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

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. Nomenclature

3. SeamWeld fatigue approach

4. Fatigue design curve generation – Test/FEM

5. Correlation on subframes / suspensions

6. Welding distortion effect on fatigue

7. Conclusion

References

In suspension design, traditionally subframes and links are welded structures, with one of the main design criteria being fatigue. Better understanding, correlation and correct design rules are vital to ensure safety and avoid over design that can lead to a reduction in competitiveness. A known mesh modelling T-joint SN method is used to design the welds and is mainly driven from standards used for thicker panels seen in the construction and ship building industry. To consider the thin sheet designs largely seen in automotive, Volvo & nCode have developed a new method based on structural stress and is known to be mesh insensitive. However, new difficulties emerged due to the clear separation between the membrane and bending behaviour needed for stress recovery. Specific tests need to be defined to ensure correct characterisation. Gestamp has generated a new standard of design rules to ensure correlation in Weld-Ends and in Mid-Weld cracks and correct toe position definition to ensure accurate stress recovery, and give more beneficial input to process by better defining the weld torch position. New test specimens have been developed and intensive testing carried out to fully characterise the bending and tension behaviour and reduce the gap in predictions between Lap Weld and Fillet Weld. This methodology has been successfully used in suspension designs with high confidence and correlation. These design rules have been used in conjunction with studying the effect of weld process and sequence on fatigue performance. Controlled distortion subframes, against high distortion subframes, have been tested to capture the benefit by reducing the fit-up gaps. Residual stresses are still a grey zone for Gestamp and further work is needed to better understand the effect.

Introduction

With the increase demand for reduction of weight and cost in the automotive industry, defining and correlating the design rules is becoming key to ensure accurate predictions in terms of fatigue performance to eliminate overengineering. There is a clear correlation between robustness and accuracy of CAE (FEM) predictions and reduction in cost and weight. Aluminium is seeing more widespread use in the automotive sector to reduce the weight and this drives a need to define new design rules and weld process control to ensure quality and safety. Critical to defining these design rules is the analysis of the limits of the current methods and develop a range of testing to correlate all the predictions. Controlling the process and especially weld distortion is more important when dealing with aluminium due to high thermal conductivity and coefficient of thermal expansion which leads to larger heat affected zones and higher distortion compared to steel. In addition, penetration and fit-up gap have more effect compared to those seen in welding steel.

1. Introduction

2. Specimen description and experimental set-up

3. Experimental tests

4. Fatigue life assessment

5. Discussion and conclusions

References

In the present work, the Fatemi-Socie approach is adopted in order to analyze the fatigue endurance of welded joints under multiaxial loads. This critical plane criterion has already been successfully applied to plain or notched components, however, it is not spread in the assessment of welded joints, yet. This work is focused on the practical implementation issues related to this particular application, which has not been discussed in the literature. The described procedure is adopted for the assessment of one hundred experimental tests and some preliminary results are shown. The specimen under investigation is a pipe-to-plate fillet joint made out of structural steel (S355JR). The tests were performed under both uniaxial and multiaxial, i.e. combined in-phase and out-of-phase bending and torsion, load conditions with a constant amplitude at the laboratories of the University of Pisa, Italy.

Introduction

The fatigue life assessment of notched components under multiaxial loads is a problem of wide practical interest since most of the components in real applications show geometrical details that produce local stress concentrations. Also, many actual applications are characterized by the presence of multiple independent loads, which produce a multiaxial non-proportional state of stress/strain. Even when a single load is applied, we can generally observe a multiaxial state of stress/strain in the volume of material surrounding a notch, even though in this case it is usually proportional. For the sake of simplicity experimental tests are often performed applying a constant amplitude uniaxial load and often times simple plain (un-notched) specimens are used in those investigations. Based on this fact, there is need to develop a procedure able to relate, in terms of fatigue damage, the actual multiaxial state of stress with the endurable (often uniaxial) stress state that is known from experiments, in order to estimate the fatigue life of real components.

