Multiple matrix cracking in a fiber-reinforced titanium matrix composite under high-cycle fatigue

An experimental study of multiple matrix cracking in a fiber-reinforced titanium alloy has been conducted. The focus has been on the effects of stress amplitude on the saturation crack density and the effects of crack density on hysteresis behavior. Comparisons have been made with predictions based on unit cell models, assuming the sliding resistance of the interface to be characterized by a constant interfacial shear stress. In addition, independent measurements of the sliding stress have been made using fiber pushout tests on both pristine and fatigued specimens. D.P. WALLS, Graduate Student, formerly with the Materials Department, University of California, Santa Barbara     

Support Mechanisms of Rammed Aggregate Piers. II: Numerical Analyses

This paper is the second of a two-part series describing an investigation of the mechanical behavior of rammed aggregate piers in supporting rigid square footings. In this paper, the performances of two pier-supported footings and three isolated piers during compressive load tests were simulated using an axisymmetric finite element model and compared to experimental data. A hardening-soil constitutive model with parameters estimated from in situ and laboratory tests was used to characterize the constitutive behaviors of the pier material and the matrix soil. Pier groups were modeled as unit cells with the tributary area determined from the center-to-center spacing. Cavity expansion modeling was used to simulate the pier installation process. Verifications of the numerical model were carried out by comparing the numerical results with the data obtained from full-scale, instrumented load tests. Interpretation of the numerical results focused on the load-deformation behavior, group effect, stress concentration ratio, and the development of stresses in the matrix soil. The distributions of vertical stress underneath the pier-supported footings are also characterized.     

Analysis of shear stress distribution in pushout process of fiber-reinforced ceramics

The interfacial shear stress distribution of a thin specimen of SiC fiber-reinforced glass matrix composite (fiber volume fraction of 0.1, 0.5 and 0.7) during a fiber pushout process was subjected to finite element analysis using a three concentric axisymmetrical model which consisted of fiber, matrix, and composite. A stress criterion was used to determine interface debonding. Effects of thermally-induced stress and a post debond sliding process at the interface were also included in the analysis. The analytical result showed that shear stress near the specimen surface was introduced during the specimen preparation process. Before the interfacial debonding, the distribution of shear stress during the pushout test was affected by the existence of thermally-induced stress in the specimen. The interfacial shear debonding initiated ≈ 30 μm below the pushing surface and the sliding at the debonded interface proceeded in the direction of both the pushing surface and back surface from the peak shear position; the debonding from the back surface initiated just before the complete debonding of the interface. The pushout load-displacement curve near the origin was straight, however, after the existence of interface sliding at the debonded interface, the curve exhibited non-linearity with the increase in applied load up to the complete debonding at the interface. This debonding process was essentially independent of the fiber volume fraction. The results indicate that the total of thermally-induced stress in the specimen and shear stress distribution generated by applied load are important for the initiation of debonding and the frictional sliding process of the thin specimen pushout test.     

Influence of fabrication technique on the fiber pushout behavior in a sapphire-reinforced NiAl matrix composite

Directional solidification (DS) of “powder-cloth” (PC) processed sapphire-NiAl composites was carried out to examine the influence of fabrication technique on the fiber-matrix interfacial shear strength, measured using a fiber-pushout technique. The DS process replaced the fine, equiaxed NiAl grain structure of the PC composites with an oriented grain structure comprised of large columnar NiAl grains aligned parallel to the fiber axis, with fibers either completely engulfed within the NiAl grains or anchored at one to three grain boundaries. The load-displacement behavior during the pushout test exhibited an initial “pseudoelastic” response, followed by an “inelastic” response, and finally a “frictional” sliding response. The fiber-matrix interfacial shear strength and the fracture behavior during fiber pushout were investigated using an interrupted pushout test and fractography, as functions of specimen thickness (240 to 730 μm) and fabrication technique. The composites fabricated using the PC and the DS techniques had different matrix and interface structures and appreciably different interfacial shear strengths. In the DS composites, where the fiber-matrix interfaces were identical for all the fibers, the interfacial debond shear stresses were larger for the fibers embedded completely within the NiAl grains and smaller for the fibers anchored at a few grain boundaries. The matrix grain boundaries coincident on sapphire fibers were observed to be the preferred sites for crack formation and propagation. While the frictional sliding stress appeared to be independent of the fabrication technique, the interfacial debond shear stresses were larger for the DS composites compared to the PC composites. The study highlights the potential of the DS technique to grow single-crystal NiAl matrix composites reinforced with sapphire fibers, with fiber-matrix interfacial shear strength appreciably greater than that attainable by the current solid-state fabrication techniques.     

