Fracture toughness of selectively reinforced Al2124 alloy: precrack tip in the aluminium alloy side

Abstract

The fracture toughness of Al2124/Al2124+SiC bimaterials is affected by thermal residual stresses, elastic/plastic mismatch, precrack tip position, and failure mechanism. When the precrack tip is in the Al2124 side, final catastrophic failure occurs when ductile fracture of the Al2124 layer between the precrack tip and the composite side takes place, followed by fracture of the composite layer. For a precrack tip 2·0 mm from the interface, K _Q(5%) values are lower than the 'Al2124 only' value due to the near crack tip tensile residual stresses and higher stress triaxiality within the Al alloy ligament. At 0·5 mm from the interface, K _Q(5%) values increase and are usually as high as the 'Al2124 only' value due to the stronger shielding of the elastic/plastic mismatch. If the precrack tip is 2·0 mm from the interface, K _crit values of the bimaterial are higher than the 'Al2124 only' value and this is deduced to be due to the elastic/plastic mismatch shielding. At 0·5 mm from the interface, K _crit values are reduced because both the near tip tensile residual stress is higher and stress triaxiality levels of the ductile ligament are higher, although the elastic/plastic mismatch shielding is also higher at this position.     

Residual stress magnitudes and related properties in quenched aluminium alloys

Abstract

The surface and through thickness residual stress magnitudes present in heat treated high strength aluminium alloy components are frequently reported to exceed the uniaxial yield stress of small specimens of the same alloy measured immediately after quenching. In thick section plate and forgings, it is proposed that these high residual stress magnitudes are a consequence of hardening precipitation that occurs during quenching which allows a greater elastic stress to be supported. To investigate this theory, a Jominy end quench technique is used to determine the hardness of aluminium alloys 7010, 7175 and 5083 as a function of distance from the quenched end. Cooling curves have been measured for Jominy end quench specimens using deeply buried thermocouples and are compared with finite element model predictions. Tensile properties are also determined for small specimens quenched into cold and boiling water. Vickers hardness and X-ray diffraction residual stress measurements are undertaken on specimens of varying size acting as a comparison with the Jominy results. These results in combination with optical and electron microscopy data do suggest that low temperature rather than high temperature precipitation during the quench leads to increased as quenched mechanical strengths, with the consequence that less quench sensitive alloys will support higher residual stress magnitudes as section thicknesses increase.     

Thermal Residual Stresses in Adhesively Bonded In-plane Functionally Graded Clamped Plates Subjected to an Edge Heat Flux

In this study we have carried out the thermal residual stress analyses of adhesively bonded functionally graded clamped plates for different edge heat fluxes. The material properties of the functionally graded plates were assumed to vary with a power law along an in-plane direction not through the plate thickness direction. The transient heat conduction and Navier equations describing the two-dimensional thermo-elastic problem were discretized using the finite-difference method, and the set of linear equations was solved using the pseudo singular value method. The plate material properties near the interfaces played an important role in the interfacial adhesive stresses. The compositional gradient affected considerably both in-plane temperature distributions and heat transfer periods. The type of in-plane heat flux had only a minor effect on the temperature profiles but affected both the temperature levels and heat transfer period. Both plates undergo considerable compressive normal strains and stresses, but shear strains were more effective. Peak equivalent strains were observed for a constant heat flux and plates with a metal-rich composition. The compositional gradient and direction played important role in the profiles and levels of normal, shear and equivalent stresses as well as strains. The equivalent stress and strains concentrated along the free edges of the adhesive layer. The adhesive layer experienced a considerable distortional deformation rather than volumetric deformation. The equivalent stress exhibited small changes through the adhesive thickness and along the overlap length. The equivalent stress remained uniform in a large region of the overlap length and increased to a peak level around the free edges of the first plate–adhesive interface, whereas it increased to a peak level in a large region of the overlap length from a minimum level around the free edges of the second plate–adhesive interface. The strains and equivalent strains were higher for a metal-rich material composition. The direction of the material composition of the plates affected both stress and strain levels; thus, the CM–CM and CM–MC plates exhibited lower strain and stress levels than those in the MC–CM and MC–MC plates. However, only the adhesively bonded CM–MC plate configuration could achieve the lowest deformations and stresses in both plates and adhesive layer.     

On the Measurement of Strain in Coatings Formed on a Wrinkled Elastic Substrate

Highly stressed coatings, such as those formedby oxidation can, on occasion, wrinkle. Such wrinklinghas been suggested as a mode of deformation by which theoverall strain energy in a compressively stressed coating can be reduced. One of the consequencesof wrinkling is that the strain in the coating does notremain independent of position, but rather varies overthe wavelength of the wrinkling. The strain variation caused by sinusoidal wrinkling iscalculated using finite-element methods and the effectson both photostimulated Cr³⁺ luminescencepiezospectroscopy measurements and X-ray measurementscalculated. Wrinkling is shown to decrease theelastic-strain-energy density in the coating. A directmeasure of the decrease is the shift in the R2Cr³⁺ luminescence line and the X-raydiffraction peaks. Wrinkling of a compressive coating also causes stressesto be created perpendicular to the coating-substrateinterface, tensile at the crests in the wrinkles, andcompressive stress at the troughs.     

