Fracture Transition Research Articles

Melt-mixed PA-6/LDPE-g-MA/nanoclay ternary nanocomposite: Micro-mechanisms from post-yield fracture kinetics and strain field analysis

The post-yield fracture behavior of nanoclay filled polyamide-6 (PA-6)/low density polyethylene-grafted-maleic anhydride (LDPE-g-MA) based ternary nanocomposites revealed a prominent semiductile-to-ductile transition in the deformation mode characterized by the essential work of fracture (EWF) parameters. The fracture parameters in terms of crack-extension (Δa) and crack tip opening displacement (CTOD; δ) were quantified and the crack-toughness aspects have been critically discussed in the realms of CTOD-rate as a measure of blunting efficiency. The crack resistance behavior revealed a three-staged crack extension in combination with a two-stepped crack tip opening displacement (CTOD) phenomena. The CTOD-rate dependent fracture transitions have been qualitatively ascertained. To ascertain the kinetic and dynamics of crack growth micro-mechanisms time-synchronized deformation data prior to failure were acquired and analyzed using digital image correlation technique followed by the integral computation of deformation-stage images using the ARAMIS-GOM software to lead to strain field contours. The strain field so obtained revealed the distinct nature of energy dissipation in the inner-fracture process zone (IFPZ) and outer-plastic deformation zones (OPDZ). Such quantification of the stress wave dissipation modes via strain field analysis demonstrates fundamentally a new approach to understand fracture-mechanics for designing materials objectively aided by convincing visualization.

A study of the stored energy in titanium under deformation and failure using infrared data

The work is devoted to the experimental study of heat dissipation caused by plastic deformation and failure processes taking place in a titanium alloy Ti-4.2Al-1.6Mn. To investigate the spatial and time evolution of temperature, a set of experiments has been carried out on plane titanium smooth specimens and specimens with pre-grown centered fatigue cracks. The original mathematical algorithm for experimental data processing has been applied to obtain the rate of heat dissipation generated by plastic deformation and stored energy. It is shown that the stored energy is accumulated in titanium specimens undergoing fatigue tests, and at the time of damage to fracture transition it is equal to zero.

Modeling flat to slant fracture transition using the computational cell methodology

Macroscopic mode I ductile crack propagation in metallic sheets or plates often starts in mode I as a flat triangle (coplanar with the precrack) whose normal corresponds to the loading direction. After some limited extension, the crack becomes slanted and propagates under local mixed mode I/III. Modeling and understanding this phenomenon is challenging. In this work, the “computational cell” methodology proposed in [1], which uses a predefined crack path, is used to study flat to slant fracture transition. The energy dissipation rate is studied as a function of the assumed crack tilt angle. It is shown that a minimum is always reached for an angle equal to 45°. This correlates well with the variation of the crack tip opening angle (CTOA) or the mean plastic deformation along the crack path. Stress and strain states in the stable tearing region hardly depend on the assumed tilt angle. A parametric study shows that flat to slant fracture transition is less likely to occur in materials having high work hardening and favored if additional damage is caused by the local stress/strain state (plane strain, low Lode parameter) in the stable tearing region.

Elastic–viscous transition in tear fracture of rubbers

There is a widely reported transition in the relationship between the critical strain energy release rate and a critical crack growth rate during the tearing of rubber materials. This transition is explained here for the first time in terms of an elastic–viscous transition phenomenon. The transition thus characterized depends on the balance of elasticity and viscosity of the material and hence the proximity of the test to the materials' glass transition temperature. Therefore two factors dominate the transition behavior, the cross-link density and the visco-elastic energy dissipation. A new elastic–viscous transition diagram is introduced to elucidate the mechanism of this phenomenon, where the diagram consists of three zones, each with a different fracture mode. These are an elastic–brittle fracture zone, a viscous–ductile fracture zone and an intermediate transition zone between the elastic and viscous zones. The transition zone characterized by stick–slip motion is caused in mechanics as results from unstable fluctuations of crack growth rate due to the energy dissipation near the glass transition temperature. The mechanism for the transition to be generated at the crack front is investigated both theoretically and experimentally with a consideration of surface roughness formation and stick–slip tear motion together with frictional sliding of the rubber at the crack tip.

