Cleavage Failure Research Articles

A unified cohesive zone approach to model the ductile to brittle transition of fracture toughness in reactor pressure vessel steels

In this study, a unified cohesive zone model is proposed to predict the ductile to brittle transition of fracture toughness in reactor pressure vessel steels. A general procedure is described to obtain the temperature and failure probability dependent cohesive zone model parameters. In order to establish the full master-curve, the procedure requires calibration at two different temperatures with one at the upper shelf for ductile fracture and one near the lower shelf for cleavage failure. In the present study, these calibrations are performed using the experimental fracture toughness values and the flow curves. Subsequently, fracture simulations are performed on one-inch thick compact tension and 3-point bend specimens at different temperatures and the resulting fracture toughness values are compared with experiments. A satisfactory agreement is observed which indicates the viability of such a unified modeling approach.

Failure of cell cleavage induces senescence in tetraploid primary cells.

Tetraploidy can arise from various mitotic or cleavage defects in mammalian cells, and inheritance of multiple centrosomes induces aneuploidy when tetraploid cells continue to cycle. Arrest of the tetraploid cell cycle is therefore potentially a critical cellular control. We report here that primary rat embryo fibroblasts (REF52) and human foreskin fibroblasts become senescent in tetraploid G1 after drug- or small interfering RNA (siRNA)-induced failure of cell cleavage. In contrast, T-antigen-transformed REF52 and p53+/+ HCT116 tumor cells rapidly become aneuploid by continuing to cycle after cleavage failure. Tetraploid primary cells quickly become quiescent, as determined by loss of the Ki-67 proliferation marker and of the fluorescent ubiquitination-based cell cycle indicator/late cell cycle marker geminin. Arrest is not due to DNA damage, as the γ-H2AX DNA damage marker remains at control levels after tetraploidy induction. Arrested tetraploid cells finally become senescent, as determined by SA-β-galactosidase activity. Tetraploid arrest is dependent on p16INK4a expression, as siRNA suppression of p16INK4a bypasses tetraploid arrest, permitting primary cells to become aneuploid. We conclude that tetraploid primary cells can become senescent without DNA damage and that induction of senescence is critical to tetraploidy arrest.

Tensile and tearing fracture properties of graphene oxide papers intercalated with carbon nanotubes

The fracture behaviors of graphene oxide (GO) and GO/multi-walled carbon nanotube (MWCNT) hybrid papers are studied in both quasi-static mode-I opening and mode-III tearing fracture. Large GO papers give rise to a higher energy release rate and tearing toughness than those made from small GO sheets: about 36% and 70% enhancements are shown for the large GO papers due to a more compact structure and better GO sheet alignment. Hybridization of GO papers with MWCNTs also improves these properties when up to 5 wt.% of MWCNTs is incorporated, attributed to the stronger GO interlayer bonds through π–π interactions with intercalated MWCNTs. Fracture surface examination indicates that cleavage failure prevails in mode-I fracture depending on the size of GO sheets: the unsorted and large GO papers fail mainly by brittle cleavage of GO sheets whereas small GO papers fail by combined brittle cleavage and minor pullout following debonding of GO bundles. In contrast, combination of cleavage and debonding failures is dominant in mode-III fracture of GO papers, regardless of GO size. The torn edges present typical characteristics of a saw-tooth wave with disordered oscillation where the large GO papers exhibit generally higher amplitude peaks than the small GO papers.

Identification and Characterization of Maizefloury4as a Novel Semidominant Opaque Mutant That Disrupts Protein Body Assembly

