Residual Stress (Engineering)

Automobile brake systems transform kinetic energy into heat for the purpose of speed control. During this operation, rapid heating and cooling cycles cause the formation of residual stress in the brake pad backing plate. In this study, the relation between residual stress magnitude and the operation period is investigated to predict the service life of brake pads. Residual stresses are calculated related to longitudinal ultrasonic wave velocity variations which are measured non-destructively using cost effective and accurate immersion ultrasonic technique. Finally, a mathematical model is developed to predict residual stress in brake pad backing plates related to operation period.

The effect, on the residual stress measurement accuracy, of the drilling speed of the end-mill during the hole-drilling
measurements was evaluated in Ti6Al4V. In spite of the well-known consideration that the highest achievable speed should
be used during hole drilling, very few experimental works exist analyzing the effects of using lower velocities. Hole-drilling
experiments were performed in this study by measuring the released strain by electronic speckle pattern interferometry. A
known stress state was generated by loading the sample in a four point bending frame up to 50% of the yield strength.
Drilling speed ranging in 5000 ÷ 50000 rpm was investigated by using an electronically controlled mill. The expected stress
field, evaluated by a numerical model in ANSYS®, was compared with the measured one at different drilling speeds.

Friction Stir Welding (FSW) has become the most important evolving welding technology for the
proposed integral aluminium structures for aerospace fuselage to avoid high cost, excessive
weight. The fatigue performances of FSW parts are superior to riveting. FSW entails a nonconsumable
rotating cylindrical tool, consisting of a shoulder and pin, being pushed in to induce
heat to plasticise the material and then traversed along the abutting surfaces of the two plates to
be welded. Several factors influence the resulting microstructure and mechanical characteristics,
and tool transverse and rotational speed are the most significant.
Characterization of selected pioneering FSW components is presented. AA5083-H111 was
selected because of its superplasticity, formability, weldability, high strength and corrosion
resistance. It has been the preferred material for manufacturing of pressure vessels for aircrafts
and spaceships, and vehicle bodies’ structures.
A CNC milling machine was converted to weld 3mm thick plates of 360mm by 120mm at
combinations of different rotational (400, 500 and 630rpm) and welding speeds (50, 60 and
70mm/min) using a tapered probe and cylindrical shoulder at zero tilt angle. Tensile testing and
bend testing were undertaken. The macrostructure and fractured surfaces were observed using an
optical microscope and an SEM in secondary electron mode.
Stress-strain relationships of the welds were different. Compared to the base material, FSW
samples had lower strain-to-failure results, owing to strain localization in the weld. H1, welded at
400rpm and 50mm/s had the highest UTS, 99.3±0.2%, whereas H2, welded at 400rpm and
70mm/s had the highest yield strength, 97.7±0.3%. Welds performed at 400 rpm were generally
flaw-free, while higher weld speeds produced welds with tunnels, due to the low forging pressure.
In the macrostructure, the weld nugget could be easily distinguished from the TMAZ (Thermo-
Mechanically Affected Zone) on the advancing side, because the grains were finer, being
recrystallized and equiaxed. However, no distinct boundary existed on the retreating side. The
joint-line remnant, which comprises amorphous clusters of alumina, was observed in the nugget,
but had no effect on mechanical properties. There was an increase in hardness in the weld zone,
towards the retreating side, due to grain refinement and better weld consolidation. Conversely,
there was a decrease in hardness in the TMAZ. The weld nugget average hardness of all flaw-free
samples was 79.8±4.6HV0.1, with no definite relationship between weld parameters and hardness
profiles. Tensile specimens fractured in the TMAZ on the advancing side at 45° to the far-field
stress, because of reduced micro-hardness, strain localization and as well as the presence of
tunnels in the flawed samples. Generally, failure occurred in a brittle mode, but areas of local
ductile shear and quasi-cleavage fracture were observed in the SEM.
This research demonstrated that good weld efficiency was obtainable on FSW of AA5083-H111
with a CNC milling machine of sufficient welding power.

A set of Ni-based single crystal superalloy aero-blade sections with electron beam physical vapour deposited thermal barrier coating (EBPVD-TBC) were used to investigate the role of substrate curvature on the microstructure and the residual stresses in the coating system. The residual stresses generated in the thermal barrier coating (TBC) and the thermally grown oxide (TGO) have been evaluated non-destructively by Raman spectroscopy (RS) and photo-stimulated luminescence piezo-spectroscopy (PLPS), respectively. In addition, residual stresses were measured in free-standing individual columns extracted from the TBCs for further understanding of the large Raman shift towards the tension side in the ceramic layer. For the TGO, three areas with varied substrate curvature were mapped by PLPS. These residual stresses are analysed as a function of the thermal exposure and the substrate curvature. Discussion focuses on the implications on how to correctly evaluate the residual stresses in EBPVD-TBCs based on Raman shifts and subsequently the role of substrate curvature on the degradation of the microstructure and evolution of residual stresses in this type of TBCs.