Effect of crystallographic texture on the anisotropy of Charpy impact behavior in pipeline steel

The current study systematically investigated the anisotropy of fracture behavior in commercial API X100 pipeline steel. The experimental results showed that the temperature for transition from ductile to brittle fracture was significantly different in samples with different orientations, defined as anisotropy of fracture behavior in this paper. The inhomogeneity distribution of microstructure (grain size) and the crystallographic texture were systematically analyzed. The difference in grain size of the samples with different orientations can be reasonably ignored in this study. Therefore, the anisotropy of fracture behavior is attributed to the crystallographic textures in which the deformation and transformation texture components were included. The effect of crystallographic texture on the anisotropy of ductile to brittle fracture transition temperature was theoretically interpreted based on the relationship among yield strength, fracture strength, and their average orientation factor.

Probabilistic fracture toughness of a duplex stainless steel in the transition range

The fracture toughness of a 2205 duplex stainless steel (DSS) heavy section forged bar in the transition range (between 0°C and −60°C) was investigated. Experimental results were used to estimate the reference temperature T0 and to define a Master Curve according to the ASTM E 1921-11 Standard.Following this Standard, that assumes the Weibull exponent b fixed and equal to 4, the reference temperature obtained using data at T=−60°C is T0=−110°C; using data at T=−40°C is T0=−103°C and using data at T=−20°C is T0=−110°C.As the description of the experimental data distribution does not follow the theoretical Weibull slope of b=4, the slope obtained by the best fit of the experimental data was also measured and its influence on the assessment of the toughness scatter in the transition range by the weakest link approach ascertained. The recalculated reference temperatures T0 for the data at −60°C, −40°C and −20°C are −114°C (with b=6.5), −107°C (with b=6), and −113°C (with b=4.8) respectively.Experimental data are well fitted by the Master Curve for all the chosen temperatures. Fracture mechanisms were also investigated by means of scanning electron microscope in order to discuss the application of the ASTM E 1921-11 Standard to duplex stainless steels whose microstructure is only partially ferritic and whose fracture transition does not result in a completely cleavage fracture at low temperatures because of the ductile behaviour of the austenite phase.

Research and Development of High Strength Architectural Heavy Plates

This paper provides a detailed description of high strength architectural heavy plates with 80mm in thickness developed at Wuhan Iron and Steel（Group）Corporation(WISCO). The chemical composition of plates contains mainly C-Mn-Nb-V-Ti with proper content of other alloys, and the thermal-mechanical controlled process and normalizing treatment were applied. The results show that the base plates manufactured at WISCO have a good match of high strength, good through-thickness characteristic, low yield ratio and low temperature toughness with fine microstructure, and the fracture transition temperature is about -40°C. The welding plate also has high strength and good low temperature toughness which comprehensively meet the technical requirement of large-scale architectural buildings.

Strength, plasticity, interlayer interactions and phase transition of low-dimensional nanomaterials under multiple fields

Atoms are hold together to form different materials and devices through short range interactions such as chemical bonds and long range interactions such as the van der Waals force and electromagnetic interactions. Quantum mechanics is powerful to describe the short range interactions of materials at the nanometer scale, while molecular mechanics and dynamics based on empirical potentials are able to simulate material behaviors at much large scales, but weak in handling of processes including charge transfer and redistributions, such as mechanical-electric coupling of functional nanomaterials, plastic deformation, fracture and phase transition of nanomaterials. These issues are also challenging to quantum mechanics which needs to be extended to van der Waals distance and larger spatial as well as temporal scales. Here, we make brief review and discussions on such kind of mechanical behaviors of some important functional nanomaterials and nanostructures, to probe the frontier of nanomechanics and the trend to multiscale physical mechanics.

Analysis of the Evaluation Indices from TSRST

AbstractBecause the thermal stress restrained specimen test (TSRST) is recommended by the Strategic Highway Research Program (SHRP) as a test method to evaluate low-temperature cracking resistance of asphalt mixtures, many studies have been carried out to investigate the performance of asphalt mixture with this test using fracture temperature as the indicator. However, few of them focus on discussing its reliability. In this paper, six kinds of asphalts mixtures are evaluated by TSRST, and it is found that the evaluation results obtained by the four indices are contradictory. It is also inferred from the results of each index coefficient of variation that fracture temperature and transition temperature are more stable when they are used to evaluate low-temperature performance of asphalt mixtures. Through the application of principal-component analysis (PCA) and the Boston Consulting Group’s matrix on the TSRST results, it is demonstrated that fracture temperature used as the indicator of low-temperature p...

