Investigation Of Fracture Surfaces Research Articles

Acrylic Nanocomposite Resins for Use in Stereolithography and Structural Light Modulation Based Rapid Prototyping and Rapid Manufacturing Technologies

AbstractA novel family of optically transparent acrylic nanocomposites containing up to 30 wt % silica nanoparticles with an average diameter of 20 nm was developed for application in structural light modulation (SLM) and stereolithography (SL) technologies. The uniform dispersion of nanoparticles affords a significantly improved toughness/stiffness‐balance of the photopolymerized and postcured nanocomposites. It is possible to increase stiffness, as expressed by Young's modulus, from 1290 to 1700 MPa without encountering the embrittlement typical for many other conventional filled polymers. Fracture behaviour is examined by means of fracture mechanics investigation and SEM analyses of fracture surfaces. According to TEM analyses and measurement of optical transmittance remarkable uniform dispersion of silica nanoparticles was achieved. The silica nanoparticle concentrations up to 17 wt % give only marginally higher viscosities and do not affect transmittance, while slightly increasing the exposure times needed in photopolymerization. Moreover, the silica nanoparticles afford materials with reduced shrinkage and improved properties. The green effective ankle splay out (EASO) measured on H‐shaped diagnostic specimens, is significantly reduced for the nanocomposite materials from 1.38 mm for the unfilled material to 0.82 mm for nanocomposites containing 30 wt % nanosilica. The building accuracy is increased significantly with increasing content of silica nanofillers.

Failure analysis of lock-pin in a gas turbine engine

Due to severe working condition in gas turbines, there are many different cases on the failure of turbine components. In this paper, one of the common catastrophic events of hot section is investigated. The failure was occurred on 1 July, 2005, in a domestic power plant at Bushehr port in the south of Iran. A 25 MW gas turbo generator after 2228 start–stops and 52,586 h operation failed abruptly and caused many detrimental results. Formerly similar cases have been reported in different power plants. Visual inspections showed a typical rubbing due to irregular ruptures of buckets and an instantly rotor unbalancing. Fracture surface investigation, and impact test results showed that there was not any problem in the microstructure and mechanical properties of disc. The metallography of blades showed that there is no significant problem in the microstructure due to fatigue, creep or hot corrosion. The preliminary results determined the mechanical specifications of lock-pins were satisfied. However, it was found some problems in metallographic inspection results of lock-pins. Fractography investigation showed typical fatigue fracture surfaces in the neck of failed lock-pins and localized pitting signs near the head of lock-pin, where crack had been nucleated. In addition, there were extensive fretting corrosion effects in the serration areas of disc and blades. It seemed the failure has been started by fatigue fracture in a lock-pin and then the bucket could move out side of disc serrations. As a correlated conclusion the likely causes of catastrophic failure in the hot section, is mainly considered to be a HCF due to blade root/head of lock-pin impact. The cause of such impact may be increases in clearance of disc serrations because of fretting wear and local microscopic fusions between blade root and lock-pins stem, after an extended period in service. In this case, the risk of failure has been clearly exacerbated by improper operation due to destroying of sealing and hot gasses leakage in these areas and finally inadequate design of lock-pins to sustain a severe hot condition in the first stage of turbine section.

Statistical scatter in the fracture toughness and Charpy impact energy of pearlitic steel

The effect of microstructure on the fracture characteristics of high carbon hypo eutectoid steel was studied under conditions of quasistatic and dynamic loading. Experimentally determined sets of fracture toughness and Charpy impact energy values were statistically treated. A relationship was found between fracture toughness and Charpy impact energy. In the very brittle domain, the fracture toughness increases slightly with increasing Charpy impact energy. In the domain where the fracture toughness is higher, the rise in fracture toughness with increasing Charpy impact energy is more pronounced. Detailed SEM examination of fractured compact tension (CT) and Charpy V-notch (CVN) specimens showed that the fracture at ambient temperature occurred almost exclusively by cleavage. There were no visible differences in the morphology of cleavage facets on the fracture surfaces of Charpy and CT specimens. Mechanisms of cleavage initiation were revealed by the fractographical investigation of fracture surfaces. Whereas the fracture surfaces of broken CT specimens exhibit a number of cleavage origins, the fracture surfaces of CVN specimens usually show only one.

