Crack Initiation Threshold Research Articles

Fatigue crack-growth in shape-memory NiTi and NiTi–TiC composites

An experimental study was conducted to examine the room-temperature fatigue crack-growth characteristics of shape-memory NiTi matrix composites reinforced with 10 and 20 vol.% of TiC particles. Microstructural characterization of these hot-isostatically-pressed materials shows that the TiC particles do not react with the NiTi matrix and that they lack any texture. Overall fatigue crack-growth characteristics were found to be similar for the unreinforced and reinforced materials. However, a slight increase in the threshold for fatigue crack initiation was noted for the composites. The fracture toughness, as indicated by the failure stress intensity factor range, was found to be similar for all materials. Neutron diffraction studies near the crack-tip of the loaded fracture NiTi specimen detected no significant development of texture at the crack-tip. These results are explained by recourse to fractographic observations. Finally, a comparison is made between the micromechanisms of fracture of metal matrix composites, which deform by dislocation plasticity, and those of the present NiTi–TiC composites, which deform additionally by twinning.

A theoretical estimation of fracture toughness of zirconium hydride

The presence of hydrogen in zirconium alloys causes the hydrides to precipitate when hydrogen solubility limits are exceeded. Hydrides in zirconium alloys can lead to a significant embrittlment effect under certain circumstances, and mechanical properties, especially crack growth resistance, may be affected. As a result, there is a phenomenon of delayed hydride cracking (DHC) of zirconium alloys which is characterized by crack propagation at low values of the stress intensity factor. One of important mechanical properties of zirconium alloys is the threshold stress intensity factor, KIH, below which cracks do not grow. Therefore, the problem of hydrogen embrittlment of zirconium alloys is a very important, particularly for structural components in the nuclear industry. For example, the gradual build-up of deuterium in the Zr-2.5Nb pressure tubes of a pressurized heavy water reactor as a result of corrosion, combined with the possibility of the generation of flaws of sufficient size, makes the tubes susceptible to the initiation of DHC [1]. In this case, the service life of a pressure tube is determined by its capability to resist the crack initiation and propagation. Because of the technological importance of this phenomenon, the crack initiation threshold due to the hydride formation has been studied. A theoretical prediction of the KIH value of zirconium alloys can be based on modelling of the process zone ahead of the crack tip (e.g. [2–4]). The process zone is treated as a zone of fracture process. On the other hand, hydrided zirconium alloy can be interpreted as zirconium-zirconium hydride mixture. In this case, to predict the value of KIH of hydrided zirconium alloy it is necessary to estimate the fracture toughness of the hydride. However, measurements of fracture toughness of δ-zirconium hydrides showed large scatter due to specimen problems because of their extreme brittleness [5]. Therefore, a theoretical estimation of the value of K H IC for bydride is very important for an analysis and optimisation of crack growth resistance in zirconium alloys. This paper presents a theoretical estimation of fracture toughness of zirconium hydrides. The theory is based on a local approach to fracture of zirconium alloys. Recently, a model for the threshold stress intensity factor, KIH, was suggested leading to an analysis of crack growth initiation in zirconium alloys during DHC [4]. Local criterion of brittle fracture and modelling of the fracture process zone were used to describe fracture initiation at the hydride platelet in the process zone ahead of the crack tip. The hydride platelet was assumed to cover the process zone and the crack growth proceeds through fracturing of the hydride. The critical condition for fracture initiation in the hydride is reached when the maximum of the local stress approaches the fracture strength of the hydride at some characteristic (critical) distance, rc, ahead of the crack tip (Fig. 1). It was shown that the theoretical KIH-estimation applied to the case of mixed plane condition within the process zone is qualitatively consistent with experimental data for unirradiated Zr-2.5Nb alloy. In the framework of the proposed model, the theoretical value of KIH for a single hydride platelet at the crack tip characterizes the ultimate lower bound value of crack resistance in hydrided zirconium alloy during DHC, whereas the minimum of the threshold stress intensity factor, (KIH)min, should be considered as the minimal capability of the hydride to resist fracture within the hydride, i.e. the fracture toughness, K H IC, of hydride. The minimum fracture toughness of structural materials can be described by a model called the Kμ-model [6]. The following equation should be valid