1. Introduction

2. Description of railway steel bridge

3. Fatigue load models

4. Numerical modelling of bridge

5. Fatigue damage assessment

6. Results

7. Conclusions

Acknowledgements

References

As the number of heavy railway traffic load increased, concern over the accurate and actual fatigue damage of the bridge is intensified. Especially for bridges which were designed for light traffic load. The fatigue damage assessment of steel bridge connections is usually based on notion of uniaxial S-N curves given in the codes of practice. Until now, there is no consensus on a method which can precisely consider non-proportional multiaxial loading. The objective of this paper is to examine the applicability and appropriateness of the critical plane approach-based C-S criterion to perform the fatigue damage assessment in welded connections in railway steel bridge. A regular U trough railway steel bridge is analyzed using finite element software ANSYS 17.2 for standard railway traffic. The averaged principal stress directions determined through appropriate weight functions are used to orient the critical plane. Prediction of fatigue damage is performed through an equivalent stress represented by a quadratic combination of the normal and the shear stress components acting on the critical plane. Applicability of the C-S criterion is studied by assessing the fatigue damage of critical welded connections and comparing with the λ – coefficient and cumulative damage method calculated according to Eurocode EN1993-1-9.

Introduction

Due to enhanced demand in Reliability, Availability, Maintainability and Serviceability (RAMS) of structural systems, there is an increase in demand to understand and estimate the precise fatigue damage in bridges. An advanced fatigue design methodology is required to recognize potential critical details introduced by specific design changes. So far, there is no precise and robust fatigue damage estimation tool particularly for welded details. Especially in bridges, stresses developed at welded connections are always multiaxial and non-proportional. Fatigue assessment based on the critical plane approach is generally accepted to be more accurate for multiaxial nonproportional loading. There are several multiaxial fatigue damage models in the literature which propose different damage parameters. In general, critical plane models require scanning over all planes intersecting the surface either orthogonally or at some inclination for maximum value of damage parameter. Also, the stress analysis has to be performed for each time step considered in the simulation of the train crossing. This complex and cumbrous task is simplified in the C-S criterion by applying some weight functions and linking the critical plane with mean principal stress directions.

1. Introduction

2. Material and methods

3. Results and discussion

4. Conclusions

Acknowledgements

References

Providing high hardness, low friction coefficient, as well as, relatively good corrosion resistance, chromium-plated coatings are widely used for steel cylinder rods in marine environment. Nevertheless, a uniform network of microcracks in chromium coating is evolving under mechanical loadings during the service-life of cylinder rods. The propagation of these microcracks is in the origin of the premature corrosion of the steel substrate. The aim of the study was to evaluate the relationship between mechanical stresses, the evolution of the microcracks network and the corrosion resistance of chromium coatings. After monotonic pre-loading tests, it was demonstrated by microscopic observations that the microcracks propagated for stress levels higher than the yield stress of the substrate (520 MPa) and have passed instantly through the whole thickness of the coating and reached the steel substrate. The density of microcracks increases with the level of total strain, the inter-crack distance go from 80 µm at 1% of total strain to approximately 65 µm at 5%. Electrochemical measurements have shown that the higher the level of plastic strain applied during the mechanical loading, the more the corrosion potential of the sample decreases until reaching that of the steel substrate of approximately -0.65 V/ECS after 2 hours of immersion. The polarization curves also evidenced an increase in the corrosion current density with the strain level. Moreover, we note the absence of the characteristic passive region of the reference samples that have not undergone any loading. After cyclic loadings, no microcracks propagation was observed after 10⁴ cycles when maximal stress was lower than the yield stress. However, a decreasing of the corrosion potential was observed for samples which were submitted to a cyclic loading. Nevertheless, the current density and the characteristic passive region were not modified.

