Prognostic analysis of fastener joints in straight attachment lugs

2017 ◽  
Vol 8 (3) ◽  
pp. 404-422 ◽  
Author(s):  
L. Chikmath ◽  
B. Dattaguru
Keyword(s):  
Crack Initiation ◽  
Crack Growth ◽  
Fatigue Loading ◽  
Computational Techniques ◽  
Contact Conditions ◽  
Content Type ◽  
Prognostic Analysis ◽  
Number Of Cycles ◽  
Lug Joints ◽  
Critical Locations

Purpose Many failures of aircraft structural components in the past were attributed to cracks emanating from joints, which are identified as the most critical locations. In cases using the recently emerging structural health monitoring (SHM) systems, continuous monitoring needs be carried out at many major joint locations. The purpose of this paper is to develop computational techniques for fastener joints, including the possible change in contact conditions and change in boundary values at the pin-hole interface. These techniques are used for the prognostic analysis of pin-loaded lug joints with rigid/elastic pin subjected to fatigue loading by estimating the residual life of the component at any given instance to assist the SHM systems. Design/methodology/approach Straight attachment lug joints with rigid/elastic push-fit pin and smooth pin-hole interface are modelled in commercial software MSC PATRAN. In each case, the joint is subjected to various types of fatigue load cycles, and for each type of cycles, the critical locations and the stress concentrations are identified from the stress analysis. Later, for each type of fatigue cycle, the number of cycles required for crack initiation is estimated. A small crack is located at these points, and the number of cycles required to reach the critical length when unstable crack growth occurs is also computed. The novelty in the analysis of life estimations is that it takes into account possible changes in contact conditions at the pin-hole interface during load reversals in fatigue loading. Findings The current work on fastener joints brings out the way the load reversals leading to change in contact conditions (consequently changing boundary conditions) are handled during fatigue loading on a push-fit joint. The novel findings are the effect of the size of the hole/lug width, elasticity of the material and the type of load cycles on the fatigue crack initiation and crack growth life. Given other parameters constant, bigger size hole and stiffer pin lead to lesser life. Under load controlled fatigue cycles, pull load contributes to significant part of fatigue life. Originality/value The analysis considers the changing contact conditions at the pin-hole interface during fatigue cycles with positive and negative stress ratios. The results presented in this paper are of value to the life prediction of structural joints for various load cycles (for both pull-pull cases, in which the load ratios are positive, and pull-push cycles, where the load ratios are negative). The prognostic data can be used to effectively monitor the critical locations with joints for SHM applications.

A study on prognostic analysis of attachment lugs under off-axis loading

2019 ◽  
Vol 10 (6) ◽  
pp. 809-824
Author(s):  
Bharath Kenchappa ◽  
Lokamanya Chikmath ◽  
Bhagavatula Dattaguru
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
Crack Growth ◽  
Structural Integrity ◽  
Fatigue Crack Initiation ◽  
Inequality Constraints ◽  
Computational Method ◽  
Engineering Structures ◽  
Content Type ◽  
Lug Joints

Purpose Lug joints with fasteners play a crucial role in connecting many major components of the aircraft. Most of the failures in the past were credited to the damages initiating and progressing from these types of joints. Ensuring the structural integrity of these fastener joints is a major issue in many engineering structures, especially in aerospace components, which would otherwise lead to fatal failure. The purpose of this paper is to adopting the prognostic approach for analysing these lug joints with fasteners subjected to off-axis loading by estimating the crack initiation and crack growth life of these joints. This data will be useful to estimate the remaining life of these joints at any given stage of operations, which is mandatory in structural health monitoring (SHM). Design/methodology/approach Straight and tapered lug joints are modelled using the finite element method in MSC PATRAN and analysed in MSC NASTRAN. These lug joints are analysed with a push fit fastener. The contact/separation regions at the pin–lug interface are carefully monitored throughout the analysis for various loading conditions. Critical locations in these lug joints are identified through stress analysis. Fatigue crack initiation and fatigue crack growth analysed is carried out at these locations for different load ratios. A computational method is proposed to estimate the cycles to reach crack initiation and cycles at which the crack in the lug joint become critical by integrating several known techniques. Findings Analysis carried out in this paper describes the importance of tapered lug joints, particularly when subjected to non-conventional way of loading, i.e. off-axis loading. There is a partial loss of contact between pin and lug upon pin loading, and this does not change further with monotonically increasing pin load. But during load reversals, there is a change in contact/separation regions which is effectively handled by inequality constraints in the boundary conditions. Crack growth in these lug joints pertains to mixed-mode cracking and is computed through the MVCCI technique. Originality/value Most of the earlier works were carried out on in-plane pin loading along the axis of symmetry of the lug. The current work considers the off-axis pin loading by loading the lug joints with transverse and oblique pin load. The significance of taper angle under such loading condition is brought in this paper. The results obtained in this paper through prognostic approach are of direct relevance to the SHM and damage tolerance design approach where the safety of the structural components is of foremost priority.

