Crack Initiation Conditions Research Articles

Aircraft Fuselage Cracking and Simulation

The fatigue failure is predominate in the aircraft fuselage. The fuselage skin consists of the shell and stingers. Fatigue plays a significant role in the crack growth of the skin structures. The proper maintenance and scheduled test intervals may avoid sudden skin failure and crack path (CP). With exiting the cracks, the propagation rate increases dramatically. Therefore, shortening the regular inspection intervals is recommended. Nevertheless, the young machines may have also unexpected skin rupture. The cracks are emanating from the notches such as rivets and the holes under the cyclic loading. The stresses concentrated around these notches. The stress concentration provides the unique conditions for crack initiation. Therefore, the cracks tend to connect with each other. Hence, the number of cycles to failure is decreased dramatically. Although the last decades, the fracture toughness, design, and new alloying element are enhanced. According to the traditional fuselage failure, the crack inspections are recommended even after a few thousand cycles.

FEM simulation for evaluation of interface crack initiation condition in EBC under thermal stress

FEM simulation for evaluation of interface crack initiation condition in EBC under thermal stress

Prediction of the Ceramic Foam Structure Failure Using a Detailed Finite Element Model

The contribution deals with modelling and prediction of failure of mechanically loaded open cell ceramic foam structures by using 3D volume FE models constructed from CT scans of real foam specimens. The condition for crack initiation in particular struts comes from the coupled stress-energy criterion which combines two fracture-mechanics parameters of the investigated material – tensile strength and its fracture toughness. By combining of both stress and energy condition one obtains information about the crack initiation length which is later used (together with the tensile strength) for determination of the strut failure in the complex 3D FE model of the ceramic foam structure. The crack onset is considered in the critical location at the moment when the (tensile) principal stress under the strut surface (in a depth corresponding to the crack initiation length) exceeded the tensile strength of the strut. Such approach enables us to define failure also on relatively coarse meshes of the FE models where potential stress concentrations are not described precisely and therefore it is not possible to decide about the failure just based upon the value of tensile stress on the strut surface.

An easy and engineering stability criterion of general singular stress concentrators

General singular stress concentrators often occur in engineering components and they represent places prone to crack initiation. Estimation of crack initiation conditions is usually performed by means of stability criteria of generalized fracture mechanics. These stability criteria require knowledge of asymptotic description of the stress state around the general singular stress concentrator. In cases of some concentrators, this analytical-numerical description can be quite difficult and does not belong among straightforward engineering approaches. This paper introduces an easy and engineering way to assess conditions for crack initiation and propagation in general singular stress concentrators using standard numerical approaches. The paper handles well-known criteria of generalized fracture mechanics of sharp and bi-material notches and simplifies them to a practical form usable for engineers. The resulting criterion fits the frame of criteria of finite fracture mechanics and extends their applicability.

Multi-parameter failure assessment of a bi-material V-notch – Crack initiation from a free-edge singularity

Multi-parameter fracture mechanics criteria are used to describe crack initiation conditions in general singular stress concentrators. In the cases of stress concentrators described by only one singular stress term, multi-parameter description provides a qualitative and quantitative increase of accuracy. In this paper multi-parameter criteria, based on an average tangential stress and average strain energy density factor, are described and used to determine a crack initiation direction and the critical load in the case of a free-edge singularity. The critical load expresses a failure/stability criterion for the studied general singular stress concentrator. Contrary to criteria employing singular stress terms only, a criterion of this kind is able to cover the influence of the direction of the external load.

Modeling soil desiccation cracking by analytical and numerical approaches

SummaryAn energy approach is proposed as a complement to the stress approach commonly considered for investigating soil desiccation cracking. The elastic strain energies before and after crack initiation are estimated by both numerical and analytical solutions. The energy released by cracking is then compared with the fracture energy to discuss crack initiation conditions. This leads to combined energy and stress conditions for crack initiation following Leguillon's theory. An approximate analytical solution is derived from a variational formulation of the porous elastic body equations. A cohesive zone model and finite element code are used to simulate crack propagation in an unsaturated porous body. This analysis shows that the energy criterion is reached before the stress criterion, and this can explain unstable crack propagation at the beginning. The approximate analytical solution allows predicting correctly the crack depth and opening in its initiation stage.

Ductile Crack Initiation Criterion with Mismatched Weld Joints Under Dynamic Loading Conditions.

