Showing papers on "Fractography published in 2005"

The effect of a simple annealing heat treatement on the mechanical properties of cold-sprayed aluminium.

Abstract: Cold spray, a new member of the thermal spray process family, can be used to prepare dense, thick metal coatings. It has tremendous potential as a spray-forming process. However, it is well known that significant cold work occurs during the cold spray deposition process. This cold work results in hard coatings but relatively brittle bulk deposits. This work investigates the mechanical properties of cold-sprayed aluminum and the effect of annealing on those properties. Cold spray coatings approximately 1 cm thick were prepared using three different feedstock powders: Valimet H-10: Valimet H-20: and Brodmann Flomaster. ASTM E8 tensile specimens were machined from these coatings and tested using standard tensile testing procedures. Each material was tested in two conditions: as-sprayed; and after a 300 C, 22h air anneal. The as-sprayed material showed high ultimate strength and low ductility, with <1% elongation. The annealed samples showed a reduction in ultimate strength but a dramatic increase in ductility, with up to 10% elongation. The annealed samples exhibited mechanical properties that were similar to those of wrought 1100 H14 aluminum. Microstructural examination and fractography clearly showed a change in fracture mechanism between the as-sprayed and annealed materials. These results indicate good potential for cold spraymore » as a bulkforming process.« less

Processing and mechanical behavior of laminated titanium–titanium tri-aluminide (Ti–Al3Ti) composites

Abstract: Ti–Al3Ti laminated composites have been fabricated through reactive sintering in vacuum using Ti and Al foils with different initial thickness. The aluminum layer was completely consumed resulting in microstructures of well-bonded metal–intermetallic layered composites with Ti residual metal layers alternating with the aluminide intermetallic layers. The MIL composites exhibit a very high degree of microstructural design and control. Microstructure characterization by scanning electron microscopy (SEM), X-ray diffractometry (XRD) and energy dispersive spectroscopy (EDX) has shown that Al3Ti is the only titanium aluminide phase due to the thermodynamics and phase selection of the reaction between Ti and Al through mass diffusion in the presence of liquid Al. The mechanical properties and fracture behavior of the fabricated laminated composites were examined through three-point bending test. The results indicated that the composites exhibited anisotropic features. When the load perpendicular to the laminates was applied, they displayed a step-like or saw-tooth load–displacement response and superior flexural strength as well as fracture toughness, which is also dependent on the number and thickness of individual layers. A non-catastropic fracture was observed in the laminated composites due to the deflection of cracks along the Ti/Al3Ti interface. The Ti layer failed by cleavage mode, showing extensive plastic deformation during the bending process.

Slow strain rate corrosion and fracture characteristics of X-52 and X-70 pipeline steels

Abstract: The susceptibility to stress corrosion cracking (SCC) in a NACE solution saturated with H2S, of the X-52 and X-70 steels was studied using slow strain rate tests (SSRT) and electrochemical evaluations. SCC tests were performed in samples which include the longitudinal weld bead of the pipeline steels and were carried out in the NACE solution at both room temperature and 50 °C. After failure, the fracture surfaces were observed in a scanning electron microscope (SEM) and the chemical analysis were obtained using X-rays energy dispersive (EDXs) techniques. The specimens tested in air, exhibited a ductile type of failure, and whereas, those tested in the corrosive solution showed a brittle fracture. Specimens tested in the NACE solution saturated with H2S presented high susceptibility to SCC. Corrosion was found to be an important factor in the initiation of some cracks. In addition, the effect of the temperature on the corrosion attack was explored. The susceptibility to SCC was manifested as a decrease in the mechanical properties. Potentiodynamic polarization curves and hydrogen permeation measurements were made. The diffusion of atomic hydrogen was related to this fracture forms. The hydrogen permeation flux increased with the increasing of temperature.

