Enhancing energy absorption through sequential instabilities in mechanical metamaterials

Structural components designed to absorb energy and shield a more valuable structure ideally require mechanical properties that combine a relatively high load-carrying capacity followed by a practically zero stiffness. This ensures that a specified energy quantity may be absorbed within a limited displacement and that any stress transfer to the valuable structure is minimized. Material damage has been historically mobilized to provide such properties, but this obviously renders such components to be single-use. By contrast, mobilization of elastic instability can also provide the desired combination of properties but without necessarily damaging the material. This reveals an intriguing possibility of such components being potentially repairable and theoretically re-usable with no significant loss in performance. A series of analytical, finite-element and experimental studies are presented for a bespoke mechanical metamaterial arrangement that is designed to buckle sequentially and behave with the desired 'high strength-low stiffness' characteristic. It is found that the various axial and rotational stiffnesses associated with the geometric arrangement and its constituent connections may be tuned to provide the desired mechanical behaviour within the elastic range and delay the onset of significant damage, thereby rendering the concept of harnessing instability to be feasible.

Unraveling the Structural Instability of Li(Ni0.80 Co0.15 Al0.05 )O2 as a Cathode Material Due to Operating a Li-ion Battery

The change of the crystal structure for Li(Ni_0.80 Co_0.15 Al_0.05 )O₂ as a cathode material in a Li-ion battery is traced. During charging and discharging, the crystallographic change of Li_x (Ni_0.80 Co_0.15 Al_0.05 )O₂ (x ≈ 1.0-0.25) is confirmed with in situ X-ray diffraction, an electrochemical measurement, and the density functional theory calculation. Li atoms after cycling do not completely return to the initial state and defects in the Li-layer generate about 5%. The effect of defects in the Li-layer reveals the transformation of crystal structure and the change of lattice constants. Upon increasing the temperature, the instability of Li_0.95 (Ni_0.80 Co_0.15 Al_0.05 )O₂ is clearly shown as the movement of transition metals using X-ray and neutron diffraction. The crystallographic values dramatically change upon increasing from 373 to 423 K, but linearly vary upon decreasing temperature. Furthermore, the result of the calculation demonstrates that the possible atom for mixing is Ni. The evolution of magnetic properties explicitly certifies the atomic movement that gives rise to a spin-glass state through the induction of ferromagnetism. In conclusion, defects are created in crystal structure during operation of the Li-ion battery and generate structural instability. The results provide the cause and mechanism of the degradation of cathode material in a Li-ion battery.

Nanoscale Imaging of Unusual Photoacoustic Waves in Thin Flake VTe2

The control of acoustic phonons, which are the carriers of sound and heat, has become the focus of increasing attention because of a demand for manipulating the sonic and thermal properties of nanometric devices. In particular, the photoacoustic effect using ultrafast optical pulses has a promising potential for the optical manipulation of phonons in the picosecond time regime. So far, its mechanism has been mostly based on the commonplace thermoelastic expansion in isotropic media, which has limited applicability. In this study, we investigate a conceptually new mechanism of the photoacoustic effect involving a structural instability that utilizes a transition-metal dichalcogenide VTe₂ with a ribbon-type charge-density-wave (CDW). Ultrafast electron microscope imaging and diffraction measurements reveal the generation and propagation of unusual acoustic waves in a nanometric thin plate associated with optically induced instantaneous CDW dissolution. Our results highlight the capability of photoinduced structural instabilities as a source of coherent acoustic waves.

Structural Instability-Enabled Mechanical Sensors Using Fiber Bragg Grating

Structural health monitoring (SHM) has been extensively used in civil infrastructures to assess structural condition and situation. Here, we develop a novel type of mechanical sensing technique using the structural instability of cylindrical cells detected by fiber Bragg grating (FBG). The cylinders are fabricated using a 3D printing technique, which are coiled by the FBG wires to detect the transverse deformation. Structural instability under axial compression is obtained in the experiments and the force–displacement relations are validated by the numerical simulations with satisfactory agreements. The wavelength variation of the FBG, caused by the structural instability, is observed and compared with the predefined threshold. Defining the variation larger than the threshold as “1” and smaller as “0”, the pattern recognition algorithm is used to convert the FBG results into binary data, which can, therefore, be analyzed to indicate the structural conditions. In the end, we envision the potential applications of the reported sensing technique, such as wireless sensors for structural health monitoring (SHM) in civil infrastructures.

NON-OPERATIVE MANAGEMENT OF INDIVIDUALS WITH NON-ARTHRITIC HIP PAIN: A LITERATURE REVIEW

BACKGROUND

Non-arthritic hip pain is defined as being related to pathologies of the intra-articular structures of the hip that can be symptomatic. A trial of non-operative management is commonly recommended before consideration of surgery for individuals with non-arthritic hip conditions. There is a need to describe a non-operative or conservative treatment plan for individuals with non-arthritic hip pain.

PURPOSE

The purpose of this literature review was to systematically examine the literature in order to identify and provide evidence for non-operative or conservative management of individuals with non-arthritic hip pain. A proposed home exercise program will be provided for individuals with non-arthritic hip pain.

STUDY DESIGN

Review of the Literature.

