Fractographic Investigation Research Articles

Fracture Characteristics of AZ31B and Cu 99 Tensile Test Specimens Joined by FSW and FSW-IG Processes

The aim of the current study is to perform a failure analysis on specimens extracted from 4 mm sheets of AZ31B, according to ASTM 4377G and 3 mm sheets of Cu99 alloys according to SR EN 1652:2000, that were joined together using FSW and FSW-IG processes. In order to promote the suitability and advantages of implementing FSW-IG processes in the automotive and aerospace industry, the papers authors performed destructive testing on the samples extracted, namely tensile strength test, following with topography analysis using scanning electron microscopy – SEM, combined with energy dispersive spectroscopy EDS, micro-hardness tests and fractography investigations. Results highlighted a considerable improvement in the ultimate tensile strength of the welded joint, a higher degree of deformability of the welded joint in the case of FSW – IG, compared to classical FSW; while maintaining the same process conditions and parameters. Experimental data is in accordant with automotive and aerospace imposed compliances, thus presented and discussed in the following scientific article.

Effect of PMMA Coupling Layer in Enhancing the Ultrasonic Weld Strength of Novel Room Temperature Curable Acrylic Thermoplastic to Epoxy Based Composites

The joining of composites can be performed in an extremely short time with more energy-efficient ultrasonic welding techniques. The current research investigated the performance optimization of ultrasonic welding of carbon/Elium® composite to carbon/epoxy composite using a polymethyl methacrylate (PMMA) coupling interlayer. The weld strength was quantified by static lap shear strength (LSS) testing. A new methodology was used by creating a PMMA coupling layer on the epoxy composite adherend to achieve an improved interphase and thus enhance the weld properties. The LSS of Elium (EL)-Epoxy (EP) _0.25_0.25 was found to be 190% higher compared to that of EL-EP, confirming the effectiveness of the strategy used for creating an interlayer thermoplastic coupling layer. The time required for welding was optimized to be 2s as compared to 10 min required for adhesive bonding. Scanning electron microscopic images of epoxy and PMMA/Elium matrix interphase were observed to have a rough surface and remained largely unaffected by welding. There was an interphase change further away from the interphase to a rougher texture. There was little to no effect on the penultimate layer on the weld strength, as no interphase change could be observed after welding. Fractography investigation revealed shear cusps, matrix plastic deformation, fiber imprints, fiber pull-out, and good adhesion between matrix and fiber, features seen for configuration with maximum LSS. The current research findings present a way to join Elium® with epoxy composites that could be used in applications that require a selective strengthening, such as in sporting goods and consumer products. Furthermore, a detailed investigation is ongoing to use different filler particles and coupling layers to reach the maximum welding performance.

Fused Filament Fabrication 3D printed polypropylene/ alumina nanocomposites: Effect of filler loading on the mechanical reinforcement

Three-dimensional (3D) printed Polypropylene (PP) reinforced with aluminum oxide (Al2O3) nanoparticles (NPs) were developed and fully characterized in this study. Nanocomposite filaments were produced initially following a melt mixing extrusion process, utillised as feedstock for the Fused Filament Fabrication (FFF) specimen manufacturing. Al2O3 NPs at 0.5, 1.0, 2.0 and 4.0 wt% loadings were melt-mixed with the PP thermoplastic matrix. Specific geometry samples were 3D printed and analysed via tensile, flexural, viscoelastic, impact, microhardness and fractographic investigations. Raman spectroscopy verified the polymeric structure and the incorporated Al2O3 NPs within the polymer matrix. Atomic Force Microscopy (AFM) of the extruded filaments revealed the nanoscale roughness induced by the alumina nanoinclusions. All 3D printed nanocomposite structures exhibited enhanced tensile, flexural and thermomechanical properties. Specifically, the best combination was found for the 1.0 wt% loaded specimen showing a tensile and flexural strength increase by approx. 4% and 19%, respectively, with a concomitant slight increase in impact and microhardness properties compared to unfilled PP. Dynamic Mechanical Analysis (DMA) revealed a stiffening mechanism for the PP/Al2O3 nanocomposites being in good agreement with the quasi-static mechanical tests. It could be envisaged that the 3D printed PP/Al2O3 nanocomposites developed herein could find numerous applications as engineered thermoplastics, where enhanced material's static and dynamic mechanical properties are required.

