1. Introduction

2. Important factors to consider in fatigue design with AM

3. Multiaxial fatigue characterization

4. Summary and perspective for future research

Acknowledgements

References

Additive manufacturing (AM) is a state of the art technology enabling fabrication of complex geometries, in addition to providing other advantages as compared to the traditional subtractive manufacturing methods. However, a wide variety of factors significantly influence fatigue behavior and structural performance of components made of AM metals. In addition to the fabrication process parameters, these include the effects of build direction, surface roughness, residual stresses, and heat treatment, and multiaxial stress states. At the microstructural level, defects such as pores and lack of fusion particles, as well as other microstructural features affect the behavior. In this paper, first a brief review of the aforementioned factors affecting the fatigue behavior will be presented. Then some experimental multiaxial fatigue data for selective laser melting (SLM), which is a powder bed fusion (PBF) metal AM process, of a common Ti alloy (Ti-6Al-4V) with applications in many industries are presented and discussed. The effects of surface finish, heat treatment, and stress state will be evaluated, as well as failure mechanisms in different life regimes and the role of defects. Finally, some additional factors that must be considered before wide acceptance of the AM technology in critical load bearing applications will be addressed.

Introduction

Additive Manufacturing (AM) is a state of the art process which enables fabrication of complex geometries and provides several advantages over the traditional subtractive manufacturing methods. Among the advantages of AM process is the possibility of fabricating complicated geometries which are difficult or impossible to build using traditional manufacturing. In contrast to subtractive manufacturing, in which a part is made by removing the material from a block, in AM the part is built with a close to final shape geometry with little or no need for material removal, therefore, there is much less material waste. On site fabrication is amongst the other advantages of AM technique. AM metallic parts are commonly fabricated via Powder Bed Fusion (PBF) and Directed Energy Deposition (DED) processes. In PBF, a metal powder is melted selectively via laser or electron beam over the previous layer, while in DED, both energy source which could be laser or electron beam, and the material which is powder or wire infuse simultaneously. For more information about the fabrication process of AM parts the reader is referred to review articles in the literature, for example to [1, 2]. Several process parameters such as laser power, scanning speed and strategy, and layer thickness are amongst the important parameters which could directly influence the material microstructure and, therefore, behavior.

1. Introduction

2. Finite element numerical modeling

3. Results and Discussions

Conclusions

References

1- Introduction

2- Online process analysis and quality estimation of the system

3- Welding process control

4- Conclusion

References

Abstract

Resistance spot welding (RSW) is frequently employed in current industrial occasions. However, the process is multi-field coupled and highly nonlinear, and full of uncertainties and disturbances. This paper presents recent primary advances and progress in process analysis and quality control of the RSW operations. Online welding process analysis, and relative online quality estimation for the welding products, are very important because they can help to save energy and improve the efficiency during actual production. It should deeply interpret the process characteristics, and then reasonable relations between selected monitoring process variables, such as dynamic resistance or electrode displacement, and quality criteria, such as nugget size or tensile-shear strength, can be established. Apart from online process analysis using mathematical tools, using different kinds of auxiliary measuring signals from external sensors and intrinsic process variables to monitor the process and obtain the quality information of the weld is presented and discussed. Then various process control works are reviewed. Besides the various parameters optimization methods, kinds of controllers, including feedback controllers, intelligent controllers and comprehensive controllers which combined the online quality estimation and control strategy application together, for obtaining welds with satisfactory quality, are respectively discussed. It can be seen that the establishment of general models to online process analysis, quality estimation and real time control system design for obtaining welds with satisfactory quality still remains a big challenge in reality. This work can provide references and enlightens for current academic researches or actual production in RSW relative area.

