Cracked Components Research Articles

Carrying capacity of edge-cracked columns under concentric vertical loads

This paper analyses the carrying capacity of edge-cracked columns with different boundary conditions and cross sections subjected to concentric vertical loads. The transfer matrix method, combined with fundamental solutions of the intact columns (e.g. columns with no cracks) is used to obtain the capacity of slender prismatic columns. The stiffness of the cracked section is modeled by a massless rotational spring whose flexibility depends on the local flexibility induced by the crack. Eigenvalue equations are obtained explicitly for columns with various end conditions, from second-order determinants. Numerical examples show that the effects of a crack on the buckling load of a column depend strongly on the depth and the location of the crack. In other words, the capacity of the column strongly depends on the flexibility due to the crack. As expected, the buckling load decreases conspicuously as the flexibility of the column increases. However, the flexibility is a very important factor for controlling the buckling load capacity of a cracked column. In this study an attempt was made to calculate the column flexibility based on two different approaches, finite element and J-Integral approaches. It was found that there was very good agreement between the flexibility results obtained by these two different methods (maximum discrepancy less than 2%). It was found that for constant column flexibility a crack located in the section of the maximum bending moment causes the largest decrease in the buckling load. On the other hand, if the crack is located just in the inflexion point at the corresponding intact column, it has no effect on the buckling load capacity. This study showed that the transfer matrix method could be a simple and efficient method to analyze cracked columns components.

The Effect of Oxygenated Fuel Properties on Mechanism of Inhibition in Particulate Matter

It is reported that the particulate matter in exhaust gas is reduced when oxygenated agents are added to diesel fuel, but the inhibition mechanisms is not established. To explain this mechanism, we investigates the amounts of particulate formation and analyzes thermal cracking and condensation polymerization components with a fluid reaction tube apparatus and nitrogen atmosphere. As a result of experiment, it became clear that inhibition mechanism of fine grain was different by fuel properties.

On dependence of the stress intensity factor and T-stress from imposed boundary conditions in a rectangular cracked plate

This paper investigates the dependence of the stress intensity factor (abbreviated as SIF) and T-stress from the imposed boundary conditions. A rectangular cracked plate is subject to a tension in the vertical direction. In the first boundary condition, the tension loading is applied at the tops of plate. In the second boundary condition, the relevant assumed displacement is applied on the edges of plate. In the study, the complex potentials are assumed in the series form with undetermined coefficients, and those coefficients can be evaluated by using the variational principle. The aim of study is to obtain the SIF and T-stress at the crack tips for two different boundary conditions. Finally, numerical results are presented. The obtained results may have their application in the fatigue test of a cracked component.

Development of Toughness Scale Diagram Considering Temperature Dependency

Fracture toughness data from the cleavage resistance test of structural steels often show a large scatter. Geometry dependency as well as the scatter makes it difficult to evaluate appropriate fracture integrity of cracked components. To address these restrictions, several stochastic models have been proposed by Beremin group, Mudry and other researchers while each of them employs specific estimation scheme and micro-mechanical parameters. The purpose of this paper is to investigate applicability of the Weibull stress model in transition temperature regime and to quantify constraint effect among different-sized CT and PCVN specimens. The constituting parameters m and σu are determined at three temperatures by maximum likelihood estimate (MLE) technique in use of FE analysis results and experimental data of PCVN specimens. Also, failure probabilities of PCVN and CT specimens are calculated from the Weibull parameters, which are used for derivation of a prototype of toughness scale diagram. The diagram provides a technical basis to resolve transferability issue in the same material under different temperatures and constraint conditions.

Damage Produced in Solder Alloys during Thermal Cycling

The anisotropy of tin is associated with significant variations in its coefficient of thermal expansion and elastic modulus, with crystallographic direction. Under pure thermal cycling (with no externally applied stress or strain), substantial strains, in excess of 100%, may develop locally, and for very small structures, such as soldered interconnections comprising a few grains, structural integrity may be adversely affected. To examine this possibility, freestanding samples of tin, Sn-3.5wt.%Ag, Sn-0.5wt.%Cu, and Sn-3.8wt.%Ag-0.7wt.%Cu, have been subjected to thermal cycling. Temperature cycles from 30°C to 125°C or from −40°C to 55°C initially caused surface cracking, with openings up to several tens of microns after 3,000 cycles. Subsequently, the surface cracks grew into the interior of the specimens, with the maximum penetration ranging from a few microns after 100 cycles to more than 200 μm after 3,000 cycles. The cracks initiated from damage accumulated along grain boundaries. For the same temperature range, less damage resulted after the lower maximum (or mean) temperature cycle, and there appears to be a thermally activated component of cracking. The microstructure produced by rapid cooling (water quenching) was slightly more resistant than that formed by air, or furnace, cooling. Apart from microstructural coarsening, no damage accrues from isothermal exposure alone.

