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simulation of a car, a team viewing an airplane model and an engineer on the computer using software for HBK Technology Days 2021

2021 HBK Technology Days | Archive

This 6-part series of 90-minute virtual seminars focus on joints & welds, wire-DED Additive Manufacturing, surface treatment, fatigue testing & characterisation for aircraft structures, automotive and ground vehicle structures and electric vehicle battery structures.

Day 1 - Fatigue of Joints & Welds

Fatigue simulation of joints in an airplane from a constructional closeup perspective

Session 1: Fatigue Simulation of Joints

A wide range of joints is used to assemble structures including welds, rivets, bolts, and adhesives, between similar and dis-similar materials - metals, polymers and composites. These joints distribute load into or through the structure, and when these loads vary during operational use they introduce fatigue cycling at and close to the joint. The local geometry detail at the joint acts as a stress raiser reducing the fatigue strength at the joint. Any evaluation of the durability of the structure must therefore place a high priority on a fatigue assessment of the joints.

This seminar presents an overview of fatigue simulation methods to predict the fatigue performance of joints and focuses on the stress severity factor method commonly used to give an indication of which locations are fatigue critical in an aircraft structural joint.

Dr. Andrew Halfpenny, Chief Technologist, HBK Prenscia

A wide range of joints is used to assemble structures including welds, rivets, bolts, and adhesives, between similar and dis-similar materials - metals, polymers and composites. These joints distribute load into or through the structure, and when these loads vary during operational use they introduce fatigue cycling at and close to the joint. The local geometry detail at the joint acts as a stress raiser reducing the fatigue strength at the joint. Any evaluation of the durability of the structure must therefore place a high priority on a fatigue assessment of the joints.

This presentation introduces fatigue of joints from their fatigue testing and supporting finite element analyses, through to their application in fatigue simulation analysis. Examples include welded joints (thin, thick, spot, seam, etc), brazed joints, bolted joints, adhesive joints, and self-piercing-rivet joints.

Mr. Steve Dosman FRAeS, AV8R Consulting

Engineers have been using Jarfal's Stress Severity Factor (SSF) concept to characterise the fatigue strength of aircraft joints since the SSF's inception in the late 1960s; however, it has often been used badly. Empirical factors have been misused or ignored and combining the method with Finite Element Model (FEM) results is fraught with difficulties. The SSF concept is described and methods to derive appropriate empirical factors are discussed. Finally, some considerations and advice for using the SSF with FEM results are provided.

Mr. Paul Roberts, Product Manager, HBK Prenscia

The stress severity factor is a method used for the fatigue assessment of a row of fasteners in a joint. The method requires determining the load beared (or carried) by the fastener and the load bypassing the fastener; often known as the bearing and bypass loads. The bearing load is the load transferred from a sheet and borne by the fastener, while the bypass load is the load remaining in the sheet that bypasses the fastener. These loads, and their resulting local stresses at the fastener, change from fastener to fastener as the load is transferred from one sheet to another through the rows of fasteners in the joint.

This presentation describes the FE modelling and results in extraction required to determine these loads and their local stresses. The similarities and differences of this method are compared with those currently used for FE modelling and fatigue analysis of spot welds.

Session 2: Fatigue Simulation of Welds

Welding is a commonly used and effective method for making structural joints between metal parts. However, the nature of the welding process means that these welds generally have a fatigue strength that is inferior to that of the parts being joined together. The result is that, even in a well-designed structure, the welded joints are likely to fatigue. Any evaluation of the durability of a welded structure must therefore place a high priority on a fatigue assessment of the welded joints.

This presents applications and methods for the fatigue life prediction of welds from finite element results. These include mesh insensitive structural stress techniques for both shell and solid elements making model generation less time-consuming. This concludes by describing the material testing and subsequent fatigue characterization required to obtain the bending and membrane weld fatigue curves for seam weld and spot weld damage models.

Fatigue simulation of welds seen in a spot contact welding of bodies of cars at an automobile plant

Mr Artur Tarasek, Durability CAE, NIO

NIO's mission is to shape a joyful lifestyle by offering smart, premium electric vehicles and providing the best user experience through technological innovation, smart design and freedom of worry-free journeys. NIO was founded in November 2014 as a global electric vehicle company, with world-class research and development, design and manufacturing centres in China, the United States, Germany and the UK.

