Damage Tolerance Assessment Research Articles

Mixed mode fatigue crack growth studies in AISI 316 stainless steel

Damage tolerance assessment requires the accurate prediction of fatigue crack growth lives. Numerical simulation techniques using mixed mode Paris' law and various equivalent stress intensity factor models (ΔKeq) are widely employed for fatigue life prediction. In the present work, mixed mode (I/II) fatigue crack growth experiments are performed using the compact tension shear specimens made of AISI 316 austenitic stainless steel for various loading angles. Finite element fatigue crack growth simulations are carried out, and the effect of ΔKeq model in fatigue life prediction is studied. To achieve this, a three parameter double exponential type best fit is proposed for fitting the experimental mixed mode crack length vs fatigue life. The performance and capability of various selected models are assessed by comparing the predicted life with the experimental results. Fractographic studies at different stages of crack propagation for different loading angles are also presented to aid the above assessments. Based on the overall consistent performance, Irwin's and Tanaka models are predicting life close to the experimental data and Richard's and Yan's models provide conservative solutions.

Damage Tolerance Assessment of a Brake Unit Bracket for High-Speed Railway Welded Bogie Frames

The brake unit bracket of a bogie frame is an important load-carrying component, particularly under emergency start/stop conditions. Conventional infinite/safe life approaches provide an over-conservative recommendation for the allowable strength and lifetime, which hinders the lightweight design of modern railway vehicles. In this study, to ensure the reliability and durability of a brake unit bracket, an attempt was made to integrate the nominal stress method and an advanced damage tolerance method. First, a complex bogie frame was modelled using solid elements instead of plate and beam elements. A hot spot stress region on the bracket was found under an eight-stage load spectrum obtained from the Wuhan–Guangzhou high-speed railway line. Based on the probability of foreign damage, a semi-elliptical surface crack was then assumed for residual life assessment. The results obtained by the cumulative damage and damage tolerance methods show that the brake unit bracket can operate for over 30 years. Moreover, even if a 2-mm depth crack exists, the brake unit bracket can be safely operated for more than 2.27 years, with the hope that the crack can be detected in subsequent maintenance procedures. Finally, an appropriate safety margin was suggested which provides a basis for the life prediction and durability assessment of brake unit brackets of high-speed railways.

A comparison of recent models for fatigue crack tip deformation

Fatigue crack propagation occupies much of the life of engineering components, particularly in the short crack regime. It is important to understand the mechanisms of propagation in order to carry out damage tolerance assessment and to predict component service life. The paper describes experiments carried out at macro- and micro-scale using digital image correlation to measure near-tip displacements. From these, various key parameters governing crack growth are extracted, and different models of fatigue crack deformation are validated. In particular, it is concluded that the Pommier and Hamam and the CJP models for fatigue crack displacement and stress fields have rather similar approaches to capturing the effects of crack tip plasticity.

Numerical assessment of cyclic J-integral △J for predicting fatigue crack growth rate

The cyclic J-integral is a significant mechanical parameter to describe the elastic-plastic fatigue crack growth rate. A equivalent domain integral method based on finite element method was proposed for evaluating the cyclic J-integral. This numerical method was verified by comparing the results of cyclic J-integral with J-integral evaluated by using the built-in functionality of ABAQUS under monotonic tensile load. Base on this numerical method, the uniqueness of extreme value of cyclic J-integral was studied through a kinked crack and a specific code was also developed to simulate the crack propagation in 304 stainless steel (304SS) compact tension (CT) specimen under mode I loading. The expression of crack growth rate was established based on the cyclic J-integral obtained by the specific code and this expression is found to be in good agreement with the experimental results. The research contents in this paper may contribute to the damage tolerance assessment of components.

Probabilistic evaluation on fatigue crack growth behavior in nickel based GH4169 superalloy through experimental data

The present paper developed a stochastic model to evaluate the scatter in fatigue crack growth (FCG) behavior of turbine disc GH4169 superalloy. Model parameters were calculated based on systematic experiments on compact tension (CT) specimens cut from the rim and installation flange locations of a turbine disk at corresponding service temperatures with stress ratios of 0.1 and 0.5. Based on the experimental data, piecewise FASTRAN model combined with Dugdale strip-yield model that includes the crack closure effect in GH4169 superalloy were proposed to formulate the deterministic FCG model. The probabilistic distribution of FCG rate was represented by a life distribution factor (LDF), combined with the deterministic model. Modeling results revealed that the variance of FCG rate followed a decrease-to-increase trend as crack propagates, which was consistent with the microstructural dependence of FCG revealed in typical polycrystalline materials. Proposed probabilistic model effectively reflected the stochastic property of FCG, offering a promising tool for probabilistic damage tolerance assessment (DTA).