Influence of mechanical working on properties of aluminium base short steel fiber reinforced composites

In the present investigation aluminium base short plain steel fiber, copper and nickel coated steel fiber reinforced composites were prepared by vortex method. These were subsequently given deformation upto 40% by hot rolling. Microstructure of deformed sample showed random orientation of fibers. Breakdown of fibers due to the hot rolling was also predominant. Density of composites increased with increase in % of deformation due to reduced porosity. The hardness and strength improvement observed is attributed to decrease in porosity and increase in the number of fibers due to fiber breakage. In case of copper-coated fiber reinforced composite some additional mechanism is observed to operate. After 40% rolling, porosity level of 5-wt% copper coated fiber composites was decreased from 3.9% to 0.7%. Hardness of as cast Al-5FeCu composite was 38.2 BHN, which increased to 45.8 BHN on 40% reduction. UTS (Ultimate tensile strength) of 5-wt% copper coated steel fiber composites increased from 124 MPa to 145 MPa on 40% deformation along with an improvement of percentage of elongation from 9.2 to 11.6. The simultaneous increase in UTS and percentage of elongation indicate that reduction in porosity is the main factor-giving rise to improvement of the properties. Fracture surface investigation showed that cracks are initiated at the fiber matrix interface, propagated through the interface and linked up with other cracks or fiber/matrix interface leading to failure. In case of as cast composites micro-porosity played a vital role in failure which is noted from SEM fractographs.

Pressureless Sintering the Al<sub>2</sub>O<sub>3</sub>/SiC Nanocomposites in Deoxidized Atmosphere

Many significant improvements have been shown in mechanical properties of ceramic nanocomposites with ceramic matrix being dispersed with second sub-micro sized (or nano-sized) phase. This study designed and fabricated Al2O3/SiC nanocomposites with a raw material used for 85% alumina as matrix and SiC grits in submicro-scale as dispersion phase via a controlled pressureless sintering in air. With coarse SiC powder as bed powder, both C and SiC powder making deoxidized atmosphere, 85Al2O3/SiC nanocomposites with different SiC contents were densified by pressureless sintering at low temperature. Fracture surface investigation indicated that dense nanocomposites with small grain size were produced. General material properties and microstructure were measured and characterized. The results showed that composites sintered in deoxidized atmosphere had better mechanical properties than those sintered in flowing argon.

Evaluation of EB-PVD deposited Ni–Cr–Al alloy foil

Ni–Cr–Al alloy foils were deposited at 900 °C from a target of Ni–Cr–Al alloy by EB-PVD, and then annealed at 760 °C in a high vacuum furnace for 16 h. Microstructure of as-deposited foil consisted of a large amount of fine grains with mean size of 90 nm. However, after annealing the grains grew obviously to an average size of 4 μm. In addition, long range chemical ordering of L12 type emerged in the annealed specimens, while no evidence of ordering structure were found in the deposited samples. The as-deposited foil exhibited a strong {111} texture on the evaporation side. Twin substructure was dominated within grains of as-deposited and annealed samples. The mechanical properties of the foils were examined using room tensile tests. The ultimate tensile strength of as-deposited and annealed samples is 780 and 767 MPa, respectively. Investigation of fracture surfaces from samples revealed that brittle fracture took place along boundaries of columnar crystals formed on the substrate side, while the equiaxed dimples were dominant on the evaporation side.

Sintering behavior of Y2O3 doped Bi2O3 ceramics

Influence of sintering temperature and soaking time on densification of Bi2O3 samples doped with 25%mol Y2O3 was investigated by shrinkage and relative density measurements. Samples were sintered in air at different temperatures in the range of 800 to 1000?C for 24 hr. The results showed that samples sintered at 950?C have the maximum relative density. Several samples were sintered at 950?C for duration of 0 to 36 hr in order to evaluate the effect of soaking time on densification of samples. It was found that the samples were sintered at 950?C for 36hr had higher relative density than others did for smaller time duration X-ray diffraction (XRD) analyses detected ? -Bi2O3 as the sole stable phase in all samples. Scanning Electron Microscopy (SEM) investigation of fractured surface of the samples showed that porosities decrease by increasing of sintering temperature and grow by further increasing of temperature.

