Application Of Fracture Mechanics Research Articles

Application of Multi-Parameter Fracture Mechanics to Study of Crack Propagation Angle in Selected Mixed-Mode Geometry

The multi-parameter fracture mechanics becomes more and more significant, because it is shown that it can help to describe fracture processes occurring in cracked specimens more precisely than conventional linear elastic fracture mechanics. In this paper, the concept based on the Williams expansion derived for approximation of stress/displacement crack-tip fields is presented and applied on a mixed-mode configuration. Two fracture criteria for estimation of the initial crack propagation angle are introduced. A parametric study is performed in order to investigate the dependence of the crack propagation angle on the stress intensity factors ratio. Influence and importance of taking into account the so-called higher-order terms of the Williams expansion are discussed and some recommendations are stated.

Outside-the-(cavity-prep)-box thinking.

Direct placement restorative materials must interface with tooth structures that are often compromised by caries or trauma. The material must seal the interface while providing sufficient strength and wear resistance to assure function of the tooth for, ideally, the lifetime of the patient. Needed are direct restorative materials that are less technique-sensitive than current resin-based composite systems while having improved properties. The ideal material could be successfully used in areas of the world with limited infrastructure. Advances in our understanding of the interface between the restoration adhesive system and the stages of carious dentin can be used to promote remineralization. Application of fracture mechanics to adhesion at the tooth-restoration interface can provide insights for improvement. Research in polymer systems suggests alternatives to current composite resin matrix systems to overcome technique sensitivity, while advances in nano- and mesoparticle reinforcement and alignment in composite systems can increase material strength, toughness, and wear resistance, foreshadowing dental application.

Theory of cyclic creep of concrete based on Paris law for fatigue growth of subcritical microcracks

Recent investigations prompted by a disaster in Palau revealed that worldwide there are 69 long-span segmental prestressed-concrete box-girder bridges that suffered excessive multi-decade deflections, while many more surely exist. Although the excessive deflections were shown to be caused mainly by obsolescence of design recommendations or codes for static creep, some engineers suspect that cyclic creep might have been a significant additional cause. Many investigators explored the cyclic creep of concrete experimentally, but a rational mathematical model that would be anchored in the microstructure and would allow extrapolation to a 100-year lifetime is lacking. Here it is assumed that the cause of cyclic creep is the fatigue growth of pre-existing microcracks in hydrated cement. The resulting macroscopic strain is calculated by applying fracture mechanics to the microcracks considered as either tensile or, in the form of a crushing band, as compressive. This leads to a mathematical model for cyclic creep in compression, which is verified and calibrated by laboratory test data from the literature. The cyclic creep is shown to be proportional to the time average of stress and to the 4th power of the ratio of the stress amplitude to material strength. The power of 4 is supported by the recent finding that, on the atomistic scale, the Paris law should have the exponent of 2 and that the exponent must increase due to scale bridging. Exponent 4 implies that cyclic creep deflections are enormously sensitive to the relative amplitude of the applied cyclic stress. Calculations of the effects of cyclic creep in six segmental prestressed concrete box girders indicate that, because of self-weight dominance, the effect on deflections absolutely negligible for large spans (>150m). For small spans (<40m) the cyclic creep deflections are not negligible but do not matter since the static creep causes in such bridges upward deflections. However, the cyclic creep is shown to cause in bridges with medium and small spans (<80m) a significant residual tensile strain which can produce deleterious tensile cracking at top or bottom face of the girder.

Crack Path Investigation Using the Generalized Maximum Tangential Stress Criterion: Antisymmetrical Four-Point Bending Specimen

A mixed-mode geometry has been chosen in order to investigate crack propagation through the specimen. There exist several criteria for estimation of the crack path, and this work enriches the concept by application of multi-parameter fracture mechanics. Particularly, the classical MTS criterion is presented in its generalized form when several initial terms of the so-called Williams series expansion are used for the tangential stress approximation. The results obtained show that the generalized approach can be important in some special cases, when the criterion shall be applied in a larger distance from the crack tip.

