Subcritical crack growth in fine grained polycrystalline graphites

Subcritical crack growth was measured in four fine grained polycrystalline graphites in air at room temperature. Filler and textural effects were observed at stress intensity factors (K_I) near the critical stress intensity (K_Ic). Scanning electron microscope fractography permitted correlation with processing conditions and microstructure. The results are compared with those for other carbon materials.     

Oxidation effects on CTE and thermal shock fracture initiation in polycrystalline graphites

The coefficient of thermal expansion (CTE) was measured as a function of oxidation for three commercial fine-grained graphites derived from petroleum cokes and coal tar pitches and fabricated by extrusion, undirectional molding, and isostatic molding. The CTE was observed to vary with the crystallite size and the preferred orientation and to decrease as much as 20% with increasing oxidation. This CTE decrease was attributed to an increase of the accommodation by Mrozowski cracks enlarged by the oxidation process. Effects on thermal shock fracture initiation were examined by estimating changes in the thermal shock resistance parameter, R. It is concluded that in spite of the continuous decrease in CTE, changes in R with oxidation are not continuous for these graphites. The complexity is a consequence of the different extents to which graphite oxidation affects CTE, strength and the Young's elastic modulus.     

High temperature slow crack growth in polyoxymethylene

Brittle failure has been observed in polyoxymethylene during long‐term low‐level tensile loading at elevated temperatures. It is argued to be associated with slow crack growth via the breakdown of the localized planar fibrillar damage zones that form under these conditions. This phenomenon has been characterized using notched compact tension specimens tested under various static loads and at different temperatures. The specimen lifetime at a given load is found to decrease strongly with increasing temperature and to increase with molar mass at a given load and temperature. The associated crack‐tip fibrillar damage zones are shown to arise from the breakdown of more localized microfibrillar deformation zones, which in turn result from interlamellar cavitation in the early stages of tensile deformation.     

Influence of the molecular structure on slow crack growth resistance and impact fracture toughness in Cr-catalyzed ethylene-hexene copolymers for pipe applications

In this study we correlate parameters describing molecular structure (molar mass distribution, short chain branching content, intermolecular heterogeneity) of different ethylene-hexene Cr-catalyzed copolymers, with slow crack growth and rapid crack propagation resistances, respectively measured with Bent Strip and Charpy tests. The PTREF technique, coupled with classical techniques, was used. Two new indices were proposed to correlate mechanical properties and molecular structure. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 916–928, 2001     

Fracture toughness and crack growth mechanism for multiphase polymers

The fracture behavior of acrylonitrile-butadiene-styrene (ABS) was investigated using the J-integral method under monotonic loading. Two ways were used to monitor the onset of crack growth: the dyeing method and the length of craze region ahead of initial notch. The blunting at the crack tip and crack growth mechanism for ABS Was observed using a scanning electron microscope. Before the onset of crack growth, the energy put into material was dissipated to create crazes ahead of the initial notch and to deform the material at the crack tip. A part of the energy was released to create a new crack surface after the onset, of crack growth.     

Statistical analysis of slow crack growth experiments

A common approach for the determination of slow crack growth (SCG) parameters are the static and dynamic loading method. Since materials with small Weibull module show a large variability in strength, a correct statistical analysis of the data is indispensable. In this work we propose the use of the Maximum Likelihood Method and a Baysian Analysis, which, in contrast to the standard procedures, take into account that failure strengths are Weibull distributed. The analysis provides estimates for the SCG parameters, the Weibull module, and the corresponding confidence intervals and overcomes the necessity of manual differentiation between inert and fatigue strength data. We compare the methods to a Least Squares approach, which can be considered the standard procedure. The results for dynamic loading data from the glass sealing of MEMS devices show that the assumptions inherent to the standard approach lead to significantly different estimates.     

Room temperature fatigue crack propagation in reactor graphites

Fatigue crack propagation studies on two reactor graphites have established a crack propagation law which is compared with fatigue models at room temperature. Predictions using this law for push-pull fatigue endurance correlate well with published experimental work.     

The anisotropy of slow crack growth in polyethylene pipes

Slow crack growth was measured in the perpendicular and parallel directions relative to the extrusion direction of the pipe. For five pipes from different manufacturers, the anisotropy factor, the lifetime to fracture in the perpendicular direction divided by lifetime in the parallel direction, varied from 1.2 to 4.7 for complete fracture and 1.4 to 4.0 for crack initiation. The degree of molecular orientation was determined by measuring the shrinkage that occured when a pipe specimen was heated near its melting point. The amount of shrinkage correlated with the anisotropy factor for slow crack growth. The shape change after shrinkage was related to the flow pattern of the resin during extrusion and the cooling rate after extrusion.     

管材树脂耐慢速裂纹增长性能快速评价方法

介绍了评价管材料耐慢速裂纹增长性能的3种实验方法:管材切口实验、全切口蠕变实验和宾夕法尼亚州切口实验的研究进展情况,并对3种方法进行深入分析和对比,可为管材料的研究和性能评价提供参考.     

