Shear properties of acrylic under high strain rate loading

Acrylic is being used in structural applications because of its higher resistance to projectile impacts. High strain rate shear loading is one of the critical conditions. In the present study, properties of typical acrylic under high strain rate shear loading are presented. Torsional Split Hopkinson Bar apparatus was used for the studies in the shear strain rate range of 290 per sec to 791 per sec. Thin‐walled tubular specimens with hexagonal flanges were used for the experimental studies. Details of specimen configuration, data acquisition, and processing are presented. Shear strength is presented as a function of shear strain rate. It is observed that the shear strength at high strain rate is enhanced up to 25% compared with that at quasi‐static loading in the range of parameters considered. Comparison of torque versus time behavior derived from signals obtained from strain gauges mounted on incident bar and transmitter bar is also presented. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Shear properties of epoxy under high strain rate loading

Shear properties of epoxy LY 556 under high strain rate loading are presented. Torsional Split Hopkinson Bar apparatus was used for the studies in the shear strain rate range of 385–880 per sec. Experimental details, specimen configuration and development, data acquisition, and processing are presented. Shear strength, shear modulus, and ultimate shear strain are presented as a function of shear strain rate. For comparison, studies are presented at quasi‐static loading. It is observed that the shear strength at high strain rate is enhanced up to 45% compared with that at quasi‐static loading in the range of parameters considered. Further, it is observed that, in the range of parameters considered, the change in shear properties with the change in shear strain rate is not significant. Comparison of torque versus time behavior derived from signals obtained from strain gauges mounted on incident bar and transmitter bar is also presented. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Acid-Enhanced Crack Initiation in Glass

Silicate glass rods containing Vickers indentations in the subthreshold-fracture region can show dramatic strength losses on brief immersion in acid solutions. The effect is most significant for "normal" glasses, notably soda-lime glass, and for HF-based acids. After extended immersion the strength begins to recover, ultimately surpassing the original (pre-immersion) level, as glass dissolution processes become dominant. The weakening effect is interpreted in terms of chemically enhanced residual-contact stress effects.     

Anodic film studies on nickel under high rate transpassive dissolution conditions

Coulometry and Auger Electron Spectroscopy (AES) were employed to study thickness and composition of anodic films formed on nickel under high rate transpassive dissolution conditions. Nickel anodes were polarized at constant current densities up to 30 A/cm² in alkaline nitrate electrolytes of different nitrate and hydroxyl ion concentration using a flow channel cell with a constant electrolyte flow velocity of 10m/sec. Results show that with increasing current density film thickness goes through a maximum. Nitrogen is detected at the apparent film metal interface in the current region where metal dissolution occurs. No correlation between anodic film thickness and dissolution efficiency is found. The data, together with previous observations, suggest that high rate transpassive dissolution takes place from film free sites.     

Crack Initiation in Unidirectional Brittle-Matrix Composites

This paper describes the initiation of matrix cracking for glass and glass-ceramic matrix composites reinforced with small-diameter silicon carbide and carbon fibers under uniaxial tensile loading. Acoustic emission, replication, and optical microscopy in conjunction with stress-strain curves are employed to detect the initiation of matrix cracking. The proportional limit of the stress-strain curve is found to overestimate the initiation of matrix cracking in the material systems studied. The matrix cracking iniates at axial strains from 0.07% to 0.15%. The ACK model overestimates the initiation of the matrix cracking for the material systems studied in this paper.     

Crack Initiation in Fiber-Reinforced Brittle Laminates

In fiber-reinforced brittle laminates, crack growth under monotonic tension generally consists of crack tunneling along the weaker ply (usually the 90° ply) followed by plane strain crack growth through the adjacent, more resistant plies (the 0° plies). In this paper, the details of this transition in crack mode are examined. The tunneling crack configuration is generalized to allow the crack to penetrate the 0° ply during tunneling. The effects of crack bridging in the 0° plies on the energetics of tunneling are computed numerically for general cases and combined with analytical results for certain limits. The nature of the transition from tunneling to plane strain cracking is found to depend on the ratio of the toughnesses of the 90° and 0° plies. Implications for laminate design are discussed.     

Electrochemical potential noise of 321 stainless steel stressed under constant strain rate testing conditions

An investigation was carried out on the features of potential electrochemical potential noise of stressed AISI321 stainless steel in solution 0.5 mol/L Na₂SO₄ + 5 × 10⁻³ mol/L H₂SO₄ under constant-strain-rate-testing (CSRT) conditions. The results showed that the strained steel exhibited a white noise feature at low frequencies and the amplitude of potential fluctuation depended on elongation of the steel. Power spectral density (PSD) of the noise increased with increasing strain level. The noise level in the elastic region of the steel was relatively low, which increased with elongation. After the steel yielded, the electrochemical noise level became higher but it increased less significantly with increasing strain. In the fracture region, the potential noise reached the maximum level. In addition to the dependence of electrochemical noise on strain level, it was also found that the electrochemical noise level increased with increasing strain rate. To interpret the generation mechanism of electrochemical noise, a simple model was proposed based on an assumption that strain results in breakdown and repairing of the passive film on the steel. With this model, the dependence of electrochemical potential noise and its power spectral density on strain level and strain rate can be successfully explained.     

