Specific Essential Work Of Fracture Research Articles

Bioinspired soft-hard interfaces fabricated by multi-material additive manufacturing: A fracture mechanics investigation using essential work of fracture

This study employs the Essential Work of Fracture (EWF) concept to evaluate the interfacial fracture toughness of bioinspired interfaces between soft and hard polymer phases. The experimental framework utilizes a model material system with Polylactic Acid (PLA) as the hard phase and Thermoplastic Polyurethane (TPU) as the soft phase. Employing the Fused Filament Fabrication (FFF) technique, bioinspired sutural interfaces are created, characterized by interpenetrating soft and hard protrusions. The modulation of a critical parameter in the FFF process enables the variation of interpenetration length of protrusions at the interface, thereby achieving a spectrum of interfacial strength and toughness. The determination of essential work of fracture from double edge notch tension samples, with varied ligament sizes, allows for the specific EWF measurement. This specific EWF is found to align with the initiation value of plane strain interfacial fracture toughness obtained through the Double Cantilever Beam (DCB) test. Therefore, the proposed approach asserts the elimination of the need for complex interfacial fracture tests, such as DCB. A noteworthy discovery is the establishment of a correlation between specific plastic energy dissipation and protrusion length. This correlation suggests an increased plastic dissipation with longer protrusions within the investigated bioinspired interface. Due to this heightened plastic energy dissipation, the shape of the interfacial nominal stress-displacement curves demonstrates substantial dependence on interface morphology, shifting from a triangular to a trapezoidal shape as protrusion length increases. These findings offer valuable insights into the mechanics of bioinspired interfaces, presenting a more efficient and nuanced approach to characterizing their fracture properties.

The essential work of fracture method for the characterisation of fusion bonding in 3D printed short carbon-fibre reinforced polyamide 6 thin films

The fusion bonding between individual filaments of 3D printed short carbon-fibre reinforced polyamide 6 (SCF/PA6) in thin films manufactured using the fused filament fabrication (FFF) process has been studied. The thin films were deposited by an ‘in-plane’ mode or an ‘interlayer’ mode. The main characterisation technique employed was the essential work of fracture (EWF) method. Undertaking EWF tests on such thin films is shown to be a feasible and efficient way to evaluate the extent and quality of fusion bonding between individual filaments of the 3D printed SCF/PA6 composites. For example, firstly, for the 3D printed SCF/PA6 thin films it was readily evident that the extent of fusion bonding, i.e. the adhesion via molecular inter-diffusion, often termed the autohesion, between the filaments is the weakest, and the most variable, in the 3D printed SCF/PA6 thin films deposited via the interlayer mode. This was indicated by the lowest measured values of both the specific essential work of fracture, we, and the coefficient of determination, R2, of 19.6 kJ/m2 and 0.05, respectively, for the interlayer printed SCF/PA6 thin films. Secondly, the quality of the 3D printed SCF/PA6 thin films deposited via the in-plane mode was significantly enhanced by the use of further compression moulding process, since the value of R2 increased from 0.77 to 0.92. The results clearly suggest the need to improve the extent and quality of fusion bonding between individual filaments printed using the FFF process when 3D printed components are made for practical applications.

Effects of deposition speed and extrusion temperature on fusion between filaments in single-layer polymer films printed with FFF

Fused filament fabrication (FFF) is an additive manufacturing technology in which a fused filament is deposited in a controlled manner over or adjoining previous deposited filaments, leading to construction of a structural part. This study aims to characterise the extent of fusion bonding between individual filaments using printed single-layer films, which helps understand the process–structure–property relationship, optimising process parameters (i.e. deposition speed and extrusion temperature) involved in the FFF process. For a brittle polylactic acid (PLA) and a ductile polypropylene (PP), single-layer double-edge notched tensile (DENT) specimens with sharp cracks parallel to the deposition direction was fabricated using FFF of different extrusion temperatures (from 200 °C to 260 °C) and deposition speeds (from 30 mm/s to 90 mm/s). The fracture toughness ( K c ), defined as the critical stress intensity factor at the critical load for brittle fracture of PLA and the specific essential work of fracture ( w e ), as an indicator of the fracture toughness for ductile fracture of PP, were characterised. The results demonstrate that this methodology proved to be an effective tool to identify the effects of process parameters on fusion bonding in the FFF process, showing the strong sensitivity of fracture toughness, either K C for PLA or w e for PP, to the extent of fusion bonding between individual filaments.

