An experimental investigation of dynamic crack propagation

The dynamic fracture behavior of polymethylmethacrylate (bdPMMA) has been investigated. The specimens were in the form of rectangular sheets with sharp notches. The elastodynamic crack tip stress field and the crack velocity were determined by the use of resistance strain gauges. An analytic expression for the dynamic crack tip stress field was used to evaluate the dynamic stress intensity factors, and the dynamic arrest toughness was also determined.The dynamic response of the stresses at the notch tip at varying loading rates was considered and some “hysteresis” fracture phenomena were observed.     

Ion migration from fluoride-releasing dental restorative materials into dental hard tissues

This study was carried out in order to determine the extent to which ions released from fluoride-containing dental restoratives migrated through the enamel and dentine of extracted teeth. A total of 40 permanent human 3rd molars were used. They were extracted for orthodontic reasons, and employed within 1 month of extraction. A cervical (Class V) cavity was prepared in each tooth, then filled with one of: a conventional glass-ionomer, a resin-modified glass-ionomer, a polyacid-modified composite resin (“compomer”) or a fluoride-releasing resin composite. Ten samples were prepared per material. After 1 month, five specimens per material were prepared and examined under SEM/EDX. Concentrations of sodium, aluminium, strontium, fluorine, magnesium, silicon, phosphorus and calcium were determined within the tooth. After 18 months, the remaining five specimens for each material were prepared and studied in the same way. The greatest extent of ion migration into the tooth was found with the conventional glass-ionomer and least migration was found for the fluoride-releasing composite, which showed no evidence of fluoride migration at all. Levels of migrating ions were generally higher in the 18 month specimens than in the 1 month specimens, and also higher in the dentine than in the enamel. Ions released by restorative dental materials have been shown conclusively for the first time to be capable of migrating into the enamel and dentine surrounding the restoration. The conventional glass-ionomer showed the highest level of ion migration whereas the fluoridated composite resin showed little if any ion migration. This suggests that the conventional glass-ionomer has the greatest caries inhibiting effects of all the materials tested, and the fluoridated composite the least.     

An experimental investigation into dynamic fracture: III. On steady-state crack propagation and crack branching

This is the third in a series of four papers in which problems of dynamic crack propagation are examined experimentally in large, thin sheets of Homalite-100 such that crack growth in an unbounded plate is simulated. In the first paper crack initiation resulting from stress wave loading to the crack tip as well as crack arrest were reported. It was found that for increasing rates of loading in the microsecond range the stress intensity required for initiation rises markedly. Crack arrest occurs abruptly without any deceleration phase at a stress intensity lower than that which causes initiation under quasi-static loading.In the second paper we analyze the occurrence of micro cracks at the front of the running main crack which control the rate of crack growth. The micro cracks are recorded by real time photography. By the same means it is shown that these micro cracks grow and turn away smoothly from the direction of the main crack in the process of branching.In the present paper we report results on crack propagation and branching. It is found that crack propagation occurs at a constant velocity although the stress intensity factor changes markedly. Furthermore, the velocity is determined by the stress wave induced intensity factor at initiation. The terminal velocity in Homalite-100 was found to be about half the Rayleigh wave speed (0.45 C _r ). These observations are analyzed in terms of a microcrack model alluded to in the second paper of this series. A mechanism for crack branching is proposed which considers branching to be a natural evolution from a cloud of microcracks that accompany and lead the main crack. These results are believed to apply to quasi-brittle materials other than Homalite-100 and the reasons for this belief are discussed briefly in the first paper of this series.In the final paper of the series the effect of stress waves impinging on the tip of a rapidly moving crack is examined. Waves affect the velocity and the direction of propagation as well as the process of crack branching.     

Fatigue crack propagation behaviour of rotor and wheel materials used in steam turbines

The fatigue crack propagation characteristics of several rotor and wheel materials that are commonly used in rotating components of steam turbines were investigated. Particular emphasis was placed on the behaviour at near-threshold growth rates, ie below 10^?5 mm/cycle, approaching the fatigue-crack propagation threshold, $\text{ΔK}{}_{\mathrm{<mtext>th</mtext>}}$. The lifetimes of the cracks of interest lie mostly in this region, and it is also the region where few data are available.The effects of load ratio on the fatigue crack growth rates were examined, as well as the tensile, Charpy V-notch and fracture toughness properties of the rotor and wheel materials. The relationship between fatigue crack propagation behaviour and fractographic features was examined. Fatigue crack growth rate data, $\text{da/d}\text{N}$ vs stress intesity range $\text{ΔK}$, were fitted with a four parameter Weibull survivorship function. This curve fitting can be used for life estimation and establishment of $\text{ΔK}{}_{\mathrm{<mtext>th</mtext>}}$. The results show that load ratio and microstructure play a role in determining the fatigue crack threshold and fatigue crack growth behaviour.     