Unification proposals for fatigue crack propagation laws

2017 ◽  
Vol 13 (2) ◽  
pp. 262-283 ◽  
Author(s):  
Vladimir Kobelev
Keyword(s):  
Differential Equation ◽  
Crack Propagation ◽  
Crack Length ◽  
Closed Form ◽  
Crack Growth ◽  
Stress Ratio ◽  
Elementary Functions ◽  
Content Type ◽  
Number Of Cycles ◽  
Cycles To Failure

Purpose The purpose of this paper is to propose the new dependences of cycles to failure for a given initial crack length upon the stress amplitude in the linear fracture approach. The anticipated unified propagation function describes the infinitesimal crack-length growths per increasing number of load cycles, supposing that the load ratio remains constant over the load history. Two unification functions with different number of fitting parameters are proposed. On one hand, the closed-form analytical solutions facilitate the universal fitting of the constants of the fatigue law over all stages of fatigue. On the other hand, the closed-form solution eases the application of the fatigue law, because the solution of nonlinear differential equation turns out to be dispensable. The main advantage of the proposed functions is the possibility of having closed-form analytical solutions for the unified crack growth law. Moreover, the mean stress dependence is the immediate consequence of the proposed law. The corresponding formulas for crack length over the number of cycles are derived. Design/methodology/approach In this paper, the method of representation of crack propagation functions through appropriate elementary functions is employed. The choice of the elementary functions is motivated by the phenomenological data and covers a broad region of possible parameters. With the introduced crack propagation functions, differential equations describing the crack propagation are solved rigorously. Findings The resulting closed-form solutions allow the evaluation of crack propagation histories on one hand, and the effects of stress ratio on crack propagation on the other hand. The explicit formulas for crack length over the number of cycles are derived. Research limitations/implications In this paper, linear fracture mechanics approach is assumed. Practical implications Shortening of evaluation time for fatigue crack growth. Simplification of the computer codes due to the elimination of solution of differential equation. Standardization of experiments for crack growth. Originality/value This paper introduces the closed-form analytical expression for crack length over number of cycles. The new function that expresses the damage growth per cycle is also introduced. This function allows closed-form analytical solution for crack length. The solution expresses the number of cycles to failure as the function of the initial size of the crack and eliminates the solution of the nonlinear ordinary differential equation of the first order. The different common expressions, which account for the influence of the stress ratio, are immediately applicable.

scholarly journals Fatigue Crack Initiation and Early Growth in GLARE 3 Fiber-metal Laminate Subjected to Mixed Tensile and Bending Loading

10.14311/252 ◽  
2001 ◽  
Vol 41 (4-5) ◽  
Author(s):  
A. Chlupová ◽  
J. Heger ◽  
A. Vašek
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
Crack Growth ◽  
Fatigue Crack Initiation ◽  
Local Stress ◽  
Fatigue Loading ◽  
Fiber Metal Laminate ◽  
Fiber Metal ◽  
Fuselage Skin ◽  
Bending Loading

A special open-hole sheet specimen of GLARE fiber-metal laminate was used to simulate mixed loading similar to the loading of a riveted hole in a real fuselage skin structure. The effect of cyclic tension and secondary bending loading was studied. The notch region was observed through a microscope in order to detect the first fatigue crack initiation during fatigue loading. The number of cycles prior to crack initiation was measured, and the crack growth rate in the surface layer of the laminate was evaluated. The specimens were subjected to fatigue damage investigation in the inner layers of the laminate after termination of the test. A significant effect of secondary bending on fatigue crack initiation and early crack growth was found. The experimental results are discussed in terms of local stress-strain conditions in the notch region evaluated by means of finite element calculation.