Effects of Grain Size on the Initiation and Propagation Thresholds of Stress-induced Brittle Fractures

The microstructure of rock is known to influence its strength and deformation characteristics. This paper presents the results of a laboratory investigation into the effects of grain size on the initiation and propagation thresholds of stress-induced brittle fracturing in crystalline rocks with similar mineralogical compositions, but with three different grain sizes. Strain gauge and acoustic emission measurements were used to aid in the identification and characterization of the different stages of crack development in uniaxial compression. Results indicate that grain size had only a minor effect on the stress at which new cracks initiated. Crack initiation thresholds were found to be more dependent on the strength of the constituent minerals. Grain size did have a significant effect, however, in controlling the behaviour of the cracks once they began to propagate. The evidence suggests that longer grain boundaries and larger intergranular cracks, resulting from increased grain size, provide longer paths of weakness for growing cracks to propagate along. This promoted degradation of material strength once the longer cracks began to coalesce and interact. Thus, rock strength was found to decrease with increasing grain size, not by inducing crack initiation at lower stresses, but through a process where longer cracks propagating along longer planes of weakness coalesced at lower stresses.

Identifying crack initiation and propagation thresholds in brittle rock

Recent work at the Underground Research Laboratory of Atomic Energy of Canada Limited in Pinawa, Manitoba, has shown that high compressive stresses near the tunnel face significantly contribute to the loss of strength, and eventual failure of the rock, through stress-induced brittle fracturing. A program of laboratory testing has been undertaken to investigate the effects of brittle fracture on the progressive degradation of rock mass strength. The work carried out in this study involves a detailed analysis of the crack initiation and propagation thresholds, two key components in the brittle-fracture process. This paper describes new techniques developed to enhance existing strain gauge and acoustic emission methodologies with respect to the detection of these thresholds and their effects on the degradation of material strength.Key words: tunnel, rock failure, brittle fracture, crack initiation, crack propagation.

Identifying crack initiation and propagation thresholds in brittle rock

Identifying crack initiation and propagation thresholds in brittle rock

Fatigue crack growth of Ni 3Al(CrB) single crystals at ambient and elevated temperatures

Fatigue crack propagation behavior of compact tension specimens of Ni 3Al(CrB) single crystals has been examined at room temperature (RT) and elevated temperature. The results indicate that crystallographic cracking occurs on two or more {111} planes simultaneously at RT. With increasing temperature, the cube slip plane becomes active and plays an important role in determining the cracking path. Orientation and temperature dependence of the fatigue crack initiation threshold from the notch root exists. Similar to the flow stress, the crack initiation threshold increases with temperature, and the threshold for ( 1 00)[010] orientation is higher than that for (110)[1 1 0]. Both orientation and temperature affect the fatigue crack growth rates (FCGRs). The FCGRs of ( 1 00)[010] are higher than that of (110)[1 1 0] in the Paris regime. A crystallographic relationship between the primary crack plane and the secondary slip plane ahead of the crack tip is suggested to explain the dependence of FCGRs on orientation. The oxidation embrittlement at the crack tip and the activated cube slip should be responsible for the acceleration of FCGRs in the Paris regime and the oxide-induced crack closure has a significant effect on the deceleration of FCGRs in the near-threshold regime at elevated temperatures. Both the relationship of orientations between octahedral slip planes and the applied load axis and the increasing activity of the cube slip plane with temperature determine the significant variation of fracture path with temperature.

Changes in acoustic event properties with progressive fracture damage

Laboratory results from uniaxial compression tests performed on pink Lac du Bonnet granite samples indicate that a number of stages in the progressive failure process of rock can be identified through the combined analysis of strain gauge and acoustic emission (AE) data. Testing focused on identifying the crack initiation (σ ci ) and crack damage (σ cd ) stress thresholds, two key components in the brittle fracture process. Results from this testing also showed that the properties of the acoustic events are markedly different throughout loading most notably before and after crack initiation. This proved valuable in substantiating observations made using strain gauge data. This paper explores the use of acoustic emissions and the interpretation of the acoustic event properties in identifying the different stages of crack development.

