Ductile-brittle Transition Point Research Articles

On the experimental research of long-term hydrostatic performance and lifespan of polyethylene pipe

ABSTRACT Long-term hydrostatic (LTH) capacity test up to 10,000 h of polyethylene pipe (PE pipe) was successfully conducted according to Arrhenius theory. It was revealed that in some cases the pipe damage points initiate within the marker line on pipe body, indicating that the addictive of marker line could somehow weaken the mechanical property of pipe base materials. Test pressure test time data was fitted using dual-parameter model under the guidance of ISO9080: 2012, and its serving life was calculated as 28.9 years, ductile-brittle transition point was found at 8456 h. Conclusions drawn from this paper could contribute to understanding the long-term mechanical behaviour of PE pipe.

Modelling of ductile fracture in pure iron considering the ductile–brittle transition at very high strain rate

At different deformation rates, a ductile material may fail by isothermal deformation fracture, thermoplastic shear instability, or even brittle fracture. The failure mode transition at sound speed deformation rate has not been well described. This paper aims to establish a new fracture model which considers the strain rate embrittlement effect (SREE) at the sound speed deformation rate, and can be used to predict the failure of the material in a large range of strain rate. Taking pure iron DT 8 as the experimental material, impact tests of pure iron specimens under different stress states, strain rates, and temperatures were carried out. Fracture strains under different stress states, strain rates, and temperatures were obtained. The strain rate was increased to 227450/s (5.36/s in log10) to obtain the ductile–brittle transition point. A new fracture model is proposed to describe the evolution of fracture strain over the wide range of strain rate. The new proposed model is experimentally validated and compared with the J-C failure model. It shows the new proposed fracture model can predict the ductile fracture behavior of a ductile material in the full strain rate range, and capture the failure mode transition at the sound speed deformation rate of a material.

Application of a Tensile Test Method to Identify the Ductile-Brittle Transition of Caramel

During cutting of foods, tensile stresses in front of the blade are responsible for the separation of the material. Therefore, tensile tests can be helpful to gain knowledge on deformation properties related to pre-fracture cutting behavior as well as on phenomena in the fracture zone, which are velocity-dependent in viscoelastic materials. The aim of this work was to apply a tensile test method for model caramels to investigate their behavior and to identify conditions where the ductile-brittle transition occurs. After executing pre-trials, tensile velocity, caramel moisture, and temperature were the parameters that were varied for this purpose. In general, increasing velocity, decreasing temperature, and decreasing moisture resulted in a stiffer response and caused a shift from a ductile to a more brittle behavior, attributable to reduced viscous contributions to the material and longer relaxation times. Fracture strain was notably lower than the maximum plastic elongation in the ductile case, but we observed equalization close to the ductile-brittle transition point for our material. This study serves as basis for an in-depth research on the complex deformation and fracture phenomena during cutting of viscoelastic food systems, including numerical modeling.

Hydrolytic Degradation of Closed Cell Polyethylene Terephthalate Foams. The Role of the Mobile Amorphous Phase in the Ductile-Brittle Transition

Polyethylene terephthalate (PET) foam is receiving increasing interest in recent years for engineering and structural applications. But for its successful and reliable implementation in real life infrastructures, knowledge on its ageing performance is critical for service life predictions. In this study the ageing of commercial closed cell PET under 90ºC and 95% RH is presented and its molecular and mechanical degradation characterized. The aim of the study is to observe the impact of the cellular structure on the hydrolysis kinetics, and provide more insights on the nature of embrittlement in PET. Structural changes are evaluated based on a three-phase model with crystalline, mobile amorphous (MAF) and rigid amorphous (RAF) fractions. The hydrolytical degradation of PET foams does not appear influenced by the cellular structure. Hydrolysys and chemicristalization appear to occur at the expense of the MAF fraction until the embrittlement occurs. The ductile-brittle transition point coincides with the achievement of the minimum MAF content. The link of embrittlement with the structural changes occurring in the mobile chains questions the occurrence of hydrolysis-induced embrittlement below Tg, which would correspond to service conditions. A micromechanical interpretation of embrittlement appears more appropriate than a molecular interpretation.