An observation of crack initiation and early crack growth under impact fatigue loading

1999 ◽  
Vol 271 (1-2) ◽  
pp. 390-394 ◽  
Author(s):  
Meng Zhang ◽  
Pinsheng Yang ◽  
Yuxu Tan ◽  
Yan Liu ◽  
Shupeng Gong
Keyword(s):  
Crack Initiation ◽  
Crack Growth ◽  
Fatigue Loading ◽  
Impact Fatigue

An experimental study of initial flaws in thin sheets of 2024 aircraft aluminium alloy

2014 ◽  
Vol 5 (2) ◽  
pp. 120-128 ◽  
Author(s):  
Wyman Zhuang ◽  
Qianchu Liu ◽  
Cathy Smith
Keyword(s):  
Experimental Study ◽  
Fatigue Crack ◽  
Crack Initiation ◽  
Crack Growth ◽  
Fatigue Crack Initiation ◽  
Optical Microscope ◽  
Life Assessment ◽  
Content Type ◽  
Essential Input ◽  
Surface Discontinuities

Purpose – One of the challenges in the prediction of fatigue crack growth is to identify representative initial flaws and defects that can cause fatigue crack initiation and subsequent crack growth. Representative initial flaws identified from this experimental study provided an essential input for the fatigue life assessment programme of the PC-9/A training aircraft currently in service. The paper aims to discuss these issues. Design/methodology/approach – This paper addresses this challenge with a critical literature review and experimental assessment of initial flaw types that may cause fatigue crack initiation, by fatigue testing and fractography analysis using optical microscope and scanning electron microscopy (SEM). Findings – With a focus on aluminium alloy (AA) 2024-T3 thin sheet, the results cover various discontinuities from microstructural constituent particles inherent from the material process to macrostructural defects and surface discontinuities (such as burrs and machining marks) introduced during the production of airframes. It was found that most fatigue cracks originated from the bore surface discontinuities of rivet holes in the PC-9 vertical stabiliser thin panels rather than microstructural material defects of AA2024-T3 inherent from the material process. Research limitations/implications – The experimental study has found that quantifying fatigue initial flaw sizes which resulted from poorly finished fastener holes with arbitrary discontinuities at the surface is a challenging topic. This topic is under the current investigation using a statistics based analysis of initial flaws in the prediction of fatigue crack growth. Practical implications – The results obtained from this experimental study provided an essential input for the empennage and aft fuselage recertification and life assessment programme for the PC-9/A training aircraft currently in service. Originality/value – This experimental study examined AA2024-T3 thin skin panels from two different PC-9/A aircraft. The post-test failure analysis using optical microscope and SEM found that machining defects dominate fatigue crack initiation that can result in subsequent crack propagation.

Prediction of the Scatter of Crack Initiation under High Cycle Fatigue

2007 ◽  
Vol 345-346 ◽  
pp. 363-366 ◽  
Author(s):  
Stephane Osterstock ◽  
Christian F. Robertson ◽  
Maxime Sauzay ◽  
Suzanne Degallaix ◽  
Veronique Aubin
Keyword(s):  
Fatigue Crack ◽  
Crack Initiation ◽  
High Cycle Fatigue ◽  
Fatigue Crack Initiation ◽  
Fatigue Loading ◽  
Anisotropic Elasticity ◽  
Dislocation Dynamics ◽  
Cycle Fatigue ◽  
Number Of Cycles ◽  
Cycles To Failure

Under fatigue loading, the number of cycles to failure and its associated scatter increase when the loading level decreases. The High-Cycle Fatigue (HCF) regime is thus characterized by a large scatter in the number of cycles to failure [1]. Cracks initiation represents an important part of the lifetime of the structures. A stochastic method is used to study the fatigue crack initiation prediction in the 316L austenitic stainless steel. The present work proposes to show that this scatter can be attributed to the random orientation of individual grains, which influences the crack initiation localization. The stresses in grains are determined by finite element computations (FEM [2]), using a configuration representative of a polycrystalline aggregate. This approach takes into account the crystallographic orientations of the grains in the aggregate as well as the deformation incompatibilities between neighbouring grains due to crystalline anisotropic elasticity and elasticplasticity [3]. Then, the scatter of the number of cycles to crack initiation is derived from the FEM stress fields using two fatigue crack initiation criteria: an usual one, Mura’s criterion [4] and a more recent one [5], based on Discrete Dislocation Dynamics (DDD) simulations and taking into account plastic slips, cross slip and stress tensor components.

