Fracture initiation in elastic brittle materials having non-linear fracture envelopes

A theory is outlined for determining the initiation of fracture and initial fracture propagation in elastic brittle materials having non-linear Mohr fracture envelopes. This theory is applied to a specific boundary value problem, i.e. a truncated quarter plane with arbitrary traction distribution on the truncated boundary and varying confining pressure. This problem simulates the chipping phase of the penetration of a wedge shaped tool into an elastic brittle material. Numerical results are obtained for two rock materials, Blair dolomite and quartzite. Results indicate that for increasing confining pressure, a limit condition is reached for both fracture initiation location and force. This limit location is closer to the boundary than the fracture initiation points at lower confining pressures, indicating smaller chips. It is also found that initial fracture propagation is less clearly defined at higher confining pressures. Both of these results have been observed experimentally.     

Investigation of fracture resistance of natural rubber/clay nanocomposites by J-testing

The present work is aimed at studying the fracture behavior of a series of vulcanized natural rubber/organoclay samples obtained by melt blending. A fracture mechanics approach based on J-testing was adopted to evaluate the material resistance to crack initiation and propagation from a J-resistance curve as experimentally obtained by a single specimen procedure. The basis of the method and the experimental procedure adopted are described. Further, the effect of the organoclay content within the elastomeric matrix on the fracture properties is analyzed. It is found that the capability of the organoclay to improve fracture resistance is rate dependent indicating the viscoelastic character of the fracture process in such filled systems.     

Validity requirements of circumferentially notched tensile specimens for the determination of the interfacial fracture toughness of coatings

A simple method was developed for evaluating the interfacial fracture toughness of coatings on substrates using circumferentially notched tensile (CNT) specimens. Mild steel cylindrical substrates of 0°, 15°, 30°, 45° and 60° notch angles with electroplated nickel were tensile tested. A well defined pre-crack was introduced at the interface for the quantitative evaluation of adhesion. In situ acoustic signals and scanning electron microscope were used to analyze the crack initiation and propagation. Finite element analyses were used to evaluate the critical interface energy release rate. The size of the plastic zone was determined for different notch angles to validate application of the linear elastic approach in determining the interfacial fracture toughness. The validity requirements have been proposed for this specimen, considering the yield strength of the coating and substrate, pre-crack position, notch angle and plastic zone size. The obtained interfacial fracture toughness values using CNT specimens was found to be very close to the values obtained by others using standard specimens.     

Crack tip damage development and crack growth resistance in particulate reinforced metal matrix composites

A study of crack tip damage development and crack growth resistance of aluminium 359/20% V_f silicon carbide and aluminium 6061/20% V_f Micral^Tm particulate reinforced metal matrix composites has been conducted. Observations of crack tip process zone development at the specimen surface have been compared with the results of fractographic examination of the centre of the specimen. Both materials were found to fracture by a process of void nucleation, growth and coalescence. Void nucleation was found to be by fracture or debonding of reinforcement particles, and/or fracture or debonding of secondary matrix particles. The preferred mode of void nucleation was found to vary depending on the constituents of the PR MMC and even the heat treatment state of the material. It was found that in these materials fractured particles identified on the fracture surface fractured during loading rather than being pre-cracked during fabrication. It was further found that observations of damage development from the specimen surface did not necessarily reflect the mechanisms prevailing in the specimen bulk. Under plane strain conditions, both materials were found to exhibit decreasing crack growth resistance as crack extension proceeded, due to the “anti-shielding” effect of damage accumulated in the process zone ahead of the crack tip. In thin specimens of the Comral-85 composite, however, dramatically improved toughness was obtained, and K_R curves have been obtained for such specimens. The method of measuring crack length was found to have a profound effect on the K_R curve; it was concluded that the K_R curve determined using the crack length measured at the specimen surface best reflected the true crack growth resistance of these materials.     

ANALYSIS AND MODELISATION OF SHORT CRACK GROWTH BY DUCTILE FRACTURE MICROMECHANISMS

Abstract— A ductile medium strength steel has been modelled by means of the Gurson model, and been used to investigate the effect of crack tip constraint in several fracture mechanics specimens. Both numerical and experimental results have been obtained, in the course of the crack extension process, for single edge notch bending specimens with different crack length-to-width ratios. The geometries with the shorter cracks always exhibited higher J values at initiation and steeper J crack growth resistance curves, and these results have been explained in terms of the stress and strain fields and damage development in the region ahead of the crack tip.     

