2021 HBK Technology Days

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.

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.

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?