Pedro Peralta

Characterization of Incipient Spall Damage in Shocked Copper Multicrystals

Correlations between spall damage and local microstructure were investigated in multicrystalline copper samples via impact tests conducted with laser-driven plates at low pressures (2—6 GPa). The copper samples had a large grain size as compared to the thickness, which was either 200 or 1000 μm, to isolate the effects of microstructure on the local response. Velocity interferometry was used to measure the bulk response of the free-surface velocity of the samples to monitor traditional spall tensile failure and to examine heterogeneities on the shock response due to microstructure variability from sample to sample. The shock pressure, dynamic yield strength and spall strength were determined from the measured velocity history via standard hydrodynamic approximations, while the effect of strength was explored via 1D hydrocode calculations. Electron Backscattering Diffraction, both in-plane and through-thickness, was used to relate crystallography to the presence of porosity around microstructural features such as grain boundaries and triple points. It was found that the dynamic yield strength measured from velocity histories in different samples correlated well with the crystallographic dependence reported for the dynamic yield strength in single crystals. Transgranular damage dominated in thin specimens with 230 μm grain size, where porosity appeared close to, but not exactly at, grain boundaries. However, a transition to dominant intergranular damage was observed as the grain size was reduced to 150 μm. Thick specimens (450 μm grain size) showed both modes, with intergranular damage found mostly where grains were smaller than average and the sites for preferred damage nucleation in these samples included grain boundaries and triple points. In particular, twin boundaries, especially tips of terminated twins, showed a large mismatch in surface displacements on the diagnostic surface as compared to the surrounding grains as well as a tendency for damage localization on the through-thickness sections.

Deformation by a kink mechanism in high temperature materials

Abstract A new double kink dislocation model has been developed to explain the temperature dependence of the yield stress in materials such as oxides and intermetallics that require high temperatures for plastic flow. The major variation in the free energy for the formation of a double kink nucleus with stress is the kink–kink interaction energy. However, there is also a stress dependence of the pre-exponential factor in the strain rate constitutive equation arising from kink diffusion. Numerical solution of the resulting equations shows that there are temperature regimes where the stress varies logarithmically with temperature. The model explains quantitatively the temperature dependence of the critical resolved shear stress on different slip systems for sapphire and spinel in terms of different activation energies for kink diffusion. The model can be modified to explain compositional softening in spinel by incorporating enhanced kink nucleation at cation vacancies. This changes both the pre-exponential term and the activation energy and explains why the critical resolved shear stress (CRSS) decreases as the inverse of the square of the vacancy concentration, as is observed.

Gaussian Process Time Series Model for Life Prognosis of Metallic Structures

Al 2024-T351 fatigue specimens have been modeled using a kernel-based multi-variate Gaussian Process approach. The Gaussian Process model projects fatigue affecting input variables to output crack growth by probabilistically inferring the underlying nonlinear relationship between input and output. The Gaussian Process approach not only explicitly models the uncertainty due to scatter in material microstructure parameter but it also implicitly models the loading sequence effect due to variable loading. The loading sequence effect is modeled through the Gaussian Process optimal hyperparameters by using the crack length data observed over the entire domain of spectrum loading. The performance in the crack growth prediction is evaluated for two covariance functions, a radial basis-based, anisotropic, covariance function and a neural network-based isotropic covariance function. Furthermore, the performance of different types of scaling, used to scale the input—output data space, is tested. It is found that for the radial basis-based anisotropic covariance function with normalized scaling, the prediction error is consistently lower compared to other combinations. In addition, the Gaussian Process model allows determination of the collapse load condition, which is a desirable feature for the online health monitoring and prognosis.

A multidisciplinary approach to structural health monitoring and damage prognosis of aerospace hotspots

The health monitoring and damage prognosis of aerospace hotspots is important for reducing maintenance costs and increasing in-service capacity of aging aircraft. One of the leading causes of structural failure in aerospace vehicles is fatigue damage. Based on the physical mechanism of damage nucleation and growth, a physics-based multiscale model is considered for fatigue damage assessment in metallic aircraft structures. A guided-wave based sensing approach is utilised to enable effective damage detection in a common structural hotspot: a lug joint. Finite element analysis is carried out with piezoelectric wafers bonded to the host structure and the simulated sensor signals are analysed. A damage classification strategy is developed, which integrates physically motivated time-frequency approaches with advanced stochastic modelling techniques. In particular, a variational Bayesian learning scheme is used to estimate the optimal model complexity automatically from the data, adapting the classifier for real-time use. Classification performance is studied as a function of signal-to-noise ratio and results are reported for the detection of fatigue crack damage in the lug joint. An adaptive hybrid prognosis model is proposed, which estimates the residual useful life of structural hotspots using damage condition information obtained in real-time.