混凝土Ⅰ型裂缝扩展准则比较研究

采用小跨高比三点弯曲梁研究了混凝土的断裂性能。基于对跨高比为2.5和4的三点弯曲梁断裂参数计算公式的线性插值,给出了计算小跨高比三点弯曲梁断裂参数的公式。通过开展3种小跨高比三点弯曲梁试验及文献中的试验数据对该文公式进行了验证,结果表明：随试件高度从100 mm增大至200 mm,基于小跨高比三点弯曲梁所得的起裂断裂韧度基本保持不变,而失稳断裂韧度则逐渐增大,但增幅较小。该文所给公式计算所得的双K断裂参数与文献中已有方法所得结果基本相同。解析法得到的起裂断裂韧度与试验法所得结果吻合较好。通过对文献中的试验数据的分析,进一步验证了该文所给的方法可以用于研究小跨高比三点弯曲梁的断裂性能。     

An investigation into the effect of grain size on fracture behaviour of a steel using crack path analysis

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Fatigue crack propagation in polymeric materials

In order to gain a better understanding of matrix-controlled fatigue failure processes in non-metallic materials a series of fatigue tests were performed on several different polymer materials representing different classes of mechanical response. Fatigue crack propagation rates between 5×10^–6 in. cycle^–1 (127 nm cycle^–1) and 4×10^–4 in. cycle^–1 (10 300 nm cycle^–1) were measured in nylon, polycarbonate, ABS resin, low-density polyethylene and polymethyl methacrylate. A strong correlation was found between the fatigue crack propagation rate and the stress intensity factor range prevailing at the advancing crack tip. Whereas metals exhibit comparable fatigue growth rates for a given stress intensity range when normalised with respect to their static elastic modulus, the polymer materials exhibited a 1300-fold difference in crack growth rate for a given normalised stress intensity range. This observation dramatically illustrates the importance of understanding molecular motion and energy dissipation processes in polymer materials as related to their chemistry and architecture. The relative behaviour of the different polymer materials could be generally correlated with their reported damping characteristics.     

On the use of partial properties to interpret the bulk crack propagation behaviour of coarse two-phase materials

The principle of partial properties is applied for the derivation of bulk properties of materials which contain more than one structural element. Partial properties are the particular properties of these elements from which together with volume portion, morphological dimension and geometry properties concerned with critical and subcritical crack growth are derived for certain micro-structures in metals as well as polymers.     

An analysis of slow crack propagation data in PMMA and brittle materials

Slow crack propagation data in PMMA previously published by Atkins et al. are reanalyzed in terms of a thermodynamic model in which the activation area is inversely proportional to the crack extension force. Important thermodynamic activation parameters are derived from the data. A correlation between the activation area and the crack extension force is observed in many brittle materials; it is compared to Li's general correlation for activation areas in creep.

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Résumé Les résultats expérimentaux sur la propagation lente d'une fissure dans le polyméthacrylate de méthyle, publiés préalablement par Atkins et col, sont analysés de nouveau en fonction d'un modèle thermodynamique dans lequel l'aire d'activation est inversement proportionnelle à la force d'extension de la fissure. Les importants paramètres thermodynamiques d'activation sont derivés à partir des données expérimentales. Pour de nombreux matériaux fragiles, on observe une corrélation entre l'aire d'activation et la force d'extension de la fissure; cette corrélation est comparée à celle de Li pour les aires d'activation en fluage.

     

An experimental and analytical investigation of axial crack propagation in long pipes

Experimental pressure histories, axial and circumferential strains, and crack-opening shapes were compared with the predictions of a simplified model of a fracturing pipe. This model, based upon previous analytical and numerical studies, assumed that the axial strains were negligible. The comparison showed that this assumption was valid only for relatively thickwalled pipes. An analytical energy analysis, based upon experimental data, indicated that the primary energy absorption in thin-wall pipes was the axial stretching of the pipe flaps formed by the propagating crack. The pressure profiles and crack shapes developed by modifying the model to account for the axial strain energy compared well with the experimental results. This good agreement indicates that accurate predictions of dynamic axially propagating cracks may be made using a ring model which will consider the axial energy and the frictional shear forces which exist between longitudinal sections.     

