Determination Of Fracture Mechanics Parameters Research Articles

Selective laser sintered Pipe Ring Notched Tension specimens for examination of fracture properties of pipeline materials

In order to develop a non-standard method for determining the resistance to fracture and damage of pipeline materials, a new geometry of ring-shaped specimens with sharp notches or cracks has been defined. The need to develop a new method for testing of the specimens cut from the pipes arises due to the difficulties in determination of fracture mechanics parameters on thin-walled pressure pipelines, especially those with a smaller cross-section, by application of the standard procedures/specimens. Previous studies dealing with the topic of pipeline testing by non-standard methods are presented in the introductory part of the paper. In the experimental part of the research, tests on PRNT (Pipe Ring Notched Tension) and SENT (Single Edge Notched Tension) specimens are performed. Samples of these specimens were produced by SLS (Selective Laser Sintering) technique of additive production from PA12 (polyamide PA 2200) material. For the purpose of this study, a tool that is protected at the national level (in Serbia) as intellectual property is used for testing of the ring-shaped PRNT specimens. Tensile testing of both types of specimens is monitored by Aramis GOM 2M system; its operation is based on the method of digital image correlation, DIC. Additionally, finite element analyses are conducted on the PRNT and SENT geometries, enabling the calculation of the fracture mechanics parameter – Stress intensity factor K. It is concluded that the presented procedure based on the PRNT specimen has a good potential for use as a non-standard method for fracture resistance examination of pipeline materials. It can be performed on the specimens cut directly from the pipes (new or from exploitation). The dependence of the fracture resistance on the stress concentrator size is not pronounced, which means that the results depend dominantly on the material properties, rather than on geometry.

Linear translaminar fracture characterization of additive manufactured continuous carbon fiber reinforced thermoplastic

This work presents the experimental determination of fracture mechanics parameters of composite specimens manufactured by fused filament fabrication (FFF) with continuous carbon fiber reinforced thermoplastic filaments, based on Linear Elastic Fracture Mechanics (LEFM). The critical mode I translaminar fracture toughness (KIc) and the critical energy release rate (GIc) are found for unidirectional and cross-ply laminates. The specimens were submitted to quasi-static tensile testing. Digital Image Correlation (DIC) is used to find the stress field. The stress fields around the crack tip are compared to linear elastic finite element simulations. The results demonstrate the magnitude of fracture toughness is in the same range as for polymers and some metals, depending on lay-up configuration. Besides, fractographic analyses show some typical features as river lines, fiber impression, fiber pulls-out and porosity aspects.

Numerical Modelling of Cylindrical Specimen under Mixed-Mode Loading Conditions

Mixed mode loaded fatigue specimens are not used very frequently. The main reason is that there are still very few qualitative results, which would help with better understanding of how the mode II and mode III are affecting the overall crack propagation. On the other hand, there is considerable number of parts loaded in mixed mode. Studying fatigue failure of roller bearing elements made of polymer material was a motivation to design a new experimental specimen for study of fatigue behaviour of the material loaded in mixed mode. The contribution presents developed FEM model of such specimen with focus on determination of fracture mechanics parameters of propagating cracks.

Full‐field characterisation of crack tip deformation and fatigue crack growth using digital image correlation—a review

AbstractIn recent years, digital image correlation has been widely used in the analysis of crack problems. This review will examine digital image correlation, as a full‐field measurement technique, in the studies of crack tip mechanical behaviour under cyclic loading conditions. In particular, topics including determination of fracture mechanics parameters and evaluation of crack closure will be discussed. Micromechanical aspects of crack growth under cyclic loading will also be explored in crack driving and attenuation effects.

Fracture parameters of concrete C40/50 and C50/60 determined by experimental testing and numerical simulation via inverse analysis

The use of stochastic non-linear computational mechanics in real-world applications faces a fundamental obstacle: the lack of knowledge regarding the stochastic properties of material parameters involved in the problem. The paper describes the results of an experimental program which was focused on the determination of fracture-mechanical parameters and their stochastic models of different concrete types after different hardening durations. The investigated types of concrete, C40/50 and C50/60, are used industrially for the production of pre-fabricated concrete elements (e.g. double T-shaped beam elements for roofing). The fracture-mechanical parameters were determined by wedge splitting tests on cubic specimens with notch. Moreover, numerical simulations were carried out for the identification of material parameters by using artificial neural networks. All results obtained from the individual tests will here be presented, compared and discussed. Finally, recommendations for stochastic models of selected parameters of the analysed concrete will be given.