scholarly journals The structural integrity of nanoclay filled epoxy polymer under cyclic loading

2017 ◽  
Author(s):  
◽  
Sathievelli Chetty
Keyword(s):  
Cyclic Loading ◽  
Crack Initiation ◽  
Fatigue Failure ◽  
Crack Density ◽  
Fatigue Crack Initiation ◽  
Fatigue Loading ◽  
Fatigue Cycle ◽  
Crack Initiation And Propagation ◽  
Initiation And Propagation ◽  
Number Of Cycles

Fatigue crack initiation and propagation behaviour of CFRP have been of great importance because such composites are often used in engineering components that are subjected to continuous cyclic loading. The objective of this thesis work was to investigate the damage characteristics of the fatigue properties of CFRP composites by the modification of the polymer matrix with nanoclay addition. Carbon fibre reinforced epoxy was produced via vacuum assisted resin infusion moulding method (VARIM) with nanoclay concentrations of 0wt%, 1wt%, 3wt% and 5wt%. Tension-tension fatigue tests were conducted at loading levels of 90%, 75% and 60%. The frequency that was used was 3Hz with R value of 0.1. The results showed that at nanoclay percentages of 0wt%, 1wt% and 3wt% there was a consistent trend, where the number of cycles increased in fatigue loading percentages of 90%, 75% and 60%. At 5wt% nanoclay percentage the number of fatigue cycles dropped significantly at the 90% fatigue loading. The brittle nature of the 5wt% laminate became dominate and the sample fractured early at low fatigue cycle numbers. At the 75% fatigue loading, the number of cycles increased and at 60% fatigue loading the 5wt% nanoclay sample exceeded the number of cycles of all the nanoclay percentages by 194%. This was due to the intercalated arrangement of the nanoclays favouring the slow rate of surface temperature increase, during fatigue testing, at low fatigue cycle loading. The Crack Density analysis was performed and showed that at the same time in the fatigue cycle life, the 1wt% had 55 cracks, 3wt% had 52 cracks and the 5wt% had 50 cracks, for the 60% fatigue loading. This proved that it took longer for the cracks to initiate and propagate through the sample as the nanoclay percentage increased. Impact and hardness testing showed that the 5wt% exhibited brittle behaviour, which contributed to the results above. Scanning electron microscopy examination highlighted that the agglomeration of nanoclays delayed the crack initiation and propagation through the specimen and that the extent of fatigue damage decreased as the nanoclay percentage increased. A fatigue failure matrix was developed and showed that delamination, fibre breakage and matrix failure were the predominate causes for the fatigue failure.

Crack Growth Under Cyclic Loading and Plasticity Conditions

2014 ◽  
Author(s):  
Jaroslaw Szwedowicz ◽  
Piotr Bednarz ◽  
Christoph Meilgen ◽  
Jeff Samuelson
Keyword(s):  
Experimental Data ◽  
Renewable Energy ◽  
Cyclic Loading ◽  
Fracture Mechanics ◽  
Crack Initiation ◽  
Crack Growth ◽  
Linear Elastic ◽  
Paris Law ◽  
Elastic Fracture ◽  
Number Of Cycles