Microstructure and mechanical properties of PA6/NBR nanocomposites fabricated by friction stir processing

Thermoplastic elastomer (TPE) based on polyamid6 (PA6) / acrylonitrile-butadiene rubber (NBR) containing 5% wt nanoclay (Closite 30B) have been prepared via friction stir process (FSP). In this study, the essential work of fracture (EWF) approach was employed to investigate the fracture behavior of PA6/NBR nanocomposites. Also, the modulus strength of specimens was modeled by response surface methodology (RSM), considering three input variables including rotational speed (ω), traverse speed (S), and shoulder temperature (T). Thus, a quadratic mathematical model between input variables (ω, S and T) and response (modulus strength) of PA6/NBR/clay nanocomposites was achieved. Moreover, the morphology of the PA6/NBR blends containing 5% wt was investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that a sample of PA6/NBR thermoplastic elastomer (TPE) containing 5% wt nanoclay at maximum tensile strength exhibited the maximum specific essential work of fracture (we) and specific non-essential work of fracture (wp). Also, the results of the RSM method demonstrate that the optimum condition of the process was found to be at including rotational speed (ω), traverse speed (S), and shoulder temperature (T) of 1200 rpm, 25mm/min, and 146 ℃, respectively. Thus, under the condition optimum, maximum modulus strength of 658 MPa was obtained.

Clay-Based Polymer Nanocomposites: Essential Work of Fracture.

This work details the general structure of the clays used as a reinforcement phase in polymer nanocomposites. Clays are formed by the molecular arrangement of atomic planes described through diagrams to improve their visualization. The molecular knowledge of clays can facilitate the selection of the polymer matrix and achieve a suitable process to obtain clay-based polymer nanocomposite systems. This work highlights the development of polymer nanocomposites using the melt intercalation method. The essential work of fracture (EWF) technique has been used to characterize the fracture behavior of materials that show ductility and where complete yielding of the ligament region occurs before the crack propagation. In this sense, the EWF technique characterizes the post-yielding fracture mechanics, determining two parameters: the specific essential work of fracture (we), related to the surface where the actual fracture process occurs, and the specific non-essential work of fracture (wp), related to the plastic work carried out in the outer zone of the fracture zone. The EWF technique has been used successfully in nano-reinforced polymers to study the influence of different variables on fracture behavior. In this work, the fundamentals of the EWF technique are described, and some examples of its application are compiled, presenting a summary of the most relevant contributions in recent years.

Extruded-Calendered Sheets of Fully Recycled PP/Opaque PET Blends: Mechanical and Fracture Behaviour.

This work presents the experimental results of the mechanical and fracture behaviour of three polymeric blends prepared from two recycled plastics, namely polypropylene and opaque poly (ethylene terephthalate), where the second one acted as a reinforcement phase. The raw materials were two commercial degrees of recycled post-consumer waste, i.e., rPP and rPET-O. Sheets were manufactured by a semi-industrial extrusion-calendering process. The mechanical and fracture behaviours of manufactured sheets were analyzed via tensile tests and the essential work of fracture approach. SEM micrographics of cryofractured sheets revelated the development of in situ rPP/rPET-O microfibrillar composites when 30 wt.% of rPET-O was added. It was observed that the yield stress was not affected with the addition of rPET-O. However, the microfibrillar structure increased the Young’s modulus by more than a third compared with rPP, fulfilling the longitudinal value predicted by the additive rule of mixtures. Regarding the EWF analysis, the resistance to crack initiation was highly influenced by the resistance to its propagation owing to morphology-related instabilities during tearing. To analyze the initiation stage, a partition energy method was successfully applied by splitting the total work of fracture into two specific energetic contributions, namely initiation and propagation. The results revelated that the specific essential initiation-related work of fracture was mainly affected by rPET-O phase. Remarkably, its value was significantly improved by a factor of three with the microfibrillar structure of rPET-O phase. The results allowed the exploration of the potential ability of manufacturing in situ MFCs without a “precursor” morphology, providing an economical way to promote the recycling rate of PET-O, as this material is being discarded from current recycling processes.

