Daren Peng

Monash University (Australia) · Department of Mechanical and Aerospace Engineering, Clayton

The durability assessment of additively manufactured parts needs to account for both surface-breaking material discontinuities and surface-breaking porosity and how these material discontinuities interact with parts that have been left in the as-built state. Furthermore, to be consistent with the airworthiness standards associated with the certific...

It is now well-known that the interaction between surface roughness and surface-breaking defects can significantly degrade the fatigue life of additively manufactured (AM) parts. This is also aptly illustrated in the author’s recent study on the durability of wire and arc additively manufactured (WAAM) 18Ni 250 Maraging steel specimens, where it wa...

Motivated by the need for an efficient fatigue crack growth prediction infrastructure for both legacy and novel materials, we have initiated the development of an automated computational framework capable of determining crack growth model parameters for an equationally defined model. As a first step in addressing this need, the present paper focuse...

USAF MIL-STD 1530D states that the design and certification approval of military aircraft requires analytical tools that are capable of modeling crack growth, and that the role of testing is to validate or correct the durability and damage tolerance analyses. As such analysis and experimental testing are intimately interconnected. As a result this...

MIL‐STD 1530D requires that the certification of an aircraft part employ analytical tools that are capable of modeling crack growth. It is further stated that the durability and damage tolerance (DADT) analyses should be based on linear elastic fracture mechanics (LEFM) and follow a building block approach. This paper illustrates the durability ana...

The airworthiness certification of military aircraft requires a durability analysis be performed using linear elastic fracture mechanics (LEFM). Furthermore, such analyses need to use a valid small crack growth equation. This paper focuses on the effect of rough surfaces and the effect of machining the surface on the durability of AM parts using LE...

The Advance Of Science Involves A Constant Interplay Between Theory And Experimental Observations, Each Of Which Informs The Other. In this context the presentation first illustrates how the effect different manufacturing processes and post build treatments on crack growth in metals is controlled by just two parameters. The Materials Science Commun...

The fracture problem of piezoelectric materials in multi-field coupling is a very attractive research topic in engineering. There is important theoretical value and practical significance. This paper focuses on solving the fracture problem of two collinear cracks subjected to the combined quadratic thermo-electro-elastic loads. Under the assumption...

: This paper studies a cracked orthotropic solid under symmetric heat flow, and symmetric mechanical loading is studied. A modified partially impermeable crack model is employed to simulate thermal load transfer. With the application of the Fourier transform technique and superposition theory, the related physical quantities and fracture parameters...

This paper addresses problem of two collinear cracks in an infinity orthotropic solid subject to combined thermal and mechanical loads. Based on a model called ' improved partially conductive crack model', the Fourier transform and superposition theorem, the analytical solutions to some physical quantities and fracture parameters are given. It is r...

This paper first discusses the accuracy of the standard adhesively-bonded joint design code, the analytical A4EI code, that is used in the aircraft industry to determine the strain-energy density in a complex step-lap joint under shear loads. This joint geometry is then used to assess the ability of the A4EI code, and a standard finite-element anal...

Tis presentation first outlines the state of the art in the durability assessment of AM parts and then addresses the effect of machining rough surfaces.

This paper summarises the findings of an investigation into the durability of cold spray repairs, also known as supersonic particle deposition or SPD repairs, to simulated corrosion damage in AA7075-T7351 aluminium alloy specimens. A feature of this paper is that it is the first to show how to perform the mandatory durability analysis of repaired c...

An improved partially permeable crack model is put forward to deal with the problem of a single crack embedded in an orthotropic or isotropic material under combined unsymmetric thermal flux and symmetric linear mechanical loading. With the application of the Fourier transform technique (FTT), the thermoelastic field is given in a closed form. Nume...

The United States Air Force (USAF) Guidelines for the Durability and Damage Tolerance (DADT) certification of Additive Manufactured (AM) parts states that the most difficult challenge for the certification of an AM part is to establish an accurate prediction of its DADT. How to address this challenge is the focus of the present paper. To this end t...

The problems of crack formation in orthotropic materials under 2i order polynomial function heat flow and 2j order polynomial function mechanical loading are considered. An extended local insulation crack model is proposed, and fracture analysis is carried out for the above problems. Utilizing Fourier transform technique (FTT) and principle of supe...