Synthesis of La-modified ultra stable zeolite L and its application to catalytic cracking catalyst

A new type of zeolite La-USL (ultra stable zeolite L (zeolite USL) modified by La), which has superior activity, stability and selectivity in catalytic cracking of hydrocarbons and thus can be used as an active catalyst component, is reported in this paper. The zeolite L with relative crystallinity of above 90% was synthesized by the hydrothermal crystallization method under optimum conditions and characterized by means of XRD, NH3-TPD and isotherm adsorption techniques. The in-situ synthesized zeolite L with a SiO2/Al2O3 mole ratio of 5–6 was modified by cation ion exchange, hydrothermal dealumination and chemical modifications with La in order to prepare La-containing USL with a higher framework SiO2/Al2O3 mole ratio of 15–30. The modified zeolite La-USL was used as an active additive component of fluid catalytic cracking (FCC) catalyst and the resulting catalysts were evaluated by microactivity test (MAT) and fixed-fluidized bed (FFB) experiments using heavy oil as feedstock. The influence of La content in La-USL on cracking product distribution, gasoline group composition and research octane number (RON) was investigated. The results showed that when La content in La-USL was 0.8 wt%, the addition of the corresponding La-USL could result in a FCC catalyst that produced significant improvement in product distribution and gasoline quality.

FKM Guideline “Fracture Mechanics Proof of Strength for Engineering Components” — Overview and Extension Topics

The German guideline “Fracture Mechanics Proof of Strength for Engineering Components” has been released in 2001 as a result of activities sponsored by the Research Committee on Mechanical Engineering (FKM), task group “Component Strength”. The guideline describes basics for the integrity assessment of cracked components subjected to static or cyclic loading and provides a step-by-step computational procedure for the use in engineering practice. The guideline was formulated based on a number of national and international reference documents, in particular SINTAP, R6, BS 7910 and DVS-2401, recent research results and some own key aspects. Since 2004 it is also available in English. The procedures and solutions of the guideline are implemented in the computer program FracSafe. The latest 3rd edition of the guideline (2005) includes several new topics. These allow for the consideration of special effects at cyclic loading, mixed mode loading, dynamic (impact) loading, stress corrosion cracking, probabilistic aspects in fracture mechanics calculations. In addition, the compendium of the stress intensity factor and limit load solutions is extended and adjusted according to the state-of-the-art. Some new examples and case studies are included to demonstrate the application of the procedure to engineering problems. This paper gives an overview of the guideline and describes new features available since its 1st edition.

Fatigue life prediction of heavy mining equipment. Part 1: Fatigue load assessment and crack growth rate tests

Frequent fatigue failure of heavy mining equipment has created the need for fatigue life prediction of its cracked components. As the first step, field monitoring of a cable shovel boom that had a history of chronic fatigue cracking was conducted. A set of load spectra for the boom was constructed from the collected field data. A finite element model of the boom was prepared and calibrated against the test data to predict the stress and strain histories. Eight crack growth rate coupon tests were carried out to obtain the fatigue properties of the material used. Constant amplitude equivalent stress ranges were calculated based on Palmgren–Miner’s cumulative damage rule and crack growth rate tests results.

On unique determination of partially coated polyhedral scatterers with far field measurements

This work is a continuation of our early study in Liu and Zou (2006 Uniqueness in an inverse acoustic obstacle scattering problem for both sound-hard and sound-soft polyhedral scatterers Inverse Problems 22 515–24; 2006 Uniqueness in determining multiple polygonal or polyhedral scatterers of mixed type Technical Report 2006-03(337) The Chinese University of Hong Kong) and addresses the unique determination of partially coated polyhedral scatterers in (N ⩾ 2) along with their surface impedance from far field data. Two global uniqueness results are established for this inverse problem with a scatterer consisting of multiple solid polyhedra: the first one is to determine such a scatterer of mixed sound-soft and impedance type by a single incident plane wave and the other is to determine such a scatterer of mixed sound-soft, sound-hard and impedance type by N different incident waves in the N-dimensional case with N ⩾ 3 and by only one incident wave for the two-dimensional case. Then we present some examples to show that as long as a scatterer admits the presence of (sound-hard) crack-type obstacles, then one cannot determine the scatterer uniquely by any less than N different incident plane waves. These examples also reveal that the uniqueness results achieved earlier in [15, 16] for polyhedral scatterers are optimal. Finally, the uniqueness results that have been solved or are still unsolved for the polyhedral-type scatterers with both solid and crack components are summarized in the conclusion.