MATERIALS/METHODS

A literature search of PubMed, Medline, SPORTSDiscus, and CINAHL was conducted. Keywords included: "hip" AND "femoroacetabular impingement" OR "labral tear." Studies were included if they described non-operative management for individuals with non-arthritic hip pain. Studies were excluded if they recommended a trial of conservative treatment without specific management or interventions and/or activity modification without specific details for intervention.

RESULTS

A total of 49 studies met the eligibility criteria and were included in the review. Rehabilitation recommendations were identified from manuscripts including clinical trials, case series, discussion articles, or systematic reviews related to the non-operative or conservative management of non-arthritic hip pain. Rehabilitation interventions focused on patient education, activity modification, limitation of aggravating factors, an individualized physical therapy protocol, and use of a home exercise program.

CONCLUSIONS

Rehabilitation should address biomechanical deficiencies with neuromuscular training of the hip and lumbopelvic regions. While the current literature on non-operative management is limited, future randomized control trials will establish the effectiveness of specific physical therapy protocols for individuals with non-arthritic hip pain.

LEVEL OF EVIDENCE

3b.

Skin aging as a mechanical phenomenon: The main weak links

From a mechanical point of view, human skin appears as a layered composite containing the stiff thin cover layer presented by the stratum corneum, below which are the more compliant layers of viable epidermis and dermis and further below the much more compliant adjacent layer of subcutaneous white adipose tissue (sWAT). Upon exposure to a strain, such a multi-layer system demonstrates structural instabilities in its stiffer layers, which in its simplest form is the wrinkling. These instabilities appear hierarchically when the mechanical strain in the skin exceeds some critical values. Their appearance is mainly dependent on the mismatch in mechanical properties between adjacent skin layers or between the skin and sWAT, on the adhesive strength and thickness ratios between the layers, on their bending and tensile stiffness as well as on the value of the stress existing in single layers. Gradual reduction of elastic fibers in aging significantly reduces the skin's ability to bend, prompting an up to 4-fold reduction of its stability against wrinkling, thereby explaining the role of these fibers in skin aging. While chronological and extrinsic aging differently modify these parameters, they lead to the same end result, reducing the critical strain required for the onset of instabilities. Comparing of mechanical properties of the skin presented as a bi-, tri- or tetra-layer structure demonstrates the particular importance of the papillary dermis in skin aging and provides the arguments to consider the undulations on the dermal-epidermal and dermal-sWAT interfaces as the result of mechanical bifurcation, leading to structural instabilities inside of the skin. According to this model, anti-aging strategies should focus not as much on the reinforcement of the dermis, but rather aim to treat the elastic mismatch between different adjacent layers in the skin and sWAT as well as the adhesion between these layers.

Strain and stability of ultrathin Ge layers in Si/Ge/Si axial heterojunction nanowires

The formation of abrupt Si/Ge heterointerfaces in nanowires presents useful possibilities for bandgap engineering. We grow Si nanowires containing thick Ge layers and sub-1 nm thick Ge "quantum wells" and measure the interfacial strain fields using geometric phase analysis. Narrow Ge layers show radial compressive strains of several percent, while stress at the Si/Ge interface causes lattice rotation. High strains can be achieved in these heterostructures, but we show that they are unstable to interdiffusion.

Prediction of incident hip fracture risk by femur geometry variables measured by hip structural analysis in the study of osteoporotic fractures

The role of bone tissue's geometric distribution in hip fracture risk requires full evaluation in large population-based datasets. We tested whether section modulus, a geometric index of bending strength, predicted hip fracture better than BMD. Among 7474 women from the Study of Osteoporotic Fractures (SOF) with hip DXA scans at baseline, there were 635 incident hip fractures recorded over 13 yr. Hip structural analysis software was used to derive variables from the DXA scans at the narrow neck (NN), intertrochanter (IT), and shaft (S) regions. Associations of derived structural variables with hip fracture were assessed using Cox proportional hazard modeling. Hip fracture prediction was assessed using the C-index concordance statistic. Incident hip fracture cases had larger neck-shaft angles, larger subperiosteal and estimated endosteal diameters, greater distances from lateral cortical margin to center of mass (lateral distance), and higher estimated buckling ratios (p < 0.0001 for each). Areal BMD, cross-sectional area, cross-sectional moment of inertia, section modulus, estimated cortical thickness, and centroid position were all lower in hip fracture cases (p < 0.044). In hip fracture prediction using NN region parameters, estimated cortical thickness, areal BMD, and estimated buckling ratio were equivalent (C-index = 0.72; 95% CI, 0.70, 0.74), but section modulus performed less well (C-index = 0.61; 95% CI, 0.58, 0.63; p < 0.0001 for difference). In multivariable models combining hip structural analysis variables and age, effects of bone dimensions (i.e., lateral distance, subperiosteal diameter, and estimated endosteal width) were interchangeable, whereas age and neck-shaft angle were independent predictors. Several parsimonious multivariable models that were prognostically equivalent for the NN region were obtained combining a measure of width, a measure of mass, age, and neck-shaft angle (BMD is a ratio of mass to width in the NN region; C-index = 0.77; 95% CI, 0.75, 0.79). Trochanteric fractures were best predicted by analysis of the IT region. Because section modulus failed to predict hip fracture risk as well as areal BMD, the thinner cortices and wider bones among those who fractured may imply that simple failure in bending is not the usual event in fracture. Fracture might require initiation (e.g., by localized crushing or buckling of the lateral cortex).