Application of the eigenstrain approach to predict the residual stress distribution in laser shock peened AA7050-T7451 samples

Laser Shock Peening allows the introduction of deep compressive residual stresses into metalliccomponents. It is applicable to most metal alloys used for aerospace applications. The method is relativelyexpensive in application, and therefore development studies often rely heavily on Finite Element Modellingto simulate the entire process, with a high computational cost. A different approach has been used recently,the so-called eigenstrain approach. The present study looks at the feasibility of applying the eigenstrainmethod for prediction of the residual stress in a sample that contains curved surface features. Theeigenstrain is determined from a simple geometry sample, and applied to the more complex geometry topredict the residual stress after Laser Shock Peening. In particular the prediction of residual stress at acurved edge, and for different values of material thickness, have been studied. The research hasdemonstrated that the eigenstrain approach gives promising results in predicting residual stresses whenboth the thickness and the geometry of the peened surface is altered

Effect of laser shock peening on residual stress and fatigue life of clad 2024 aluminium sheet containing scribe defects

Laser peening at a range of power densities has been applied to 2-mm-thick sheets of 2024 T351 aluminium. The induced residual stress field was measured using incremental hole drilling and synchrotron X-ray diffraction techniques. Fatigue samples were subjected to identical laser peening treatments followed by scribing at the peen location to introduce stress concentrations, after which they were fatigue tested. The residual stresses were found to be non-biaxial: orthogonal to the peen line they were tensile at the surface, moving into the desired compression with increased depth. Regions of peen spot overlap were associated with large compression strains; the centre of the peen spot remaining tensile. Fatigue lives showed moderate improvement over the life of unpeened samples for 50 μm deep scribes, and slight improvement for samples with 150 μm scribes. Use of the residual stress intensity Kresid approach to calculate fatigue life improvement arising from peening was unsuccessful at predicting the relative effects of the different peening treatments. Possible reasons for this are explored

An evaluation of the effect of non-setting calcium hydroxide on human dentine: a pilot study.

AIM: To evaluate the effect of non-setting calcium hydroxide (NSCH) on the hardness and elastic modulus of dentine from extracted permanent premolar human teeth. METHODS: 30 freshly extracted single rooted human premolar teeth were decoronated and the roots then sectioned longitudinally into equal halves. In the experimental group a thin layer of NSCH was applied whilst the control group had no medicament. After 1, 3 and 6 months, nanoindentation was used to assess dentine hardness and the modulus of elasticity. Scanning Electron Microscopy (SEM) was used to visualize the depth of penetration of NSCH into the dentinal tubules. RESULTS: SEM images showed that there were no structural changes in the dentine slabs that had NSCH application after 1, 3 or even 6 months. However, penetration of NSCH into the dentine tubules was seen at both 3 and 6 months with a significant reduction in the hardness of dentine observed at 3 (p<0.02) and 6 months (p<0.01). The modulus of elasticity was significantly lower (p<0.01) at 6 months. CONCLUSION: It appears that there is a significant reduction in the hardness of dentine with increasing periods of calcium hydroxide application. Prolonged application of NSCH could have a detrimental effect on dentine, making the dentine more prone to fracture

All-ceramic and porcelain-fused-to-metal fixed partial dentures: a comparative study by 2D finite element analyses

All-ceramic fixed partial dentures (FPDs) have an esthetic approach for oral rehabilitation. However, metal-ceramic FPDs are best indicated in the posterior area where the follow-up studies found a lower failure rate. This 2D finite element study compared the stress distribution on 3-unit all-ceramic and metal-ceramic FPDs and identified the areas of major risk of failure. Three FPD models were designed: (1) metal-ceramic FPD; (2) All-ceramic FPD with the veneering porcelain on the occlusal and cervical surface of the abutment tooth; (3) All-ceramic FPD with the veneering porcelain only on the occlusal surface. A 100 N load was applied in an area of 0.5 mm² on the working cusps, following these simulations: (1) on the abutment teeth and the pontic; (2) only on the abutment teeth; and (3) only on the pontic. Relative to the maximum stress values found for the physiological load, all-ceramic FPD with only occlusal veneering porcelain produced the lowest stress value (220 MPa), followed by all-ceramic FPD with cervical veneering porcelain (322 MPa) and metal-ceramic FPD (387 MPa). The stress distribution of the load applied on the abutments was significantly better compared to the other two load simulations. The highest principal stress values were low and limited in a small area for the three types of models under this load. When the load was applied on the pontic, the highest stress values appeared on the connector areas between the abutments and pontic. In conclusion, the best stress values and distribution were found for the all-ceramic FPD with the veneering porcelain only on the occlusal surface. However, in under clinical conditions, fatigue conditions and restoration defects must be considered