Tough-to-brittle transition in multiwalled carbon nanotube (MWNT)/polycarbonate nanocomposites

Morphology and fracture behaviour of polycarbonate (PC)/multiwalled carbon nanotubes (MWNT) composites have been studied by AFM and essential work of fracture approach (EWF). A maximum in the non-essential work of fracture was observed at 2 wt.% MWNT demonstrating enhanced resistance to crack propagation compared to pure PC. DMA analysis shows an increase in the T g and broadening of the tan δ peaks with the increase in the MWNT content suggesting confinement of the PC chain mobility. Fracture kinetics studies reveal that the crack growth mechanism is broadly dominated by crack propagation rather than crack blunting. Crack resistance curves ( R-curves) demonstrate a transition in the crack growth mechanism (depending on crack extension) from pure crack-tip blunting/crack initiation mode to mixed mode of crack blunting and crack initiation/propagation. Above 2 wt.% MWNT a tough-to-brittle transition has been observed which is also in agreement with the substantially higher (almost 10 times) speed of crack growth without any fracture instability before the failure. The time-resolved in situ strain field analysis revealed that the onset of crack initiation has been shifted to shorter time for nanocomposites with 4 wt.% MWNT than that of 2 wt.% MWNT and thus explaining the existence of a tough-to-brittle transition in these nanocomposites. Fracture surface investigation by SEM shows bridging of the nanotubes across the radial and transverse cracks imparting higher toughness at 2 wt.% of MWNT. The main conclusion of our study is that the resistance against crack initiation ( J 0.5, EWF) and the resistance against crack propagation ( T J, βw p) in these nanocomposites are structurally correlated with the matrix behaviour (state of percolation) and morphology (distance between MWNT and their dispersion) respectively.

Recovery of tensile properties in helium implanted EUROFER97 by post-implantation annealing

Reduced activation martensitic stainless steel EUROFER97 was homogeneously implanted at 250 °C with 0.25 at.% helium, and was subsequently annealed in vacuum for 10 h at 550 °C and 750 °C, respectively. Tensile testing at room temperature and at 250 °C showed increase in strength and decrease of ductility by the implantation. Both changes recovered during annealing: While the strength was reduced by annealing to values between unimplanted and implanted condition, ductility was increased even beyond that of virgin material. This means, the implanted plus annealed material showed improved behaviour even when compared to the virgin material: higher strength and greater ductility. Including the results on fracture surface and microstructural investigations, a tentative model is presented.

Morphology and fracture behaviour of polyethylene/Mg–Al layered double hydroxide (LDH) nanocomposites

Fracture behaviour of polyethylene (PE)/Mg–Al layered double hydroxide (LDH) based nanocomposites has been studied by essential work of fracture (EWF) approach. Transmission electron microscopy (TEM and X-ray diffraction (XRD) analysis have been used to investigate the morphological features of these nanocomposites. A maximum in the non-essential work of fracture was observed at 5 wt.% LDH demonstrating enhanced resistance to crack propagation compared to pure PE. Morphological analyses of the nanocomposites show that the dispersed LDH platelets are partially exfoliated and also forms clusters with polymer chains remaining entrapped within. Rheological analyses show that the typical low-frequency Newtonian flow behaviour, as observed in unfilled polymer, shifts to shear-thinning behaviour with increasing LDH concentration. At 5 wt.% LDH a ductile-to-brittle transition has been observed. Fracture surface investigation by SEM reveals the arresting of the plastic crack growth by the LDH particle clusters, which is more significant at 5 wt.% LDH content. At higher LDH concentrations, the number of such particle clusters increases causing decrease in the average distance between them. As a result large-scale plastic deformation of the matrix at higher LDH concentration is effectively arrested favouring small strain failure and this in turn reaffirms the possible existence of a ductile-to-brittle transition. The study in general reveals that the resistance against crack initiation (essential work of fracture: EWF) and crack propagation (non-essential work of fracture: βw p) in these nanocomposites are structurally correlated with the matrix behaviour and the morphology (state of LDH particle dispersion) respectively.

Failure analysis of styrene reactor tubes

The analysis of failure of the styrene reactor tubes made of stainless steel has been reported where the tubes contained catalyst as well as ethyl benzene in vapour form along with superheated steam at 570 °C. The tubes were exposed to flue gases causing heating of tubes to 790 °C in the upper part where cracking and failure were mostly noticed. The studies included microstructural examinations of cracked and uncracked tubes, fracture surface investigations and estimation of creep rupture strength, etc. The degradation in microstructure such as extensive grain coarsening, sigma phase formation, carbide formation along grain boundaries, etc., led to development of cracks/voids within the grains and along the grain boundaries and also excessive oxidation of tubes. Possible chemical attack by the feed on the inner surface of tubes further deteriorated the tube life. Suggestions for avoidance of failure have been listed.

