•The adiabatic shear failure process of solids was investigated for the first time by dynamic tests synchronically combined with high-speed photography and infrared temperature measurement.•The key ...characteristics of ASB, such as temperature, critical strain, propagation speed and cooling rate were systematically studied.•The experimental results shows that the apparent temperature rise might have occurred after ASB initiation, indicating it might not be the causation but the consequences of ASB.•The discovery might help to clarify the causality of ASB and serve as the starting point for further physical, mechanistic and mathematic studies of ASB.
One of the most important issues related to dynamic shear localization is the correlation among the stress collapse, temperature elevation and adiabatic shear band (ASB) formation. In this work, the adiabatic shear failure process of pure titanium was investigated by dynamic shear-compression tests synchronically combined with high-speed photography and infrared temperature measurement. The time sequence of important events such as stress collapse, ASB initiation, temperature rise and crack formation was recorded. The key characteristics of ASB, such as width, critical strain, temperature, propagation speed and cooling rate were systematically studied. The maximum propagation velocity of ASB is found in this work to be about 1900 m/s, about 0.6Cs (Cs is the shear wave speed). The maximum temperature within ASB is in the range of 350–650 °C, while the material close to ASB is also heated. The cooling rate of ASB is on the order of 106 °C/s, indicating that it needs a few hundreds of microseconds for the ASB to cool down to the ambient temperature. One important observation is that the apparent temperature rise occurs after ASB initiation, which indicates that it might not be the causation but the consequences of ASB. Further efforts are called for confirmation of this notion because of its significance.
The unusual properties of mechanical metamaterials are determined by the configuration of artificial periodic structures. However, the mechanical performance of conventional metamaterials is ...irreversible and cannot perceive and respond to the changes in the environment. In present study, a zero Poisson's ratio metamaterial with intelligent switching mechanical properties and vibration isolation effect is proposed. Based on a 4D printing method of shape memory polymer, this metamaterial is created that can sense temperature changes and switch mechanical properties. The macroscopic deformation and the morphology change of the metamaterial during compression tests are analyzed using experimental and finite element methods. The irregular buckling distortion of the metamaterial is eliminated by cylindrical design, and controllable and adjustable local deformation and stress-strain curve are achieved based on microstructure gradient design. Subsequently, this work focused on the vibration isolation performance of metamaterials, and found fascinating shock absorption performance. Compared with traditional linear spring, this metamaterial spring can effectively reduce the vibration amplitude of certain frequency bands before reaching the resonance peak, which provides a new realization method for low-frequency vibration isolation design.
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•A 4D printed zero-Poisson's ratio metamaterial with programmable shapes, thermally tunable mechanical performance is created.•A cylindrical design is proposed to eliminate buckling distortion in unit cells during compression,.•Based on gradient design, controllable and defect-insensitive local deformation can be achieved in the metamaterial.•The metamaterial can effectively reduce the vibration amplitude of certain frequency bands before the resonance peak.
•A variety of geometries of specimens, including the modified shear-compression, shear-tension, and thin-walled tubular specimen are designed to create different stress states.•A hybrid ...experimental–numerical method is developed to evaluate quantitatively the effect of stress stare on the initiation of ASB within Ti–6Al–4V.•A phenomenological model of ASB initiation considering stress triaxiality and Lode parameter is proposed. The proposed model can predict the initiation of ASB with good accuracy under the test condition of this work.
Shear failure is frequently accompanied with the formation of an adiabatic shear band (ASB) under dynamic loading condition. The results obtained by a previous study (Guo et al., 2019) indicate that temperature increase does not play a primary role in the formation of ASB. Moreover, the shear strain to ASB initiation is closely related to the stress state. In this paper, a hybrid experimental–numerical method is developed to evaluate quantitatively the effect of stress stare on the initiation of ASB within Ti–6Al–4V. A variety of geometries of specimens, including the modified shear-compression, shear-tension, and thin-walled tubular specimen are designed to create different stress states. Combined with high-speed photography, split Hopkinson bar systems are utilized to measure the mechanical response of these specimens. Critical shear strains of ASB initiation are acquired based on the high- speed photos. The stress state for each test condition is obtained by numerical simulation. A phenomenological model of ASB initiation considering stress triaxiality and Lode parameter is proposed. Compared to the experimental results, the proposed model can predict the initiation of ASB with good accuracy under the test condition of this work.
We give a concise tutorial on knowledge discovery with linear mixed model in movie recommendation. The versatility of mixed effects model is well explained. Commonly used methods for parameter ...estimation, confidence interval estimate and evaluation criteria for model selection are briefly reviewed. Mixed effects models produce sound inference based on a series of rigorous analysis. In particular, we analyze millions of movie rating data with LME4 R package and find solid evidences for a general social behavior: the young tend to be more censorious than senior people when evaluating the same object. Such a social behavior phenomenon can be used in recommender systems and business data analysis.
As a typical failure behavior, the dynamic fracture is critical for material safety assessment and design, whose rapid evolution in the crack tip over a short time causes tremendous challenges in ...characterizing the evolution of the local temperature field. Although researchers have pointed out that temperature significantly affects fracture toughness, the temperature rise and the role of thermal softening in the crack tip region during the dynamic crack initiation need to be clarified. This work aims to evaluate the temperature rise effect at crack initiation in TC4 (Ti-6Al-4V) alloy under impact loading. We developed a thermal-mechanical coupled
in-situ
measurement system to obtain synchronized COD (crack-tip opening displacement) and temperature field evolution for the dynamic mode I fracture with high temporal and spatial resolution. The evolution of synchronous COD and temperature field during crack initiation under dynamic loading was investigated. The corresponding simulation was conducted to analyze the thermal softening effect at dynamic crack initiation. In TC4 alloy, the maximum temperature detected at crack initiation is about 130 °C, and the thermal effect on the material property is minor. The results of this work fulfill data support for investigating the coupled thermal-mechanical fracture behavior of metal.
High-entropy alloys (HEAs) have attracted considerable attention in recent years because of their unique mechanical properties. In this work, the mechanism of dynamic shear banding (also called ...adiabatic shear bands, ASBs) in a BCC HEA HfNbZrTi was investigated combining dynamic experiments and numerical simulations. The temperature evolution during dynamic shear banding, which has been believed to play a dominant role during ASB formation in the literature, was measured using high-speed infrared thermal detectors synchronized with a split Hopkinson pressure bar system. The dynamic mechanical behavior of the BCC HEA was described using the Johnson-Cook model accompanied by damage accumulation. The process of ASB formation, considering potential contributions from thermal softening and damage softening, was numerically investigated by controlling the activation of each softening mechanism separately. Based on the results of experimental observation and numerical analysis, dynamic shear banding in this BCC HEA is proposed to be dominated by damage softening, and thermal softening only plays a secondary role, which differs from the thermal-softening-dominated ASB formation in typical FCC HEAs such as the Cantor alloy.
One of the most important issues related to adiabatic shear failure is the correlation among temperature elevation, adiabatic shear band (ASB) formation and the loss of load capacity of the material. ...Our experimental results show direct evidence that ASB forms several microseconds after stress collapse and temperature rise reaches its maximum about 30 μs after ASB formation. This observation indicates that temperature rise cannot be the cause of ASB. Rather, it might be the result of adiabatic shear localization. As such, the traditional well-accepted thermal-softening mechanism of ASB needs to be reconsidered.
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