0

IN THIS ISSUE

Research Papers

J. Eng. Mater. Technol. 2008;130(4):041001-041001-7. doi:10.1115/1.2969250.

Large displacement micro-indentation tests have been performed on various polymeric solids to measure the plastic properties. Cylindrical flat-ended indenters with diameter in the range of $10–90 μm$ are mostly used. The mechanism of large-strain indentation has been examined with optical microscopy and finite element simulations. Results show that under a flat-tipped indenter, the material can quickly reach a fully plastic state. The size (diameter) of the plastic zone is constant in large-strain regions and unaffected by the exact tip profile (flat, spherical, and conical). The indentation stress-displacement curve at large strains is linear as a result of the steady-state plastic flow, from which the mean indentation pressure, a measure of yield strength, can be readily extrapolated. The indentation stress-displacement response is independent of the indenter diameters but strongly dependent on the strain-hardening behavior of the material and the friction between a material and an indenter. Compared with other shaped indenters, the flat-ended indenter requires the least penetration depth in order to probe the plastic properties of a material or structure.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(4):041002-041002-6. doi:10.1115/1.2969253.

The fatigue behavior and residual strength of postimpacted GLARE 4-3/2, GLARE 5-2/1, and monolithic aluminum 2024-T3 alloy were investigated experimentally. Drop-weight impact was applied at a variety of energy levels to inflict a barely visible impact damage, a clearly visible impact damage, and a penetration damage. After the impact test, constant-amplitude tension-tension fatigue was done to delineate the modes of damage initiation and growth and the effect of damage on fatigue life and residual strength. The results showed that GLARE laminates exhibit superior postimpact fatigue durability when compared with the monolithic 2024-T3 aluminum alloy.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(4):041003-041003-9. doi:10.1115/1.2969256.

Laser welding of advanced high strength steels for fabrication of tailor welded blanks is of increasing interest for continued improvements in vehicle performance and safety without an increase in weight. Experimental results have shown that formability of welded dual-phase (DP) steels is significantly reduced by the formation of a softened region in the heat-affected zone (HAZ). In this study, a finite element simulation of welded DP980 samples undergoing transverse uniaxial tensile testing was used to evaluate the effects of soft zone width and strength on formability characteristics. Both the strength and the ductility of laser welded blanks decreased compared with those of unwelded blanks due to the formation of a softened outer-HAZ, where strain localization and final fracture occurred during tensile testing. The magnitude of softening and the width of the HAZ depend on the laser specific energy. It was observed from tensile test experiments and numerical simulations that both a decrease in strength and an increase in width of the softened HAZ were responsible for a decrease in the overall strength and ductility of the welded blanks.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(4):041004-041004-9. doi:10.1115/1.2969246.

Limiting dome height (LDH) tests were used to evaluate the formability of laser butt welded blanks of the dual phase 980 steel in comparison with the base metal. Two different lasers were used: diode and Nd:YAG, giving a wide range of welding thermal cycles. A sharp decrease in LDH was observed in the welded specimens due to the formation of a softened zone in the outer heat affected zone. Softened zone characteristics were correlated with the LDH. Larger softened zones led to a larger reduction in the LDH. The welding orientation relative to the rolling direction or to the punch surface did not influence the formability, as the softened zone dominated the formability behavior. It was observed that in both uniaxial and biaxial strain tests, the fracture occurred in the softened zone of the welded samples consistently slightly farther out from the weld centerline than in the location of the minimum hardness.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(4):041005-041005-16. doi:10.1115/1.2969247.

The polymer spray-on foam insulation used on NASA’s Space Shuttle external fuel tank is analyzed via the high-fidelity generalized method of cells micromechanical model. This model has been enhanced to include internal pore pressure, which is applied as a boundary condition on the internal faces of the foam pores. The pore pressure arises due to both ideal gas expansion during a temperature change as well as outgassing of species from the foam polymer material. Material creep and elastic stiffening are also incorporated via appropriate constitutive models. Due to the lack of reliable properties for the in situ foam polymer material, these parameters are backed out from foam thermomechanical test data. Parametric studies of the effects of key variables (both property-related and microstructural) are presented as is a comparison of model predictions for the thermal expansion behavior of the foam with experimental data.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(4):041006-041006-7. doi:10.1115/1.2969251.

