Single-layer aluminium alloy cylindrical shells are established using the powerful finite element (FE) software package ABAQUS. Furthermore, the probability distribution models of different random parameters in structural modelling are summarized. Forty seismic ground motion records are selected to consider the uncertainty of earthquakes. Sensitivity analysis of modelling parameters is conducted to determine the parameters with the greatest influence on seismic responses. The incremental dynamic analysis (IDA) method is performed on aluminium alloy cylindrical latticed shell structures with different structural parameters. After applying the damage index and structural performance levels of the latticed shells proposed in this paper, the probabilistic seismic demand model and probabilistic seismic capacity model are established. The seismic performance and the collapse capacity of different aluminium alloy cylindrical latticed shells are discussed based on the FE results. Furthermore, vulnerability curves are obtained according to the IDA results, which can be utilized to predict the failure probability and to evaluate the structural performance of aluminium alloy cylindrical latticed shells under different levels of earthquakes.

Introduction

Aluminium alloys have become a widely applied construction material in transportation applications in the aviation, aerospace, automobile, mechanical manufacturing, shipbuilding and chemical industries [1]. In recent years, they have developed into an important substitute of structural steel, especially for light structures, such as long-span spatial structures, tower structures and bridges [2]. The increase of aluminium alloys in spatial structures, such as reticulated shells, is a result of their superior characteristics to structural steel, such as their low density, high strength-to-weight ratio, low maintenance costs and good recyclability. These alloys also provide comparable ease of manufacturing and superior aesthetics. Many representative aluminium reticulated spatial structures have been established in the United States, Europe and China [3–4]. Most aluminium reticulated spatial structures are single-layer reticulated shells. Due to their importance and specialty to building functions, once the reticulated shells are damaged or collapse, a large direct or indirect economic loss or casualty results. However, the Chinese seismic code only aims to achieve the performance level of life safety, which is not capable of satisfying users’ needs. The analysis of the seismic fragility of aluminium reticulated shells both provides theoretical support to the performance-based seismic design and lays the foundation for the evaluation of seismic loss.

1. Introduction

2. Experimental description

3. Results and Discussion

4. Conclusion

References

Conclusions

References

The aim of this study is to evaluate a full integrated modelling strategy to evaluate the influence of casting defects on the fatigue life directly from process simulation. We have shown that defects characterized by their size and the microstructure characterized by the SDAS, are the main parameters that control the fatigue limit. A fatigue criterion that already takes into account for the effect of the defect on the fatigue limit was modified to introduce the effect of SDAS. This improved criterion has been employed to predict the Kitagawa diagram for multiaxial loading for different loading cases. The simulation of the modified criterion showed that the reduction of the fatigue limit with the defect size and SDAS is well described. In the last part a numerical model was developed to perform a simulation of the fatigue limit starting from the simulation of the casting process. Using this numerical model, we simulated the defect size and SDAS depending on the solidification time, the fatigue limit is simulated using the improved criterion. We proposed in this part a mold which let to obtain samples with two different microstructures. In this study, a second fatigue tests was carried out on these samples to validate the numerical simulation on the proposed mold. It turns out that the numerical model provides reasonably well the obtained experimental results.

HCF studies on cast aluminum alloys have shown [1, 2] that the effect of defect size is in competition with other microstructural features such as SDAS/DAS to limit fatigue life. It has been clearly observed that casting defects have a detrimental effect on fatigue limit above a critical size [3, 4]. In addition, the influence of microstructure defined by DAS or SDAS on the fatigue behaviour of cast aluminum alloy cannot be neglected [5-7]. It has been demonstrated [6] that the eutectic phase can cause micro-cracking due to strain accumulation in proximity to Siparticles. As reported in [1], for small defect sizes, there is a prominent interaction between defects and microstructure on the fatigue limit of cast A356-T6 aluminum alloy. Therefore, the simulation of defect size and microstructure distributions within a cast aluminum alloy component are critical inputs for fatigue design. In this context, several studies [8-12] have proposed models to simulate the solidification process and the formation of porosity. A number of models combine the shrinkage pressure and hydrogen concentration to describe porosity formation. In these models the defects were explicitly assumed to be spherical [13-15]. Atwood et al [11] assumed that during solidification, due to solid-liquid balance, pore shapes become complex as they grow between grains or between dendrite arms. Several studies [8, 13] have suggested that it is more appropriate to consider an equivalent pore size rather than trying to characterize the complex 3D shape. This assumption is useful in simplifying the fatigue design for casting aluminum alloy components. In some studies [8, 10-12], the effects of pressure, cooling rate and the distribution of hydrogen in the casting on the formation of pores has been studied.