Brittle failure of high toughness steel structures tends to occur after ductile crack initiation/propagation. Damages to steel structures were reported in the Hanshin Great Earthquake. Several brittle failures were observed in beam-to-column connection zones with geometrical discontinuity. It is widely known that triaxial stresses accelerate the ductile fracture of steels. The study examined the effects of geometrical heterogeneity and strength mismatches (both of which elevate plastic constraints due to heterogeneous plastic straining) and loading rate on critical conditions initiating ductile fracture. This involved applying the two-parameter criterion (involving equivalent plastic strain and stress triaxiality) to estimate ductile cracking for strength mismatched specimens under static and dynamic tensile loading conditions. Ductile crack initiation testing was conducted under static and dynamic loading conditions using circumferentially notched specimens (Charpy type) with/without strength mismatches. The results indicated that the condition for ductile crack initiation using the two parameter criterion was a transferable criterion to evaluate ductile crack initiation independent of the existence of strength mismatches and loading rates.

Numerical analysis of a testing technique to investigate the dynamic crack propagation in armour ceramic

Ceramic materials are numerically studied to understand their fracturing behaviour upon dynamic conditions and impact loadings. During a ballistic impact of a projectile against a ceramic armour system, an intense fragmentation composed of numerous oriented cracks, develops in the target. It is the reason why the conditions of crack initiation, propagation and arrest in these materials need to be investigated. In the present work, a dynamic testing configuration has been developed in order to characterise the dynamic fracture toughness (K1,d), considering a single crack that propagates from the specimen notch tip. The “Rockspall” testing technique, which employs a two-notch specimen loaded in a spalling experiment, was used. Thanks to the reflection of a compression wave into a tensile load from the sample free-end, a single dynamic crack is triggered. The sample geometry is optimised by means of a series of FE numerical simulations involving an anisotropic damage model.

Multi-parameter average strain energy density factor criterion applied on the bi-material notch problem

The stress field near a bi-material notch tip has the singular character. The power of singularity is lower in comparison to the case of a crack. The stress field can be described by the asymptotic stress series, in which each term contains generalized stress intensity factor and stress term exponent. The terms can be either singular or non-singular depending on the value of stress term exponent. The exponents are determined as solution of eigenvalue problem and depend on material properties and local geometry. The factors are calculated by analytical-numerical approaches and depend on global geometry and applied loading. The singular terms become unbounded when approaching the bi-material notch tip while the non-singular terms vanish. The non-singular terms increase precision in stress description on larger distances from the notch tip. The crack initiation conditions from the notch tip can be assessed by the average strain energy density factor (SEDF) criterion. In the case of bi-material notch problem, the crack can initiate in the direction of global minimum, the local minimum or the interface. The actual crack initiation load is calculated for each of abovementioned directions considering the individual fracture toughnesses. Contrary to the case of a crack, the direction of minimum of SEDF changes in dependence on the distance from the tip, therefore the average value over specific distance is used. The multi-parameter criterion considers singular and non-singular stress terms. Therefore in cases, when the specific distance is large, the non-singular terms provide means to improve precision of prediction of crack initiation parameters.

Multi-parameter average strain energy density factor criterion applied on the sharp material inclusion problem

The tip of a sharp material inclusion (SMI) embedded in a parent material is considered as the singular stress concentrator. The SMI tip is modelled as a special case of a multi-material junction. The singular stress behaviour is caused by the material mismatch and geometrical discontinuity. The power of singularity is lower in comparison to the case of a crack. The stress field can be described by the asymptotic stress series, in which each term contains generalized stress intensity factor and stress terms exponent. The exponents are determined as a solution of eigenvalue problem. The factors are calculated by combination of analytical and numerical approaches. The terms can be either singular or non-singular depending on the stress term exponent. When approaching the concentrator, the singular terms become unbounded while the non-singular terms vanish. The non-singular terms increase precision of stress description on larger distances from the point of singularity. In some cases they provide the only means to describe the stress field well. Because of the singular stress behaviour near SMI tip, this location is prone to crack initiation. The crack initiation conditions are calculated by the average strain energy density factor (SEDF) criterion. Contrary to the case of a crack, the direction of minimum of SEDF changes with distance from the singular point. Therefore, an averaged value over specific distance is used. If the specific distance is relatively large, the employment of non-singular terms in multi-parameter criterion can significantly improve the critical parameters prediction. Thanks to that e.g. the particle composite design can be optimized.

