Showing papers on "Fractography published in 2007"

Cyclic Void Growth Model to Assess Ductile Fracture Initiation in Structural Steels due to Ultra Low Cycle Fatigue

Abstract: A new model is proposed to simulate ductile fracture initiation due to large amplitude cyclic straining in structural steels, which is often the governing limit state in steel structures subjected to earthquakes. Termed the cyclic void growth model (CVGM), the proposed technique is an extension to previously published models that simulate ductile fracture caused by void growth and coalescence under monotonic loading. The CVGM aims to capture ultra low cycle fatigue (ductile fracture) behavior, which is characterized by a few (generally, less than 20) reverse loading cycles to large inelastic strain amplitudes (several times the yield strain). The underlying mechanisms of low-cycle fracture involve cyclic void growth, collapse, and distortion, which are distinct from those associated with more conventional fatigue. The CVGM represents these underlying fracture mechanisms through plastic strain and stress triaxiality histories that can be modeled at the material continuum level by finite-element analyses. Development and validation of the CVGM is substantiated by about 100 notched bar tests, with accompanying finite-element analyses, metallurgical tests, and fractographic examinations of seven varieties of structural steels.

Suppression of fatigue crack growth in carbon nanotube composites

Abstract: Fatigue is one of the primary causes for catastrophic failure in structural materials. Here, we report an order-of-magnitude reduction in the fatigue crack propagation rate for an epoxy system with the addition of ∼0.5wt.% of carbon nanotube additives. Using fractography analysis and fracture mechanics modeling, we show that the crack suppression is caused by crack bridging, which results in an effective crack-closing stress due to the pull out of nanotube fibers in the wake of the crack tip. Carbon nanotubes therefore show potential to significantly enhance the reliability and operating life of structural polymers that are susceptible to fatigue failure.

Effects of microporosity on tensile properties of A356 aluminum alloy

Abstract: The effect of microporosity on the tensile properties of A356 alloy was investigated through systematic experimental approaches, with a constitutive prediction that takes into account the strain rate sensitivity and strain-hardening exponent. The strain rate sensitivity was measured through the incremental strain rate change method, and the volumetric porosity and fractographic porosity were obtained from the measurements of bulk density and the quantitative fractography analyses on the fractured surface, respectively. The UTS and elongation exhibit a strong dependence upon the variation in microporosity, with a linear and inverse parabolic relationship, respectively. The constitutive prediction based on the fractographic rather than the volumetric porosity can more accurately predict the overall tensile properties of A356 alloy. The constitutive model should necessarily take into account the strain rate sensitivity and strain-hardening exponent for an exact theoretical prediction of the tensile properties.

Effect of Grain Size on Mechanical Properties of Submicrometer 3Y-TZP: Fracture Strength and Hydrothermal Degradation

Abstract: This paper considers fracture strength, fracture origins, and hydrothermal degradation of 3Y-TZP with grain sizes in the range of 110–480 nm. Biaxial fracture strength testing was used to show that the fracture strength increases with grain size and is governed by the concurrent change in fracture toughness. Hydrothermal degradation was studied by means of fractography, Raman microscopy and its effect on fracture strength. Up to 200 nm grain size, hydrothermal degradation of strength is limited. Larger grain sizes exhibit either premature failure or an increase in strength. A surface transformation zone was found to be responsible for both phenomena.