Effect of the initial-flaw on crack-propagation in two-step cutting of monocrystalline sapphire

Abstract Though superior multifunctional performances of sapphire, heavy surface/subsurface damage during the process limit applications of the material. Many efforts have been made to improve the machinability of the material, by utilizing the ductile-regime cutting. Here, in addition to the efforts on control of material behavior, machining strategies consisting of aggressive rough cutting and fine finish cutting were utilized in the machining of sapphire to improve the process throughput. To achieve this goal, it is required to investigate the effects of existing flaws in materials because their shapes and size significantly influence crack propagation in subsequent cutting. Considering this, machining behavior of primary and subsequent cutting was analyzed in terms of cutting direction of each, and the effects of existing cracks were investigated with respect to the crystal orientation and stress at the ductile-brittle transition point. Experimental results showed that existing flaws distinctively influence the crack propagation in subsequent cutting; the ductile cutting regime varied up to about 2 times with the combination of different primary and subsequent cutting directions. Based on the experimental results, a two-step cutting was performed as a proof of concept to optimize the material removal rate.

Modeling and simulation of phase transformation and crack formation during scribing of mono-crystalline silicon

This paper presents a simple two-parameter constitutive model to describe the material deformation behavior during scribing of mono-crystalline silicon (Si) – a process characterized by phase transformation and crack formation. The effect of phase transformation and volume densification of diamond-cubic Si (Si-I) to metallic Si (β-Si) during the loading stage is described by a modified Drucker-Prager yield surface. During the unloading stage, β-Si is completely transformed to amorphous Si (a-Si) due to the high unloading rate. The volume expansion accompanying the amorphization is described by a newly defined yield surface. The initiation and propagation of cracks are modeled using the extended finite element method (XFEM). The two constitutive model parameters, namely the cohesion parameter d and the initial yield stress ps, are calibrated using data from nano-indentation tests. The model is shown to be capable of predicting the load-displacement curve during nano-indentation of Si, including the elbow feature experimentally observed during the unloading stage. The material model is then used in a finite element model of scribing of mono-Si, and shown to successfully predict the residual depth of the scribed groove as well as the crack formation observed at the ductile-brittle transition point.

Investigation of the Ductile Cutting Behavior of Monocrystalline Yttria-Stabilized Zirconia During Ultra-Precision Orthogonal Cutting

Manufacturability of advanced ceramics has been a challenging issue mainly because of their brittle behaviors and high hardness. One approach to solving this issue is enabling ductile regime cutting, which can also be used to enhance the quality of the surface and accuracy of the final product. There have been many studies investigating how to control and prolong the ductile response regime during cutting; however, it still lacks a straightforward explanation that enables us to predict the transition of the material response from the ductile regime to the brittle regime. In this study, the processing of monocrystalline yttria-stabilized zirconia was investigated to predict material behavior during cutting. Here, it is aimed to confirm that stress intensity factor analysis can be applied with a wide variety of process parameters and investigate the effect of varying the process parameters on the ductile–brittle material response transition. Experimental results showed that negative rake angle and higher cutting speed prolonged the ductile cutting regime. However, the cutting stress at the ductile–brittle transition point remained constant regardless of the process parameters which enabled us to predict the transition point with respect to the stress intensity factor. It is expected that the results of this research can contribute to the development of machining strategies with improved throughput and thus to increasing the utilization of ceramic materials.

Theoretical model of transverse and longitudinal surface roughness and study on brittle-ductile transition mechanism for turning Fluorophlogopite ceramic

The material is removed by brittle fracturing during turning ceramics, leaving characteristic pits on the machined surface and seriously deteriorating surface quality. This study quantified the geometric features (i.e. slope angle and length) of characteristic pits based on maximum tensile stress criterion for brittle crack propagation and energy conservation principle of fracture chip unit, respectively. Theoretical models of longitudinal and transverse surface roughness for turning ceramic process was presented based on the geometric interference of tool-workpiece and brittle fracture characteristics of ceramic materials. Turning experiment of Fluorophlogopite was conducted on a CNC lath using PCD tool under different process parameters. The research revealed that as the cutting speed or cutting depth increases, the material removal method will be changed from ductile shearing to brittle fracturing. The critical cutting speed and cutting depth of ductile-brittle transition are 47.1 m/min and 0.05 mm, respectively. The material removal method has great influence on turning force and surface morphology. The turning force increases by 736% at the ductile-brittle transition point. In brittle mode, turning force fluctuates violently and periodically, the work done by cutting force is equal to the strain energy of chip, the surface energy of new fracture surfaces and the thermal energy generated by friction between tool-workpiece contact interface. The lowest surface roughness, equalled to Rat = 1.53 μm and Ral = 1.1 μm have been obtained during turning with cutting depth ap = 0.05 mm. The theoretical surface models only apply to brittle mode, because they only consider the brittle fracture and do not consider the ductile shearing of workpiece material.