Producing composites from natural fibers is known to be common. These fibers benefit from their mechanical performances, low density, and their biodegradability. However, it is necessary for the fibers to form adhesion in the matrix. Therefore, it is necessary to apply a chemical process to the surface of the fibers. In this study, four different processes in conventional and ultrasonic energies were applied on luffa cylindrical fibers. At the end of the application, a composite structure was formed on the fibers that were obtained by using unsaturated polyester resin. The changes in the characteristics of the composite structure were recorded by mechanical tests, Fourier transform infrared, X-ray diffractometer, and their morphological characteristics by means of scanning electron microscopy. Considering all the results, formic acid and acetic acid process results were found to adequately modify the fiber surfaces.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(4):041007-041007-10. doi:10.1115/1.2975233.

A process to improve formability and spring-back was developed for AA5xxx-H temper sheets based on the surface friction stir (SFS) method. In the SFS method, a rotating probe stirs the sheet surface so that material flow and heat, which result from plastic deformation and friction, change the microstructure and macroscopic mechanical properties of the stirred zone and therefore, ultimately, the formability and spring-back performances of the whole sheet. When applied to AA5052-H32 sheets, the process improved formability and spring-back, as experimentally and numerically confirmed in the limit dome height and unconstrained bending tests.

Topics: Friction , Probes , Springs
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(4):041008-041008-9. doi:10.1115/1.2969255.

The prediction of fatigue life for metallic components subjected to complex multiaxial stress states is a challenging aspect in design. Equivalent-stress approaches often work reasonably well for uniaxial and proportional load paths; however, the analysis of nonproportional load paths brings forth complexities, such as the identification of cycles, definition of mean stresses, and phase shifts, that the equivalent-stress approaches have difficulties in modeling. Shear-stress based critical-plane approaches, which consider the orientation of the plane on which the crack is assumed to nucleate, have shown better success in correlating experimental results for a broader variety of load paths than equivalent-stress models. However, while the interpretation of the ancillary stress terms in a critical-plane parameter is generally straightforward within proportional loadings, there is often ambiguity in the definition when the loading is nonproportional. In this study, a thorough examination of the variables responsible for crack nucleation is presented in the context of the critical-plane methodology. Uniaxial and multiaxial fatigue data from Ti–6Al–4V and three other alloys, namely, Rene’104, Rene’88DT, and Direct Age 718, are used as the basis for the evaluation. The experimental fatigue data include axial, torsional, proportional, and a variety of nonproportional tension/torsion load paths. Specific attention is given to the effects of torsional mean stresses, the definition of the critical plane, and the interpretation of normal stress terms on the critical plane within nonproportional load paths. A new modification to a critical-plane parameter is presented, which provides a good correlation of experimental fatigue data.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(4):041009-041009-14. doi:10.1115/1.2931152.

This article analyzes the formability data sets for aluminum killed steel (Laukonis, J. V., and Ghosh, A. K., 1978, “Effects of Strain Path Changes on the Formability of Sheet Metals  ,” Metall. Trans. A., 9, pp. 1849–1856), for Al 2008-T4 (Graf, A., and Hosford, W., 1993, “Effect of Changing Strain Paths on Forming Limit Diagrams of Al 2008-T4  ,” Metall. Trans. A, 24A, pp. 2503–2512) and for Al 6111-T4 (Graf, A., and Hosford, W., 1994, “The Influence of Strain-Path Changes on Forming Limit Diagrams of Al 6111 T4  ,” Int. J. Mech. Sci., 36, pp. 897–910). These articles present strain-based forming limit curves $(ϵFLCs)$ for both as-received and prestrained sheets. Using phenomenological yield functions, and assuming isotropic hardening, the $ϵFLCs$ are transformed into principal stress space to obtain stress-based forming limit curves $(σFLCs)$ and the principal stresses are transformed into effective stress and mean stress space to obtain the extended stress-based forming limit curves (XSFLCs). A definition of path dependence for the $σFLC$ and XSFLC is proposed and used to classify the obtained limit curves as path dependent or independent. The path dependence of forming limit stresses is observed for some of the prestrain paths. Based on the results, a novel criterion that, with a knowledge of the forming limit stresses of the as-received material, can be used to predict whether the limit stresses are path dependent or independent for a given prestrain path is proposed. The results also suggest that kinematic hardening and transient hardening effects may explain the path dependence observed in some of the prestrain paths.

