Fracture Toughness Behavior Research Articles

Influence of Ti addition on fracture behaviour of HSLA steel using TIG melting technique

The welding process is a critical stage in the production of structural parts and the microstructure and mechanical properties of the welded joints must be appropriate in order to guarantee the reliability and durability of the components. The fracture toughness behaviour, which accounts for the residual strength of the component in the presence of flaws or cracks, is one of the most important properties to be evaluated in terms of microstructure and mechanical properties. In this present study, the surface of high strength low alloy (HSLA) steel was surface modified with the preplacement of pure Titanium (Ti) powder using a tungsten inert gas (TIG) arc heat source, at 100 ampere current with a voltage 30 V and a constant traversing speed of 1.0 mm/s using Argon shielded gas. The effect of preplaced Ti powder on the strength and toughness properties of the modified HSLA steel surface was investigated. The results indicated that the tensile and yield strength of HSLA steel decreased by ∼12% and ∼14%, respectively. While the impact toughness increased by ∼33% and the ductility decreased by ∼50%. The fractography analysis results by scanning electron microscopy (SEM) were also presented in this paper.

Optical, mechanical and fractographic response of transparent alumina ceramics on erbium doping

Alumina ceramics found their utilisation in many applications which can be further extend by attaining functional properties; in our case the transparency obtained through precise processing and photoluminescence due to erbium (Er) doping. In order to examine the optical, mechanical and fractographic response of transparent alumina on Er doping, slip casted samples containing 0–0.15at.% of erbium nano-oxide were pre-sintered by two-step sintering regime and then hot isostatically pressed. Prepared samples exhibited fully dense submicron microstructure and corresponding high transparency (RIT up to 60%). Positive influence of doping on the Vickers hardness resulted in values up to 27GPa (at 10N load). Moreover, the comparison of the Vickers hardness determined at different loadings with literature data showed that the Er doped alumina is one of the hardest material in this category. The samples were characterised also in terms of fracture toughness and fractographic behaviour.

Modification of fracture toughness Master Curve considering the crack-tip Q-constraint

Engineering structures may have significant lower crack-tip constraint compared with those in the standard fracture toughness test specimens. This paper considers a methodology to extend the application of fracture toughness master curve approach for structures with low crack-tip constraint. This is conducted by considering the Q-parameter as a suitable crack-tip constraint parameter, and study its effects on the reference temperature T0 in the master curve approach. The experimental fracture toughness data from the literature as well as supplemental experiments are used for this purpose. The relation between Q and T0 is investigated by using numerical analyses of experiments covering various types of geometry, loading condition, thickness and crack depth. The effects of loading amplitudes and material properties on the relation between Q and T0 are also studied. Based on this study, a relationship between Q and T0 is derived and experimentally verified. It is shown that the proposed relationship has a considerable ability to predict the apparent fracture toughness behavior in the ductile to brittle region of complex structures without need of conducting additional experimental fracture toughness tests.

Fracture Toughness Behavior of 38MnSiVS5 Microalloyed Steel After Isothermal Transformation and Thermomechanical Processing

Present work deals with the study of structural changes of medium carbon micro alloyed steel during two heat treatment cycles as isothermal transformation and thermomechanical processing followed by isothermal processing. In isothermal transformations, the steel immediately after austenitization at 1250°C for 20 minutes was transferred a to salt bath at a preselected temperature of 365, 400, 435, 470 or 500°C and after holding for 35 minutes followed by their quenching in water. In the other heat treatment cycle, after austenitization at 1250°C for 20 minutes the steel was subjected to thermomechanical treatment by hot rolling at a finish temperature of 750°C followed by isothermal treatment under the similar conditions as mentioned above.The third heat treatment, Two Step Cooling (TSC), involved ϕ30 mm forged bar of 38 MnSiVS5 soaked at 1200 °C (30 min) → hot rolled at finish rolling temperature of 800 °C in 6 to 8 passes to ϕ 17 mm → air cooling up to 700 °C followed by water quenching (Two Step Cooling) → annealed at 400-550 °C (in steps of 50 °C) for 90 minutes. The effect of microstructure on the fracture toughness behavior of heat treatment routes has been studied and related to their microstructures. The cost effectiveness of this steel for critical aeronautical field is discussed.