Polycarbonate toughening with reduced graphene oxide: Toward high toughness, strength and notch resistance

The toughening effect of graphene sheets on polycarbonate (PC) was investigated to fabricate PC composites with excellent balanced toughness, notch resistance as well as strength. The reduced graphene oxide (RGO) was incorporated into PC matrix through melt compounding. A maximum toughening effect was observed in PC/RGO (PCG) composites with 0.03wt% or 0.07wt% RGO. Particularly, the tensile fracture toughness of PCG composites with 0.03wt% RGO was enhanced by 89%. The notched impact strength and KIC of PCG composite with 0.07wt% RGO was increased by 46% and 58%, respectively. The point of 0.1wt% was found to be the ductile-brittle transition point in PCG composites. Meanwhile, the yield strength of these novel materials was increased by around 12% as well at loading of 0.07wt%. Microcrack, resulting from interfacial debonding between PC and RGO as well as breakage and pulling out of graphene layer, was proposed to be the main toughening mechanism contributing to the great enhanced fracture toughness and notch resistance. Apart from the microcrack, crack pinning, crack deflection and crack arresting were also found and proposed to be toughening mechanism in notch-fractured process. This work not only provides us a novel strategy to fabricate advanced PC nanocomposites but also gives us a deep understanding on the toughening role of graphene on polymers.

Study on ductile mode machining of single-crystal silicon by mechanical machining

Nano patterns on single-crystal silicon are generally manufactured by photolithography, which can form limited cross-sectional shapes such as U-shapes or rectangular channels. Though V-shaped patterns are widely used in the optical industries because they concentrate light, they are challenging to manufacture by conventional photolithography. Mechanical machining is useful in manufacturing various kinds of cross-sectional shapes including V-shapes with various apex angles, but is hard to apply to single-crystal silicon due to its brittle fracture. Here we suggest a novel way of mechanical machining of single-crystal silicon that suppresses brittle fracture below the critical point (the ductile-brittle transition point) as determined by nano-scratch testing. We find that the first drop point of the cutting force corresponds to a critical point and define the critical forces as the thrust force and the cutting force at the critical point. The critical forces are varied by the applied force per unit length, which is the possibility that the cutting tool interacts with mechanically weak atomic bonds. When the applied force per unit length is zero (a general condition of mechanical machining), the cutting speed does not affect the variation of the critical forces or the quality of the machined pattern. Based on analysis of the experimental results, we suggest that the single-crystal silicon can be mechanically machined without brittle fracture at high cutting speed if the thrust force is smaller than the critical force of zero applied force per unit length.

Theoretical study on brittle–ductile transition behavior in elliptical ultrasonic assisted grinding of hard brittle materials

The elliptical ultrasonic assisted vibration can lead to a higher comprehensive performance with better machined quality and lower grinding force during brittle materials machining. The undeformed chip thickness in grinding can be defined as the distance between two consecutive grinding surfaces formed by the adjacent abrasive grain trajectories. The ductile–brittle transition point of chip formation in brittle materials machining is believed to be represented by the threshold value of undeformed chip thickness (critical undeformed chip thickness). In this paper, an energy based method is proposed to predict the critical undeformed chip thickness in elliptical ultrasonic assisted grinding (EUAG). The effects of vibration parameters on grinding force and specific grinding energy are analyzed in detail. Results show that the axial vibration amplitude leads to a slight reduction in grinding force, whereas the vertical vibration amplitude results in a significant reduction of grinding force. The increase of wheel speed and ultrasonic vibration frequency can reduce the grinding force as well. Nevertheless, the grinding force ratio remains relatively steady fluctuating from 1.37 to 1.56. Additionally, the critical undeformed chip thickness increases with the increasing grinding speed, axial vibration amplitude and ultrasonic vibration frequency, while firstly increases and then decreases with the increasing vertical vibration amplitude. Especially, a ductile-mode grinding in micron-level can be achieved when the axial vibration amplitude is above 3μm and a totally brittle fracture occurs when the axial vibration does not exist. Thus, a reasonable selection of vibration parameters in EUAG process is required. The above theoretical evaluations coincide well with the variable experimental results.

