Spatiotemporal Evolution of Stress Field during Direct Laser Deposition of Multilayer Thin Wall of Ti-6Al-4V

The present work seeks to extend the level of understanding of the stress field evolution during direct laser deposition (DLD) of a 3.2 mm thick multilayer wall of Ti-6Al-4V alloy by theoretical and experimental studies. The process conditions were close to the conditions used to produce large-sized structures by the DLD method, resulting in specimens having the same thermal history. A simulation procedure based on the implicit finite element method was developed for the theoretical study of the stress field evolution. The accuracy of the simulation was significantly improved by using experimentally obtained temperature-dependent mechanical properties of the DLD-processed Ti-6Al-4V alloy. The residual stress field in the buildup was experimentally measured by neutron diffraction. The stress-free lattice parameter, which is decisive for the measured stresses, was determined using both a plane stress approach and a force-momentum balance. The influence of the inhomogeneity of the residual stress field on the accuracy of the experimental measurement and the validation of the simulation procedure are analyzed and discussed. Based on the numerical results it was found that the non-uniformity of the through-thickness stress distribution reaches a maximum in the central cross-section, while at the buildup ends the stresses are distributed almost uniformly. The components of the principal stresses are tensile at the buildup ends near the substrate. Furthermore, the calculated equivalent plastic strain reaches 5.9% near the buildup end, where the deposited layers are completed, while the plastic strain is practically equal to the experimentally measured ductility of the DLD-processed alloy, which is 6.2%. The experimentally measured residual stresses obtained by the force-momentum balance and the plane stress approach differ slightly from each other.

The study of redistribution in residual stresses during fatigue crack growth

Numerical and experimental study was conducted on fatigue crack growth (FCG) of metallic components to investigate the redistribution of mechanical residual stresses during FCG. To this end, the compact tension specimens of an aluminium alloy were used. In addition, mechanical residual stresses were introduced near the crack tip by applying compressive and tensile loads, followed by visually observing the side-surface of the specimens to estimate the crack growth length. In the numerical simulation, cyclic J-integral was used as the crack growth fracture parameter and a good agreement was observed between the numerical and experimental results. The results of the finite element method demonstrated a clear redistribution of mechanical residual stresses during FCG. After a few cycles, the residual stress field around the crack tip reached a lower magnitude value confined in a smaller zone, although this zone was stable during the remaining fatigue process. Finally, present study evaluated the effect of stress ratio, load amplitude, and initial residual stresses level on the redistribution of residual stresses. It was observed that the residual stresses are mainly released during the first steps of fatigue loading.

NUMERICAL AND EXPERIMENTAL STUDY ON PLATE FORMING USING THE TECHNIQUE OF LINE HEATING

Nowadays manual and experiential technique patterns of line heating process could not meet the requirement of modern shipbuilding. Therefore, the automatic forming method is being an active research topic in manufacturing. An accurate and practical predicting method is an essential part of the automatic plate forming system. In the present work a numerical elasto-plastic thermo-mechanical model has been developed for predicting the thermal history and resulting deformation and residual stress field of line heating process. A moving Gaussian distributed heat source was used in the modelling to create a realistic simulation of the process. The transient temperature distributions were predicted using temperature-dependent material properties. The deformation and residual stress field were predicted based on the transient temperature distributions of line heating. Experiments were conducted to prove the validity of the numerical thermo-mechanical model. The final numerical results of temperature, deformation and residual stresses are in good agreement with experiment results. The proposed method presents a valuable reference for the study of similar thermal process.

Radial dynamics of an encapsulated microbubble with interface energy

In this work a mathematical model based on interface energy is proposed within the framework of surface continuum mechanics to study the dynamics of encapsulated bubbles. The interface model naturally induces a residual stress field into the bulk of the bubble, with possible expansion or shrinkage from a stress-free configuration to a natural equilibrium configuration. The significant influence of interface area strain and the coupled effect of stretch and curvature is observed in the numerical simulations based on constrained optimization. Due to the bending rigidity related to additional terms, the dynamic interface tension can become negative, but not due to the interface area strain. The coupled effect of interface strain and curvature term observed is new and plays a dominant role in the dominant compression behaviour of encapsulated bubbles observed in the experiments. The present model is validated by fitting the experimental data of $1.7\,\mathrm {\mu }$ m, $1.4\,\mathrm {\mu }$ m and $1\,\mathrm {\mu }$ m radii bubbles by calculating the optimized parameters. This work also highlights the role of interface parameters and natural configuration gas pressure in estimating the size-independent viscoelastic material properties of encapsulated bubbles with interesting future developments.

Effect of vibrations on the residual stress field formation during milling of high-strength aluminum alloys

The authors examine the influence of high-speed milling on the distribution of residual stresses in parts made of structural high-strength aluminum alloys Al-Cu-Mg, which are the main structural materials in the aerospace industry. Milling was carried out at high cutting speeds. Different tool settings were used to balance the instrument. Plastic deformation occurred in the part’s surface layers. Residual stresses were measured by the X-ray method. It was found that high-speed milling creates residual compressive stresses that are favorable for the operation of the part. The depth of the residual stresses depends on the cutting mode. The article shows the relationship between residual stresses and the type of metalworking tool, processing conditions in structural parts made of high-strength aluminum alloys.

An Idea for Predicting Residual Stress Field of Roll Tensioned Circular Saw Blade

In this paper, a concise and fast 2D model of the roll tensioning process was built using the finite element method. Elastic thermal expansion is used to simulate rolling plastic deformation. A 3D model considering contact between roller and circular saw blade was also built. Through comparison of residual stress results obtained by the 2D model, 3D model, and X-ray stress test method, the correctness and feasibility of the 2D model were proven. While accounting for the diversity of circular saw blade structure, this paper provided an idea for rapidly predicting the residual stress field of a roll-tensioned circular saw blade.

A Data-Driven Method for Predicting Deformation of Machined Parts Using Sparse Monitored Deformation Data

In the aircraft industry, where high precision geometric control is vital, unexpected component deformation, due to the release of internal residual stress, can limit geometric accuracy and presents process control challenges. Prediction of component deformation is necessary so that corrective control strategy can be defined. However, existing prediction methods, that are mainly based on the prediction or measurement of residual stress, are limited and accurate deformation prediction is still a research challenge. To address this issue, this paper presents a data-driven method for deformation prediction based on the use of in-process monitored deformation data. Deformation, which is caused by an unbalanced internal residual stress field, can be accurately monitored during the machining process via an instrumented fixture device. The state of the internal stress field within the part is first estimated by the using the part deformation data collected during machining process, and then, the deformation caused by a subsequent machining process is predicted. Deep learning is used to establish the estimating module and predicting module. The estimating module is used to infer the unobservable residual stress field as vectors by using sparse deformation data. The inferred vector is then used to predict the deformation in the predicting module. The proposed method provides an effective way to predict deformation during the machining of monolithic components, which is demonstrated experimentally.