Fracture Initiation Sites Research Articles

Further study on the scattering of the local fracture stress and allied toughness value

In this investigation, the values and their scatters of the local fracture stressof and the fracture load Pf and allied toughness values of base and weld metals were measured, and the main reasons for scattering were analyzed. The results showed that the scatters in σf*, in general, were the smallest. Measuring the fracture initiation sites with a scanning electron microscope and calculating the location of maximum stress and the range covered by the 95 pct maximum stress did not reveal a “characteristic distance” with a statistical feature, over which the local fracture stress must be exceeded by normal stress. Instead, a combined criterion of a critical strain for initiating a crack nucleus and a critical value and triaxiality of stress for its propagation must be satisfied. The present study discovered the phenomenon that the fracture initiation site could locate at the left side (shorter distance side) of the location of the maximum stress. The reasons for this phenomenon and for the minimum crack opening displacement (COD) value not being zero even though a characteristic distance did not exist were proposed.

Fracture prediction in plastic deformation processes

This paper describes the use of the finite-element technique to predict fracture initiation in a range of simple metalforming operations. These cover typical processes and enable deformation and fracture initiation to be examined under several different loading conditions. Three types of metalforming operation are considered, simple upsetting, axisymmetric extrusion, and strip compression and tension. Using a program developed to deal with metalforming analysis, the local stress and strain distributions are determined. These values are then used with previously published continuum fracture criteria to predict fracture initiation sites. The predictions so obtained are subsequently compared with experimental observations. Only one criterion has successfully predicted the fracture initiation site found experimentally in all the operations examined in this paper: that of a critical value of generalized plastic work per unit volume. This criterion was also used to predict the level of experimental deformation at which fracture should occur, but only with partial success.

Prevention by surrounding media (rolling emulsion) of thermal fatigue damage of cylinders from hot rolling mills: Kedim, O.El., Fluzin, P., Clausse, M., Hilaire, M. and Beranger, G. Mem. Etud. Sci. Rev. Metall. Feb. 1989 86, (2), 77–83 (in French)

The objective of this paper is to study the influence of the second peak temperature during real and simulated welding on properties of the subcritically (S), intercritically (IC) and supercritically (SC) reheated coarse grained heat affected (CGHAZ) zones. The X80 high strength pipeline microalloyed steel was subject to processing in a double-pass tandem submerged arc welding process with total heat input of 6.98 kJ/mm and thermal cycles to simulate microstructure of reheated CGHAZ zones. This involved heating to a first peak temperature (TP1) of 1400 °C, then reheating to different second peak temperatures (TP2) of 700, 800 and 900 °C with a constant cooling rate of 3.75 °C/s. Toughness of the simulated reheated CGHAZ regions were assessed using Charpy impact testing at 0 °C, −25 °C and −50 °C. The microstructure of the real and simulated reheated CGHAZ regions was investigated using an optical microscope and field emission scanning electron microscope. Morphology of the martensite/austenite (MA) constituent was obtained by the use of a field emission scanning electron microscope. The blocky and connected MA particles, along prior-austenite grain boundaries, act as a brittle phase for the initiation site of the brittle fracture. Charpy impact results indicated that IC CGHAZ had less absorbed energy with higher transition temperature and hardness. The SC CGHAZ region showed higher absorbed impact energy with lower hardness. Design of multipass weld joints with less IC CGHAZ regions can result in a higher toughness property.

The K- S fatigue test of chains: stochastically censored Weibull treatment : de Crombrugghe, O. Rev. Mec. Tijdschr. June 1988 32, (3–4), 117–130 (in Dutch)

The objective of this paper is to study the influence of the second peak temperature during real and simulated welding on properties of the subcritically (S), intercritically (IC) and supercritically (SC) reheated coarse grained heat affected (CGHAZ) zones. The X80 high strength pipeline microalloyed steel was subject to processing in a double-pass tandem submerged arc welding process with total heat input of 6.98 kJ/mm and thermal cycles to simulate microstructure of reheated CGHAZ zones. This involved heating to a first peak temperature (TP1) of 1400 °C, then reheating to different second peak temperatures (TP2) of 700, 800 and 900 °C with a constant cooling rate of 3.75 °C/s. Toughness of the simulated reheated CGHAZ regions were assessed using Charpy impact testing at 0 °C, −25 °C and −50 °C. The microstructure of the real and simulated reheated CGHAZ regions was investigated using an optical microscope and field emission scanning electron microscope. Morphology of the martensite/austenite (MA) constituent was obtained by the use of a field emission scanning electron microscope. The blocky and connected MA particles, along prior-austenite grain boundaries, act as a brittle phase for the initiation site of the brittle fracture. Charpy impact results indicated that IC CGHAZ had less absorbed energy with higher transition temperature and hardness. The SC CGHAZ region showed higher absorbed impact energy with lower hardness. Design of multipass weld joints with less IC CGHAZ regions can result in a higher toughness property.

