Crack Extension Behavior Research Articles

腐食防食小特集 破壊じん性値がき裂速度に依存するぜい性材料の安定破壊条件と擬安定現象

By assuming that the fracture toughness value (crack resistance force) R is expressed by a constitutive relation as R (a, a) ∝ R1(a)·am*, (a and a; crack length and crack velocity, m*; rate sensitivity parameter), the stability condition of cracked specimens is analytically discussed for rate sensitive brittle materials such as glasses and ceramics. The stability criterion obtained is identical in its form to the one reported by Gurney and Hunt in which the rate effect is not taken into account. However, by introducing the rate effect into the analysis, it was found that an initial crack begins to extend before the load maximum appears. This fact that load gets larger, nevertheless crack extends, is named “pseudo stable phenomena (PSP)”.Load-displacement and crack extension behaviour were simulated by numerical calculations including the rate effect. The PSP was confirmed to appear in rate sensitive brittle materials. Experiments on Pyrex glass also showed the appearance of PSP, which is prominent when EBΛ* gets larger (E and B; Youngs modulus and thickness of the specimen, Λ*; the compliance of the testing machine).

Fatigue crack extension behaviour in porous plasma spray 80Ni-20Cr material: the influence of the R-ratio : Bulloch, J.H. and Schwartz, I. Theor. Appl. Fract. Mech. July 1991 15, (2), 143–154

Fatigue crack extension behaviour in porous plasma spray 80Ni-20Cr material: the influence of the R-ratio : Bulloch, J.H. and Schwartz, I. Theor. Appl. Fract. Mech. July 1991 15, (2), 143–154

Static and Dynamic Fracture Toughness of Ductile Cast Iron.

In the present study, crack extension behavior and fracture toughness of ductile cast iron are examined by three-point bending tests with various detection methods of crack initation under static and dynamic loading conditions. Loading on specimens is interrupted at various displacement points, and fracture surfaces of the specimens are observed by scanning electron microscopy (SEM). Each method is effective in detecting the strict crack initiation at the middle of specimen thickness under static and dynamic loading conditions. J (mid) is proposed to define the fracture toughness of ductile cast iron. Crack tip opening displacement(CTOD) under the dynamic loading condition is smaller according to the results of calculation and SEM observation. It is considered that crack initiation under the dynamic loading condition is accelerated since the plastic deformation cannot follow up deflection of the specimen.

Understanding and eliminating crack extension behaviour in feed water deaerator systems—A critical review

A background to deaerator cracking is given in the form of operational considerations, crack extension characteristics and the possible mechanisms involved in this particular cracking phenomenon. The reader is then introduced to modern day advances and concepts of environmental assisted crack (EAC) extension behaviour, which has been developed in the nuclear engineering field over the past, almost two, decades. Such modern day EAC concepts are considered and have been applied to the area of crack extension in feed water deaerator systems and this has resulted in a sophisticated electrochemical approach to this particular problem.

Interrelation among flaw resistance, KR-curve behavior and thermal shock strength degradation in ceramics. II. Experiment

Indentation-induced flaw resistance, toughness behavior upon crack extension and thermal shock behavior of a range of different ceramics are investigated. The results are analysed and compared with previous theoretical considerations. An attempt is made to establish an interrelation among the above three properties of ceramics. Für eine Reihe unterschiedlicher keramischer Werkstoffe wurde das R- bzw. K R -Kurvenverhalten sowie die Thermoschockbeständigkeit untersucht. Die Ergebnisse wurden analysiert und mit früheren theoretischen Überlegungen verglichen. Es wird der Versuch unternommen, zwischen den drei oben erwähnten Eigenschaften von keramischen Werkstoffen einen Zusammenhang abzuleiten. Nous avons étudié, pour un éventail de céramiques, la résistance à la fissuration induite par indentation, la ténacité par propagation de fissures et le comportement aux chocs thermiques. Les résultats sont analysés en vue d'être confrontés aux considérations théoriques préalables. On essaie notamment d'établir une corrélation entre les trois propriétés pré-citées.

Threshold fatigue crack extension characteristics of a low alloy steel: The influence of environment and microstructure

The present investigation is aimed at examining the influence of environment and microstructure on the fatigue crack extension behaviour of a low alloy steel. Significant environmental effects were recorded under low R-Ratio testing but not under high R-Ratio conditions, viz., the ΔK th values recorded in salt solution were significantly larger than those for a dry air environment and a constant value of 10 MPa√ m was recorded irrespective of microstructure (yield strength). Such effects were attributed to oxide induced closure effects. Above threshold, environmental assisted cracking behaviour in the salt solutions was the result of the appearance of transgranular cleavage facets on the fatigue surfaces. The extent of this environmentally enhanced cracking was simply related to the extent of cleavage failure.

