Nacre-like alumina with unique high strain rate capabilities

Nacre-like alumina manufactured using spark plasma sintering shows a strikingly different mechanical behaviour compared to conventional alumina. A range of sintering conditions were applied to micron-sized alumina platelet powders to form alumina with different nacre-like microstructures, density, grain size and flexural strength. We show that a microstructure of aligned sintered platelets not only mitigates the typical issue of brittleness, but also has extraordinary energy absorption capabilities. It can withstand an impact with up to three times the kinetic energy required to break monolithic alumina while maintaining structural integrity. The high-rate compressive strength is shown to be more than 50% higher than that of monolithic alumina and we show energy absorption mechanisms such as crack deflection and branching to be present. Our approach provides a fast and effective way of manufacturing aligned nacre-like ceramic microstructures that maintain structural integrity through energy dissipation and interlocking mechanisms.     

Strain hardening behaviors and mechanisms of polyurethane under various strain rate loading

The uniaxial tension experiments are performed on thermoplastic polyurethane to investigate its mechanical behaviors and related potential mechanisms, and the loading strain rate is designing to be wide ranging from 0.0001 to 1 s⁻¹. It is found that the polyurethane presents an obvious rate-dependence, and the stress strain curves share distinct strain hardening characteristics under the investigated strain rates. Furthermore, the strain hardening ratios are sharing nearly same trends and appear to be influenced by both strain rate and the induced adiabatic heating. Besides, the ratio is also strain-dependent on previous loading history. Then, a two-dimension unit cell model is built to investigate potential equivalent mechanisms, of which the hard phase as inclusion is equivalent with crystallization zone, crosslinking sites, and so forth. The simulation results facilitate to explain the distinct strain hardening ratios, even for the matrix from the extrapolated curves under super-low strain rate loading. Finally, the analogic mechanisms of equivalent hard inclusions are proposed, which can reasonably explain the strain rate- and strain-dependence characteristics of polyurethane mechanical behaviors.     

Nanoindentation and scratch behaviour of Ni–P electroless coatings

ABSTRACT

In the present paper, the mechanical properties and the scratch failure mechanisms of Ni–P electroless coatings are described. The material microstructure was studied in as-deposited and annealed conditions through SEM and EDS analyses. Nanoindentation measurements on the coatings showed a remarkable hardening due to the crystallization and precipitation behaviour produced by annealing. The scratch tests, conducted by increasing the load during scratch, revealed the coating failure mechanisms in a broad range of applied stresses up to delamination.     

Residual strain in cadmium telluride/gallium arsenide (001) heterostructures as a function of temperature

Optical studies of residual strain in cadmium telluride (CdTe) films grown using molecular beam epitaxy on gallium arsenide (GaAs) substrate have been performed using photoreflectance techniques. Measurements have been conducted to determine the fundamental transition energy, heavy-hole and light-hole transition energy critical-point parameters in a range of temperatures between 12 and 300 K. There are problems inherent in the fabrication of optoelectronic devices using high-quality CdTe films, due to strain effects resulting from both the lattice mismatch (CdTe: 14.6%) and the thermal expansion coefficient difference. The CdTe film exhibits compressive stress causing valence-band splitting for light and heavy holes. We have used different models to fit the obtained experimental data and, although the critical thickness for the CdTe has been surpassed, the strain due to the lattice mismatch is still significant. However, the strain due to the thermal expansion is dominant. We have found that the fundamental transition energy, E₀, is affected by the compressive strain and the characteristic values are smaller than those reported. In addition, the total strain is compressive for the full measured range, since the strain due to the lattice mismatch is one order of magnitude higher than that calculated from the thermal expansion.     

Structural fatigue crack growth on a representative volume element under cyclic strain behavior

By introducing parameters λ and ω into the crack tip field, a unified cyclic stress and strain field was first formulated by using the Hutchinson-Rice-Rosengren (HRR) field and the Rice-Kujawski-Ellyin (RKE) field under plane stress states in the present study. On the basis of the plastic strain energy and the linear damage accumulation, two fatigue crack growth models without any artificial parameters were then proposed from a representative volume element of cyclic strain behavior. The fatigue crack growth model included parameters λ and ω which showed the effect of two singularity fields. In addition, a simplified structural fatigue crack growth model was eventually established in terms of the fatigue life of each point on the crack front and the non-self-similar shape evolution law. Finally, the predictions of models are compared with the experimental data and the agreement is found to be fairly good.     

Assessment of local strain field in adhesive layer of an unsymmetrically repaired CFRP panel using digital image correlation

The present study focuses on experimental investigation of through the thickness displacement and strain field in thin adhesive layer in single sided (unsymmetrical) patch repaired CFRP (carbon fiber reinforced polymer) panel under tensile load. Digital image correlation (DIC) technique is employed to acquire the displacement and strain (longitudinal, peel and shear) field. Experimental determination of shear transfer length based on shear strain field obtained from DIC is introduced to estimate the optimum overlap length which is an essential parameter in patch design for the repair of CFRP structures. Further, DIC experiment with magnified optics is performed to get an insight into complex and localized strain field over thin adhesive layer especially at critical zones leading to damage initiation. The failure mechanism, load displacement behavior, damage initiation and propagation are closely monitored using DIC. The influence of patch edge tapering on strain distribution in adhesive layer is also investigated. The DIC successfully captures the global and localized strain field at critical zones over thin adhesive layer and further helps in monitoring the damage based on strain anomalies. Strains are found to have maximum magnitude at the patch overlap edge and the shear strain level in adhesive layer is higher than the peel strain. Normal tapering increases the peel strain and has negligible influence on shear strain level in adhesive layer. The recommended overlap length is found to be consistent with the recommendation in the literature. Whole field strain pattern and the overlap length obtained from experiment are further compared with the finite element analysis results and they appear to be in good coherence.     

高速钢轧辊表面氧化膜研究

借助Gleeble 3500模拟机模拟了高速钢轧辊服役时表面工作层温度的升高,得到了循环加热冷却30次、50次及100次后试样表面的氧化膜。为对比分析,对高铬铸铁轧辊进行了循环50次的试验。利用扫描电子显微镜、X射线衍射分析仪、原子力显微镜对循环氧化后的试样表面氧化产物及其形貌进行了分析。结果表明:随着循环次数增加,高速钢轧辊氧化程度加剧,氧化膜更加致密;同样循环次数下,高铬铸铁轧辊氧化程度要比高速钢严重且氧化膜的粗糙度也大于高速钢。     

Damage accumulation modeling under uniaxial low cycle fatigue at elevated temperatures

The paper presents a fatigue damage accumulation model, which allows us to predict fatigue life under low cycle uniaxial loadings at elevated temperatures. The structure of the model has been based on the stress–strain curves obtained during the experimental study. The model has been verified experimentally by applying experimental studies carried out on ENAW-2024T3 aluminum alloy and 2Cr–2WVTa steel. Moreover, a comparison between the results of fatigue life prediction using the proposed damage accumulation model was done with the results obtained on the basis of various generally applied models, based on the Manson–Coffin dependency. Furthermore this paper presents the results of experimental studies carried out on the aluminum alloy ENAW 2024 T3 under uniaxial low cycle fatigue loadings in the conditions of elevated temperatures. In the course of the study, material constants and the parameters of the stress–strain curve in the range of low cycle fatigue for four levels of temperatures (20, 100, 200 and 300 °C) were set.