Indentation and microcutting fracture damage in a silicon carbide coating on an Incoloy substrate

The fracture damage morphology for static indentation tests and controlled microcutting tests using a Vickers indentor was investigated for an amorphous silicon carbide coating deposited on an Incoloy substrate. Crack initiation thresholds were detected for both testing modes using an acoustic emission sensor. The fracture damage morphology for static indentation consisted of Hertzian-like cracks surrounding the indentation site with no lateral crack-chipping occurring up to the maximum indentation load of 8 N. In contrast, microcutting generates lateral cracks at the microcutting groove entrance for loads as low as 0.3 N. For loads up to 1 N, the groove damage was confined to the 5 μm thick coating and the fracture response is similar to that occurring in a monolithic brittle solid. At higher loads, extensive lateral crack chipping occurred along the microcutting grooves accompanied by coating decohesion at the root of the chip.

Split rate fatigue propagation in polymer blends

The long-term properties of PαSAN/PMMA blends have been examined by conducting fatigue tests on a fracture mechanics-type specimen. Initial testing on the individual constituents showed that the crack initiation threshold was higher in the PMMA polymer than in the PαSAN system. Alloying these polymers resulted in the crack initiation thresholds between those of the two base polymers. A scanning electron microscopy study of the fracture surfaces indicated that fatigue loading resulted in the formation of well defined striations, the spacing of which depended on the crack velocity and morphology. The fatigue striations in a fine-structured blend remained straight and did not appear to be modified by the presence of the secondary phase. Crack extension in a coarser structure depended upon the characteristics of the particular phase in which the crack was extending. It was observed that the striations in the less ductile PαSAN phase were more closely spaced as well as being curved. It is believed that the crack propagated more rapidly in this phase resulting in a split rate crack velocity within the polymer blend.

Comparative ultrasonic analysis of damage in CFRP under static indentation and low-velocity impact

Non-destructive testing plays an important role in analyzing composite damage by supplying useful information on damage initiation and propagation. However, for the scientist, the main problem to be solved is the choice of how best to detect the damage. For impact damage to carbon/epoxy plates, widespread methods such as X-ray inspection are not easy to use because the damage does not always penetrate completely through the thickness. The choice of an ultrasonic facility designed to dimension the damage correctly and give access to interface delaminations has thus proved to be necessary. The ultrasonic method has been applied here for a comparative analysis of the damage detected after static indentation and after low-velocity impact on a composite with quasi-isotropic lay-up and a comparative analysis of the damage caused by static indentation for two different lay-ups (quasi-isotropic and cross-ply) on T300/914 material. The results show the damage process. A crack initiation threshold, crack propagation, delamination initiation threshold and delamination propagation are observed. The propagation of the delaminated area is similar for static and impact damage versus the maximum load. Similarly, propagation of the delaminated areas obtained in through-transmission mode is similar for the two lay-ups analyzed. By using a ply-by-ply ultrasonic analysis the authors observed a slightly different internal delamination process comparing cross-ply and quasi-isotropic laminates under static indentations. Two other methods, fractography and accelerometry, were used in association with the ultrasonics to complete the results. The influence of parameters, such as the indenter diameter and the velocity and form of loading, was also evaluated under local bending loads.