Tuning thermo-mechanical properties of poly(lactic acid) films through blending with bioderived poly(alkylene furanoate)s with different alkyl chain length for sustainable packaging

To favor a transition toward a more sustainable society, it is fundamental to find renewable and effective alternatives to current packaging materials. Here, fully renewable polymer films are produced by blending poly (lactic acid) (PLA) and poly (alkylene furanoate)s (PAFs). The work aims at discussing their microstructural and thermomechanical properties as a function of PAF concentration (5–30 wt%) and type (4–10 carbon atoms in the alkyl chain length of the repeating unit). For neat PAFs, an increase in the alkyl chain length promotes a decrease in Tg and Tm, but also an increasing tendency to crystallize, due to an enhanced molecular mobility. The Tg of PLA does not vary upon PAF addition, which accounts for blend immiscibility, in good agreement with SEM and FTIR. Tensile tests highlight that the addition of 5 wt% of any of the considered PAFs leads to a strong enhancement of the strain at break, which increases of approx. 10 times in the blend containing 5 wt% of poly (decylene furanoate) (PDeF-5). This remarkable increase in ductility is investigated more in detail with the essential work of fracture (EWF) approach, which evidences that the specific essential work of fracture increases from 16.2 kJ/m2 of neat PLA up to 32.9 kJ/m2 of PDeF-5 (+103%). This occurs especially thanks to the remarkable increase in essential work of fracture propagation (+222%). This work highlights the beneficial effect of finely dispersed PDeF domains on the fracture resistance of PLA, which is a fundamental aspect for many industrial applications.

Fracture Resistance of Advanced High-Strength Steel Sheets for Automotive Applications

The fracture resistance of different advanced high-strength steel (AHSS) sheets for automotive applications is investigated through conventional tensile tests, fracture toughness measurements, and hole expansion tests. Different fracture-related parameters, such as the true fracture strain (TFS), the true thickness strain (TTS), the fracture toughness at crack initiation (wei), the specific essential work of fracture (we), and the hole expansion ratio (HER), are assessed. The specific essential work of fracture (we) is shown to be a suitable parameter to evaluate the local formability and fracture resistance of AHSS. The results reveal that fracture toughness cannot be estimated from any of the parameters derived from tensile tests and show the importance of microstructural features on crack propagation resistance. Based on the relation fracture toughness-local formability, a new AHSS classification mapping accounting for global formability and cracking resistance is proposed. Furthermore, a physically motivated fracture criterion for edge-cracking prediction, based on thickness strain measurements in fatigue pre-cracked DENT specimens, is proposed.

Microstructural effects on fracture toughness of ultra-high strength dual phase sheet steels

