, a linear-exponential traction-separation law is proposed, where the fracture toughness and the increment in the fracture energy could be considered separately in the model. This model is then implemented in the finite element simulation of the delamination process in the composite laminate. Numerical results indicate that with the incorporation of the fibre bridging effect leads to a well-predicted force-displacement response of the composite laminates.

Slow crack growth, its modeling and crack-layer approach: A review

A review of empirical equations for slow crack growth under fatigue and creep conditions is presented. The crack propagation rate is commonly expressed as a function of stress intensity factor or energy release rate. A concept of crack driving force and crack stability analysis is employed to predict crack behavior under different loading conditions; the concept of crack growth resistance, such as an effective surface energy, is used. It is shown that for a stable crack propagation (the energy release rate is a decreasing function of crack length) crack growth equation results from the crack equilibrium condition, if the crack resistance is assumed to be constant. Experimental studies demonstrate that crack growth resistance is not constant, but changes with crack propagation. Formation of the so-called process zone appears to be responsible for the changes. Process zone (PZ) is a material reaction to stress concentration at crack tip and it is commonly observed under fatigue and creep conditions in most engineering materials. For an unstable crack growth (the energy release rate is an increasing function of crack length) the effect of PZ even more dramatic; for example, in a perfectly homogeneous elastic solid slow crack growth would be impossible without the stabilizing effect of PZ. A system of a crack and PZ is referred to as crack-layer (CL). CL driving forces that are thermodynamic forces associated with crack and PZ growth are introduced. CL growth equations are employed to explain the observed variations of crack growth resistance commonly known as R-curve behavior. The evolution of the process zone in a complex stress field is illustrated by an experiment demonstrating how process zone accelerates and decelerates crack growth. Modeling of the crack–process zone interaction is a challenging problem; recent advances in computational techniques make it solvable. The CL approach is illustrated on an example of engineering thermoplastics. Applications to lifetime prediction of structural components, as well as toughness and durability of materials are discussed.

Crack stability in edge-notched clamped beam specimens: modeling and experiments

Stability of a fracture toughness testing geometry is important to determine the crack trajectory and R-curve behavior of the specimen. Few configurations provide for inherent geometric stability, especially when the specimen being tested is brittle. We propose a new geometrical construction called the single edge notched clamped bend specimen (SENCB), a modified form of three point bending, yielding stable cracking under load control. It is shown to be particularly suitable for small-scale structures which cannot be made free-standing, (e.g., thin films, coatings). The SENCB is elastically clamped at the two ends to its parent material. A notch is inserted at the bottom center and loaded in bending, to fracture. Numerical simulations are carried out through extended finite element method to derive the geometrical factor f(a/W) and for different beam dimensions. Experimental corroborations of the FEM results are carried out on both micro-scale and macro-scale brittle specimens. A plot of vs a/W, is shown to rise initially and fall off, beyond a critical a/W ratio. The difference between conventional SENB and SENCB is highlighted in terms of and FEM simulated stress contours across the beam cross-section. The `s of bulk NiAl and Si determined experimentally are shown to match closely with literature values. Crack stability and R-curve effect is demonstrated in a PtNiAl bond coat sample and compared with predicted crack trajectories from the simulations. The stability of SENCB is shown for a critical range of a/W ratios, proving that it can be used to get controlled crack growth even in brittle samples under load control.

Processing, spark plasma sintering, and mechanical behavior of alumina/titanium composites

This paper focuses on the study of the processing and mechanical properties, (flaw tolerance and R-curve behavior) of alumina–titanium ceramic–metal composites produced by spark plasma sintering. In order to obtain homogenously dispersed composites, a rheological study was carried out by measuring the flow behavior in different conditions of solid content, amount of dispersant and shear stress. It has been found that, with the suitable conditions (80 wt% solids and 3 wt% deflocculant), a ceramic–metal homogeneously dispersed (Al2O3–Ti) composite can be obtained. After sintering, the composites were mechanically tested and the cermet showed an important improvement in the flaw tolerance and R-curve behavior when compared with the monolithic material. It has been demonstrated by scanning electronic microscopy that this improvement is a consequence of the reinforcement mechanisms provided by the metallic particles that interact with the crack producing a notable increase in toughness up to ~8 MPa m1/2.