Showing papers on "Fractography published in 1988"

Crack growth from internal hydrogen—temperature and microstructural effects in 4340 steel

Abstract: Internal hydrogen effects on stage II crack growth rates in AISI 4340 steel have been studied as a function of test temperature. A model is developed that is physically based in that classical thermodynamics relates to solubility and trapping and Fick’s second law controls hydrogen transport. Both of these are microstructurally related to how trapping affects both the crack initiation site and diffusion to it. For two tempered conditions of 4340 steel, it is shown that there is a test temperature,T 0, for stage II crack growth, above which the crack does not grow. The fractography associated with test temperatures approachingT 0 tends toward 100 pct intergranular for both 1340 MPa and 1620 MPa strength levels. At lower test temperatures, there is as much as 50 pct microvoid coalescence or 30 pct quasi-cleavage. In the lower strength condition, hydrogen traps at oxysulfide particles with a binding energy near 75 kJ/mol. Where these intersect the prior austenite grain boundaries, this promotes fingers of intergranular fracture which later triggers tearing of 100 μm size ligaments by microvoid coalescence. For the higher strength material, it is proposed that hydrogen traps along martensite lath intersections with prior austenite grain boundaries, the binding energy being near 27 kJ/mol. This promotes 1 μm size striations along intergranular facets. In both cases the fractography is consistent with a proposed model of stress field concentration of hydrogen, further concentration along trap sites, fracture nucleation at trap sites, and local, discontinuous fracture instabilities.

The influence of hydride size and matrix strength on fracture initiation at hydrides in zirconium alloys

Abstract: The effects of matrix strength (yield stress) on hydride fracture and alloy ductility have been studied as a function of stress state, hydride content, hydride size, and precipitation stress. Uniaxial and triaxial states of stress were investigated by using smooth and notched tensile specimens, respectively, containing 0.18 or 0.90 at. pct H, with the longest hydride platelet dimension varying from 5 to 400 μm. The majority of the hydrides in the specimens had their plate normals oriented parallel to the tensile axis direction. Crack initiation at hydrides was monitored using acoustic emission, finiteelement calculations were employed to determine the stresses and strains in the notched specimens, and metallographic and fractographic analyses were carried out to determine the state of fractured hydrides/voids near and on the fracture surface. These techniques showed that, up to a hydride platelet length of ∼50 to 100 μm and regardless of the stress state, a critical plastic strain, independent of matrix strength, controls the initiation of fracture in hydrides. The amount of plastic strain needed to fracture hydrides decreases as (a) the average hydride length increases and (b) the axiality of stress increases. The equivalent plastic strain to fracture small hydrides is ∼ 1 pct under a triaxial as opposed to ∼5 pct under a uniaxial state of stress. When the average hydride platelet lengths are longer than ∼50 to 100 μm, negligible plastic deformation is required to fracture hydrides. A critical applied stress then is the governing factor in all three materials, ranging from 750 to 850 MPa, depending on the stress state.

Fractography of unfilled and particulate-filled epoxy resins

Abstract: The objective of this work was to analyse and understand the types of fracture surface morphology found in unfilled and particulate-filled epoxy resins in the light of the thermomechanical history of the specimen (loading rate or duration of loading, temperature, strain at break). Short-term tensile tests and long-term creep tests were conducted at four different temperatures. The fracture surface features were analysed using the scanning electron and optical microscopes and, where suitable, an image analyser. In order to correlate these morphologies with certain regimes of crack velocity, fracture mechanics tests were also conducted, varying the crack speed between 10−7 and 102 m sec−1. In the case of the filled resin, the lifetime under static loading is governed by a phase of slow, sub-critical crack growth which is manifested by resin-particle debonding. Thereafter, the crack accelerates and finally may reach terminal velocities depending on the amount of stored elastic energy available at the moment of fracture.

