Part 1: Short-answer quiz
Instructions: Review each question prompt and evaluate its metallurgical principles before expanding the panel to verify your understanding against the answer key.
1. How is residual stress defined in a stationary body?
Residual stress is defined as that which remains within a body which is stationary and at stable equilibrium with its immediate surroundings. It originates fundamentally from localized microstructural misfits in the natural shape or volume between different internal regions, structural phases, or component layers.
2. Distinguish between Type I and Type II residual stresses based on their characteristic length scales.
Type I residual stresses (macrostresses) vary continuously over macroscopic dimensions and self-equilibrate across the structural scale of the entire component. Type II residual stresses (intergranular microstresses) vary across single grain dimensions and self-equilibrate over a local volume spanning only a few crystal grains.
3. Why is shot peening used to improve the fatigue resistance of a material?
Shot peening introduces highly beneficial compressive in-plane residual macrostresses within the near-surface region of an engineering component. These near-surface compressive stresses directly counteract external tensile service stresses, effectively lowering the mean stress amplitude experienced over a cyclic loading programme and delaying fatigue crack initiation.
4. How do residual stresses in thermally toughened glass prevent failure from surface flaws?
Rapid thermal quenching generates severe compressive macrostresses in the exterior glass layers, counterbalanced by internal tensile stresses within the core. Because any propagating surface flaws are tightly clamped by this in-plane compression, they cannot open or extend under low levels of applied service load, shielding the component from brittle failure.
5. Explain the basic principle behind mechanical stress measurement methods like hole drilling?
Mechanical relaxation techniques rely on measuring the localized deformation or distortion of a component when internal stresses are allowed to relax via targeted material removal. In hole drilling, a fine cylindrical cavity is machined into the substrate, and the resulting microstructural strain relaxation is recorded by surface gauges to back-calculate the pre-existing macrostress profile.
6. What is the "time of flight" method used in neutron diffraction?
This method utilises a pulsed polychromatic neutron beam where the scattering Bragg angle ($2\theta$) is fixed while the incident wavelength ($\lambda$) varies. The specific velocity and wavelength of each detected particle are deduced directly from its travel time relative to the initial pulse generation event, enabling non-destructive elastic lattice strain determination.
7. In the context of thin films, what is the difference between extrinsic and intrinsic stresses?
Extrinsic stresses originate from external boundary changes after film deposition, most notably due to thermal expansion coefficient mismatches ($\Delta\alpha$) during cooling. Intrinsic stresses develop during the atomic deposition process itself, driven by non-equilibrium growth phenomena such as coherent epitaxial mismatch, atom tracking defects, or rapid splat quenching.
8. Define the "effectively stress-free temperature" ($T_{\text{esf}}$) as it relates to composite materials.
The effectively stress-free temperature ($T_{\text{esf}}$) is a virtual model parameter defining the theoretical point from which a composite must be cooled to reproduce its exact ambient thermal macrostresses assuming entirely elastic behavior. It captures the real thermal expansion misfit while safely accounting for high-temperature plastic relaxation mechanisms like creep and matrix yielding.
9. Contrast reconstructive and displacive phase transformations regarding their associated strains.
Reconstructive transformations proceed via uncoordinated long-range atomic diffusion, typically generating macroscopic strain states that are isotropic and purely dilatational. Displacive transformations occur via a coordinated lattice shear deformation, inducing an invariant-plane strain (IPS) characterised by a large shear component along the habit plane and a dilatational strain normal to it.
10. How do phase transformations in steels affect the development of macro residual stresses during cooling?
Solid-state transformations like the decomposition of parent austenite into bainite or martensite involve transformation plasticity and significant volume expansion. This expansion actively counteracts and cancels out localized thermal contraction strains, meaning that long-range macrostresses only begin to accumulate after transformation is complete and cooling resumes down to ambient temperature.
Part 2: Essay questions
Instructions: Review the detailed composition options below. Hints outlining critical parameters and Fickian constraints can be expanded for guidance.
1. The role of scale in stress measurement
Compare and contrast diffraction-based measurement techniques (X-ray and Neutron) with mechanical relaxation methods (Hole Drilling and Curvature). How does the "sampling volume" of a technique determine which types of residual stress (Type I, II, or III) it can accurately record?
Key points for formulation: Contrast the mechanical gauge fields of hole drilling, which average macroscopic strains across millimetre blocks (resolving Type I stresses), with the sub-nanometre crystal lattice gauges evaluated via diffraction. Show that while laboratory X-rays evaluate surface fields, deep neutron beams penetrate bulk sampling volumes ($1\text{--}10\,\text{mm}^3$) to resolve phase-specific Type II and Type III intergranular strain tensors.
2. Residual stresses in multiphase materials
Analyze the origins of Type II mean phase microstresses in metal matrix composites. How do mismatches in thermal expansivity, yield stress, and stiffness contribute to load transfer between the matrix and reinforcement?
Key points for formulation: Detail the generation of microstructural eigenstrains when cooling a composite containing phases with dissimilar thermal expansion coefficients ($\Delta\alpha$). Discuss how thermal misfits push stiffer ceramic reinforcements into hydrostatic compression while generating counterbalancing tensile microstresses within the ductile metal matrix, and evaluate how matrix yielding establishes a strict plastic limit on these fields.