Influence of Cr and W alloying on the fiber-matrix interfacial shear strength in cast and directionally solidified sapphire nial composites

Sapphire-reinforced NiAl matrix composites with chromium or tungsten as alloying additions were synthesized using casting and zone directional solidification (DS) techniques and characterized by a fiber pushout test as well as by microhardness measurements. The sapphire-NiAl(Cr) specimens exhibited an interlayer of Cr rich eutectic at the fiber-matrix interface and a higher interfacial shear strength compared to unalloyed sapphire-NiAl specimens processed under identical conditions. In contrast, the sapphire-NiAl(W) specimens did not show interfacial excess of tungsten rich phases, although the interfacial shear strength was high and comparable to that of sapphire-NiAl(Cr). The postdebond sliding stress was higher in sapphire-NiAl(Cr) than in sapphire-NiAl(W) due to interface enrichment with chromium particles. The matrix microhardness progressively decreased with increasing distance from the interface in both DS NiAl and NiAl(Cr) specimens. The study highlights the potential of casting and DS techniques to improve the toughness and strength of NiAl by designing dual-phase microstructures in NiAl alloys reinforced with sapphire fibers. R. TIWARI, formerly Research Associate, Department of Chemical Engineering, Cleveland State University     

On the cyclic behavior of a directionally solidified eutectic alloy containing ductile fibers

A study of the low cycle fatigue behavior of the unidirectionally solidified eutectic alloy Ni₃Al(γ′)-molybdenum has been carried out. A difference in thermal contraction characteristics of fiber and matrix led to the development of residual stresses on the composite which influenced the initial flow processes as well as the shape of hysteresis loops. The fatigue tests were carried out under reversed strain-control conditions, and for lives less than 10⁴ cycles fiber buckling led to a curvature of the log strain-log life plot. Dislocation arrays in both matrix and fibers were examined by TEM. Of particular interest was a unique dislocation arrangement in the fibers which consisted of a dense tangle of dislocation debris in the core of the fiber and a relatively dislocation-free region between the core and outer surface of the fiber. The development of this core structure was attributed to the operation of multiple dislocation sources and the annihilation of trailing screw segments of dislocation loops within the fibers. formerly Post-doctoral Research Associate, Department of Metallurgy, University of Connecticut     

Fatigue and creep-fatigue damage of

The objectives of the present study are to observe and model physical damage induced by cyclic multiaxial (tension-torsion) loading of 316L stainless steel both at room temperature and at elevated temperature (600 °C). Four types of experiments were carried out on thin tubular specimens: (a) continuous pure fatigue (PF) tests; (b) PF sequential tests with different sequences of push-pull and torsional loading; (c) creep-fatigue (CF) tests with superimposed hold time at maximum tensile strain; and (d) sequential tests involving sequences of PF and CF loadings. Optical microscopy and scanning electron microscopy (SEM) were used to study quantitatively the damage, in particular, to determine the orientation of cracks and to measure the kinetics of crack nucleation and crack growth. It is shown that in pure fatigue at 600 °C, the classical crack initiation stage I is bypassed due to a strong interaction between cyclic plasticity, oxidation, and cracking. Intense slip bands act as diffusional short circuits, leading to the formation of external (Fe₂O₃) and internal ((FeCr)₃O₄) oxide scales. The orientation of the microcracks during initiation and propagation stages, which is strongly affected by oxidation effects, explains qualitatively the significant deviations observed in the sequential tests from the Miner linear damage cumulative rule. It is also shown that creep-fatigue damage, which involves intergranular damage, is a complex process rather than a simple superposition of fatigue and creep damage. A stochastic model based on a Monte-Carlo simulation is developed. This model, which accounts very well for the situations in which crack initiation and crack propagation are coplanar, includes damage equations based on quantitative metallographical observations. Damage is modeled as the continuous nucleation of a population of growing cracks which eventually coalesce to lead to final fracture. It is shown that this simulation is able to reproduce with a good accuracy the fatigue lives measured under multiaxial continuous and sequential tests. formerly with Ecole des Mines, is Visiting Scientist, Ice formerly with Ecole des Mines, is Visiting Scientist, Ice formerly with Ecole des Mines, is Visiting Scientist, Ice     

Softening and microstructural change following the dynamic recrystallization of austenite

