Showing papers on "Fractography published in 1996"

Mechanisms of fatigue in short glass fiber reinforced polyamide 6

Abstract: An adaptation to existing failure models for fatigue fracture of short fiber reinforced thermoplastics is presented, based on results using some new experimental methods. These results lead to the following conclusion: Cracks in polyamide remain bridged (by plastically drawn matrix material and/or fibers) until just before final fracture. Important is the conditioning of the polyamide: conditioned to equilibrium water content, this mechanism occurs, but not when it is dry as molded. Fatigue damage measurements were done on thin foils cut from the fatigued specimen. When tensile tested, these foils show a change in both strength and fracture strain after fatigue. Further observations during the experiments and SEM fractography strengthen the conviction that fatigue damage initiates and grows in the form of bridged cracks. A correlation between tensile strength and fatigue strength was found; the degree of fiber alignment has a similar effect on both tensile and fatigue properties.

Reinforcement shape effects on the fracture behavior and ductility of particulate-reinforced 6061-Al matrix composites

Abstract: Particle shape effects on the fracture and ductility of a spherical and an angular particulate-reinforced 6061-Al composite containing 20 pct vol Al2O3 were studied using scanning electron microscopy (SEM) fractography and modeled using the finite element method (FEM). The spherical particulate composite exhibited a slightly lower yield strength and work hardening rate but a considerably higher ductility than the angular counterpart. The SEM fractographic examination showed that during tensile deformation, the spherical composite failed through void nucleation and linking in the matrix near the reinforcement/matrix interface, whereas the angular composite failed through particle fracture and matrix ligament rupture. The FEM results indicate that the distinction between the failure modes for these two composites can be attributed to the differences in the development of internal stresses and strains within the composites due to particle shape.

Loading rate and test temperature effects on fracture of In Situ niobium silicide-niobium composites

Abstract: Arc cast, extruded, and heat-treatedin situ composites of niobium suicide (Nb5Si3) intermetallic with niobium phases (primary—Nbp and secondary—Nbs) exhibited high fracture resistance in comparison to monolithic Nb5Si3. In toughness tests conducted at 298 K and slow applied loading rates, the fracture process proceeded by the microcracking of the Nb5Si3 and plastic deformation of the Nbp and Nbs phases, producing resistance-curve behavior and toughnesses of 28 MPa√m with damage zone lengths less than 500μm. The effects of changes in the Nbp yield strength and fracture behavior on the measured toughnesses were investigated by varying the loading rates during fracture tests at both 77 and 298 K. Quantitative fractography was utilized to completely characterize each fracture surface created at 298 K in order to determine the type of fracture mode (i.e., dimpled, cleavage) exhibited by the Nbp. Specimens tested at either higher loading rates or lower test temperatures consistently exhibited a greater amount of cleavage fracture in the Nbp, while the Nbs, always remained ductile. However, the fracture toughness values determined from experiments spanning six orders of magnitude in loading rate at 298 and 77 K exhibited little variation, even under conditions when the majority of Nbp phases failed by cleavage at 77 K. The changes in fracture mode with increasing loading rate and/or decreasing test temperature and their effects on fracture toughness are rationalized by comparison to existing theoretical models.

Mechanical Properties and Failure Analysis of Alumina‐Glass Dental Composites

Abstract: Strength measurements and fractography were used to investigate the failure of alumina-glass dental composites containing 75 vol% alumina and 25 vol% glass. Alumina compacts were prepared by slip casting and sintering at 1100°C for 2 h. Dense composites were made by infiltrating partially sintered alumina with glass at 1150°C for 8 h. Young's modulus and the hardness of the composites were 270 GPa and 12 GPa, respectively. The mean strength (460 MPa) and fracture toughness (4.0 MPa·m1/2) of the composites were insensitive to the glass thermal expansion coefficient (αglass= 5.9 × 10−6 to 7.8 × 10−6°C−1). Typical flaws were pores and cracklike voids formed by poor particle packing and differential sintering near agglomerates of alumina in the composite. Crack deflection and crack bridging were observed in indentation cracks. Fracture toughness was single-valued because the alumina particle size was small (∼3 μm). Alumina-glass composites are promising new ceramics for dental crown and bridge applications, because their strength and fracture toughness are ∼2 times greater than those of current dental ceramics.