Effects of different curing conditions on slow crack growth in cement paste

Portland cement paste was prepared in the following five ways: (1) cured in fresh water, (2) made with fresh water, cured in sea water, (3) made and cured in sea water, (4) low-pressure steam cured, (5) high-pressure steam cured. The water-cement ratio was 0.4 in all cases. Slow crack growth was studied by the double torsion method, and plots were made of crack velocity versus stress intensity. Fracture toughness and the modulus of elasticity were also measured by flexural beam methods. The steam cured specimens had lower fracture toughness and were somewhat more sensitive to static fatigue than the room-temperature cured specimens. There was evidence that crack growth is aided by the presence of water in the environment or by increased water-cement ratio.

Abstract

Le ciment Portland hydraté a été préparé d'aprés les cinq méthodes suivantes: (1) traité dans l'eau fraîche, (2) préparé avec de l'eau fraîche, traité dans l'eau de mer, (3) préparé et traité dans l'eau de mer, (4) traité avec de la vapeur d'eau à basse pression, (5) traité avec de la vapeur d'eau à haute pression. Dans chaque cas la proportion eau/ciment était 0.4. La lente propagation de la fissure à été étudiée d'après la méthode de la double torsion et al vitesse de propagation de la fissure à été représentée graphiquement en fonction de l'intennsité de torsion. La résistance à la rupture et le module d'élasticité ont aussi été mesurés d'après la méthode de flexion des poutres. Les échantillons traités avec de la vapeur d'eau avaient une résistance inférieure à la rupture et étaient un peu plus susceptibles à la fatigue statique que les échantillons traités à temperature ambiante. Il semble que la propagation de la fissure est aidée par la présence de l'eau dans l'environment ou par une augmentation de la proportion eau/ciment.     

A sensitive mechanical test for slow crack growth in polyethylene

Slow crack growth (SCG) in a wide variety of polyethylenes has been investigated by a constant tensile load test (the PENT test) for a single edge notched specimen. The PENT test is very sensitive to the changes in molecular structure and morphology of polyethylene. The resistance to SCG depends on the density of the tie molecules and the strength of the crystals.     

Very slow crack growth during osmosis in epoxy and in polyester resins

It has been demonstrated that osmotic pressure filled cracks in both epoxy and polyester resins are elastic cracks. Use of classical formulas for elastic cracks has enabled estimates to be made of the time dependence of Young's modulus for both resins. Use of linear elastic fracture mechanics formulas has enabled stress intensity factors to be determined from measurements of crack profiles. Radical crack growth rates are small, in the range 10^?12–10^?9 ms^?1 for hot water tests, and remain constant over a wider range of stress intensity factor, from 0.3 to 0.8 MPa.m^1/2. To a first approximation, constant radial growth rate is compatible with a diffusion controlled mechanism. However, analysis of the data indicates an activation energy of ～50 kcal. Some evidence is presented for concluding that, in polyesters at least, the true nature of crack propagation can be by way of slip/stick.     

The effect of microstructure on the slow crack growth resistance in polyethylene resins

The slow crack growth (SCG) in high density polyethylene (HDPE) is a phenomenon dominated by crazing. In this work, the crazing was analyzed from a microstructural point of view. PENT (Pennsylvania Edge Notched Tensile) test was chosen to study the evolution of the craze with time for different resins from PE‐80 up to PE‐100 grades. Two different geometries, the standard and an alternative named CDNT (Circumferentially Deep Notched Tensile), were employed. Failure times were correlated with intercrystalline parameters like tie molecules and the molecular weight between entanglements. Experimental results showed good correlations using both direct SCG test (standard PENT and CDNT geometries). Finally, the strain hardening modulus was correlated with PENT failure times. The results disclosed an outstanding correlation for several polyethylene grades from blow molding up to PE‐80, PE‐100, and higher resistant to crack grades. These results permitted an easy‐classifying and ranking method as much to the old polyethylene grades as to the new generation of HDPE resins with a very high SCG resistance. POLYM. ENG. SCI., 55:1018–1023, 2015. © 2014 Society of Plastics Engineers     

Theory of slow,steady state crack growth in polymer glasses

Cracks in polymer glasses can grow slowly preceded by a craze, a narrow zone of plastic cavitation. The craze widens by drawing more polymer from its surfaces into its fibrils but the fibrils themselves fail by local creep. When the crack tip moves at velocity v the loading at the crack tip can be described by a local stress intensity factor K which is the sum of the ‘apparent’ stress intensity factor K_A and a plastic contribution K_p (usually negative). K_p is found to be $\text{?KP(K)}\text{v}$ where P(K) is an integral over the craze boundary displacement law. Fibril failure by local creep leads to a power law, v ∞ K^m. From these relations K and v can be determined as a function of K_A. The plot of K vs. K_A is multiple-valued with a stable branch (at high K) and an unstable branch (at low K) separated by a minimum value of K_A which represents a threshold for stable, steady state crack growth. There is also a v threshold, below which cracks will not grow steadily. These predictions, the form of the v?K_A curve and implications for slip-stick crack growth are compared with recent experiments.     