Introduction

Resistance spot welding (RSW) is extensively employed in joining sheet metal components, such as in the manufacturing of automobiles, trucks trailers, buses, recreational vehicles, office furniture and appliances, railway vehicles, airplane structures, aeronautical and space applications and many other products [1–5]. Especially in the automobile industry, over 90% of assembly work in a car body is completed by RSW [6]. The automotive structural assemblies use groups of spot welds to transfer load through the structure during a crash. Typically, a modern vehicle includes 2000–5000 spot welds [7]. These are the main reasons that the RSW is employed extensively currently. The main advantage of the RSW is that the process can be automated and robotized in high volume for high production rate operations. However, RSW process involves interactions between electromagnetic, thermal, mechanical, fluid flow and metallurgical phenomena across faying interfaces and so that is very complicated [8]. The process is difficult to properly control because the final weld quality is determined by interaction between various operational parameters and mechanical/electrical characteristics of the machine and equipment involved. In general, a modern automotive production line of high volume models produces approximately 7 million welds per day [9]. Hence, to ensure the integrity of the welded structure and improve the efficiency of the welding production, the recent advances of some important aspects during the RSW process, including online process analysis and monitoring, online non-destructive weld quality estimation and control system design to guarantee the high quality, are considered in this paper. During the traditional RSW working process, two or more parent metal sheets are pressed together by electrode force, which is usually controlled by air pressure in the pneumatic cylinder, or servo actuator. After the parent metal sheets are fixed, external electrical energy is delivered into the welding system and the welding current goes through the parent metal sheets, then heat energy is initially generated at the interface of the metal energy due to contaminations and surface asperities, so that the interface has the largest resistances at the beginning of the process [10,11].

1. Introduction

2. Weld quality

3. Measurement systems for quality assurance

4. Improved fatigue strength using alternative methods

5. Fatigue design for improved weld quality

6. Fatigue and quality aspects of cut edges

Conclusions

References

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. Experimental procedure

3. Fatigue tests results

4. Discussion

5. FEA considering the effect of defects

6. Conclusion

References

Since a decrease of the fatigue strength may result from punching operations, this study proposes a methodology for designing punched parts against high cycle fatigue crack initiation. To reach this goal, high cycle fatigue tests are performed on different specimens configurations with either punched or polished edges. Due to punching effects, the fatigue strength of punched specimens is significantly decreased. Fracture surfaces observations reveal that crack initiation occurs always on a punch defect. Additional investigations are combined to characterize how the edges are altered by the punching operations. High tensile residual stress levels along the loading direction are quantified using X-Ray diffraction techniques. Furthermore, micro-hardness measurements and X-Ray diffraction results reveals a strong hardness gradient due to punching operation. For a better understanding of crack initiation mechanisms, the edge geometries have been scanned with 3D optical microscopy, allowing us to identify the most critical defect (and its real geometry) by comparing the edges before and after fatigue failure. Finally, FEA are performed on identified defects. A non-local high cycle multiaxial fatigue strength criterion has been used as post-processing of FEA to take into account the effect on the HCF strength of defects and the strong stress-strain gradients around them.

Introduction

New electric motors generations require high-performance electrical steel sheets. These materials, which present better magnetic properties, are suitable for several applications especially high speed electric motors. The improved magnetic properties of Fe-Si alloys are provided by reducing thickness, adjusting silicon content and increasing the grain size. Electric motors are made from stacks of thin sheets. Punching process is usually used to produce these components because it offer a good production rate and low cost. However, this process generates different effects influencing the durability of components. Several studies have highlighted the influence of these effects which are hardening, tensile residual stresses and defects [1‒4]. In order to study the effect of punching process on the high cycle fatigue strength of electric motor components and to take into account these effects when designing pieces, an experimental and numerical procedure are curried out. Fatigue tests have been performed on different specimen’s configurations. Punched, polished and punched-annealed specimens are used to quantify the contribution of each effect induced by the process on the high cycle fatigue strength of the studied alloy. SEM observations reveal that fatigue crack initiation occurs on the specimen’s edge. In order to study the fatigue crack initiation mechanisms, SEM observations and 3D surface topography of the specimens have been performed. Results show damaged edges with many types of defects. A specific procedure is used to identify the critical defects [5-6].