NIO, as a start-up company, has a unique opportunity to re-think what the user’s experience of owning an electric vehicle should feel like which is reflected in the product design and engineering. Nevertheless, being a relatively new company creates some challenges as NIO might not have the luxury of having legacy data and processes that more established OEMs possess which was the motivation for the CAE capability enhancement discussed here.

Modern, lightweight vehicle manufacturers require a wide range of joining technologies to provide a mass-efficient and cost-efficient product that withstands the variable loading that the structure is exposed to during the life of the vehicle. Given the ease and speed with which the resistance spot welding process can be automated, it is a primary joining process in high-volume steel vehicle manufacturing.

Due to the complex loading that the joint might be exposed to it is important to understand the failure modes that might occur during extensive vehicle testing and develop a suitable durability prediction technique to avoid costly, late design changes.

A significant amount of joining configurations in the modern vehicle’s car body requires an approach that will provide good quality data with a reduced scope of testing. Good practices and challenges associated with creating a resistance spot weld’s durability prediction methodology will be discussed.

Mr. Jeff Mentley, HBK Prenscia

Welding is a commonly used and effective method for making structural joints between metal parts. However, the nature of the welding process means that these welds generally have a fatigue strength that is inferior to that of the parts being joined together. The result is that, even in a well-designed structure, the welded joints are likely to fatigue. Any evaluation of the durability of a welded structure must therefore place a high priority on a fatigue assessment of the welded joints.

This presentation examines multiple approaches for the fatigue life prediction of welds from finite element results. Mesh insensitive structural stress techniques are discussed for spotweld, shell seam weld and solid seam weld modelling. These stresses can be obtained from quasi-static analyses, or from dynamic analyses in either the time or frequency domains enabling the correct loading environment to be accurately represented. Corrections applied to the stresses and material definitions for seam weld analysis will be examined, and their application in the fatigue life prediction process discussed.

Mr. Rob Plaskitt, HBK Prenscia 

Fatigue assessment of a welded joint using the structural stress approach requires stress-life (SN) fatigue curves derived from fatigue test specimens of representative weld geometry and manufacture. To obtain these SN curves requires fatigue test specimens that are designed and modelled by finite element analysis to cause the desired weld failure mode by a single axis fatigue test. Example failure modes for spot welds include cracks growing through the sheet or the spot weld nugget, and for a seam, welds cracks growing from the weld toe or weld root.

This presentation shows representative fatigue test specimens for thin sheet spot and seam welded joints and how different camera systems are used to obtain images and videos to validate the failure mode. Consequent fatigue tests are conducted at constant load cycles to plot applied load against the number of cycles to failure, with appropriate failure criteria, for characterisation into SN curves.

Day 2 - Wire-Arc AM | Surface Treatment

Large steel part from Sonia at Cranfield that shows wire-arc additive manufacturing

Session 3: Wire-Arc Additive Manufacturing

Cranfield University is leading a five-year research programme with multiple UK universities for New Wire Additive Manufacturing “NEWAM”. This is a directed energy deposition additive manufacturing technology, with a research focus on process, material and structural integrity. Linear “walls” of material are manufactured for cutting and machining into test specimens for non-destructive and destructive tests.

Coventry University is leading the “Material Performance and Structural Integrity” of NEWAM, determining structural integrity through fatigue initiation, fatigue fracture, and residual stress. Prenscia is contributing mechanical fatigue testing services and material characterization services to this NEWAM research programme.

Prof. Stewart Williams, Director of the Welding Engineering and Laser Processing Centre, Cranfield University

An introduction to the NEWAM research programme, focused on the process, material and structural integrity of wire-based directed energy deposition AM (w-DEDAM) processes. Four universities (Cranfield U., U. of Manchester, Strathclyde U., and Coventry U.) have joined forces to deliver an ambitious research programme over five years with EPSRC funding, and considerable industry support from AM equipment supply chain, service providers and end users.

  • Why NEWAM? – current w-DEDAM processes have limitations, relatively low deposition rate, materials and alloys not designed for AM, and no online monitoring system to guarantee the structural integrity of the built parts.
  • What does NEWAM propose? - using multiple energy sources to achieve independent control of deposited bead geometry (height and width) and thermal field or microstructure. This is targeting increasing the deposition rate, achieving net shape finish with precise control of microstructure and therefore properties. Synergistic design of processes and new materials and alloys for AM to achieve properties similar to or better than the raw material. Physics-based quality control using new in-process non-destructive testing techniques for quality control and in-process monitoring for quality assurance.
  • How will NEWAM deliver? – through a fully integrated multi-disciplinary research programme delivered by four leading Universities. Process design, monitoring and modelling at Cranfield, material expertise and modelling at Manchester, material performance and behaviour at Coventry and in-process NDT at Strathclyde.