A predictive model of the critical undeformed chip thickness for ductile–brittle transition in nano-machining of brittle materials

There is a distinct transition in the mode of material removal in machining of brittle materials if the undeformed chip thickness is below a critical threshold of submicron scale. It is believed that at such small scale of material removal, the energy required to extend pre-existing flaws in the microstructure of brittle material exceeds the energy required to mobilize the micro-structural dislocations and hence plastic deformation serves as the dominant mode of material removal. It is postulated that a transition in the mode of material removal in machining of brittle materials is accompanied by a corresponding shift in the representative mode of energy expenditure. Hence, machining energy is a viable parameter to characterize the modes of material removal in machining of a brittle material. This paper presents a specific cutting-energy based model to predict the ductile–brittle transition point in ultra-precision machining of brittle materials. The energy expended in brittle and ductile modes of machining is modeled as a function of work-material intrinsic properties, tool geometry and process parameters. The transition point is identified in terms of undeformed chip thickness at which the mode of energy undergoes a transition from the plastic deformation based one to the fracture based one. The validity of the proposed model is verified by single-edge cutting tests on single-crystal silicon and BK7 glass. The experimental results are found in good agreement with model results.

Effects of core‐shell acrylate particles on impact properties of chlorinated polyethylene/polyvinyl chloride blends

AbstractThe impact properties of core‐shell acrylate (CS‐ACR)/chlorinated polyethylene (CPE)/poly(vinyl chloride) (PVC) blends under different temperatures were investigated. The fracture surface morphologies of the blends were observed by scanning electron microscopy (SEM). The results show that there exists significant synergistic effect between CS‐ACR particles and CPE in toughening PVC, and the impact properties of the blends generally correlate well with SEM morphologies. Besides, with increasing CS‐ACR content, ductile–brittle transition point of the ternary blends remarkably shifts to a lower temperature. Dynamic mechanical analysis exhibited that intensity and area of low‐temperature tan δ peaks of the CPE/PVC blends increase obviously after the addition of CS‐ACR particles, which to some extent are just in line with the changes in impact strength and ductile–brittle transition point of the blends. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Evaluation of surface and subsurface cracks in nanoscale-machined single-crystal silicon by scanning force microscope and scanning laser microscope

Studies of nanoscale machining of brittle materials, such as silicon, have come into prominence in recent years. Material removal with plastic deformation, known as ductile mode machining, has been achieved by a very small depth of cut. When surface cracks occurred with increasing cut depth, the machining has been commonly considered to be of brittle mode. In the present study, the ductile–brittle transition point in the nanoscale machining of single-crystal silicon was examined by the observations not only on the surface cracks but also on the subsurface cracks. A scanning force microscope was used to measure the depth of the grooves and to observe the surface cracks. The distance from the surfaces to the deepest point of the subsurface cracks on obliquely sectioned and then etched surfaces were measured by a scanning laser microscope. The transition point determined by the subsurface cracks was found to occur at a shallower depth of cut compared with the transition point from the surface cracks. Therefore, evaluation of subsurface cracks is important especially in the finish shape of machining.