COMPUTER METHODS AND PROGRAMS IN BIOMEDICINE

The finite element method (FEM) is applied for studying the hardness (H) and yield strength (Y) of dentin subjected to a nano-indentation process. The nanoindentation experiments were simulated with the ABAQUS finite element software package. This test, performed with a spherical indenter, was simulated by axisymmetric finite element analysis. The load versus displacement was calculated during loading-unloading sequence for different elastic modulus (E) and yield strength. Hardness and maximum principal compressive and tensile stresses were plotted for different elastic modulus depending on yield strength. The dentin was assumed to be isotropic, homogenous and elasto-ptastic. The theoretical results outlined in this study were compared with the experimental works reported in the literature and then hardness and yield strength of dentin was estimated. (c) 2004 Elsevier Ireland Ltd. All rights reserved

Stress analysis in a post-restored tooth utilizing the finite element method

This study utilized the finite element method (FEM) to predict distribution of stresses in dentin of an endodontically treated tooth, restored with cast post and cores. For this investigation an axisymmetric model of a maxillary second pre-molar that included an alveolar bone was analysed. The three tooth models evaluated were Ti-Ti alloy, NiCr-AuPd alloy and Ti-NiCr alloy as post-material and crown material with porcelain. A load of 200 N at an angle of 45degrees to the longitudinal axis was applied on the occlusal margin of each model. The tooth was assumed isotropic, homogenous and elastic. The author prepared a calculation program using fortran 77. Investigation of the stress distributions was made in five regions; namely bottom of post, top of post, cole, metal-cement interface and metal-porcelain interface. The distributions of radial and axial stresses were plotted with length of radial

Residual stresses induced by laser peening of thin aluminium plates

Laser shock peening offers potential advantages over conventional peen technologies in terms of the depth of the residual stresses that can be induced, and improvements in surface roughness. In this study the application of laser peening to thin aluminium plates such as are used in aerospace applications is investigated. Peening of thin plates presents challenges in balancing the peen intensity to prevent overpeening that will actually lower the stress field. Strain profiles for different laser peening parameters were obtained using synchrotron X-ray diffraction at the ESRF, France. Results are presented and discussed of the residual strain profiles in terms of the laser power density and the number of peen passes. When the power density and number of passes are increased the compressive strain magnitudes are also increased, as has been observed in previous studies. However, the strain components longitudinal and transverse to the peen line are not identical to each other, with the transverse component being much less compressive

Determination of multiple near-surface residual stress components in laser peened aluminum alloy via the contour method

In this study, residual stress fields, including the near-surface residual stresses, were determined for an Al7050-T7451 sample after laser peening. The contour method was applied to measure one component of the residual stress, and the relaxed stresses on the cut surfaces were then measured by X-ray diffraction. This allowed calculation of the three orthogonal stress components using the superposition principle. The near-surface results were validated with results from incremental hole drilling and conventional X-ray diffraction. The results demonstrate that multiple residual stress components can be determined using a combination of the contour method and another technique. If the measured stress components are congruent with the principal stress axes in the sample, then this allows for determination of the complete stress tensor

COMPUTERS & STRUCTURES

In this study, it was proposed that the residual stresses within steel bars after quenching in water from 600 degreesC could be calculated by using the finite element method (FEM) and an artificial neural network (ANN) algorithm. Three modelled cylindrical specimens of AISI 1020 steel were heated and then quenched in water. Using FEM, temperature distribution with time and thermal residual stress values in the samples were calculated after cooling. The analysis was extended to elastic-plastic deformation during the quenching of steel cylinders of various diameters. The calculated temperature and thermal residual stress values were used in training a multi-layer, feed forward, back propagation ANN algorithm. The results obtained via the ANN algorithm method have been compared with the FEM results. Comparison showed good agreement. (C) 2002 Elsevier Science Ltd. All rights reserved