Vertical cylindrical vessel-chambers as a part of coal — drying plants, whose purpose is to collect wastewater, are supported at 3 points in upper dish head area and are made of fine-grained Mn steel plates, joined by welding. Significant thinning and leaking in upper dish head area of the vessel occurred due to original design provoking an intensive abrasion, cracking and rupture. After reconstruction, in the upper zone of cylindrical shell, two new joints were made in sity by manual arc welding, with subsequent, local post-weld heat treatment. However, cracks appeared firstly in new welded zones in radial and axial joints, and then in zones of openings. Later, the similar failure features began to appear in the area of original welded joints. All of these cracks were repaired by properly specified technology. Unfortunately, after some period of exploitation the initiation of new cracks was observed, at first by the visual inspection. This problem was detected in the repaired areas in all (16) vertical cylindrical vessels. Cracks have propagated in different directions with various penetration depths, up to 3 mm. It is interesting to note that crack appeared in the HAZ of vertical joints, while in the area of radial welded joints the cracks were randomly distributed in a larger zone. For cracks up to 3 mm, deep grinding was applied whereas for greater crack depths repair welding with local post-weld heat treatment was used. Since welding with subsequent heat treatment could only be used twice to repair the welded joints [1, 2], it is clear that in the areas repaired several times (up to 7) proper mechanical characteristics of the material is rather difficult to maintain.

Hole drilling (HDM) is the most widespread approach for measuring residual stress profile and it is, at the moment, the only technique to be ruled by a standard. Many improvements were introduced along the years and recently research papers investigated the feasibility to use optical systems to replace strain gage rosette as measurement tool of the strain field generated by the drilled hole. Generally speaking, the accuracy of the measurement is influenced by the proper choice of the experimental and analysis parameters. In this paper the effects of the choice of a proper analysis area, to be intended as the portion of the image captured by the CCD camera whose data are used to calculate the strain values, are shown. Furthermore the influence of the drilling rotation speed of the cutter used for drilling the hole is investigated. Experimental tests were performed on a Ti6Al4V specimen loaded in a four point bending frame up to 50% of the yield strength. Drilling speeds ranging between 5000 ÷ 50000 rpm were investigated by using an electronically controlled mill. Results obtained at different speeds and for different locations of the analysis area were compared with the theoretical expected value and with results from a numerical model implemented in ANSYS®.

Selection of a proper material for fabricating an
industrial product through welding process is not only
one of the initial necessities but also plays an important
parameter in achieving a safe design. At the same time,
researchers have shown that part of the residual stresses
in weldments could be generated under the influences of
joints material. Thus, values of the residual stresses that
produced due to variation of the characteristics of
different materials can be a basic criterion for reaching
this goal. In a similar condition, each case in
comparison with the others that begets less residual
stresses can be selected. On the other hand, prediction
of the behavior of the residual stresses is complex and
time-consuming in experimental methods and even in
numerical models. Therefore, in this work, entropy
generation methodology is presented as an efficient tool
to estimate the distribution of residual stresses,
qualitatively. In this approach, the behavior of the
induced residual stresses caused by the variation of
different parameters such as weldment material is
examined based on the behavior of generated entropy.
To verify the application of the proposed method, a 3D
model is employed to study the similar behavior of
residual stresses and entropy generation for three
different steels that are in frequent use. Furthermore, the
metallurgy analysis of the steels is performed in welding
processes and the results are compared with the residual
stresses and entropy generation ones. Based on the
outcome of the present work, one can find that the
behavior of entropy generation and residual stresses
with respect to different weldment materials are very
similar. Also, the entropy generation and residual
stresses results are comparable and in-line with the
metallurgy analysis ones. Finally, in the proposed
methodology, studying the induced residual stresses
without trying to obtain residual stresses directly which
used to be very complicated and time-consuming is
possible as well.

In this work the numerical simulation of the Additive Manufacturing (AM) process is addressed. The coupled thermo-mechanical framework used to solve the balance equations, as well as the constitutive laws to describe the material behavior in the entire temperature range are presented. The numerical model has been calibrated through the experimental campaign carried out at the State Key Laboratory of Solidificati on Processing (SKLSP) where a Laser Solid Forming (LSF) machine is operated. This machine makes use of the blown powder technique to perform the Metal Deposition (MD) process in a layer-by-layer manner. Both the software and the machine read the same scanning sequence given through Common Layer Interface (CLI) format, that is, the sequence of polylines and hatching to cover the entire section of the component for each layer. The power absorption coefficient and the Heat Transfer Coefficients (HTC) for both heat convection and heat radiation laws have been calibrated to capture the temperature evolution at the different locations where the thermocouples have been placed. The response of the thermo-visco-elastic-visco-plastic constitutive model has been calibrated by comparing the distortion of the supporting plate at different locations monitored during the full duration of the manufacturing process. Remarkable agreement between experimental and numerical results is shown.

This work describes the local-global strategy proposed for the computation of residual stresses in Friction Stir Welding (FSW) processes. A coupling strategy between the analysis of the process zone nearby the pin-tool (local level analysis) and the simulation carried out for the entire structure to be welded (global level analysis) is implemented to accurately predict the temperature histories and, thereby, the residual stresses in FSW. As a first step, the local problem solves the material stirring as well as the heat generation induced by the pin and shoulder rotation at the heat affected zone. The Arbitrary Lagrangian Eulerian (ALE) formulation is adopted to deal with the rotation of complex pin shapes. A thermo-rigid-viscoplastic constitutive law is employed to characterize the viscous flow of the material, driven by the high strain rates induced by the FSW process. A mixed temperature-velocity-pressure finite element technology is used to deal with the isochoric nature of the the strains. The output of this local analysis is the heat generated either by plastic dissipation or by friction and it is used as the power input for the welding analysis at structural (global) level. The global problem is tackled within the Lagrangian framework together with a thermo-elasto-viscoplastic constitutive model. Also in this case the mixed temperature-displacement