Showing papers on "Fractography published in 2012"

Hydrogen embrittlement phenomena and mechanisms

Abstract: Abstract Mechanisms of hydrogen embrittlement in steels and other materials are described, and the evidence supporting various hypotheses, such as those based on hydride formation, hydrogen-enhanced decohesion, hydrogen-enhanced localised plasticity, adsorption-induced dislocation emission, and hydrogen-vacancy interactions, are summarised. The relative importance of these mechanisms for different fracture modes and materials are discussed based on detailed fractographic observations and critical experiments.

Hot extruded carbon nanotube reinforced aluminum matrix composite materials.

Abstract: Carbon nanotube (CNT) reinforced aluminum (Al) matrix composite materials were successfully fabricated by mechanical ball milling followed by powder hot extrusion processes. Microstructural analysis revealed that the CNTs were well dispersed at the boundaries and were aligned with the extrusion direction in the composites obtained. Although only a small quantity of CNTs were added to the composite (1 vol%), the Vickers hardness and the tensile strength were significantly enhanced, with an up to three-fold increase relative to that of pure Al. From the fractography of the extruded Al–CNT composite, several shapes were observed in the fracture surface, and this unique morphology is discussed based on the strengthening mechanism. The damage in the CNTs was investigated with Raman spectroscopy. However, the Al–CNT composite materials were not only strengthened by the addition of CNTs but also enhanced by several synergistic effects. The nanoindentation stress–strain curve was successfully constructed by setting the effective zero-load and zero-displacement points and was compared with the tensile stress–strain curve. The yield strengths of the Al–CNT composites from the nanoindentation and tensile tests were compared and discussed. We believe that the yield strength can be predicted using a simple nanoindentation stress/strain curve and that this method will be useful for materials that are difficult to machine, such as complex ceramics.

Investigation of cold rolling influence on the mechanical properties of explosive-welded Al/Cu bimetal

Abstract: In this study, effect of cold rolling process on the mechanical properties and bond strength of Al/Cu/Al bimetal has been investigated. The bimetal raw material has been fabricated by the explosive welding process. Then, cold rolling process was used to reduce the thickness of explosive-welded plates. The mechanical properties of the raw materials and cold-rolled samples were experimentally measured using the tensile, tensile-shearing and hardness tests along the thicknesses of the samples. Moreover, the fractography of the surfaces after the tensile tests were examined by the electron microscope (SEM). The obtained results show that, with the increase of thickness reduction, the ultimate strength and hardness have been increased significantly, while the elongation percentage has been diminished. Also, the bond strength confirms the relevant known standard criterion which is higher than the strength of the aluminum layers. Examination of the fracture surfaces reveal that, due to the brittle nature of the intermetallic compounds at the joining interface, the nucleation and propagation of micro cracks have been accelerated under tension and plastic deformation.

Friction Welding of Titanium to 304 Stainless Steel with Electroplated Nickel Interlayer

Abstract: Friction welding is a solid state joining process and it is best suited for joining dissimilar metals. It overcomes the problems associated with the conventional fusion welding processes. The joining of dissimilar metals using fusion welding processes produce brittle intermetallic precipitates at the interface which reduce the mechanical strength. Various aerospace, nuclear, chemical and cryogenic applications demand joints between titanium and stainless steel. Direct joining of these metals results in brittle intermetallics like FeTi and FexTiy, at the weld interface, which is to be avoided in order to achieve improved properties of the joints. Present study involves joining of two industrially important dissimilar metals such as commercially pure titanium and 304 stainless steel by friction welding with electroplated nickel coating as interlayer that can prevent the brittle intermetallic formation. Microstructural details of the interfaces of the friction welded joints were studied by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) technique and X-ray diffraction (XRD). Microhardness survey was carried out across the joints and tensile test was conducted to assess the mechanical properties of the joints. Fractography studies were carried out on the fracture surfaces of the joints to know the region of failure as well as the mode of failure. XRD patterns indicate the presence of intermetallics in the friction welded joints. These two metals were successfully joined by having electroplated nickel as interlayer. The weld interface on titanium side contained Ti-Ni intermetallics layers, in which the hardness of the weld metal showing the higher value than the base metals. Fractography study conducted on the fracture surfaces created due to pull test reveals that the failure is by brittle fracture and occurred at the intermetallics layer. The maximum strength of the joints achieved for 30 μm and 50 μm thick electroplated nickel interlayers are 242 MPa and 308 MPa, respectively.

