Magnetization reversal mechanism of anisotropic HDDR Nd2Fe14B-based magnet powder

The coercivity mechanism of anisotropic Nd₂Fe₁₄B-based magnetic powders prepared by hydrogenation–decomposition–esorption–recombination process was studied. Polarization and corresponding differential susceptibility curves of the powders in its thermally demagnetized state were measured. Microstructure and constituents of the powders were investigated by means of transmission electron microscope and scanning electron microscope with energy dispersive X-ray detector. In addition, theoretical calculation of the intrinsic coercive force of the magnetic powders was performed. It is concluded that the magnetic hardening mechanism of the powders is the pinning of domain walls at grain boundaries of the Nd₂Fe₁₄B main phase and Nd-rich phase that distributes homogeneously around some conglomerations composed of fine Nd₂Fe₁₄B grains. The coercive force of the powders is mainly determined by the pinning of domain walls by Nd-rich boundary phase.     

Failure in the junction region of T-stiffeners: 3D-braided vs. 2D tape laminate stiffeners

Strain distributions and failure mechanisms are compared for a three-dimensionally (3D) braided T-stiffener (preform designed and supplied by 3TEX Inc.) and a conventional two-dimensional (2D) tape laminate T-stiffener, bonded onto a tape laminate skin. The strain distributions in a pull-off test are measured by laser speckle interferometry and calculated by computational simulations. With good agreement between experiment and theory, substantial differences are found between the two classes of stiffener. The tape laminate stiffeners exhibit large strain concentrations across the noodle region and in the adjacent radii, which correlate well with observed first cracking events. The 3D-braided stiffeners show relatively uniform strain distributions throughout the flanges, the web, and the flange/web junction region. Strain concentrations are modest at the corner of the junction and absent along the interface between the flanges and the skin. Failure in the 3D-braided stiffeners does not occur within the junction region, but by a sequence of cracking events, first next to the junction region and then at the end of one flange. The pull-off load at the first failure event is substantially higher for the 3D-braided stiffener than the tape laminate stiffener, which is attributed mainly to the relative absence of strain concentrations in the former.In predicting strain distributions, account is taken of the details of the 3D architecture of the 3D-braided stiffeners as specified by the supplier (3TEX Inc.) by using the Binary Model of textile composites, whose formulation has been described previously. Comparison of the predicted and measured spatial distributions of strain constitutes a critical test of the Binary Model. It is found to perform well in this case.     

Mechanisms of stress transfer and interface integrity in carbon/epoxy composites under compression loading. Part II: Numerical approach

The finite element method is used to get an insight into the micromechanics of the compressive behaviour of carbon fibre composites. First the developed model is validated with existing experimental data and good agreement between predictions and experiments was found. Then the FE model is used to derive the complete stress field in the fibre and the matrix in the vicinity of a fibre fracture location. It was found that the perturbation of the stress field occurs mainly in the direction transverse to the fibre axis and this could explain the failure modes observed in composites tested in compression. Finally, a parametric study was performed on the effect of matrix modulus and matrix yield stress on the compressive fragmentation process.     

Abnormal ripple patterns with enhanced regularity and continuity in a bulk metallic glass induced by femtosecond laser irradiation

Two types of abnormal ripple patterns: classical ripples (C-ripples) with continuous ridges and unclassical ripples (U-ripples) with regular micropores were investigated in a Zr-based bulk metallic glass (Zr-BMG) after femtosecond laser beam irradiation. U-ripples with a period of $\sim $ 1,600 nm and the orientation nearly parallel to the polarization of laser beam were observed to form gradually on the top of C-ripples with the effective pulse number. Micropores created by the superposition of C-ripples and U-ripples had an average diameter of $\sim $ 750 nm nearly equal to the period of C-ripples ( $\sim $ 720 nm) and ran along parallel lines in the grooves of U-ripples. Both U-ripples and C-ripples in Zr-BMG showed significant microstructural differences comparing with those in a conventional Zr-based alloy with the same composition and processing conditions, including much more regular and continuous ridges. The results indicate that the amorphous structure in Zr-BMG plays a key role in the uniform morphology of laser-induced structures.     