MODELO DE EVOLUCIÓN DEL DAÑO SOBRE HORMIGÓN, AFECTADO TEMPRANAMENTE POR ATAQUE DE SULFATO Y SU VERIFICACIÓN EXPERIMENTAL

The disturbance during the early age of the setting and hardening stage of concrete usually creates microcracks in the concrete matrix and thus decreases its strength and durability. Among the factors that affect the durability of concrete, sulfate attack is one of the important factors that cause damage. To analyze the damage development of early disturbed concrete under sulfate attack environment, the residual coefficient of flexural strength of concrete was tested by the perturbation of horizontal vibrating table. Using erosion damage factor to characterize the degree of erosion deterioration of concrete, the sulfate attack tests were carried out on the disturbed concrete. Starting from the crack initiation condition of sulfate attack, and based on the theory of microcrack nucleation and growth in the process of erosion, the statistical evolution equation of sulfate corrosion damage of concrete was obtained by using micro crack propagation to characterize the mechanical behavior of concrete after corrosion damage. Considering the influence of early disturbance on the critical stress of concrete microcrack nucleation, the damage evolution model of concrete with different disturbed conditions under sulfate attack was established, and the corrosion degree of disturbed concrete at different erosion ages was predicted. Based on the experimental data of damage factors, the damage evolution equation was fitted and the model was validated. Results demonstrate that the damage model curve fits well with the test curve of damage factor, and the correlation coefficients are above 0.98, which can reflect the damage evolution law of disturbed concrete sulfate attack. This method provides a good reference for evaluating the effect of early disturbance on the sulfate resistance of concrete. Keywords: Concrete, Disturbance, Sulfate attack, Microcrack nucleation, Damage evolution

Numerical simulation of thermal stress-induced crack initiation condition in environmental barrier coatings with columnar structure layer for ceramics

Numerical simulation of thermal stress-induced crack initiation condition in environmental barrier coatings with columnar structure layer for ceramics

Research on crack initiation mechanism and fracture criterion of rock-type materials under compression–shear stress

With the focus on the failure of single-cracked rocks under compression–shear stress, the form and the critical conditions of crack initiation by adopting theoretical derivation and numerical exper...

Generalised fracture mechanics approach to the interfacial failure analysis of a bonded steel-concrete joint

Steel-concrete joints are often made by welded shear studs. However, this connection reduces the fatigue strength, especially in situations where locally concentrated loads occur with a large number of load cycles e.g. in bridge decks. In this paper the shear bond strength between steel and ultrahigh performance concrete (UHPC) without welded mechanical shear connectors is evaluated through push-out tests and a generalized fracture mechanics approach based on analytical and finite element analyses. The connection is achieved by an epoxy adhesive layer gritted with granules. In the tests, specimens made with various manners of preparation of the epoxy interlayer are tested experimentally. Numerical-analytical 2D and 3D modelling of a steel-concrete connection is performed without and with the epoxy interlayer. The model of a bi-material notch with various geometrical and material properties is used to simulate various singular stress concentrators that can be responsible for failure initiation. Thus conditions of crack initiation can be predicted from knowledge of the standard mechanical and fracture-mechanics properties of particular materials. Results of the fracture-mechanics studies are compared with each other and with experimental results. On the basis of the comparison, the 2D simulation of the steel-concrete connection without the epoxy interlayer is shown to be suitable for the estimation of failure conditions.

A general model for hydrogen trapping at the inclusion-matrix interface and its relation to crack initiation

The role of non-metallic inclusions in hydrogen-induced failure of structural materials has been a controversial topic for many years. In this paper, hydrogen trapping and its relation to the crack initiation at the inclusion-matrix interfaces are studied by considering the interfacial structure and the interaction between the dissolved hydrogen atoms and the elastic strains produced by lattice matching and misfit dislocations. A model is proposed to analyse the change of interfacial structure with inclusion size and its relation to hydrogen trapping. Hydrogen accumulation at the interfaces is quantitatively analysed. The obtained results are in good agreement with the experimental observations. The multiple factors, such as interfacial structure, chemical composition, elastic properties of matrix and inclusions, crystallographic relationship between inclusions and matrix, inclusion morphology and size, simultaneously control hydrogen trapping. In addition, the mechanism of hydrogen-induced crack initiation at the interface is investigated. A criterion is proposed to determine critical conditions for crack initiation. For the first time, the inherent relationship between hydrogen trapping and hydrogen-induced cracking at the interface is clarified. This work paves a way for an in-depth understanding of the effects of inclusions on hydrogen-induced degradation of mechanical properties.

The effect of thermo-mechanical loading on fracture-related parameters of austenitic steel

Abstract Failure analysis shows that 80–85% of emergency boiler shutdowns at power plants result from heating surface damage. When in service, the tubes are exposed to alternating thermo-mechanical loads, which trigger phase change affecting the individual service life. The service life sometimes differs significantly from its estimated value. This research is based on the hypothesis about the role and influence of internal structural stresses on the actual strength and long-term performance of tube products made of steels and alloys. The purpose of this work is to determine the limit state region of internal stresses of the first kind, in which microdamage will not lead to fracture. For that, we did a set of experiment studies to model the accelerated aging processes by thermal cycling and cold cyclic deformation. We chose a tube made of austenitic steel 10Cr13Mn12Si2Ni2Cu2Nb (Di59) as the object of research. The methods used were XRD analysis, microhardness testing, X-ray spectroscopy, and microstructure analysis. Due to redistributing properties, steel has no stable states: all its states are short-term and dependent on external disturbances. In accordance with the suggested hypothesis, the research findings make it possible to forecast the trends and direction of changes in the material properties. This allows us to evaluate the achievement of the limit state by the object and to use relaxations of internal stresses as a sign determining the conditions of crack initiation and propagation. The results are confirmed by the data obtained from the microstructure analysis of fractured tubes of a superheater from a functioning boiler.