Development of B4C–HfB2 composites by reaction hot pressing

Abstract: This paper reports on the development of HfB2 reinforced B4C composite prepared through reaction hot pressing of B4C and HfO2. These composites were characterized for phase composition, microstructure, mechanical and physical properties. Full dense composites were obtained by hot pressing at 1900 °C and a pressure of 40 MPa. HfB2 phase was identified as a reaction product in the composites by XRD analysis. Hardness of all the composites was measured to be in the range of 28–35 GPa. High elastic modulus (525 GPa) and shear modulus (196 GPa) were measured for composite corresponding to 10 wt.% HfO2. Fractography of the composites indicates transgranular mode of fracture. Fracture toughness of the composites was in the range of 4–7 MPa.m1/2 which is higher than that of monolithic B4C (2.43 MPa.m1/2). Microstructural observation of crack propagation patterns indicates the major toughening mechanism as crack deflection, crack arrest and crack bridging.

Fatigue behavior of a Hi-Nicalon™/SiC–B4C composite at 1200 °C in air and in steam ☆

Abstract: Effects of steam environment on fatigue behavior of a non-oxide ceramic composite with a multilayered matrix were investigated at 1200 °C. The composite was produced via chemical vapor infiltration (CVI). The composite had an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide and was reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. Tensile stress–strain behavior and tensile properties were evaluated at 1200 °C. Tension-tension fatigue tests were conducted at 0.1 Hz and at 1.0 Hz for fatigue stresses ranging from 100 to 140 MPa in air and in steam. Fatigue run-out was defined as 105 cycles at 0.1 Hz and as 2 × 105 cycles at 1.0 Hz. Presence of steam had little influence on fatigue performance at 1.0 Hz, but noticeably degraded the fatigue lifetimes at 0.1 Hz. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. Prior fatigue in air and in steam caused significant reduction in tensile strength and modulus. Composite microstructure, as well as damage and failure mechanisms were investigated.

Room temperature fracture processes of a near-α titanium alloy following elevated temperature exposure

Abstract: Near-α titanium alloys are used at higher temperatures than any other class of titanium alloys. As a consequence of thermal exposure, these components may develop locally elevated oxygen concentrations at the exposed surface which can negatively impact ductility and resistance to fatigue crack initiation. In this work, monotonic and fatigue fracture mechanisms of Ti–6Al–2Sn–4Zr–2Mo–0.1Si samples exposed to laboratory air at 650 °C for 420 h were identified by means of a combination of quantitative tilt fractography, metallographic sectioning, and electron backscatter diffraction. These mechanisms were compared and contrasted with those operative during similar tests performed on material is the as-received condition with uniform oxygen content. While faceted fracture was not observed during quasi-static loading of virgin material, locally elevated concentrations of oxygen near the surfaces of exposed samples were shown to change the fracture mode from ductile, microvoid coalescence to brittle facet formation and grain boundary separation at stresses below the macroscopic yield point. Similar features and an increased propensity for facet formation were observed during cyclic loading of exposed samples. The effects of this time-dependent degradation on monotonic and cyclic properties were discussed in the context of the effect of oxygen on crack initiation and propagation mechanisms.