1. Introduction

2. Material behavior

3. Fatigue criteria

4. Conclusions

References

Numerical simulation is more and more used in automotive industry to reduce design time and cost. Moreover, for high performance engines, the development of cylinder heads requires well known and adapted materials to ensure their reliability. The estimation of adapted fatigue criteria based on few experiments is thus an important challenge: to obtain more predictive model on more complex mechanical phenomena. This paper describes the methodology adopted by Renault to estimate fatigue life of a new aluminum-silicon alloy. A low copper aluminum-silicon alloy has been developed for a new generation of cylinder heads. Those kinds of alloys provide a good compromise between the two main failure modes encountered for cylinder heads: high cycle fatigue of water jacket and low cycle fatigue of fire deck. However, those materials are subjected to thermal ageing that can affect mechanical behavior, such as yield stress and hardening, but also fatigue mechanisms at higher temperatures. This article presents first the selected constitutive equations that introduce thermal ageing effect in a cyclic elasto-viscoplastic model. The low cycle fatigue criterion is then calibrated on the relevant database, which includes isothermal and non-isothermal tests. The fatigue criterion is a specific version of a classical model previously developed at Onera. The critical variables for the fatigue part are the stress amplitude and the mean stress, both of them being normalized by an ultimate stress which depends on temperature and ageing state. Finally, the constitutive and damage models have been applied on 3D cylinder head numerical analysis. The paper proposes a brief comparison of those results with more classical simulations made on cylinder heads and highlights the perspectives offered by this new aluminum-silicon alloy.

Introduction

Cylinder head is a central and complex part on an internal combustion engine as it has to ensure many important functions: closure of the combustion chamber, admission of fresh gas, exhaust of hot burned gas, cooling of the engine. Consequently to that central position, it supports 3 kinds of loadings:  Static loading due to tightening on cylinder block and interference fitting of seats  Thermal loading due to gas combustion flow Because of the engine power variation, that loading is cyclic and therefore creates thermo-mechanical stresses located on the fire deck, and then low cycle fatigue cracks.  Pressure loading at each combustion in the cylinder Variation of pressure (with / without), combined with mean stresses coming from static and thermal loadings, induces high cycle fatigue damage, especially on the water jacket. Those complex loadings generate strong material mechanisms that must be understood to design the cylinder head. An important challenge of our industry is to define the adapted experiments that allow to estimate an accurate fatigue criteria. Considering in another hand that the number of experiments has to be optimized to reduce the new alloy’s cost development. This is the aim of the collaborative work presented in this paper which was conducted by different specialists from casting industry, material characterization, material behavior and motorist.

1. Introduction

2. Materials & experimental conditions

3. Assessment of average critical defect size

4. Towards a fatigue design procedure for cylinder heads accounting for the size effect

5. Conclusion

Acknowledgement

References

The fatigue strength of cast aluminium alloys is known to be greatly affected by different defect types related to the manufacturing process, particularly microshrinkage pores created during the solidification phase of the casting process. Even if certain classification procedures are given in the standard ASTM E155-15 [1], the presence of defects is not readily related to the capacity of a component or a structure to meet the requirements of the mechanical technical specifications. The present study aims at establishing a clear link between certain microstructural features and the average fatigue strength. This is possible by looking for the average size of critical defects and using a relevant statistical analysis. More exactly, the Murakami approach based on the statistics of extremes is employed. The main originality of this work lies in the application of this approach to the case of a real structure submitted to high cycle fatigue damage: engine cylinder heads, used in the automotive industry. Indeed, both fatigue tests and microstructural characterizations are carried out on cylindrical specimens and real structures. The specimens are subjected to uniaxial and multiaxial loading conditions [2]. Original fatigue tests, developed by PSA to load in-service critical regions, are carried out on cylinder heads. Systematic analyses of fatigue failure surface are conducted to obtain the statistics of critical defects at the origin of the failures for both specimens and structures. In parallel, critical regions and the associated local loading mode in the structure are characterized by an appropriate high cycle fatigue analysis. The latter, combined with the fatigue test data and the statistical analysis of the critical defects, leads to a discussion about the size effect and an approach is proposed for a relevant fatigue design procedure.