STUDY OF THE PHYSICAL PROPERTIES OF POLYETHYLENE POLYMERS THAT AFFECT CRACK INITIATION AND PROPAGATION

High-speed photography has been used to study the behaviour of polymer specimens subjected to high velocity impacts that excite the specimen into strong, complex vibrations. These photographic data are correlated to the outputs of sensors located on the specimen, striker and supports. Optical and scanning electron-microscope studies of the crack initiation zone and the size and texture of other features on the fracture surfaces are also correlated to other fracture data. To obtain better information about the threshold of crack initiation of these materials and other data difficult to obtain by impact testing, a new experimental method has been developed. To assist in predicting and analysing experimental data a desk-top computer model has been devised with good visibility of the computing steps and how these relate to crack propagation in polymeric materials.     

Survey of current fracture mechanics studies at AMMRC

The fracture and fatigue studies at AMMRC encompass both experimental and analytical thrusts. This presentation will cover a few representative activities.

The status of a test program on fracture behavior of unidirectionally reinforced composites using double cantilevered type specimens will be reported. Material systems include ‘S’ glass/epoxy and graphite/epoxy. Limited fracture toughness data is available. Additionally, some effort has been initiated on cross ply specimens.

Fatigue studies at AMMRC in metals reflect a continuing interest in both initiation and propagation aspects. A technique has been developed for detection of crack initiation using an automated photographic process for recording, periodically, potential initiation sites. The observations are discussed in conjunction with damage criteria. Crack propagation studies have focussed in developing a ‘law’ which accounts for the extremes of propagation rates at threshold levels of stress intensity and at the higher levels associated with unstable growth. Additionally, crack propagation experiments are described which are aimed at exploring the transient effects on crack propagation of sudden changes in stress intensity levels.

Supplementing these experimental studies are extensive analytical efforts directed toward defining the stress states in anisotropic material when used in lap and/or mechanical joint configurations. Additionally, extensive effort by one team of investigators has been devoted to analytical techniques for crack analysis. These techniques have been applied to a wide variety of geometric configurations and to a range of material types including anisotropic. This effort will be described briefly.     

An enhanced probabilistic model for cleavage fracture assessment accounting for local constraint effects

Objective of the present study is the development of an enhanced model for the probabilistic cleavage fracture assessment of ferritic materials considering the conditions for both, nucleation and propagation of micro defects. In a first step, the local load and deformation history at the cleavage initiation spot is analysed numerically for a variety of fracture mechanics specimens. The experimental data base includes experiments on standard deep and shallow crack specimens with different geometries as well as novel small scale cruciform bending specimens. These specimens enable the application of an additional stress component along the crack front. Based on the results, a two-criteria concept for cleavage initiation is proposed, assuming that the propagation of existing micro defects is controlled by the maximum principal stress whereas the nucleation of potentially critical micro defects is governed by a combination of the local plastic strain and the local stress triaxiality at the respective material point. Based on these assumption, a probabilistic cleavage fracture model is formulated and validated against the experimental data base.     

Hygrothermal influence on delamination behavior of graphite/epoxy laminates

The hygrothermal effect on the fracture behavior of graphite/epoxy laminates has been investigated as part of an overall effort to develop a methodology for damage-tolerance predictions in advanced composite materials. Several T300/934 laminates were tested using a number of specimen configurations (double cantilever, compact tension, and cracked lap shear) in order to evaluate the effects of temperature and humidity on delamination fracture toughness under Mode I and Mode II loading. The specimens were exposed to different humidity levels and temperatures for varying periods of time prior to testing. The preexposed specimens were tested under room conditions, and fracture energies during initiation and propagation were estimated. Acoustic emission was used to detect crack initiation. It was indicated that moisture has a slightly beneficial influence on fracture toughness or critical strain energy release rate during Mode I delamination but a slightly deleterious effect on Mode II delamination and Mode I transverse cracking. The failed specimens were examined by scanning electron miscroscope (SEM) and topographical differences due to fracture modes were identified. The effect of moisture on fracture topography could not be distinguished.     