The effect of texture on the low cycle fatigue property of Inconel 718 by selective laser melting

In recent decades, additive manufacturing (AM) technology has shown its great advantages to produce end-use products with complex design and high-added value. However, the AM-specific characters, such as inherent material anomalies (porosity, lack of fusion defects, or inclusions), anisotropy, location-specific properties and residual stresses, prevent AM from widely adoption in safety-critical parts. Therefore, the damage tolerance assessment of AM parts is desperately necessary. In this study, the impact of residual stress and the induced texture (columnar/equiax grain structure) after different heat treatment on the low cycle fatigue (LCF) behavior of Inconel 718 fabricated through selective laser melting (SLM) is investigated. The results showed that the texture of AMed parts can be controlled by suitable heat treatment, based on the residual stress during AM processing acting as the drive force to recrystallization. For SLMed Inconel 718 samples with columnar grains, anisotropic LCF properties exist, while no obvious sensitivity to orientations is shown for samples with equiaxed grains. This work is significantly meaningful to speed up the design-to-product transformation of safety-critical AM parts and optimize the orientation of components for various applications.

The lead crack concept applied to defect growth in aircraft composite structures

Currently, the cyclic fatigue growth and residual strength of damaged aircraft composite structures under operational loads is not fully understood. This leads to structures generally being designed to a no damage growth criterion with many knock down factors included to cover unknown/untested effects. Thus, full optimisation of composite aircraft structures is unlikely to be achieved under the no damage growth criterion. In 2009 the US Federal Aviation Administration (FAA) introduced a slow growth approach to certifying composite, adhesively bonded structures and bonded repairs which could improve the situation and is worthy of further investigation. In this paper the growth of some (limited) damage types available in the literature are reviewed and a framework proposed to address the damage tolerance assessment of these structures.

Error-controlled adaptive extended finite element method for 3D linear elastic crack propagation

We present a simple error estimation and mesh adaptation approach for 3D linear elastic crack propagation simulations using the eXtended Finite Element Method (X-FEM). A global extended recovery technique (Duflot and Bordas, 2008) is used to quantify the interpolation error. Based on this error distribution, four strategies relying on two different mesh optimality criteria are compared. The first aims at homogenizing the error distribution. The second minimizes the total number of elements given a target global error level. We study the behaviour of these criteria in the context of cracks treated by an X-FE approach. In particular, we investigate the convergence rates at the element-level depending its enrichment type. We conclude on the most suitable refinement criterion and propose and verify a strategy for mesh adaptation on 3D damage tolerance assessment problems.

Damage tolerance assessment of the interface strength gradation in thermoplastic composites

Abstract The newly developed hybridization approach for thermoplastic composites, based on the gradation of the interlaminar interface strength (IGIS design), was used to prepare polypropylene/glass fibre composites by properly alternating layers of reinforcing fabric with compatibilized and not compatibilized polymeric films. Maleated polypropylene was used to improve the matrix/fibre interface strength. The flexural and low-velocity impact characterizations showed that the use of the coupling agent improved the quasi-static flexural properties through the strengthening of the matrix/fibre interface, but considerably lowered the low velocity impact resistance of the laminate. The use of the IGIS design, which grades the interface strength through the laminate thickness, enabled the fabrication of composites with a favourable combination of flexural properties and impact resistance and proved to be effective in better preserving the integrity of the fibres. The damage after impact has been assessed by means of micro-computed tomography, which elucidated the improving effect of the matrix hybridization on the impact resistance, and acoustic emission analysis, which allowed the identification of the mechanisms responsible for impact energy absorption. The role played by the matrix hybridization sequence was pointed out along with the effective role of IGIS design in the overall mechanical performance of laminates.