Microstructures relevant to brittle fracture initiation at the heat-affected zone of weldment of a low carbon steel

Abstract: Charpy toughness of the heat-affected zone (HAZ) of weldment of a low carbon steel has been investigated by means of an instrumented Charpy test and fractographic analysis. Microstructures were varied with thermal cycles simulating double-pass welding. The ductile-brittle transition temperature is the most deteriorated at an intermediate second-cycle heating temperature. The origin of the difference in the transition temperatures has been analyzed to exist in the brittle fracture initiation stage. Fractographic examination correlating with microstructural features has revealed that the brittle fracture initiation site is associated with the intersection of bainitic ferrite areas with different orientations rather than the martensite-austenite constituents. The role of the constraint of plastic deformation on the brittle fracture initiation is discussed.

The fracture toughness of niobium-based,in situ composites

Abstract: The fracture resistance of Nb-Cr-Ti alloys orin situ composites of three different compositions, Cr2Nb, and a Nb-10Siin situ composite was studied at ambient temperature. The crack-tip deformation and fracture behaviors were characterized using near-tip measurement techniques and fractographic analyses. The relevant fracture and toughening mechanisms were identified and related to the microstructure. Despite fracture by a combination of cleavage and slip band decohesion, the Nb solid-solution alloy exhibited a resistance-curve behavior with a relatively high toughness and local ductility. The source of toughness was modeled and explained in terms of a cracking process that involved alternate slip band decohesion and cleavage. Thein situ composites, on the other hand, exhibited cleavage fracture but considerably lower toughness with little or no resistance-curve behaviors. The difference in the fracture behavior appears to arise from two factors: (1) the presence of a high constraint in the Nb solid-solution matrix in thein situ composites, and (2) the lack of plastic flow associated with cleavage of the constrained Nb solid-solution matrix.

Ice/metal interfaces: fracture energy and fractography

Abstract: Results are presented of the fracture tests of ice/metal interfaces in an attempt to utilize fracture mechanics to characterize the failure of ice/solid adhesion. The four-point bending delamination specimen was used to measure the fracture energy of ice/aluminium and ice/steel joints at — 15 °C. The interfacial fracture energy was found to be dependent on ice type and formation procedure of the ice/metal composites. Crack growth was in a manner of asymmetrical bursting, and both cohesive and adhesive failure mechanisms were observed. Although the fracture of ice/metal interfaces was brittle in nature, the evidence of dislocation slip in ice crystals, as revealed by etching and replicating, suggests that microplastic deformations occur in the ice component.

Influence of grinding direction on fracture strength of silicon nitride

Abstract: It is generally found that grinding transverse to the tensile stress direction in flexure bars subjected to four-point bending results in a lower strength compared to grinding in the longitudinal direction. In the present study, standard flexure specimens made from a reaction-bonded and a sintered reaction-bonded silicon nitride (RBSN and SRBSN) were surface ground under three different conditions in both longitudinal and transverse directions to assess the effect of grinding direction on strength. Four-point flexure tests were performed on the specimens and Weibull parameters were calculated. The results showed that while the strength was not affected by the grinding condition when grinding was performed in the longitudinal direction, the strength of the samples ground in the transverse direction was reduced as the material removal rate was increased by a factor of 30. This result was confirmed by fractography, which showed that almost all the fracture initiation sites in the longitudinally ground samples were associated with near-surface microstructural features, whereas in the transverse ground samples fracture initiated from damage introduced by grinding. The strength reduction by grinding in the transverse direction was found to be material dependent, and was larger for SRBSN than for RBSN.