Shear deformation under impact conditions

The deformation behavior that results from falling weight impact testing with flat-headed darts is analyzed in terms of elasticity and plasticity. From a comparison of the time and temperature dependence of the yield stress that is observed during a falling weight test with the same dependences of the yield stress during simple shear tests, it is concluded that the use of flat-headed darts results in strong shear loading. Consequently most of the plastic deformation takes place by shear flow. Of practical interest is that this leads to a simple dependence of the yield stress during the impact test and the impact energy on the plate thickness. Furthermore, for a number of cases this result allows a direct conversion of temperature effects to time effects. In practice this means that it is possible to simulate the impact behavior at very high impact rates simply by performing impact tests at lower temperatures.     

Crack Initiation in Borosilicate Glass-SiC Fiber Composites

The initiation of matrix microcracking was investigated in unidirectional glass matrix composites having controlled fiber spacing. Observations were taken from composites consisting of regular arrays of TiB₂-coated SIGMA 1240 and carbon-coated SCS-6 monofilament SiC fibers in a series of borosilicate glasses. The thermal expansion mismatch between the fibers and glass matrix was varied such that the resulting radial stresses after processing ranged from tensile to compressive. The glass strongly bonds to the TiB₂-coated SIGMA 1240 fiber but weakly bonds to the carbon coating of the SCS-6 fiber, allowing the investigation of the effects of bonding at the fiber/matrix interface. The observed crack initiation stresses of the various composites are compared to predictions based on a previously developed semiempirical model and used to study the influence of the volume fraction of fibers, residual stress state and interface strength.     

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.     

Indentation Crack Initiation and Propagation in Tempered Glass

Indentation crack initiation and propagation in tempered glass surfaces are examined and the results compared to those in annealed and in ion-exchange-strengthened glasses. The presence of surface compression due to tempering inhibits median crack initiation during the loading cycle of the indentation and depresses the radial crack initiation load during unloading. However, the extent of lateral cracking is enhanced in tempered glass surfaces. In situ crack propagation experiments reveal that the compressive stress tends to weakly stabilize crack extension prior to failure. The degree of crack stabilization is considerably lower than expected from a theoretical analysis.     

Compressive mechanical properties of sawdust/high density polyethylene composites under various strain rate loadings

This article is concerned with the static and dynamic mechanical properties of high‐density polyethylene (HDPE) reinforced with sawdust (SD) at a strain rate of up to 10³ s^?1. In this study, the static and dynamic properties of HDPE/SD composites with different filler loadings of 5, 10, 15, 20, and 30 wt% SD were deliberated at different levels of strain rates (0.001, 0.01, 0.1, 650, 900, and 1100 s^?1) using a conventional universal testing machine and the split Hopkinson pressure bar apparatus. The results showed that the stress–strain curves, yield behavior, stiffness, and strength properties of the HDPE/SD composites were strongly affected by both the strain rate and the filler loadings. Furthermore, the rate sensitivityof the HDPE/SD composites showed a great dependency on the applied strain rate, increasing as the strain rate increased. However, the thermal activation values showed a contrary trend. Meanwhile, for the postdamage analysis, the results showed that the applied strain rates influenced the deformation behavior of the tested HDPE/SD composites. Moreover, for the fractographic analysis at dynamic loading, the composites showed that all the specimens underwent a severe catastrophic deformation. J. VINYL ADDIT. TECHNOL., 24:162–173, 2018. © 2016 Society of Plastics Engineers     

Hertzian Ring Crack Initiation in Hot-Pressed Silicon Carbides

The use of Hertzian indentation to measure ring crack initiation force (RCIF) distributions in four hot-pressed silicon carbide (SiC) ceramics is described. Three diamond indenter diameters were used with each SiC; the RCIF in each test was identified with the aid of an acoustic emission system; and two-parameter Weibull RCIF distributions were determined for all 12 combinations. RCIF testing was found to be an effective discriminator of contact damage initiation and response. It consistently produced the same ranking of RCIF between the four SiCs, with all three different indenter diameters, which is noteworthy because Knoop hardness and fracture toughness measurements were only subtly different or equivalent for the four SiCs. However, because RCIF, like hardness, is a characteristic response of a target material to an applied indentation condition (e.g., a function of indenter diameter) and not a material property, the implications and possible limitations should be acknowledged when using RCIF to discriminate the target material response.     