Determination of Stress Intensity Factors in Low Pressure Turbine Rotor Discs

An attention in this paper is focused on the stress analysis and the determination of fracture mechanics parameters in low pressure (LP) turbine rotor discs and on developing analytic expressions for stress intensity factors at the critical location of LP steam turbine disc. Critical locations such as keyway and dovetail area experienced stress concentration leading to crack initiation. Major concerns for the power industry are determining the critical locations with one side and fracture mechanics parameters with the other side. For determination of the critical locations in LP turbine rotor disc conventional finite elements are used here. For this initial crack length and during crack growth it is necessary to determine SIFs. In fatigue crack growth process it is necessary to have analytic formulas for the stress intensity factor. To determine analytic formula for stress intensity factor (SIF) of cracked turbine rotor disc special singular finite elements are used. Using discrete values of SIFs which correspond to various crack lengths analytic formula of SIF in polynomial forms is derived here. For determination of SIF in this paper, combinedJ-integral approach and singular finite elements are used. The interaction of mechanical and thermal effects was correlated in terms of the fracture toughness.

Determination of fracture mechanics parameters for bodies gyration in unsteady dynamic problems

On the basis of semi-analytical final elements method data validity research and research of effective determination of fracture mechanics parameters for bodies of revolution in non-stationary dynamics problems were conducted. New method of J-integral calculation and crack approximation under dynamic impact are presented.

Fracture Mechanical Investigation of Elastomeric Material

AbstractThe increasing use of elastomeric components in advanced engineering applications requires a thorough understanding of the complex material properties and a reliable assessment of the quality and durability of the products. This contribution concentrates on the computational determination of fracture mechanical parameters for rubber material using the material force method. For dissipative, inelastic material, a distinction between two fracture mechanical parameters is presented. The time‐dependent behaviour of these fracture mechanical parameters is illustrated by an application to the dwell‐effect. (© 2009 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Application of indentation fracture mechanics approach for determination of fracture toughness of brittle polymer systems

The analysis of cracks formed close to hardness impressions is a part of the current trend which deals with the development of simplified approaches for determination of fracture mechanics parameters of brittle polymer materials. However, up to now only very limited experimental knowledge exists of how to calculate the fracture toughness of these materials based on indentation fracture mechanics. Within the scope of this work, fracture toughness values for polymethyl methacrylate (PMMA) and polystyrene (PS) determined on the basis of a method proposed by Laugier have been found to be in good agreement with conventionally determined values. Furthermore, the fracture toughness of a number of other materials such as PMMA/SiO 2 nanocomposites and syndiotactic PS has been determined using this Laugier approach.

Crack toughness behavior of binary poly(styrene-butadiene) block copolymer blends

Fracture behavior of binary blends comprising styrene-butadiene block copolymers having star and triblock architectures was studied by instrumented Charpy impact test. The toughness of the ductile blends was characterized by the dynamic crack resistance concept (R curves). While the lamellar thermoplastic star block copolymer shows elastic behavior (small scale yielding and unstable crack growth), adding 20 wt% of a triblock copolymer (thermoplastic elastomer, TPE) leads to a strong increase in crack toughness. The stable crack propagation behavior of these blends was described by the crack resistance curve (R) concept of elastic-plastic fracture mechanics. This concept allows the determination of fracture mechanics parameters as resistance against stable crack initiation and propagation. Two brittle to tough transitions (BTT) are observed in the binary block copolymer blend: BTT1 at 20% TPE and BTT2 at about 60% TPE. The strong increase of toughness at 60 wt% TPE indicates a ‘tough/high-impact’ transition as a measure for the protection against stable crack initiation.

Determination of fracture mechanics parameters J and [formula omitted] by finite element and reference stress methods for a semi-elliptical flaw in a plate

The fracture mechanics parameters J and C ∗ used, respectively, to describe ductile fracture and creep crack growth can be determined either by finite element methods or reference stress techniques. In this paper solutions for a partially penetrating semi-elliptical flaw in a plate subjected to tension and bending loading are considered. Estimates of J and C ∗ are obtained from finite element calculations for a range of work-hardening plasticity and power law creep behaviours and from reference stresses derived from ‘global’ collapse of the entire cracked cross-section. Comparisons are made with solutions taken from the literature for a range of loading conditions, plate geometries and crack sizes and shapes. Generally it is found that although there are significant variations between the different finite element solutions, satisfactory estimates of J and C ∗ that are mostly conservative are obtained when the reference stress procedure is adopted.