Analysis of variability in tensile ductility of a semi-solid metal cast A356 Al-alloy

Abstract: Variability in the ductility of tensile test specimens of semi solid process cast A356 Al-alloy (Al-7 wt.% Si-0.5 wt.% Mg-base alloy) is examined. The variability in the ductility does not correlate to the global average microstructural parameters such as dendrite cell size, Si particle size, and amount of porosity in the three-dimensional microstructure. Tensile fracture surfaces contain micro-defects that are essentially residues of modifiers, fluxes, grain refiners, and mold release agents: the energy dispersive analysis shows presence of elements such as O, Na, K, C, Cl, Ca, Fe, Ti, and S in these defects. The fracture path preferentially goes through the regions containing the defects. Scanning electron microscopy, quantitative fractography was used to estimate the total area fraction of such defects in the fracture surfaces of the tensile test specimens. The percent tensile ductility e shows strong quantitative correlation with the area fraction of the defects f in the corresponding fracture surfaces, which can be represented by the following simple equation. e = e 0 [ 1 − f ] n In this equation, e0 and n are empirical constants. For the present data set, e0 is equal to 11.5% and n is equal to 41.66. Thus, the variability in the ductility can be decreased through a reduction in the amount of processing defects via better process control.

Fractography, mechanical properties, and microstructure of commercial silicon nitride-titanium nitride composites

Abstract: Commercial-grade Si3N4–TiN composites with 0, 10, 20, and 30 wt% TiN content have been characterized. Submicrometer grain-size Si3N4 was reinforced with fine TiN grains. Density, Young's modulus, coefficient of thermal expansion, and fracture toughness increased linearly with TiN content. Increased strength was observed in the Si3N4+20 wt% TiN, and Si3N4+30 wt% TiN composites. Fractography was used to characterize the different types of fracture origins. Improvements in toughness and strength are due to residual stresses in the Si3N4 matrix and the TiN particles. A threefold improvement in dry wear resistance of the Si3N4+30 wt% TiN composite over the Si3N4 matrix was observed.

Microstructure and tensile properties of squeeze cast magnesium alloy AM50

Abstract: High-pressure die cast magnesium alloy AM50 is currently used extensively in large and complex shaped thin-wall automotive components. For further expansion of the alloy usage in automobiles, novelmanufacturing processes need to be developed. In this study, squeeze casting of AM50 alloy with a relatively thick cross section was carried out using a hydraulic press with an applied pressure of 70 MPa. Microstructure and mechanical properties of the squeeze cast AM50 with a cross-section thickness of 10 mm were characterized in comparison with the die cast counterpart. The squeeze cast AM50 alloy exhibits virtually no porosity in the microstructure as evaluated by both optical microscopy and the density measurement technique. The results of tensile testing indicate the improved tensile properties, specifically ultimate tensile strength and elongation, for the squeeze cast samples over the conventional high-pressure die cast parts. The analysis of tensile behavior show that the strain-hardening rate during the plastic deformation of the squeeze cast specimens is constantly higher than that of the die cast specimens. The scanning electron microscopy fractography evidently reveals the ductile fracture features of the squeeze cast alloy AM50.

Fracture of open- and closed-cell metal foams

Abstract: Two closed cell aluminium foams and one open cell nickel-chromium foam were subjected to microstructural characterization, in situ fracture tests and fractography. The failure process of the open cell foam was observed to be rather ductile, while that of the closed cell foams was found to be brittle. The ductility was related to the purity of the nickel chromium alloy, resulting in necking to be the dominant source of energy dissipation during failure. The brittleness of the closed cell foams was related to the presence of precipitates and particles in the cell wall microstructure, limiting the amount of plastic dissipation. The embrittling phases were traced back to the alloy composition, viscosity enhancing additions and foaming agent.