Quantitative fractography: A modern perspective

Abstract: The important aspects of quantitative fractography as a new analytical tool for understanding material fracture are discussed. The special considerations that rise in the quantification problems are examined from the purview of stereology. Two major experimental techniques for obtaining geometrical information about the fracture surface topography are critically evaluated. These methods are based on stereophotogrammetry or vertical sectioning procedures-both of which yield estimates of the true fracture surface area. The profile and surface roughness parameters which are required to transform measurements made on flat SEM fractographs to the true quantities in the fracture surface are introduced. The two roughness parameters are related by a simple parametric equation, permitting the fracture surface area to be calculated from the experimental measurement of the profile roughness parameter. Alternatively, it is shown how the fracture surface area can be obtained from the angular distribution of the profile elements by employing a transform procedure. The concept of “fractals” as it applies to quantitative fractography is introduced. Recently developed relationships which describe the true variation of the profile and surface roughness parameters with the size of the measuring unit are presented. Calculations are made of the mean area and perimeter length of dimples in the fracture surface of a 4340 steel. Three fracture surface configurations are examined: (1) an assumed flat- , (2) an assumed randomly-oriented- , and (3) the actual partially-oriented fracture surface. Significant differences are demonstrated between the true and the assumed situations, illustrating the importance of quantitative methods in fractography. By means of examples, it is shown how the quantitative methods permit detection of subtle changes in the fracture surface topography as influenced by the materials’ microstructure.

The effects of tempering and test temperature on the dynamic fracture initiation behavior of an AISI 4340 VAR steel

Abstract: An investigation was conducted into the effect of tempering, test temperature and loading rate on the fracture initiation behavior of an AISI 4340 VAR steel. The fracture initiation tests were conducted by stress wave loading of a prefatigued circumferential crack in a notched round bar. The geometry provides for plane strain conditions at the fracture site, and the instrumentation gives records of average stress at the fracture site and of crack opening displacement, both as functions of time. A stress intensity rate KI of about 2 × 10 6 Mpa m 1 2 s −1 is attained by this technique, and a comparison is presented with results obtained in static loading with the same geometry. Finally, the dynamic and quasi-static stress-strain behavior in shear was determined by loading a thin-walled tubular specimen in a torsional Kolsky bar. The strain rates were γ = 103s−1 and γ = 10−4s−1. In addition, Charpy specimens of the various tempers were tested. These showed a toughness trough indicative of tempered martensite embrittlement in those specimens tempered at 350°C. However, in the fracture initiation tests with the notched round bar the embrittlement trough was observed only in tests conducted below room temperature. Extensive quantitative fractography was performed on the plane strain fracture specimens to identify the dominant mode of fracture initiation for each temper at the different rates and test temperatures.

An analytical electron microscope investigation of the phase transformations in a simulated heat-affected zone in alloy 800

Abstract: The heat-affected zone (HAZ) hot-cracking behavior of Alloy 800 was investigated with hot ductility (Gleeble) testing during a simulated HAZ thermal cycle. Microstructural analyses were performed by optical metallography, fractography, electron microprobe analysis, and analytical electron microscopy on specimens that were water quenched from selected temperatures during this thermal cycle. Analysis of analytical electron microscopy (AEM) and Auger electron spectroscopy (AES) data suggests that incipient melting of the grain boundaries occurs at temperatures of 1300 °C and above. The HAZ hot-cracking mechanism was consistent with aspects of a constitutional liquation phenomenon involving a nonequilibrium eutectic-type reaction between grain boundary Ti(C, N) and the austenitic matrix. The extent of expected HAZ cracking would be low as the liquation of Ti(C, N) was localized and no lower melting intermetallic solidification products(e.g., Laves) were observed. The mechanistic observations were consistent with classical thermodynamic and solid state diffusion models.

Determining the Critical Strain Energy Release Rate of Plasma-Sprayed Coatings Using a Double-Cantilever-Beam Technique

Abstract: A double-cantilever-beam (DCB) method for determining critical strain energy release rate (GIc) values from plasma-sprayed coatings is described in detail. This approach, involving acoustic emission (AE) methodology, yielded up to 25 results per specimen and was successful in providing cohesive GIc values for plasma-sprayed coatings of Al2O3-2.5 wt% TiO2, Ni-20 wt% Al, as well as two ostensibly identical, 99.5% commercially pure Al2O3 coatings. Adhesive GIc data from Al2O3-40 wt% TiO2 coatings was also obtained. Results showed a GIc dependence on crack length, and a number of possibilities, based on fractography, the AE response of the coatings during testing, and crack velocity measurements, are advanced to explain this occurrence. Differences in GIc values between coatings were found to correlate with differences in powder/coating properties. The DCB method was also used to investigate batch differences and the effect on toughness of sealing an alumina coating. Problems associated with this method of testing are addressed.