Mechanical property evaluation of Ti-6Al-4V parts made using Electron Beam Melting

Abstract: Cylindrical Ti-6Al-4V parts were built in vertical and horizontal orientations using electron beam melting. Tensile tests and fatigue tests were carried out. The specimens were tested in as-built and machined conditions to understand the effect of surface finish on mechanical properties. The fracture surfaces were analyzed using scanning electron microscopy and the fractography results were correlated with the mechanical properties. Based on the results the effects of part orientation and surface finish on mechanical properties are discussed.

The fracture of brittle materials : testing and analysis

Abstract: Preface ix Acknowledgments xi 1. Introduction 1 2. Fracture Mechanics Background 6 3. Environmentally Enhanced Crack Growth 19 4. Fracture Mechanics Tests 32 5. Strength Testing 68 6. Quantitative Fractography 99 7. Microstructural Effects 131 8. Predicting Reliability 145 9. Concluding Remarks 158 References 161 Name Index 175 Subject Index 179

Microstructure and mechanical properties of joints in sintered SiC fiber-bonded ceramics brazed with Ag Cu Ti alloy

Abstract: Active metal brazing of a new high thermal conductivity sintered SiC-polycrystalline fiber-bonded ceramic (SA-Tyrannohexs) has been carried out using a Ti-containing Ag Cu active braze alloy (Cusil-ABAs) The brazed composite joints were characterized using scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM EDS) The results show that this material can be successfully joined using judiciously selected off-the shelf active braze alloys to yield metallurgically sound joints possessing high integrity Uniform and continuous joints were obtained irrespective of differences in the fiber orientation in the substrate material Detailed interfacial microanalysis showed that the titanium reacts with C and Si to form TiC layer and a Ti Si compound, respectively Furthermore, the evaluation of shear strength of the joints was also conducted at ambient and elevated temperatures in air using the single-lap offset (SLO) shear test The perpendicular-type SA-Tyrannohex joints exhibited apparent shear strengths of about 42 MPa and 25 MPa at 650 1C and 750 1C, respectively The fracture at the higher temperature occurred at the interface between the reactionformed TiC layer and braze This might be caused by generation of stress intensity when a shear stress was applied, according to m-FEA simulation results

Microstructure transformation and mechanical properties of TiC–TiB2 ceramics prepared by combustion synthesis in high gravity field

Abstract: By conducting combustion synthesis in high-gravity field, the solidified TiC–TiB2 with a series of TiB2 mole content were prepared through adjusting the mole ratio of C and B elements in combustion system. XRD, FESEM and EDS results showed that with increasing TiB2 mole content, the matrix of TiC–TiB2 composite ceramics transformed a number of fine TiB2 platelets from the TiC spherical grains, and fine-grained even ultrafine-grained microstructures were achieved in solidified TiC–50%TiB2 due to the coupled quasi-eutectic growth under rapid solidification of the ceramic. Properties showed that relative density, Vickers hardness and flexural strength of TiC–50%TiB2 simultaneously reached the maximum values of 98.7%, 21.5 ± 1.5 GPa and 860 ± 35 MPa, whereas the maximum fracture toughness of 13.5 ± 1.5 MPa m0.5 was achieved in TiC–66.7%TiB2. FESEM fractography analyses of the solidified ceramics indicated that the highest flexural strength achieved in TiC–50%TiB2 benefits from not only the lowest content of Al2O3 inclusions and Cr–W–Ti borides, but also the achievements of fine-grained microstructure in the near-full-density ceramic and high fracture toughness contributed from a intensive coupled mechanism of crack deflection, crack-bridging and pull-out by a large number of fine TiB2 platelets.