Effect of heat treatment on microstructures and tensile properties of Ni-base superalloy M963

Abstract: The effect of solution treatment (ST) on tensile properties of M963 Ni-base superalloy tested at 800 degrees C has been investigated. The detailed microstructures, fracture surfaces and dislocation structures are examined through energy dispersive X-ray analysis (EDAX), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). With increasing solution treated temperature, the yield strength (YS) and ultimate tensile strength (UTS) increase, however, the elongation decreases. Microstructural observations show that the morphologies of carbide, primary gamma' and re-precipitated gamma' change significantly with increasing solution treated temperature. The main deformation mode is gamma' by-pass when solution treated temperature is lower than 1220 degrees C, and changes to gamma' shearing at 1230 degrees C. The interface of carbide with matrix is the main site of crack initiation and propagation under all testing conditions. (c) 2005 Elsevier B.V. All rights reserved.

Observation of hydrogen effects on fatigue crack growth behaviour in an 18Cr-8Ni austenitic stainless steel

Abstract: The hydrogen effect on crack growth behaviour in a type 304 austenitic stainless steel was investigated and the following results were obtained. The crack growth rate in hydrogen gas is accelerated compared with that in air. In order to clarify the mechanism of the acceleration, the growth behaviours of a crack propagating in a grain and propagating along the boundary to be a fracture facet were investigated. Slip behaviour, opening displacement and fractography showed that the slip-off mechanism in fatigue crack growth is valid even in hydrogen gas. Hydrogen mainly affects slip behaviour such that slip bands concentrate at a crack tip and result in acceleration of the growth rate. The facets are not significantly responsible for the acceleration. The ratio of facets to the entire area is low, and a crack nearly compensates for the temporary acceleration by the facets with subsequent deceleration.

The role of defects in the fracture of an Al–Si–Mg cast alloy

Abstract: A study of the tensile behaviour of test coupons that were extracted from cast plates of D357 aluminium is described. The effects of specimen thickness, heat treatment, structural integrity and hot isostatic pressing (HIP) were investigated. Chevron marks were found on the fracture surfaces of the specimens that failed under plane strain conditions. In most cases, structural defects, such as shrinkage pores and oxide films, were found at the initiation sites for fracture. It is shown that the structural defects that were found at the origin of the chevron patterns are large enough to explain the fractures, and that the tensile failure of cast Al–7Si–Mg alloys can be explained in terms of the principles of fracture mechanics. The influence of structural defects on the elongation to fracture is discussed, together with some implications for quality control programs involving cast Al–7Si–Mg components.

Creep rupture of lead-free Sn-3.5Ag and Sn-3.5Ag-0.5Cu solders

Abstract: The aim of this study is to investigate the creep rupture behavior of lead-free Sn-3.5Ag and Sn-3.5Ag-0.5Cu solders at three temperatures ranging from room temperature (RT) to 90 °C, under a tensile stress range of σ/E=10−4 to 10−3. The ultimate tensile strength (UTS) and creep resistance were found to be decreased with increasing temperature for each given lead-free solder. Both the binary and ternary Ag-containing alloys exhibited superior UTS and creep strength to the conventional Sn-37Pb solder at a similar temperature. Due to a more uniform distribution of eutectic phases and a larger volume fraction of intermetallic compounds (IMCs), the Sn-3.5Ag-0.5Cu alloy had greater UTS and creep strength than did the eutectic Sn-3.5Ag solder at each testing temperature. The stress exponents (n) of minimum strain rate (˙emin) were decreased from 7 and 9 at RT to 5 and 6 at 60 and 90 °C, for the binary and ternary lead-free alloys, respectively. Fractography analyses revealed typical rupture by the nucleation and growth of voids/microcracks at IMCs on the grain boundaries. Both Monkman-Grant and Larson-Miller relationships showed good results in estimating the rupture times under various combinations of applied stress and temperature. A model, using a term of applied stress normalized by Young’s modulus, was proposed to correlate the rupture times at various temperatures and could explain the rupture time data reasonably well for the given two lead-free solders.

