Development Of Residual Strains Research Articles

Measurements of residual strains and stresses in laser formed plates of Ti-6Al-4V using synchrotron X-ray diffraction

The synchrotron X-ray diffraction technique is used to examine the development of residual strains in laser formed plates of the α-β titanium alloy Ti-6Al-4V. The following conditions were studied: (A) one pass of the laser beam; (B) three passes with intervals of 10 s between scans; (C) three passes but with 90 s intervals; (D) five passes with 10 s intervals; and (E) five passes with 90 s intervals. Measurements were made in both the longitudinal and transverse directions using the {1013} and {1120} lattice planes. The sample with five passes and 90 s intervals exhibited the largest residual stresses in the longitudinal direction, and the sample with a single pass had the lowest. The width of the zone of tensile strain was largest in the sample with five passes at 10 s intervals; next was the sample with three passes at 10 s intervals. Optical microscopy and microhardness tests demonstrated that the temperature-time history associated with condition D was sufficient to partially anneal the material, and hence low strains were observed. Needle probe distortion measurements showed that condition D resulted in the largest distortion angle (~7.0°) while the smallest distortion (~0.4°) was observed after condition A.

Measurement of Stress in Phosphated-Iron Oxide Layers by In-Situ Diffraction of Synchrotron Radiation

In the present work the development of residual strains in the iron-oxide layers growing on α-Fe and phosphated α-Fe at 400°C in artificial air at 1 atm was investigated by X-ray diffraction of synchrotron radiation, both in-situ during oxide growth and at room temperature after cooling. The oxidation kinetics of α-Fe and phosphated α-Fe at 400°C and the microstructural state of the oxide layers are first presented. Then a detailed study of the residual strains in the oxides layers is undertaken. Correlations between the residual stresses measurements and the successive parabolic oxidation stages for phosphated α-iron are established. It leads to a better understanding of the oxidation behaviour.

Cure Cycle Effect on Composite Structures Manufactured by Resin Transfer Molding

An experimental investigation was conducted of the effect of curing cycle on the development of material properties, residual strains and stresses in composite parts during the resin transfer molding (RTM) process. The material investigated consisted of AS4 carbon fibers as the preform anda three-part epoxy system. Unidirectional and crossply carbon/epoxy laminates were prepared by the RTM process in aluminum molds. Several different curing cycles were designed by changing the peak cure temperature andheat up rate basedon a cure kinetic model. Strains were measured in the composite laminates during curing using embedded fiber optic strain sensors andelectrical resistance strain gages. It was foundthat significant strain was developed by interference between composite and mold during cure, resulting in constraint-induced strain. The magnitude of residual stresses was also assessedby measuring the warpage curvature of asymmetric cross ply laminates. The data showed that the residual stress was significantly dependent on the cure cycle andthe interference.

Intergranular strains and plastic deformation of an austenitic stainless steel

Intergranular strains due to tensile plastic deformation were investigated in a sheet material of austenitic stainless steel. The objective was to study the development of residual intergranular strains in samples unloaded from the intermediate and large plastic deformation regimes for which few theoretical and experimental studies were available. By using neutron diffraction, residual lattice strain distribution as a function of sample direction was mapped for a number of crystallographic planes. Deformation microstructures were examined by both transmission electron microscopy and the electron back scattering pattern technique. Residual intergranular strains were observed in samples deformed significantly beyond the elastic limit and the strains varied with sample directions as well as the amount of applied plastic strain. In addition, a different tendency of intergranular strain evolution was observed after large plastic deformation, which could be attributed to the change of dominant plastic deformation mode from slip to mechanical twinning. The results are discussed based on the observed deformation microstructure studies.

Development of thermal residual strains in a single sided composite patch

The use of composite patching to reinforce aircraft structure is on the increase as it offers advantages over those of rivet fastened metal patches in application. The objective of this research is to determine the thermal residual strains in graphite/epoxy composites on Al-2024 T3 substrate. To realize this, four specimens were produced, of which one was instrumented by bonding several strain gauges on the patch and Al-substrate. Thermal and thermal residual strains were measured as a function of operating temperatures via those gauges.

Biomechanical consequences of an isolated overload on the human vertebral body.

The biomechanical consequences of an isolated overload to the vertebral body may play a role in the etiology of vertebral fracture. In this context, we quantified residual strains and reductions in stiffness and ultimate load when vertebral bodies were loaded to various levels beyond the elastic regimen and related these properties to the externally applied strain and bone density. Twenty-three vertebral bodies (T11-L4, from 23 cadavers aged 20-90 years) were loaded once in compression to a randomized nominal strain level between 0.37 and 4.5%, unloaded, and then reloaded to 10% strain. Residual strains of up to 1.36% developed on unloading and depended on the applied strain (r2=0.85) but not on density (p = 0.25). Percentage reductions in stiffness and ultimate load of up to 83.7 and 52.5%, respectively, depended on both applied strain (r2 = 0.90 and r2 = 0.32, respectively) and density (r2 = 0.23 and r2 = 0.22, respectively). Development of residual strains is indicative of permanent deformations, whereas percentage reductions in stiffness are direct measures of effective mechanical damage. These results therefore demonstrate that substantial mechanical damage-which is not visible from radiographs-can develop in the vertebral body after isolated overloads, as well as subtle but significant permanent deformations. This behavior is similar to that observed previously for cylindrical cores of trabecular bone. Taken together, these findings indicate that the damage behavior of the lumbar and lower thoracic vertebral body is dominated by the trabecular bone and may be an important factor in the etiology of vertebral fracture.

On the oxidation ofα-Fe andε-Fe2 N1−z II. Residual strains and blisters in the oxide layer

The development of residual strains in the iron-oxide layers growing on α-Fe and ε-Fe2N1−z at 673 K in O2 at 1 atm was investigated by X-ray diffraction at room temperature. After correction for thermal-strain development due to cooling after oxidation, it was found that tensile growth strains occur in magnetite and compressive growth strains occur in hematite. The growth strains in the oxides on α-Fe are (in absolute sense) 2–3 times as large as those in the oxides on ε-Fe2N1−z. Buckling of the oxide layer occurs in the case of an α-Fe substrate, which is attributed mainly to relaxation of the growth strains in magnetite and hematite. Thermal-strain development during cooling enhances the tendency for buckling. Buckling is not observed for oxide layers on ε-Fe2N1−z, which could be due to the smaller values of absolute strain in the oxide layer on ε-nitride. The absolute values of the growth strains in the oxide layer on ε-nitride being smaller is attributed to microstructural changes in the nitride layer during oxidation.