Damage Tolerance Assessment for Fatigue-Critical Locations of Wing Structure of Aged Aircraft

Damage Tolerance Assessment for Fatigue-Critical Locations of Wing Structure of Aged Aircraft

Finite Element Modeling of Fatigue Crack Propagation Using a Self Adaptive Mesh Strategy

A new finite element model has been developed to predict fatigue crack growth in arbitrary 2D geometries under constant amplitude loading. The purpose of this model is on the determination of 2D crack paths and surfaces as well as on the evaluation of components Lifetimes as a part of the damage tolerant assessment. Throughout the simulation of crack propagation an automatic adaptive mesh is carried out in the vicinity of the crack front nodes and in the elements which represent the higher stresses distribution. The fatigue crack direction and the corresponding stress-intensity factors are estimated at each small crack increment by employing the displacement correlation technique under facilitation of singular crack tip elements. The propagation is modeled by successive linear extensions, which are determined by the stress intensity factors under linear elastic fracture mechanics (LEFM) assumption. The stress intensity factors range history has to be recorded along the small crack increments. Upon completion of the stress intensity factors range history recording, fatigue crack propagation life of the examined specimen is predicted. Verification of the predicted fatigue life is validated with relevant experimental data and numerical results obtained by other researchers. The comparisons show that this model is capable of demonstrating the fatigue life prediction results as well as the fatigue crack path satisfactorily. Copyright © 2008 Praise Worthy Prize S.r.l. - All rights reserved.

Calculation method of fracture toughness of materials at transition state

The fracture toughness of materials is very important for damage tolerance design and assessment technology,while the test method to obtain the fracture toughness in the transition state is costly and time-consuming. In order to overcome these disadvantages,a determination method of fracture toughness based on calculation in the transition state was presented. The triaxial stress constraint parameter for the three-dimensional crack tip was analyzed,the relation between fracture toughness and material tensile properties was researched with the energy criterion for material fracture,meanwhile the relation expression in the three-dimensional stress state for I type crack fracture and the determination method of fracture toughness of materials based on calculation in the transition state were obtained. The fracture toughness in the transition state for2219-T87 aluminum alloy was calculated with the determination method and empirical formula most of the theoretical calculation values were safer compared with test results and smaller than the values calculated by the empirical formula.

Rail Bogie Structural Assurance

Two types of cast steel rail bogies have been affected by cracking and defects. The cast steel design was from a period when bogie loads were lower than the endurance limit. However, the bogies as manufactured included a large number of casting defects and 60 years of operations has resulted in many repairs being applied. Thus like all ageing structure, the original basis for safe operations may be invalid. Structural assurance instructions were developed based on the following work. A condition data survey was undertaken to obtain configurations and defects within the fleet. This was used to assist in identifying critical areas, types and shapes of defects. Two bogies were instrumented to obtain operational loads that were compared to modern standards to provide both a spectrum for future operations and loads for Finite Element Analysis (FEA), Fatigue and Damage Tolerance Assessment (DTA). Finite Element Models (FEM) were developed to provide internal bogie loads and stresses for use in the fatigue and DTA and identify additional critical locations not identified in the condition data survey. DTA were undertaken to determine maximum flaw sizes and to verify the safe period of operation; given the loads, usage and stress data of the previous tasks. Based on the work undertaken, instructions for continued management documenting acceptable flaw criteria, life limits, and ongoing inspections were provided.

C-130H Centre Wing Lower Surface Tang Blend Repair Management

The RAAF found significant corrosion on the C-130H fleet Centre Wing Lower Surface (CWLS) panels at the tangs adjacent to the rainbow fittings. Repair of this corrosion involves blends and spot facing, and often requires the addition of a doubler to reinforce the region. All RAAF C-130H aircraft had various combinations of spot faces, blends and in some cases doublers at this location. Due to the number and combination of repairs, providing fleet wide management advice is problematic. The fleet condition was assessed from damage maps, repairs and previous analyses. From this a number of worst case configurations were determined. A Finite Element Model was developed and used to determine the bearing and by-pass loads in each fastener row of the panel tangs. Stress intensity correction factors were developed for cracks growing from or to a spot face using Stress Check. These correction factors were applied on top of geometry factors for the baseline configuration. A Damage Tolerance Assessment (DTA) was performed to assess the impact of spot face and blend repairs on the centre wing lower surface panel tangs, in order to develop a fleet wide management strategy. Based on the results for the repair cases, it was shown that the repairs identified in the damage maps could be managed within the existing safety by inspection program.

Cost Effective Safety from Full System Teardowns

This paper describes techniques for affordable teardowns on aircraft to extract condition data for assessment of system integrity durability and damage tolerance. Studies of fleet accidents show a significant proportion are due to failures in systems other than structures and engines but knowledge of their degradation with usage and age are less well understood. Forensic results from a full teardown provide robust data that supplement design and service data to support safety assessments. Fleet retirement is a golden opportunity to carry out a full system teardown if it is cost-effective. This permits a condition assessment of all critical systems after decades of Australianspecific configuration, roles and environment. Rapid dismantling techniques that do not destroy key information are being refined. Performance measurements are being compared with original design specifications and forensic data can be compared with non-destructive inspection data.

