Mode Fracture Behavior Research Articles

Plane Stress Fracture Toughness Testing of Freestanding Ultra-Thin Nanocrystalline Gold Films on Water Surface.

Fracture toughness, which is the resistance of a material to crack propagation, is a critical material property for ensuring the mechanical reliability of damage-tolerant design. Recently, damage-tolerant design is introduced to flexible electronics by adopting micro-cracked ultra-thin nanocrystalline (NC) gold films as stretchable electrodes in a plane stress state. However, experimental investigation of the plane stress fracture toughness of those films remains challenging due to the intrinsic fragility from their sub-100nm thicknesses. Here, a quantitative method for systematically evaluating the plane stress fracture toughness of freestanding ultra-thin NC gold film on water surface platform is presented. After effectively fabricating single-edge-notched-tension samples with femtosecond laser, mode I stress intensity factors are measured in the plane stress state on water surface. Moreover, investigation regarding the effect of notch length, notch sharpness, and notch tip plasticity validates this method based on linear elastic fracture mechanics theory. As a demonstration, the thickness-dependent plane stress fracture toughness of ultra-thin NC gold films is qualitatively unveiled. It is revealed that the thickness confinement effect on grain boundary sliding induces a transition in fracture behavior. This method is expected to further clarify the fracture-related properties of various ultra-thin films for next-generation electronics.

Small-scale measurement of the transition in fracture behavior of marine sediments

Bubbles grow and burrows extend through cohesive, muddy marine sediments by fracture. In contrast, sands are non-cohesive, granular materials. Natural sediments comprised of heterogeneous mixtures of muds and sands are common in coastal areas and provide important habitat for infaunal animals. To explore the transition from cohesive to non-cohesive mechanical behavior of natural sediments, we modified a probe designed for measuring fracture toughness (KIc). The helical probe is rotated and translated into sediment to grip a plug of sediment, then translated upward to break off the plug while force is measured. Fracture toughness is calculated from the peak net force. The probe shows clearly distinct results in muddier sediments, in which fracture occurs, and in sandier sediments, in which no fracture occurs. The modified probe is limited to near-surface sediments, but it provides a novel method for distinguishing cohesive sediments with tensile strength from non-cohesive sediments on scales relevant for burrowing animals or bubble growth. This measurement allows for comparison of surface and subsurface cohesion and for assessing how tensile strength depends on other properties of sediments.

Phase evolution and mechanical properties of an intercritically-annealed Fe–10Ni–7Mn (wt. %) martensitic steel severely deformed by high-pressure torsion

Experiments were conducted on an Fe–10Ni–7Mn (wt. %) martensitic steel processed by high-pressure torsion (HPT) after intercritical annealing in the dual phase (α+γ) region at 600 °C for 2 h. Microstructural examination and phase analysis using electron backscatter diffraction (EBSD) confirmed that post-HPT processing led to a phase transformation of austenite to α′-martensite (γ→α′ and γ→ε→α′) such that after 20 turns of HPT the microstructure included multi-phases of α′-martensite, austenite and ε-martensite. The HPT processing also produced a significant reduction in the α′-martensite and austenite grain sizes from initial values of ~3.8 and ~3.9 μm to ~225 and ~60 nm after 20 HPT turns, respectively, while the ε-martensite final grain size was ~44 nm. Micro-hardness measurements and tensile testing showed a significant increase in hardness and in the ultimate tensile strength to about 680 Hv and 2235 MPa after 20 turns, respectively. Finally, the fracture surfaces of samples after HPT processing showed a combination of dimples, cracks and shear bands which confirmed a transition in fracture behavior from ductile to a mixture of a ductile and brittle mode.

Mechanical properties and fracture behaviour of agave fibers bio-based epoxy laminates reinforced with zinc oxide

Biolaminates of Ixtle and Henequen natural fibers reinforced bio-based epoxy resin were prepared using Vacuum Assisted Resin Infusion process. ZnO nanoparticles were added to the bio-based epoxy resin at 1, 2 and 3 wt. % content before impregnation process. The viscoelastic and mechanical properties, as well as the fracture behavior, were evaluated and related to the nature of the fibers and filler content. The viscoelastic results indicated the ZnO particles are effective fillers just at low concentrations, and induce different reinforcement mechanisms attributed to the interaction between the nature of fibers and nanoparticles. The mechanical properties of the Ixtle biolaminates decreased at higher filler concentrations, while Henequen biolaminates showed better mechanical properties just above the 2 wt. % of ZnO. The fracture behavior in mode I registered moderate changes in toughness, related to the ZnO fraction, which promoted different behaviors on the interlaminar adherence of the layers. The results point to the need to continue evaluating the potential application of these green composites for their use in construction and automotive industries.