Influence of mean stress on the fatigue crack growth characreristics of [formula omitted] CrlMo steel tube at elevated temperature

The fatigue crack growth characteristics of 2- 1 4 CrlMo steel have been investigated at 861 K over the R-ratio range 0.1–0.7 utilising a dwell time of 10 min. at maximum load. All tests were conducted under load control in a laboratory air environment. It was established that the R-ratio significantly affected the fatigue crack extension behaviour inasmuch that with increasing R-ratio, the critical ΔK level for the onset of creep fatigue interactive growth, ΔK IG, decreased from 20 to 7 MPa√m and the threshold stress intensity, ΔK th, decreased from 9 to about 3 MPa√m. At intermediate ΔK levels, i.e. between ΔK th and ΔK IG, the fatigue crack extension rates, for all R-ratio values, resided on or slightly below the CTOD line, which represents the upper bound for contrnuum controlled fatigue crack growth. Creep fatigue interactive growth was typified by crack extension rates that reside above the CTOD line with a ΔK IG dependence; the attainment of some critical creep condition or crack linkage condition which causes the abrupt change in crack extension behaviour at ΔK IG; and crack extension occurs almost exclusively in an intergranular manner. The R-ratio and ΔK IG followed a linear relation. A literature review concerning the effect of temperature on the threshold fatigue crack growth characteristics of low alloy ferritic steels demonstrated powerful effects of temperature; the magnitude of these effects, however, were dependent upon the testing temperature regime and R-ratio level. The effect of R-ratio on ΔK th was greatest at temperatures >400°C, significant at ambient temperatures and least in the temperature range 90°C to <300°C. The relationship between temperature and ΔK th, at a given R-ratio, exhibited a through and a minimum ΔK th value was observed in the temperature range 200–250°C. The magnitude of the temperature effects on ΔK th decreased with increasing R-ratio. Such effects of temperature and R-ratio on ΔK th was reasonably explained in terms of crack closure effects. Finally, the present elevated temperature fatigue crack growth data exhibited massive crack extension enhancement values when compared to ambient near-threshold fatigue crack growth data for 2- 1 4 CrlMo steel. Such large enhancement values were the combined effects of temperature (environment) and frequency.

Fiber Bridging Effect on In-Plane-Shear Mode Crack Extension Resistance of Unidirectional Fiber-Reinforced Composites

The fiber bridging effect, which affects the crack extension resistance in fibrous composites, is discussed in the case of in-plane-shear mode crack extension parallel to fibers in unidirectional fiber-reinforced composites. We first make a model of the bridging of a single fiber and estimate the force acting on the crack surface through a bridging fiber. Then, introducing the stochastic process of fiber breakage, we obtain the quantitative relationship between the relative crack surface displacement and the equivalent cohesive stress which is the probabilistic expectation of forces acting on the crack surface through a large number of bridging fibers. We numerically simulate the crack extension behavior with the equivalent cohesive stress acting on the crack surface. Then the simulated results are consistent with the experimental results. We finally conclude that the in-plane-shear mode crack extension is greatly affected by the stochastic process of fiber breakage.

A review of the fatigue crack extension behaviour of ferritic reactor pressure vessels materials in pressurized water reactor environments: Bulloch, J.H. Res. Mech. 1989 26, (2), 95–172

A review of the fatigue crack extension behaviour of ferritic reactor pressure vessels materials in pressurized water reactor environments: Bulloch, J.H. Res. Mech. 1989 26, (2), 95–172

Analysis of crack extension in anisotropic materials based on local normal stress

The normal stress ratio theory is applied to predict crack extension behavior in center-notched unidirectional graphite-epoxy of arbitrary fiber axis orientation, subjected to arbitrary far-field planar loading. The theory is applied within analytical solutions for two infinite plate geometries: a plate with a sharp center crack, and a plate with an elliptical center flaw. A critical analytical case is identified suggesting that application of the theory within a stress solution modelling crack tip shape may increase the accuracy of crack growth direction predictions. Crack extension direction, location of crack extension, and critical stress predictions of the theory are compared to those obtained from experiments on specimens subjected to tensile, shear, and mixed-mode far-field loading. The comparison shows that, applied within each analytical solution, the normal stress ratio theory provides verifiable predictions of crack growth behavior. By modelling actual notch tip shape, the elliptical notch solution is able to provide accurate qualitative predictions of the origin of crack extension along the periphery of a cut notch tip in a way that the sharp crack analysis cannot. The sharp notch solution appears to provide slightly more accurate crack growth direction predictions, however. Also, in predicting critical applied far-field stresses, the sharp crack solution appears to exhibit a stronger ability to model subtle experimental trends.