Microstructural processes of fracture of rubber-modified polyamides

The microstructural processes of fracture in rubber-modified polyamides were studied in blends of polyamide 66/ethylene—propylene—diene rubber (PA66/EPDR) in Izod bending impact experiments. Izod tests were performed at various temperatures to determine the ductile—brittle transition as a function of temperature, rubber weight fraction and particle size. Subsequent analysis of the fracture surfaces by scanning electron microscopy revealed unique morphologies for various regions of toughness: in the brittle region, the fracture surface is patchy; in the transition region, there are occasional striations present on the fracture surface, along with the brittle fracture morphology; in the tough region, the fracture surface is fully covered by striations which penetrate only ≈2 μm beneath the surface in the form of shallow cracks perpendicular to the surface. The striations lie parallel to the main crack front and are a signature of the effective toughening of the polyamide through the incorporation of rubber particles. This drastic change in the fracture surface morphology and formation of striations at the ductile—brittle transition point is the result of elastic—plastic buckling of previously highly stretched material layers on the crack flank as they undergo a constrained accommodation. Morphological studies of the process zone, mainly below the crack flanks, have confirmed this hypothesis of buckling of surface layers. The spacing between the striations observed on the fracture surface of tough specimens varied with rubber content, particle size, temperature and strain rate. These effects are discussed in terms of local adiabatic heating and variations in the plastic resistance of the matrix material. A model for striation formation based on plastic buckling of a thin surface layer on a porous elastic foundation is provided that serves as a scaling relation accounting well for the temperature-dependent changes in striation spacing.

Microstructural processes of fracture of rubber-modified polyamides

The microstructural processes of fracture in rubber-modified polyamides were studied in blends of polyamide 66/ethylene—propylene—diene rubber (PA66/EPDR) in Izod bending impact experiments. Izod tests were performed at various temperatures to determine the ductile—brittle transition as a function of temperature, rubber weight fraction and particle size. Subsequent analysis of the fracture surfaces by scanning electron microscopy revealed unique morphologies for various regions of toughness: in the brittle region, the fracture surface is patchy; in the transition region, there are occasional striations present on the fracture surface, along with the brittle fracture morphology; in the tough region, the fracture surface is fully covered by striations which penetrate only ≈2 μm beneath the surface in the form of shallow cracks perpendicular to the surface. The striations lie parallel to the main crack front and are a signature of the effective toughening of the polyamide through the incorporation of rubber particles. This drastic change in the fracture surface morphology and formation of striations at the ductile—brittle transition point is the result of elastic—plastic buckling of previously highly stretched material layers on the crack flank as they undergo a constrained accommodation. Morphological studies of the process zone, mainly below the crack flanks, have confirmed this hypothesis of buckling of surface layers. The spacing between the striations observed on the fracture surface of tough specimens varied with rubber content, particle size, temperature and strain rate. These effects are discussed in terms of local adiabatic heating and variations in the plastic resistance of the matrix material. A model for striation formation based on plastic buckling of a thin surface layer on a porous elastic foundation is provided that serves as a scaling relation accounting well for the temperature-dependent changes in striation spacing.

Effects of temperature and void on elasto-plastic fracture toughness of porous and polymeric material.

The J-integral resistance curve method is applied to RIM-polyurethan from which the skin layer is removed, as a porous and polymeric material. We have studied how the fracture toughness Jin and Jc values are affected by temperature and void. It is found that the stable fracture appears in the temperature range from -40 to 20°C and that the unstable fracture appears in the temperature range from -120 to -60°C. The Jin values are almost constant in the temperature range from -40 to 20°C and the Jc value have a maximum value at the temperature of about -60°C. It is considered that the ductile-brittle transition point exists at a temperature between -60 and -40°C. The Jin values increase with an increase of void content from 5 to 17% and decrease with an increase of void diameter on a constant void content at a temperature of 20°C. The Jc values decrease with an increase of void content from 5 to 17% and are not affected by the void diameter on a constant void content at the temperature of -80°C.

The impact strength and the transition to cold brittleness in molybdenum

1. The ductile-brittle transition point, determined from the impact strengths, is higher for recrystallized molybdenum than for deformed molybdenum (400–480 and 100–220°C, respectively). 2. Increase in the sharpness of the notch does not alter the temperature corresponding to the transition to the cold-brittle state in molybdenum. 3. With deformed molybdenum in the brittle state rupture takes place a cross the specimen, while in the region of the transitional temperatures part of the specimen ruptures and the other part bends. The proportion of the bent part increases with increase in temperature. The sharpness of the notch does not affect the ratio of the bent and ruptured parts of the section. 4. The specific energy expended on the tearing of the fibers was determined. It decreases with decrease in temperature and, with low ductile-brittle transition points, becomes close to zero.