溶湯鍛造アルミニウム合金鋳物の室温および４２３Ｋにおける破壊靭性と疲労強度

Fracture toughness, S-N curve and fatigue crack propagation behavior in melt-forged aluminum casting alloys such as AC4C-T6 and AC8A-T6 were investigated at room temperature and 423K, and were compared with these in a commercial 6061-T6 wrought alloy. The static or dynamic fracture toughness, JIC or Jd for AC4C-T6 and AC8A-T6 alloys was lower than that for a 6061-T6 alloy. The S-N curves for AC4C-T6 and AC8A-T6 alloys were situated lower than that for a 6061-T6 alloy. This was because of a pre-existed surface flaw effect by coarse Si particles in the casting alloys. The fatigue crack propagation in AC4C-T6 and AC8A-T6 was unstable in a high stress intensity range, because a large number of fracture-initiation sites such as Si and compounds were distributed in these alloys comparing with a 6061-T6 alloy. On the contrary, the value of ΔKth increased in a lower stress intensity range because Si particles and compounds acted as obstacles against crack propagation.

Development of high toughness in austempered type ductile cast iron and evaluation of its properties

In order to increase the toughness of austempered ductile cast irons, we attempted to strengthen the fracture initiation sites such as graphite-matrix interfaces and eutectic cell boundaries in a way of the microsegregation of alloying elements. For instance, the retained austenite which is stable under external stresses may be introduced preferentially into these sites by the addition of Ni, which segregates to a graphite periphery and of Mn, which partitions mainly to eutectic cell boundaries. Following this concept, the effects of various austempering processes on toughness are also in-vestigated. The cast iron alloying with Ni and Mn shows the best fracture toughness when it is heat-treated by either QB' or B' process; here, the QB' means the oil-quenching from an austenite γ phase range followed by austempering from a ferrite α plus γ range and the B' means austempering from a (α + γ) range. In the newly developed iron, there is a mixed microstructure composed of the ferrite, bainitic ferrite, and austenite. Abnormal elongation due to the TRIP effect in the austenite phase is found to have occurred at about 198 K. Moreover, it is shown that this TRIP effect may be caused by the formation of deformation twins.

Microstructural effects on the cleavage fracture stress of fully pearlitic eutectoid steel

The microstructural parameter(s) controlling the critical cleavage fracture stress, σF, of fully pearlitic eutectoid steel have been investigated. Independent variation of the pearlite interlamellar spacing,Sp, and the prior austenite grain size were accomplished through heat treatment. Critical cleavage fracture stresses were measured on bluntly-notched bend specimens tested over the temperature range -125 °C to 23 °C. The cleavage fracture stress increased with decreasingSp, and was independent of prior austenite grain size. Fine pearlitic microstructures exhibited temperature, strain-rate, and notched-bar geometry independent values for σF, consistent with propagation-controlled cleavage fracture. Coarse pearlitic specimens exhibited temperature-dependent values for σF over a similar temperature range. Inclusion-initiated fractures were generally located at or beyond the location of the peak normal stress in the bend bar, while cracking associated with pearlite colonies was observed to be closer to the notch than the predicted peak stress location. The calculated values for σF were independent of both the type and location of initiation site(e. g., inclusion, pearlite colony). Thus, although inclusions may provide potent fracture initiation sites, their presence or absence does not necessarily change σF in fully pearlitic microstructures.

Statistical distribution of strength of fiber‐reinforced composite materials

AbstractThe Weibull distribution is presented as a physically consistent description of strength distribution in brittle composites. Methods are developed for obtaining point and one‐sided interval estimates for the parameters of the Weibull distribution. Strength data are obtained for 25 replicates of nine different sheet molded and injection molded composites, and the developed statistical methods are applied to these data. Details of the materials and processing are given. Strength results indicate that the reliability calculations using the one‐sided interval estimates are uniformly conservative for all materials. In addition, it is shown that composites produced with a less automated production process show more strength variability. Machining of an injection molded composite surface appears to lower strength and increase strength variability by introducing flaws which act as fracture initiation sites.