Anwendung des instrumentierten Kerbschlagbiegeversuchs zur Ermittlung von Referenztemperaturen nach dem Master-Curve-Konzept

de

Geometrieunabhängige bruchmechanische Werkstoffkenngrößen - Voraussetzung für die Zähigkeitscharakterisierung von Kunststoffen

Starting from the history of introducing the Charpy test to the characterization of plastics, polymer-specific requirements for the specimen geometry relevant for the determination of fracture mechanics parameters of plastics are summarized. Here, the influence of the specimen thickness, the specimen configuration and size, and a/W-ratio are considered. In order to characterize the geometry independence of fracture mechanics parameters, in this connection, geometrical factors are used to estimate requirements for the specimen geometry without an experimental determination of the geometry influence. For plastics, these experimentally determined factors vary in a wide range. For this reason, the use of constant geometrical factors must cause a pronounced under- or over-evaluation of the requirements on the specimens. The relationships between the geometrical factors and the fracture mechanics parameters those are universally valid for plastics are independent from the kind of loading (static, dynamic) and the crack propagation behaviour (stable, unstable).

Investigation of Fracture Processes in Elastomeric Structures under Conditions of Nonlinear Viscoelastic Deformation

A procedure for the determination of fracture mechanics parameters, which is based on the J-integral method, is proposed for the solution of nonlinear viscoelasticity problems. Stress intensity factors and the value of the J-integral for elastomeric cracked structures under stress relaxation and creep conditions have been determined using the finite-element method.

A contact zone approach for an interface crack in a piezoelectric anisotropic bimaterial

AbstractAn exact analytical solution for an interface crack with an artificial contact zone situated between two piezoelectric semi‐infinite spaces has been found. The classical interface crack model and the contact zone model can be obtained as particular cases of this solution. Analytical relationships between the fracture mechanical parameters of these different models are given. On the base of these relations a numerical algorithm has been developed for the determination of fracture mechanical parameters at an interface crack tip situated in the material interface of a finite‐sized anisotropic piezoelectric bimaterial.

Effects of Initial Crack Length and Specimen Thickness on Fracture Toughness of Compact Bone.

This paper deals with the determination of fracture mechanics parameters, such as critical stress intensity factor, Kc, and critical strain energy release rate, Gc, of compact bone. Haversian bone samples taken from fresh bovine femurs were machined into small compact tension (CT) specimens with various initial crack lengths and thicknesses, and then tested under quasi-static mode I loading condition at room temperature. Factor Kc was determined following the ASTM standard E399-90, and the compliance method was used for the determination of Gc. The values of Kc and Gc were larger in the specimens with transversely oriented initial cracks than in those with longitudinally oriented initial cracks. Both Kc and Gc were dependent on initial crack length as well as specimen thickness, and these fracture characteristics could be interpreted by a hypothesis based on the observation of damage accumulation, or microcracking ahead of the main crack front.

Determination of geometry-independent fracture mechanics values of polymers

Abstract Specific requirements on the specimen geometry necessary for the determination of fracture mechanics parameters for polymers by using the instrumented Charpy impact test are summarized. As examples, the influences of the specimen thickness and a/W ratio are considered. For characterizing the geometry independence of fracture mechanics parameters, geometrical factors are used, which make possible an estimation of the requirements on the specimen geometry without any further experiments. For polymers, the experimentally determined geometrical factors vary in a wide range; this means that the use of constant geometrical factors must cause a pronounced under-or overevaluation of the requirements on the specimens. The relationships between geometrical factors and fracture mechanics parameters, which were found to be generally valid for polymers, are independent of the kind of loading (static, impact-like) and the crack propagation behaviour (stable, unstable).

Energy transformation during dynamic crack initiation and arrest

This contribution deals with the experimental determination of fracture mechanics parameters concerning dynamic crack initiation, crack propagation and crack arrest demonstrated on reactor pressure vessel steels 20 MnMoNi 5 5 and 22 NiMoCr 3 7. Appropriate measuring methods are available to determine the impact fracture toughness K Id for CT specimens and CCP specimens. However, for small scale specimens there are still experimental and theoretical problems to be met with when determining the fracture heat of a propagating crack and ascertaining the parameters of arrest.

Determination of Fracture Mechanics Parameters with Contact Conditions

Determination of Fracture Mechanics Parameters with Contact Conditions