Strength Distributions in Polycrystalline Silicon MEMS

Abstract: Safe and reliable design of MEMS components requires a statistical description of the material properties that are associated with failure. To this end, a series of microscale tensile tests was performed on polysilicon MEMS structures fabricated using Sandia National Laboratories' SUMMiT Vtrade process. Tensile bars were fabricated from each of the four freestanding polysilicon layers, with gage lengths ranging from 30 to 3750 mum. A two-parameter Weibull distribution appeared to adequately characterize the observed tensile strength distributions. The strength distribution was found to be dependent on the length of the tensile structures, as expected by the Weibull size effect, and unexpectedly strongly dependent on the layer from which the tensile bar was constructed. Specifically, the topmost polysilicon layer in the deposition process (poly4) was more than twice the strength of the bottom freestanding polysilicon layer (poly1). The mechanistic source of this layer-dependent strength appears to originate, at least in part, from process-dependent surface roughness, although other factors such as layer-dependent variations in microstructure, residual stress, and doping are also considered. A fracture mechanics analysis of the strength distributions suggests that the size of the critical flaws is in the vicinity from 50 to 150 nm. Fractography revealed crack origins along the sidewalls, corners, and top surfaces. Weibull strength distributions were also established at elevated temperatures: 200, 400, 600, and 800 degC in air and nitrogen environments. These results revealed the onset of ductility and reduction in strength at elevated temperatures: at 600 degC strength was less than 40% of the room temperature value. Most of the strength was regained if the material was tested at room temperature after a high-temperature exposure. In the discussion, we briefly review concepts for incorporating these observed strength distributions into probabilistic safe design of MEMS components

Shear strength and interfacial microstructure of Sn–Ag–xNi/Cu single shear lap solder joints

Abstract: This study investigates composite lead-free solders fabricated by adding between 0.5 and 3 wt% of Ni particles in situ to Sn–3.5 wt%Ag lead-free solder. The single lap shear strength, fracture behavior and microstructural evolution characteristics of the as-reflowed specimens are examined and compared with those of specimens thermally aged at 150 °C for various aging times. In general, it is found that the single lap shear strength of the joints increases with increasing Ni addition in the as-reflowed condition, but decreases with increasing storage time in the aged specimens. For Ni additions of 0.5 and 1 wt%, the specimens fracture in the solder near the intermetallic compound (IMC) layer/solder interface, which suggests that the solder matrix has a lower strength than the IMC layer. The presence of elongated dimple-like structures on the fracture surfaces of these specimens is indicative of a ductile failure mode. For Ni additions of more than 1 wt%, the specimens fracture with brittle characteristics at the solder/IMC interface, which indicates that an increased Ni addition increases the strength of the solder matrix beyond that of the interfacial layer.

Determination of crystallographic orientation of dwell-fatigue fracture facets in Ti-6242 alloy

Abstract: A technique to determine the crystallographic orientation of the fracture facets has been described. The spatial orientation of the facet plane is determined in a scanning electron microscope (SEM) using a quantitative tilt fractography technique. The crystallographic orientation of the grain, across which a particular fracture facet had been produced, is determined using the electron backscattered diffraction (EBSD) technique in an SEM. These two pieces of information were combined to obtain the crystallographic orientation of the fracture facet normal. This technique was used for the characterization of dwell-fatigue fracture facets at the crack-initiation site in Ti-6242 alloy. Our results indicate that these facets are not exactly aligned with the basal plane, but are inclined at ∼10° to it.

Influence of TiN Inclusions on the Cleavage Fracture Behavior of Low-Carbon Microalloyed Steels

Abstract: Toughness is a major concern for low-carbon microalloyed steels. In this work, the impact fracture behavior of two low-carbon Ti-V microalloyed steels was investigated in order to better understand the role of TiN inclusions in the toughness of the steels. The steels had similar chemical compositions and were manufactured by the same rolling process. However, there was an obvious difference in the ductile brittle transition temperature (DBTT) in the Charpy V-notch (CVN) impact tests of the two steels; one (steel 1) possessing a DBTT below −20 °C, while the DBTT of the other (steel 2) was above 15 °C. Scanning electron microscopy (SEM) fractography revealed that there were TiN inclusions at the cleavage fracture initiation sites on the fracture surfaces of steel 2 at both low and room temperatures. It is shown that the TiN inclusions had nucleated on Al2O3 particles and that they had pre-existing interior flaws. A high density of TiN inclusions was found in steel 2, but there was a much lower density in steel 1. Analysis indicates that these inclusions were responsible for the shift of DBTT to a higher temperature in steel 2. A mechanism is proposed for understanding the effect of the size and density of TiN inclusions on the fracture behavior, and the cleavage fracture initiation process is analyzed in terms of the distribution and development of stresses ahead of the notch tip during fracture at both low and room temperatures.