Large Toughening Effect in Biomimetic Geopolymer Composites via Interface Engineered 3D Skeleton

Green and eco-friendly geopolymers with high thermal/acid resistance represent potential candidates for the replacement of traditional Portland cement in construction, as well as many other applications; however, the intrinsic brittleness and low toughness typical of ceramic hinders widespread adoption of this material in various applications. In this work, we fabricated a new type of geopolymer composites by impregnated with interface engineered 3D skeleton resembling the lotus root structure. Highly porous melamine foam was selected as the 3D skeleton and its interior surface was coated with elastomeric polydimethylsiloxane–polyurea block copolymer. Under loading, the interfacial elastomer could deform and absorb large amount of energy concurrently with crack deflection of melamine foam and delamination of interfaces, thus the toughness was substantially improved as results indicated a transition of fracture behavior from brittle failure mode to a more ductile one. With as low as 2.5 wt % elastomer, the...

Rate-dependent shear banding and fracture behavior in a ductile bulk metallic glass

The rate-dependent shear banding and fracture behavior of a ductile bulk metallic glass (BMG) under compressive loading are investigated in this paper. The continuous deformation process of the BMG specimens is captured by using the high-speed camera under both quasi-static and dynamic compression. Their characteristic shear banding and fracture behavior are synchronized with corresponding mechanical responses. The rate effect on initiation of shear bands is attributed to an increase in the coalescence rate of free volume. Rate-dependent shear banding behavior, i.e., the transition from multiple shear banding at lower strain rate to single shear banding at higher strain rate, are resulted from an increase in free-volume concentration level with increasing loading rate. The transition of fracture behavior from shear-dominated progressive sliding along slip planes under quasi-static compression to crack-dominated failure under dynamic compression is controlled by the patterns and time-scale of shear banding at various strain rates.

Size-dependent fracture behavior of silver nanowires

Silver (Ag) nanowires have great potential to be used in the flexible electronics industry for their applications in flexible, transparent conductors due to high conductivity and light reflectivity. Those applications always involve mechanical loading and deformations, which requires an in-depth understanding of their mechanical behavior and performance under loadings. However, current understanding on the mechanical properties of Ag nanowires is limited, especially on their size-dependent fracture behavior. In this work, mechanical properties of Ag nanowires with diameters ranging from 50 to 300 nm were systematically studied by in situ TEM tensile testing for the first time. The size effect was clearly found, with the increasing of the diameter of Ag nanowires, the ultimate tensile stress decreased. More importantly, the fracture behavior of Ag nanowire was studied and a brittle-to-ductile transition in fracture behavior was observed at the diameters around 100 nm which could be attributed to the dislocation activities within the geometry confinement. This work could give insights for understanding nanosized Ag wires and the design of Ag nanowire-based flexible devices and touchable panels.

On the Effect of Natural Aging Prior to Low Temperature ECAP of a High-Strength Aluminum Alloy

Severe plastic deformation (SPD) can be used to generate ultra-fine grained microstructures and thus to increase the strength of many materials. Unfortunately, high strength aluminum alloys are generally hard to deform, which puts severe limits on the feasibility of conventional SPD methods. In this study, we use low temperature equal-channel angular pressing (ECAP) to deform an AA7075 alloy. We perform ECAP in a custom-built, cooled ECAP-tool with an internal angle of 90° at −60 °C and with an applied backpressure. In previous studies, high-strength age hardening aluminum alloys were deformed in a solid solution heat treated condition to improve the mechanical properties in combination with subsequent (post-ECAP) aging. In the present study, we systematically vary the initial microstructure—i.e., the material condition prior to low temperature ECAP—by (pre-ECAP) natural aging. The key result of the present study is that precipitates introduced prior to ECAP speed up grain refinement during ECAP. Longer aging times lead to accelerated microstructural evolution, to increasing strength, and to a transition in fracture behavior after a single pass of low temperature ECAP. These results demonstrate the potential of these thermo-mechanical treatments to produce improved properties of high-strength aluminum alloys.