Cohesive laws X-FEM association for simulation of damage fracture transition and tensile shear switch in dynamic crack propagation

This paper is devoted to the formulation of transitions in fracture for quasi static and dynamic crack propagation. It is organized in three parts. The first one describes a general way to construct a cohesive law which is thermodynamically equivalent to a damaging bulk material. The second part is devoted to the proposition of a unique dynamic crack propagation law which discriminates between tensile and shear cracking in case of moderate plasticity at crack tip. The third part of the paper compares experiments with simulation in both cases.

Non-fracture prediction of a C–Mn weld joint in the brittle-to-ductile fracture transition temperature range: Part I: Experimental results and numerical study

This paper evaluates the brittle fracture risk for a C–Mn weld in the upper shelf of the brittle-to-ductile transition: the criterion considered is based on a critical stress σ th, with the failure probability related to the volume around the crack where the maximum principal stress exceeds σ th. The weld shows a complex microstructure with two types of melted zone. SEM observations showed that the main cleavage sites are located in the coarse grain zone, near an inclusion. In test analyses, the material’s heterogeneity in the weld metal is considered to estimate local fields at the weld pass scale.

Effect of Meso to Micro Transition in Morphology Dependent Fracture of SiC Ceramics

Silicon carbide (SiC) is an important ceramic material usually found in polycrystalline form with grain boundary thickness ranging from a few nanometers to a few hundred nanometers and grains with multiple orientations with sizes of the order of few micrometers. The present work focuses on analyzing how the interplay between different orientations of SiC grains and different grain boundary thicknesses can be exploited for targeted improvement in the fracture resistance properties of SiC. Crack propagation simulations using the cohesive finite element method (CFEM) are performed on the finite element meshes developed on experimentally processed SiC morphologies. Analyses were performed at two different length scales: 300 μm × 60 μm (scale-1:Microscale) and 75 μm × 15 μm (scale-2:Mesoscale). Lower resolution microstructure at scale-1 does not explicitly consider the presence of grain boundaries (GBs). Higher resolution microstructure at scale-2 explicitly models GBs. Results indicate that the effect of change in grain orientation is on crack path only. The fracture resistance is not significantly affected. The presence of GBs may directly aid in strengthening a microstructure’s fracture resistance. However, indirectly it may weaken a microstructure by favoring the formation of microcracks. Significantly higher crack formation in grain interior while lower interfacial energy dissipation in comparison to interfaces indicates overall lower fracture strength of grain interiors in comparison to interfaces. If GBs are not accounted for, the second most influencing factor affecting fracture strength is the average grains size. Overall, it is mainly the GBs not the grain orientation distribution and grain size that significantly affects fracture strength.

Flat to slant ductile fracture transition: Tomography examination and simulations using shear-controlled void nucleation

Tomography examination of arrested cracks at crack initiation, flat to slant transition and slant propagation indicates a clear change in micromechanisms from high-stress triaxiality void growth to shear-dominated coalescence in the slant regime. A Gurson-type model is used and a shear void nucleation term based on the Lode parameter for strain rate is suggested for simulation of a fully meshed sample, which achieves flat to slant crack transition at loads close to experimental results for aluminium.

Strength-Mismatch Effect on Steel Weld HAZ-Toughness in CTOD and Charpy Tests

The effects of strength-mismatch between the base and weld metal in welded joints on CTOD and Charpy impact toughness of heat-affected zone (HAZ) are investigated in this study. Both toughness properties are compared, in consideration of the effect of the local brittle zone (LBZ) in HAZ, and strength-mismatch effects are discussed analytically. Two types of welded joints for 490 MPa strength class structural steel with the same thickness of 25 mm, which were strength-matched and overmatched joints, are prepared under the same welded conditions (heat input = 4 KJ/mm) with different welding consumables. The critical CTOD of the HAZ for the overmatched joint exhibits the lower values in wide temperature range of ductile-to-brittle fracture transition than that for the matched joint, whereas microstructures and total LBZ size along crack-tip are consistent with each other. On the contrary, significant difference between the matched and overmatched joints in the lower temperature range cannot be observed in the Charpy impact toughness. The reason why the strength-mismatch effects on the fracture toughness are different between the CTOD and Charpy impact tests is discussed.