Fracture initiation in a lamellar alloy

The possibility of slip bands providing sites for fracture initiation within composites of lamellar construction is investigated. It is proposed that the tips of these bands, which terminate at the composite's bimetallic interfaces, are the specific locations of fracture initiation. Application of a dislocation intensity factor criterion allowed the applied stress at which cracks may be initiated to be calculated. The predictions compare favorably with experimental data. The dependence of the fracture initiation stress calculated from this model is shown to follow a Hall-Petch relationship.

Experiments on the impact bending of continuous and notched steel beams

The results of quasi-static four-point bend tests and dynamic bend tests free-free beams, both continuous and notched, are reported. Angles of bend and rotation are measured as a function of time after impact, where the angle of bend is defined as the angle of deflection of one part of the beam relative to a neighbouring segment of the beam, and the angle of rotation is the angle of deflection relative to a datum line through the initial axis of the beam. The effect of notches is to change the strain profile in the beams, localize plastic formation and provides sites for fracture initiation. The influence of the notch depends on its position and depth; however, the responses of beams in terms of angles of bend and rotation are similar for notched and unnotched cases and are also insensitive to notch depth, being governed primarily by inertia of the rotating sections rather than the strength of the joint. Elastic deformation and wave propagation are shown to be important. Comparisons of angles of bend and rotation as functions of time are made with a rigid-plastic solution and with two-dimensional finite-element computations, both of which slightly underestimate the angle of bend at any time after impact.

ＡｒガスアトマイズされたＮｉ基超合金粉末の特性

The fatigue properties of P/M superalloys can be significantly damaged by defects, which act as sites for fatigue fracture initiation. These defects can be introduced as inclusions either during the melting operation, during powder atomization or during subsequent handling.Equipments for producing cleaner powders by Ar gas atomization have been designed and installed, and the properties of produced MERL76 superalloy powders were examined. As a result, this method was capable of producing fine and clean powders with an average grain size of 56 μm and an oxygen content of less than 100 ppm. Results also indicated that the amount of non-metallic inclusions of produced powders was less than those in the commercialized powders, which was detected by the new water elutriation method.

フライスによる断続切削における超硬工具の初期欠損開始位置について

フライスによる断続切削における超硬工具の初期欠損開始位置について

Effect of inclusions on LCF life of HIP plus heat treated powder metal rené 95

Effects of inclusions on 538 °C (1000 °F) strain control low cycle fatigue life of hot isostatically pressed and heat treated powder metal Rene* 95 compacts were evaluated. Size and location (surface or internal for the test bar) effects along with inclusion types and sources are categorized. Five types of inclusions were identified based on fracture initiation site appearance, although only two major types commonly contribute to significant life low cycle fatigue life degradation. Prior particle boundary decoration reactive type inclusions typically cause the most severe low cycle fatigue life degradation, and those are followed by the discrete ceramic type inclusions. Known potential contaminant seeding study evaluations were used to confirm sources for specific inclusion types. Attempts to minimize the sources for introduction of these contaminants in the argon gas atomization process facilities were only partially successful. An advanced processing approach for the manufacture of Rene 95 to achieve superior low cycle fatigue life has been proposed based on the improved understanding of the inclusion problem.

The Effect of Stress Concentration on the Reliability of Composite Materials

A model is presented for predicting reliability of brittle composite structures with complex multiaxial stress states. This model allows all of the stress components to con tribute to the probability of material fracture. The model is applied to an off-axis geometry and to a center through-hole off-axis geometry using several different material systems. Results for the off-axis geometry demonstrate the importance of using the correct specimen configuration when performing a reliability analysis. Stresses induced by the end-constraint may cause an over prediction of reliability or an underutilization of material strength, depending on the overall stiffness and strength characteristics of the material. Results for the center-notched off-axis geometry demonstrate the effect of strength variability on the localization of fracture prob ability. Low variability materials exhibit a localization of high probability of failure regions, implying a high repeatability of fracture initiation site. High variability materials exhibit a smoothing of the high failure probability regions, which denotes less sensitivity to the stress concentration. Regions which contribute heavily to a loss in structure reliability may be identified before testing, so that a redesign may be effected.

Chemical reaction of boron with liquid aluminum

1. Chemical reaction products have been determined by electron diffraction analysis; at the original fiber surface there is B2O3; at the interphase boundary of an aluminized fiber there is AlB2; α-AlB12, and β-AlB12. 2. By comparing the results of ethcing the aluminum matrix in different reagents it was established that aluminum diboride dissolves in 20% KOH solution, and it is retained on the surface of a boride fiber by using a 20% NaOH solution and a 10% HCl solution. 3. The strength of aluminized fiber depends on the amount of reaction products, since boride phases formed at the interphase boundary reduce boron fiber corss section and may be the site of brittle fracture initiation.