Fracture behavior of nanostructured heavily cold drawn pearlitic steel wires before and after annealing

In situ micro-cantilever fracture testing is used to demonstrate changes in fracture behavior of nanostructured, heavily cold drawn pearlitic steel wires as a function of drawing strain and annealing conditions. It is shown that these steels exhibit a sharp transition in fracture behavior between a drawing strain of 320% and 520% with a drop in fracture toughness from 7.5 to 4MPam1/2. This is confirmed from the nature of fracture which is stable with some degree of plasticity at drawing strains below 320% and changes to catastrophic cleavage fracture at drawing strains of 420% and above. This transition and associated brittleness is attributed to structural (cementite decomposition and strain induced increase in tetragonality) and microstructural (increasing nanocrystallinity and dislocation density) evolution that these steels undergo at higher drawing strains. On heat treating the 420% strained sample, brittle cleavage fracture continues for low temperature (200°C) annealing with no visible changes in microstructure, while crack growth is suppressed and large-scale plasticity is recovered for high temperature (500°C) annealing with accompanying grain coarsening, and re-precipitation of spherodized cementite at grain boundaries.

Mixed Mode (I/III) Fracture Behaviour of an Aerospace Grade Aluminium Alloy in Different Ageing Conditions

The effect of ageing on the mixed mode fracture toughness (I/III) was evaluated for the aluminium alloy AA2219 in limited underaged (LUA), underaged (UA) peakaged (PA) and overaged (OA) conditions. Opening mode fracture toughness (Mode I) test was conducted. Sheet tensile samples were prepared and tested in all the ageing conditions. Optical microscopy has been carried out and fractographic studies have been conducted to study and determine the mode of failure.

Super-toughening polyamide-612 by controlling dispersed phase domain size: Essential work of fracture assessment

A new pathway to super-toughen polyamide-612 (PA-612) by incorporating domains of soft poly(octene-co-ethylene)-g-maleic anhydride (POE-g-MA) via melt blending leading to more than ∼1100% increase in notched Izod impact strength vis-à-vis fracture toughness enhancement is demonstrated. Fourier transform infra red (FTIR) studies showed effective phase interactions between PA-612 and POE-g-MA whereas dynamic mechanical analysis (DMA) revealed a reduction in loss-peak intensity at ∼45°C with increase in the soft phase fraction. The optimal dependence of fracture-toughness (in plane-stress) on domain-size (Dn) of dispersed-phase in the form of a reduction in resistance to crack initiation indicated by essential work of fracture (we) and linear increase in resistance to crack propagation indicated by non-essential work of fracture (βwp) of the blends ⩾10wt% of POE-g-MA content is correlated to an increase in domain-size ⩾∼0.3μm. Fracture surface morphology indicated crazing to be responsible for the transition in fracture behavior, i.e. remarkable toughening of PA-612 at the critical rubber phase domain size range of ∼0.2–0.3μm.

Crack propagation behavior and toughness of V‐notched polyethylene terephthalate injection moldings

AbstractThe role of the skin and core regions in controlling the effects of V‐notches, on the fracture behavior of PET injection‐moldings, was correlated with their tensile and impact properties. Investigations revealed that there were three distinct fracture behaviors: ductile, semiductile, and brittle fracture transitions. The notch sensitivity factor for strength (KS) in the ductile and semiductile transitions indicates that the fracture strength was not sensitive to ≤1.5 mm deep notches, which is considered the skin region. The introduction of core‐deep notches (>1.5 mm) resulted in a rapid increase in KS. On the other hand, the notch sensitivity factor for energy (KT) shows that the fracture energy was not sensitive at ≤0.5 mm deep notches. However, KT increased drastically when notches >0.5 mm deep were introduced. The development of an anisotropic skin‐core structure in injection moldings is well acknowledged. This is revealed in a constant fracture behavior between 0.6 and 1.0 mm deep notches. Notably, there was a drastic change in the fracture pattern from ductile to semiductile at a critical 0.6 mm deep notch. The specimens experienced a mixed fracture behavior at 1.5 mm deep notch, which marks a transitional fracture pattern at the interface between the skin and core regions. Lastly, a constant fracture behavior was observed at notch depths ≥1.5 mm. Results show that crack opening, in the samples that had semiductile fracture, was a postnecking phenomenon. Before shear yielding, two shear lines that intersected at 45° were seen to originate from the crack root when a 1.2 kN load was applied. Conversely, crack opening and failure occurred simultaneously in brittle fractures. It is obvious that V‐notches provided a gradual transition in fracture behavior from the skin to the core regions, which confirms that the fracture behavior of PET injection moldings can be dependent on the skin and core structure. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Variation in Dislocation Pattern Observed in SCS Films Fractured by Tensile Test: Effects of Film Thickness and Testing Temperature