A comparative assessment of the fracture toughness behavior of ferritic-martensitic steels and nanostructured ferritic alloys

The Fe-Cr alloys with ultrafine microstructures are primary candidate materials for advanced nuclear reactor components because of their excellent high temperature strength and high resistance to radiation-induced damage such as embrittlement and swelling. Mainly two types of Fe-Cr alloys have been developed for the high temperature reactor applications: the quenched and tempered ferritic-martensitic (FM) steels hardened primarily by ultrafine laths and carbonitrides and the powder metallurgy-based nanostructured ferritic alloys (NFAs) by nanograin structure and nanoclusters. This study aims at elucidating the differences and similarities in the temperature and strength dependences of fracture toughness in the Fe-Cr alloys to provide a comparative assessment of their high-temperature structural performance. The KJQ versus yield stress plots confirmed that the fracture toughness was inversely proportional to yield strength. It was found, however, that the toughness data for some NFAs were outside the band of the integrated dataset at given strength level, which indicates either a significant improvement or deterioration in mechanical properties due to fundamental changes in deformation and fracture mechanisms. When compared to the behavior of NFAs, the FM steels have shown much less strength dependence and formed narrow fracture toughness data bands at a significantly lower strength region. It appeared that at high temperatures ≥600 °C the NFAs cannot retain the nanostructure advantage of high strength and high toughness either by high-temperature embrittlement or by excessive loss of strength. Irradiation studies have revealed, however, that the NFAs have much stronger radiation resistance than tempered martensitic steels, such as lower radiation-induced swelling, finer helium bubble formation, lower irradiation creep rate and reduced low temperature embrittlement.

Fracture toughness and fatigue crack behaviour of A3003/SUS304 lap friction stir welded joints

Friction stir welding (FSW) can weld dissimilar metal joints without a thick and brittle intermetallic compound layer at the weld interface. In this study, the dissimilar lap joint of A3003 aluminium alloy and SUS304 stainless steel was successfully welded by FSW, and the joint obtained was tested to examine the properties of fracture toughness and fatigue crack growth rate. Its fracture toughness was different by the directions of crack propagation. The fracture toughness of advancing side (AS) to retreating side (RS) was stronger than that of RS to AS, and that of cryogenic temperature was stronger than that of room temperature. Its fatigue crack growth rate also showed the same tendency as its fracture toughness. These data were compared with the past data and discussed.

Toughness Enhancement of Commercial Poly (Hydroxybutyrate-co-Valerate) (PHBV) by Blending with a Thermoplastic Polyurethane (TPU)

Poly(hydroxyl butyrate-co-valerate) (PHBV) is a biopolymer synthesized by microorganisms that is fully biodegradable with improved thermal and tensile properties with respect to some commodity plastics. However, it presents an intrinsic brittleness that limits its potential application in replacing plastics in packaging applications. Films made of blends of PHBV with different contents of thermoplastic polyurethane (TPU) were prepared by single screw extruder and their fracture toughness behavior was assessed by means of the essential work of fracture (EWF) Method. As the crack propagation was not always stable, a partition method has been used to compare all formulations and to relate results with the morphology of the blends. Indeed, fully characterization of the different PHBV/TPU blends showed that PHBV was incompatible with TPU. The blends showed an improvement of the toughness fracture, finding a maximum with intermediate TPU contents.

Indentation fracture toughness behavior of FeCo-based bulk metallic glass intrinsic composites

Indentation fracture toughness of (Fe0·5Co0.5)69−xNb6B25+x (x=0, 2, 4) bulk metallic glass alloys in the partially crystalline (intrinsic composite) state has been studied. Corner cracking was observed on indentation in all the compositions above a certain critical load. High hardness with values in excess of 1100HV was obtained for all the alloy compositions. Indentation fracture toughness measured through various empirical relations was found to be <5MPa√m for all the alloys irrespective of crack morphology. Palmqvist crack morphology was observed within the materials, for the applied indentation loads upto 9.8N. The change of crack morphology from Palmqvist to half-penny is generally expected to happen at a c/a (crack length plus half diagonal length to half diagonal length) value of 2.5 but the Plamqvist crack morphology prevailed even to a c/a of 3. The values of crack length to half diagonal length ratio (l/a) remained below 2 for all the loadings. Indentation fracture toughness values of 1.85MPa√m, 2.1MPa√m, 2.9MPa√m obtained were based on the observed crack morphology. The values of energy release rate (GC), surface energy (γS) and brittle index are also given.