Effects of hygrothermal aging on the fracture and failure behavior in short glass fiber-reinforced, toughened poly(butylene terephthalate) composites

Abstract: The fracture response of injection molded short glass fiber (GF) reinforced and rubber-toughened poly(butylene terephthalate) (PBT) composites has been characterized by the fracture toughness (K c ) and energy (G c ), measured on static-loaded compact tension (CT) specimens. The related failure of the composites with 30 wt% GF reinforcement in as-received (AR), hygrothermally aged (HA) and re-dried (RD) states, respectively, was studied by acoustic emission (AE) and fractography. Tougheners were functionalized ethylene/acrylate (EAF), crosslinked acrylate (XAR) and core-shell type (CSR) rubbers, at 20 wt% in the composites. It was shown that both K c and G c decrease with hygrothermal aging at 90°C, and their values cannot be restored by subsequent drying. This is attributed to severe hydrolysis degradation of the PBT matrix. Deterioration in the fracture parameters was affected by the composition of the rubbery toughener : The toughness retention by EAF was superior to the other modifiers. The difference in the failure mode of the GF-PBT composites before and after hygrothermal aging was revealed by viewing the fracture surface of the CT-specimens in scanning electron microscope (SEM). Based on the fractographic results, changes in the AE amplitude envelopes are interpreted and discussed.

The interfacial compounds and SEM fractography of squeeze-cast SiCp/6061 Al composites

Abstract: The effects of the addition of silica binder during preform preparation on the development of interfacial compounds in squeeze-cast SiC p /6061 Al composites (containing silicon carbides with particle sizes of 85, 30 and 14 μm) were examined using energy dispersive spectroscopy (EDS) X-ray mapping, scanning electron microscopy (SEM) fractography and optical microscopy (OM). Various interfacial compounds, Al 2 O 3 , MgO and MgAl 2 O 4 , were observed. The effects of the various interfacial compounds on the SEM fractography and mechanical properties of the composites were investigated and a mechanism was suggested. The micromechanism of crack initiation, crack propagation and final fracture was discussed using SEM fractography and OM observations.

Fracture toughness of eutectic Al-Si casting alloy with different microstructural features

Abstract: The static fracture toughness of a series of eutectic Al-Si casting alloy with different microstructural features has been evaluated. The dominant influence of eutectic silicon in controlling the fracture toughness is thus clarified. The relationship between the fracture toughness and the microstructure was established. Fracture toughness was found to be strongly associated with the size and morphology of silicon particles. The other feature which greatly influences the fracture toughness is the ratio (λ/DE)Si, i.e. the silicon particle spacing divided by the equivalent particle diameter, rather than the silicon particle spacing, λSi. Fracture toughness also correlates well with the void growth parameter, VGP (=σy (λ/DE)Si), proposed by the authors. The results of the present work can be used to develop an understanding of the variation of fracture toughness with the microstructural features of eutectic Al-Si alloys.

Fatigue damage and its localization in particulate metal matrix composites

Abstract: A series of high and low cycle fatigue tests have been conducted on Al 2 O 3 particulate-reinforced 6061 aluminum alloy composite(Al 2 O 3 /6061 Al) Fatigue damage in the composite, as observed by optical and scanning electron microscopes, is primarily in the form of particle debonding, fractured particles and matrix cracks Mesoscale reinforcement defects, such as a clump of large particles, cause damage localization which may lead to short crack initiation and extension The cyclic plastic strain amplitude appears to be an appropriate low cycle fatigue damage parameter as indicated by the hysteresis loop variation of strain-controlled cylindrical specimens

Critical Hydrogen Concentration for the Brittle Fracture of AISI 430 Stainless Steel

Abstract: The introduction of hydrogen to AISI 430 stainless steel by electrochemical charging was studied. Tensile tests were employed to elucidate the mechanical behavior of the alloy responding to the effect of aging after cathodic charging, dynamic charging, and interrupted charging. The fractography was investigated by scanning electron microscopy. It was found that the hydrogen embrittlement of AISI 430 stainless steel was mainly due to the trapping of hydrogen at dislocations, and its fracture type is mainly transgranular with a small fraction of intergranular fracture. Through mathematical modeling, with the diffusivity and surface concentration of hydrogen obtained by permeation measurements, the critical concentration of hydrogen required to change the fracture mode from ductile microvoid coalescence to brittle transgranular cracking was determined to be 1.45 {times} 10{sup {minus}5} mol/cm{sup 3}.