On the Anisotropic Impact Behavior of an Additively Manufactured AlSi10Mg Alloy in Different Heat Treatment Conditions

The present work deals with the anisotropic high-strain rate behavior of laser-powder bed fusion (L-PBF) produced AlSi10Mg alloy in different heat treatment conditions. Impact specimens were produced with different orientations towards building platform and U-notch positions to assess the anisotropic properties. Besides the as-built material, several heat treatments were considered, including annealing, standard T6, hot isostatic pressing (HIP), HIP plus T6, and a recently proposed T6 at high pressure. The high-strain rate behavior was investigated by conducting Charpy impact tests, while material characterization was performed by scanning electron microscopy and x-ray diffraction. Results show that as-built and annealed alloys display significant anisotropic impact properties, whereas samples heat-treated at high temperatures generally have more consistent behavior. A coupled microstructural and fractographic investigation highlights that mitigation of anisotropy descends from the recovery of microstructural heterogeneity of the Si phase after heat treatment at high temperatures. This does not happen for both grain morphology or crystallographic structure, which are not significantly altered after the heat treatment. The present study aims to fill the gap in the literature regarding the anisotropic high-strain rate behavior of additively manufactured Al alloys and provide useful insights for mitigation of anisotropy by heat treatment.

Experimental investigation of high strain-rate, large-scale crack bridging behaviour of z-pin reinforced tapered laminates

Significant research exists on small-scale, quasi-static failure behaviour of Z-pinned composite laminates. However, little work has been conducted on large-scale, high strain-rate behaviour of Z-pinned composites at structural level. Small-scale testing is often at an insufficient scale to invoke the full crack bridging effects of the Z-pins. Full-scale testing on real components involves large length scales, complex geometries and resulting failure mechanisms that make it difficult to identify the specific effect of Z-pins on the component failure behaviour. A novel cantilever soft body impact test has been developed which is of sufficient scale to invoke large-scale delamination, such that behaviour in Z-pin arrays at high strain-rates can be studied. Laminates containing Z-pin arrays were subjected to soft-body gelatine impact in high-speed light gas-gun tests. Detailed fractographic investigation was carried out to investigate the dynamic failure behaviour of Z-pins at the microscopic scale.

Fracture toughness and fractographic investigation of polybutylene terephthalate/thermoplastic polyurethane binary blends reinforced by multi-walled carbon nanotubes using essential work of fracture approach

In this paper, fracture toughness evaluation of polybutylene terephthalate (PBT)/thermoplastic polyurethane (TPU) binary blends and PBT/TPU/carbon nanotubes (CNTs) ternary nanocomposites have been conducted using both Izod impact and quasi-static fracture tests. Essential work of fracture (EWF) approach is used to study the fracture properties in details. The results of EWF tests revealed an effective role of TPU and CNTs in toughening mechanism of binary blends and ternary nanocomposites. According to EWF results, both the crack resistance and plastic deformation energies promoted in all compounds as compared to neat PBT. Energy dissipation in the yielding and tearing stages determined by the energy partitioning method. The obtained results indicated that displacement up to the failure point increased by increasing the TPU content, while inclusion of CNTs reduced this quantity. The specific non-essential work of fracture [Formula: see text] , [Formula: see text], and [Formula: see text] increased with increasing the TPU contents which is confirmed by load-displacement curves. Whereas, addition of CNTs reduced [Formula: see text] and [Formula: see text] values as compared to reference binary blend, however, ternary nanocomposites still have higher values as compared to pure PBT. In contrast with EWF results, high strain rate of impact test prevents the activation of toughness improving mechanisms that readily occurs in quasi-static loading.

Specimen size effect on evaluation of strength properties of 3Cr1MoV and Incoloy 800H using small punch test

Standardization of small punch test (SPT) is being carried out, but the specimen thickness is not unified in different laboratories and specimen size effect on deformation mechanism still remains unclear. In this paper, SPT specimens with thicknesses of 100 μm–500 μm were used to investigate specimen size effect on strength properties and fracture morphology of 3Cr1MoV and Incoloy 800H. The grain sizes of two materials were 10 μm (3Cr1MoV) and 50 μm (Incoloy 800H). Yield and ultimate tensile strengths were extracted from SPT data based on a combined-scheme of finite element simulations and experiments. Fractography of specimens was studied using a scanning electron microscope. The results showed that the strength value of Incoloy 800H increased with the decrease of thickness, the size dependence of yield strengths was stronger than that of ultimate strengths, and the strength value of 3Cr1MoV was thickness independent. Fractographic investigations found that failure was caused by ductile fracture for all the specimens. A theoretical model considering geometrically necessary dislocations densities, available dislocation sources and grain sizes was proposed to explain the different mechanical responses of investigated materials to specimen thicknesses. This work clarifies the SPT specimen thickness and the grain size of studied materials should be taken into consideration before sampling.