The influence of microstructure on the fracture toughness of two industrially processed 1000 MPa dual-phase (DP) steel grades is investigated. Crack initiation and propagation resistance are evaluated by means of the essential work of fracture (EWF) methodology and the main damage and fracture mechanisms are investigated. The results are discussed in terms of the proportion and distribution of the different microstructural constituents, which is assessed by scanning electron microscopy (SEM), high-resolution electron backscatter diffraction (HR-EBSD) and nanoindentation hardness measurements. The investigations show that the strain-induced transformation of retained austenite to martensite (TRIP effect), may be detrimental to cracking resistance, even though it increases tensile properties. This phenomenon is attributed to a "brittle" network effect generated by the presence of hard fresh martensite islands in the fracture process zone. The connectivity of the hard secondary phases and the proportion of soft phase (ferrite) also have a major role in fracture toughness. The DP steel with the larger volume fraction of ferrite and homogeneously distributed martensite islands shows significantly higher crack propagation resistance. The contribution of necking to the ductile fracture process is evaluated by means of thickness measurements in fractured DENT specimens and the correlation between the specific essential work of fracture (we) and tensile properties is investigated. It is concluded that the global formability and cracking resistance of high strength DP steels can be balanced through microstructural tailoring.

Characterisation of fusion bonding between filaments of thin 3D printed polyamide 6 using an essential work of fracture method

The extent and quality, i.e. consistency, of the fusion bonding between filaments that occurs during the fused filament fabrication (FFF) process play an important role in determining the mechanical performance of 3D printed polymeric parts. This present research is designed to quantify these parameters of the fusion bonding between individual filaments deposited during the 3D printing process. Polyamide 6 (PA6) specimens were prepared from both 3D printed single-layer and single-filament films. Also, to provide a benchmark material, 3D printed PA6 films prepared by the FFF process were further processed by using compression moulding (CM) to investigate whether the fusion bonding could be enhanced. An essential work of fracture method was employed to evaluate the fusion bonding between the filaments by determining the value of the specific essential work of fracture, we, and the associated experimental reproducibility. The higher value of we, and the relatively low degree of scatter determined for the benchmark material revealed that the extent and quality of fusion bonding attained when using only the FFF process clearly needed to be improved when parts are fabricated for practical applications. To study the effect of temperature on the fusion bonding, the structural detail and fracture surfaces of thin polymer films were studied, with the thermal history of the deposited filaments also being measured using a thermal camera.

Fracture mechanisms and micromechanical deformation processes of polyethylene/calcium carbonate nanocomposite films

AbstractThe fracture behavior of medium density polyethylene/calcium carbonate (MDPE/CaCO3) nanocomposites was evaluated using the essential work of fracture (EWF) test. Fracture surfaces and the deformation zone in front of the precrack were studied using different microscopy techniques. Microscopic observations proved that extensive shear deformation of matrix due to debonding of nanoparticles plays the most important role on deformation mechanism of polyethylene nanocomposite. Large quantities of energy can be absorbed by plastic deformation of matrix polymer. This is why the specific plastic work (wp) of nanocomposites is higher than that of polyethylene. The presence of voids formed due to debonding of nanoparticles, reduces the load carrying capacity of the matrix and consequently leads to the reduction of the specific EWF (we). Besides, it causes easily crack propagation that may restricts the area where plastic deformation takes place and results in the reduction of plastic zone shape factor (β).

Effect of the Temperature and of the Drawing Conditions on the Fracture Behaviour of Thermoplastic Starch Films for Packaging Applications

In this work the effects of testing temperature and drawing orientation on the fracture behaviour of biobased and biodegradable thermoplastic starch based films, currently applied in the food packaging field, were investigated. At this aim, the Essential Work of Fracture (EWF) approach was applied on samples tested at 0 °C, 25 °C and 50 °C both in machine (MD) and in cross (CD) directions. The specific essential work of fracture (we) values strongly decreased with the testing temperature, and we values of CD samples were systematically higher than those of the corresponding MD samples. Considering that photograms of the CD samples taken during the EWF tests highlighted that the yielding zone became progressively opaque as an effect of the strain induced crystallization, it was hypothesized that in CD specimens part of the tensile energy applied during the straining process was utilized for the orientation of the macromolecules along the strain direction, rather than for the propagation of the crack in the ligament zone. Tensile tests under quasi-static conditions on dumbbell specimens highlighted that, regardless to the drawing direction, the stiffness and the resistance at yield and at break decreased with the temperature, while the strain at break was considerably enhanced. Moreover, because of the strong anisotropy induced by the molecular orientation in the drawing direction, MD tested samples had higher elastic modulus, yield resistance, and stress at break compared to CD samples.