Stress Corrosion Cracking and Life Prediction Evaluation of Austenitic Stainless Steels in Calcium Chloride Solution

Abstract: The stress corrosion cracking (SCC) susceptibility of austenitic stainless steels (SS) in calcium chloride (CaCl2) solutions was studied using a constant-load method Initiation and propagation of stress corrosion cracks were examined using fractography The distribution of cracks was classified A physical cracking was introduced, and creep deformation measurements were performed The steady-state strain rate (E˙SS) obtained from the corrosion elongation curve (elongation-vs-time curve) showed a linear function of time to failure (tf) This implied that E˙SS can be applied as a parameter for prediction of tf Furthermore, E˙SS below which no failure occurs within a laboratory time scale was estimated Based on results obtained, the critical values of stress (σ) below which no SCC occurred were evaluated Based upon creep measurements in a noncorrosive environment, the influence of environment on E˙SS was more than fivefold Cracking characteristics were divided into three categories according to

Hydrogen-plasticity interactions in pearlitic steel: A fractographic and numerical study

Abstract: A fractographic and numerical approach is presented to analyze hydrogen-plasticity interactions in pearlitic steel and to elucidate the main hydrogen transport mechanism in this material under triaxial stress states produced by notches. Fractographic analysis showed that the microdamage produced by the hydrogen was clearly detectable by scanning electron microscopy (SEM), through a specific microscopic topography associated with hydrogen effects (tearing topography surface or TTS). Numerical computations obtained by using an elastic-plastic finite element program gave the progressive spreading of the plastic zone, closely associated with the movement of dislocations. In the majority of cases, the plastic zone (PZ) clearly exceeds the hydrogen affected region (TTS) and has no relation with it. In some tests, however, the hydrogen-induced micro-damage surpasses the only region in which there is dislocation movement, and in this case the net macroscopic transport of hydrogen cannot be attributed to dislocation dragging, but only to a random-walk stress-assisted diffusion. Therefore, in spite of the fact that dislocational transport of hydrogen has been sufficiently demonstrated on a microscopic scale, it might not be a hydrogen embrittlement mechanism per se, detectable with loss of fracture load, on a macroscopic scale.

Notch fracture in γ -titanium aluminides

Abstract: The notch fracture behavior of twoγ-titanium aluminide alloys, having duplex and fully lamellar microstructures, has been investigated as a function of notch geometry and test temperature. The unnotched tensile properties and notch fracture loads are used to perform finite element analysis (FEA) to determine triaxial tensile stresses and effective plastic strains in the vicinity of notch roots. These results, together with fractographic examinations of notch failures, indicate that a crack nucleates in the triaxial tensile field when the effective von Mises stress just exceeds the uniaxial tensile yield stress. The high tensile stress component then propagates the nucleated microcrack to failure with local stress intensity reaching the toughness of the material. Thus, both plasticity and high tensile stress are required to cause notch failure.

Studies on the influence of metallurgical variables on the stress corrosion behavior of AISI 304 stainless steel in sodium chloride solution using the fracture mechanics approach

Abstract: Stress corrosion data on a nuclear grade AISI type 304 stainless steel in a boiling solution of 5M NaCl+ 0.15M Na2SO4+ 3 mL/L HC1 (bp 381 K) for various metallurgical conditions of the steel are presented in this article. The metallurgical conditions used are solution annealing, sensitization, 10 pct cold work, 20 pct cold work, solution annealing + sensitization, 10 pct cold work + sensi-tization, and 20 pct cold work + sensitization. The fracture mechanics approach has been used to obtain quantitative data on the stress corrosion crack growth rates. The stress intensity factor,K 1, andJ integral,J 1, have been used as evaluation parameters. The crack growth rates have been measured using compact tension type samples under both increasing and decreasing stress intensity factors. A crack growth rate of 5 X 10-11 m/s was chosen for the determination of threshold para-meters. Results of the optical microscopic and fractographic examinations are presented. Acoustic signals were recorded during crack growth. Data generated from acoustic emissions, activation energy measurements, and fractographic features indicate hydrogen embrittlement as the possible mechanism of cracking.

Reliability Analysis of Uniaxially Ground Brittle Materials

Abstract: The fast fracture strength distribution of uniaxially ground, alpha silicon carbide was investigated as a function of grinding angle relative to the principal stress direction in flexure. Both as-ground and ground/annealed surfaces were investigated. The resulting flexural strength distributions were used to verify reliability models and predict the strength distribution of larger plate specimens tested in biaxial flexure. Complete fractography was done on the specimens. Failures occurred from agglomerates, machining cracks, or hybrid flaws that consisted of a machining crack located at a processing agglomerate. Annealing eliminated failures due to machining damage. Reliability analyses were performed using two and three-parameter Weibull and Batdorf methodologies. The Weibull size effect was demonstrated for machining flaws. Mixed mode reliability models reasonably predicted the strength distributions of uniaxial flexure and biaxial plate specimens.