Effect of crack layer on dynamic crack propagation behaviour in polystyrene

Dynamic crack propagation in thin, edge-notched polystyrene specimens was studied by the method of dynamic caustics. During crack propagation, an intensive zone of crazing surrounds and precedes the propagating crack. Therefore, an active zone ahead of the crack tip is developed. This active zone is related to the velocity of crack propagation and the strain rate of loading. The velocity of the crack and the stress intensity factor, K_I, or the energy release rate, G_I, were strongly influenced by the development of the active zone at the crack tip.     

An XFEM method for modeling geometrically elaborate crack propagation in brittle materials

We present a method for simulating quasistatic crack propagation in 2‐D which combines the extended finite element method (XFEM) with a general algorithm for cutting triangulated domains, and introduce a simple yet general and flexible quadrature rule based on the same geometric algorithm. The combination of these methods gives several advantages. First, the cutting algorithm provides a flexible and systematic way of determining material connectivity, which is required by the XFEM enrichment functions. Also, our integration scheme is straightforward to implement and accurate, without requiring a triangulation that incorporates the new crack edges or the addition of new degrees of freedom to the system. The use of this cutting algorithm and integration rule allows for geometrically complicated domains and complex crack patterns. Copyright © 2011 John Wiley & Sons, Ltd.     

An investigation into the dynamic indentation response of metallic materials

The dynamic indentation response of several ductile metallic materials [Al(111), polycrystalline copper, Fe, and Ti6Al4V] has been investigated using a pendulum-based nano-impact test. The impact process involves repetitive contact cycles until finally coming to rest in the material. Each cycle includes four phases: acceleration, indentation, rebound, and deceleration. The dynamic indentation resistance of the metallic materials scales with their hardness determined under quasi-static conditions. However, through a one-dimensional analytical model, it has been shown that the relationship between the dynamic resistance and the depth during indentation cannot be adequately described using the quadratic relationship commonly found under quasi-static conditions. A power law relationship with a reduced index was proposed and it is found the index is around 1 when the quasi-static and dynamic compliance are similar. A linear relationship between impact resistance and depth has been found during rebound, where the released elastic energy is much higher than that produced by quasi-static nanoindentation.     

High frequency cyclic crack propagation in ceramic materials

Techniques and instrumentation for studying high frequency cyclic crack propagation in ceramic materials are described. Tests performed on a silicate glass show that the cyclic slow crack growth mechanism up to 600 Hz is identical to the quasi-static mechanism. Conversely, a strong cyclic effect on the crack growth rate is observed in a tungsten carbide-cobalt material.

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Résumé On décrit les techniques et l'instrumentation utilisées pour l'étude de la propagation des fissures sous sollicitations cycliques à haute fréquence dans des matériaux céramiques.Des essais exécutés sur un verre au silicate montrent que le mécanisme de croissance cyclique lente de la fissure est identique à un mécanisme quasi-statique, pour les fréquences allant jusqu'à 600 Hz.Par contre, un effet important de la fréquence sur la vitesse de croissance de la fissure a été observé dans le cas d'un alliage pour outil de coupe au cobalt et carbure de tungstène.

     

Curved crack propagation in homogeneous and graded materials

Deflection and deviation of cracks commonly occurs because of asymmetry in crack‐tip stresses in both homogeneous materials and functionally graded materials (FGMs); yet the analysis of curved cracks has been limited to simple crack shapes, otherwise the analysis would involve extensive levels of computation. The present study investigates the approximation of curved cracks with simplified shapes. A simple analytical model justifying the use of crack‐shape approximations, developed in an earlier study on stationary curved cracks in homogeneous materials, is outlined. Then, the approach is applied to propagating cracks in both homogeneous and graded material structures. Results are presented from finite element (FE) simulations of crack propagation using exact and simplified crack shapes. The use of an approximated crack shape can provide basic estimates for crack propagation path and critical load. However, systematic divergence can occur between predictions for exact and approximated crack shapes, particularly in inhomogeneous material configurations, and so the development of solutions for non‐straight cracks in FGMs would be expedient.