Toughening of polycarbonate: Effect of particle size and rubber phase contents of the core-shell impact modifier

Abstract: The toughening behavior of polycarbonate modified with core-shell type particles was investigated. The alloys were found to exhibit maximum impact strength upon addition of a modifier with a poly(butyl acrylate) rubbery core of 0.25 μm diameter. The incorporation of particles with diameter greater than 0.25 μm resulted in decreased impact strength. The influence of rubber phase contents on toughness was also studied. It was observed that the alloys exhibited maximum impact strength upon addition of 4 wt % rubber phase. Further increase in the rubber phase content resulted in reduced impact strength. Fractography of the samples showed that, below 4 wt % rubber phase content, the fracture occurs mainly by internal crazing and, from 4 wt % onward, only by shear deformation. When the effect of dual particle size distribution was analyzed, it was found that there was only a moderate increase in toughness compared with alloys containing monosized particles. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 748–755, 2005

Evolution of Young's Modulus, Strength, and Microstructure during Liquid‐Phase Sintering

Abstract: This comparative study addresses the influence of microstructural evolution on mechanical properties of porous alumina prepared by partial densification with and without addition of a glassy phase. The composition used for liquid-phase sintering is heated above and, in order to freeze the starting defect distribution, below the glass transition point. The ensuing densification rate is compared with measurements of the temperature and time-dependent wetting angle. The strength of the alumina is correlated with microstructural characteristics, Young's modulus, fracture toughness, and crack tip toughness as a function of porosity. Linkage of the crack tip toughness to elastic modulus, in combination with fractography and a description of a linearly shrinking critical defect, yields a predictive capacity to determine strength of porous ceramics.

Shear fracture behavior of Ti3SiC2 induced by compression at temperatures below 1000 °C

Abstract: Polycrystalline titanium silicon carbide (Ti 3 SiC 2 ) samples with grain size of 20–50 μm were synthesized from 2Ti/2Si/3TiC powder at 1300 °C for 60 min through the pulse discharge sintering (PDS) technique. Compressive tests were conducted on the Ti 3 SiC 2 specimens at temperatures up to 930 °C in vacuum. The results showed that the Ti 3 SiC 2 specimens displayed obvious shear fracture behavior and monotonically decreasing fracture strength from 935 MPa (25 °C) to 640 MPa (930 °C). On the macro-scale, the shear fracture surfaces make an angle of 23–31° with respect to the stress axis. In micro-scale, the main deformation and damage modes consist of sliding, buckling and fracture of grains, kinking and intergranular cracking. The macro-shear fracture mechanism of Ti 3 SiC 2 is quite analogous to other brittle materials, such as polycrystalline ice and bulk metallic glasses (BMGs). Based on the results available, the shear fracture mechanism of Ti 3 SiC 2 is discussed and related to the behavior of other brittle materials.

Effect of Precrack “Halos” on Fracture Toughness Determined by the Surface Crack in Flexure Method

Abstract: The surface crack in flexure method, which is used to determine the fracture toughness of dense ceramics, necessitates the measurement of precrack sizes by fractographic examination. Stable crack extension may occur from flaws under ambient, room-temperature conditions, even in the relatively short time under load during fast fracture strength or fracture toughness testing. In this article, fractographic techniques are used to characterize evidence of stable crack extension, a “halo,” around Knoop indentation surface cracks. Optical examination of the fracture surfaces of a high-purity Al2O3, an AlN, a glass-ceramic, and a MgF2 reveal the presence of a halo around the periphery of each precrack. The halo in the AlN is merely an optical effect due to crack reorientation, whereas the halo in the MgF2 is due to indentation-induced residual stresses initiating crack growth. However, for the Al2O3 and the glass-ceramic, environmentally assisted slow crack growth is the cause of the halo. In the latter two materials, this stable crack extension must be included as part of the critical crack size to determine the appropriate fracture toughness.