To characterize the dynamic recrystallization behavior of austenite, continuous-torsion tests were carried out on a Mo steel over the temperature range 950 ‡C to {dy1000} ‡C, and at strain rates of 0.02, 0.2, and 2 s^-1. Interrupted-torsion tests also were performed to study the characteristics of postdynamic recrystallization. Quenches were performed after increasing holding times to follow the development of the postdynamic microstructure. Finally, torsion simulations were carried out to assess the importance of metadynamic recrystallization in hot-strip mills. The postdynamic microstructure shows that the growth of dynamically recrystallized grains is the first change that takes place. Then metadynamically recrystallized grains appear and contribute to the softening of the material. The rate of metadynamic recrystallization and the meta-dynamically recrystallized grain size depend on strain rate and temperature and are relatively independent of strain, in contrast to the observations for static recrystallization. True dynamic recrystallization-controlled rolling (DRCR) is shown to require such short interpass times that it does not occur in isolation in hot-strip mills. As these schedules involve 20 to 80 pct softening by metadynamic recrystallization, a new concept known as metadynamic recrystallization-controlled rolling (MDRCR) is introduced to describe this type of situation. 1 C. ROUCOULES, formerly with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada     

Strength and Failure Behaviour of Spark Plasma Sintered Steel‐Zirconia Composites Under Compressive Loading

Several composites, consisting of a metastable austenitic steel matrix and varying amounts of MgO partially stabilized zirconia particles (Mg‐PSZ), were produced through spark plasma sintering (SPS). Compression tests were carried out at room temperature in a wide range of strain rate (4 · 10^?4 s^?1, 2 · 10^?3 s^?1, 10^?1 s^?1, 1 s^?1, 10² s^?1). In conjunction with subsequent microstructural investigations, the mechanical material behaviour was clarified. All composites showed a good ductility and a high strength. The strength increased with an increase of the ceramic content and with higher strain rates. Both, the martensitic transformation of the steel matrix and of the ceramic particles, could be proved at all strain rates. In this study no significant influence of the strain rate on the amount of transformed ceramic could be detected while the steel matrix showed less α′‐martensite after compression at rising strain rates. Local material failure occurred around 0.3 true compressive strain depending on the applied strain rate and the amount of the Mg‐PSZ powder. The main reason for the damage is the relatively weak ceramic‐ceramic interface within the ceramic clusters.     

Fabrication and creep properties of superalloy-zirconia composites

Graded composite interfaces have been proposed as a means to reduce thermally induced stresses between dissimilar materials. This is expected to be useful in applications such as ceramic thermalbarrier coatings (TBCs) on superalloy substrates. The interfaces, in such cases, are metal-matrix composites containing the ceramic phase within the superalloy matrix, whose creep properties during elevated-temperature service become critically important. This study was carried out to assess the creep properties of a typical superalloy-ceramic combination, namely, a René 95 alloy containing partially stabilized zirconia. Composites of these materials were prepared via powder metallurgy. Microscopy and X-ray work revealed that the zirconia reacted with γ′ (Ni₃Al) to form Al₂O₃, which resulted in the depletion of γ′ from the matrix. The creep behavior of the composites was markedly different from that of the unreinforced matrix. In addition to showing different stress exponents, the composites were stronger than the unreinforced material at low strain rates and weaker at the higher strain rates. A composite load-transfer model is used to isolate the effect of particles on strengthening. It is found that strengthening by the ceramic particles is smaller than strengthening arising from the change in chemistry of the matrix due to the addition of ZrO₂.     

A low-cost vacuum casting equipment for aluminium alloys

Some of the advantages inherent in the vacuum casting technique such as improved mechanical properties and elimination of shrinkage have been replicated in a low-cost design. Melting and vacuum requirements of the equipment were determined based on a furnace capacity of 15 kg of aluminium. Clay-based refractory and insulating bricks were used for furnace construction. The vacuum system was made from locally available materials and designed for efficiency required for vacuum casting. The equipment was used to melt and cast small-sized specimens using the lost-wax technique and compared with green sand and ceramic mould cast specimens. Tensile and shrinkage tests were carried out on all specimens. The results of the tensile tests were 123, 98 and 113 MPa for the vacuum cast, green sand cast and ceramic mould cast specimens, respectively. Furthermore, shrinkage defect which is common to both green sand and ceramic mould casting was eliminated in vacuum casting. The vacuum casting equipment cost an equivalent of about U.S $1000.00 (One thousand dollars) to construct and can be easily scaled-up for large-sized castings by increasing furnace capacity and the size of the moulding flask.     