Introduction

Since 1924 [1], chromium-electroplating process is a well-established practice in industrial needs, such as aerospace, automotive and general engineering [2]. This fact is due to a combination of properties offered to steel by a chromium coating, such as, high hardness, low coefficient of friction and corrosion resistance [3]. These properties depend highly of electroplating parameters: temperature of plating solution, plating current density, concentrations of chemical compounds in plating solution and duration of process [3-4]. Nevertheless, after many decades of practice, some aspects of this process are still not fully understood. One of them is an appearance and evolution of microcracks network during electroplating process [3-5]. The origin of microcracks initiation is related to the residual tensile stresses, when the thickness of chromium coating reaches 0.5 µm [6]. The first explanation of residual tensile stresses is due to the release of trapped atoms of hydrogen during electrolysis, with the following shrinkage of chromium coating [4]. In [3], authors highlight that these trapped atoms of hydrogen could play a role of catalyst to accelerate the chromium phase transformation. Therefore, β-Cr with HCP or FCC crystal arrangement will be transformed in more stable α-Cr (BCC) with shrinkage of 15 % vol. [5]. It has been proved that the presence of these microcracks is favorable for penetration of corrosion agents, such as chlorides [7- 9].

1. Introduction

2. Fatigue of Welded Joints in API 579-1/ASME FFS-1

3. Ratcheting

4. Fatigue Assessment of Welds Using the Equivalent Structural Strain Method

5. Summary

Acknowledgements

Appendix A. Derivation of the Structural Strain Method

References

The 3rd Edition of API 579-1/ASME FFS-1 2016 Fitness-For-Service includes a new Part 14 dedicated to fatigue assessment. An important section in this part covers the fatigue assessment of welded joints. In this paper, an overview of the fatigue methods for welded joints is provided and extensions are recommended. First, an overview is given of the classical fatigue method used in the ASME B&PV Code based on smooth bar fatigue curves in conjunction with a fatigue strength reduction factor. In addition, the mesh insensitive structural stress method is outlined using an equivalent stress parameter based on fracture mechanics considerations in conjunction with a master S-N curve based on the analysis of over 2000 high and low cycle S-N test data. The resulting master S-N curve approach is applicable to high cycle fatigue and low cycle fatigue if a Neuber correction is introduced. In this paper, a new structural strain method is presented to extend the early structural stress based master S-N curve method to the low cycle fatigue regime in which plastic deformations can be significant while an elastic core is present. With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure can be eliminated, such as its use of Neuber’s rule in approximating structural strain beyond yield. The earlier mesh-insensitive structural stress based master S-N curve method can now be viewed as an application of the structural strain method in the high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke’s law. Thus, both low cycle and high cycle fatigue behavior can now be treated in a unified manner. In the low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy a linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. A PVRC Joint Industry Project is currently sponsoring work on the structural strain method that will lead to its incorporation in the next edition of API 579-1/ASME FFS-1.

Introduction

In a previous paper, Osage [1] provides an overview of the third edition of API 579-1/ASME FFS-1 Fitness-ForService that was published in 2016. The 2016 Edition includes a new Part 14 covering fatigue assessment procedures for in-service components. Fitness-For-Service (FFS) assessments are quantitative engineering evaluations that are performed to demonstrate the structural integrity of an in-service component that may contain a flaw or damage, or that may be operating under a specific condition that might cause a failure. The API 579- 1/ASME FFS-1 Standard was specifically written to cover in-service pressurized equipment typically found in the refining and petrochemical industries as well as the fossil utility industry. Part 14 provides methods used to estimate the time to crack initiation using a strain-life approach and is written as a multi-level approach covering screening, current design code methods, and advanced methods that take into account the latest in technology. The advanced methods include fatigue assessment of welded joints using the equivalent structural strain and Master S-N Curve Method and a new smooth-bar fatigue assessment method that incorporates a multi-axial fatigue criterion with a critical plane approach. Cycle counting methods for both welded joint and smooth-bar fatigue methods are also provided. Methods to evaluate fatigue in the subcritical crack-growth regime in API 579-1/ASME FFS-1 using a fracture mechanics approach are also covered.