Extrinsic Flaws Research Articles

Yield strength of “brittle” metallic glass

“Brittle” metallic glass (MG) usually fractures catastrophically in a shattering mode under macroscopic compression, because cleavage cracking of splitting that originates from extrinsic flaws dominates the failure of such alloys, which brings challenges for studying yield strength. Here we show that the plastic yielding behavior in a brittle Fe-based MG can be successfully activated by decreasing the sample size to micrometer scale to avoid the possible large tensile stress concentrators. The yield strength was found to be at least 33% higher than the fracture strength measured with bulk samples for the present brittle MG. The results further demonstrate that the critical stresses for shear band initiation and propagation are size-independent, while the required stress for cleavage cracking increases with decreasing sample size. The competition of thermodynamic driving forces between the two processes of shear banding and cleavage cracking hence leads to the size-induced brittle-to ductile-transition. These findings clarify the physical nature of the strength of “brittle” MG, implying the great opportunity for using high-strength brittle MGs in devices with small dimensions.

Intrinsic strength and the structure of glass

Intrinsic strength is the strength of a glass without extrinsic flaws. Knowledge of the composition dependence of the intrinsic strength is important both technologically for the development of high strength glass compositions and theoretically as it may provide important structural information. Intrinsic strength is easy to define but difficult to measure experimentally. In this paper we discuss intrinsic (σ*) strength measured in normal laboratory conditions as well as the inert intrinsic (σ0*) strength. We define these terms in a rigorous manner and indicate how to gauge if they have been measured by developing a set of criteria. Based on these criteria, we tabulate those values of intrinsic strengths that have been measured to date.

Intrinsic and extrinsic flaws of the nomogram predicting bone-only metastasis in women with early breast cancer: An external validation study

BackgroundThe recently developed MDACC nomogram purports to predict the risk of bone-only metastasis in women with early breast carcinoma based on five clinical and pathological characteristics. We set out to externally validate and assess its robustness using a tertiary breast cancer centre database. MethodsAll consecutive women treated for early breast cancer in our centre between January 1989 and December 2013 and who had all the nomogram variables documented were eligible for analysis. ResultsWe identified 1255 eligible women for external validation analysis. The median follow-up was 54 months (range: 1–312) and time to initial metastasis 20 months (range: 1–80). The correspondence between the actual bone-only metastasis and the nomogram predictions implied poor calibration of the nomogram in the validation cohort, be it in the whole cohort or when stratified by breast cancer subtype. ConclusionThis external validation study of the MDACC nomogram showed limitations in its generalizability to a new and independent European patient population.

Cast defects induced sample-size dependency on compressive strength and fracture toughness of Mg–Cu–Ag–Gd bulk metallic glass

In this work, Mg59.5Cu22.9Ag6.6Gd11 bulk metallic glass (BMG) is selected to investigate the specimen-size dependency of the compressive fracture strength and notch toughness. As the diameter of cylindrical sample increases, apparent fracture strength is reduced by 25% from ∼950MPa at 1-mm down to ∼710MPa at 10-mm sample, which is associated with the population and size of as-cast porosity in the BMG rods as indicated by X-ray computed tomography (CT). Meanwhile, it is inappropriate to describe the size dependence of fracture strength using Weibull or Gumbel statistics owing to the inhomogeneous distribution of cast defects in different sized rods. Similar to the compressive fracture strength, cast defects in Mg59.5Cu22.9Ag6.6Gd11 BMG also yield specimen-size dependency of the measured notch toughness, when the plate-specimen thickness varies within the range of 1–8mm. With three-parameter Weibull analysis, threshold toughness Kμ of as-cast Mg59.5Cu22.9Ag6.6Gd11 BMG can be estimated, which represents a minimum toughness value below which the cracks arrest. Using finite element analysis, it is confirmed that the specimen-thickness dependency of notch toughness does not result from the transition from plane-stress to plane-strain in the specimen, but from the extrinsic flaws.

