Natural Flaws Research Articles

Nondestructive Characterization of Planar Defects Using Laser-Generated Ultrasonic Shear Waves

This paper introduces a laser scanning technique to characterize internal planar defects in a specimen with parallel boundaries. Based on the principles of laser-based ultrasonic shear waves and shadowing, a procedure to determine flaw location, size, and orientation is described. The key feature of this scheme is the use of an optimum wave propagation angle where the maximum shear wave propagates. The feasibility of the approach is evidenced by testing specimens with various controlled and natural internal flaws. The experimental results are promising, in that the flaw characteristics can be determined with good accuracy. It is found that the scheme is especially useful for characterizing transverse-type flaws. The limitations of the technique are also addressed.

Nondestructive Characterization of Planar Defects Using Laser-Generated Ultrasonic Shear Waves

This paper introduces a laser scanning technique to characterize internal planar defects in a specimen with parallel boundaries. Based on the principles of laser-based ultrasonic shear waves and shadowing, a procedure to determine flaw location, size, and orientation is described. The key feature of this scheme is the use of an optimum wave propagation angle where the maximum shear wave propagates. The feasibility of the approach is evidenced by testing specimens with various controlled and natural internal flaws. The experimental results are promising, in that the flaw characteristics can be determined with good accuracy. It is found that the scheme is especially useful for characterizing transverse-type flaws. The limitations of the technique are also addressed.

Fatigue mechanics of WC–Co cemented carbides

The fracture and fatigue behavior of a fine-grained WC–10 wt% Co hardmetal is investigated. Mechanical characterization included flexural strength and fracture toughness as well as fatigue limit and fatigue crack growth (FCG) behavior under monotonic and cyclic loads, respectively. Considering that fatigue lifetime of cemented carbides is given by subcritical crack growth of preexisting defects, a linear elastic fracture mechanics (LEFM) approach is attempted to assess fatigue life–FCG relationships for these materials. Following the experimental finding of an extremely high dependence of FCG rates on the applied stress intensity for the hardmetal studied, the LEFM analysis is concentrated, from a practical design viewpoint, on addressing the fatigue limit–FCG threshold correlation under infinite fatigue life conditions. Thus, fatigue limit associated with natural flaws is estimated from FCG threshold experimentally determined for large cracks under the assumptions that (1) similitude on the FCG behavior of small and large cracks applies for cemented carbides, and (2) critical flaws are the same, in terms of nature, geometry and size, under monotonic and cyclic loading. The reliability of this fatigue mechanics approach is sustained through the excellent agreement observed between estimated and experimentally determined values for the fatigue limit under the different load ratios investigated.

Natural crack recognition using inverse neural model and multi-frequency eddy current method

In this paper a Multi-Frequency Excitation and Spectrogram Eddy Current System and an inverse neural model were used to detect and identify natural flaws in steam generator tubes. It is shown that the applied dynamic neural model of the ECT sensor offers very high speed of operation and guarantees reliability of the recognition results.

Fatigue of casting flaws at a notch root under an SAE service load history

Smooth and notched specimens of a 319 cast aluminium alloy were fatigue tested under a Society of Automotive Engineers service load history in the as‐cast and hipped conditions. The hipping process, which includes subjecting the cast material to a high pressure at high temperature and then slowly cooling down to eliminate internal flaws, decreased the flaw size and improved the fatigue life of cast Al 319 smooth specimens. A 0.6‐mm‐diameter hole was drilled at the notch root of notched specimens to simulate a natural flaw at the notch root. Specimens with two different notch sizes were tested. Circular edge notches reduced the fatigue strength and a 0.6‐mm‐diameter drilled hole at the notch root resulted in a further reduction.The fatigue lives of smooth specimens, notched specimens and notched specimens with a flaw at the notch root subjected to the service load history were predicted using the strain‐life approach, an effective strain‐life approach and a strain‐based intensity factor crack growth model. In crack growth modelling of the fatigue life of smooth cast aluminium specimens the flaw was modelled as a circular edge notch having the same diameter as the flaw. However, in the case of a flaw at a notch root the flaw was modelled as a three‐dimensional cavity subjected to the notch stress field and the crack length was predicted in the longitudinal and transverse directions of the specimen cross‐section. The strain‐life approach was unconservative for all specimen geometries studied. The effective strain‐life approach gave good predictions for smooth and blunt notched specimens but gave very conservative predictions for the specimens with flaws in the notch roots. The crack growth calculations gave accurate predictions for all the specimen geometries.