Fatigue strength and crack initiation mechanism of very-high-cycle fatigue for low alloy steels

Abstract: The fatigue strength and crack initiation mechanisms of very-high-cycle fatigue (VHCF) for two low alloy steels were investigated. Rotary bending tests at 52.5 Hz with hour-glass type specimens were carried out to obtain the fatigue propensity of the test steels, for which the failure occurred up to the VHCF regime of 108 cycles with the S-N curves of stepwise tendency. Fractography observations show that the crack initiation of VHCF is at subsurface inclusion with “fish-eye” pattern. The fish-eye is of equiaxed shape and tends to tangent the specimen surface. The size of the fish-eye becomes large with the increasing depth of related inclusion from the surface. The fish-eye crack grows faster outward to the specimen surface than inward. The values of the stress intensity factor (K I ) at different regions of fracture surface were calculated, indicating that the K I value of fish-eye crack is close to the value of relevant fatigue threshold (ΔK th ). A new parameter was proposed to interpret the competition mechanism of fatigue crack initiation at the specimen surface or at the subsurface. The simulation results indicate that large inclusion size, small grain size, and high strength of material will promote fatigue crack initiation at the specimen subsurface, which are in agreement with experimental observations.

Influence of Aging and Thermomechanical Treatments on the Mechanical Properties of a Nanocluster-Strengthened Ferritic Steel

Abstract: This study investigated the effect of aging and thermomechanical treatments on the mechanical properties of a nanocluster-strengthened ferritic steel, Fe-1.5Mn-2.5Cu-4.0Ni-1.0Al (wt pct). The effect of thermomechanical treatments on the microhardness and tensile properties were measured at room temperature and correlated with microstructural features. Cu-rich precipitates were characterized by transmission electron microscopy and were found to coarsen slowly during long-time aging. The microhardness measurements indicate a typical precipitation hardening behavior during aging at 773 K (500 °C). Tensile tests showed that thermomechanical treatments can improve the mechanical strength and ductility of the nanocluster-strengthened ferritic steel significantly compared with those without the treatments. Fractography results indicated that the high yield strength resulted from precipitation hardening makes the steel more susceptible to grain-boundary decohesion, which can be suppressed by grain refinement. Atmosphere adsorption and diffusion along grain boundaries were found to intensify brittle intergranular fracture, and this embrittlement can be avoided by vacuum heat treatment.

Fractographic analysis of tensile failures of aerospace grade composites

Abstract: This paper describes fractographic features observed in aerospace composites failed under tensile loads. Unidirectional Carbon Fibre Reinforced Plastic (UD CFRP) and Unidirectional Glass Fibre Reinforced Plastic (UD GFRP) composite specimens were fabricated and tested in tension. The morphology of fractured surfaces was studied at various locations to identify failure mechanism and characteristic fractographic features. CFRP composites displayed transverse crack propagation and the fracture surface showed three distinct regions, viz., crack origin, propagation and final failure. Significant variations in the fractographic features were noticed in crack propagation and final failure regions. Crack propagation region exhibited brittle fracture with chevron lines emanating from the crack origin. The entire crack propagation region exhibited radial marks on the individual fibre broken ends. On the other hand, the final fracture region revealed longitudinal matrix splitting and radial marks in majority of locations, and chop marks at some locations. The change in fracture mode in the final fracture was attributed to superimposition of bending loads. GFRP composites exhibited broom like fracture with extensive longitudinal splitting with radial marks present on individual fibre broken ends. Transverse fracture was observed at a few locations. These fracture features were analyzed and correlated with the loading conditions.

Effect of microstructure on the tensile property of porous Ti produced by powder metallurgy technique

Abstract: Porous Ti with the porosity of 50% and the average pore size of 200 μm was prepared by powder metallurgy technique using polymethyl methacrylate (PMMA) as space holder. The microstructure, tensile deformation and final fracture behavior were investigated. Porous Ti reveals two kinds of pores: open interpenetrated macro-pores in the range of 30–260 μm and micro-pores with the average size of 9 ± 2 μm. The stepped surface of macro-pores consists of polygons with angles of 30°, 60° or 90°. The ultimate tensile strength of porous Ti is 81 MPa. Porous Ti fails with the formation of shear bands at 45° to the tensile axis. The cracking of the struts on porous Ti is controlled primarily by the macro-pores. Fractography shows evidence of the brittle cleavage fracture in porous Ti. The failure mechanism has been discussed by taking the intrinsic microstructural features into consideration.