This paper presents a detailed study into the damage tolerance and durability of supersonic particle deposited (SPD), also known as cold spray, repairs to AA7075-T7351 aluminium alloy wing skins. Recent studies have shown that failure in 7075 and 6061 cold spray repairs is often as a result of the nucleation of multiple small cracks in the substrat...

One of the challenges in aircraft sustainment is to develop additively manufactured (AM) replacement parts for legacy aircraft. This is particularly important for fixed and rotary wing aircraft that operate in in aggressive environments, i.e. off carriers, in a marine environment, etc. On the other hand the United States Air Force (USAF) have now a...

This paper discusses two approaches that have been proposed to account for the data scatter observed in delamination growth tests under cyclic-fatigue loading and thereby enable an estimate of a worst-case delamination growth curve for use in the damage tolerance and durability assessment of composite and adhesively-bonded airframes. The two approa...

The certification requirements for additively manufactured (AM) replacement parts requires the ability to predict the growth of small sub mm cracks. To this end, it is shown that the Hartman-Schijve crack growth equation proposed in a prior paper for the growth of small cracks in AM Ti-6Al-4V manufactured specimens also accurately predicts the grow...

The present paper shows that the variability in the cyclic fatigue crack growth (FCG) rate in structural adhesives that is frequently observed in laboratory test results can be captured by using a Hartman-Schijve methodology. To this end the Hartman-Schijve equation has been used to access an 'upper-bound' FCG rate curve that (a) encompasses all th...

This paper presents an experimental study into the analysis required for the durability assessment of 7075 and 6061 cold spray repairs to military aircraft. To this end, it is first shown that provided the bulk stress in a 7075 cold spray coating can be kept beneath approximately 150 MPa, then the coating should not crack. A range of examples are p...

Sustainment issues associated with military helicopters have drawn attention to the growth of small cracks under a helicopter flight load spectrum. One particular issue is how to simplify (reduce) a measured spectrum to reduce the time and complexity of full-scale helicopter fatigue tests. Given the costs and the time scales associated with perform...

This paper uses a Fourier transform technique, which uses dislocation density functions, to solve the problem of layered media with finite cracks perpendicular to the interface. This approach reduces the problem to the solution of two coupled integral equations each with a singular kernel which are the solved using Cauchy-Chebyshev polynomials. Two...

The USAF requirements for the durability and damage tolerance certification for additively manufactured (AM) aircraft structural parts, which are detailed in Structures Bulletin EZ-19-01, raise a number of new and, as yet, unanswered questions. The present paper attempts to address three questions: How to perform a fracture mechanics-based analysis...

This paper firstly reveals that when assessing if a bonded joint meets the certification requirements inherent in MIL-STD-1530D and the US Joint Services Standard JSSG2006 it is necessary to ensure that: (a) There is no yielding at all in the adhesive layer at 115% of design limit load (DLL), and (b) that the joint must be able to withstand design...

This paper focuses on the problem of skin corrosion on the upper wing surfaces of rib-stiffened aircraft. For maritime and military transport aircraft this often results in multiple co-located repairs. The common approach to corrosion damage in operational aircraft is to blend out the corrosion and rivet a mechanical doubler over the region. In par...

Composite and bonded aircraft structures are currently designed such that any delamination/disbond will not grow. However, it is now known that despite this design limit, delaminations and disbonds can both arise and grow when subjected to operational flight loads. In this context in 2009, the FAA Advisory Circular 20-107B introduced a slow growth...

Additive manufacturing (AM) is now being used both to manufacture replacement parts and to repair damage associated with corrosion and wear in ageing military aircraft. To meet the damage tolerance requirements outlined in the US certification standard MIL-STD-1530D, the present paper first discusses how to capture the variability in the crack grow...

This paper is motivated by the need to better understand the potential of Additively Manufactured (AM) Ti6-Al-4V replacement parts for future use on operational aircraft. Unfortunately, the interaction between surface and near surface (sub-surface) breaking material discontinuities, i.e. porosity due to and lack of fusion or keyholing, and the roug...

Composite airframes are currently designed such that any delamination will not grow. However, despite this design limit requirement, there have been a number of instances where delaminations and disbonds have grown in operational aircraft. To address this discrepancy between as designed and actual performance, in 2009 the FAA Advisory Circular 20-1...

The present paper examines crack growth in a range of structural adhesive joints under cyclic-fatigue loadings. It is shown that cyclic-fatigue crack-growth in such materials can be modelled by a form of the Hartman and Schijve crack-growth equation which aims to give a unique and linear 'master' representation for the fatigue data points that have...