Effect of microstructural constituents on the thermal fatigue life of A319 aluminum alloy

Abstract: In this investigation, the effect of microstructural constituents on thermal fatigue resistance of A319 aluminum casting alloy was studied. For this purpose, both thermo-mechanical fatigue and thermal-shock fatigue experiments were performed in thermal cycling range between 40 and 250 °C. Following the results, samples with finer SDAS, lower porosity volume fraction and lower brittle intermetallic contents, as well as higher Si modification showed better thermal fatigue resistance. Moreover, T6 and T7 heat treatments appeared to be highly beneficial for thermal fatigue performance. According to the fractography analysis, porosities and coarse intermetallic phases were determined as the most prime locations for crack propagation.

High cycle fatigue behavior of a single crystal superalloy at elevated temperatures

Abstract: Smooth and notched specimens of single crystal (SC) superalloy SRR99 with [0 0 1] orientation were subjected to high-cycle fatigue (HCF) loading at temperatures of 700 °C, 760 °C, 850 °C and 900 °C in ambient atmosphere. The results demonstrate that the fatigue strength of smooth specimens reached the maximum at 760 °C and decreased with increasing temperature. The alloy became more notching sensitive with increase of temperature while the notch sensitivity declined at 900 °C. Analysis on fracture surfaces of both smooth and notched specimen shows that a transition from ductile fracture at lower temperatures to cleavage mode at higher temperatures were observed. Evolution of the microstructure was investigated by SEM and TEM observation. With the process of cyclic plastic deformation at elevated temperatures, the primary cuboidal γ′ precipitates tended to dissolve into the matrix channels, meanwhile a larger number of secondary γ′ particles were formed in the γ matrix. In addition, different types of dislocation structures were developed during the cyclic deformation, which would have a significant impact on the fatigue life of the material.

Dynamic compressive deformation and failure behavior of Zr-based metallic glass reinforced porous tungsten composite

Abstract: The dynamic compressive deformation and fracture behavior of the Zr-based metallic glass reinforced porous tungsten composite were investigated at room temperature by means of the Split Hopkinson Pressure Bar (SHPB). Both fracture stress and fracture strain increased significantly compared to the pure metallic glass phase. The deformation behavior of the composite was found to be dominated by the ductile W phase and the 3D net structure of the W phase. It was found that the composite appeared to exhibit some work hardening during the dynamic compressive deformation. The failure mode of the specimen is a mixture of one major shear band and axial splitting, and the shear plane inclined similar to 56 degrees with respect to the loading axis. Scanning election microscope (SEM) was used to evaluate damage initiation and propagation. It was found that the increase of fracture stress and fracture strain is due to the interaction between localized shear banding and axial splitting, promoting additional fracture surface area, and large volume fraction of ductile W phase. The dynamic compressive deformation and fracture behavior of the composite are discussed by taking the effect of the complex stress state within the composite into account. (c) 2006 Elsevier B.V. All rights reserved.

Relationships between tensile and fracture mechanics properties and fatigue properties of large plastic mould steel blocks

Abstract: Moulds for plastic automotive components such as bumpers and dashboards are usually machined from large pre-hardened steel blocks. Due to their dimensions, the heat treatment produces mixed microstructures, continuously varying with the distance from the quenched surface, at which fracture toughness and fatigue properties are not well known and generally lower than those corresponding to a fully quenched and tempered condition. The response of the mould to defects (for example, microcracks due to improper weld bed deposition) and stresses during service depends on steel properties, that in turn depend upon the heat treatment and the microstructure. A pointwise determination of the tensile, Charpy V-notched, fracture toughness and rotating bending fatigue properties was carried out in a large block. High cycle fatigue was investigated by the stair-case method. The samples were obtained from different depths of the blooms. The relationship between mechanical properties, fracture surfaces morphology and microstructure was also investigated.