Quasi-static and dynamic fracture toughness of a γ-TiAl alloy: Measurement techniques, fractography and interpretation

Intermetallic titanium aluminides (TiAl) offer an enormous potential for high temperature applications due to their high specific strength, creep and oxidation resistance. However, their use for structural components is limited by their low ductility and fracture toughness. During service, along with a high number of load cycles, the material may also be subjected to unexpected impact loads, which requires therefore knowledge about dynamic material parameters for a damage-tolerant component design. Here we present a feasibility study to determine the fracture toughness of a third generation TiAl-alloy over a wide range of loading rates. Quasi-static crack resistance curves (R-curves) were determined using the direct current potential drop (DCPD) technique. The dynamic experiments were performed with a drop tower with impact velocities ranging from 1 to 10 m/s on pre-cracked SENB specimens. To exclude the influence of inertial forces, the measurements were carried out with strain gauges applied in the elastic near-field of the crack tip. Despite a critical consideration of possible systematic errors of this measurement method, it was found that the dynamic fracture initiation toughness slightly increases up to loading rates of K̇I = 106 MPa√m/s. The fracture behaviour of the material and possible reasons for the toughness increase are discussed based on comprehensive fractographic investigations as well as crack path analyses within the microstructure.

Mechanism of HCF-creep interaction in a type 316LN stainless steel

HCF-creep interaction in type 316LN stainless steel is investigated at 923 K by conducting HCF tests well below the endurance limit with high mean stress (high R-ratio). High mean stress inflicts creep damage at 923 K, which when imposed on a small alternating stress, leads to significant HCF-creep interaction. Depending on the magnitude of mean stress (R-ratio), three distinct regimes are identified, viz. (a) HCF regime (b) HCF-creep interaction regime and (c) creep regime. Detailed fractographic investigation revealed distinct fracture characteristics pertaining to each of the three regimes. HCF-creep interaction regime is marked by significant intergranularity at low values of mean stress which changes to predominantly dimpled fracture with increasing mean stress. The above observations are corroborated through detailed EBSD investigations which bring out the underlying deformation mechanisms responsible for failure under HCF-creep interaction.

Fractographic investigation of Low Cycle Fatigue behaviour of IN718 coated with NaCl at 550°C

Nickel base super alloys due to their superior properties are used in different parts of gas turbines, in jet engines, and as well as in marine application from 250°C to 650°C. At such high temperature sulphur and vanadium as impurities in fuel oil get oxidised in SO2 and V2O5. Further, sulphur oxide reacts with NaCl to form Na2SO4. These salts cause high temperature corrosion which causes stress corrosion cracking of engine components of marine gas turbine. Therefore, LCF resistance of the material becomes an important consideration in design of turbine. Fatigue samples both coated with NaCl salt and uncoated samples were tested in low cycle fatigue. The fatigue tested samples were analyzed under Scanning Electron Microscope (SEM). Since micrographs analysis is an important tool with the testing of materials to evaluate the various properties of the materials. In present study micrographs analysis helped to a great extent for evaluation the Low cycle fatigue behaviour of Nickel base super alloy INCONAL 718 (IN718) at high temperature which is 550°C.

Investigation of microstructure-tensile behavior relationship in Hastelloy X Ni-based superalloy processed by laser powder-bed fusion: Insights into the elevated temperature ductility loss

The microstructure-tensile response correlation in a crack-free laser powder-bed fusion (LPBF) Hastelloy X Ni-based superalloy is investigated. The LPBF process parameters in terms of volumetric energy density were optimized to obtain the maximum relative densities, as well as the lowest defect size. The microstructure and tensile behavior of samples were evaluated. Microstructural characterizations showed that the columnar grains of the austenite phase consisted of a submicron cellular microstructure with a weak texture along <100> direction. The as-built sample had an acceptable tensile behavior at the ambient temperature, as compared to the AMS 5536 N standard specification. However, at 760 °C, it was observed that the plastic deformation of the as-built sample was influenced by an intergranular brittle fracture, leading to the elevated temperature ductility loss. The results of fractography investigations, performed by means of electron backscatter diffraction analysis, indicated that grain boundaries were precipitation-free and the cracks were formed in the areas with a high Taylor factor value. The investigations also demonstrated that a high degree of grain-to-grain incompatibilities, especially in the grains having <110> parallel to the building direction orientation, resulted in the stress concentration in the grain boundaries, leading to inferior elevated temperature tensile properties.