Intrinsic strength of sodium borosilicate glass fibers by using a two-point bending technique

Flaws existing on glass surface can be divided into two types, extrinsic and intrinsic. Although the extrinsic flaws are generated during processing and using, the intrinsic flaws are regarded as structural defects which result from thermal fluctuation. It is known that the extrinsic flaws determine glass strength, but effects of the intrinsic flaws on the glass strength are still unclear. Since it is considered that the averaged bond-strength and the intrinsic flaw would affect the intrinsic strength, the intrinsic strength of glass surely depends on the glass composition. In this study, the intrinsic failure strain of the glass fibers with the compositions of 20Na2O-40xB2O3-(80-40x)SiO2 (mol%, x = 0, 0.5, 1.0, 1.5) were measured by using a two-point bending technique. The failure strength was estimated from the failure strain and Young's modulus of glass. It is elucidated that two-point bending strength of glass fiber decreases with increasing B2O3 content in glass. The effects of the glass composition on the intrinsic strength are discussed in terms of elastic and inelastic deformation behaviors prior to fracture.

The effect of sample preparation on the mechanical properties of nylon 66

AbstractCutting test specimens from molded plaques is commonly used in mechanical testing. The mechanical properties of these cut specimens may be affected by cutting process as it could introduce extrinsic flaws and thermal effects on cut edge surfaces. The objective of this experimental research is to determine how band saw cutting affects the flexural and impact strengths of 33% short glass fiber (GF) reinforced polyamide 66 (PA66) and unreinforced PA66. The specimens for the flexural and impact tests were obtained by cutting molded plaques using different blade types, blade speeds, feed rates, and levels of polishing. Results were compared with those from uncut specimens. Surface morphologies of the specimens' cut edges (photographs and roughness) were assessed using Scanning Electron Microscopy and PRK Perthometer, respectively. The results indicated that higher flexural and impact strengths of cut specimens of 33% GF reinforced PA66 were achieved with high blade speed, low work piece feed rate and using a high number of teeth per unit length. For unreinforced PA66, higher impact strengths were achieved at low blade speeds and work piece feed rates.

Statistical analysis of failure of SiC fibres in the presence of bimodal flaw populations

Statistical parameters pertinent to silicon carbide fibres were estimated from experimental distributions of failure data (including strengths and strains) using the linear regression and maximum likelihood estimators. Strengths were evaluated using the average diameters of fibres, diameters at failure locations measured by SEM, and taking into account the diameter variations along the fibre axis, as evidenced by laser diffractometry. It was shown that gauge-length insensitive statistical parameters were estimated when considering that fibre fracture was dictated by two populations of partially concurrent flaw populations, including intrinsic and extrinsic flaws.