1. Introduction

2. Trailer FEA model

3. Fatigue enhanced structural stress analysis

4. Vehicles inspection and comparison

5. Conclusion

Acknowledgements

References

A FE seam weld model has been built for advanced fatigue structural stress analysis. The shell element model allows both weld toe and weld root fatigue assessment. In order to validate the model, this project deals with the complete analysis of a last decade manufactured vehicle for which experience feedback is possible. The vehicle structure is mainly built with steel sheets and the assembly is performed by seam welding. Shell element model is appropriate for FEA. The entire FEM preparation is presented hereafter and each step is detailed: 3D CAD weld building, mid-surface idealization, weld leg imprint creation, weld mesh connection and load application. Fatigue analysis is performed based on a fatigue limit approach and IIW FAT data. For every structure location a Haigh diagram is built. Different diagrams are presented for plain metal, cut edge, weld toe and weld root. The FEA results are post processed and each appropriate tensor component is taken into account. For plain metal, stress variation along the worst principal stress orientation is calculated. In welds, for both toe and root locations, the structural stress computation based on grid point forces extraction is described. Longitudinal shear stress in weld and at weld toe as well as weld longitudinal normal stress calculations are detailed. Fatigue results are displayed for each location over the whole chassis model. Finally, vehicles built during the last decade are inspected back from the customers experience field. Observed crack types and locations are compared to fatigue analysis predictions.

Introduction

LOHR Industrie products car carrier vehicles, railway semi-trailer transport systems and urban electric people transportation solutions. All products are steel made and assembled by welding. For vehicle structure analysis, LOHR Industrie has developed FEA models for assemblies relative to seam weld and bolt [1]. Seam weld fatigue assessment model has been described in numerous publications [2, 3, 4] and validation based on simple assembly specimens has been detailed. A project has been launched in order to verify the proposed FEA model and the associated fatigue behaviour prediction by comparison with long mileage vehicle structure inspections. An old generation car carrier trailer model is built. The fatigue analysis based on advanced structural stress approach is performed.

1. Introduction

2. Surface integrity customization

3. Rotary bending fatigue tests

4. Discussion

5. Conclusions

6. References

This works aims at identifying the influence of surface integrity parameters on fatigue life of a 15-5PH. The effect of residual stress profile, surface roughness and microstructure are investigated. Various cutting and superfinishing processes (turning, ball burnishing and belt finishing) are used so as to reach various engineered surface integrities and as a consequence to highlight the influence of each surface integrity parameter. Rotary-bending tests have been carried out on samples to determine the average fatigue strength at 2 million cycles. It is shown that the influence of a deep compressive sublayer is by far more important than the ones of surface roughness or microstructure. The so-called ‘white-layer’ brings also a slight improvement of fatigue resistance.

Introduction

The 15-5PH is a martensitic stainless steel widely used for critical parts in the aeronautic or energy fields. The table 1 below presents its composition. In these sectors the machining sequences are critical and cannot be changed easily by authorities’ certifications. The reason for this is that the machining operations modify the surface integrity parameters responsible of the fatigue strength of the steel and the parts and slight changes in the process can have major influences on the performances and lifetime. [1], [2]. [3], [4] In previous works Chomienne [5] has machined customized parts trying to identify the influence of roughness and surface residual stress on fatigue behavior. The picture below (figure 1) presents the 4 sets of surface characteristics that were tested. In the article, the conclusions were that surface compressive residual stresses associated with a very low surface roughness increases the fatigue strength. Nevertheless, a higher roughness seems to have a non-negligible effect on the fatigue strength even if there are large compressive residual stresses. In fact it seems that the improvement of fatigue strength due to compressive residual stresses decrease with an increase of surface roughness.

1. Introduction

2. Taira’s Damage Model

3. Modification of Taira’s Damage Model

4. Modified Taira’s Damage Model for TMF

5. Conclusion

Acknowledgements

References

This study investigated a fatigue life prediction method based on extensive experiment results of cast aluminum alloy Al319-T7 subjected to repeated thermal and mechanical loading. Cyclic tests and fully reversed fatigue test results of the material were obtained from the specimens subjected to three different strain rates (5×10^-5, 5×10^-4 and 5×10^-3) and various temperature conditions. At each strain rate the specimens were subjected to room temperature (25°C), 150°C, 200°C, 250°C and 300°C. Thermo-mechanical fatigue (TMF) tests were also conducted for in-phase and out-of-phase conditions of the temperature and mechanical loading. During the thermo-mechanical fatigue tests, the effect of loading phases and dwell time on fatigue life of the specimens was also observed. This study modified Taira’s fatigue damage model for thermo-mechanical loading condition to include the strain rate effect on the fatigue damage. Taira assumed that fatigue damage per reversal is proportional to the damage factor, λ(T), and plastic strain range powered by n, (Δε_p)ⁿ. The relationship between the plastic strain range (Δε_p) and the number of cycles to failure (N_f) is presented as λ(T)•(Δε_p)ⁿ•N_f = C. Where C is a temperature independent material constant. The temperature effect is included in the damage factor, λ(T) that can be determined from the ratio of λ(T)/λ(T_o) for low cycle fatigue test results at various isothermal conditions. T_o is the reference temperature and can be determined by experiment. This study used stress range applied instead of plastic strain range in the original equation. Furthermore, the modified equation includes the effect of strain rate, phase, and dwell time. The new fatigue damage equation was well correlated with the experiment results.