Effects of alkali-metal impurities on fracture toughness of 2090 Al-Li-Cu extrusions

Abstract: The effects of alkali-metal impurity (AMI) content, temperature, and crack-mouth-opening displacement (CMOD) rate on the fracture toughness of 2090-T8 Al-Li-Cu alloy extrusions were studied, particularly for short-transverse (S-L) orientations. Decreasing AMI content resulted in increasing room-temperature fracture toughness, especially for underaged S-L and T-L specimens. Unlike most Al-Li based alloys, material with very low (<2 wt. ppm) AMIs produced by vacuum refining had a high S-L fracture toughness (up to 38 MPa√m for proof strengths ∼440 MPa) as well as high toughness in other orientations. The increase in room-temperature fracture toughness with decreasing AMI content was associated with a decrease in the proportion of brittle intergranular and cleavagelike islands, and a corresponding increase in the proportion of high energy dimpled fracture modes, on fracture surfaces. Both the present and previous studies indicate that the brittle islands result from liquid-metal embrittlement due to the presence of discrete sodium-potassium rich liquid phases. For medium to high AMI contents (5 to 37 wt ppm), S-L fracture toughness increased with decreasing temperature due to solidification of these phases and a consequent decrease in the mobility of embrittling atoms. The ability of embrittling atoms to keep up with crack tips also depended on crack velocity so that CMOD rate influenced fracture toughness. The grain structure (degree of recrystallization) appeared to be another important parameter affecting fracture toughness.

Effect of particle size and matrix aging condition on toughness of particle reinforced aluminium based metal matrix composites

Abstract: Room temperature fracture toughness and tensile tests were carried out on metal matrix composites based on the aluminium alloy 7075, and also on monolithic 7075. The particulate reinforcements used were SiC in three nominal sizes: 5, 13, and 60 μm. Three aging conditions were studied: peak aged, underaged, and overaged conditions of equivalent matrix micro hardness values.The addition of 5 and 13 μm particles increased the 0·2% proof stress and tensile strength, and reduced the ductility, compared with the monolithic material. Composites containing 60 μm particles had lower 0·2% proof stress and tensile strength, and very low tensile ductility. In all cases the toughness of the composites was lower than that of the unreinforced material. However, in contrast to the tensile ductility, the material containing 60 μm particles was the toughest of the composites. The failure mechanism is believed to be one of particle fracture and/or decohesion at low applied stress intensities, followed by ductile fa...

Solution heat treatment of 319 aluminium alloy containing ~0.5wt% Mg. Part 1- solidification and tensile properties

Abstract: The present work was carried out to establish the appropriate solution heat treatment for 319 aluminium alloy containing ~0.5 wt% Mg. Several solution treatments (single-stage and two-stage) were t...

Cyclic Deformation and Low Cycle Fatigue Behavior in a 6061Al/22vol% SiC Whisker Composite

Abstract: A 6061Al matrix composite reinforced with 22vol% SiC whiskers and the unreinforced matrix alloy in different aged conditions were examined at the ambient temperature. Total strain controlled cyclic deformation under fully-reversed loading and low cycle fatigue properties were measured. The mechanical test results demonstrated that the composite materials in different aged conditions cyclically hardened at the applied strain amplitudes, typically at 0.4 %, 0.6 % and 0.8 %. The underaged composite specimens showed the most pronounced cyclic hardening, while the overaged composite only hardened to a limited amount at the first a few cycles. It has been found that the addition of SiC whiskers into the 6061Al increased the cyclic stress to the largest extent in the underaged condition while to the smallest in the overaged. The low cycle fatigue resistance of the composite to cyclic straining was found to be inferior to that of the unreinforced matrix alloy. When the saturation cyclic stress amplitude is considered, however, the composite material showed superior fatigue strength to the matrix alloy in this low cycle fatigue region. The experimental phenomena are interpreted in terms of the result of the electron microscopy and the fractography of the fatigue failed samples.