Mechanical Properties and Morphology of Melt‐mixed PA6/SWNT Composites: Effect of Reactive Coupling

AbstractAn interfacial reaction during melt mixing of maleic anhydride copolymer (SMA) encapsulated single wall carbon nanotubes (SWNT) and polyamide 6 (PA6) was used in order to disperse SWNT homogeneously and to enhance interfacial adhesion. The intended reactive coupling between PA6 and SMA was evident from IR spectroscopy. Nanocomposites with SMA encapsulated SWNT showed increased elongation at break as compared to PA6/SWNT composites. SEM investigation of tensile fractured surfaces of PA6/SWNT+SMA composites indicated enhanced interfacial adhesion between PA6 and SMA modified SWNT.

Amine-terminated poly(ethylene glycol) benzoate (ATPEGB)-modified epoxy: mechanical and thermal properties

Amine-terminated poly(ethylene glycol) benzoate (ATPEGB), synthesized from the esterification reaction of poly(ethylene glycol) (PEG) with 4-amino benzoic acid, was used to modify the toughness of bisphenol-A diglycidyl ether epoxy resin (DGEBA) cured with room temperature curing agent, triethylene tetramine. ATPEGB was characterized by FT-IR and H-NMR spectroscopies, viscosity measurements, solubility parameter calculation and molecular weight determination with gel-permeation chromatography (GPC). The modified epoxy network was evaluated for its impact, adhesive, tensile, flexural and thermal properties. Improvement in mechanical properties depends upon the concentration of the ATPEGB modifier. The optimum properties were obtained at 12.5 phr (parts per hundred parts of resin) concentration of the modifier. The ATPEGB modified cured epoxy was thermally stable up to 315°C. The morphology of cured epoxy was also analyzed by scanning electron microscopy (SEM) investigation of fracture surfaces.

Microwave preheating of anisotropic conductive adhesive films for high-speed flip chip on flex bonding

Anisotropic conductive adhesive film (ACF) can be preheated by microwave (MW) radiation in order to reduce the bonding time for flip-chip technology. Due to sluggish and nonuniform curing kinetics at the beginning of the curing reaction, thermal curing of epoxy is more time consuming. Therefore, MW radiation may be more effective, due to its uniform heating rate during the cycle. In this paper, MW preheating (for 1–4 sec) of ACF prior to final bonding has been applied to determine the electrical and mechanical performance of the bond. Powers of 80 and 240 W MW were used to study the effect of the MW preheating. A final bonding time of 6–7 sec can be used for flip chip on flex bonding instead of 10–15 sec (standard time for flip chip bonding) for MW preheating time and power used in this study. The contact resistance (as low as 0.01) is low in these samples, whereas the standard resistance is 0.017 ohm (bonded at 180°C for 10 sec without prior MW preheating). The shear forces at breakage were satisfactory (0.167–0.183 KN) for the samples bonded for 6–7 sec with MW preheating. This is very close and even higher than the standard sample (0.173 KN). For MW preheating power of 80 W and sweeping time of 2 sec, final bonding at 6 sec can also be used because of its low contact resistance (0.019 ohm). Scanning electron microscope (SEM) investigation of microjoints and fracture surface shows uneven distribution of conductive particles and thick bond lines in samples bonded for 5 sec (with MW preheating). Samples treated with MW radiation (80 W and 2–3 sec time) serve as evidence that well-distributed particles along with thin bond lines cause low contact resistance and high joint strength.