Increasing the limit of usability of CFRP tubes by built-in stresses

The limit of usability of a CFRP structure is often determined by first ply failure (FPF) indicated by interfibre/matrix cracking. It will be shown how for certain types of CFRP structures this crack initiation threshold can be delayed. The process used here is prestraining in analogy to that of prestressed concrete. It is a so-called thermo-mechanical after treatment, inducing advantageous compression built-in stresses, i.e. negative strains transverse to the fibres. Cylindrical tube specimens were wound, prestrained and submitted to hydraulic pressurisation or spin load testing. Acoustic emission monitoring and crack counting following sectioning confirmed an increase in crack threshold level for this specimen type from ϵ hoop ≈ 0·8% to 1·3%, which is higher than the operational strain, normally defined as tensile failure strain e ∥ t of the fibre divided by the usual design factor j u = 1·5. Theoretical investigations were done in support of the design of the prestraining device and of the tube specimen, as well as an evaluation of experimental data.

Crack initiation threshold as a parameter for characterizing fused alumina abrasive grains

The crack initiation threshold F c , which is determined using the Vickers indentation technique, was used to estimate abrasion strengths of different fused alumina abrasive grains. Assuming Weibull distributed strength, the dependence of the crack initiation probability on the indentation load F according to F − 1 4 was verified. The crack initiation thresholds of Al 2O 3 crystals correlate with the strengths of the abrasive grains which were determined by static and dynamic methods. The contents of chromium, iron, manganese and titanium in the alumina crystals were estimated by electron paramagnetic resonance and energy dispersive X-ray spectrometry measurements; the results indicate that the crack initiation thresholds are not essentially determined by these impurities.

Markov modelling of the fatigue damage in welded structures under in-service inspection

This paper describes a probabilistic cumulative fatigue damage model based on a simple Markov chain approach. The objective is to develop a tool for a reliability assessment and a strategy for periodic inspection of welded joints in marine steel structures. The model is a unit-jump discrete-state/discrete-time Markov chain where damage states are related to the fatigue crack depths. The model parameters are estimated from the experimental statistics of the fatigue crack initiation and growth in fillet-welded joints tested under uniaxial constant-amplitude (CA) loading. By the use of an automatic crack monitoring system the statistics of the lifetime to reach given crack depths are obtained. Based on a fracture mechanics approach and the statistics of S-N data the model is extended to deal with variable-amplitude loading. The possibilities of taking into account crack initiation and crack threshold are discussed. The practical engineering utility of the model is emphasized and a case study for an articulated column in the North Sea is included.

Cyclic J-integral in relation to fatigue crack initiation and propagation

Fatigue crack initiation (threshold) and propagation are associated with an appreciable sensitivity to one basic factor, among others (environment, microstructure, frequency, temperature etc.), namely load ratio R. The crack closure concept has frequently been hypothesized as a possible explanation for this. Nevertheless, existing experimental results with respect to crack closure are somewhat conflicting due to significant variabilities of such factors as specimen geometries and stress intensity factor ranges. This paper intends to describe the R- dependent fatigue threshold as well as fatigue crack growth from a different point of view, i.e. from the global energy equivalence concept. The generalized driving force attached to a fatigue crack tip is in the first instance re-examined with the introduction of a new cyclic J-integral. The original definition of a cyclic J-integral is shown to be inadequate as it fails to account for the total energy flux into the crack tip region from external loading mechanisms. Under small scale yielding conditions, the newly defined cyclic J-integral is equivalent to the range of elastic energy release rate, ΔG, for a linear elastic crack independent of loading processes. It is proposed to use ΔJ as an appropriate criterion for fatigue crack growth so long as the crack does not extend during the unloading portion of one load reversal. ΔJ as such is also interpreted as the source supplier for the energy flow rate dissipated on crack tip cyclic plastic deformations. It is then assumed that, under fixed test conditions, the minimum specific work of fracture required for fatigue crack initiation, ΔJ th , is a material property independent of the load ratio R. The so-predicted values of fatigue threshold, ΔK th , are correlated favorably with experimental measurements. For fatigue crack growth (FCG), a unified formulation is capable of being derived from the thermodynamic theory of irreversible processes, if the cracked surface area is taken as an internal variable and the rate of energy dissipated on FCG depends on ΔJ only. The applicability of the formulation for producing a master FCG diagram is examined for both metals and non-metals, including steel, aluminum alloys, PMMA and PVC, etc.