Zeins are the major seed storage proteins in maize (Zea mays). They are synthesized on the endoplasmic reticulum (ER) and deposited into protein bodies. Failure of signal peptide cleavage from zeins can cause an opaque endosperm in the mature kernel; however, the cellular and molecular mechanisms responsible for this phenotype are not fully understood. In this study, we report the cloning and characterization of a novel, semidominant opaque mutant, floury4 (fl4). fl4 is caused by a mutated z1A 19-kD α-zein with defective signal peptide cleavage. Zein protein bodies in fl4 endosperm are misshapen and aggregated. Immunolabeling analysis indicated that fl4 participates in the assembly of zeins into protein bodies, disrupting their proper spatial distribution. ER stress is stimulated in fl4 endosperm, as illustrated by dilated rough ER and markedly up-regulated binding protein content. Further analysis confirmed that several ER stress pathways are induced in fl4 endosperm, including ER-associated degradation, the unfolded protein response, and translational suppression by the phosphorylation of eukaryotic translational initiation factor2 α-subunit. Programmed cell death is also elevated, corroborating the intensity of ER stress in fl4. These results provide new insights into cellular responses caused by storage proteins with defective signal peptides.

ROCK inhibition prevents early mouse embryo development

ROCK is a Rho-GTPase effector that is important for actin assembly and is involved in various cellular functions, including cell contraction, migration, motility, and tumor cell invasion. In this study, we investigated ROCK expression and function during early mouse embryo development. Inhibiting ROCK by Y-27632 treatment at the zygote stage resulted in first cleavage failure, and most embryos failed to develop to the 8-cell stage. When adding Y-27632 at the 8-cell stage, embryos failed to undergo compaction and could not develop into blastocysts. In addition, fluorescence staining intensity analysis indicated that actin expression at blastomere membranes was significantly reduced. After ROCK inhibition, two or more nuclei were observed in a cell, which indicated possible cytokinesis failure. Moreover, after ROCK inhibition with Y-27632, the phosphorylation levels of LIMK1/2, a downstream molecule of ROCK, were decreased at blastomere membranes. Thus, our results showed conserved roles for ROCK in this mammalian embryo model and indicated that a ROCK-LIMK1/2-actin pathway might regulate cleavage and blastocyst formation during early mouse embryo development.

Influence of omnidirectional pressure on the development of porosity during necking under stretching of metals and alloys

The influence of omnidirectional pressure on microporosity developing in the neck of 45-μm-thick copper sheets tested for creep is considered. Comparing these data with a porosity measured in cylindrical specimens leads us to conclude that the transition from cleavage failure to shear fracture observed in copper under pressure is related to a decrease in the mean hydrostatic tension in the neck.

Effect of hydrogen charging on the mechanical properties of advanced high strength steels

The present work investigates the influence of hydrogen on the mechanical properties of four multiphase high strength steels by means of tensile tests on notched samples. This was done by performing mechanical tests on both hydrogen charged and uncharged specimens at a cross-head displacement speed of 5 mm/min. A considerable hydrogen influence was observed, as the ductility dropped by 8–60%. In order to demonstrate the influence of diffusible hydrogen, some parameters in the experimental set-up were varied. After tensile tests, fractography was performed. It was found that hydrogen charging caused a change from ductile to transgranular cleavage failure near the notch with a transition zone to a fracture surface with ductile features near the centre.

Micromechanical Modeling of Crack Propagation with Competing Ductile and Cleavage Failure

Typical engineering metals exhibit a change of the failure mechanism with decreasing temperature. In the range of room temperature a ductile mechanism is observed. Thereby, voids nucleate, grow by plastic deformations of the surrounding matrix and finally the voids coalesce. In contrast in the low temperature regime, cleavage failure occurs, a mechanism which is associated with macroscopically brittle behavior.In the present study the crack initiation and propagation is investigated in the ductile-brittle transition region by means of a microscopic model. The voids in the process zone in front of the crack tip are resolved discretely. Possible void growth in the surrounding plastic zone, which may induce an important shielding effect, is taken into account in a homogenized way by means of the GTN-model. In contrast to comparable studies in the literature not only cleavage crack initiation is addressed but the material degradation by the cleavage mechanism is incorporated explicitly by means of a cohesive zone model. The limit case of small- scale yielding is investigated. This model allows to simulate all stages of crack initiation and propagation at all temperatures. A systematic study of the effects of the model parameters is performed.