Effect of Transition Metal Oxides Addition on Yttria - stabilized Zirconia for improving Physical and Mechanical Properties

Mechanical properties of Y2O3-containing tetragonal ZrO2 polycrystals(Y-TZP) were investigated. Several additives were used to modify the hardness and fracture toughness of Y-TZP. The effects of these individual additives were discussed and their interactions were also analysed. Each additive, such as CoO, Fe2O3, MnO2 was found to deteriorate the mechanical properties of Y-TZP when it was used singly. But the fracture toughness of Y-TZP was significantly improved when these additives and Al2O3 were added in combination at a certain ratio. The addition of CoO, Fe2O3 and MnO2 into Y-TZP resulted in the more complex behavior of fracture toughness and hardness. The specimen with 1.5 wt%-Fe2O3, 3.0 wt% -Al2O3 and 1.5 wt%-CoO showed the monoclinic to tetragonal phase ratio of 18% and the highest toughness of <TEX>$10.8MPa{\cdot}m1/2$</TEX> with Vickers hardness of 1201 kgf/mm2. However, the toughness decreased as the ratio increased and macrocracks developed beyond the ratio of 25%. Sample No. 16 is improved high Physical and Mechanical Properties.

Stress Corrosion Cracking Behavior of 4.19%Zn-1.34%Mg (A7N01S-T5) Aluminum Alloy Welded Joints

The fracture toughness behavior of Al-Zn-Mg alloy, A7N01S-T5, welded joints was investigated using three points bending test with the J integral approach. The microstructures of the welded joints were examined using optical microscopy, microhardness, and transmission electron microscopy. The heat affected zone (HAZ) had the best crack propagation resistance, and the weld zone (WZ) and base metal (BM) behaved similarly. The stress corrosion cracking (SCC) behavior of the welded joints was investigated under a constant loading condition. The threshold stress intensity factor of SCC and the cracks’ propagation rate were tested. The microstructure of the fracture was examined using optical microscope and scanning electron microscope. The results showed that both the BM and the HAZ were susceptible to SCC and the HAZ was smaller than the BM, but no cracks were detected in the WZ. A greater amount of H+ and a higher concentration of Zn and Mg accumulated at the crack front tips, which led to the SCC resistance ...

Fracture toughness in some hetero-modulus composite carbides: carbon inclusions and voids

ABSTRACTStructure and mechanical characteristics of dense ceramic composites synthesised by reactive hot pressing of TiC–B4C powder mixtures at 1800–1950°C under 30 MPa were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM and EDX). The results show that during hot pressing solid-phase chemical reaction 2TiC + B4C = 2TiB2 + 3C has occurred with final products like TiB2–TiC–C, TiB2–C or TiB2–B4C–C hetero-modulus composite formation with around one micrometer size carbon precipitates. The fracture toughness depends on the amount of graphite precipitation and has a distinct maximum K1C = 10 MPa m1/2 at nearly 7 vol.-% of carbon precipitate. The fracture toughness behaviour is explained by the developed model of crack propagation. Within the model, it is shown that pores (voids) and low-modulus carbon inclusions blunt the cracks and can increase ceramic toughness in some cases.

On the fracture toughness behavior of in-situ Al-Ti composites produced via mechanical alloying and hot extrusion

Mechanical alloying and hot extrusion techniques were employed to produce in-situ fully dens Al-Ti composites without applying any heat treatment. The focus of this study is the investigation of fracture toughness in produced composites in order to improve it and to explain the mechanisms responsible for the fracture. The morphology of milled powders was studied by Scanning Electron Microscopy (SEM). The microstructure of the extruded composites was examined by Optical Microscopy (OM) and Field Emission Scanning Electron Microscopy (FESEM). Moreover, the density and hardness of samples were investigated. Three point bending test of the single edge notched beam (SENB) samples was applied to examine the fracture toughness behavior under the quasi-static condition. In order to find the effective mechanism of fracture, the crack propagation paths and the fracture surfaces were studied using optical microscopy and SEM, respectively. The results showed that the morphology of the milled powders has a significant impact on the final mechanical properties of the extruded samples. The best bonding between particles corresponds to the powders with equiaxial morphology. The high energy vibratory mill was used to fabricate Al-20Ti-4hrV nano composite sample while low energy planetary mill was used to produce Al-20Ti-20hrP and Al-10Ti-60hrP composites. The results indicated the improved distribution of particles with steady state equiaxial morphology and the formation of in-situ nanometric intermetallics in the ultrafine Al matrix in Al-20Ti-4hrV sample. Accordingly, the highest density, the best mechanical properties and fracture toughness were obtained in Al-20Ti-4hrV sample. The use of low energy planetary mill led to the formation of flattened particles, low density and subsequently, poor mechanical properties in Al-20Ti-20hrP and Al-10Ti-60hrP composite samples. The fracture mechanism of Al-20Ti-4hrV sample was distinguished as micro void coalescence (MVC) whereas those of Al-20Ti-20hrP and Al-10Ti-60hrP samples were fracture and debonding of Ti particles, respectively. In addition, the good agreement between the experimental results of fracture toughness and the Rice & Johnson model predictions attained which confirms that this model is applicable for describing the fracture toughness of these composites.