Analysis and Testing of Mixed-Mode Interlaminar Fracture Behavior of Glass-Cloth∕Epoxy Laminates at Cryogenic Temperatures

Abstract: This paper presents results from an analytical and experimental study of the effect of temperature and mixed-mode ratio on the interlaminar fracture toughness in glass-cloth/ epoxy laminates. Mode I, mode II, and mixed-mode tests were conducted by the double-cantilever beam, end-notched flexure, and mixed-mode bending lest methods at room temperature, liquid nitrogen temperature (77 K), and liquid helium temperature (4 K). A finite element model was used to perform the delamination crack analysis. Mode I, mode II, and mixed-mode energy release rates at the onset of delamination crack propagation were computed using the virtual crack closure technique. The fracture surfaces were examined by scanning electron microscopy to correlate with the interlaminar fracture properties.

Characterizing ceramics and the interfacial adhesion to resin: I - The relationship of microstructure, composition, properties and fractography.

Abstract: The appeal of ceramics as structural dental materials is based on their light weight, high hardness values, chemical inertness, and anticipated unique tribological characteristics. A major goal of current ceramic research and development is to produce tough, strong ceramics that can provide reliable performance in dental applications. Quantifying microstructural parameters is important to develop structure/property relationships. Quantitative microstructural analysis provides an association among the constitution, physical properties, and structural characteristics of materials. Structural reliability of dental ceramics is a major factor in the clinical success of ceramic restorations. Complex stress distributions are present in most practical conditions and strength data alone cannot be directly extrapolated to predict structural performance.

Determination of dynamic delamination toughness of a graphite‐fiber/epoxy composite using Hopkinson pressure bar

Abstract: Modified dynamic three-point-bending and compact shearing test configurations based on Hopkinson pressure bar (HPB) and crack detection gage (CDG) (Vishay Intertechnology, Inc) were used for the determination of the dynamic mode I and mode II delamination-initiation toughness of a unidirectional graphite-fiber/epoxy composite made of P7051S-20Q-1000 prepregs (Toray Composites America). The transient loading history was recorded precisely by the HPB installed with a high-resolution digital oscilloscope, and the crack initiation and delay time were captured using the CDG. By means of dynamic finite-element analysis (FEA) of the impact processes with the loading history and crack initiation time as input, the critical dynamic stress intensity factors (DSIFs) (KIDC/KIIDC) were extracted from numerical results of the crack opening displacements (CODs). Results show that under the present transient loadings, the KIDC value is about 80–90% of the static one, while the KIIDC value is nearly unchanged. Dynamic failure mechanisms of the composite specimens were evaluated by fractography using a scanning electron microscope (SEM). POLYM. COMPOS., 26:165–180, 2005. © 2005 Society of Plastics Engineers

Mixed mode fracture toughness of lead–tin and tin–silver solder joints with nickel-plated substrate

Abstract: This study uses modified compact tension specimens to evaluate the fracture toughness of solder joints under a combination of tensile and shear loads. Copper substrates were coated with electroless nickel plating before they were soldered together. The samples were reflowed to peak temperatures of 215 and 240 °C for Sn–37Pb and Sn–3.5Ag, respectively, with appropriate preheating temperatures, holding time and cooling rates. Fracture toughness tests were conducted as per ASTM E399-90. Cross-sectional microstructural and fractography studies were carried out on the tested samples. Earlier fracture studies which were based on J integral as per ASTM E813-89 did not produce valid results because of brittleness of the solder joints. The current results showed that the fracture of the solder joints follows the general principles of a mixed mode fracture mechanism map and shear is the preferred mode of failure regardless of different mode of loadings, type of solders and substrate coatings.

Forming limit diagram for interstitial free steels: Part I

Abstract: The suitability of interstitial free (IF) steels of thickness 0.9 and 1.2 mm for press forming operations were examined. The microstructural aspects, tensile properties and formability parameters were studied. Forming limit diagrams were evaluated for the above sheet metals and they were compared. Strain distribution profiles were obtained from the forming experiment. The fracture surfaces of the formed samples were viewed using scanning electron microscope (SEM). Using the fractography, the fracture behaviour and formability were analyzed. The tensile properties and formability parameters were correlated with the FLD. From the analysis, it was found that IF steel having 1.2 mm thickness is superior compared to the other one.