Application Of Fracture Mechanics Methods Research Articles

Influence of pores on the lifetime of die cast aluminium alloys studied by fracture mechanics and X-ray computed tomography

Pores and other defects play a crucial role in aluminium die cast alloys. They critically affect the fatigue properties and lifespan of components. The application of fracture mechanics methods is a cornerstone in the calculation of high cycle fatigue properties of such components. In recent research failure behaviour and internal structure of defects are analysed and predicted based on X-ray computed tomography scans (CT) and machine vision integrated into the simulation models. Here, we present the analysis of defects of the die cast aluminium alloy EN-AC46200. In the first step, we use CT scans to analyse pores and defect structures. Afterwards, the CT scans are evaluated for pores and defects using modern 3d image analysis software. Additionally, the samples are tested by high cycle fatigue until failure. To determine the stress intensity, the pore analysis is combined with the help of machine vision. Furthermore, a novel clustering method based on 3d volume data is proposed to predict critical regions of the samples. We show that by this approach we can augment the fracture mechanics of the defects and identify critical pores in combination with stress hot spots in the aluminium die cast alloy.

Fracture mechanics approach to the problem of subsidence induced by resource extraction

A new area of application of Fracture Mechanics methods is proposed: the determination of surface deformation induced by internal volume change (e.g. due to underground hydrocarbon extraction). The finite layer-like zone of volume alteration is modelled as a crack, that is as a continuous distribution of dislocation loops. This approach allows the determination of surface deformation and can serve as the basis for monitoring of the volume alteration from surface measurements. In a limiting case of nearly uniform deformation of the reservoir boundaries the model consists of a single dislocation loop (an inserted cylindrical body of a constant height) in half-space and results in simple approximate expressions for the normal and tangential displacement components of the free surface. For the example of subsidence in the Groningen gas field the proposed method produces more realistic results describing reality better than the traditional models based on representing the reservoir as a collection of independent dilation centres. Also, analytical expressions are proposed for the nuclei – the basic Green’s function type expressions that allow representation of a planar horizontal reservoir layer of an arbitrary shape as a distribution of infinitesimal dislocation loops. For the case of a cylindrical reservoir, the nuclei integration recovers the exact distribution of the surface displacements.

Measurement of CTOD along a surface crack by means of digital image correlation

The experimental measurement of fracture toughness and crack driving force are the main steps for the application of fracture mechanics methods in structural integrity assessment. Crack Tip Opening Displacement (CTOD), defined as the opening of a defect at its crack front upon loading a test specimen or structure, is one of the most important crack driving force parameters. However, due to its subsurface nature direct measurement of CTOD is very cumbersome. CTOD is commonly measured using single or double clip gauge techniques and applying a plastic hinge model. However, clip gauge CTOD calculations merely provide information related to the center of the defect. For the case of a finite-length surface breaking defect, where CTOD is variable along the defect front, exact knowledge of CTOD over the entire front is not possible by the clip gauge measurement techniques. In the present paper a novel technique is proposed based on full field three-dimensional profile measurement of the notched surface by means of stereoscopic Digital Image Correlation (3D-DIC) to measure the CTOD over the entire crack front. The proposed method is verified by measurement of silicone replicas cast inside the notch in addition to a supporting Finite Element (FE) investigation.

Application of fracture mechanics methods to rail design and maintenance

Railway organizations are facing continuous increase in train frequency, running speed and axle load with ever limited financial resources. This is why methods broadening knowledge of external loads and internal rail stresses are of higher importance to address rail design or maintenance process. The following paper explains how the development and application of such methods has been conducted by a european partnership (called NOVUM) between railway organizations (SNCF, DB AG, RATP) and rail producer (CORUS) sharing their specific own competences, experience and material resources. Research institutes and universities (BAM, GKSS, INRETS, INSA, LMS) have been in charge of building sound numerical simulations or experimental tests and tools. Theoretical aspects of the method are described in independent paper “A global approach for modelling fatigue and fracture of rails” [1]. A first task group of the program is dedicated to the determination of external forces coming from wheel/rail and from rail/support interactions. The second and third task groups are about material properties and internal stresses inside rail section. Laboratory and track validations are undertaken in the fourth task group. Two characteristic cases are then chosen to check the developed models.