Residual stresses in polymers. II. Their effect on mechanical behavior

AbstractThe distribution of density and tensile properties in quenched modified poly(phenylene oxide) specimens was investigated. Quenching was carried out from temperature level above Tg to below Tg temperatures. Simultaneous to buildup of residual stresses, profiles of density and tensile properties were observed. The profiles were obtained using the layer removal technique, which was found not to affect the measured properties. Quenching of the material results in a steep density gradient in the surface layers. Correspondingly, the tensile modulus increases significantly from the surface to the inner layers and so are also the ultimate tensile properties. This behavior could be accounted for neither by the conventional packing volume approach nor by superposition of internal and external stresses. However, observations of the fracture surfaces are very supportive and indicate that the fracture initiation sites are influenced by the residual stresses. Hence, the mechanical behavior is strongly affected by both density and residual stresses profile. Density is the controlling factor in determining the elastic properties whereas residual stresses determine the ultimate strength and fracture mechanism.

Fractography of Reaction-Sintered Si3N4

Fractography studies were accomplished on two reaction-sintered Si3N4 materials (Norton NC-350 Si3N4 and KBI (Kawecki-Berylco) Si3N4) which had been tested in four-point bending at room temperature, 1200, 1350, and 1500°C in air. At room temperature NC-350 exhibited uniform planar features representing a transgranular fracture mode, uniformly distributed small pores having a size range of 1–2 μm or less, and a few isolated large pores of ∼10 μm. In contrast, the KBI Si3N4 at room temperature exhibited nonuniform, diffuse, and ill-defined fracture features, small pores having a size range of 2–5 μm, and many large pores of ∼50 μm in size. At high temperatures, the fracture appearance of NC-350 Si3N4 was ill defined and diffuse, indicating a change in fracture mode. The fracture originating flaws in NC-350 were primarily the large pores or pore agglomerates. At all temperatures, in the KBI material, it was not possible to observe the mirror and, therefore, the site of fracture initiation. For NC-350, KIC values were 1.2–1.6 MN/m3/2 at room temperature and 2.1–2.4 MN/m3/2 at high temperatures.

Direct prediction of fracture for two-dimensional plane stress structures

This paper presents a procedure for the direct prediction of fracture in two-dimensional structures subjected to plane stress conditions as the result of monotonically increasing loads. The procedure is based on standard finite element methods coupled with incrementally linear element material properties and a zero modulus-unload reload scheme to advance the fracture surface. Comparisons of numerical and experimental data show that the procedure can accurately predict load-deflection curves, load and deflection at fracture, and fracture initiation sites for two-dimensional structures with either mild or severe stress concentrations located anywhere in the structure. Plasticity zones and stable or unstable fracture can also be predicted. The procedure is shown to highly dependent on the mesh density along anticipated crack paths for severe stress concentrations.

Identification of fracture zones in porcelain-veneered-to-metal bond test specimens by ESCA analysis

The ESCA technique provides a useful approach to identifying probable sites of fracture initiation in ceramic-fused-to-metal systems in which the analyzed fracture area is 1 mm2 or larger. The primary disadvantages of this technique include high equipment cost, the need for meticulous removal of superficial oxide layers formed at room temperature, and susceptibility to surface contamination by carbon, nitrogen, and oxygen. From this study the following may be concluded: 1. Judgment of the composition of any fracture surface (except that which occurs exclusively in porcelain) by naked-eye examination or optical microscopic inspection may be erroneous. 2. Generalizations on the nature of adherence zone failures for nonprecious allow-ceramic systems should not be made in the absence of fracture surface or adherence zone composition data determined by means of an acceptable analytical technique. 3. It appears unlikely that fractures originating in one of the regions of the adherence zone will have a composition analysis identical to that of metal, metal oxide, or bonding agent standards, since considerable interdiffusion or chemical interaction between these regions will have altered the original chemical compositions.

Initial crack paths in glass: influence of temperature and composition

Fracture initiation sites in soda-lime glass are similar for fracture at room temperature and at −196° C, where slow crack propagation should be absent. For most samples the measured depths of these sites were greater than the actual depths of the fracture initiating flaws. Micrographs of fracture initiation sites in Pyrex borosilicate glass and a lithium aluminosilicate glass-ceramic are also shown.