A Method to Apply the Reference Stress Approach to Simplified Estimates of Inelastic J-integral Under Displacement-Controlled Loading

In the assessment of structural integrity and remaining life of a power component operated at high temperatures, evaluation of crack-like damage is frequently required. When estimating crack propagation rates under creep-fatigue loading, J-integral type inelastic fracture mechanics parameters including elastic-plastic J-integral for elastic-plastic body and creep J-integral for creeping body are needed. Although these parameters may be estimated by detailed inelastic finite element analysis of a cracked component, this kind of analysis is usually laboring and costly. Therefore, employing simplified methods such as the reference stress approach is practical. Since the reference stress approach has been developed for an inelastic body subjected to external loading, a method to apply this to a component under secondary stress has not been established. This paper proposes an approach to apply the reference stress method to a cracked component subjected to displacement-controlled loading together with a clear definition of elastic follow-up factor

Three-Dimensional Finite Element Model for Evaluation the Stress Intensity Factors for Different Fracture Modes of Homogeneous Bimaterial

The failure of cracked components is governed by the stresses in the vicinity of the crack tip. The singular stress contribution is characterized by stress intensity factors. The stress intensity factors depend on the geometry of the component and on loading condition. This paper addresses the evaluation of stress intensity factors (SIFs) of opening, shearing and tearing modes for compact tension specimens for bonding homogeneous materials. The aim of this work is to find the optimal pre-cracked length, crack notch radius and crack notch angle in order to eliminate the effect of them on the SIF and then on the Fracture toughness. To achieve this goal, a three-dimension finite element analysis (FEA) model using ANSYS program is constructed for specimens made of homogeneous materials such as stainless steel bonding with epoxy as a filler material. The effects of notch angle, notch tip radius and pre-crack length on the stress intensity factors are studied for different fracture modes. The results for the stress intensity factors KI , KII and KIII are obtained using linear elastic fracture mechanics (LEFM) approach.

The Post‐Yield Behaviour of an Internally Pressurised Cylinder having a Pad Attachment

Abstract: A study of pressurised cylinders having attached pads was made. The possibility of a fatigue crack developing along one side of the pad was noted. For both defect‐free components and components having axial crack‐like defects, finite element and limited experimental studies of the local and global deformations were made and results were found to be in close agreement. Both welded and integrally machined pads were studied. It was observed that for defect‐free components, the method of pad attachment has little effect on the burst pressure. The presence of a pad was found to increase the burst pressure of cracked components by a small but significant margin. A finite element‐based parametric study of pipes with part‐penetrating defects and a range of pad geometries has demonstrated that pad size has little influence on the limit load for any of the cracked cases considered. However, pipes with penetrating defects were strengthened by the presence of pads, particularly for thin‐walled sections.

Effects of lateral load on warm prestressing in a center crack plate

The load bearing capacity of cracked components increases if an appropriate compressive residual stress field is created near the crack tip. Warm prestressing is a well-known technique for introducing such a residual stress field. In this paper, the finite element method is used to study the effect of the load applied parallel to the crack (or the lateral load) in warm prestressing of a cracked specimen. The specimen is a square plate containing a center crack and the lateral load is applied in the preloading stage. The numerical results suggest that the lateral load in the preloading stage can influence significantly the apparent fracture toughness after the warm prestressing. It is shown that the improvement in the apparent fracture toughness due to a compressive lateral load is more significant than the improvement due to a tensile lateral load.

Threshold stress intensity factor and crack growth rate prediction under mixed-mode loading

A new mixed-mode threshold stress intensity factor is developed using a critical plane-based multiaxial fatigue theory and the Kitagawa diagram. The proposed method is a nominal approach since the fatigue damage is evaluated using remote stresses acting on a cracked component rather than stresses near the crack tip. An equivalent stress intensity factor defined on the critical plane is proposed to predict the fatigue crack growth rate under mixed-mode loading. A major advantage is the applicability of the proposed model to many different materials, which experience either shear or tensile dominated crack growth. The proposed model is also capable to nonproportional fatigue loading since the critical plane explicitly considers the influence of the load path. The predictions of the proposed fatigue crack growth model under constant amplitude loading are compared with a wide range of fatigue results in the literature. Excellent agreements between experimental data and model predictions are observed.

Constraint CorrectedJ-R- Curve and Its Application to Fracture Assessment for X80 Pipelines

AbstractSingle specimen J-R curve testing for X80 pipeline steel was conducted using single edge notched bend (SENB) and single edge notched tension (SENT) specimens with various crack lengths. Test data indicate that the J-R curves for this steel are strongly constraint dependent. To facilitate transferability of experimental J-R curves to those for actual cracked components, this paper develops a constraint corrected J-R curve for X80 steel. A modified J-Q theory that can consider the global bending stress influence is proposed so as to correctly quantify constraint effect on the crack-tip fields and the J-R curves. Results show that the modified J-Q solution can well match numerical crack-tip fields for bending specimens, with Q being a load-independent constraint parameter under large scale yielding. Based on the experimental data and numerical analysis, a constraint corrected J-R curve is formulated as a function of the parameter Q and crack extension Δa for X80 steel. A general procedure to predict J-R curves for actual cracked components is then outlined. Comparison indicates that the predicted J-R curves developed in this paper agree well with the experimental data for both SENB and SENT specimens. To demonstrate its application in failure assessment, the constraint corrected J-R curve for X80 steel is used to determine failure loads for a surface cracked pipeline. Reasonable agreement to available analytic solution is achieved.