Catastrophic failure of submerged slopes in normally consolidated sediments

The application of fracture mechanics methods to shear band growth in granular materials requires careful consideration of the elasto-plastic nature of the material behaviour. In this paper, the Palmer and Rice fracture mechanics approach is applied to the problem of catastrophic shear band propagation in a submerged slope built of normally consolidated sediments. This allows for catastrophic (unstable) and progressive (stable) types of soil failure to be properly distinguished. Many submerged landslides are triggered by earthquakes, via the shear strength degradation, due to the excess pore water pressure accumulation. The conventional analysis assumes that the excess water pressure zone develops simultaneously along the entire landslide length, often reaching several kilometres. When a mechanism of the catastrophic shear band propagation is considered, a much shorter excess water pressure zone (e.g. several hundred metres) can still cause a full-scale landslide (often generating a tsunami wave of considerable height). This mechanism also suggests possible measures for mitigation of the landslide hazard.

Catastrophic failure of submerged slopes in normally consolidated sediments

The application of fracture mechanics methods to shear band growth in granular materials requires careful consideration of the elasto-plastic nature of the material behaviour. In this paper, the Palmer and Rice fracture mechanics approach is applied to the problem of catastrophic shear band propagation in a submerged slope built of normally consolidated sediments. This allows for catastrophic (unstable) and progressive (stable) types of soil failure to be properly distinguished. Many submerged landslides are triggered by earthquakes, via the shear strength degradation, due to the excess pore water pressure accumulation. The conventional analysis assumes that the excess water pressure zone develops simultaneously along the entire landslide length, often reaching several kilometres. When a mechanism of the catastrophic shear band propagation is considered, a much shorter excess water pressure zone (e.g. several hundred metres) can still cause a full-scale landslide (often generating a tsunami wave of considerable height). This mechanism also suggests possible measures for mitigation of the landslide hazard.

Toughness Optimization of SBR Elastomers – Use of Fracture Mechanics Methods for Characterization

AbstractElastomer materials are used in a wide application range and subjected to different loading from which failure of the material results. Because this failure is caused by initiation and propagation of cracks, the application of fracture mechanics methods for the assessment of the material is obvious. A short summary of the methods of technical fracture mechanics including possibilities of determination of crack resistance curves is given. Vulcanizates on the basis of SBR 1500 with various sulfur and carbon black contents were investigated. For describing the crack initiation and crack propagation behavior, several fracture mechanics examination methods were applied. Tear‐analyzer results were used to assess the crack propagation behavior under fatigue‐like loading conditions. Furthermore, for the characterization of the crack resistance of the materials under impact‐like loading conditions, instrumented tensile‐impact tests were performed. To obtain information about the initiation and propagation of a stable crack, quasi‐static fracture mechanics tests were applied. The results of the several tests are discussed in dependence on sulfur and carbon black contents. We found a non‐monotonous behavior of the toughness as a function of carbon black loading. An explanation is given in connection with a percolation‐like transition in filler morphology on larger length scales. Schematic crack propagation curve for characterizing the fatigue behavior of the vulcanizates recorded in a TFA test.magnified imageSchematic crack propagation curve for characterizing the fatigue behavior of the vulcanizates recorded in a TFA test.

Characterization and analysis of delamination fracture in composites: An overview of developments from 1990 to 2001

Delamination is a major failure mode in laminated composites and has received much research attention. A huge amount of literature has been published on this subject, resulting in considerable improvement in our understanding of delamination behavior. A rather comprehensive review paper was published by Garg in 1988. That paper contains 136 references from 1971 to 1988, a period spanning roughly two decades. A more recent survey by Pagano and Schoeppner contains critical reviews of many selected papers, especially the pioneering works on delamination research. An overview of Russian and Soviet literature on this subject is also given by Bolotin. The present paper aims to review major developments in the analysis and characterization of buckling driven delamination from 1990 to the present. Its abridged version was presented in a plenary lecture at the 13th International Conference on Composite Materials, Beijing. Particular emphasis is placed on the application of fracture mechanics methods in the analysis of delamination. The question of the extent of fracture mechanics’ success in characterizing delamination, both in laboratory-based specimens and in more realistic structural composite components, is discussed. Some recent numerical formulations for efficient computational modeling and analysis of delaminations are also reviewed. This review lists 270 major works covering a period of about a decade, indicating that there is still considerable research interest and activity in this field.

Application of fracture mechanics methods in safety analysis of piping components in subcreep and creep behavior

Application of fracture mechanics methods in safety analysis of piping components in subcreep and creep behavior

Compilation of references, data sources and analysis methods for LMFBR primary piping system components

A survey and review program for application of fracture mechanics methods in elevated temperature design and safety analysis has been initiated in December of 1976. This is the first of a series of reports, the aim of which is to provide a critical review of the theories of fracture and the application of fracture mechanics methods to life prediction, reliability and safety analysis of piping components in nuclear plants undergoing sub-creep and elevated temperature service conditions.