A Theoretical Improved Adhesively Bonded Bi-material Beam Model for FRP–RC Hybrid Beams

In this paper we present an improved bi-material beam theory with adhesive interface, which has been applied to the study of the interfacial behavior in a concrete beam reinforced by an externally bonded fibre reinforced polymer (FRP) plate. The work explicitly considers the interfacial slip effect on the structural performance by including the effect of adherend shear deformations. This new method needs only one differential equation to determine both shear and normal interfacial stress whereas the others solutions in the literature need two differential equations. Compared with previously published analytical results, this one improves the accuracy of predicting the interfacial stresses and the solution is in a closed form. This research is helpful in the understanding of the mechanical behavior of the interface and design of FRP–reinforced concrete (RC) hybrid beams.     

Internal stresses and adhesion of thin silicon oxide coatings on poly(ethylene terephthalate)

The effect of internal stresses on the cohesion and adhesion of a thin silicon oxide (SiO_x) oxygen-barrier coating, evaporated on a poly(ethylene terephthalate) (PET) film substrate was studied. Internal stresses were generated during annealing in the temperature range for recrystallization of the PET,during calendering in a multilayer structure where two SiO_x /PET films were laminated together with a polypropylene film, and during long-term thermal aging below the glass transition temperature of the polymer. The cohesion of the coating and its adhesion to the polymer substrate were derived from fragmentation tests, in which the failure of the oxide coating was analyzed as a function of the applied stress during uniaxial tensile loading of the substrate. The intrinsic coating strength at critical length and the interfacial shear strength were found to be equal to 1350 MPa and 73 MPa, respectively. It was found that none of the thermal treatments investigated altered the interfacial interactions. Rather, these treatments induced shrinkage of the PET substrate, which increased the coating internal compressive stress and the SiO_x /PET interfacial shear strength. A linear relationship between the SiO_x /PET interfacial shear strength and the coating internal stress was determined from a stress transfer analysis. The coefficient of this linear relationship, equal to-1.34 · h _c/l _c, where h _c is coating thickness and l _c is the critical stress transfer length, reproduces the experimental data with good accuracy.     

Hydrodynamic analysis of rough curved pivoted porous slider bearings with couple stress fluid

Abstract

The purpose of this paper is to study the effect of surface roughness on the performance of curved pivoted porous slider bearings lubricated with couple stress fluid. The modified B–J slip boundary condition is utilised at the porous/fluid film interface to derive the Reynolds type equation for the problem under consideration. To mathematically model the surface roughness due to non-uniform rubbing of bearing surfaces, a stochastic random variable with non-zero mean, variance and skewness is considered. The closed form solution is obtained for the averaged Reynolds equation, and the compact expressions for the mean fluid film pressure mean load carrying capacity, frictional force and the centre of pressure are obtained. The numerical computations of the results show the improved performance due to the couplestresses and the presence of negatively skewed surface roughness. However, the presence of porous facing and positively skewed surface roughness affects the performance of the pivoted porous slider bearing.     

Generation of compressive residual stresses by high speed water quenching

Abstract

According to the literature, rapid water quenching can create compressive residual stresses (RSs) near the surface and thereby produce a significant increase in the fatigue limit. This technique is called 'intensive quenching'. In the present paper, some results from a research project will be presented. The project was initiated by the technical committee 'quenching' of the German Heat Treatment Association to deal critically with this issue.

The focus of this paper is on the influence of the martensite transformation on RS generation. A process window was determined, within which one needs to quench fast enough, to get the surface temperature below the M_S temperature, before the maximum temperature gradient in a cylinder is reached, in order to get compressive RSs on the surface.     

A novel mathematical procedure to evaluate the effects of surface topography on the interfacial state of stress. Part II. Verification of the method for scarf interfaces

A mathematical procedure was developed to utilize the complementary energy method, by minimization, in order to obtain an approximate analytical solution to the 3D stress distributions in bonded interfaces of dissimilar materials. The stress solutions obtained predict the stress jumps at the interfaces, which cannot be captured by the current FEA methods. As a novel method, the penalty function is used to enforce the displacement boundary conditions at the interfaces. Furthermore, the mathematical procedure developed enables the integration of different interfacial topographies into the solution procedure. In order to incorporate the effects of surface topography, the interface is expressed as a general surface in Cartesian coordinates, i.e. F(x, y, z) = 0. In this paper, the scarf interface problem, i.e. y = x/2 surface is considered for verification of the method by comparison with finite element analysis (FEA) results. Comparison of the results reveals our new mathematical procedure to be a promising and efficient method for optimizing interface topographies.