Microstructure and mechanical behavior of Ni xAl y–Al 2O 3 in situ composites by pre-oxidation followed by hot-pressed reactive sintering

An in situ composite Ni x Al y –Al 2O 3, with a near fully dense microstructure, is fabricated from Ni and Al elemental powders using the pre-oxidation and subsequent hot-press reactive synthesis (PORS) technique. The exact formula of the formed Ni x Al y phase in the excessive Al-containing PORS1 and PORS2 samples deviates from the nominal stoichiometric one, dependent on the initial ratio of Ni to Al in the starting powder mixtures. Only NiAl compound is formed in the Ni50–Al50 (at.%) and Ni50–Al50 (at.%)–20 wt.% Al 2O 3 reaction systems during the hot-press reactive sintering process. The composites exhibit superiority to the NiAl monolithic compound in both flexural strength and fracture toughness at room temperature. Specifically, the in situ PORS1 composite has the highest room temperature flexural strength and fracture toughness with values of 453 MPa and 10.2 MPa m , respectively. The toughening effect might be attributed to the interaction between the crack tip and components in the microstructure.

T‐stress solutions for two‐dimensional crack problems in anisotropic elasticity using the boundary element method

ABSTRACTThe importance of a two‐parameter approach in the fracture mechanics analysis of many cracked components is increasingly being recognized in engineering industry. In addition to the stress intensity factor, the T stress is the second parameter considered in fracture assessments. In this paper, the path‐independent mutual M‐integral method to evaluate the T stress is extended to treat plane, generally anisotropic cracked bodies. It is implemented into the boundary element method for two‐dimensional elasticity. Examples are presented to demonstrate the veracity of the formulations developed and its applicability. The numerical solutions obtained show that material anisotropy can have a significant effect on the T stress for a given cracked geometry.

Cracking of AFNOR 7020 aluminium alloy component: A metallurgical investigation

Aluminium alloy of Al–4.5Zn–1.5Mg system, belonging to age hardenable, high strength category is being used for the fabrication of various components. The Al alloy component was part of a conduit line for filling the liquid chemical to its storage tank. The component consisted of an extruded tube, TIG welded to a bulb, which was fabricated from Al alloy forgings. A crack was observed on the conduit tube at the region where tube was welded to the bulb. Detailed investigations were carried out on the cracked component, which revealed insufficient working of the ingot during forging, resulting in remnant cast structure along with a heavy network of low melting point compound throughout the material. The cracks, which were initiated under the stresses induced during thermal treatment, machining and assembly, were found propagating through the eutectic network. This paper highlights the investigations carried out on the failed components.

Trends in High Temperature Structural Integrity Assessment

Abstract Failure in components that may contain defects and that operate at elevated temperatures can occur by creep, fatigue, or fast fracture, or some combination of these processes. Frequently, these components include welds and repairs to welds. Codes and standards are available for assessing the structural integrity of these types of components. This paper addresses the main issues of concern in making safety assessments and reliable lifetime predictions for situations where creep processes dominate. Emphasis is placed on the creep component of cracking. Initially, models of crack initiation and growth are reviewed. The creep fracture mechanics parameter C* is used to make predictions. The influence of constraint on the mechanisms responsible for void nucleation and growth is considered and creep crack growth rate laws derived. Methods of calculating C* in components from the plastic fracture mechanics parameter J or, approximately, from stress intensity factor K and limit analysis using reference stress concepts are described. A procedure for making remaining life assessments for cracking into progressively deteriorating material is considered. Techniques for dealing with an initial transient phase, or incubation period, during which damage builds up at a crack tip are presented. An indication of the scatter to be expected in creep crack growth data, for use in sensitivity studies, is provided. The role of residual stress on the early stages of cracking and subsequent crack propagation is discussed.

The Effect of Oxygenated Fuel Properties on Mechanism of Inhibition in Particulate Matter

It is reported that the particulate matter in exhaust gas is reduced when oxygenated agents are added to diesel fuel, but the inhibition mechanisms are not established. To explain this mechanism, we investigates the amount of particulate formation and analyzes thermal cracking and condensation polymerization components with a fluid reaction tube apparatus and nitrogen atmosphere. As a result of experiment, it became clear that inhibition mechanism of fine grain was different by fuel properties.