Crack stability and strength variability in alumina ceramics with rising toughness-curve behavior

Aluminas with four distinct microstructures have been fabricated to investigate the influence of grain size and grain morphology on strength variability. The four microstructures comprise two grain size scales and are characterized as either “equiaxed” with a narrow size distribution or “elongate” with a higher aspect ratio and a broader size distribution. Indentation-strength tests indicate that only the coarse-grain, elongate microstructure exhibits a strong rising toughness-curve ( T-curve or R-curve). Furthermore, in situ measurements demonstrate that the coarse-grain, elongate microstructure is the only one that displays significant stable crack extension from annealed indentation flaws free of contact-induced residual stress. Strength tests on polished specimens indicate that the highest mean strength is achieved in the fine-grain, equiaxed material with little or no T-curve. The lowest strength variability, however, is exhibited by the coarse-grain, elongate alumina and is rationalized in terms of the strong rising T-curve and its associated influence on crack stability. The study suggests that maximum reliability is achieved when the T-curve is sufficiently strong to stabilize the propagation of natural flaws en route to failure.

An experimental study of the effect of a flaw at a notch root on the fatigue life of cast Al 319

Smooth and notched specimens of a 319 cast aluminum alloy containing casting defects were fatigue tested in the as cast and hipped conditions. Hipping is a process in which the material is subjected to a high pressure at high temperature to eliminate flaws and then slowly cooled. The materials were tested under fully reversed constant amplitude loading and under a variable amplitude load history consisting of underloads followed by constant amplitude small cycles. The stress ratio and the number of constant amplitude small cycles following an underload were adjusted so that the crack did not close and remained fully open for all the small cycles. Three notch sizes of 1.0 mm, 3.0 mm and 6.0 mm in diameter, with a natural flaw at the center of the notch root, were examined under constant and variable amplitude loading. In another series of tests the natural flaws in the as cast material were modeled in the hipped material by a notch made with a circular drill at the notch root. The fatigue lives observed showed the following: 1. A notch size effect is observed for notched as cast and hipped Al 319 specimens having a 0.6 mm diameter flaw at notch root under constant and variable amplitude loading. The fatigue limit stress ranges for hipped Al 319 under constant amplitude loading were 117 MPa, 90 MPa, and 80 MPa for 1.0 mm, 3.0 mm, and 6.0 mm diameter edge notches respectively. The corresponding values under variable amplitude loading were 25 MPa, 18 MPa, and 8 MPa. 2. The effect of natural flaws or casting defects, on fatigue strength, in the 319 cast aluminum alloy can be modeled by a drilled hole of the same size. A crack growth analysis based on a fracture mechanics approach was used to model the fatigue behavior of the cast aluminum material under variable amplitude loading. In calculating the fatigue life of the cast aluminum, the flaw was modeled by a spherical cavity having the same diameter as the natural flaw. The size and location of the flaw at the notch root were varied to examine their effect on fatigue life. Fatigue life predictions were in good agreement with the experimental results.

Crack propagation in nitrogen-implanted glassy carbon observed during scratch testing

The purpose of this work was to study the mechanism of crack propagation under friction and wear of glassy carbon (GC) in the virgin state and when implanted with a high dose of nitrogen. The experiments were carried out on a scratch tester with a diamond Vickers indenter and with a steel ball. A sliding velocity of V=0.2 mm s −1, and loads from 0.5 to 4 N, during 50–300 cycles were applied in laboratory air (humidity ∼50%). Mechanisms of crack propagation in brittle materials, such as GC were considered on the atomic-molecular and the macroscale levels. The pre-existing flaws are likely to be responsible for the tear formation on the atomic-molecular scale. Hertzian ring cracks were observed under friction and wear of GC caused with a diamond indenter and with a steel ball when high loads were applied. A characteristic load, P*, of the transition from the inherent natural flaws to the Hertzian ring/cone cracks was estimated. The formation of this debris along the wear track is shown to be due to the extension of lateral vent cracks. Subsequently, larger wear particles are developed.

