Dynamic Impact Toughness Research Articles

Impact toughness and fractography of diverse microstructure in Al-Cu alloy fabricated by arc-directed energy deposition

To investigate the effects of heterogeneous microstructure, precipitates, and pore defects on dynamic impact toughness of Al-Cu alloy fabricated by arc-directed energy deposition (Arc-DED). Samples with diverse microstructures were prepared by adjusting the forming parameters, namely air cooling (AC), water cooling (WC), and the post-treatment, hot isostatic pressing (HIP), and heat treatment (T6) samples. The impact fracture behavior of samples with Charpy V-notched was studied by an instrumented drop hammer impact testing machine which can provide a load-displacement curve. The important load parameters and the absorbed energy at different stages can be determined by the load-displacement curve. The results show that the impact absorbed energy of the AC/WC/HIP/T6 sample was 1.33 ± 0.20, 1.96 ± 0.26, 1.91 ± 0.18, and 2.54 ± 0.21 J, respectively. The impact fracture mechanism was discussed as the corresponding fractographic observations. Brittle fractures with inhomogeneous dimples were observed in all the samples. The key determinant of the impact toughness was the block-shaped brittle θ phase that precipitated along the grain boundaries and pore defects. The equiaxed/columnar heterogeneous microstructure affects not only the coordinated deformation ability of grains but also the distribution of brittle θ phases. Therefore, compared with the AC sample, the impact toughness of the WC sample with a low-volume fraction θ phase, the HIP sample with low porosity, and the T6 sample with a large amount of ductile θ″/θ' phases was significantly improved. This study provides valuable insights for improving the dynamic impact resistance of Arc-DED fabricated Al-Cu alloy.

Dynamic Mechanical Properties of Red Sandrock-Polypropylene Fiber Reinforced Concrete Composite under Impact Load

To investigate the dynamic mechanical properties and fracturing behavior of the red sandrock-polypropylene fiber reinforced concrete (R-PPFRC) composite, R-PPFRC composite specimens with different Polypropylene fiber (PPF) dosage were fabricated. Conducted quasi-static compression experiments and dynamic experiments by using Split Hopkinson pressure bar (SHPB) apparatus and testing machine. The fracture characteristics and process of composite specimens were analyzed with the assistance of ultra-high-velocity camera photography. It can be concluded that the dynamic increase factor and the dynamic compressive strength of composite specimens R-PPFRC increase with the increase of load level, indicating a remarkably strain rate strengthening effect. The optimal PPF content is determined finally. The dynamic impact toughness, ultimate toughness, and dissipation energy of R-PPFRC are significantly affected by the load level and the PPF content. The fracturing process of the composite specimen is divided into four phases, crack initiation, crack expansion, crack cluster and ultimate failure. After going through these phases, the ultimate failure pattern is mainly shear and split mixed failure.

Dynamic mechanical properties of cementitious composites with carbon nanotubes

This paper studied the effect of different types of multi-walled carbon nanotubes (MWCNTs) on the dynamic mechanical properties of cementitious composites. Impact compression test was conducted on various specimens to obtain the dynamic stress-strain curves and dynamic compressive strength as well as deformation of cementitious composites. The dynamic impact toughness and impact dissipation energy were, then, estimated. Furthermore, the microscopic morphology of cementitious composites was identified by using the scanning electron microscope to show the reinforcing mechanisms of MWCNTs on cementitious composites. Experimental results show that all types of MWCNTs can increase the dynamic compressive strength and ultimate strain of the composite, but the dynamic peak strain of the composite presents deviations with the MWCNT incorporation. The composite with thick-short MWCNTs has a 100.8% increase in the impact toughness, and the composite with thin-long MWCNTs presents an increased dissipation energy up to 93.8%. MWCNTs with special structure or coating treatment have higher reinforcing effect to strength of the composite against untreated MWCNTs. The modifying mechanisms of MWCNTs on cementitious composite are mainly attributed to their nucleation and bridging effects, which prevent the micro-crack generation and delay the macro-crack propagation through increasing the energy consumption.

Dynamic impact behaviors and constitutive model of super-fine stainless wire reinforced reactive powder concrete

The dynamic impact behavior of super-fine stainless wire (SSW) reinforced reactive powder concrete (RPC) was studied through split hopkinson pressure bar test with the strain rate range from 94/s to 926/s in this paper. The modification mechanisms of SSW to the dynamic impact performance of RPC were revealed via computed tomography and scanning electron microscope analysis. Experimental results showed that the dynamic impact compressive strength of SSW reinforced RPC increases with the strain rate. The dynamic increase factor of compressive strength is decreased and the strain-rate strengthening effect of RPC is weakened by SSW. The maximum dynamic peak strain of SSW reinforced RPC reaches up to 34,070 µε at the strain rate of 305/s. The limit strain of RPC is decreased because of the lateral confinement effect of SSW. The stress-strain curves of SSW reinforced RPC include elastic stage, elastic-plastic stage and descending stage. The addition of SSW leads 43.5% and 58.2% of increase in dynamic impact toughness and impact dissipate energy of RPC, respectively. Increasing SSW volume fraction makes more SSW to inhibit the generation and propagation of cracks in RPC, thus leading to the decrease of destruction degree. The formation of inter-anchored interface among bundling SSW increases the resistance of RPC to crack development. The dynamic impact constitutive model established on the basis of revised visco-elastic and damage theory can well describe the stress-strain relationship of SSW reinforced RPC at different strain rates, in which the strain threshold is governed by strain rate and SSW volume fraction simultaneously.