Topics: Stress , Steel , Hardening
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(4):041010-041010-15. doi:10.1115/1.2975234.

Fiber reinforced polymer composites are two component material systems in which fibers are embedded in a polymer matrix. Such a system inherently has an interface where the two components meet. Adjacent to the interface extending beyond the fiber surface is the “interphase region.” Properties within the interphase vary due to variations in the chemistry. The study of mechanical property variations with changing chemistry will help in better understanding and tailoring of the composite properties. The present work concentrates on the investigation of nanomechanical properties within the interphase of a glass fiber embedded in polyester matrix system. The glass fibers were coated with two types of silanes to produce a strong and a weak bond at the fiber-matrix interface. Nanoindentation techniques coupled with atomic force microscopy imaging capabilities have been used for this investigation. Two different tips were employed for indenting, one being a Berkovich diamond tip supplied by Hysitron, Inc., Minneapolis, MN and another being a parabolic tungsten tip, which was made in the laboratory. Indentations were performed within the interphase region, also in the bulk matrix, and on the glass fiber. The variation in mechanical properties such as modulus, stiffness, hardness, and penetration depth were obtained within the interphase by indenting at the fiber surface outward. Variations of the elastic modulus in the interphase region and its relation to the chemistry are presented. The results obtained using two different tip shapes have been compared. Phase imaging was performed using tapping mode atomic force microscopy to qualitatively identify the presence of an interphase near the glass fiber-polyester interface. These experiments show that when no coupling agent is used the interphase thickness is less than $0.1 μm$, and its exact determination is limited by the spatial resolution of the tips employed and the process of indentation. Phase imaging results with composite samples made of coated glass fibers corroborate the results obtained from nanoindentation experiments.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Eng. Mater. Technol. 2008;130(4):044501-044501-5. doi:10.1115/1.2969249.

In this paper, the multiscale thermodynamic basis of the plastic potential theory is addressed within the irreversible thermodynamic framework with internal variables by Rice (1971, “Inelastic Constitutive Relations for Solids: An Integral Variable Theory and Its Application to Metal Plasticity  ,” J. Mech. Phys. Solids, 19, pp. 433–455). It is shown that the condition of free or complementary energy equivalence leads to the full equivalence of the microscale and macroscale thermodynamic formulations as soon as the multiscale kinematic relation is prescribed. The condition of dissipation equivalence by Rice is not an independent condition. The thermodynamic significance and counterparts of plastic potentials and multipliers, Koiter’s and Mises’ flow rules, the intrinsic time in the work of Valanis (1975, “On the Foundations of the Endochronic Theory of Viscoplasticity  ,” Arch. Mech., 27, pp. 857–868), the viscoplasticity, and the $J2$ plasticity can all be revealed and recovered within the multiscale thermodynamic framework.

Commentary by Dr. Valentin Fuster

Discussions

J. Eng. Mater. Technol. 2008;130(4):045501-045501-2. doi:10.1115/1.2975231.
FREE TO VIEW

In a recent paper (1), the authors revisited a basic question of interfacial fracture mechanics, namely, the choice of the characteristic reference length$lc$ in the open model of interface cracks (2), and introduced a new procedure for an estimation of $lc$ based on the interface fracture toughness measurements. It is well known that the choice of a reference length $l$ is related to the position along the abscissa axis of the interface fracture toughness curve for an elastic bimaterial, which represents the interfacial fracture toughness $Γint(ψ)$ as a function of the local phase angle $ψ$ of the stress intensity factor (3). The following two comments are essential with regard to the procedure for an estimation of $lc$ proposed by the authors in Sec. 3.2 and applied to their experimental results in Sec. 5.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 2008;130(4):045502-045502-1. doi:10.1115/1.2975232.
FREE TO VIEW

The method proposed by Agrawal and Karlsson (1) is correct when assuming a symmetric interfacial fracture toughness curve. Based on the resolution (e.g., the scatter in the data) from the experimental results, an antisymmetric interfacial fracture toughness curve cannot be assumed, as suggested by Mantic (2). However, if one anticipates an antisymmetric curve, the methodology outline in the first comment by Mantic (2) may be appropriate. Additional experiments are needed to achieve such results, so the characteristics of the minimum of the curve can be established.

Commentary by Dr. Valentin Fuster