Constitutive modeling of polycarbonate during high strain rate deformation

A constitutive model is presented for large strain deformation of polycarbonate (PC) at high strain rates (above 10² s^?1). The proposed model considers the primary process (α) and the two secondary rate‐activated processes (β and γ). It is shown that the secondary transitions in the material affect the yield and post yield behavior of the material at high strain rates. The constitutive model has been implemented numerically into a commercial finite element code through a user material subroutine. The experimental results, obtained using a split Hopkinson pressure bar, are supported by dynamic mechanical thermal analysis (DMTA) and DSR (Decompose/Shift/Reconstruct) method. These are employed to gain understanding of the material transitions, and to further the linkages between material viscoelastic, yield, and stress–strain behavior. Comparison of model predictions with experimental data demonstrates the ability of model to capture the characteristic features of stress–strain curve of the material such as initial linear elasticity, global yield, strain softening, and strain hardening at very high strain rates (up to 10,000 s^?1). POLYM. ENG. SCI. 2013. © 2012 Society of Plastics Engineers     

Cyclic-Fatigue Crack Initiation and Propagation in Smooth Alumina Specimens

Crack-initiation sites and crack-propagation rates of small cracks in smooth specimens of alumina with two grain sizes have been studied. The principal results that have been obtained are as follows: (i) in most cases, the crack-initiation life comprises a large portion of the fatigue life, (ii) small cracks that are initiated in smooth specimens propagate with stress intensity factors that are much lower than the apparent threshold stress intensity factors ( K _Imax) of artificial cracks that are 200 µm in length, and (iii) the critical crack lengths beyond which the crack-propagation rate is described uniquely by K _Imax are 120 µm for fine-grained alumina and 250 µm for coarse-grained alumina.     

Crack initiation of natural rubber under high temperature fatigue loading

Vulcanized natural rubber (NR) under quiescent thermal oxidation aging and high temperature fatigue loading with small strain amplitude was investigated by the infrared spectroscopy, scanning electron microscopy, and positron annihilation lifetime spectroscopy measurements. IR results demonstrated the thermal oxidation degradation process of vulcanized NR at 85°C. During high temperature fatigue loading, nanoscale cracks and voids that are generated by the combined impact of thermal oxidation and cyclic loading were detected. Further investigation suggests that the nucleation effect of dissolved vapor and gas in the low molecular weight domains of the NR under fatigue loading accounts for the appearance of nanocracks. This work provides some new insight into the crack initiation mechanism of NR during high temperature fatigue loading, which has not been clearly understood. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Multiple crazing at crack tips in PVC under plane strain conditions

The nature of the yield zone at the crack tip of poly(vinyl chloride) (PVC) pipe materials has been investigated. Microscopy studies employing a plasma etching technique reveal the presence of multiple crazes ahead of the crack tip in the interior of specimens of pure PVC, CaCO₃ filled PVC, and PVC pipe compound. The craze zone and the fracture toughness of blade-notched specimens are compared with those of fatigue pre-cracked specimens. Both types of specimens have similar fracture toughness values and form multiple crazes upon loading, suggesting that multiple crazing Is an intrinsic property of the material. The kinetics of craze initiation and the development of the multiple craze zones have also been explored.     

Electric-Field-Induced Crack Initiation From a Notch in a Ferroelectric Ceramic

Crack initiation of ferroelectrics under a cyclic electric field was studied using DCB specimens, which contained through-thickness notches of different lengths. An electric field larger than the coercive field strength of the material was applied and resulted in pop-in of a crack in the direction perpendicular to the field. The length of the pop-in varied with the length of the notch. Finite Element modeling was used to quantify the stresses occurring in the specimen under the application of the electric field. The model took into account the fully coupled ferroelectric and ferroelastic hysteresis behavior. Stress intensity factors were derived from the resulting stress distribution using the weight function method. The results were found to agree well with the experimental data.     

Effect of strain rate on the behavior of three-dimensional polymers of oligoesteracrylates under the conditions of the uniaxial elongation

For three-dimensional polymers based on the dimethacryloil derivatives adipic and sebasinoil acids and dicarbonyloxyethylene we have established the influence of the deformation velocity (V _ε) on their behaviour under the conditions of the uniaxial elongation. With the increase of V _ε up to a distinct value of V _εl , the ultimate stress σ _m for of the polymers grows, but at V _ε > V _εl the value of σ _m decreases. This phenomenon is explained. With the rise of V _ε, the module of elasticity E ₁ increases and the curves σ(ε) are shifted to the left along the ε axis. The change of the character of the structural organization of three-dimensional polymers in the deformation process of their samples was established.