Achieving high strength and toughness in a Zr–2.3Nb alloy by the formation of duplex microstructure

A duplex microstructure, which displays improved strength and toughness compared with a starting Widmanstätten microstructure, was obtained in a Zr–2.3Nb alloy. The duplex microstructure was generated via plastic deformation and subsequent recrystallization annealing for no less than 30min. Air cooling was an important factor after the recrystallization process. The duplex microstructure possesses not only excellent static mechanical properties of high tensile strength (∼746MPa), high elongation (∼16%), and high static toughness (∼119MJ/m3), but also high dynamic impact toughness (∼134J/cm2) which was nearly three times as great as that of the Widmanstätten microstructure (∼48J/cm2). The crack propagation during tension and impact fracture of the duplex microstructure are both in a ductile dimple feature, revealing an excellent static and impact toughness.

Experimental study of dynamic mechanical properties of reactive powder concrete under high-strain-rate impacts

The dynamic mechanical properties of reactive powder concrete subjected to compressive impacts with high strain rates ranging from 10 to 1.1×102 s−1 were investigated by means of SHPB (split-Hopkinson-pressure-bar) tests of the cylindrical specimens with five different steel fiber volumetric fractions. The properties of wave stress transmission, failure, strength, and energy consumption of RPC with varied fiber volumes and impact strain rates were analyzed. The influences of impact strain rates and fiber volumes on those properties were characterized as well. The general forms of the dynamic stress-strain relationships of RPC were modeled based on the experimental data. The investigations indicate that for the plain RPC the stress response is greater than the strain response, showing strong brittle performance. The RPC with a certain volume of fibers sustains higher strain rate impact and exhibits better deformability as compared with the plain RPC. With a constant fiber fraction, the peak compressive strength, corresponding peak strain and the residual strain of the fiber-reinforced RPC rise by varying amounts when the impact strain rate increases, with the residual strain demonstrating the greatest increment. Elevating the fiber content makes trivial contribution to improving the residual deformability of RPC when the impact strain rate is constant. The tests also show that the fiber content affects the peak compressive strength and the peak deformability of RPC in a different manner. With a constant impact strain rate and the fiber fraction less than 1.75%, the peak compressive strength rises with an increasing fiber volume. The peak compressive strength tends to decrease as the fiber volume exceeds 1.75%. The corresponding peak strain, however, incessantly rises with the increasing fiber volume. The total energy Edisp that RPC consumed during the period from the beginning of impacts to the time of residual strains elevates with the fiber volume increment as long as the fiber fraction is not larger than 2%. It turns to decrease if the fiber volume exceeds 2%. The added fibers make various contributions to enhancing the capability of RPC to consume energy at different loading stages. If the fiber fraction is not larger than 2%, the added fibers make more contribution to enhancing the energy consumption ability of RPC in the period before the peak strain than in the period after the peak strain. The impact strain rate, however, distinctively affects the total energy that RPC consumed and the energy consumed in the different loading periods. The higher the impact strain rate, the more the energy consumed in the stages and therefore the higher the dynamic impact toughness. The empirical relationships of the peak compressive strength, corresponding peak strain, residual strain, total consumed energy and the energy consumed in the varied periods with the impact strain rate and the fiber fraction are derived. Four generalized forms of the dynamic impact stress-strain responses of RPC are formulated by normalizing stresses and strains as the generalized coordinates and by taking account of the influences of impact strain rates and fiber volumetric fractions.

Relationship between dynamic and static toughness of flake and compacted graphite cast irons

The very little plastic deformation exhibited in the fracturing of flake graphite (FG) and compacted graphite (CG) cast irons qualified them as brittle materials. The fatigue pre-cracking of these brittle material in K IC plane strain fracture toughness testing is a difficult task. Opposed to this static toughness K IC, the dynamic toughness obtained by impact testing does not have such problems. Since the stress–strain behavior of brittle material is essentially linear and the fracture appearance of the specimens after impact testing must largely be flat without shear-lip, these conditions also satisfy the linear-elastic requirement in K IC testing. Thus, it is of interest to find out if there’s any linear relationship existed between these two toughness properties of brittle material. Annealing, normalizing, and austempering heat treatment were applied to FG and CG irons to alter their matrix structures so as to obtain a range of static ( K IC) and dynamic (impact) toughness values. A third toughness, calculated by integrating the area under the load versus CGD (Clip Gage Displacement) curve in K IC testing, was termed ‘calculated toughness’ and was also listed for comparison purposes. It was found, among dynamic impact toughness ( X), static K IC ( Y), and calculated toughness ( Z), linear relationships exist for flake graphite cast iron as: Y=4.02 X−0.81 Z=1.75 X+2.99 and for compacted graphite cast iron as: Y=0.86 X+20.2 Z=0.77 X+4.77.