Fracture and failure behavior of fabric-reinforced all-poly(propylene) composite (Curv®)

Abstract: The in-plane static fracture of a fabric reinforced all-poly(propylene) (all-PP, Curv®) composite was studied at ambient temperature using the concept of the linear elastic fracture mechanics. The apparent fracture toughness was determined on single-edge notched tensile specimens (SEN-T) considering the maximum load. The related values did not differ much from those determined by the resistance curve (KR) method. The crack growth, requested to construct the KR curves, was traced by the movement in the center of gravity of the cumulative amplitude of the located acoustic emission (AE) events. The quality of consolidation of the all-PP composite was reflected by the force-displacement curve (appearance of pop-in), course of the cumulative AE events during loading, extension and change of the estimated damage zone during fracture. The failure behavior was studied also by fractography and is discussed. Copyright © 2006 John Wiley & Sons, Ltd.

Corrosion–fatigue studies of the Zr-based Vitreloy 105 bulk metallic glass

Abstract: The purpose of this study was to characterize the stress-life behavior of the Vitreloy 105 BMG alloy in the four-point bending configuration in a 0.6 M NaCl electrolyte. At high stress amplitudes, the corrosion-fatigue life was similar to the fatigue lives observed in air. The environment became increasingly detrimental with decreases in stress, and the corrosion-fatigue endurance limit decreased to about 50 MPa, an 88% decrease relative to testing in air. Similar to the tests conducted in air, oxide particles were found on the fracture surfaces but did not appear to significantly affect the corrosion-fatigue lives. However, wear and the resultant corrosion at the outer loading pins resulted in crack initiation in most of the samples. Thus, these results are considered conservative estimates of the corrosion-fatigue behavior of this BMG alloy. Monitoring of the samples and the open-circuit potentials revealed that the onset of significant crack growth occurred at an average of 92% of the total fatigue life. The mechanism of corrosion-fatigue degradation was found to be anodic dissolution.

Effects of loading rate on damage and fracture behavior of TiAl alloys

Abstract: Based on the results of tensile tests and notch 3PB tests with various loading rates and the observation of fracture surfaces, the effects of loading mode and rate on damage and fracture mechanisms of fully lamellar (FL) and duplex phase (DP) TiAl alloys are indicated: (1) For the FL specimen fractured in tensile test, a number of interlamellar cracks occur before final fracture, which is produced by the cracking of the area remained between the existing cracks on a most weakened cross-section. However the DP specimen in tensile test is fractured by the propagation of a crack with a critical length acting as a Griffiths crack in brittle materials. In the 3PB tests of notched specimens the fracture mechanism is different with that in the tensile tests. Crack initiates at the notch root and propagates along a strip around the center line where the normal stress is highest. For FL specimen a more tortured path through low-resistance-interlamellar cracks can be taken at a low loading rate. Because of the low resistance and the rate-dependence of the interlamellar cracking, the loading rate affects significantly the fracture mode. (2) Based on the variation of the fracture mechanisms, the reason why the FL alloy shows inferior tensile properties but matching even superior fracture toughness to the DP alloy is explained further incorporating the effects of loading rate. (3) The tensile strength and fracture toughness show a decreasing trend with lowering loading rate and it is associated with rate-dependent interlamellar cracks involving in the fracture process.

Micromechanisms of cleavage fracture initiation from inclusions in ferritic welds: Part I. Quantification of local fracture behaviour observed in notched testpieces

Abstract: Micromechanisms of cleavage fracture have been investigated in high strength weld metals with two types of microstructure. The local fracture stress was measured using notched bend testpieces, and by combining FEM predictions of local variations of local tensile stress and fractographic observations of initiation sites. The value of σ F ( X 0 ) is significantly higher for weld metals with a lath-like microstructure. Inclusions have been found to be present in 82% of the initiation sites and undoubtedly they were usually responsible for the initiation of catastrophic cleavage fracture in both type of microstructures studied here. Local plasticity appears to be required to initiate cleavage fracture. An analysis of the potential difference between the local yield stress and the mean yield stress (from tensile testing) and the accuracy of predicted size of the plastic zone, suggests that these initiation sites are actually likely to be located inside the plastic zone. For a classical microstructure the value of the effective surface energy, γ p , was deduced to be approximately 9 J m −2 ; and for lath-like microstructures it was found to be approximately 13 J m −2 .