Fracture Toughness Analysis of Automotive-Grade Dual-Phase Steel Using Essential Work of Fracture (EWF) Method

Fracture toughness determination of dual-phase DP450 steel using the essential work of fracture (EWF) methodology is the major focus of this research work. The EWF method is used for the determination of fracture toughness of thin sheets in a plane stress dominant condition. The EWF method is discussed in detail with the help of DP450 steel experimental results. Double edge notched tension (DENT) specimens with fatigue pre-crack and without fatigue crack (notched) have been used for testing. Specific essential work of fracture (we), crack tip opening displacement (δce) and crack tip opening angle (ψe) parameters were used for the comparative analysis. High-intensity laser beam cutting technology was used for the preparation of notches. Fracture toughness values of fatigue pre-cracked and notched samples were compared. The effect of notch tip radius and fatigue crack on the fracture toughness values were analysed. Digital image correlation (DIC) technology was used for the identification of local strain distribution and validation of the methodology. Fractured surfaces were examined by a scanning electron microscope (SEM) to analyse the fracture morphology and stress state.

Characterization of Specific Essential Work of Fracture for various Fracture Modes on Thermoplastic Materials

Characterization of Specific Essential Work of Fracture for various Fracture Modes on Thermoplastic Materials

Essential work of fracture analysis for surface modified carbon fiber/polypropylene composites with different interfacial adhesion

AbstractThe fracture behavior of polypropylene/carbon fiber composites with different interfacial adhesion was evaluated using the essential work of fracture method. All samples exhibited ductile fracture characteristics, while the specific essential fracture work, we, was obviously decreased by carbon fiber incorporation, indicating the impaired ductility and toughness for polypropylene. The cracks were easily formed and expanded at the fiber‐matrix interface under external force for polypropylene/original carbon fiber composites, since the weak interfacial adhesion between the fiber and matrix could not prevent the generation of new crack. Introduction of compatibilizer as well as surface modification of fibers effectively improved the interfacial adhesion between the fiber and matrix, so that the value of we increased and the elastic deformation of the composites became more prominent. Therefore, the improvement of interfacial adhesion between the fiber and matrix influenced both elastic and plastic deformation of the composites.

Identification of fracture toughness parameters to understand the fracture resistance of advanced high strength sheet steels

The fracture toughness of four advanced high strength steel (AHSS) thin sheets is evaluated through different characterization methodologies, with the aim of identifying the most relevant toughness parameters to describe their fracture resistance. The investigated steels are: a Complex Phase steel, a Dual Phase steel, a Trip-Aided Bainitic Ferritic steel and a Quenching and Partitioning steel. Their crack initiation and propagation resistance is assessed by means of J-integral measurements, essential work of fracture tests and Kahn-type tear tests. The results obtained from the different methodologies are compared and discussed, and the influence of different parameters such as specimen geometry or notch radius is investigated. Crack initiation resistance parameters are shown to be independent of the specimen geometry and the testing method. However, significant differences are found in the crack propagation resistance values. The results show that, when there is a significant energetic contribution from necking during crack propagation, the specific essential work of fracture (we) better describes the overall fracture resistance of thin AHSS sheets than JC. In contrast, energy values obtained from tear tests overestimate the crack propagation resistance and provide a poor estimation of AHSS fracture performance. we is concluded to be the most suitable parameter to describe the global fracture behaviour of AHSS sheets and it is presented as a key property for new material design and optimization.

