J. Eng. Mater. Technol. 2003;125(4):345. doi:10.1115/1.1605772.
Topics: Durability
Commentary by Dr. Valentin Fuster


J. Eng. Mater. Technol. 2003;125(4):346-352. doi:10.1115/1.1605107.

Recent studies have shown that Pt-aluminide—a common bond coat material in thermal barrier coatings—undergoes martensitic transformations during thermal cycling. The transformations are associated with both large transformation strain and a strain hysteresis, leading to accumulation of a mismatch strain. Thermal barrier systems based on Pt-aluminide bond coats are susceptible to interfacial morphological instabilities. In this study, we investigate how the cyclic martensitic transformation influences the morphology. Two key results are: (i) the morphological instabilities are highly sensitive to the thermo-mechanical properties of the substrate due to the martensitic transformation; (ii) the hysteresis associated with cyclic martensitic transformation cannot drive the morphological instabilities; the strains associated with the formation of the thermally grown oxide do.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(4):353-360. doi:10.1115/1.1605108.

The complex fracture behavior of a cross-ply composite cantilever beam with artificially embedded delamination is investigated analytically, numerically, and experimentally. The analysis of the cantilever beam is divided into two geometric configurations: the global bending of the undelaminated cantilever, and the local buckling of the delaminated part. A finite element model developed in ANSYS is used to concurrently analyze the effects of contact zone and delamination in the aforementioned asymmetrically loaded structure. The obtained experimental data are correlated and compared with the findings of the FEM simulations. All numerical, analytical, and experimental results illustrate that the fracture behavior of the laminate cantilever beam is dominated by mode II, mainly due to the effect of a large contact zone. The latter is determined by geometric and loading parameters. The dominance of mode II over mode I, leads to the initiation and propagation of an interfacial crack rather than an intralayer one. Furthermore, experimental evidence indicates that crack kinking during propagation depends on the architecture of the specimens.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(4):361-367. doi:10.1115/1.1605109.

MEMS (MicroElectroMechanical Systems) are composed of thin films and composite nanomaterials. Although the mechanical properties of their constituent materials play an important role in controlling their quality, reliability, and lifetime, they are often found to be different from their bulk counterparts. In this paper, low-k porous silica thin films spin coated on silicon substrates are studied. The roughness of spin-on coated porous silica films is analyzed with in-situ imaging and their mechanical properties are determined using nanoindentation. A Berkovich type nanoindenter, of a 142.3 deg total included angle, is used and continuous measurements of force and displacements are acquired. It is shown, that the measured results of hardness and Young’s modulus of these films depend on penetration depth. Furthermore, the film’s mechanical properties are influenced by the properties of the substrate, and the reproduction of the force versus displacement curves depends on the quality of the thin film. The hardness of the studied low-k spin coated silica thin film is measured as 0.35∼0.41 GPa and the Young’s modulus is determined as 2.74∼2.94 GPa.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(4):368-371. doi:10.1115/1.1605110.

In this paper, the dynamics of the spatial impact of a slender beam is analyzed. The equations of motion are calculated using Kane’s impact method. The generalized momentum and generalized impulse of the beam are considered to find the equations of motion of the beam. The frictional phenomenon at the contact point is analyzed. For the case of impact without slipping, it is used the assumption that the tangential component of the velocity of separation is null. In the case with slipping, the tangential impulse (at the plane of impact) is computed. The sliding direction after impact is calculated. A simulation of the impact of beam with a surface is developed and the velocity of separation, force of impact and kinetic energy of the beam after impact are studied for different incident angles of the beam. The incident angle is varied from 0 deg to 57 deg. The results are function of the incident angle of impact.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(4):372-377. doi:10.1115/1.1605111.

A 1200 W diode laser was used to modify the surface properties of a fully-sintered ferrous PM material. Two modifications were investigated; transformation hardening of selected areas and sealing of surface porosity by re-melting. The diode laser was used because the beam footprint is large (5×0.5 mm) and the wavelength short (0.94 μm). Processed samples were examined using metallographic and hardness testing techniques. Results indicated that hardness comparable to induction hardening could be achieved and re-melting could be controlled to seal porosity. Further work now needs to be undertaken to convert this demonstrated potential into a commercial reality.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(4):378-384. doi:10.1115/1.1605112.

Although the abrasive reinforcement in MMCs primarily controls their machining behavior, the properties of the matrix also exert an influence. A 1200 W diode laser was used, due to the large footprint (5×0.3 mm) and the short wavelength (0.94 μm) to pre-treat a 2618 (18% SiC) alloy. The laser heating and self-quenching of the material modified the matrix properties. Machining performance was then assessed by measuring tool wear and edge condition, cutting forces, surface finish, and sub-surface damage. Results indicated that pre-treatment gave less wear, lower forces, and less sub-surface damage although abrasion remained the primary wear mechanism.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(4):385-393. doi:10.1115/1.1605771.