Fracture of nanoscale Cu/Ag bimaterials with an interface crack

The fracture of nanoscale metallic bimaterials with an interface crack is investigated using atomistic simulations under uniaxial tensile loading conditions. Two Cu/Ag bimaterials with different crystal orientations are considered to examine the direction-dependent interface fracture behaviors. The effects of crack tip states on the fracture toughness and fracture behavior are also studied. The overall features for the selected models of both bimaterials are brittle; however, the fracture patterns are slightly different. Three methods were adopted to calculate the fracture toughness including atomistic simulations, linear elastic fracture mechanics, and Griffith theory. The results from different methods show good consistency because the models of the two bimaterials exhibit a linear elastic response under the applied tensile loading conditions. The scatter of the fracture toughness for the same bimaterials obtained from atomistic simulations is attributed to the different initial states of the models induced by the discrete nature of the materials on the atomic scale.

Strength and Fracture Behavior of a Particle-Reinforced Transformation-Toughened Trip Steel/ZrO2 Composite

A newly developed particle-reinforced composite based on a high-alloy metastable CrMnNi TRIP steel was investigated concerning its fracture toughness behavior. The particle reinforcement was done using 10 vol.% of metastable MgO-partially-stabilized ZrO 2 (Mg-PSZ), which has the capability of a stress-induced transformation from the tetragonal to the monoclinic phase. Moreover, the alloying concept of the steel matrix enables a strain-induced transformation from the metastable γ‑austenite phase to the α’‑martensite phase leading to an increase in strength and ductility. Both effects in combination are intended to dissipate energy and increase the fracture toughness of the composite material (R‑curve behavior). To evaluate the mechanical performance of the composite, tensile and fracture mechanics tests according to ISO 12135 were performed, followed by microstructural investigations. The fracture process was analyzed in an in situ tensile test with simultaneous recording of SEM micrographs and subsequent optical analysis of deformation. The results obtained show that the toughness of the composite is primarily determined by the presence of reinforcement particles. The low interfacial strength between the steel and ceramic associated with small interparticle spaces leads to an accelerated fracture process and a low overall toughness. This behavior is amplified as soon as particle clusters are formed during processing.

Master curve in upper region of ductile brittle transition: a modification based on local damage approach

The fracture behaviour of ferritic/ferritic martensitic steels in Ductile to Brittle Transition (DBT) region is well captured by Master Curve approach, except in the upper region of transition due to ductile tearing prior to cleavage. The fracture toughness behavior in the upper region of DBT is generally censored by Master Curve. In this work the Master Curve approach is modified to extend its applicability to the upper region of transition. The ductile tearing in the upper region of transition, increased sampled volume and constraint increment are addressed in this work using constraint parameter Tstress and numerical analyses using GTN damage.

Fatigue and Fracture Resistance of 316H Stainless Steel With Prior Creep Damage

316H stainless steel is widely used in high temperature components for advanced gas cooled reactors. Some plats have been de-rated to temperatures below the creep regime, due to the presence of creep damage in some components. However, such damaged or defected components must still satisfy structural integrity criteria for fatigue crack growth and fracture toughness behaviour. Therefore work has been performed to examine the fatigue and fracture toughness resistance of prior creep damaged material. A large block of material has been globally creep damaged (GCD) at 550 °C to the onset of tertiary creep behaviour. In this work, half sized compact tension C(T) specimens have been extracted from 316H blocks which were both pre-compressed and prior creep damaged at 550 °C. Fatigue crack growth and fracture toughness tests were subsequently performed on them. Note that prior to creep, this block was pre-compressed (PC) to 8% plastic strain at room temperature in order to work harden the material and limit the influence of crack tip plasticity in subsequent creep crack growth tests. The results, when compared to those previously obtained from as-received (AR), pre-compressed (PC) and locally creep damaged (LCD) standard sized C(T) samples, show an overall reduction of the fracture energy J. However, the global creep damage method does not introduce substantial changes on the fatigue crack growth behaviour of the material.