A Method for Dynamic Fracture Initiation Testing of Ceramics

Abstract: : An experimental method is described whereby the dynamic fracture initiation toughness of ceramics and ceramic composites can be measured in pure tension or pure torsion at stress intensity factor rates of 100,000 to 1,000,000 MPA sq rt m/s. In this procedure, circumferentially-notched cylindrical rods are subjected to uniaxial cyclic compression at room temperature to introduce a self-arresting, concentric Mode I fatigue pre-crack, following the technique presented by Suresh et al. (1987) and Suresh and Tschegg (1987). Subsequently, dynamic fracture initiation is effected by stress wave loading with a sharp- fronted pulse which subjects the specimen to a dynamic load inducing either Mode I or Mode III fracture. Instrumentation appropriate to the loading mode provides a record of average stress at the fracture site as a function of time. The capability of this method to yield highly reproducible dynamic fracture initiation toughness values for ceramics is demonstrated with the aid of experiments conducted on a polycrystalline aluminum oxide.

Wave and Fracture Phenomena in Impacted Ceramics

Abstract: : Experiments have been performed to investigate the fracture behaviour of pressure wave loaded alumina tiles. The waves were produced by impacting steel projectiles at a speed range between 100 and 1200m/s. The investigation of the damage generation phase was made possible by a newly developed special loading and observation arrangement. The tiles were impacted edge on and observed was one of the large surfaces by a photographic high speed technique in combination with the shadow optical method. The aim was to observe the fracture appearance and, based on this, to understand energy absorbing processes. The results were compared with those of earlier experiments with glass slabs. It was found that alumina behaves quite different than glass. From a fracture mechanics point of view it was most surprising that no distinct terminal crack velocity could be found as was expected and was experienced with glass. Keywords: Alumina; High speed photography; Shadow optical method; Terminal crack velocity; Specific wave energy; Energy absorption mechanism; Crack nucleation; Fractography.

Tensile and fatigue properties of a thermomechanically treated 7475 aluminium alloy

Abstract: High-purity Al-Zn-Mg alloy was thermomechanically treated. The process included solution treatment, pre-ageing, cold-working by rolling and final ageing. Pre-ageing was carried out at 100°C (TAHA1) and room temperature (TAHA2). Experimental results indicated that the TAHA1 process improved the tensile strength significantly while the TAHA2 process improved the fatigue life more substantially. Fatigue crack initiation sites were examined carefully by scanning electron microscopy. A correlation between fatigue crack initiation, fatigue striation, tearing ridge, dimple distribution and fatigue life was observed. The experimental results are discussed in terms of substructure and are also compared with the tensile and fatigue properties of a thermomechanically treated 7075 Al-Zn-Mg alloy which were previously reported by one of the authors.

Fracture Toughness Variations in Alloy 718

Abstract: The effect of product-form variations within a single heat on the J{sub Ic} fracture toughness behavior of Alloy 718 was examined at 24, 427 and 538{degree}C using the multiple-specimen J{sub R}-curve method. Three product forms (plate, round bar and upset forging) were tested in both the conventional and modified heat-treatment (CHT and MHT) conditions. In CHT material, the fracture toughness response was different for the three product forms -- plate having the highest toughness, bar the lowest. The MHT was found to improve the overall fracture resistance for each product form. In this condition, plate and forging had very similar toughness values, but J{sub Ic} levels for the bar were considerably lower. These results and WHC data previously reported for four other Alloy 718 heats were unalloyed statistically to establish minimum-expected J{sub Ic} values based on tolerance limits bracketing 90% of a total population at a 95% confidence level. Metallographic and fractographic examinations of the seven material lots were performed to relate key microstructural features and operative fracture mechanisms to macroscopic properties. Generally, coarse {delta} precipitates controlled fracture properties in CHT material by initiating secondary dimples that pre-empted growth of the primary dimples nucleated by broken carbide inclusions. The MHTmore » dissolved the coarse {delta} particles and thereby suppressed secondary microvoid coalescence. This generally enhanced the fracture resistance of Alloy 718, except when alternate secondary fracture mechanism, such as channel fracture and dimple rupture at {delta}-phase remnants, prematurely interrupted primary microvoid growth. 25 refs., 12 figs., 12 tabs.« less

Investigation by fractography of the high-temperature oxidation of the 20%Cr-25%Ni-Nb stainless steel