Subcritical crack growth and crack tip driving forces in relation to material resistance

AbstractBasic concepts, related to the crack tip driving forces in relation to the material resistance, are analyzed for the elastic and elastic-plastic crack growth condition. This defines the crack initiation and growth conditions, as well as for crack arrest. Environment provides an additional driving force, thereby reducing the mechanical driving force required for the crack to grow. It is shown that (a) crack initiation and its growth are inseparable and (b) the magnitude of the applied and/or internal stresses; their gradients are also important for initiation and continuous growth of a crack. Elastic-plastic crack growth is also analyzed using the discrete dislocation models. The results show that its behavior is similar to that of an elastic crack. These concepts are valid for all subcritical crack growth. Mechanical and mechanical equivalent of chemical forces are defined for estimating the life prediction of a component in service. Failure diagrams are defined based on the extension of classical Kitagawa-Takahashi diagram that bridges the behavior of smooth and fracture mechanics specimens. Connections between crack initiation, growth, arrest, and overload fracture are established via these failure diagrams. Application of these diagrams for engineering components in service is outlined for diagnostic and prognostic purposes.

Determination of the crack-initiation and propagation conditions in a styrene-acrylonitrile water-filter housing with external ribs based on destructive pressure tests

Abstract Fatigue cracks were initiated during the operation of a water-filter housing made from styrene-acrylonitrile (SAN). Using a destructive pressure test, it was confirmed that the sharp transition between the wall and the vertical external ribs of the filter housing at the point of the increased external diameter is the critical area of the water-filter housing. This is a consequence of the stress concentration due to the combined action of the sharp edges of the vertical external ribs and the increased stiffness due to the thicker wall and the mounting of the upper part of the housing onto the filter head. During the destructive pressure tests a crack was initiated on the external surface in the same place and spread in the same direction as the fatigue crack during the operation of the water filter. At the critical point the actual loads due to the stress concentrator are approximately 3.2 times higher than that expected based on the water pressure in the pipeline. The tensile strength of the styrene-acrylonitrile from the water-filter housing was comparable with the literature data. The fracture toughness of styrene-acrylonitrile is low and comparable to the fracture toughness of polymethylmethacrylate. The critical crack depth for the occurrence of an uncontrolled rupture for a continuously increasing load is approximately 100 μm, while the plane-strain condition occurs at a thickness of about 1 mm. A rapidly growing crack in the wall tends to grow in a layer of plane-strain conditions, and from the internal- to the external-wall surface. The possibility of determining the fracture toughness with a destructive pressure test of the housing or a pressure vessel with an external surface stress concentrator was achieved.

The influence of various types of small defects on the fatigue limit of precipitation-hardened 17-4PH stainless steel

Fatigue tests were performed on 17-4PH stainless steel specimens containing small artificial defects, with area ranging from 30 to 900μm at stress ratios, R, of −1, 0.05 and 0.4. An investigation was thus conducted to examine the influence of various types of small artificial defects on fatigue strength, including circumferential notches, corrosion pits, drilled holes and pre-cracked holes. The fatigue limit was determined by the threshold condition for the propagation of a crack emanating from the defects. The threshold stress intensity factor range, ΔKth, exhibited a defect size dependency for area≤80µm , and it became a constant value for area>80µm independent of R. Based on the area parameter model and a material constant of ΔKth for long cracks, the fatigue limit could be predicted as a function of R, with the exception of drilled holes with relatively large diameters of 100 and 300μm, for which the fatigue limit was determined by the critical condition for crack initiation. When artificial defects were absent or non-detrimental, intrinsic defects, such as non-metallic inclusions, were found to control the fatigue strength and, in addition, were responsible for the scatter in the fatigue limit. The proposed method enables the quantitative evaluation of the lower bound of the scatter as a function of the number of test specimens, or the overall control volume of fatigue-loaded components.

Assessment of asymmetric crack initiation in open‐hole plates using a coupled stress and energy criterion

AbstractAn application of the finite fracture mechanics concept to open‐hole plates subject to combined tensile and bending loading is presented. In finite fracture mechanics, the simultaneous satisfaction of both, a stress and an energy criterion, is enforced as a condition for crack initiation. Efficient modeling and closed‐form expressions for the dependence of the stress and energy quantities on governing structural and material parameters allow for a comprehensive numerical analysis of the onset of asymmetric crack patterns. The obtained failure load predictions are found to agree well with a cohesive zone model and experimental data from literature. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)