Fiber bridging effect on in-plane shear mode crack extension resistance of unidirectional fiber-reinforced composites.

The fiber bridging effect, which affects the crack extension resistance in fibrous composites, is discussed in the case of in-plane-shear mode crack extension parallel to fibers in unidirectional fiber-reinforced composites. We first make a model of the bridging of a single fiber and estimate the force acting on the crack surface through a bridging fiber. Then, introducing the stochastic process of fiber breakage, we obtain the quantitative relationship between the relative crack surface displacement and the equivalent cohesive stress which is the probabilistic expectation of forces acting on the crack surface through a large number of bridging fibers. We numerically simulate the crack extension behavior with the equivalent cohesive stress acting on the crack surface. Then the simulated results are consistent with the experimental results. We finally conclude that the in-plane-shear mode crack extension is greatly affected by the stochastic process of fiber breakage.

一方向連続繊維強化複合材料中の開口型き裂のき裂進展抵抗に及ぼす繊維橋渡し効果

In this paper, discussion is focused on the mechanical modelling of fiber bridging which affects the resistance of opening-mode crack extension parallel to fibers in unidirectional fiber-reinforced composites. Firstly, the formation process of a bridging fiber is considered and a model of this single fiber bridging is made to characterize precisely the behavior of bridging fibers under crack opening. By using this model, the forces acting on crack surfaces through a bridging fiber are estimated and the relationship between crack opening displacement and equivalent cohesive stress due to large number of bridging fibers is obtained. An attempt is made to derive equations which describe the crack extension with the cohesive stress acting on crack surfaces. By solving numerically these equations, the crack extension behavior is simulated. The simulated result shows agreement with the experimental result of a unidirectional glass fiber-reinforced epoxy composite.It is concluded that the fiber bridging discussed here accounts for the opening-mode crack extension behavior parallel to fibers in the unidirectional fiber-reinforced composites.

Crack extension resistance of unidirectional fiber-reinforced composites and a cohesive force model of crack surface interactions.

Crack extension resistance in fiber-reinforced composites is considered from the view point of crack surface interactions, which may be induced by some micromechanisms such as fiber bridging behind the crack tip. Using a concept of interactive stress, equivalent cohesive stress is evaluated as a function of the relative displacement between upper and lower crack surfaces. With some experimental results of crack extension parallel to fibers in a unidirectional continuous fiber-reinforced epoxy composite, we conclude that this inherent relationship between cohesive stress and relative crack surface displacement completely determines the crack extension behaviour of the composite. We also compare opening mode crack extension with in-plane-shear mode crack extension, and find that in-plane-shear mode cohesive stress is much larger than that of opening mode. Furthermore, in-plane-shear mode cohesive stress is shown to decline sharply under large relative crack surface displacement, while opening mode cohesive stress tends to remain a constant value. This difference of cohesive stresses accounts for the characteristic behaviour of crack extension in each mode.

Effect of anisotropy on mixed mode crack extension behavior in granite.

This paper describes an experimental investigation of the effect anisotropy on mixed mode crack propagation in granite by the use of asymmetric three-point bend specimens. Acoustic emission techniques were employed during the fracture tests to detect the onset of kinked crack extension. In order to characterize the fracture strength anisotropy, uniaxial tension tests and Brazilian tests were conducted, and fracture strength was measured as a function of direction with respect to the anisotropy. Based on the fracture strength data, the results of the mixed mode fracture tests were analyzed with regard to the fracture angle by means of linear elastic fracture mechanics. It is shown that the concept of a critical tensile stress at a characteristic distance, modified to account for the anisotropy of fracture strength, can explain the observed extension behaviors mixed mode cracks.