Adiabatic effects on the temperature and rate dependency of the fracture toughness of an ethylene-fluoroethylene film

In this work, the essential work of fracture (EWF) test has been applied to determine the temperature and strain-rate dependency of the fracture toughness of ethylene-fluoroethylene (ETFE) thin films.EWF tests were conducted at different temperatures (23, 50 and 80 °C) and different cross-head rates (1, 10, and 100 mm/min). At 23 °C, the specific essential work of fracture (we) values slightly increase as the strain rate increases, while at 50 °C and 80 °C we values decrease. This trend is associated to an increase of the specific essential work for crack propagation (we,prop) while the specific essential work for crack initiation (we,init) decreases with strain rate with a magnitude related to the testing temperature. Thermal observations with an infrared camera revealed an increase of the temperature at the crack tip for the highest strain rate, thus suggesting, for the sample tested at the lowest temperature, the concurrent effects of strain-rate embrittlement and adiabatic heating. Digital image correlation (DIC) analysis revealed an ovoid shape of the outer plastic zone (OPZ) prior to the start of the crack propagation. DIC also indicated that the extension of the OPZ increases linearly with the square of the ligament length.

Characterization and Research on Mechanical Properties of Bamboo Plastic Composites.

The focus of this study was to observe the mechanical properties of bamboo plastic composites (BPCs) with bamboo pulp fiber (BPF) or white mud (WM). The essential work of fracture (EWF) methodology was used to characterize the impact toughness of BPCs. The results revealed an increase in flexural, tensile and impact properties, when adding the BPF in the BPCs. While the flexural properties of WM-reinforced BPCs revealed increasing, there was a decrease in tensile and impact strength. In an impact strength analysis study, BPF-filled BPCs showed excellent impact property over WM-filled BPCs; scanning electron microscopy (SEM) helps to explain impact fracture behavior of BPCs. EWF analysis of impact results showed that the specific essential work of fracture (we) increased significantly with the amount of BPF used in BPCs but decreased with the increase of WM in the BPCs. There was similar variation for the non-essential plastic work (βwp) of BPCs. This result indicates that the fracture initiation and fracture propagation of BPCs are different.

Notch effect on the fracture of a polymeric film

The fracture behavior of a bio-based thermoplastic copolyester film has been studied by combining the essential work of fracture, the J-integral, and the crack tip opening displacement characterization methods on double edge notched tension specimens, in an attempt to provide a better understanding of the details that play an important role in the repeatability and the reproducibility of the essential work of fracture test, with particular attention to the effect of the quality of the notches generated by two different notch sharpening techniques. Specifically, the femtosecond pulsed laser ablation and the classic razor blade sliding techniques have been applied. The equivalence, reported and discussed in the literature, between the specific essential work of fracture and the J value at crack initiation, as well as their relationship with the crack tip opening displacement, have been confirmed. It is also identified the parabolic shape of the stress-displacement curves, which modeled relates the specific non-essential work of fracture with the stress at initiation and the extension ratio during the crack growth.

Phase morphology, fracture toughness and failure mechanisms in super-toughened PLA/PB-g-SAN/PMMA ternary blends: A quantitative analysis of crack resistance

The effect of poly(methyl methacrylate) (PMMA) content on the fracture resistance, failure mechanisms and micromechanical deformation of poly(lactic acid) (PLA)/polybutadiene-g-poly(styrene-co-acrylonitrile) (PB-g-SAN) blend was studied. The fracture toughness of samples was characterized by the essential work of fracture (EWF) method. The specific EWF (we) linearly increased with PMMA content up to 30 wt%, indicating improved fracture toughness of ternary blends with PMMA content. The specific plastic work (βwp) increased at first with PMMA content up to 15 wt% and then decreased at higher loadings of PMMA. The energy dissipated during yielding (we,y) and subsequent tearing (we,t) stages was determined by the energy partitioning approach. Both we,y and we,t components monotonically increased with PMMA content in ternary blends, suggesting enhanced material resistance against crack initiation and subsequent crack propagation. Fracture behavior and toughening mechanisms were studied. Suitable interfacial adhesion, cavitation of rubber particles and irreversible plastic growth of micro-voids caused massive shear yielding of matrix material in the super-toughened blends.