Professor Xiang Zhang, Coventry University

Coventry University is leading the “Material Performance and Structural Integrity” of NEWAM, determining structural integrity through:

  • residual stress - modelling, contour & diffraction test methods, and residual stress intensity;
  • fatigue initiation - defects, defect tolerance, testing & characterization and modelling;
  • fatigue fracture - testing & characterisation, crack growth and fracture mechanics.

This presentation describes the process induced residual stress and fatigue crack growth behaviour for the first material, the high strength titanium alloy Ti-6Al-4V. The following presentation presents the fatigue initiation and microstructure research.

Material was manufactured with three different deposition strategies; single pass, parallel pass, and oscillation wave builds. The single pass build is limited to a maximum thickness of around 8 mm, whereas the other two methods can build thicker materials as well as parts with variable thickness. All samples were tested in the as-built condition with standard surface machining and polishing.

Dr. Abdul Khadar Syed, Coventry University & Mr. Rob Plaskitt, HBK Prenscia

This presentation describes the fatigue initiation and microstructure research for the high-strength titanium alloy Ti-6Al-4V material additively manufactured by w-DED for the NEWAM research programme. This includes the material behaviour and properties in fully reversed low cycle fatigue (LCF) and high cycle fatigue (HCF) regimes in as-deposited conditions. X-ray computed tomography was used to detect defects in the test samples. A detailed microstructure and fractography were performed to understand the role of microstructure on crack initiation and fracture. The following findings are presented with respect to the characteristics of the microstructure:

  • Wider columnar primary β grains consisting α+β in Widmanstätten and colony morphology;
  • Cyclic stress-strain curves obtained from fatigue hysteresis loops show typical cyclic softening. Horizontal build orientation samples, with the loading axis perpendicular to the columnar β grains, showed higher material performance. This finding aligns with static tensile test data where horizontal samples showed higher yield and tensile strengths;
  • In the LCF region (<104 cycles), vertical build orientation samples, loaded along the columnar β grains, are superior. This is considered to be linked to the microstructure along the grain boundary α;
  • In the HCF region (>104 cycles), vertical and horizontal build orientation sample performance is similar, though with considerable scatter in the data.

Session 4: Surface Treatment of Aircraft Structures

Aircraft landing gear manufacturing and overhaul/maintenance processes alter surface material properties that influence fatigue life. As aging aircraft continue to be pushed beyond their originally intended service life, it has become increasingly critical to characterize specific surface processing conditions. For this reason, Select Engineering Services, General Atomics and Prenscia are conducting research/testing to develop tools and methods that incorporate surface treatment effects in USAF landing gear fatigue models.

Laser shock peening is an emerging technology used for the enhancement of the fatigue performance of safety-critical components and structures. This is achieved through the introduction of a beneficial compressive residual stress field in the near-surface layer of the component which counteracts applied tensile stresses and so extends the fatigue life. In the aerospace industry laser, shock peening has been applied to the root of engine turbine blades and wing attachment lugs.

airplane structure with a ladder on the side to show a surface treatment of aircraft structures

Mr. Andrew Clark, Landing Gear Systems, USAF

This is a short presentation to introduce the following presentation. These are the USAF requirements and objectives for the continued safe life of landing gear for aging aircraft through overhaul and maintenance processes, for the development of fatigue tools and methods to incorporate surface treatment effects in USAF landing gear fatigue models.

Mr. Ben Griffiths, Chief Engineer, Select Engineering Services

Landing gear manufacturing and overhaul/maintenance processes alter surface material properties that influence fatigue life. The effect of these processes was not accurately accounted for in legacy United States Air Force (USAF) landing gear designs. As aging aircraft in the USAF fleet continue to be pushed beyond their originally intended service life, it has become increasingly more critical to characterize specific surface processing conditions. For this reason, Select Engineering Services (SES), General Atomics Systems Integration (GA-SI), and the HBK Advanced Materials Characterization & Test (AMCT) Facility are conducting research/testing to develop tools and methods that incorporate surface treatment effects in USAF landing gear fatigue models.

Surface treatment factors (k factors) are used in nCode DesignLife software to accurately debit strain-life analysis curves and account for material surface conditions. SES has collected 24 datasets to derive 19 k factors for 3 common landing gear materials. In order to facilitate efficient data management and k factor application in nCode DesignLife simulations, SES has developed the Material Assessment and Predictive Analysis (MAPA) tool. The purpose of this presentation is to provide an overview of material and surface treatment characterizations, demonstrate the capabilities of the MAPA tool, and establish the significance of completed research and development.