Effect of artificial aging on the mechanical performance of (Al-Cu) 2024 and (Al-Cu-Li) 2198 aluminum alloys

Al-Cu-Li-Mg based alloys exhibit an excellent combination of low density, high elastic modulus and high specific strength and have already being used as structural materials for aerospace applications. The mechanical properties of these alloys are often associated with the addition of Li which enables the formation of several strengthening precipitates including δ΄ (Al3Li), θ΄ (Al2Cu), δ (AlLi) and T1 (Al2CuLi). Other precipitates have also been reported in these alloys that include GP zones, θ (Al2Cu), Ω (Al2Cu), S΄ (Al2CuMg), and β΄ (Al3Zr), e.g. [1-3]. Nevertheless, the quasi-static mechanical properties of AA2198 are scarcely reported in the open literature while quite few for the fracture toughness. For example, Chen et al. [4] performed tests on two different heat treated AA2198 (namely T351 and T851) and investigated their plastic and fracture behavior. Steglich et al. [5, 6] investigated experimentally and analytically the anisotropic deformation of AA2198-T8 occurred during mechanical loading with and without the presence of artificial notches. In a recent publication [7], a combination of transmission electron microscopy, atom probe tomography and high-energy X-ray diffraction was employed to investigate the influence of local microstructural changes on strengthening in AA2198 in different aging conditions.As AA2198 is supposed to replace AA2024 in aerostructures designed with the damage tolerance philosophy, the authors in the present work report and compare their tensile mechanical and fracture toughness behavior under different aging conditions to simulate the natural aging parameter. To this end, a comparison of both alloys of their tensile mechanical behavior is reported for different stages of aging, including conditions of under-aging (UA), peak-aging (PA) and over-aging (OA). Typical results of yield stress as well as elongation at fracture for the case of AA2024 can be seen in the diagrams of Figure 1. The effect of artificial aging on the fracture toughness is also assessed for both alloys. Although the peak-aged temper of AA2198 is of application interest, a detailed investigation of the structure-property relationships of other temper states is attempted since it provides the basis for understanding the influence of process-induced microstructural changes on the post-processing properties of the alloy. To this end, structural characterization as well as relationship between yield stress, elongation at fracture and fracture toughness is reported and discussed in the manuscript for all investigated aging conditions.

An overall assessment of a new adhesive bonding process for composite materials, with regard to quality and cost

An assessment of innovative adhesive bonding process has been performed with regard to quality and cost. In this frame, the effect of two different atmospheric pressure plasma surface treatment conditions on the fracture toughness behaviour of adhesively bonded joints was experimentally investigated. Furthermore, the mechanical performance of a newly developed aerospace structural adhesive has been characterised experimentally in order to assess the quality of the bonded elements. To assess the feasibility of the new process, a complete cost-estimation analysis of the process has been carried out based on the activity-based costing modelling approach, thus serving to the estimation of the total cost/duration of the process. To this end, the newly developed process is assessed with regard to quality and cost. It could be shown that the new process offers tempting alternatives to the existing adhesive bonding and joining processes used in the aeronautic industry.

Evaluation of Fracture Toughness Behavior of Polyethylene Pipe Materials1

The main purpose of the present paper is to investigate the effect of crosshead speed, specimen thickness, and welding on the fracture toughness. The material of the investigated pipe is a high density polyethylene (HDPE), which is commonly used in natural gas piping systems. The welding technique used in this study is butt-fusion (BF) welding technique. The crosshead speed ranged from 5 to 500 mm/min and specimen thickness ranged from 9 to 45 mm for both welded and unwelded specimens at room temperature, Ta = 20 °C. Curved three point bend (CTPB) specimens were used to determine KQ. Furthermore, the results of fracture toughness, KQ, will be compared with the plane–strain fracture toughness, JIC, for welded and unwelded specimens. The experimental results revealed that KQ increases with increasing the crosshead speed, while KQ decreases as the specimen thickness increases. The investigation reveals that the apparent fracture toughness, KQ, for HDPE pipe of unwelded specimen is greater than that of corresponding value for welded specimen. The same trend was observed for the plane-strain fracture toughness, JIC. At lower crosshead speeds there is a minimum deviation in KQ between welded and unwelded specimens, while the deviation becomes larger with increasing crosshead speed.

Recent advances in epoxy resin, natural fiber-reinforced epoxy composites and their applications

The versatile characteristic of epoxy and its diversity made it suitable for different industrial applications such as laminated circuit board, electronic component encapsulations, surface coatings, potting, fiber reinforcement, and adhesives. However, the pervasive applications in many high-performance field limited the epoxy use because of their delamination, low impact resistance, inherent brittleness, and fracture toughness behavior. The limitations of epoxy can be overcome by incorporation and modification before their industrial applications. Currently, modified epoxy resins are extensively used in fabrication of natural fiber-reinforced composites and in making its different industrial products because of their superior mechanical, thermal, and electrical properties. Present review article designed to be a comprehensive source of recent literature on epoxy structure, synthesis, modified epoxy, bio-epoxy resin, and its applications. This review article also aims to cover the recent advances in natural fiber-based epoxy composites and nanocomposites research study, including manufacturing techniques and their different industrial applications.