Abstract: Simple procedures have been developed for the preparation, by fracture, of transverse corroded metal sections for subsequent examination of the microstructure and microchemistry of the corrosion attack by scanning electron microscopy. Examination of 20%Cr-25%Ni-Nb (20/25/Nb) stainless steel following 120-hr oxidation in carbon dioxide at 1000°C confirmed that the essentially three-dimensional picture revealed by fracture provided additional microstructural information not apparent from optical microscopy of standard metallographically mounted transverse sections. The size and morphology of the oxide grains constituting the various layers of the uniform and the pitting types of external scale were established. Nearest to the underlying steel, a band of the inner Cr2O3 layer contained a high but variable silicon concentration as well as substantial voidage. Beneath the scale, silica intrusions dramatically influenced the mechanical properties of the 20/25/Nb stainless steel to the depth of this intergranular attack, as fracture at 78 K occurred by a brittle mode. Steel unaffected by oxidation, however, continued to remain ductile.

Corrosion fatigue crack arrest in aluminium alloys

Abstract: The economic life of airframes is largely determined by widespread initiation and growth of fatigue cracks at low stress intensities. Any factor reducing the low stress intensity fatigue damage rate could, if controllable, increase the structural durability. One such factor is corrosion fatigue crack arrest. Corrosion product build-up within a crack can lead to retardation or arrest of fatigue cracks that would readily grow in air. Corrosion fatigue crack retardation and arrest in two high strength aluminium alloys was investigated. Results indicate that two mutually competitive processes, hydrogen embrittlement and crack tip blocking by corrosion products, have opposing effects on low stress intensity fatigue. Crack blocking is dominant at the lowest stress intensities. Prospects for enhancing corrosion fatigue crack arrest, and hence the durability of aircraft structures, are reasonable. It is suggested that multifunctional inhibitors could be added to the environment and metal surface treatments to promote corrosion product build-up and prevent hydrogen entry at the crack tip. This paper has been prepared for the ASTM Symposium on Evaluation and Techniques in Fractography, Atlanta, November 1988

Fracture Toughness of Stainless Steel Welds

Abstract: The effects of temperature, composition, and weld-process variations on the fracture toughness behavior for Types 308 and 16-8-2 stainless steel (SS) welds were examined using the multiple-specimen J-resistance-curve procedure. Fracture characteristics were found to be dependent on temperature and weld process, but not on filler material. Gas-tungsten-arc (GTA) welds exhibited the highest fracture toughness, a shielded-metal-arc (SMA) weld exhibited an intermediate toughness, and submerged-arc (SA) welds yielded the lowest toughness. Minimum expected fracture properties were defined from lower bound fracture toughness and tearing modulus values generated here and in previous studies. Fractographic examination revealed that microvoid coalescence was the operative fracture mechanism for all welds. Second-phase particles of manganese silicide were found to be detrimental to ductile fracture behavior because they separated from the matrix during the initial stages of plastic straining. In SA welds, the high density of inclusions resulting from silicon pickup from the flux promoted premature dimple rupture. The weld produced by the SMA process contained substantially less manganese silicide, while GTA welds contained no silicide inclusions. Delta ferrite particles, present in all welds, were substantially more resistant to local failure than the silicide phase. In welds containing little or no manganese silicide, delta ferrite particles initiated microvoid coalescence, but only after extensive plastic straining.

Failure modes and fractographic study of glass−epoxy composite under dynamic compression

Abstract: Unidirectional glass-epoxy composite has been tested under dynamic compressive loading conditions to study the different modes of failure and characterize them fractographically. Specimens of six fibre orientationsθ = 0, 10, 30, 45, 60 and 90° with respect to the loading axis were loaded on Kolsky bars at an average strain rate of 265 sec−1. The failure occurs on three different types of plane such that the fibre direction is preserved in all cases. Type A planes are tensile split planes and 0° specimens fail only in this mode. 10, 30 and 45° specimens shear on Type B planes by the combined action of normal and shear stresses. 60° and 90° specimens also fail by shear by the combined action of normal and shear stresses but on different types of planes called Type C planes. In these specimens the normal of the failure plane is found to make an angle lying between 55° and 70° with respect to the loading axis. The fractographs indicate intense matrix deformation and breaking up of fibre-matrix bonds for shear failure and comparatively clean fracture surfaces for tensile failure.

Room-temperature forming limit diagram and tensile behaviour up to 200°C of an AI-Ca-Zn superplastic alloy

Abstract: The forming limit diagram and strain distribution under punch stretching at room temperature of an AI-Ca-Zn (superplastic) alloy have been evaluated. Tensile behaviour up to 200° C is reported. The fracture surfaces have been examined by scanning electron microscopy and the results are analysed to support the failure criterion proposed earlier by Marciniaket al.