This paper focuses on the problem of skin corrosion on the upper wing surfaces of rib stiffened aircraft. For maritime and military transport aircraft this often results in multiple co-located repairs. The common approach to corrosion damage in operational aircraft is to blend out the corrosion and rivet a mechanical doubler over the region. Unfort...

This paper analyses the problem of a crack in an orthotropic material subjected to combined quadratic mechanical loading and a quadratic thermal flux. Attention is focused on the problem of a partially insulated crack. Applying the Fourier transform technique and the superposition principle enables closed form solutions to be obtained for the tempe...

This paper investigates the combined effect of corrosion and fatigue on the growth of cracks that arise from natural corrosion in steel bridges. It is shown that if these two effects need to be simultaneously analyzed. If not, then the resulting life is not conservative. Consequently, to enable a better understanding of the remaining life of steel...

This paper studies the combined effect of corrosion and fatigue on the growth of cracks in aircraft and on the effect of skin corrosion and stress corrosion cracking on the load bearing capacity of rib stiffened aircraft wings. In this context it is shown that the growth of cracks from surface pitting, and also from intergranular cracking at a fast...

In machine component stress analysis, it usually assumed that the geometry specified in CAD provides a fair representation of the geometry of the real component. While in particular circumstances, tolerance information, such as minimum thickness of a highly stressed region, might be taken into consideration, there is no standard practice for the re...

In the context of aircraft sustainment, multiplicative manufacturing is defined as the simultaneous application of subtractive and additive manufacturing processes to extend the fatigue life of an operational structure. In this chapter, we illustrate the potential of multiplicative manufacturing via the LAU7 missile launcher housing. It is shown th...

All repairs to airframes now need to be assessed as to their effect on the damage tolerance the aircraft. To this end this chapter first discusses difference between the analysis tools needed for ab initio design and aircraft sustainment. It is shown that using small or physically short-crack da/. dN versus Δ. K data results in reduced through life...

This chapter presents simple computational tools for determining the stress intensity factors associated with small sub-mm cracks in realistic complex geometries under arbitrary flight loads. Attention is also focused on small initial cracks that first grow as a complex three-dimensional (3D) shape and then transition into a through-the-thickness f...

In the past decades, particularly since the Aloha Airlines (AA243) in 1989, corrosion has become one of the primary considerations in both military and commercial aircraft. Numerous authors have noted that corrosion can impact heavily on the economics, maintenance and safety of aircraft fleets. A number of different types of corrosion have been det...

The problem of two collinear cracks in an orthotropic solid under antisymmetrical linear heat flow is investigated. It is assumed that there exists thermal resistance to heat conduction through the crack region. Applying the Fourier transform, the thermal coupling partial differential equations are transformed to dual integral equations and then to...

This paper discusses the stress and strain states in adhesively bonded step lap joints from the perspective of the strain energy density solutions determined via the elastic and elastic-plastic solutions obtained using the computer code A4EI, which as outlined in the US Defence Departments Composite Materials Handbook CMH-17-3G, is the industry sta...

This paper studies the prior effect of corrosion on fatigue on the growth of cracks that arise from natural corrosion in steel bridges. It is shown that these 2 effects need to be simultaneously analysed. If not, then the resulting life is not conservative. This paper presents a simple methodology for performing this coupled analysis.

This paper focuses on the growth of cracks that arise from natural corrosion in steel bridges. It is shown that these two effects of corrosion and stress, need to be simultaneously analysed. A methodology used to compute the growth of such cracks in bridge steels is presented. A better understanding of the remaining life of steel bridges would help...

This paper investigates the ability of the USAF characteristic K approach to represent the growth of cracks from small naturally occurring material discontinuities. The examples discussed in this paper are associated with two different load spectra. The first case studied involves crack growth in AA7050-T7451 under a combat aircraft load spectrum....

A hopper wagon is a type of railroad freight car used to transport many types of materials such as coal, ore, grain, ballast and minerals and bulk cargo etc. It is a complicated welded structure. When performing a safety assessment of cracks in a hopper wagon, it is necessary to consider fatigue crack propagation and fracture among the possible mod...