Structural integrity aspects of reactor safety

The overall goal of nuclear power plant safety is to protect individuals, society and the environment from undue radiological hazard so that nuclear power production does not significantly add to the health risks to which individuals and society are already exposed. This paper addresses the safety principles followed during the design phase of life cycle of a nuclear power plant. The principles followed such as safety classification, design rules based on failure modes, detailed stress analysis, stress categorization, consideration of design basis events, failure probability, flaw tolerance, leak-before-break are described. Engineering structures always contain flaws, albeit of very small size. Fatigue and fracture are the two important failure modes affected by flaws. Thus flaw tolerance becomes very important. This is assessed by applying fracture mechanics principles. The R6 procedure, which is used for evaluation of structures containing flaws, has been incorporated in the software BARC-R6. Improvements by way of shell-nozzle junction pull-out, adoption of hot wire GTAW with narrow gap technique have been brought out. Post Fukushima incidence, resistance to seismic loading and containment design have assumed great importance. The paper describes these aspects in detail. Regulatory aspects of seismic design regarding siting, Seismic margin assessment, base isolation, retrofitting are the aspects covered under seismic design. Under the action of seismic loading, the piping in a nuclear power plant piping is vulnerable to a phenomenon called ratcheting. The process of seismic margin assessment and consideration of ratchetting has been backed up by a large experimental data. The experiments carried out on structures and piping components form a part of the paper.

Application of fracture mechanics for energetic effects predictions while wood sawing

In the classical approach, energetic effects (cutting forces and cutting power) of wood sawing process are generally calculated on the basis of the specific cutting resistance, which is in the case of wood cutting the function of more or less important factors. On the other hand, cutting forces (or power—more interesting from energetic point of view) could be considered from a point of view of modern fracture mechanics. Cutting forces may be employed to determine not only toughness but also shear yield strength, which are then applied in the models. Furthermore, forecasting of the shear plane angle for the cutting models, which include fracture toughness in addition to plasticity and friction, broadens possibilities of energetic effects modelling of the sawing process even for small values of the uncut chip. Mentioned models are useful for estimation of energetic effects of sawing of every kinematics. However, for band saws and circular sawing machines, the chip acceleration power variation as a function of mass flow and tool velocity ought to be included in analysis of sawing at larger cutting speeds.

Application of thermoelastic stress analysis for the experimental evaluation of the effective stress intensity factor

In recent years, the advent of staring array detectors has made Thermoelastic Stress Analysis (TSA) a technique with considerable potential for fatigue and fracture mechanics applications. The technique is noncontacting and provides full field stress maps from the surface of cyclically loaded components. In addition, the technique appears to have a great potential in the evaluation of the effective stress intensity factor range during fatigue since fracture mechanics parameters are derived directly from the temperature changes in the vicinity of the crack tip rather than from remote data. In the current work TSA is presented as a novel methodology for measuring the effective stress intensity factor from the analysis of thermoelastic images. ?K values inferred using TSA have been employed to estimate an equivalent opening/closing load at different R-ratios in a cracked aluminium 2024 CT specimen. Results have been compared with those obtained using the strain-offset technique showing a good level of agreement.

Proposed Revisions to the Current Charpy V-Notch Requirements for Structural Steel Used in U.S. Bridges

AbstractThe purpose of the research presented in this paper is to reexamine the current Charpy V-notch (CVN) requirements for steel bridges in the United States using state-of-the-art applied fracture mechanics analysis. Proposed revisions to the current CVN requirements are introduced. It is shown that, in general, the typical fracture toughness implied by current minimum CVN requirements results in a trend of decreasing resistance to brittle fracture as the minimum specified yield strength grade increases. The results also show that fracture resistance for the current minimum CVN requirements for the three AASHTO temperature zones is inconsistent for different high-performance steel (HPS) grades. A realignment of the current CVN test temperatures and required notch energy is proposed by normalizing all grades to the crack tolerance associated with the AASHTO yield strength grade 36 ksi steel CVN requirements.