Effect of 475°C Embrittlement on Fracture Resistance of Cast Duplex Stainless Steel

Abstract: Cast duplex stainless steel (CF3M) which contained 15% ferrite was aged at 500°C for 1000h, and fracture toughness tests were conducted at various loading rates and temperatures. Fracture resistance of the aged material reduced largely compared with that of unaged material. The fracture resistance of the aged material had strain rate and temperature dipendence. Fracture morphology in the aged material also changed by loading rate and test temperature. The role of embrittled frerrite phase on toughness reduction of duplex stainless steel have described qualitatively based on scanning electron microscope observations.

Fatigue cracking of titanium tubes in an industrial heat exchanger

Abstract: The fatigue fracture of commercially pure (cp) titanium tubes at relatively low temperatures [120° C (250° F)] in a shell-and-tube heat exchanger was studied. The cp titanium tubes were subjected to flow-induced vibration and compression axial loading in service. The combined stresses resulted in numerous intergranular cracks in circumferential direction. Some cracks reached a size that resulted in complete rupture of tubes. Fractographic analyses indicated that the fracture was caused by high-cycle fatigue. A large portion of the tube wall indicated a lack of characteristic ductility of cp titanium. The reduced ductility was a result of second-phase particle inclusion and hydrogen absorption in tube material. The secondphase particles provided easy sites for initiation and coalescence of microvoids. Hydrogen absorption of titanium resulted in an increase in hydride content and thus a decrease in ductility.

Analysis of the velocity dependence of fracture toughness in pulsed loading of steel 40Kh

Abstract: 1. The velocity dependence of dynamic fracture toughness can be evaluated by the method of quantitative stereoscopic fractography within a broad range of loading rates. 2. For steel 40Kh the ascending branch of the velocity dependence of KId is determined by fragmentation of the structure causing ramification of the crack which, in the final analysis, increases fracture toughness. The reduced size of the microcrack in fragmentation is directly connected with the weakening of the dependence of crack resistance on the loading rate [19], which in our experiments corresponds to the range of impact speeds of 300–600 m/sec. 3. Removal of microsegregations from the boundaries forming upon fragmentation of blocks in the structure (by means of diffusion) increases the surface energy of such boundaries, impedes the formation of microcracks on them, and consequently increases fracture toughness.

Fractography of the bonding between light-cured resin and tooth substrates.

Abstract: The failure of bonding between tooth substrates and the light-cured bonding resin containing 6-methacryloyloxyethyl naphthalene-1, 2, 6-tricarboxylate anhydride (MENTA-126) was examined by fractography. The dependence of the bond strength on the infiltration of the monomer into dentin was also investigated.Scanning electron microscopic fractographs of failed bonding to treated enamel showed a cohesive fracture in the resin. Fractographs of the bonding to treated dentin suggested a complex failure including an interface fracture between the resin and the dentin and cohesive fractures in the resin and just under the resin-infiltrated dentin layer. The bond strength was slightly dependent on the infiltration of the monomer only when dentins treated with an aqueous solution of 10% citric acid and 3% FeCl3 were employed. In the other cases examined, there was not any significant dependence.

凝固前面の挙動および凝固割れの発生・伝播過程の検討 動的直接観察測定法(miso法)による溶接凝固割れ理象の解明(第1報)

Abstract: A new technique named MISO has been invented by the authors, in which solidification front and/or hot cracking during welding is photographed with camera mounted on optical microscope and analyzed with film analyzer The MISO technique was applied to welding of stainless steels, Inconel alloy 600 and plain carbon steels The technique revealed next phenomena: (1) The apparent solidification range where liquid was clearly observed with cellular dendrite growing was only 20 to 35% of the true solidification range measured as brittleness temperature range (BTR) (2) Initiation of solidification crack in a high strain rate with tensile hot cracking test occurred near the solidification front These were well explained by the calculation of solid fraction vs temperature relation and fractography Furthermore, all these suggested that so-called liquid film stage occurs comparatively near the liquidus temperature, and that the about lower half of BTR is liquid droplet stage where the liquid phases locate discontinuously at interdendritic boundary Based on these, a modification of the "Generalized theory" seems to be necessary in terms of crack susceptibility vs temperature relation in solid-liquid coexistent region