Introduction

The cast aluminium alloys are widely used in automotive industry due to their relative good strength combined with a low density and an excellent conductivity. These alloys are commonly employed for chassis and engine components which are, for the most part, subjected to mechanical cyclic loads and present a serious risk of mechanical fatigue. In order to avoid fatigue crack initiation and ensure the component resistance, appropriate design methodologies have to be developed. They are mostly based on an appropriate fatigue criterion, whose identification is carried out by fatigue tests performed on test specimens, who’s size is generally quite different to the size of the fatigue critical zones found on the real manufactured components. The fatigue strength of cast aluminium alloys is known to be greatly affected by different types of microstructural heterogeneities due to the manufacturing process. According to this process and subsequent treatment, the microstructural features can be significantly modified especially in terms of eutectic components (silicon particles and iron-based intermetallics) and casting defects, notably micro-shrinkage pores and gas porosity. Despite the significant effort to characterize and understand the influence of these microstructural heterogeneities on the High Cycle Fatigue (HCF) behaviour of these alloys [2][7], the development of design methodologies taking into account the effect of defects on the fatigue life prediction of structures is still under progress [8][10].

1. INTRODUCTION

2. MATERIAL AND EXPERIMENTAL METHOD

3. Results and discussions

4. CONCLUSION

Acknowledgements

References

The aim of this study is to identify the impact of High Speed Machining defects on the fatigue behaviour of the Al7050 aluminium alloy. A vast experimental campaign under fully reversed plane bending loads containing different surface states has been undertaken to characterize the effect of the surface topography on the fatigue behaviour. The results show that the fatigue strength decreases only when the surface roughness is significantly degraded. It is also pointed out that manual grinding eliminates the effect of the machining defects on the fatigue behaviour. In order to predict the influence of the surface condition on the fatigue behaviour, a numerical approach based on the real surface topology has been developed. It is shown that the numerically identified crack initiation sites are in agreement with the experimental results. A probabilistic approach based on the weakest link concept, associated with the definition of a stress based crack initiation threshold has been integrated in a FE model. This approach leads naturally to a probabilistic Kitagawa type diagram, which in this case explains the relationship between the size defect and the scale effect on the fatigue strength.

INTRODUCTION

This study is part of a French research project that deals with the control of the machining and grinding of large structural aeronautical components. The material under investigation is the 7050-T7451 aluminium alloy. During HSM (High Speed Machining) of aircraft parts, macro-geometrical defects can be created. Mismatch may appear due to the gap left between two consecutive passes of the machining tool. Chatter can also be created due to the tool/part vibrations which may occur during different types of machining. In order to respect the strict specifications imposed by the aeronautical industry, expensive manual grinding is often performed to remove the machining defects. By the local thermomechanical effects induced, machining can change the surface integrity of manufactured parts and therefore change their fatigue strength. Shahzad [1] and Guillemot [2] showed that changing the cutting conditions can generate a change in surface topology and therefore a change in the fatigue life. Furthermore, Shahzad pointed out that the fatigue resistance shows a significant decrease when surface roughness increases. Even if HSM is known to introduce residual stresses in the machined parts, Tang [3] and Rao [4] showed that these residual stresses are mainly located in the first 50 micrometers below the surface. A variation of the microstructure in the upper surface (hardness variation, grain recrystallization) can also be detected after HSM. Campbell [5] showed that for the Al7000, these changes are located in the first 30 micrometers below the surface.

1. Introduction

2. Experimental approach

3. Results and discussions

4. Conclusions

5. Acknowledgement

6. References

Due to complex microstructure and defect inherited from this ALM process, it is necessary to assess the fatigue resistance of the material constitutive of manufactured parts prior to certification. This work is precisely tackling this issue with a special attention paid to the role of microstructural parameters and defects on fatigue life. Specimens were built with two configurations (0° and 90°) in order to evaluate the impact of the induced anisotropy of microstructure on fatigue properties. X-Ray 3D tomography was used to characterize defect population by their size. Microstructure is furthermore characterized by considering four characteristic scales [1-3]: melt-pools, crystallographic grains, dendritic structure and the precipitates. The fatigue properties are determined by establishing S-N curves for as-built and heat-treated samples for R= -1. The defect size responsible for the fatigue damage initiation is determined in each sample so as to establish a relation between the fatigue limit and the defect size by means of Kitagawa type diagrams. It is shown that the defect size is the first order parameter in terms of the fatigue resistance. Through the Kitagawa diagrams for as-built and heat treated samples, we quantify the improvement of the fatigue resistance due to the peak hardening treatment.