Crack velocity-dependent dynamic tensile behavior of concrete

Experiments have consistently shown that the tensile strength of concrete increases with increasing strain rate. The reasons for this phenomenon are not yet well understood and several hypotheses have been proposed in the past to explain it. This study offers additional insight through the application of dynamic fracture mechanics. The relationship between crack velocity and strain rate of concrete is first investigated using a cohesive zone model and fitted to available experimental data. The obtained relationship is then implemented into two different versions of crack-speed dependent dynamic fracture models. Both models show that computed strength versus strain rate responses compare favorably to well-established test data, suggesting that strain rate sensitivity is strongly associated with the characteristics of dynamic crack growth and inertial effects at the boundaries of the crack. A constitutive modeling scheme that incorporates the obtained dynamic fracture models into a meso-mechanical model is also proposed to predict stress–strain behavior of concrete under dynamic tensile loading. Comparisons between model predictions and published experimental data are provided to show the accuracy of the proposed framework.

A new direction in computational fracture mechanics in materials science: Will the combination of probabilistic and fractal fracture mechanics become mainstream?

Continuum mechanics-based approximation is too often unsatisfactory for solution of real material problems especially in experimental as well as computational fatigue applications. Various methods of classical-deterministic analyses often produce inconclusive or conflicting estimates of the fatigue life of a component. In addition, the classical Griffith–Irwin–Orowan concept that assumed the phenomena based on homeomorphism mathematics cannot be developed any closer to the experimental results anymore. It has already reached its saturation point. This note discusses the fundamental reasons of the limitations of classical fracture mechanics and subsequently predicts alternatives. Application of classical fracture mechanics to engineering problems is discussed along with possible alternatives employing probabilistic and fractal fracture mechanics in materials engineering.

Fracture Mechanics Study on Stress Corrosion Cracking Behavior under Corrosive Environment

This paper deals with applicability of non-linear fracture mechanics to crack growth by stress corrosion cracking (SCC) under large-scale yielding and in a plastically deformed area. Crack growth test by compact tension specimen is carried out to evaluate crack growth rate under small-scale and large-scale yielding conditions. To evaluate the crack growth behavior from a crack initiated in a plastically deformed area, crack growth test is also carried out for a very short pre-crack in a plastically deformed four-point bending specimen. Conventional stress intensity factor (K) and equivalent stress intensity factor (KJ) defined by J integral are used as fracture mechanics parameters which characterize the crack growth rate. On da/dt-K diagram, a data band shows wide scatter, especially the crack growth rate in a plastically deformed area is higher than that under small-scale yielding condition. On the other hand, da/dt-KJ diagram exhibits narrower scatter on a data band than da/dt-K diagram. The equivalent stress intensity factor is appropriate for characterization of crack growth rate by SCC under small-scale yielding through large scale yielding conditions and in a plastically deformed area.

Conditions for Failure of Normal Section in Flexural Reinforced Concrete Beams of Rectangular Cross-Section

The stress in longitudinal tensile reinforcement is one of the main important parameters while examining the technical state of under- reinforced concrete structures. The most important issue is to determine whether the external loads cause the close to yield stress in the main reinforcement. Yield stress in tensile reinforcement could be treated as the start of incipient failure of the flexural structure. The state of tensile reinforcement of flexural reinforced concrete structures could be examined by observing the properties of the normal cracks. The application of fracture mechanics of solids could be used for determining the actual damage to the structure by knowing only the measured height of the normal crack.

1513 シリコンナノ構造体のへき開き裂進展に対する破壊力学の適用可能性に関する分子静力学解析

1513 シリコンナノ構造体のへき開き裂進展に対する破壊力学の適用可能性に関する分子静力学解析

Application of Fracture Mechanics to Investigate the Influence of Compressive and Tensile Load on Durability of Concrete

Article Application of Fracture Mechanics to Investigate the Influence of Compressive and Tensile Load on Durability of Concrete was published on December 1, 2012 in the journal Restoration of Buildings and Monuments (volume 18, issue 6).