Inherent reinforcement of ceramic microstructures by grain boundary engineering

Experiments with Al 2 O 3 /TiC (ATC) and with tetragonal ZrO 2 (Y 2 O 3 )/Al 2 O 3 (TZP-A) reveal a basically different distribution of amorphous phase at the grain boundaries. However, in both types of ceramic, the grain facets are free of flaws like micropores (which are typically observed in sintered aluminas) and are, hence, characterized by a high grain boundary toughness. On the other hand, the macroscopic toughness of TZP-A is in the same range as that of transformation-free ATC ceramics, because transformation toughening, though being the one important dissipation process in TZP-A, makes rather a small contribution. Therefore the comparison of materials with different grain boundary structures and toughening mechanisms like ATC and TZP-A gives rise to the idea that their high strength is not achieved by an enhanced macroscopic toughness but by an inherent reinforcement of their microstructures, resulting in an increased grain boundary toughness which provides a similar strength-improving mechanism independent of dissipative processes: limitation of the usual subcritical growth of extrinsic flaws due to the improved perfection of grain boundary structures. Experimente mit Al 2 O 3 /TiC (ATC) und mit tetragonalem ZrO 2 (Y 2 O 3 )/Al 2 O 3 (TZP-A) zeigen eine grundlegend verschiedene Verteilung der amorphen Phase an den Korngrenzen. In beiden Keramiktypen sind die Kornfacetten frei von Fehlern wie Mikroporen (die aber typischerweise in gesintertem Aluminium-oxid beobachtet werden). Diese Werkstoffe sind daher durch eine hohe Zähigkeit der Korngrenzenbereiche gekennzeichnet. Andererseits aber liegt die makroskopische Zähigkeit von TZP-A in der gleichen Größenordnung wie die makropische Zähigkeit von unwandlungsfreien ATC-Keramiken, da die umwandlungsbedingte Zähigkeitssteigerung nur einen verhältnismäßig geringen Beitrag liefert, obgleich sie den wichtigsten Dissipations-Vorgang in TZP-A darstellt. Der Vergleich von Werkstoffen mit unterschiedlichen Korngrenzenstrukturen und zähigkeits-steigernden Mechanismen, wie der Vergleich von ATC und TZP-A, führt daher zu dem Gedanken, daß die hohe Festigkeit dieser Werkstoffe nicht aufgrund einer verbesserten makroskopischen Zähigkeit erreicht wird, sondern durch eine inhärente Verstärkung ihrer Gefüge entsteht, die zu einer gesteigerten Zähigkeit der Korngrenzenbereiche führt, die ihrerseits einen ähnlichen festigkeitssteigernden Mechanismus darstellt, der unabhängig von dissipativen Vorgängen ist: die Begrenzung des üblichen unterkritischen Wachstums von extrinsischen Fehlern infolge der erhöhten Perfektion der Korngrenzenstruktur. Des expériences menées conjointement sur Al 2 O 3 /TiC (ATC) et sur de la zircone tétragonale ZrO 2 (Y 2 O 3 )/Al 2 O 3 (TZP-A) mettent en évidence une distribution radicalement différente de la phase amorphe aux joints de grain. Cependant, dans les deux types de céramiques, les facettes de grain sont exemptes de défauts tels que des micropores (qui sont typiquement observés dans les alumines frittées) et sont, en conséquence, caractérisées par une ténacité élevée des joints de grain. Par ailleurs, la ténacité macroscopique de la céramique TZP-A est du même ordre que celle des céramiques ATC ne comportant pas de transformation, car le renforcement par transformation de phase, bien qu'étant le seul mécanisme de dissipation important dans TZP-A, représente une contribution plutôt faible. Donc, en comparant des matériaux comme ATC et TZP-A, possédant des structures de joints de grain et des mécanismes de renforcement différents, an arrive à la conclusion que leur résistance mécanique élevée n'est pas due à une ténacité macroscopique améliorée, mais à un renforcement intrinsèque de leurs microstructures, dont découle une ténacité des joints de grain améliorée qui fournit un mécanisme similaire d'augmentation de la résistance indépendent des processus dissipatifs: la limitation de la croissance subcritique habituelle des défants extrinsèques, due à l'amélioration des structures de joint de grain.

Toughening mechanisms in high impact polystyrene

In situ scanning electron microscope crack propagation experiments have been performed on a number of polystyrene and high impact polystyrene blends so that dynamic observations can be made of the mechanisms of failure. Brittle fracture is observed in low rubber phase volume systems, whereas high rubber phase volume systems exhibit a ductile tearing mode of fracture. As the rubber phase volume is increased there is an increased density of crazes, which leads to a reduction in width of material between them. The subsequent failure of the crazes leaves bridging ligaments. Under increasing load these fail in a manner dependent on their thickness such that there is a brittle-ductile transition at a ligament thickness around 3μm. We argue that this alteration in mechanism could be caused by either the loss of the triaxial stress state or the reduced probability of extrinsic flaws being found in the smaller ligaments, resulting in inhibition of crazing. The stress required for failure at the crack tip consequently increases from that for craze formation to the yield stress of the dense polymer. This in turn allows a larger crazed deformation zone (already increased due to the stress relief effects of crazing) to form, hence a further toughness increase.

On the estimation of the Weibull modulus

It is well documented that nominally identical specimens of brittle materials, e.g. ceramics, show a large variation of tensile fracture stresses and in order to use brittle materials as engineering materials the strength has to be characterized. The most widely used expression for characterization is the cumulative distribution function proposed by Weibull [ 1 ]. The Weibull function is also known to statisticians as Fisher-Tipper Type Ill distribution of smallest values or as the third asymptotic distribution of smallest extreme value [2]. The Weibull statistics is based on the the "weakest link-hypothesis" which means that the most serious flaw in the specimen will control the strength. The most serious flaw is not necessarily the largest one because its severity also depends on where it is situated. In other words, the flaw which is subjected to the highest stress intensity factor will be strength controlling. The flaws initiating fracture can conveniently be classified as intrinsic or extrinsic [3]. The intrinsic flaws are introduced during fabrication and are predominantly inclusions and voids. The extrinsic flaws are stress-induced cracks, such as surface cracks introduced during machining and microcracks resulting from large residual stresses, e.g. due to thermal contraction