The Effect of Low Temperatures on the Fatigue of High-strength Structural Grade Steels

It is well-known that for fracture, ferritic steels undergo a sudden transition from ductile behavior at higher temperatures to brittle cleavage failure at lower temperatures. However, this phenomenon has not received much attention in the literature on fatigue. The so-called Fatigue Ductile-Brittle Transition (FDBT) has been identified in the literature as the point at which the fracture mode of the fatigue cracks changes from ductile transgranular to cleavage and/or grain boundary separation. The current paper contributes to understanding this phenomenon by presenting both ductile to brittle fracture transition data and fatigue crack growth rate curves for two modern high strength steel base plate materials: S460 and S980. The data in this paper suggests that fatigue at lower shelf temperatures may have a higher rate than in the transition or upper shelf temperatures for Regions I and II of the da/dN versus ΔK curve.

Improved Numerical Determination of the Effective Stress within the Framework of Local Approach Cleavage Models

For the determination of the Weibull stress or analogous effective stress measures in the assessment of the cleavage failure probability based on finite element results, robust and numerically stable procedures are necessary. For the quantitative assessment of the numerical errors in the approximate integration procedures for the determination of the Weibull stress or corresponding stress measures, an appropriate error indicator in analogy to adaptive finite element meshing methods was derived. The mathematical considerations reveal that an evaluation of the cleavage probability by means on the classical Beremin (1983) model using the standard one-point Gauss integration scheme may result in distinct numerical errors. Therefore, an enhanced numerical integration formula was derived and implemented. This formula facilitates a numerical evaluation of the integrals with improved accuracy.

181 CYTOSKELETAL AND CHROMOSOMAL ORGANIZATION IN DEVELOPMENTALLY ARRESTED EQUINE ZYGOTES AFTER INTRACYTOPLASMIC SPERM INJECTION

Intracytoplasmic sperm injection (ICSI) has been developed as a clinical procedure in the equine industry. Not all sperm-injected oocytes develop into embryos; however, the causes of developmental failure after equine ICSI have not been studied. Our objective was to use confocal microscopy to evaluate sperm-injected equine oocytes from young and old mares that did not cleave in a clinical ICSI program, to study the hypothesis that cleavage failure is associated with cytoskeletal and chromosomal alterations and affected by mare age. Oocytes were collected from the dominant follicles of young (Y; 5–15 years old) and old (O; 20–24 years old) oestrous mares approximately 24 h after administration of deslorelin and hCG. At approximately 44 h after deslorelin and hCG administration, oocytes were injected using frozen-thawed sperm from various stallions. Injected oocytes (n = 15) that failed to cleave by 24 to 48 h were fixed in microtubule stabilization buffer extraction fixative (MTSB-XF) and stained for DNA, α/β tubulin, acetylated α-tubulin, and f-actin. Confocal z-stacks were obtained using a Zeiss LSM 5 microscope (Carl Zeiss Microscopy LLC, Thornwood, NY), and images were assessed for cytoskeletal structures and chromosome organisation using the Zeiss LSM image browser. Potential zygotes (Y: n = 6 and O: n = 9) were categorized for (1) 2 sets of chromosomes, (2) misaligned chromosomes, (3) oversized spindles (approximately twice the size of an MII spindle), and (4) actin bubbling. The number of zygotes in each category for Y and O mares were compared by Fisher's exact test. Some samples were included in more than 1 morphological category. Two separate sets of chromosomes were observed in 1 of 6 and 4 of 9 injected oocytes from Y and O mares, respectively (P = 0.58), with development stopping before fusion of male and female pronuclei. Misaligned chromosomes were present in 2 of 6 and 7 of 9 oocytes from Y and O mares, respectively (P = 0.14). Oversized spindles were observed in more Y than O potential zygotes (5 of 6 and 2 of 9, respectively, P = 0.04), representing fusion of male and female pronuclei before developmental arrest. Multiasters were associated with the oversized spindles in potential zygotes from Y (2 of 6) and O (2 of 9) mares (P = 1), with microtubule activity potentially associated with the first mitotic division. Structures associated with actin bubbling tended (P = 0.12) to be higher in uncleaved zygotes from Y than O mares (5 of 6 and 3 of 9, respectively). Of the 15 injected oocytes, 10 arrested after fusion of male and female pronuclei. We have previously observed a high incidence of chromosomal misalignment in metaphase II oocytes from old mares. In the small number of potential zygotes that were studied, chromosomal misalignment tended (P = 0.14) to be higher for old mares. Other observations, such as oversized spindles and actin bubbling, appeared to be more prevalent in the potential zygotes of Y than O mares; these observations could be associated with normal development or post-ovulatory aging of the oocyte. Additional work will focus on the roles of actin and microtubules in remodelling the cytoplasm and organizing the chromosomes after fertilization in the horse and on the role of maternal aging in fertilization failure.