Dr. Niall Smyth, Coventry University

Laser shock peening (LSP) is a surface treatment used for the enhancement of the fatigue performance of safety critical components and structures. This is achieved through introduction of a compressive residual stress field in the material that counteracts applied tensile stresses thus potentially extending the fatigue life. This presentation will provide an overview of the recent applications of LSP studied at Coventry University such as use of peening strips as a replacement for bonded crack retarders providing a potential weight saving, and using laser peened residual stress fields as a tool to control the crack growth direction away from safety critical areas. Finally, some of the application challenges for laser peening that hinder wider industrial application will be discussed.

Day 3 - Material Characterisation | EV batteries

AMCT lab with people

Session 5: Advanced Materials Characterisation & Testing

All of the previous seminar sessions in this HBK Technology Days virtual seminar series have described or used results from fatigue tests performed by the Prenscia Advanced Materials Characterisation & Testing (AMCT) facility. This seminar presents these facilities and fatigue testing capabilities of the AMCT, and subsequent characterisation into strain-life, stress-life and/or load-life fatigue curves.

The AMCT specialises in strain-controlled fatigue testing in the low cycle fatigue (LCF) and high cycle fatigue (HCF) regions, typically between 500 and 5,000,000 cycles. Above this, very high cycle fatigue (VHCF) testing introduces additional challenges.

Dr. Michelle Hill, Head of Materials Testing, HBK Prenscia

The Prenscia Advanced Materials Characterization & Test facility (AMCT) is located in Rotherham, United Kingdom. Conducting materials testing for over two decades, the AMCT facility is focused on providing customers with fully characterized and interpreted material parameters ready for use in mechanical finite element analysis and/or fatigue analysis. Testing services offered include:

  • Tensile and compression tests with a determination of basic mechanical properties;
  • Strain-controlled axial fatigue tests for Basquin, Coffin-Manson, Smith-Watson-Topper and Ramburg-Osgood curves with a full evaluation of regression and design curves;
  • Load-controlled axial fatigue tests for Basquin curves with a full evaluation of regression and design curves;
  • High temperature tensile and fatigue tests in strain or load control between -40 to 900°C;
  • Analysis of iso-thermal stress-life (SN) and strain-life (EN) curves or analysis of advanced Chaboche thermo-mechanical fatigue properties;
  • Fatigue crack growth testing and generation of da/dN curves and calculation of rho* at ambient conditions;
  • Torsional fatigue testing for generation of Wohler curves and ‘staircase’ testing at ambient and elevated temperatures;
  • Fatigue testing of sub-element specimens, jointed materials or assemblies in axial, bending or custom tests.

Material types include metallics, engineering polymers, composites, civil engineering elastomeric materials and joining materials. With in-house design and manufacture of fixturing and full specimen preparation from raw material or components, and testing conducted to recognized standards, in-house customer standards or development of bespoke test methods for advanced materials and components.

Dr-Ing. Yevgen Gorash, Weir Advanced Research Centre, University of Strathclyde

There is limited data on VHCF for structural steels and their weldments for >10^7 cycles. Unalloyed low-carbon steel grades S235, S275 and S355 (EN 10025) are common structural materials for the components made for minerals and mining applications. The purpose of the current research is an investigation of the gigacycle domain for structural steel grades that are expected to perform for years at normal frequencies (10-20 Hz) of loading with low-stress amplitudes. The work focuses on ultrasonic fatigue testing of steels in both as-manufactured and pre-corroded conditions. As vibration-induced heating is a massive challenge for ultrasonic fatigue testing, especially in the case of structural steels attributed with a pronounced frequency effect, temperature control arrangement is crucial for proper implementation of testing. However, it becomes not straight forward with the introduction of air-cooling guns, which performance has shown to be unstable.

The frequency effect is assessed by comparing the fatigue test data at 20kHz and the conventional frequency of 15-20Hz. Its contribution is found to be significant because there is no overlap between the stress ranges of interest. The ultrasonic fatigue data is intended to be applied to the fatigue assessments of the equipment operating at normal frequency for 10^10 cycles. Therefore, the effect of frequency sensitivity is quantified by calculating the difference in terms of stress amplitude between corresponding SN curves. The whole set of technical challenges associated with both implementation of testing and interpretation of the obtained results has been discussed in this lecture.