Failure behavior of alumina and alumina/silicon carbide nanocomposites with natural and artificial flaws

Alumina/silicon carbide nanocomposites with 5 vol.% SiC nanoparticles were produced by slip casting, pressureless sintering, and hot isostatic pressing. The grain size dependence of both the bend strength and fracture toughness have been investigated. The strength exceeds 1 GPa at a grain size of 1.7 μm. Crack opening displacements (COD) were measured, revealing that crack tip toughness is considerably lower than in pure alumina and an R-curve behavior is unlikely to occur. By introducing artificial pores with a size of 60 μm, the micromechanical fracture process has been studied in both pure alumina and nanocomposites. In contrast to alumina, where an annular precrack forms prior to fracture, it is suggested that precrack formation is strongly impeded in the nanocomposites and failure is controlled by microcrack initiation. The high strength of Al2O3/SiC nanocomposites seems to be the result of an unusually high crack initiation stress.

Generation and characterization of longitudinal and transverse cracks in thick weldments of pressure vessel steel

The detection, characterization and eyaluation of flaws that are crack-like, in thick weldments of pressure vessels, are important, Usually this work is performed through non-destructive evaluation (NDE). Volumetric NDE techniques use the standard references to provide an approximate assessment, but specimens with natural flaws can give more accurate reference data in accordance with actural situations. The present research focuses attention on: (i) a methodology to generate definite-sized and oriented cracks; (ii) the effect of new material used to introduce cracks on the microstructure and mechanical property; and (iii) characterization of cracks. 20 and 125 mm thick weldments of carbon-manganese pressure vessel steel (SA-299/ASTM standard) were provided for the investigation. This paper discusses both the use of an innovative technique to generate cracks in weldments and also the crack characterization by NDE.

Complicating A Simple Story: Inchbald's Two Versions of Female Power

Females have been insulated, as it were; and while they have been stripped of the virtues that should clothe humanity, they have been decked with artificial graces that enable them to exercise a shortlived tyranny. Love, in their bosoms, taking place of every nobler passion, their sole ambition is to be fair, to raise emotion instead of inspiring respect; and this ignoble desire, like the servility in absolute monarchies, destroys all strength of character. Liberty is the mother of virtue, and if women be, by their very constitution, slaves, and not allowed to breathe the sharp invigorating air of freedom, they must ever languish like exotics, and be reckoned beautiful flaws in nature.

Discrimination of multiaxiality criteria with the Brazilian disc test

A fixture was developed which allows us to test circular discs of high strength ceramic materials under diametral compression. This so-called Brazilian disc test can be used to determine the failure behaviour of natural flaws contained in ceramic materials under multiaxial loading, as a stress state with both negative and positive principal stresses is induced in a disc during the test. The results of the test series performed with low and high strength ceramics are analysed using the multiaxial Weibull theory. Comparison of the Weibull distributions of the fracture stresses obtained with the Brazilian disc test and the outcome of the other test series clearly shows that a shear-insensitive failure criterion is more suitable to determine the critical flaw size under mixed mode loading than a shear-sensitive criterion.

CYCLIC FATIGUE OF ZIRCONIA

AbstractTwo partially‐stabilized zirconia materials, Mg‐PSZ and Y‐PSZ, were investigated in static and cyclic fatigue tests. The crack growth rates were determined as a function of the stress intensity factor (static tests) and the stress intensity factor range (cyclic tests). For all R‐ratios investigated the crack‐growth relations are sufficiently described by power laws. An influence of frequency is obvious. Finally, the crack growth behaviour of natural flaws is compared with that of macroscopic cracks

Influence of time-dependent phase transformations on bending tests

PSZ-zirconia shows the effects of stress-induced phase transformations with the tetragonal phase changing to the monoclinic modification. This transformation occurs spontaneously when the local stresses reach a critical value. In addition, the time dependent contributions of the transformation are quoted in the literature. The phase transformation behaviour exhibit the well-known consequences on the fracture mechanical properties (R-curves) determined by macroscopic cracks. In case of uncracked specimens (i.e. specimens with natural flaw populations) the phase transformations will influence the stress state and will affect the strengths and lifetimes. This will be significant in bending loading, where spontaneous and time-dependent transformations will change the stress distribution. Therefore, the investigation deals with uncracked bending specimens. The numerical treatment is discussed and an example is described in detail with reference to data measured by Liu and Chen, Fatigue deformation mechanisms of zirconia ceramics, J. Amer. Ceram. Soc., 75 (1992) 1191–1204, and Finlayson et al., Creep of Mg-PSZ at room temperature, J. Amer. Ceram. Soc., 77 (1994) 617–24.