Thermal Oxidation of Al2O3-SiC Whisker Composites: Mechanisms and Kinetics

AbstractIt has been shown that SiC whisker-reinforced Al2O3 is susceptible to thermal oxidation because of a series of reactions in which SiC is oxidized and then reacts with Al2O3 to form a whisker-free layer of mullite and silica glass. In this paper, the kinetics of the reaction are analyzed as a function of the composition and whisker content of the composite. The mechanism of the reaction is described in terms of the evolution of phases within the reaction layer on the basis of transmission electron microscope observations and concentration profiles determined by electron microprobe analyses. The reaction is initiated by oxidation of SiC, forming silica glass and graphitic carbon. The silica glass penetrates the Al2O3 matrix along grain boundaries, and reacts to form mullite. As the reaction proceeds, Al2O3 is consumed, leaving residual intragranular Al2O3 islands, and a population of transgranular microcracks in the mullite. It has also been shown that Al2O3-SiC composites are susceptible to crack nucleation and growth under cyclic compressive loads. The presence of the mullite reaction layer on the surface of the composite reduces its resistance to crack initiation under these conditions. The threshold for crack initiation in cyclic compression is measured as a function of reaction layer thickness.

On the expression of fatigue crack initiation life considering the factor of overloading effect

In the present study attempts are made to give an expression of the fatigue crack initiation life of notched elements with the consideration of overloading effects. This expression may be used to predict the fatigue crack initiation life of notched element under variable amplitude loading. Experimental work on LY12CZ alloy show that the test results of fatigue crack initiation life after tension overloading can be well fitted by the formula developed before for fatigue crack initiation life. Tension overloading increases the fatigue crack initiation threshold but has no effect on the coefficient of the resistance to fatigue crack initiation. The overloading ratio has no markable effect on crack initiation life. The increase of the crack initiation threshold results in the increase of crack initiation life, in particular, in long life range. The same results are also obtained by reanalysing some existing test results of overloading effect on crack initiation life given in literature. Consequently, the expression of the fatigue crack initiation life can be obtained by the method given in this paper. However, the overloading stress should be determined from the theoretical stress concentration factor of notched element and the maximum nominal stress in the load spectrum of elements.

Surface crack growth behaviour under tensile cyclic loading

An experimental investigation of LY12R plate specimens with various notches under tensile fatigue loading has been carried out. In accordance with the a- N and c- N curves, a classification for the surface crack is proposed. Surface crack initiation, growth behaviour and surface crack threshold are studied. An empirical expression to predict the surface crack depth is obtained.

A further study on fatigue crack initiation life — mechanical model for fatigue crack initiation

In the present study, a new formula for the fatigue crack initation life is developed based on recent progress in the study of fatigue damage and crack initiation, and is substantiated experimentally. The new formula reveals a correlation between the fatigue crack initiation life, the notched element geometry, the cyclic loading condition, the tensile properties and the fatigue crack initiation threshold, which is indicated as an important parameter in describing the fatigue damage and crack initiation. The correlation between the fatigue crack initiation threshold, the endurance limit and the tensile properties is also given. The threshold for fatigue crack initiation can be obtained by the regression analysis of the fatigue crack initiation life test data without any additional test.

Effect of Water Content on Mechanical Properties of Na2O‐SiO2 Glasses

Na2O‐SiO2 glasses with high water contents (up to =72 wt%) were prepared under high‐pressure hydrothermal conditions. Hardness, crack initiation, and glass‐transition temperature were investigated. The hardness and the glass‐transition temperature decreased and the crack initiation threshold increased with increasing water content, which suggests that dissolved water promotes deformation by plastic flow.

The crack-initiation threshold in ceramic materials subject to elastic/plastic indentation

The threshold for indentation cracking is established for a range of ceramic materials, using the techniques of scanning electron microscopy and acoustic emission. It is found that by taking into account indentation plasticity, currant theories may be successfully combined to predict threshold indentation loads and crack sizes. Threshold cracking is seen to relate to radial rather than median cracking.