A His6-SUMO-eXact tag for producing human prepro-Urocortin 2 in Escherichia coli for raising monoclonal antibodies

This is a first report of recombinant production of human prepro-Urocortin 2 in Escherichia coli by N-terminal fusion with a triple His6-SUMO-eXact tag and its subsequent use as an antigen for the production and screening of very high affinity monoclonal antibodies. The rationale for this combinatorial construct is that the His tag allows first step protein purification of insoluble and soluble proteins, the SUMO tag enhances protein expression level and solubility, while the eXact tag facilitates anion-triggered on-column cleavage of the triple tag to recover pure native proteins in a simple two-step protein purification procedure. Compared with an eXact fusion alone, the presence of the SUMO moiety enhanced overall expression levels by 4 to 10 fold but not the solubility of the highly basic prepro-Urocortin 2. Insoluble SUMO-eXact-preproUCN2 was purified in milligram quantities by denaturing IMAC and solubilized in native phosphate buffer by on-column refolding or step-wise dialysis. Only a small fraction of this solubilized protein was able to bind onto the eXact™ affinity column and cleaved by NaF treatment. To test whether binding and cleavage failure was due to improperly refolded SUMO-eXact-preproUCN2 or to the presence of N- and C-terminal sequences flanking the eXact moiety, we created a SUMO-eXact-thioredoxin construct which was overexpressed mainly in the soluble form. This protein bound to and was cleaved efficiently on the eXact™ column to yield native thioredoxin. Solubilized SUMO-eXact-preproUCN2 was used successfully to generate two high affinity mouse monoclonal antibodies (KD~10−10 and 10−11M) specific to the pro-region of Urocortin 2.

Profiling of the Mammalian Mitotic Spindle Proteome Reveals an ER Protein, OSTD-1, as Being Necessary for Cell Division and ER Morphology

Cell division is important for many cellular processes including cell growth, reproduction, wound healing and stem cell renewal. Failures in cell division can often lead to tumors and birth defects. To identify factors necessary for this process, we implemented a comparative profiling strategy of the published mitotic spindle proteome from our laboratory. Of the candidate mammalian proteins, we determined that 77% had orthologs in Caenorhabditis elegans and 18% were associated with human disease. Of the C. elegans candidates (n=146), we determined that 34 genes functioned in embryonic development and 56% of these were predicted to be membrane trafficking proteins. A secondary, visual screen to detect distinct defects in cell division revealed 21 genes that were necessary for cytokinesis. One of these candidates, OSTD-1, an ER resident protein, was further characterized due to the aberrant cleavage furrow placement and failures in division. We determined that OSTD-1 plays a role in maintaining the dynamic morphology of the ER during the cell cycle. In addition, 65% of all ostd-1 RNAi-treated embryos failed to correctly position cleavage furrows, suggesting that proper ER morphology plays a necessary function during animal cell division.