Study on Crack Resistance Performance of Continuous Glassfiber Reinforced Polyurethane Foam (2 nd Report)

The purpose of this study is to make a quantitative evaluation of the effect of glassfibers on the increased crack resistance performance of reinforced PUF (polyurethane foam), which was developed to meet the requirements of insulation material for the TGZ Mark III cargo containment system of LNG carriers. In this report, the crack extension behavior and the fracture toughness of reinforced PUF (R-PUF) have been examined based on compact tension tests at room and low temperature in comparison with those of non-reinforced PUF (G-PUF) following the first report, in which the three-point bending test results at room temperature were shown and discussed.In particular, a different behavior in crack extension modes and fracture toughness of R-PUF has been experimentally characterized depending on direction of a notch, tempearture and crosshead speed in the compact tension tests. The acoustic emission analyses and the finite element simulations proposed in the previous report have been shown to be effective in comparing the fracture behavior of R-PUF with that of G-PUF in the compact tension tests as well as the case of three-point bending tests.

Study on Crack Resistance Performance of Continuous Glassfiber Reinforced Polyurethane Foam (1st Report)

Rigid PUF (polyurethane foam) is one of materials for in-ground LNG storage tanks which have been successfully constructed since 1970. For LNG carriers, however, it is necessary to use a material having sufficient strength to withstand cyclic load and impact load caused by hull motion in waves and hull deformation etc., and having excellent resistance to crack propagation as well. To meet this requirement, reinforced PUF consisting of continuous glassfiber uniformly dispersed in rigid PUF has been developed especially for TGZ Mark III cargo containment system.The present paper aims at making a quantitative evaluation of the reinforcing effect of glassfiber in reinforced PUF from a viewpoint of increased crack resistance performance. In this report, crack extension behavior and fracture toughness of reinforced PUF have been examined in detail in comparison with those of rigid PUF under a three-point bending test on notched beam specimens at room temperature. Acoustic emission analyses and finite element simulations have been shown to be successful in comparing the fracture behavior of reinforced PUF with that of rigid PUF.

Surface Acoustic Wave Measurements of Surface Cracks in Ceramics

An investigation of scattering from surface cracks has been conducted. In particular, the change in the reflection coefficient of a Rayleigh wave incident on a surface indentation crack has been measured as the sample is stressed to fracture. The acoustic measurements have been correlated with the stable crack extension that precedes final failure. The crack extension behavior of as-indented specimens was found to differ appreciably from that of annealed specimens. Cracks in the annealed samples exhibited partial crack tip closure, but little stable extension, whereas cracks in the as-indented samples displayed both crack closure and irreversible crack growth. This behavior has been rationalized by invoking concepts based upon the residual stresses created by indentation.

A combination of statistics and subcritical crack extension

The change in fracture probability after a subcritical crack extension process within a ceramic material is demonstrated. The theoretical treatment of the problem is based on four assumptions which include the initial fracture probability and a materials function concerning the subcritical crack extension behaviour. Concepts such as the “initial defect strength”, the “strength function” and “statistics-mechanics-synthesis” are introduced and explained. An experimental verification is given to enable the theoretical predictions to be compared with the data evaluated from ceramic materials. Good agreement between theory and experiment was found.

Surface acoustic wave measurements of surface cracks in ceramics

An investigation of scattering from surface cracks has been conducted. In particular, the change in the reflection coefficient of a Rayleigh wave incident on a surface indentation crack has been measured as the sample is stressed to fracture. The acoustic measurements have been correlated with the stable crack extension that precedes final failure. The crack extension behavior of as-indented specimens was found to differ appreciably from that of annealed specimens. Cracks in the annealed samples exhibited partial crack tip closure, but little stable extension, whereas cracks in the as-indented samples displayed both crack closure and irreversible crack growth. This behavior has been rationalized by invoking concepts based upon the residual stresses created by indentation.

Adhesive bond durability with conversion coatings

The effect of processing time during conversion coating of 2024-T3 clad aluminum alloy on adhesive bond strength and durability was investigated. Iridite 14-2, both by immersion (leached and non-leached) and brush application methods, and alodine 1500 processes were studied. Bond durability and strength were determined on both primed and unprimed surfaces by performing wedge and T-peel tests, respectively. The morphology and thickness of the conversion coatings prepared by varying the processing time were studied by scanning transmission electron microscopy (STEM). The chemical composition of the surfaces was determined by electron spectroscopy for chemical analysis (ESCA). It was found that wedge test crack extensions for brush iridite 14-2 (both primed and unprimed) and primed leached iridite 14-2 surfaces were comparable to those of FPL-prepared surfaces when recommended application times were observed. Unfavorable crack extensions for primed iridite 14-2, alodine 1500 (both primed and unprimed) and unprimed leached iridite 14-2 surfaces dictate that those surface preparation methods should not be used where adhesive bond durability is desired. All crack extension behavior observed can be explained by radiations in surface roughness. Surface roughness seems to be critical in achieving durable adhesive bonds.