Detection of Fatigue Cracks and Torque Loss in Bolted Joints

Fatigue crack growth during the service life of aging aircraft is a critical issue and monitoring of such cracks in structural hotspots is the goal of this research. This paper presents a procedure for classification and detection of cracks generated in bolted joints which are used at numerous locations in aircraft structures. Single lap bolted joints were equipped with surface mounted piezoelectric (pzt) sensors and actuators and were subjected to cyclic loading. Crack length measurements and sensor data were collected at different number of cycles and with different torque levels. A classification algorithm based on Support Vector Machines (SVMs) was used to compare signals from a healthy and damaged joint to classify fatigue damage at the bolts. The algorithm was also used to classify the amount of torque in the bolt of interest and determine if the level of torque affected the quantification and localization of the crack emanating from the bolt hole. The results show that it is easier to detect the completely loose bolt but certain changes in torque, combined with damage, can produce some non-unique classifier solutions.

Adaptive residual useful life estimation of a structural hotspot

In conventional approaches to life prognosis, damage tolerance and fatigue life predictions are obtained based on assumed structural flaws, regardless of whether they actually occur in service. Consequently, a large degree of conservatism is incorporated into structural designs due to these uncertainties. In a real time environment, keeping track of the damage growth in a complex structural component manually is quite difficult and requires automatic damage state estimation. The current research on structural health monitoring (or on-line damage state estimation) techniques offers condition-based damage state prediction and corresponding residual useful life assessment. The real-time damage state information from an on-line state estimation model can be regularly fed to a predictive model to update the residual useful life estimation in the event of a new prevailing situation. This article discusses the use of an adaptive prognosis procedure, which integrates an on-line state estimation algorithm with an off-line predictive algorithm to estimate the condition-based residual useful life of structural hotspots such as a lug joint.

Fatigue Damage Prediction in Metallic Materials Based on Multiscale Modeling

This paper addresses the problem of predicting fatigue damage accumulation in metallic materials accounting for local crystal orientation effects using a multiscale model. Single crystal plasticity is introduced to describe crystalline material behavior. At the mesoscale level, different material properties and crystal orientations are assigned to individual grains in a finite element model. Finally, an average method is used to compute the material properties at the mesoscale, which are then applied to a macroscale representative test structure. To predict fatigue damage evolution, a comprehensive fatigue damage criterion is modified to account for single crystal plasticity.

Dynamic response of materials on subnanosecond time scales, and beryllium properties for inertial confinement fusion

During the past few years, substantial progress has been made in developing experimental techniques capable of investigating the response of materials to dynamic loading on nanosecond time scales and shorter, with multiple diagnostics probing different aspects of the behavior. These relatively short time scales are scientifically interesting because plastic flow and phase changes in common materials with simple crystal structures—such as iron—may be suppressed, allowing unusual states to be induced and the dynamics of plasticity and polymorphism to be explored. Loading by laser-induced ablation can be particularly convenient: this technique has been used to impart shocks and isentropic compression waves from ∼1to200GPa in a range of elements and alloys, with diagnostics including line imaging surface velocimetry, surface displacement (framed area imaging), x-ray diffraction (single crystal and polycrystal), ellipsometry, and Raman spectroscopy. A major motivation has been the study of the properties of be...

Crystallographic effects on the fatigue fracture of copper-sapphire interfaces

Abstract Interfacial fatigue cracks were propagated in copper-sapphire bicrystals with the boundary perpendicular to the load axis and (110)Cu||(1010)Al2O3 – [001]Cu||[0001]Al2O3 to study the effect of crystallography in the fracture process. Cylindrical samples with a circumferential notch were loaded in compression–compression and compact tension specimens in tension–tension. Three interfacial cracks in the cylindrical sample nucleated simultaneously at sites corresponding to the maximum slip length, under local single slip conditions, for three of the four slip vectors expected for the <110> loading axis in the copper crystal. These cracks arrested with continued cycling, while two new cracks nucleated at 0° and 180° from [110]Cu, which also self-arrested. Then another crack started at 90° from [110]Cu and grew with an inclined front. Striations could be observed on the copper fracture surfaces; however, they did not coincide macroscopically with traces of {111} slip planes. Large areas were also relatively free of features. Elastic analysis of the anisotropic near-tip stress fields for the interfacial crack revealed that the dominant crack growth direction had the highest energy release rate, whereas the second crack direction had the minimum mode 11 mix. A model to account for the non-crystallographic striations is proposed.

Superhard Nanobuttons: Constraining Crystal Plasticity and Dealing with Extrinsic Effects at the Nanoscale

The compressive plastic strength of nanosized single-crystal metallic pillars is known to depend on their diameter D. Herein, the role of pillar height h is analyzed instead, and the suppression of the generalized crystal plasticity below a critical value h(CR) is observed. Novel in situ compression tests on regular pillars as well as nanobuttons, that is, pillars with h < h(CR), show that the latter are much harder, withstanding stresses >2 GPa. A statistical model that holds for both pillars and buttons is formulated. Owing to their superhard nature, the nanobuttons examined here underline with unprecedented resolution the extrinsic effects-often overlooked-that naturally arise during testing when the Saint-Venant assumption ceases to be accurate. The bias related to such effects is identified in the test data and removed when possible. Finally, continuous hardening is observed to occur under increasing stress level, in analogy to reports on nanoparticles. From a metrological standpoint the results expose some difficulties in nanoscale testing related to current methodology and technology. The implications of the analysis of extrinsic effects go beyond nanobuttons and extend to nano-/microelectromechanical system design and nanomechanics in general.