Introduction

Additive manufacture is considered to be a process that breaks with the existing ones, insofar as it offers, in particular, the possibility of producing parts with complex geometry and topographically optimized. In the aeronautical context, the realization of the parts involves them qualification which means the understanding of the fatigue behavior due to integrity of the material. So, the knowledge of the microstructure of the material so as the metallurgical defects inherited to the manufacturing process is therefore necessary in order to quantify the impact of each material parameter on the fatigue behavior of such parts. Thus it will be possible to tend towards the optimization of the process regarding the fatigue behavior. In this work, we are talking about a hypo eutectic aluminum alloy developed by the ALM process. Some studies have been carried out on the microstructural characterization and defects, as well as the anisotropy effects due to the direction of growth of the specimens in relation to the statics mechanical properties such as UTS, Ys and Ef [3, 4]. Concerning the fatigue, Brandl et al [5] showed that anisotropy due direction of production is observable for the platform temperature of 30 ° C. According to the previous author by coupling a platform temperature of 300°C and a T6 heat treatment, no difference is visible anymore due to the direction of sample growth. Maskery et al [6] evaluated the impact of T6 treatment on fatigue performance for as built manufacturing surfaces; they work showed a significant increase in the fatigue limit due to T6 treatment. According to Aboulkhair et al [7], the specimens machined from the T6 heat treated bars have a greater fatigue resistance. Few studies quantify the impact of defects on the fatigue limit.

Abstract

In the present work, SiCp/2024 composite powders were produced by low energy ball milling a mixture of centrifugally atomized 2024 particles and β-SiC reinforcements having a size range between 2 and 6 μm. The experimental parameters studied were the milling time (between 1 and 24 h) and the volume proportion of β-SiC in the starting powder mixture (between 5 and 35%). These SiCp/2024 composite powders were consolidated by hot extrusion using optimized processing parameters. The microstructure of the extruded composites showed a uniform distribution of β-SiC particles in the 2024 aluminum matrix. Moreover, the mechanical properties (yield strength and Young’s modulus) improved with the SiC content although some reduction in ductility was observed.

Introduction

For more than two decades, aluminum-matrix composites have been made by different methods including casting [1–9], spray atomization and codeposition [9,10], pressure-infiltration of porous ceramic preforms [11], and powder metallurgy techniques like densification of mixtures of metal and ceramic powders [12–17], or of composite powders made by mechanical processing [13,18–34]. The mechanical properties of these composites are influenced significantly by the quantity and mode of distribution of the reinforcements and also by the nature of the interfaces between the reinforcements and the matrix.

مقاله سال 2016

در این مطالعه، کامپوزیت های ماتریس فلزی آلومینیوم تقویت شده با نانوذرات ZrO2 در کسر حجمی 0.5، 0.75 و 1 درصد از طریق فرایند (ARB) اتصال نورد تجمعی تولید می شود. نتایج حاصله از ریزساختار نشان دهنده توزیع ذره ZrO2 عالی در ماتریس آلومینیوم پس از 10 سیکل از فرایند ARB است. همچنین نتایج انکسار اشعه ایکس نشان می دهد که کامپوزیت نانوذرات Al/ZrO2 با متوسط اندازه کریستالیته  از 48.6 نانومتر بطور موفقیت آمیزی پس از 10 سیکل از فرایند ARB به دست می آید. تست های کشش در نوارهای ARBed انجام می شوند. استحکام کششی 2.15 بار بیشتر از مقدار اولیه افزایش می یابد. کشیدگی بطور ناگهانی در سیکل نخست رها می شود، و سپس کمی افزایش می یابد. مشاهدات تصاویر SEM از سطح شکستگی نشان می دهد که پس از 10 سیکل از فرایند ARB شکست تقریبا حالت شکست را با منافذ ریز و کشیده شده برش می دهد.