Cyclic fatigue crack growth of SiCw/Y-TZP composites: long- and short-crack behaviour

Cyclic fatigue crack growth behaviour has been investigated for 10 vol % SiCw/Y-TZP composites with grain sizes varying from 1.50–2.00 μm in a residual stress field. To investigate the effect of precracking procedure, cyclic fatigue tests were performed on unannealed specimens using the four-point bending method under two conditions: (a) with a sharp crack and precracking procedure, and (b) with a natural sharp flaw. For all specimens in both conditions, an overall V-shaped da/dN versus K app relation was obtained. However, for the specimens without precracking, the da/dN had an unusual dependence on the applied stress intensity, giving a zigzag V-shaped curve. Explanations for these different results for the two conditions are discussed in terms of crack-tip shielding effects and residual stress field.

Grain size and porosity dependence of ceramic fracture energy and toughness at 22 °C

A review of the fracture energy and toughness data for dense ceramics at 22 °C shows maxima commonly occurring as a function of grain size. Such maxima are most pronounced for non-cubic materials, where they are often associated with microcracking and R-curve effects, especially in oxides, but often also occur at too fine a grain size for association with microcracking. The maxima are usually much more limited, but frequently definitive, for cubic materials. In a few cases only a decrease with increasing grain size at larger grain size, or no dependence on grain size is found, but the extent to which these reflect lack of sufficient data is uncertain. In porous ceramics fracture toughness and especially fracture energy commonly show less porosity dependence than strength and Young's modulus. In some cases little, or no, decrease, or possibly a temporary increase in fracture energy or toughness are seen with increasing porosity at low or intermediate levels of porosity in contrast to continuous decreases for strength and Young's modulus. It is suggested that such (widely neglected) variations reflect bridging in porous bodies. The above maxima as a function of grain size and reduced decreases with increased porosity are less pronounced for fracture toughness as opposed to fracture energy, since the former reflects effects of the latter and Young's modulus, which usually has no dependence on grain size, but substantial dependence on porosity. In general, tests with cracks closer to the natural flaw size give results more consistent with strength behaviour. Implications of these findings are discussed.

Grain Size Distributions and Strength Variability of High‐Purity Alumina

Two batches of high‐purity, alumina ceramics having an equiaxed grain morphology were manufactured with a mean grain size of approximately 10 μm. A low‐temperature heat treatment performed prior to sintering on one batch of specimens resulted in a narrow grain size distribution. The other batch of specimens was fired in a conventional manner and exhibited a broader grain size distribution. Mechanical tests performed on these specimens indicated little difference in the flaw tolerance of T‐curve behavior of the aluminas, despite the presence of coarser grains in the conventionally fired alumina. Observations of cracks in these high‐purity aluminas revealed that large grains ruptured transgranularly and therefore did not act as effective bridging sites. Strength tests on polished specimens indicated that the alumina with the broad grain size distribution exhibited greater strength variability than the alumina with a narrow grain size distribution. A simple analysis shows that, because of the shallow T‐curve behavior observed in both aluminas, stable crack extension should not occur from natural flaws. The strength of these high‐purity aluminas is therefore controlled by the initial flaw size and the initial toughness. The increased strength variability in the alumina with the broad grain size distribution is rationalized in terms of a wider distribution of critical flaws.

プラスチック成型加工における圧力効果 マイクロセルラープラスチックの成形プロセスにおける圧力効果

Microcellular plastics are a unique cellular material in which voids, smaller than the natural flaw size pre-existing in polymers, are nucleated: this process results in material costs savings without significant reductions and sometimes improvements in mechanical properties. They are characterized by average cell diameter sizes on the order of 0. 1 to 10 μm, cell densities on the order of 109 to 1015 cells/cm3, and specific density reductions (relative to the unfoamed plastic) from 5 to 95 percent. In this paper, the basic concept of foaming for microcellular plastics is described and the process technology of microcellular plastics based on that concept is introduced. The effect of pressure in processing of microcelullar plastics is discussed.

Discrimination of multiaxiality criteria using brittle fracture loci

The statistical distribution of brittle fracture loci was measured in the Brazilian disc test. A logical circuit was used to identify the fracture event in addition to fractographic examination. The empirical distribution of brittle fracture loci is compared with the predictions of the multiaxial Weibull theory. The results indicate that a shear-insensitive criterion is more suitable than a shear-sensitive criterion to describe the fracture behaviour of natural flaws in ceramic materials.

Reliability of structural ceramics

In this study the influence of R -curve effects on reliability are investigated assuming natural flaws as being peripherally cracked spherical voids. Various experimental literature as well as own results are presented, including Al 2 O 3 , Si 3 N 4 , Z r O 2 and Al reinforced Al 2 O 3 .