Structural designs for composite laminated systems can be optimized for a fail-safe in-service performance by introducing the built-in cumulative-damage-indicators for the progressive degradation of material properties. This design methodology is based on the concepts of the characteristic failure signature (CFS), cumulative-damage states and a load-drop sequence that characterize the stress-strain response and progressive accumulation of damage. The cumulative damage mechanics is based on the three-dimensional laminate analysis that is used to predict nonlinear response of composites, accumulation of damage and failure behavior. An earlier-developed nonlinear analysis involves an incremental formulation that couples the three-dimensional laminate analysis with a progressive ply-failure methodology, which has been tested in the World-Wide Exercise on Composites Failure Theories. The failure signatures are shown to have unique “safety features” that depend on the ply stacking sequence and predominant loading. To refine the analysis of micromechanical damage a model for the macro-to-micro coupling is introduced. Various examples of failure envelopes, characteristic failure signatures, a safety criterion and the “safe” CFSs that lead to the desired controlled failures are discussed for symmetric balanced laminates.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(4):394-401. doi:10.1115/1.1605113.

Experimental investigations were carried out on unidirectional Graphite/Epoxy laminate samples under dynamic compression loading using a modified Split Hopkinson Pressure Bar. High strain rate testing was carried out at room and elevated temperatures. 30 layered graphite/epoxy unidirectional laminates made using DA 4518U unidirectional prepregs system were fabricated. Tests were carried out on samples at room, 51.7°C, 121.1°C, and 190.6°C temperatures. Additional high strain rate tests were conducted on samples that were subjected to moist/freeze conditioning for 42 days. Failure modes were studied through scanning electron microscopy. Results of the study indicated plasticizing of matrix which was reflected through increased ductility of the samples as well as reduced slope of the stress-strain curves with the increase in temperature. Similar effect was evident in the samples that were subjected to moist/freeze conditioning.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(4):402-405. doi:10.1115/1.1605114.

The fatigue strength of two types of FRP/metal adhesive joints at low temperature, a double lap joint and an embedded joint, was evaluated analytically and experimentally. First, the stress singularity parameters of the delamination edges under mechanical and thermal loadings were analyzed by FEM for various delamination lengths. The delamination propagation rate of the double lap joint under mechanical cyclic loadings at room temperature was measured. Using the relationship between the measured propagation rates and the analyzed ranges of stress singularity intensity, we estimated the fatigue strength of the embedded joint, which coincided well with the measured one. Second, we developed an evaluation method that separates the effects of temperature on fatigue strength into two effects: thermal residual stress and low temperature. Third, the fatigue strengths of the double lap joints were measured for various mean stresses. Fatigue limit of adhesive joints was experimentally measured and compared with analytical intensity of stress singularity. A method for evaluating the fatigue strength of adhesive joints by taking mean stress into account was developed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(4):406-411. doi:10.1115/1.1605115.

The continuous indentation test, which applies an indentation load to a material and records the indentation depth, yields indentation tensile properties whose accuracy can vary depending on such experimental parameters as number of unloadings, unloading ratio, maximum depth ratio and indenter radius. The Taguchi method was used to quantify their effects and to determine their optimum values. Using signal-to-noise ratio calculated from the error in the indentation tensile properties, the criterions and the optimum values for the experimental parameters were presented. The indentation tensile properties evaluated with the optimum parameters were in better agreement with the tensile properties.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(4):412-417. doi:10.1115/1.1605116.

A damage model developed by Johnson and Holmquist is implemented into a dynamic finite element code. This is then used to study the effect of grading of the phenomenological damage parameters on the propagation of damage through the material. The numerical results for two one-dimensional example problems with different boundary conditions are presented, wherein the effect of a gradient in the intact strength of the material on damage propagation is studied. The results show that introducing different strength gradients can alter the location of the site of maximum damage. This may have important implications in the design of impact resistant materials and structures.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2003;125(4):418-425. doi:10.1115/1.1605117.

A previously published computational model of textile composites known as the Binary Model is generalized to allow systematic study of the effects of mesh refinement. Calculations using different meshing orders show that predictions of local strains are mesh independent when the strains are averaged over gauge volumes whose dimensions are greater than or equal to approximately half the width dimensions of a single tow. Strains averaged over such gauges are favored for use in failure criteria for predicting various mechanisms of failure in a textile composite, including transverse cracking within tows, kink band failure in compression, tensile tow rupture, and shear failure. For the highest order representations (infinitely dense meshes), the generalized formulation of the Binary Model necessarily approaches conventional finite element meshing strategies for textile composites in its predictions. However, the work reported here implies that usefully accurate predictions of spatially averaged strains can be obtained even at the lowest level of mesh refinement. This preserves great simplicity in the model set-up and rapid computation for relatively large features of structural components. Calculations for some textile structures provide insight into the strength or relative absence of textile effects in local strains for different loading configurations.

Commentary by Dr. Valentin Fuster

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