Effect of notch severity on fatigue fracture in a rubber-modified glassy polymer

The research concerned the effect of notch severity on the fatigue behaviour of series of rubber-modified glassy polymers, consisting of a styrene-acrylonitrile copolymer with different amounts of an olefin rubber. Tests were conducted under displacement control and two different loading conditions. Both stages of fatigue lifetimes, that is, fracture initiation and crack propagation, were examined. It was observed that the initial notch severity determines the duration of the crack-initiation stage, while crack propagation does not depend on it. The crack velocity appears to be controlled by the maximum applied stress intensity factor, and the correlation does not depend on the rubber content. The results obtained have been interpreted by considering three different zones in the specimen during the fracture process: a far-field viscoelastic continuum, a process zone and a failure zone.     

Effect of hydrogen charging on the torsional fatigue behaviour of 2024 aluminium

Torsional fatigue tests have been carried out on overaged and hydrogen charged specimens of 2024 aluminium in gaseous hydrogen and humid air. Hydrogen charging was found to significantly increase the number of fatigue crack initiation sites compared with uncharged specimens tested in argon, resulting in an overall reduction in fatigue life. Fatigue testing in gaseous hydrogen and humid air influenced both initiation and propagation of cracks. The fracture sites of both charged and uncharged specimens were similar, and the fracture mode was predominantly tensile in all specimens. However, specimens tested in humid air showed small amounts of longitudinal and transverse fracture, with ≈5% shear at low humidity and 10% at high humidity.     

Interlaminar fracture of carbon-thermoplastic polyimide composites

Mode I interlaminar fracture of a novel amorphous thermoplastic polyimide reinforced with unidirectional carbon fibre has been studied experimentally using double cantilever beam specimens and scanning electron microscopy. Three kinds of composite were manufactured from different monomeric reactant solutions which were prepared by using different alcohol solvents. The values of fracture toughness of these three composites were measured to construct the crack growth resistance curves (R curves). The contributions from various failure processes to the total fracture toughness were separated, and approximate calculations of these contributions were conducted based on several simplifying assumptions and some data obtained from the fracture surfaces. Though fibre peeling and fibre breakage are observed, interlaminar fracture in the composites studied is primarily controlled by fracture and deformation in the matrix. It is found that the measured fracture toughnesses of the composites differ from each other not only in the propagation values but also in the initial values. A possible reason for this may be variations of matrix ductility in the three composites.     

A theory of crack initiation and growth in viscoelastic media II. Approximate methods of analysis

Starting with equations developed in Part I for the opening mode of displacement, simple, approximate relations are derived for predicting the time of fracture initiation and crack tip velocity in linearly viscoelastic media. First we use the assumption that the second derivative of the logarithm of creep compliance with respect to logarithm of time is small (which is normally valid for viscoelastic materials); we next derive a relation between instantaneous values of tip velocity and stress intensity factor. This result is then used to examine some characteristics of crack growth behavior. Finally, some results are obtained for the separate problem of predicting the time at which propagation initiates.     

Study of polyethylene pipe resins by a fatigue test that simulates crack propagation in a real pipe

A fatigue test that simulates the step-wise crack propagation found in pipes in the field, and uses a standard compact-tension specimen, was employed to study and rank crack resistance of various pipe resins. The thermal history during compression moulding of the test specimens strongly affected fracture kinetics. It was found that crack-resistant properties of in-service pipe were best reproduced if compression-moulded plaques were fast cooled under load. This procedure was used to prepare specimens from candidate pipe resins for fatigue testing. The resins were compared in terms of discontinuous crack growth kinetics. The ranking based on resistance to fatigue crack propagation correlated with results of a standard PENT creep test. However, fatigue failure times were an order of magnitude less than the standard creep times. After comparing the initiation and failure times of the resins with detailed kinetics of step-wisse crack propagation, a simplified and rapid procedure is proposed which calls for evaluating only the first jump after initiation.     

Finite element simulation of fracture behaviour for aged duplex stainless steels

In this paper, the local approach model developed by Gurson–Tvergaard has been applied to simulate both the crack initiation and the crack growth of aged duplex stainless steel. The parameters of the Gurson–Tvergaard model have been obtained, from axisymmetric notched specimen testing, as a function of the ageing time at 400°C, the ferrite content of the steel and the stress triaxiality. After that, to simulate the fracture of CT specimens, finite element (FE) calculations have been effected in order to obtain the stress triaxiality value at each point on the process zone ahead of the crack tip of these specimens. The adequate damage parameters concerning triaxiality are determined from the ones obtained at the notched specimens, in order to be used in FE simulations of fracture behaviour. With them, the corresponding J−Δa curves have been simulated as representative of both the crack initiation and crack propagation stages, and compared with experimental results in order to validate the methodology proposed.     