Influence of shot peening on the fatigue life of Cu–Ni austempered ductile iron

Abstract: Shot peening (SP) was carried out in order to improve the fatigue life of an austempered ductile iron alloyed with copper and nickel austenitised at 900 °C for 2 h and austempered at 360 °C for 1 h. The optimal peening intensity using S330 steel shot, was found to be 0.38 mmA producing residual compressive stresses with a maximum of approximately 1000 MPa at the surface. The microhardness at the surface was increased by SP from 370 to 535 HV. The fully reversed fatigue tests of SP-specimens showed a 61% increase in fatigue life over that of polished samples. The improvement in fatigue life is explained by the compressive residual stresses in the near surface shifting crack initiation to sub-surface. Fractography analysis showed multiple crack nucleation sites at the surface for the polished specimens, while SP-specimens showed sub-surface crack nucleation. In addition, a mixed fracture mode was also identified, with the graphite nodules and copper islands affecting crack propagation.

Glass Thickness and Fragmentation Behavior in Stressed Glasses

Abstract: Breakage patterns, residual stress, and fractured surfaces on tempered glasses are investigated to find the correlation among glass thickness, tempered level, and the number of fragments, particularly when the glass thickness is less than 4 mm. Relatively thin glasses require high compressive stress for producing fragments, and the required compressive stress is increased with decreasing glass thickness (3.2 to 2.1 mm). By analyzing the residual stress of glasses before and after the fragmentation test, we observe that a relatively thin glass spends more stored energy to generate a new fracture surface and stores less energy for the second cracking as compared to thick glasses. Fractography shows that all glasses have a similar characterization on the fractured surface irrespective of glass thickness. However, the only dif- ference is the depth of the compressive layer. By reducing the depth of the compressive layer to less than approx. 20% of the glass thickness, it is observed that the possibility of producing small fragments is dramatically decreased. There- fore, this study confirms that the compressive stress and its depth are essential as key factors contributing to the achievement of a relatively high fragmentation using a thin glass.

A new method for soldering particle-reinforced aluminum metal matrix composites

Abstract: Soldering of aluminum metal matrix composites (Al–SiC) to other structural materials, or even to themselves, has proved unsuccessful mainly due to the poor wetting of these composites by conventional soldering alloys. This paper reports a new approach, which improves the wetting properties of these composites by molting solder alloys to promote stronger bonds. The new approach relies on nickel-plating of the composite's faying surface prior to application of a solder alloy. Based on this approach, an aluminum metal matrix composite containing 55 vol.% SiC particles is successfully soldered to a Fe–Ni–Co alloy (commercially known as Kovar 4J29). The solder material is a zinc-based alloy (Zn–Cd–Ag–Cu) with a melting point of about 400 °C. Microscopic examinations of the aluminum metal matrix composites (Al-MMCs)–Kovar interfaces show that the nickel-plating, prior to soldering, could noticeably enhance the reaction between the molten solder and composites. The fractography of the shear-tested samples revealed that fracture occurs within the composite (i.e. cohesive failure), indicating a good adhesion between the solder alloy and the Al–SiC composite.

Micro- and macro-mechanical testing of transparent MgAl2O4 spinel

Abstract: Advanced transparent ceramics with high chemical and thermal stability are gaining increasing interest as replacement of glass-based materials in technical window applications. The mechanical reliability and performance of transparent MgAl2O4 with a grain size of 5 μm has been characterized at ambient temperature using micro-mechanical indentation and macroscopic bending tests. The measurements focused on elastic modulus, fracture toughness, crack kinetics, and strength, the latter analyzed with Weibull statistics. The effect of slow crack growth is assessed using a strength–probability–time plot. Complementary fractography by optical, confocal and scanning electron microscopy provided a correlation between failure origin and fracture stress. The results and reliability aspects are discussed in terms of linear elastic fracture mechanics.