보강재 본딩접합 복합재 적층판구조 피로손상 균열진전 수명예측에 대한 연구

본 연구에서는 적층판 시편의 피로손상 균열진전 시험결과와 적층보강판 구조의 응력강도 해석결과를 기초로 충격손상을 모사한 원공과 노치손상을 내재한 보강재 본딩접합 적층보강판 구조의 피로손상 균열진전 수명예측에 대하여 고찰하였다. 그리고 적층보강판 구조시편에 대한 손상허용 시험결과와 손상진전 수명예측 해석결과를 비교분석한 결과 손상균열 길이 변화에 따라 최종파단에 대한 잔여강도를 예측하고 손상허용성 평가를 할 수 있었다. The prediction and analysis procedure of fatigue damage crack growth life for a stiffener bonded composite laminate panel including center hole and edge notch damage, was studied. It was performed on the basis of fatigue damage growth test results on a laminated skin panel specimens and the analysis results of stress intensity factor for the stiffener bonded composite panel. According to the comparison between experimental test and prediction results of fatigue damage growth life, it was concluded that the residual strength and damage tolerance assessment can be carried out along to the edge notch crack growth.

Next generation 3D mixed mode fracture propagation theory including HCF–LCF interaction

The damage tolerance assessment of complex aerospace structural components requires the capability of effective modeling of 3D cracks and their associated propagation and velocity and path under fatigue loads. A 3D mixed mode crack propagation theory is presented which includes the effect of KI, KII, and KIII, as well as non-proportional loading, elastic and fracture resistance anisotropy, and fracture mode asymmetry (viz. the ability to transition between competing tensile and shear modes of propagation). A modified strain energy release rate criterion including the modeling of crack closure is developed and presented for a representative problem. An elementary, mode I characterization of closure is used, leaving shear mode closure as fertile ground for further study.Use of the model is presented for an example problem with steady–vibratory interaction.

Damage Tolerance and Reliability Assessment under Random Markovian Loads

Accounting for load uncertainties plays an important role in the design of safe structural components of aircrafts under damage tolerance requirements. The purpose of this paper is to develop a reliability assessment technique for cracked structures submitted to non-stationary random fatigue loads modeled by first-order Markov chains with discrete state space and identified from in-flight measurements. The strategy based on a multi-level version of the cross-entropy method consists in progressively updating the tran- sition probability matrix in order to generate load sequences of increasing severity which are likely to cause failure. The proposed method is applied to a cracked M(T) specimen under the defined random fatigue loads. Load cycle interactions and retardation effects are accounted for by means of the PREFFAS crack closure model. The efficiency of the proposed approach in terms of computational cost is clearly observed for rare failure events in comparison with direct Monte Carlo simulations. In addition to the failure probability estimate, the multi-level cross-entropy method provides the analyst with information on the most probable load sequences at failure.

Adaptive Finite Element Modeling of Fatigue Crack Propagation

An adaptive finite element interactive program has been developed for fatigue crack propagation simulation under constant amplitude loading condition. The purpose of this model is on the determination of 2D crack paths and surfaces as well as on the evaluation of components Lifetimes as a part of the damage tolerant assessment. As part of a linear elastic fracture mechanics analysis, the determination of the stress intensity factor distribution is a crucial point. The fatigue crack direction and the corresponding stress-intensity factors are estimated at each small crack increment by employing the J-integral technique. The propagation is modeled by successive linear extensions, which are determined by the stress intensity factors under linear elastic fracture mechanics assumption. The stress intensity factors range history has to be recorded along the small crack increments. Upon completion of the stress intensity factors range history recording, fatigue crack propagation life of the examined specimen is predicted. Verification of the predicted fatigue life is validated with relevant experimental data and numerical results obtained by other researchers. The comparisons show that this model is capable of demonstrating the fatigue life prediction results as well as the fatigue crack path satisfactorily.

Fatigue crack growth in nano-composites

This paper discusses the potential of a variant of the Hartman–Schijve equation to represent fatigue crack growth in a range of nano-composites. It is found that when expressed in this form the exponent of this variant is approximately 2 and, as such, is considerably lower than the exponent in “Paris like” power law representations for delamination growth in composites. As such we see that, in these examples, the present variant of the Hartman–Schijve representation of delamination growth in nano-composites is similar to that seen for crack growth in metals, delamination growth in composites and the environmental degradation of adhesive bonds. This suggests that the present formulation may be useful for the damage tolerant assessment of small naturally occurring defects in nano-composite structures and for the ranking of nano-composites.