An experimental and numerical study of cyclic deformation in metal-matrix composites

The cyclic stress-strain characteristics of discontinuously reinforced metal-matrix composites are studied both experimentally and numerically. The model systems used for investigation are aluminum alloys reinforced with SiC particulates and whiskers. Finite element analyses of the fatigue deformation of the composite are performed within the context of axisymmetric unit cell formulations. Two constitutive relations are used to characterize the matrix of the composite: the fully dense Mises model of an isotropically hardening elastic-viscoplastic solid and the Gurson model of a progressively cavitating elastic-viscoplastic solid (to simulate ductile matrix failure by the nucleation and growth of voids). The brittle reinforcement phase is modeled as elastic, and the interface between the ductile matrix and the reinforcement is taken to be perfectly bonded. The analyses provide insights into the effects of reinforcement shape and concentration on (1) constrained matrix deformation under cyclic loading conditions, (2) cyclic hardening and saturation, (3) the onset and progression of plastic flow and cavitation within the matrix, and (4) cyclic ductility. The numerical predictions of flow strength, strain hardening, evolution of matrix field quantities, and ductility under cyclic loading conditions are compared with those predicted for monotonic tensile deformation and with experimental observations. formerly Visiting Scientist, Division of Engineering, Brown University     

The use of single fibre pushout testing to explore interfacial mechanics in SiC monofilament-reinforced Ti—II. Application of the test to composite material

Single fibre pushout testing has been used to measure the load needed to displace a fibre, as a function of its aspect ratio. This has been done for SiC monofilaments, having duplex carbon/TiB₂ coatings, embedded in a matrix of Ti6Al4V. Wedge-shaped specimens have been used, allowing pushout of fibres with a range of aspect ratios from a single specimen. Partially pushed-out fibres have also been pushed back into the matrix. Specimens have been examined in the as-fabricated form and also after subsequent heat treatments. Analysis of the results indicates that in all cases it was the resistance to the onset of frictional sliding which was determining the pushout load. Values of the interfacial shear stress necessary for frictional sliding, τ_fr, have been established, although it was not possible to measure separately the coefficient of static friction or the residual radial compressive stress. The value of τ_fr was found to increase progressively on heat treating the composite. Preliminary chemical analysis work suggests that this results from an interfacial reaction, possibly one which causes the carbon layer to become reduced in thickness.     

Combustion synthesis and mechanical properties of dense NiTi-TiC intermetallic-ceramic composites

Combustion synthesis (SHS) coupled with a quasi-isostatic densification step was employed to produce dense NiTi-TiC composites. The synthesis and characterization of five composites are presented, including ceramic-intermetallic (≥50 pct ceramic) composites and intermetallic-ceramic (≥50 pct intermetallic) composites. Particle size, X-ray diffraction (XRD), and scanning electron microscopy (SEM) analysis was conducted to characterize the microstructure of the composites. Refractory TiC and NiTi intermetallic phases become more stoichiometric and the TiC particle size decreases with increasing intermetallic content. Micro- and nanoindentation and quasi-static compression tests were performed, to determine mechanical and material properties. The Vickers hardness decreases as the matrix shifts from ceramic to intermetallic. Modulus and compressive strength decreases with increasing amounts of Ni-Ti intermetallic. The SEM photomicrographs of fractured surfaces are included.     

Nucleation Criteria for the Formation of Aluminum Nitride in Aluminum Matrix by Nitridation

Synthesis of in situ metal matrix composites using liquid metallurgy technique involves processing the melt to generate ceramic particles and entrap them into the matrix during solidification. The process parameters such as temperature, holding time and buoyancy effect generated in the molten bath are crucial factors governing the size and distribution of ceramic particles into the matrix. Temperature being most important parameter and its role is easy to comprehend for in situ formation of ceramic particles using established nucleation criteria. In the present research paper, thermodynamic study is carried to understand the role of temperature on formation of aluminum nitride by nitridation of aluminum melt using ammonium chloride and calcium oxide. Mechanism of aluminum nitride formation by heterogeneous nucleation and its growth has also been discussed based on the nucleation theory. An attempt is also made to correlate nucleation theory for the formation of AlN in Al matrix at different temperatures. The mechanical behavior under compressive loading is carried out on synthesized composites to understand its capability to withstand static loading. Composite strength is analyzed and attempt is made to correlate nucleation dynamics of AlN particles.     