AbstractThis paper reports a transition in the fracture behavior of micron-sized single-crystal-silicon (SCS) film in an MEMS structure for various film thicknesses and ambient temperatures. The mean fracture toughness of 4-µm-thick SCS films was 1.28 MPa at room temperature (RT), and the value increased as the film thickness decreased, reaching 2.91 MPa for submicron-thick films. The fracture toughness of 4-µm-thick film did not change for ambient temperatures ranging from RT to 60ºC. However, it drastically increased at 70ºC and reached 2.60 MPa at 150ºC. Enhanced dislocation activity in the SCS crystal near the fracture surface was observed on 1-µm-thick film at RT and 4-µm-thick film at 80ºC by high-voltage electron microscopy. This change in dislocation activity seemed to correlated with the transition in fracture behavior.

Physical aging and time–temperature behavior concerning fracture performance of polycarbonate

The effects of physical aging on fracture and yielding behavior are polycarbonate are considered. Two groups of Bisphenol A-based polycarbonate, consisted of extruded PC sheets (thickness of 0.25 mm) and injection molded PC bars (thickness of 3.18 mm) are used. These samples were annealed at various temperatures ranging from 60 to 120 °C, for different times varying up to 240 h. For PC sheets the essential work of fracture (EWF) method was used to analyze fracture behavior. The results are compared to the strain energy density with aging time and aging temperature in the ranges investigated. This effect is confirmed by the change in fracture toughness, as measured by three-point bending tests. The concept of fictive temperature ( T f) was used to characterize the degree of aging in the sample. T f of a glass in an aged state at a time t is defined as the temperature at which the volume would be equal to the equilibrium volume at T f if the sample were instantaneously removed to that temperature. Differential scanning calorimetry (DSC) was used to determine T f. The variations of T f with aging time and aging temperature are in agreement with both the strain energy density measurement and the three-point bending tests. These results contradict the effects of aging on fracture toughness observed by the essential work of fracture approach. The latter showed anomalous regions of increasing fracture toughness with aging, leading to spurious conclusions. The brittle–ductile transition in fracture behavior is analyzed by an activation energy approach. Aging increases the brittle–ductile transition temperature and the effect is more pronounced for the lower molecular-weight sample. Fracture tests also showed a decrease in the entropy with aging, confirming the results observed previously from tension and compression tests.

Finite Element Investigation of Quasi-Static Crack Growth in Functionally Graded Materials Using a Novel Cohesive Zone Fracture Model

This work studies mode I crack growth in ceramic/metal functionally graded materials (FGMs) using three-dimensional interface-cohesive elements based upon a new phenomenological cohesive fracture model. The local separation energies and peak tractions for the metal and ceramic constituents govern the cohesive fracture process. The model formulation introduces two cohesive gradation parameters to control the transition of fracture behavior between the constituents. Numerical values of volume fractions for the constituents specified at nodes of the finite element model set the spatial gradation of material properties with standard isoparametric interpolations inside interface elements and background solid elements to define pointwise material property values. The paper describes applications of the cohesive fracture model and computational scheme to analyze crack growth in compact tension, C(T), and single-edge notch bend, SE(B), specimens with material properties characteristic of a TiB/Ti FGM. Young’s modulus and Poisson’s ratio of the background solid material are determined using a self-consistent method (the background material remains linear elastic). The numerical studies demonstrate that the load to cause crack extension in the FGM compares to that for the metal and that crack growth response varies strongly with values of the cohesive gradation parameter for the metal. These results suggest the potential to calibrate the value of this parameter by matching the predicted and measured crack growth response in standard fracture mechanics specimens.