Introduction

In automotive industry, the durability of engine components at high temperature is one of the major considerations in the design phase since they are usually running under the service environment of cyclic loading and high temperature. While most engine components were made of cast iron in the past, many cast iron components have been replaced by aluminum alloys nowadays to reduce the vehicle weight. Along with the use of light weight components in the vehicle, thermal loadings as up as to 300˚C can be applied to engine components [1]. With the strong demand to understand the thermos-mechanical behavior of aluminum alloy engine parts and predict their service life, many efforts have been made in the past decades. Thomas et al. [2] proposed a complete design approach and lifetime prediction method for aluminum alloy cylinder heads under thermos-mechanical fatigue (TMF) loading. Engler-Pinto et al. [3] investigated the thermosmechanical fatigue behavior of cast 319 aluminum alloys used in cylinder blocks. Kang et al. [4] introduced a fatigue life prediction method for thermo-mechanical fatigue damage under variable temperature and loading amplitudes using a rainflow cycle counting technique. Wei et al. [5, 6] reviewed the up-to-date methods for damage modelling and life assessment of components under thermal fatigue loading and developed a general life assessment procedure under variable amplitude thermal-mechanical loadings, emphasizing on hold-time effect. Santacreu et al. [7-9] proposed a TMF damage model of stainless steels based on the maximal temperature and plastic strain amplitude reached during a thermal cycle.

1. Introduction

2. Material for testing

3. Test specimens production

4. Fatigue test performance

5. Test results

6. Conclusions

Acknowledgements

References

In this paper, the results of the fatigue tests of the P92 and 15CH2NMFA steels are presented and discussed. The fatigue S – N curves were obtained by means of both standard and miniature test specimens. The Small Fatigue Test (SFT) specimens were manufactured partially by machining and also by waterjet cutting. The fatigue tests were performed on Amsler 10 HFP 5100 ZWICK//Roell high-frequency pulsator at room temperature and R = 0,1 and R = -1 cycle asymmetries. The results obtained on standard and SFT specimens are compared and in case of P92 steel are also compared the two methods of SFT specimens manufacturing. In case of 15CH2NMFA steel, the results of standard and SFT specimens are in a very good agreement both within the time limit fatigue range and the fatigue limit. After taking the stress concentration of SFT specimens into consideration, the results of traditional fatigue test samples and SFT samples are similar. In case of P92 steel, the leaning branch of S-N curves is much steeper for SFT specimens and the fatigue limits lower than it could be expected after the stress concentration is taken into account. The fatigue limits for equal cycle asymmetry show the same ratio of standard to SFT fatigue limits, for R = 0,1 it is 1,31, for R = 1 it is 1,32. This very good agreement can be used for the evaluation of fatigue behaviour of P92 steel by means of SFT specimens. The comparison of the two ways of SFT samples manufacturing revealed a scatter of fatigue test results, nevertheless the fatigue limits differ by 5 MPa only, so that even the influence of the manufacturing seems to be negligible.

Introduction

At present, a great interest is given to the Small Punch Test method. Its greatest advantage is the almost non-destructive intervention into the integrity of mechanical components thanks to the small amount of the removed material. This “new” (also called) semi-destructive method makes it possible to evaluate the current status of operating components on small samples which do not disrupt the integrity of the operating components and makes it also possible to evaluate the current status without long outages. To perform the fatigue tests, the fatigue specimens “SFT” (Small Fatigue Test) according to [1], [2], see Fig.1, were used.

1. Introduction

2. Specimens

3. Experimental equipment and loading

4. Theory and methods

5. Results and discussion

6. Conclusions

Acknowledgements

References