Contribution of Galvanizing Layer to Hydrogen Induced Cracking Failure of AISI 4140 Bolt for Padeye Fixing in Marine Environment

A high strength bolt with hot dip galvanizing treatment failed shortly after being installed for two days on a padeye of mooring dolphin. The bolt was installed with increasing stress on the second day in which the value was nearly twice. Investigation results showed the bolt fractured with the nut head separated from the pin. Corroded fracture surface with brittle characteristic and no plastic deformation observed dominating the failed area. Characterization of hardness and chemical composition followed with microstructure and fractography observation on the fracture surface then conducted in order to analyze the reason for this brittle fracture occurrence. Results indicate that, while the bolt conforms to the material specification in term of chemical composition, the hardness value was high. The microstructure observation reveals a transgranular crack propagation and cleavage failure occurred. The cleavage failure was clearly observed under fractography observation using scanning electron microscope. Failed galvanize layer due to mechanical failure becomes preferential site for hydrogen evolution in marine environment, which leads to hydrogen diffusion into the matrix, thus results in hardness increase. The increasing stress during installation become detrimental to the bolt and facilitate the hydrogen induce cracking. Detrimental effect of hot dip galvanize layer is pointed out in the application of high strength material in marine environment.

Effect of tungsten content on microstructure and mechanical properties of swaged tungsten heavy alloys

This paper describes the effect of tungsten content on microstructure and mechanical properties of swaged Co-containing tungsten heavy alloys with varying tungsten (90W–7Ni–2Fe–1Co, 93W–4.9Ni–1.4Fe–0.7Co and 95W–3.5Ni–1Fe–0.5Co). With increasing tungsten while tensile strength goes through a maximum, both percent elongation and impact energy decrease. Microstructure and fractographic analyses have been carried out in order to explain the trends in mechanical properties. Predominant transgranular fracture of tungsten grains combined with ductile dimple failure of the matrix in 90% W alloy is responsible for superior properties of this alloy in comparison to the alloy with 95% W. The highest tensile strength attained in 93% W alloy is attributed to predominant cleavage failure of W-grains. The results clearly indicate that the matrix volume fraction, contiguity and matrix mean path greatly influence the mechanical properties of tungsten heavy alloys.

Effect of Processing Parameters on the Mechanical and Structure Properties of 93W–4.9Ni–2.1Fe Tungsten Heavy Alloy

The objective of this experimental study is to investigate the effect of the main compaction and sintering parameters on the micro-structure and the mechanical properties of the liquid phase sintered 93%wt.W–4.9%wt.Ni–2.1%wt.Fe tungsten heavy alloy aiming at determining the optimum values of these parameters. Elemental powders were mixed using planetary mixer for 5 hours to ensure suitable homogeneity. Uni-axial compaction was applied to obtain standard tensile and impact specimens using compaction pressures ranging from 50MPa to 300MPa. Vacuum liquid phase sintering was carried out under different temperatures from 1460ᵒC up to 1500ᵒC and sintering time from 30 minutes up to 120 minutes. The effect of these parameters was characterized in terms of density, hardness, impact resistance and tensile properties. Microstructure variations, notably grain size, matrix volume fraction and contiguity were measured and used to explain the effect of sintering temperature and time on the properties. The obtained results indicated that optimum hardness and impact resistance can be obtained at a compaction pressure of about 200 MPa. As the sintering temperature increases, the grain size and volume fraction of matrix increase, while the contiguity decreases. As a result of these micro-structural changes, strength and hardness decrease. On the other hand, ductility and impact resistance increase with sintering temperature to some maximum at 1480ᵒC. At further increase of this temperature, grain growth becomes the dominant factor leading to a sensible decrease in all properties. The effect of sintering time on mechanical properties isrelated to grain growth and pore coarsening mechanisms. It is generally observed that strength and ductility decrease after sintering over 90 minutes at 1480ᵒC. It was also noticed that the ductility is more sensitive to sintering time and is reduced sharply with prolonged holding at the sintering temperature. It can be concluded that the mechanical properties of tungsten heavy alloys are sensitive to the processing cycle and are adversely affected by residual porosity. Moreover, tungsten grain size plays an important role in dictating the failure mode during tensile testing. If tungsten grain size is large interface failure is predominant. However, as the grain size decreases, fracture mode changes gradually from interface failure to matrix failure and then to tungsten grain cleavage failure.