Ductility of an aluminum–4.4 wt. pct. magnesium alloy at warm- and hot-working temperatures

An AA5182 aluminum alloy sheet, containing 4.4 wt. pct. magnesium, was subjected to tensile testing at temperatures from 100 to 400 ° C under strain rates from 1 0 − 3 up to 3 × 1 0 − 2 s −1. Flow stress, tensile elongation and reduction-in-area were measured and are correlated with deformation and fracture mechanisms. At slow strain rates and elevated temperatures, solute-drag creep produces large tensile elongations, up to 247%, and large reductions in area, up to 91%. Tensile elongation is greatest when the Zener–Hollomon parameter is in the range of 1 0 9 to 1 0 10 s −1. Ductility decreases at the slowest strain rates and highest temperatures because of cavity interlinkage leading to fracture. As strain rate increases and temperature decreases beyond the range of peak ductility, an increased rate of flow localization, i.e. necking, reduces elongation and reduction-in-area. Ductility further decreases with increasing strain rate and decreasing temperature as deformation transitions into logarithmic creep and fracture transitions to a ductile shear mode. At the lowest temperature and fastest strain rate applied, the Portevin–Le Chatelier (PLC) effect is observed and ductility is least.

Brittle-Quasibrittle Transition in Dynamic Fracture: An Energetic Signature

Dynamic fracture experiments were performed in polymethylmethacrylate over a wide range of velocities and reveal that the fracture energy exhibits an abrupt threefold increase from its value at crack initiation at a well-defined critical velocity, below the one associated with the onset of microbranching instability. This transition is associated with the appearance of conics patterns on fracture surfaces that, in many materials, are the signature of damage spreading through the nucleation and growth of microcracks. A simple model allows us to relate both the energetic and fractographic measurements. These results suggest that dynamic fracture at low velocities in amorphous materials is controlled by the brittle-quasibrittle transition studied here.

G0300-1-3 ポリカーボネート板の延性 : 脆性破壊遷移現象について([G0300-1]材料力学部門一般講演(1):破壊)

Ductile-brittle fracture transition of notched polycarbonate plate was studied with respect to the specimen thickness t, notch root radius ρ and the ratio of K_II to K_I As a result, there was a tendency to arise the brittle fracture as increased with t and 1/ρ under mode I loading. However, failure mode of natural cracked specimen, ρ = 0, of thick plate was ductile fracture. Moreover, it was revealed that failure mode was put under the control of ductile fracture as increased with the ratio of μ = K_II/K_I under mixed-mode loading. Caustic pattern formed by the information at the crack tip was obtained, and the ratio of μ was evaluated from the configuration of this pattern.

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

Composites of photocured acrylic resins with natural and polymeric fibers

The aim of this work was to examine a new family of composites based on the matrix produced of photocrosslinked resins filled with various types of natural or highly oriented synthetic thermoplastics fibers, and finally to examine the properties of such composites in comparison with samples produced from neat photocured resins. The matrix of the composites was the UV-cured epoxyacrylate or epoxymethacrylate resin. The isotactic polypropylene (iPP) fibre, modified with ethylene vinyl acetate (EVA) copolymer, or hemp fibre was used as the reinforcement. An increase in certain mechanical properties was reached in most of the composite materials studied. In case of iPP fibres reinforced composites, a brittle to ductile fracture transition was observed.

Mechanics of nanocrack: Fracture, dislocation emission, and amorphization

Understanding the nanoscale fracture mechanisms is critical for tailoring the mechanical properties of materials at small length scales. We perform an atomistic study to characterize the formation and extension of nano-sized cracks. By using atomistic reaction pathway calculations, we determine the energetics governing the brittle and ductile responses of an atomically sharp crack in silicon, involving the competing processes of cleavage bond breaking, dislocation emission, and amorphization by the formation of five- and seven-membered rings. We show that the nanoscale fracture process depends sensitively on the system size and loading method. Our results offer new perspectives on the brittle-to-ductile transition of fracture at the nanoscale.