Dr. Andrew Halfpenny, Director of Technology, HBK nCode Products

Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. In many cases, fatigue failures occur suddenly with little warning and so it is important to design structures to resist fatigue failure. While it can be technically challenging to simulate fatigue failure, it is possible to get accurate fatigue life calculations by using specific and reliable material data.

This presentation considers how material fatigue properties used in fatigue simulation are determined from laboratory tests. We will review the very first ‘rotating-bend’ test invented by August Wöhler through to the popular ‘load-control’ and ‘strain-control’ test procedures used today. Particular consideration is paid to ‘characterizing the material’ to derive a ‘design curve’ for use in component fatigue analysis.

Such material characterisation accounts for statistical reliability and confidence, enabling quantification of answers to questions such as:

  • How many specimens to test?
  • How to validate replacement materials?
  • How to compare the performance of materials from different suppliers?

Dr. Cristian Bagni, HBK Prenscia

Previous presentations have introduced finite element modelling techniques for structural stress-based fatigue life prediction of welds, fatigue test specimen design, and failure mode validation for fatigue tests of spot welded and seam welded joints. The immediately preceding presentation has described fatigue testing and characterisation of the fatigue performance of material with statistical reliability and confidence.

This presentation describes the post-processing of welded joint fatigue test measurements to determine the number of cycles to failure for an appropriate failure criterion. It then illustrates the consequent characterisation of the fatigue performance of the joint, where:

  • The characterisation for spot welds requires optimising the factors in the structural stress equation to collapse two load-life curves, from membrane-dominated and bending-dominated fatigue tests, into a single stress-life curve;
  • The characterisation for seam welds requires isolating membrane and bending components from two load-life curves, again from membrane-dominated and bending-dominated fatigue tests, to generate a set of two stress-life curves, one to represent wholly membrane stress and the other pure bending stress.

Session 6: Electric Vehicle Battery Durability & Reliability

Like their thermal-engine counterparts, electric vehicles are susceptible to structural fatigue failures. The mechanical complexity of the battery structure and its mountings also give rise to significant additional fatigue failure issues. Insights into these structural and vibration-induced failures enable engineers to eliminate the risk of fatigue failure, improve the durability of battery structures, and increase vehicle reliability.

This seminar considers fatigue design of battery packs, accelerated vibration testing of battery packs and fatigue analysis of electric vehicle structures with industry applications.

generic electric car with battery visible x-ray charging at a public charger in a city parking lot with lens flare 3d render. Electric Vehicle Battery Durability & Reliability, session 6 from day 3 of the 2021 HBK Technology Days.

Dr. Jim Hooper, Principal Engineer – Electric Vehicle Projects, WMG, University of Warwick

Within this presentation, Dr. Jim Hooper will illustrate that vibration, which is representative of 100,000 miles of service can have a direct impact on the electrical and mechanical performance of electric vehicle battery cells and packs. The presentation will cover the work undertaken by WMG in vibration durability and will highlight test methodology that can be employed by engineers to quantify the vibration robustness of their battery packs. It will illustrate different electrical and mechanical characteristics that need to be considered in the assessment of battery cells, modules and packs and provide examples of tests that have been conducted on Tesla Model S battery modules.

Dr. Andrew Halfpenny, Director of Technology, HBK Prenscia

Battery Structural Durability

Like their thermal-engine counterparts, electric vehicles are susceptible to structural fatigue failures. The mechanical complexity of the battery structure and its mountings also give rise to significant additional fatigue failure issues. Insights into these structural and vibration-induced failures enable engineers to eliminate the risk of fatigue failure, improve the durability of electric vehicle batteries, and increase vehicle reliability. This presentation considers:

  • Fatigue design of battery packs;
  • Accelerated vibration testing of battery packs;
  • Fatigue analysis of battery structures.

 

Battery Reliability

All electric vehicle batteries degrade over time. Their performance, however, varies by model and external conditions such as usage, temperature, and charging methods. To improve the overall reliability of the battery system and avoid excessive warrantee exposure, it is important to understand both the mean life and the statistical distribution of lives for the battery. Furthermore, understanding how the battery degrades over time will lead to significant improvements in battery design, reliability, and vehicle efficiency.

This presentation considers how Prenscia's experience with mechanical fatigue testing, material characterisation, fatigue modelling and analyses are applicable to battery reliability:

  • Battery life analysis;
  • Battery performance degradation modelling and analysis;
  • FMEA for new failure modes.