Resistivity-Based Evaluation of the Fatigue Behavior of Cast Irons

Cast irons are used in particular for highly stressed components in the automotive and commercial vehicle industry, e.g., for crankcases and in the wind power industry, e.g., for rotor hubs. The mechanical properties of cast irons are strongly influenced by parameters like phase composition of the matrix, graphite shape, micro-pinholes, and micro-cracks. The measurement of the electrical resistance in the unloaded state and its change during cyclic loading offers the possibility to get detailed information about the actual defect density and the cyclic deformation behavior. In the scope of the present work, stress-controlled load increase tests and constant amplitude tests were carried out at ambient temperature with specimens of the perlitic cast irons EN-GJL-250 (ASTM A48 35B), EN-GJV-400, and EN-GJS-600 (ASTM 80-55-06). Beside electrical measurements, scanning electron microscopy (SEM) was used to characterize the microstructure and to correlate the change of microstructural details with cyclic properties.     

冶金烟气微孔陶瓷管膜研制及深度除尘应用的研究

提出了将SiC、Al2O3等原料等静压成型后烧制成多孔陶瓷基体,再通过多次浸浆、干燥达到所需膜厚度后,采用热喷涂制成非对称陶瓷复合膜管的新工艺技术.研发出能够处理高温烟气的微孔陶瓷管复合膜除尘器,应用到冶金高温烟气中进行深度除尘和净化.采用了SEM、EDS等手段对多孔陶瓷基体和微孔复合膜进行表征,结果表明其具有良好的微孔结构和物理性质.自行设计了一套测定过滤效率的冷态试验装置模型,并通过实验确定了合理的参数.     

Creep of Polymer Matrix Composites. I: Norton/Bailey Creep Law for Transverse Isotropy

This research concerns polymer matrix composite (PMC) materials having long or continuous reinforcement fibers embedded in a polymer matrix. The objective is to develop comparatively simple, designer friendly constitutive equations intended to serve as the basis of a structural design methodology for this class of PMC. Here (Part I), the focus is on extending the deformation model of an anisotropic deformation/damage theory presented earlier. The resulting model is a generalization of the simple Norton/Bailey creep law to transverse isotropy. A companion paper (Part II) by the writers deals with damage and failure of the same class of PMC. An important feature of the proposed deformation model is its dependence on hydrostatic stress. Characterization tests on thin-walled tubular specimens are defined and conducted on a model PMC material. Additional exploratory tests are identified and carried out for assessing the fundamental forms of the multiaxial creep law.     

Crack Growth Resistance of Hybrid Fiber-Reinforced Cement Matrix Composites

The effect of hybrid fiber reinforcement on fracture energy and crack propagation in cement matrix composites is examined. The crack in cement matrix composites is allowed to fracture under mode-I loading with three-point bending beam specimens. The influence of fiber types and their combination is quantified by using the toughness index and fracture energy. A proper hybrid combination of steel fibers and polyvinyl alcohol microfibers enhances the resistance to both the nucleation and growth of the crack. The micromechanical model of hybrid composites by using a fiber bridging law is emphasized, and the numerical model prediction closely matches the behavior obtained from the experiment. The influencing role of the material parameters in the fracture tests (e.g., the fracture toughness index and fracture energy) becomes more apparent than ones used in some conventional strength-based or fiber pullout tests, and these fracture parameters could screen the effect of fiber/microfiber reinforcement in enhancing the crack growth resistance of cementitious composites. This study demonstrates that fundamental fracture tests are effective to characterize and develop high-performance hybrid fiber–reinforced cement matrix composites.     

Reinforcing Mechanism of Mg‐PSZ Particles in Highly‐Alloyed TRIP Steel

Dense TRIP‐matrix composites containing 5 vol.% Mg‐PSZ as reinforcing phase were produced by employing the spark plasma sintering technique. A continuous and seamless interface between the ceramic particles and the steel matrix was achieved. Compression tests revealed better mechanical properties of the 5 vol.% Mg‐PSZ‐TRIP steel composites in comparison with both, pure and Al₂O₃ reinforced TRIP steel. The underlying deformation mechanism within the austenitic matrix entailed a pronounced martensite formation. An additional phase transformation was observed within the ZrO₂ particles. The enhanced mechanical properties of the 5 vol.% Mg‐PSZ composite are dedicated to the transformation strengthening of the ceramic particles. Finally a model of the reinforcing mechanism is proposed.