Effect of welding energy on interface zone of Al–Cu ultrasonic welded joint

Ultrasonic welding is attracting increasing attentions in joining of dissimilar materials. The effect of welding energy on joint strength, failure behaviour and microstructure of Al–Cu ultrasonic welded joint has been experimentally investigated. The results showed that joint strength increased with welding energy initially and reached its maximum at 1000 J, then dropped significantly instead. Meanwhile, the failure mode changed from interfacial debonding to nugget pullout, and then back to cleavage failure. Various microstructures with different morphologies and properties were also observed at the interfacial region. At lower energy, the joint was only partly bonded by numbers of dispersed microbonds. A swirl-like structure appeared at the weld interface and led to a mechanical interlocking between the materials at higher energies (∼1000 J). However, cavity defects and intermetallic compound (IMC) were more likely to form under excessively high energies. A 0·5 μm thick IMC layer with dominant phase of Al4Cu9 was found in 2000 J joint.

The rRNA methyltransferase Bud23 shows functional interaction with components of the SSU processome and RNase MRP

Bud23 is responsible for the conserved methylation of G1575 of 18S rRNA, in the P-site of the small subunit of the ribosome. bud23Δ mutants have severely reduced small subunit levels and show a general failure in cleavage at site A2 during rRNA processing. Site A2 is the primary cleavage site for separating the precursors of 18S and 25S rRNAs. Here, we have taken a genetic approach to identify the functional environment of BUD23. We found mutations in UTP2 and UTP14, encoding components of the SSU processome, as spontaneous suppressors of a bud23Δ mutant. The suppressors improved growth and subunit balance and restored cleavage at site A2. In a directed screen of 50 ribosomal trans-acting factors, we identified strong positive and negative genetic interactions with components of the SSU processome and strong negative interactions with components of RNase MRP. RNase MRP is responsible for cleavage at site A3 in pre-rRNA, an alternative cleavage site for separating the precursor rRNAs. The strong negative genetic interaction between RNase MRP mutants and bud23Δ is likely due to the combined defects in cleavage at A2 and A3. Our results suggest that Bud23 plays a role at the time of A2 cleavage, earlier than previously thought. The genetic interaction with the SSU processome suggests that Bud23 could be involved in triggering disassembly of the SSU processome, or of particular subcomplexes of the processome.

Fracture toughness and fracture mechanisms in Mo5SiB2 at ambient to elevated temperatures

The fracture behavior of the polycrystalline Mo5SiB2 alloy was investigated by three-point bending tests, in vacuum, in the range of 25–1400 °C at a crosshead speed of 10−3 mm/s. At room temperature (RT), the alloy exhibited a higher fracture toughness of 3.34 MPa m1/2 compared to single crystalline Mo5SiB2 with a value of ∼1.8 MPa m1/2. The improved toughness is attributed to crack overcoming the barrier effects of grain boundaries. With increasing temperature, the fracture mode underwent a conversion from transgranular cleavage at RT to a mix of transgranular cleavage and intergranular failure at elevated temperatures. At 1000 °C, the maximum increment (∼3.4 MPa m1/2) in toughness was found to be related to the brittle-to-ductile transition (BDT). At 1200 °C, the effects of extrinsic toughening mechanisms (microcracking, etc.) are limited, i.e. its toughness only increasing by ∼1.1 MPa m1/2. However, the cavities formed at triple junctions degraded toughness at 1400 °C.

Holoprosencephaly with Multiple Anomalies of the Craniofacial Bones-An Autopsy Report

Holoprosencephaly (HPE), a disorder which results from a failure of cleavage or the incomplete differentiation of the forebrain structures at various levels or to various degrees, is related to hereditary factors, chromosomal anomalies, cytogenetic abnormalities, and environmental teratogenic factors. We are reporting a case of a multiparous woman who was G3,P3,L2, who delivered a full term foetus with holoprosencephaly and multiple craniofacial anomalies. An autopsy was conducted. Multiple anomalies of the craniofacial bones, which include hypoplasia and synostosis of the frontal bone, anophthalmia, absence of the anterior cranial fossa, hypoplasia of the maxillae, an absent antrum, cleft palate, a central hare lip and arrhinia which includes absence of the nostrils and hypotelorism of the eye placodes, were noted. This case is being reported for its rarity and the available literature was reviewed in this respect.