A Strength Prediction of Wire and Arc Additive Manufactured joints

Abstract

Anticipating the benefits and potential of using Wire and Arc Additive Manufacturing (WAAM) in the construction field for structural connections to enhance automation, energy efficiency, and material utilization rate, the process to provide a reliable and efficient prediction for the strength of WAAM components is the focus of this thesis. Finite Element Analysis (FEA) is proven reliable and can be used as a method for strength prediction. ABAQUS has been chosen as a simulation tool due to its automatic and adaptive ability for computation and the dominancy in academic research. Current WAAM components research builds WAAM models with the heat source and printing path, which took an unacceptable time (could be several days) to simulate the behavior (distortion and residual stress) for meter-scale structural WAAM steel components. A semi-analytical model, which implicitly incorporated the behavior of the WAAM material by the direct use of components' mechanical test data as the FEA inputs, can speed up the strength prediction process. This research is built with a feasible process with unique steps for predicting WAAM components' strength. First, WAAM components’ lab tests from Van Bolderen [1] have been reviewed and numerically examined before being used for designing WAAM joints , which makes the data used more reliable. Four lab tests that have been reviewed and simulated are the tensile test, the compression test, the bending test, and the buckling test. Examined material non-linearity, anisotropy of the material, and the surface roughness have been applied to predict the strength of the joints. Five aims and five actions have been set and implemented in the design of WAAM printed gridshell joints. Moment-rotational curves for three joints are presented to acquire flexural stiffnesses and moment capacities. Second, the inclusion of the defined geometrical imperfections (the surface roughness and the lack of straightness) and material imperfections (anisotropy) to get better simulation results to yield closer results to lab experiments. The surface roughness is derived from the bending test, the lack of straightness is incorporated with the failure mode found from Linear Buckling Analysis (LBA), and the anisotropy is evaluated by building orthotropic material with the young's modulus from two WAAM printing directions (transverse and longitudinal) in the elastic range. Last, the codified guidance for WAAM printed components, which are defined as shell structures in EN 1993-1-6: Eurocode 3: Design of steel structures - Part 1-6: Strength and stability of shell structures, have been implemented. From the numerical simulation of four lab experiments, the tensile test simulation results show only maximum 0.5% errors (while compared yield strength, ultimate strength, and young's modulus to the lab result). Therefore, the material non-linearity from the lab test has been correctly used in the numerical simulation. The compression test results indicate the material non-linearity used in the three-dimensional objects is valid since the compression stress-strain curve fits the tensile engineering stress-stain curve well before the yield point (0.41% of errors with the comparisons of yield strength and young's modulus). The bending test can not precisely infer the errors of derived effective thickness mainly due to the lack of friction coefficient. However, the inclusion of the effective thickness reduces the error from 22.0% to 16.8%, and limited to 16.8% when compared to the lab test results, despite the fact that the value is from assumptions. The buckling test shows large buckling load errors (35%) that might due to the hand measured lack of straightness is too less, the effective thickness derived are too thick, or the defined geometrical imperfections are too simple that can't reflect the real printed conditions, which has local geometrical imperfections and deviations. The high sensitivity of the geometrical imperfections to the buckling load has also been found out from the analysis of scale factors with buckling loads. Few fraction increase of lack of straightness (0.2% to 0.4%) leads to tens of percentage error drop (19.6%) of buckling load compared to the lab results. Examined data that have been used in the analysis of joints are material non-linearity, the anisotropy of the material (build-in orthotropic manners), and the surface roughness. The target values of the flexural stiffness and moment capacity are insensitive to the lack of straightness due to the fact that the instability of the joint does not come from the buckling effect, the WAAM printed parts are not in slender shape, and the lack of straightness is small (L/791). Three WAAM printed joints have been designed, and the strength prediction have been carried out with the joint classification. At the end of this thesis, the conclusion is that there is potential to provide reliable strength prediction of the designed WAAM joints with the use of EN 1993-1-6: Eurocode 3 as guidance for building semi-analytical models for WAAM components and joints with the lab test material data and defined imperfections for two reasons. First, the simulation results yield closer values to the lab test results with the shell analysis process in EC3. In the compression test simulation, the inclusion of the possible geometrical imperfections shape found from LBA reduces the percentage errors decrease from 52.27% to 13.75%. In the bending test simulation, the inclusion of effective thickness leads to a closer value to the lab test (from 22% to around 16.8%). In the buckling test simulation, the implementation of effective thickness for surface roughness reduces around 10% to 15% error, and the inclusion of effective thickness combined with lack of straightness reduces roughly 40% of the error. Second, the parameters are examined before being used in the strength analysis of designed WAAM joints. The material non-linearity data is proven valid in the FEA with 3D geometry in tensile test and compression test simulations. The orthotropic behavior shows a minor effect in all four numerical simulations. The surface roughness combined with the use of assumed friction coefficient and the assumed equipment properties in the bending test simulation show limited error (around 17%). However, limited errors exist and undermine the strength prediction precision due to the assumptions of lacked lab test measurements and the simplification (parameterized) imperfections from WAAM components. Therefore, It is expected that more tests and works could be made to improve the strength prediction precision of WAAM components. Also, the FEA setup of the designed joints is referred to as the gridshell joints' lab experiments, therefore, providing a methodology for possible future examination of the strength of WAAM printing joints with lab experiments.