Biaxial strength and slow crack growth in porous alumina with silica sintering aid

Biaxial strength, fracture toughness and subcritical crack growth are reported for coarse grained porous alumina ceramics. The materials were prepared with a varying amount of a silica sintering aid, which resulted in the formation of a glassy secondary phase at the grain boundaries. Crystalline mullite was additionally found in the material with the highest silica content. The biaxial strength, measured by Ball-on-Ring and Ball-on-3-Balls, was highest for the material without mullite at the grain boundaries, and the biaxial strength decreased with increasing porosity. The fracture toughness of the materials was in the range of 1.7–1.9 MPa m^0.5. Measurements of subcritical crack growth by a modified lifetime method in air and aqueous environments demonstrated a higher crack growth rate in water and acid relative to in air. The effect of porosity and grain boundary phase were discussed in relation to subcritical crack growth and fracture mode in the coarse grained alumina ceramics.     

Craze morphology and molecular orientation in the slow crack growth failure of polyethylene

An optical microscopy study and a micro‐Raman spectroscopy study were carried out on polyethylene samples subjected to an environmental stress crack resistance (ESCR) test. The aim was to elucidate the molecular deformation mechanisms associated with the failure process. It has been shown that in the early stages of the ESCR test, in a regime of low local stress, failure in the craze occurs via a brittle process with limited ductility and with molecular orientation being detected. As the experiment progresses, however, extensive fibrillation takes place. The molecular orientation in these fibrils was found to be comparable to that measured in cold‐drawn samples. Moreover, the fibril molecular orientation decreased from the crack to craze tip and was found to be higher in the midrib part of the fibril (fibril failure point). As a consequence, fibril creep is the most likely mechanism of failure in the craze. Microscopy and Raman measurements showed that the extent of the brittle process is molecular weight‐dependent, that is, the brittle process seems to operate longer at higher molecular weights. These observations are in agreement with a previous work which showed that the molecular stress per macroscopic strain/stress decreases with increasing molecular weight, therefore holding the high molecular weight craze in a regime of low local stress for longer testing times. Fibrils spanning the craze are envisaged as the anchor points that hold the structure during the process of failure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 283–296, 2000     

Accelerated test for evaluating slow crack growth of polyethylene copolymers in igepal and air

The time for brittle failure by slow crack growth for 22 polyethylene copolymers was measured in Igepal and air. The notched tensile tests were conducted in Igepal and air at 50°C and 4.2 MPa and in air at 80°C and 2.4 MPa. For failure times less than 10³ min, the difference between the Igepal and air environments was not measurable. As the failure time increased beyond 10³ min, the ratio of failure in air compared to that in Igepal increased so that for the very highest failure times of 5 × 10⁵ to 108 min in air, the failure time in Igepal was reduced by 25—50 times. The correlation between the Igepal and air tests was generally good with respect to all types of polyethylene. However, a separation of the polyethylenes with respect to their comonomer, butene, hexene, or octene improved the correlation. The resistance to slow crack growth of all the current commercial polyethylene copolymers can be assessed by a notched tensile test in Igepal in about a week or less.     

Fracture strength and toughness of chemical-vapor-deposited polycrystalline diamond films

The strength of diamond films prepared by chemical vapor deposition (CVD) is usually much lower than that of natural-type IIa single-crystal diamonds. In this work, the fracture strength of free-standing diamond films deposited by direct current arc plasma jet CVD has been examined by conducting three-point bending experiments. The results obtained for both the polished and as-grown samples were in good agreement with a well-known Hall–Petch equation describing the relationship between the fracture strength and grain size, indicating that grain refinement represented an effective way of improving the mechanical properties of CVD diamond films. Furthermore, the diversification of the crystalline texture of the films achieved by polishing apparently increased their fracture strength, which was inversely proportional to the film thickness. A theoretical method for estimating the fracture strength of free-standing CVD diamond films by approximating their intrinsic strength was proposed, whereas their fracture toughness was determined by conducting simplified four-point bending tests at room temperature, 25?°C, using a single-edge pre-cracked beam method.     

Double torsion tests for studying slow crack growth of portland cement mortar

For studying slow crack growth in portland cement mortar 32″ (812.8 mm) long double torsion specimens were tested. During testing, the loading and reloading compliances, permanent (or inelastic) deformations and crack growth were measured. It was observed that the strain energy release rates calculated from elastic, secant or reloading compliances do not accurately represent the fracture behavior of this material. A modified definition of the strain energy release rate is developed here to include both the elastic and the inelastic strain energy absorbed during crack extension. For this method, in addition to the reloading compliance, the knowledge of the rate of change of permanent deformations with crack growth is necessary. Details of the analytical and experimental procedure are described in this paper.