Multiscale experimentation & modeling of fatigue crack development in aluminium alloy 2024.

The objective of this research project is to be able to understand the role of various microstructural features on Fatigue Crack Initiation (FCI) of metallic alloys. By employing a novel experimental set-up, mechanical testing was performed in situ within an SEM chamber, and the deformation of the individual grains was observed real time. A physically-based Crystal Plasticity (CP) model was then developed that accurately predicts the macro and micro mechanical behaviour for Al2024 T3. An experimentally informed FCI criterion was developed that accounts for the effect of local slip bands and the applied local strains. While ‘precious’ insights were given on the small crack growth regime observing the occurring microscale phenomena. FCI is a multiscale process and thus evaluating the microscale does not cover fully the understanding of local deformation and damage. Thus a multiscale DIC process was employed to better understand the macro and mesoscale as well. 3D Digital Image Correlation (DIC) was employed and the strain distributions (at the sample scale) were obtained for various loading conditions. High magnification camera based 2D DIC was then used and the strain measurements were also extracted at clusters of grains. Useful observations were given for the different strain components (εxx, εyy, εxy). Finally the total fatigue lifetime of the component was compared to the modeled FCI for various loading conditions

The strain fields present during the bending of ultra-high strength steels

Ultra high strength steels (UHSS) have an ultimate tensile strength of greater than 1GPa. Typically, their ambient temperature elongation is less than 10% and as a result, they are rarely used in stamping applications. However, the continuous demand for the weight reduction of structures built for the transport sector means that such materials are attractive because they can be used for parts with thinner cross-sections while maintaining required in-service performance. One way to overcome the ambient temperature ductility of these materials is to roll-form them, particularly with emerging flexible roll forming technology. Using numerically-controlled actuators, the rolls on each stand are designed with sufficient degrees of freedom to form parts that curve, vary in depth and width along their lengths. This makes flexibly roll-formed parts attractive to the transport, particularly the automotive, sector. Roll forming deforms a material through incremental, localised bending, which is known to suppress the necking response, resulting in deformations that are higher than in stretch deformation. Recent work, such as Le Maoût, Thuillier & Manach, Eng. Frac. Mech., Vol. 76, p.1202 (2009), focussed on the development of ductile fracture models to explain failure but their validation was limited to load displacement and surface strain data. This work aims to characterise the strain field during bending more comprehensively. Using the digital image correlation technique, the macroscopic strain distribution in UHSS in the thickness of the sheet and the strain partitioning in its microstructure is measured during bending. The data provides a detailed explanation of the strain distribution during bending

Experimental and modelling study of fatigue crack initiation in an aluminium beam with a hole under 4-point bending

Slip band formation and crack initiation during cyclic fatigue were investigated by in-situ experiments and non-local CPFEM simulations systematically. Experimental techniques including EBSD, digital image correlation (DIC) and SEM have been used to obtain consistent grain orientations, local strains, as well as the locations where slip bands and micro-cracks form on the sample surface. The realistic microstructure based on the EBSD map has been generated and used for finite element modelling. An advanced non-local crystal plasticity model, which considers the isotropic and kinematic hardening of the plastic strain gradient, has been adopted. The simulation results match well the corresponding experimental results. It was found that total strain and averaged slip on all slip systems, combined with accumulated slip on specific slip planes help predict the location and orientation of slip bands and micro-crack initiation correctly. Furthermore, a fatigue indicating parameter based on competition between maximum slip and the total slip has been proposed to reproduce the experimental observations

Damage in dual phase steel DP1000 investigated using digital image correlation and microstructure simulation

Microstructure failure mechanisms and void nucleation in dual-phase (DP) steels during deformation have been studied using a combination of in situ tensile testing in a scanning electron microscope (SEM), digital image correlation (DIC) and finite element (FE) modelling. SEM images acquired during in situ tests were used to follow the evolution of damage within the microstructure of a DP1000 steel. From these images, strain maps were generated using DIC and used as boundary conditions for a FE model to investigate the stress state of martensite and ferrite before the onset of the martensite phase cracking. Based on the simulation results, a maximum principal stress of about 1700 MPa has been estimated for crack initiation in the martensite of the investigated DP1000 steel. The SEM image observations in combination with the FE analyses provide new insights for the development of physically-based damage models for DP-steels

Laser Welding Dissimilar High-Strength Steel Alloys with Complex Geometries

Laser welding of dissimilar high-strength steels was performed in this study for two different geometries, flat and circular samples with material thicknesses of 5 and 8 mm. The material combinations were a low carbon to a medium or high carbon steel. Three different welding systems were employed: a Nd:YAG, a CO2 and a fiber laser. The process stability was evaluated for all the experiments. The resulting full penetration welds were inspected for their surface quality at the top and bottom of the specimens. Cross sections were taken to investigate the resulting microstructures and the metallurgical defects of the welds, such as cracks and pores. Significant hardening occurred in the weld region and the highest hardness values occurred in the Heat Affected Zone (HAZ) of the high carbon steel. The occurrence of weld defects depends strongly on the component geometry. The resulting microstructures within the weld were also predicted using neural network-simulated Continuous Cooling Transformation (CCT) diagrams and predicted the occurrence of a mixture of microstructures, such as bainite, martensite and pearlite, depending on the material chemistry. The thermal fields were measured with thermocouples and revealed the strong influence of component geometry on the cooling rate which in term defines the microstructures forming in the weld and the occurring hardness

Laser Welding Dissimilar High-Strength Steel Alloys with Complex Geometries

Laser welding of dissimilar high-strength steels was performed in this study for two different geometries, flat and circular samples with material thicknesses of 5 and 8 mm. The material combinations were a low carbon to a medium or high carbon steel. Three different welding systems were employed: a Nd:YAG, a CO2 and a fiber laser. The process stability was evaluated for all the experiments. The resulting full penetration welds were inspected for their surface quality at the top and bottom of the specimens. Cross sections were taken to investigate the resulting microstructures and the metallurgical defects of the welds, such as cracks and pores. Significant hardening occurred in the weld region and the highest hardness values occurred in the Heat Affected Zone (HAZ) of the high carbon steel. The occurrence of weld defects depends strongly on the component geometry. The resulting microstructures within the weld were also predicted using neural network-simulated Continuous Cooling Transformation (CCT) diagrams and predicted the occurrence of a mixture of microstructures, such as bainite, martensite and pearlite, depending on the material chemistry. The thermal fields were measured with thermocouples and revealed the strong influence of component geometry on the cooling rate which in term defines the microstructures forming in the weld and the occurring hardness