Reactive Compatibilization of Melt Mixed PA6/SWNT Composites: Mechanical Properties and Morphology

AbstractSummary: Styrene maleic anhydride copolymer (SMA) encapsulated single wall carbon nanotubes (SWNT) were melt mixed with polyamide 6 (PA6) utilizing an interfacial reaction in order to disperse SWNT homogeneously and to enhance interfacial adhesion. The intended reactive coupling between PA6 and SMA was evident from IR spectroscopy. Raman spectroscopy was used to characterize the SWNT and the nanocomposites. Studies on crystallization behavior showed the nucleating behavior of SWNT in the composites, as found using DSC and WAXD. The encapsulation of SWNT by SMA copolymer leads to increased elongation at break in the composites, which can be attributed to the enhanced interfacial adhesion between PA6 and SMA modified SWNT as observed from the SEM investigation of tensile fractured surfaces of PA6/SWNT+SMA composites. The glass transition temperature of PA6 increased significantly in the composites.Tensile fractured surface of SMA encapsulated PA6/SWNT composites.magnified imageTensile fractured surface of SMA encapsulated PA6/SWNT composites.

Adhesion between polyurethane coating and EPDM rubber compound

The effect of the bulk modification of the EPDM rubber compound on the adhesion performance with polyurethane coating (PU) was studied. Ethylene propylene diene monomer (EPDM) was bulk modified using maleated EPDM, polynorbornene (PNR) or different curatives to improve adhesion to polyurethane coating. The coating–rubber composite adhesion performance was examined by peel, crack and abrasion tests followed by microscopic investigation of fracture surfaces and their chemical nature. The adhesion between the coating and the rubber substrate was improved using maleated EPDM or PNR, as well as different curing systems. The crack stress for the delamination of the polyurethane coating from the flexible bulk-modified rubber surface follows the trend: maleated EPDM > PNR > different curing systems > control.

An investigation of weld fracture surfaces and mechanical properties of two gamma titanium aluminide alloys

Two gamma titanium aluminide sheet materials were investigated to characterize the fusion zone cracking and mechanical properties that occur for specimens subjected to gas tungsten arc welding (GTAW). The materials used in this study consisted of essentially the same composition; however, one was received in the primary annealed (PA) condition and the other in the designed fully lamellar (DFL) condition. For both materials, all of the specimens welded with a stationary torch (spot welds) cracked catastrophically immediately after welding. The PA specimens exhibited a different mechanism of fracture in the fusion zone compared with the DFL specimens. The fractures that occurred in the PA specimens followed an interdendritic path through regions that did not completely solidify prior to the occurrence of cracking, whereas for the DFL specimens the primary mechanism of fracture was cleavage. For the butt welded specimens, no cracking occurred. For both materials, specimens welded parallel to the rolling direction exhibited more distinct preferred growth of columnar grains than those welded perpendicular to the rolling direction. The weld structure/property relationship is also discussed.

An Investigation of Weld Fracture Surfaces and Mechanical Properties of Two Gamma Titanium Aluminide Alloys

An Investigation of Weld Fracture Surfaces and Mechanical Properties of Two Gamma Titanium Aluminide Alloys

Application of the Master Curve Method and the Engineering Lower Bound Toughness Method to Laser Welded Steel

Abstract The fracture toughness of laser beam welded ferritic steel has been measured in the ductile-to-brittle transition regime using precracked bend specimens. The precracked welded specimens showed a complex fracture behavior, which involves crack path deviation. It was found that the large scatter of the cleavage fracture toughness and its temperature dependence can be characterized by the Master Curve Method (MCM). The results promote the validity of Engineering Lower Bound Method (ELBM) to derive lower bound toughness of laser beam welded steel. To enable a successful application of the Methods, it was necessary to add fracture toughness data censoring to the method, which takes into account the phenomenon of crack path deviation. The application of the censoring criterion requires additional fracture surface investigations, which can be performed easily using a conventional low magnification light microscope.

Resistance of glass fiber-reinforced thermoplastics to water hammer

The resistances of three kinds of short glass fiber or glass bead-reinforced plastics (polyphenyleneether (GFPPE), polyphenylenesulfide (GFPPS), and polyoxymethylene (GBPOM)) to water hammer were studied. The effect of shape of the specimen of these materials on the test results was also studied. The fracture surfaces and the cross sections of the fracture surfaces were observed to investigate the fracture mechanism. PPE showed poor resistance to water hammer compared to PPS and POM. The resistance to water hammer was found to be strongly affected by the shape of the specimen. Investigation of the fracture surfaces showed that their surfaces were rather flat and hardly any glass fibers were observed at the fracture surfaces. It was found that the characteristic fracture surfaces were attributable to the breaking of the glass fibers at the fracture surface which is hardly observed in other modes of fracture, such as tensile fracture.