AbstractIn the aerospace industry, thermo-mechanically processed aluminium alloy structures are commonly used in fatigue critical applications. Such structures, possessing complex crystallographic texture and grain structure are usually surface treated with techniques such as shot peening, to enhance their fatigue life. Of the many peening techniques, laser shock peening (LSP) has many advantages over other peening processes and is being heavily explored for applicability.
Firstly, this thesis investigates the effects of LSP on surface roughness, hardness, texture and grain structure of aluminium alloys, 7050-T7451 and 2099-T8. Secondly, it investigates the
influence of texture and grain structure on the stability of LSP induced compressive residual stresses under mechanical fatigue using experimentation and crystal plasticity based finite element (CPFE) simulation. Details of the investigations presented are briefed below.
Results from the peened region are compared against those of the unpeened region. These results included surface profile, grain structure, texture, hardness, residual stress and residual
strain accumulation. LSP induced changes to surface profile were measured using the technique of White Light Interferometry. Investigations into grain structure and crystallographic
texture were made using Electron Back Scatter Diffraction (EBSD). Depth dependent hardness variations in the material were measured using Vickers indentation. Residual strain accumulation was studied using both neutron transmission and EBSD derived data was also used to investigate. Residual stresses were measured using X-ray diffraction intermittently during fatigue loading of peened samples.
Comparison of various parameters of the above results, revealed that LSP can induce significant and measurable changes to the surface profile, grain structure, crystallographic texture,hardness, residual stresses and residual strain accumulation. Texture and grain structure results investigated and their presentation in the thesis are briefed below.
Changes to both macro-texture and micro-texture were investigated. LSP induced changes in Euler space are explained in terms of texture parameters such as intensity and volume
fractions of the texture features. Evidence of LSP induced texture evolution is presented andits difference is highlighted in comparison to that induced by deformation processes such as
rolling and extrusion. Insights into stability of dominant rolled texture orientations, under LSP treatment are also presented. LSP induced changes to misorientation statistics in the grain
structure are presented. Investigations into LSP induced changes to texture are then made in in the physical space of the grain structure. Results from the investigations into texture component dependent LSP induced plastic strain is presented. The hardness and residual stress results investigated are briefed below.
Hardness measurements were used to derive depth dependent Yield strength in the material. To study residual stress relaxation, a four-point bending relaxation sample was used. Residual
stress measurements were made at different locations on the relaxation sample. Such points on the sample were chosen for their differences in texture and grain structure. Intermittent
residual stress measurements at such points were made during fatigue loading, and the relaxation was tracked with load and number of cycles for different texture and grain structures.
Brief results and insights pertaining to LSP induced changes are presented below.
LSP induced surface roughness was found to be dependent on LSP coverage. LSP can alter the volume fraction and intensity of important macro-texture components in AA7050-T7451.
Grains in the peened region were found to be significantly sub-structured. Microtexture changes were found to stem from the evolution of deformation bands both inside the grains
and at the grain boundaries. LSP induced hardness change was found to vary between
AA7050-T7451 and AA2099-T8. Neutron transmission based residual strain estimation in AA7050-T7451 revealed that LSP could induce plastic strain deep below the peened surface.
Such a study could not be successfully carried out in AA2099-T31 due to the high neutron cross section of the chemical element Lithium in AA2099. Following results pertain to AA2099-
T831. It is found that, a high Goss and low Brass texture, together with large un-substructured grains are beneficial to achieve a high LSP induced compressive residual stress. Results from
relaxation experiments are briefed below.
Fatigue experiments revealed that residual stress relaxation is spatially heterogeneous even when the bending induced stresses are spatially homogeneous. Such a heterogeneity is shown to be related to heterogeneities in texture and grain structure of the sample. This relationship was then investigated to understand the stability of residual stresses in terms of texture and grain structure parameters. Those texture and grain structure parameters supportive of residual stress stability and those detrimental to it are then provided. A Location-Event-Relaxation hyperspace is introduced to compactly describe spatial distribution of stability of LSP induced residual stresses and consequently explain residual stress stability in
2099-T8 aluminium alloy.
A texture and material calibrated crystal plasticity based finite element (CPFE) model was made to study partitioning of mechanical behaviour with different texture components and insights from the results are used to further explain the experimental findings. Schmidt factors are calculated and expressed as a function of texture in Euler space. Texture partitioned stress-strain behaviours were then derived to understand the texture partitioning of mechanical behaviour.
Results show that residual stresses in Cube and Goss grains can relax much earlier than others. Though both Cube and Goss texture components contribute to instability, the instability
offered by Goss can be triggered by a much narrower range of induced stresses. Brass grains need an overall higher applied stress to develop grain internal plasticity and hence resist
relaxation over higher applied stresses.
Results from experiment and CPFE simulations conclusively suggest that mechanical fatigue induced relaxation of LSP induced residual stress in aluminium alloy AA2099-T831, is both texture and grain structure dependent.
|Date of Award||2022|
|Supervisor||Michael Fitzpatrick (Supervisor), Xiang Zhang (Supervisor) & Kashif Khan (Supervisor)|