This report presents the results of a detailed three dimensional non-linear a finite element study into SPD repairs to corrosion damage in P3C upper wing planks. This analysis, which allowed for both large deformation and material non-linearity (yielding), first reveals that the effect of skin corrosion on the load carrying capacity of rib stiffene...

Purpose The purpose of this paper is to describe the results of a combined numerical and experimental study into the ability of supersonic particle deposition (SPD) to restore the load carrying capacity of rib stiffened wing planks with simulated stress corrosion cracking (SCC). Design/methodology/approach In this context the experimental results...

This paper summarises the findings of an international collaborative programme that focuses on the problem of the growth of cracks that arise from natural corrosion in bridge steels. The experimental data presented in this paper confirm that the bridge steel da/dN versus ΔK relationship is similar to that seen by the high-strength aerospace steels...

This paper discusses how cracks that grow from small naturally occurring material discontinuities under operational load spectra behave. The growth of small cracks under a representative maritime aircraft flight load spectrum is discussed first. The results of this study, when taken in conjunction with the authors previous studies into cracks growi...

This paper focuses on problems associated with aircraft sustainment related issues and illustrates how cracks that grow from small naturally occurring material discontinuities in operational aircraft behave. The example discussed in this paper, which is associated with crack growth under a representative maritime aircraft load spectrum, when taken...

The growth of cracks from small naturally occurring material discontinuities plays a major role in the operational lifespan of aircraft structures. Calculating the life of an airframe structure requires the determination of the crack path(s) which for complex real life geometries can often be highly complex. This paper presents a simple method base...

Ageing aircraft fleets commonly experience environmentally induced degradation. This requires an accurate assessment of fatigue performance including any influence of corrosion damage. Since the majority of the life of a fatigue crack is spent in propagation of small cracks, the predictive capabilities of crack growth laws must ideally capture smal...

This paper focuses on problems associated with aircraft sustainment-related issues and illustrates how cracks, that grow from small naturally occurring material and manufacturing discontinuities in operational aircraft, behave. It also explains how, in accordance with the US Damage Tolerant Design Handbook, the size of the initiating flaw is mandat...

The ability to understand the factors that contribute to the scatter inherent in the fatigue lives of metallic structures is critical both to ensuring continued airworthiness and to designing optimum structures. This paper focuses on the effect of variations in a material’s cyclic stress intensity threshold (ΔKthr) on fatigue crack growth. It then...

This paper uses a Fourier transform technique to solve the problem of two interacting collinear unequal cracks in a finite width plate. This approach reduces the problem to the solution of two coupled integral equations each with a singular kernel which is the solved using Cauchy-Chebyshev polynomials. The solution is first validated by comparing t...

. Squats are becoming a major form of rail damage for railways worldwide. The development of squat defects has become a major concern in numerous railway systems throughout the world. Infrared thermography is a relatively new nondestructive inspection technique used for a wide range of applications. However, it has not been used for rail squat dete...

This paper describes the results of a study into a new class of multi-site damage problems, viz: the interaction between multi-layer intergranular cracks and cracks that grow from small naturally occurring material discontinuities at a hole. The nature of the intergranular cracks, which are perhaps better thought of as delaminations rather than cra...

This paper provides a method for addressing thermal fatigue crack growth in railway wheels, allowing for rail chill effects under block braking. A 3D non-linear finite element model has been used to evaluate the thermal stress allowing for rail chill and for its influence on the crack growth in a rail wheel. Both stop and drag braking are examined....

Experimental studies of fatigue crack growth in aluminium alloys have shown that, at the low-to-mid stress intensity factor range, there is often a log-linear relationship between the crack length and the fatigue life. These observations have led to the development of the generalised Frost–Dugdale crack growth law, which allowed the accurate predic...

This paper summarises recent developments in the formulation and application of the generalised Frost–Dugdale crack growth law. We first reveal the relationship between the generalised Frost–Dugdale crack growth law, dislocation based crack growth laws, the two parameter crack growth model, and fractal fatigue concepts. We then show that a range of...

A simplified finite element analysis on the squats growth simulation and the effect different contact stresses has been presented. This analysis is based on the element removal study to simulate squat growth in a rail track under cyclic loading. The major principal stress (maximum principal stress failure theory) has been used as failure criteria....

This paper describes the results of the development of squats under alternating loads. An evolutionary program has been developed to simulate squats growth. The code is based on genetic algorithms and the whole field of evolutionary computation. Evolution strategies are derived from the biological process of evolution, to find squats growth path so...