Mechanical and fatigue properties of pearlitic ductile iron castings characterized by long solidification times

In this paper, the fatigue behaviour of heavy section pearlitic ductile iron castings has been investigated. The inoculation treatment has been changed for each casting in order to investigate its influence on the mechanical and fatigue properties of the materials.Tensile tests and axial fatigue tests under nominal ratio R = 0.01 have been performed on specimens taken from the core of casting components characterized by long solidification times. Scanning Electron Microscopy has been used to investigate the fracture surface of the broken samples in order to identify crack initiation points and fracture mechanisms. Metallographic analyses have been carried out to measure nodule count and nodules dimensions and to identify matrices structures.It has been found that fatigue behaviour is strongly influenced by defects, such as microshrinkages or degenerated graphite particles near to specimens' surface. It has been also found that inoculation process influences the microstructure and the fatigue resistance of heavy section pearlitic ductile iron castings.     

Fracture toughness of concrete determined on large specimens

It has often been questioned whether linear elastic fracture mechanics can be applied to describe crack propagation and failure of concrete. An important argument is that most test results are obtained on specimens too small to be representative of a material with a composite structure such as concrete. Large specimens with four different geometries have been prepared and tested. Crack length was increased under controlled conditions to at least 250 mm. It was found that fracture toughness increases initially as a crack propagates, but that a length-independent value is reached asymptotically. Within the range of accuracy, asymptotic values obtained with the four different geometries were the same. It is concluded that failure of large size concrete elements can be predicted realistically on the basis of linear elastic fracture mechanics. For comparatively small specimens, however, an approach which takes total fracture energy into consideration (for instance the fictitious crack model) is more appropriate. It is pointed out that the role of subcritical crack growth on fracture toughness needs further investigation.     

PREDICTING THE EFFECTS OF CRACK TIP CONSTRAINT ON MATERIAL RESISTANCE CURVES USING DUCTILE DAMAGE THEORY

Abstract— In recent years much interest has been focused on the geometry dependence of the resistance to stable ductile crack growth of engineering materials, and in particular, in explaining this in terms of "constraint" effects. This paper describes the results of work using the Rousselier ductile damage model in finite element studies to simulate the growth and coalescence of voids, and hence the mechanics of ductile crack growth, to predict the effect of constraint on resistance to fracture. Using the modified boundary layer solution, where constraint is controlled by the application of remote displacements, it was possible to simulate resistance curves for different constraint conditions. This has produced a "net" of resistance curves, within which the curve for any specimen geometry can be found from a knowledge of the crack tip constraint for that specimen. This has been tested by comparing the results with those obtained from two specimens for which the constraint conditions are known. Good agreement has been achieved.
The results show that, although constraint has very little effect on conditions at the crack tip at initiation of crack growth, beyond that constraint plays an important part in defining the resistance curve. For low constraint geometries there is a very large loss in crack tip constraint which results in a large increase in the slope of the resistance curve. On the other hand, high constraint geometries exhibit very little dependence on crack tip constraint.     

Fracture resistance of a glass-fibre reinforced rubber-modified thermoplastic hybrid composite

Toughening mechanisms in a hybrid amorphous thermoplastic composite containing both distributed rubber particles and rigid glass fibres have been investigated. Tensile properties were measured for a range of materials with varying rubber particle and glass-fibre contents, and different rubber particle sizes. Fracture toughness was characterized by separating the overall fracture into its initiation and propagation components. Deformation and fracture modes at crack tips were optically characterizedin situ during loading. The results indicate that both initiation and propagation toughness are enhanced by rubber particle additions to the glass-fibre reinforced composite. Synergistic effects between glass fibres and rubber particles are identified: for example, glass fibres inhibit crazing at rubber particles, and rubber particles tend to promote crazing at fibre/matrix interfaces and also void initiation at fibre ends. Toughening mechanisms are discussed in the light of available models.