Low-velocity impact fracture behaviour of impact-resistant polymer matrix composite laminates under mixed mode loading

The mixed mode (I+II) fracture behaviour of the newly developed 'interlayer-toughened' carbon-fiber/epoxy composite system, T800H/3900-2 (Toray), was studied over a wide range of loading rate including low-velocity impact region (displacement rate: 10 -6 -10 1 m/s). The MMF (Mixed Mode Flexure) specimen and SHPB (Split Hopkinson Pressure Bar) system were employed for measuring the mixed mode fracture toughness under impact loading. The experimental results showed that the mixed mode fracture toughness was sensitive to loading rate; the impact fracture toughness was about 30-38 % lower than the static value. The microscopic fracture morphology was rather sensitive to loading rate; the impact fracture surface was somewhat smoother than the static fracture surface. The effects of mode mixture and their rate dependence were discussed on the basis of the above and previous results by the authors. The validity of the estimation method for mixed mode impact fracture toughness was also discussed on the basis of computational results.

Influence of triblock copolymer interfacial modifiers on fracture behavior of polystyrene/ethylene‐propylene rubber blends

AbstractIn a previous study, two triblock copolymers of styrene/ethylene‐butylene/styrene (SEBS), of different molecular weights, were used to compatibilize a blend of 80 vol% polystyrene (PS) and 20% ethylene‐propylene rubber (EPR). The emulsification curve, which relates the average minor phase particle diameter to the concentration of interfacial agent added, was used to quantify the effect of the interfacial agents on the blend morphology. Links between morphology, interface, and properties were established by combining the emulsification curve with a fracture mechanics approach. The aim of this work is to foster the understanding of the effects of these two triblock copolymers on the fracture behavior of the blend over various loading rates and temperatures. The focus is on the brittle‐ductile transition in fracture behavior, which is a critical condition for the application of these materials. It has been found that adding an interfacial agent lowers the temperature at brittle‐ductile transition. However, this effect is much more pronounced for the copolymer with a lower molecular weight. The time‐temperature dependence of fracture performance of the blend is also affected by the interface and morphology. When loading rate increases, the shift of the temperature at brittle‐ductile transition is less significant for the blend with an interfacial agent having a lower molecular weight. The effect of loading rate and temperature on the brittle‐ductile transition in fracture performance of the blends is controlled by an energy activation process. Adding the interfacial agents results in a plasticizing effect of the polystyrene matrix and a reduction in the energy barrier controlling the fracture process. With the addition of interfacial agent, the yield stress slightly increases at low concentration, attains a maximum value, and then decreases. The increase in yield stress confirms the coupling role of the copolymer and is in agreement with the observed emulsification curves. The reduction of yield stress and increase in ultimate strain with the copolymer concentration demonstrate the plasticizing effect of the interfacial agent. The result of stress relaxation tests also confirms the above effects of the interfacial agent.

Mode II delamination behavior of carbon/epoxy composites

The end-notched flexure (ENF) test was employed in an investigation of the interlaminar fracture behavior in mode II static and fatigue loading for carbon/epoxy composites T300/M10 and HTA/6376. The mode II interlaminar fracture toughness was determined by a compliance calibration method. The tests were performed in the range of the a/L ratio from 0.5 to 0.9. Both composites, T300/M10 and HTA/6376, exhibited brittle unstable and brittle stable behavior of crack growth. Stable mode II crack growth was observed during fatigue loading with R = 0.1 in a large range of cyclic strain energy release rate. A consistent relation of the crack growth rate to the cyclic strain energy release rate was obtained for both T300/M10 and HTA/6376 composites. Within the range of moderate crack growth rate, the data of mode II fatigue crack growth for the composites obeyed a power law. The trend of reduction in the crack growth rate data with degradation in the cyclic strain energy release rate suggests the presence of a thre...

Analytic model for scaling of breakdown.

A theory is presented for the time-dependent breakdown of a network of spring (fuse) elements where the probability of breaking an element under load-sigma is sigma-eta. For all eta, it predicts the system-size scaling of the number of broken elements at breakdown round in simulations. The breakdown is shown to be percolationlike for eta less-than-or-equal-to 2 but is due to the dominance of one large growing crack, despite the absence of a failure threshold, for eta textgreater 2. This transition in fracture behavior and in scaling at eta textgreater 2 is found to be directly related to the dependence of crack tip stress enhancement on the square root of crack size.