The increasing use of renewable energy sources to produce electricity requires additional operational flexibility from fossil-fuel gas and steam turbines. To compensate for renewable energy fluctuations in the electrical grid, a gas turbine (GT) engine needs to be more flexible, operating in peaking and partial loading modes as well as the base-load operation mode. Understanding how these different modes affect the lifetime of turbine components is critical to ensuring favourable RAM (Reliability, Availability, and Maintainability). Component lifetimes in peaking modes are limited by the number of thermo-mechanical cycles that a component can experience before crack initiation. The useful lifetime of some components can be increased by basing the predicted lifetime on the number of cycles for crack initiation plus the number of cycles for the crack to reach its maximum allowable length based on the fracture toughness K1C criterion for linear elastic fracture mechanics (LEFM). This is usually accomplished by using the Paris law to predict the rate of crack growth. Once cracks are formed, further propagation depends on the states of stress and strain near the cracks. These factors, which drive crack growth, can be quantified by the energy release rate. The Paris law predicts crack growth as a function of the energy release rate under linear elastic conditions, commonly for load controlled tests with load ration R>0. However, large thermal and mechanical loading can result in plastic deformation under cyclic loading conditions. Most GT components operate under strain controlled conditions generated by thermal loading. In this paper, a novel method is used to characterize crack growth under cyclic strain conditions in regions under plastic strain. The experimental data reveal that the rate of crack growth changes under plastic conditions in comparison with the linear elastic case. Especially compared to very high stress intensities ΔK of load controlled tests, here the allowable displacement limiting strain control matters. Applying experimental data from material tested under cyclic loading and elastic-plastic material response, component lifetime has been reliably predicted. Hereafter the developed method is referred to as elastic plastic fracture mechanics (EPFM) lifetime assessment. The EPFM approach more closely predicts the observed rate of crack growth than linear elastic fracture mechanics. LEFM over-predicts component lifetime for cracks growing in plastic regions under cyclic loading and could lead to catastrophic failure of a component. Therefore, the lifetime of a highly loaded component is more reliably assessed using the EPFM approach, as demonstrated for two alloys in this paper.

scholarly journals Low-Cycle Fatigue Crack Initiation Simulation and Life Prediction of Powder Superalloy Considering Inclusion-Matrix Interface Debonding

Materials ◽  
2021 ◽  
Vol 14 (14) ◽  
pp. 4018
Author(s):  
Shuming Zhang ◽  
Yuanming Xu ◽  
Hao Fu ◽  
Yaowei Wen ◽  
Yibing Wang ◽  
...  
Keyword(s):  
Finite Element ◽  
Crack Initiation ◽  
Life Prediction ◽  
Damage Mechanics ◽  
Low Cycle Fatigue ◽  
Fatigue Crack Initiation ◽  
Fatigue Loading ◽  
Interface Debonding ◽  
Damage Evolution Equation ◽  
Finite Element Software

From the perspective of damage mechanics, the damage parameters were introduced as the characterizing quantity of the decrease in the mechanical properties of powder superalloy material FGH96 under fatigue loading. By deriving a damage evolution equation, a fatigue life prediction model of powder superalloy containing inclusions was constructed based on damage mechanics. The specimens containing elliptical subsurface inclusions and semielliptical surface inclusions were considered. The CONTA172 and TARGE169 elements of finite element software (ANSYS) were used to simulate the interfacial debonding between the inclusions and matrix, and the interface crack initiation life was calculated. Through finite element modeling, the stress field evolution during the interface debonding was traced by simulation. Finally, the effect of the position and shape size of inclusions on interface debonding was explored.

scholarly journals Theoretical and Experimental Fatigue Strength Calculations of Lips Compensating Circumferential Backlash in Gear Pumps

Energies ◽  
2021 ◽  
Vol 14 (1) ◽  
pp. 251
Author(s):  
Piotr Osiński ◽  
Grzegorz Chruścielski ◽  
Leszek Korusiewicz
Keyword(s):  
Maximum Stress ◽  
Theoretical Calculations ◽  
Fatigue Loading ◽  
Minimum Thickness ◽  
Bending Tests ◽  
External Gear Pumps ◽  
Gear Pumps ◽  
In Series ◽  
Number Of Cycles ◽  
Technical Solutions

This article presents theoretical and experimental calculations of the minimum thickness of a compensation lip used in external gear pumps. Pumps of this type are innovative technical solutions in which circumferential backlash (clearance) compensation is used to improve their volumetric and overall efficiency. However, constructing a prototype of such a pump requires long-lasting research, and the compensation lip is its key object, due to the fact that it is an element influenced by a notch and that it operates in unfavorable conditions of strong fatigue stresses. The theoretical calculations presented in this article are based on identifying maximum stress values in a fatigue cycle and on implementing the stress failure condition and the conditions related to the required value of the fatigue safety factor. The experimental research focuses on static bending tests of the lips as well as on the fatigue loading of the lips in series of tests at increasing stress values until lip failure due to fatigue. The tests allowed the minimum lip thickness to be found for the assumed number of fatigue cycles, which is 2.5 times the number of cycles used in wear margin tests.

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