Mechanisms of stress transfer and interface integrity in carbon/epoxy composites under compression loading: Part I: Experimental investigation

Raman spectroscopy is used to get an insight into the microstructural aspects of the compressional behavior of carbon fiber composites. This is done by a comparative assessment of the stress transfer efficiency in tension and compression in single-fiber discontinuous model geometries. It was found that axial stress is transferred in the fiber through the generation of shear stresses at the interface for both tension and compression loading. Experimental evidence is presented to verify that the values of the maximum interfacial shear stress that the system sustains is a function of the applied strain and independent of the type of loading. However, compressive failure is quite different as fiber fragments remain in contact, thus can still bear load.     

Acoustic damping characterization and microstructure evolution in nickel-based superalloy during creep

We studied the microstructure evolution of a nickel-based superalloy, Waspaloy, subjected to tensile creep at 1073 K through monitoring of shear-wave attenuation and velocity using electromagnetic acoustic resonance (EMAR). Contactless transduction based on the Lorentz force mechanism is the key to establishing a monitor for microstructural change in the bulk of the metals with a high sensitivity. There is a clear relationship between the attenuation and the life fraction. In the interval, 35 to 40% of the creep life, attenuation experiences a peak, being independent of the applied stress. This novel phenomenon is interpreted in terms of drastic change in dislocation mobility and the coarsening of γ′-precipitates, which is supported by SEM and TEM observations. At this period, dense dislocations start tangling to γ′-precipitates and the coarsening and condensation of γ′-precipitates become saturated. The EMAR has a potential to assess the damage advance and to predict the remaining creep life of metals.     

Repetitive impact exposure and characterization of stress-whitening of an American football helmet outer shell material

Mechanical stress exerted upon impact-modified polycarbonate (PC) and poly(ethylene terephthalate) (PET) blends has been reported to generate microscopic voids via rubber-toughener (RT) particle cavitation which can macroscopically manifest to visibly whiten the material. Previous work has revealed a whitening phenomenon in collegiate American football helmet outer shells after a single season and in helmet-grade plaques following linear impact testing. The purpose of this research was to quantify the effects of repetitive linear drop exposures on the (i) impact performance; (ii) physical and thermal properties; and (iii) surface and tensile mechanical properties of a stress-whitened American football helmet outer shell material. Statistically significant changes in plaque impact performance corresponded to substantial stress-whitening that penetrated up to 40–45% into the plaque thickness and elicited shifts in surface and tensile mechanical properties. Nanoscale microscopy revealed elongation of the PC/PET matrix and delamination at the RT-matrix interface. Thermal property analysis suggested the concomitant occurrence of RT cavitation and strain-induced PET crystallization. Overall, the research identified a battery of diagnostic tools to characterize material property changes of stress-whitening in rubber-toughened helmet outer shell materials.     

Abnormality in using cyclic fatigue for ranking static fatigue induced slow crack growth behavior of polyethylene pipe grade resins

Slow crack growth behavior in polyethylene pipe grade resins were studied using both static fatigue (stress-rupture) and cyclic fatigue tests. This was done to better understand the applicability of cyclic fatigue in the prediction of slow crack growth ranking determined from the static fatigue test. In all polyethylene pipe grade resins tested at 80 °C, reduced crack growth failure times were exhibited when the cyclic fatigue test was employed. However, when applied to rank the resins through their slow crack failure times, the cyclic fatigue results did not always confirm those obtained from the static fatigue test. That is, in some cases, a resin with higher slow crack resistance ranking (longer failure times) than another resin in static fatigue exhibited lower ranking (shorter failure times) in the cyclic fatigue test. This abnormality of reversal in ranking is not a general observation but does occur. Based on the data obtained so far, when resins with smaller differences between static fatigue and cyclic fatigue slow crack growth failure times are compared with those resins having larger differences, the chances of correctly predicting the ranking obtained from static fatigue using cyclic fatigue tend to decrease. Hence, it is suggested that one needs to practice caution when using cyclic fatigue to predict the static fatigue ranking of resins for slow cracking resistance. Some insight into the cause of such abnormality is discussed with reference to creep-fatigue interactions.