Fracture mechanics in aircraft failure analysis: Uses and limitations

Fracture mechanics is generally accepted as essential to the design of metallic aircraft structures, notably for predicting and validating the fatigue crack growth and residual strength properties. Applications of Linear Elastic Fracture Mechanics (LEFM) began in the early 1970s, and would be expected by now to have reached a mature status. Hence one might also expect that LEFM methods can often be used for analysing service fatigue failures. However, the DSTO’s and NLR’s experience is that only non-LEFM analyses are directly useful for determining service failure fatigue crack growth (FCG) behaviour. LEFM analyses can assist in analysing FCG in full-scale, component and specimen tests, and these analyses can be used – within limits – to predict some aspects of the FCG behaviour of in-service structures and components. The DSTO has developed a hybrid method, the Effective Block Approach (EBA) using both LEFM and non-LEFM analyses, from full-scale testing. The EBA enables a wide range of FCG predictions for high performance aircraft.

Extended Finite Element Method for Fracture Mechanics and Mesh Refinement Controlled by Density Function

This paper discusses the combination of element enrichment by mesh refinement controlled by density function with the extended finite element method and its application in fracture mechanics. Extended finite element method (XFEM) is an effective numerical method for solving discontinuity problems in the finite element work frame. A numerical example of fracture mechanics is analyzed at the end of this paper to show the application of the above method.

Characterising the fracture toughness of polymers at moderately high rates of loading with the use of instrumented tensile impact testing

The more common use of instrumentation in impact testing opened up the possibility to apply fracture mechanics testing methods to polymers at moderately high impact rates. Tensile impact testing offers some advantages over the more common three-point bending and compact tension geometries. Most importantly the dynamic effects encountered at (moderately) high impact rates are less significant due to inherent damping within the specimens. Consequently, the fracture toughness can be determined directly from the force signal without having to employ more sophisticated instrumentation or more elaborate analysis. Despite some unresolved issues the first results on fracture mechanics based tensile impact testing are promising: the toughness values found are comparable to those found using other loading geometries and show less scatter.

The LEFM Concept for Interfacial Cracks Application to the Problem of Coating Delamination on Cylindrical Substrates

In this paper the delamination of coating subjected to compressive stress on a cylindrical substrate is considered. This problem is particularly interesting in oxide coatings on wire elements exposed to extreme temperatures and in ceramic coatings on turbine engine blades or other components that operate at high temperatures. Using the results of Hutchinson (Hutchinson, 2001, “Delamination of Compressed Films on Curved Substrates,” J. Mech. Phys. Solids, 49, pp. 1847–1864) the aforementioned problem is discussed from the aspect of application of the linear elastic fracture mechanics (LEFM) concept for an interfacial crack. The energy release rate and mode mixity for the case of the coating delamination in the axial and radial directions are determined. It is shown that the results also depend on whether the substrate is convex or concave. Delamination in the radial direction in the case of the concave substrate is harder, but it is more likely when the substrate is convex. Delamination in the axial direction is equally likely in both cases. The results presented in this paper justify the application of the concept of linear elastic fracture mechanics for an interfacial crack for explaining the influence of the elastic characteristics of the substrate on the buckling delamination of the coating.

Application of Fracture Mechanics in Ocean Engineering Structures Quality Control

This paper introduced the application of Engineering Critical Assessment (ECA) technology based on fracture mechanics in ocean engineering structures. The application actualities and developing trend of ECA evaluation criterion in domestic and foreign ocean engineering structures were described. The content and process of ECA evaluation at home and abroad were compared, and the key technologies of ECA evaluation in practical projects were summarized.

Evolution of stress intensity factors in mixed mode with presence of mismatch effect

In this study, numerical models based on finite-element method are developed for several applications of fracture mechanics. The use of cracked specimens formed by different material properties has allowed us to analyse the effect of the crack position to the interface and to have a well understanding of mismatch effect for the case of mixed mode of fracture. In other hand, we have noticed that it is very interesting to see the effect of the interface position and loading for the case of symmetrically cracked ring specimen under compression loading.