Manufacturing of Al2O3/Ni/Ti Composites Enhanced by Intermetallic Phases.

In this study, ceramic–metal composites in the Al2O3/Ti/Ni system were fabricated using the slip casting method. Two series of composites with 15 vol.% metal content and different solid phase contents were obtained and examined. A proper fabrication process allows obtaining composites enhanced by intermetallic phases. The microstructure of the base powders, slurries, and sintered composites was analyzed by scanning electron microscope. Analysis of the sedimentation tendency of slurries was carried out. The phase composition of the sintered samples was examined by X-ray diffraction analysis. A monotonic compression test was used to investigate the mechanical properties of the composites. A fractography investigation was also carried out. The research conducted revealed that the slip casting method allows the obtaining of composites enhanced by intermetallic phases (TiNi, Ni3Ti). The results show the correlation between solid-phase content, microstructure, and mechanical properties of the composites.

Development and Mechanical Characterisation of Al6061-Al2O3-Graphene Hybrid Metal Matrix Composites

MMC based on aluminium (Al) were produced for light-weight applications especially in aviation and automobile areas. Present paper deals with the fabrication and mechanical performance of AA6061 matrix composites fortified with Al2O3 (alumina) and graphene particulates. Fluid metallurgy method namely stir casting route was employed for fabricating the hybrid composites. Al2O3p and graphene powder are mixed in different weight fractions in which graphene (1 wt. %) particle reinforcement is held consistent and Al2O3 reinforcement is differed freely with 5, 10 and 15 wt. %. Using optical analyser and SEM equipment, microstructural examination is carried out and the result reveals that the graphene and Al2O3 particles prevalently are homogeneously appropriated on the grain limits of Al matrix and Al2O3 particles are disseminated between graphene in the as-cast AA6061 MMC’s. Detailed analysis on investigation of the microstructure and mechanical aspects of Al6061-graphene-Al2O3p composites is presented by following ASTM guidelines; results uncovered that with increment in reinforcement particles, there is an enhancement in the hardness, ultimate strength, yield strength and a decline in the elongation values was however noticed when contrasted with Al6061 alloy. Fractography investigation revealed dimples in unreinforced alloy and the composite.

Failure Analysis of a Diesel Engine Crankshaft

Abstract A six-cylinder crankshaft in a 12L diesel engine was locked after testing for about 840 hours in the bench test. The fractography investigation indicates that fatigue is the dominant failure mechanism of the crankshaft. It is found that the fatigue crack mainly initiated at the fillet of the crankweb between the 6th main journal and 6th crankpin. The fatigue crack initiation area lies outside the induction surface hardened zone. From detailed metallographic inspection, abnormal microstructure containing Widmannstatten was found in the fatigue crack initiation area and the 5th main journal, while that was not found at the severely deformed crankweb. Since the region containing Widmannstatten has lower hardness, the root cause of the failure may be that the abnormal microstructure lowered the fatigue strength at the stress concentrated fillet. The crankshaft prematurely fractured under the complex stress condition in the bench test.

Failure analysis of a marine intermediate shaft

A bulk-ship was not working normally when anchoring to wait for entering port. Inspection in site found that the intermediate shaft fractured. The intermediate shaft had accumulatively serviced for 43,000 h before fractured. Fractographic investigation indicated that the multiple origins fatigue fracture was the dominant failure mechanism of the shaft. The cracking propagated on two fracture planes at approximately ± 45° with respect to the shaft axis. The material of failed shaft was found to be a low carbon steel of type ASTM 1020, rather than the specified the medium carbon steel of type ASTM 1035. The average values of yield strength and ultimate strength of shaft material were respectively 16% and 20% lower than the lower limit of technical specification due to low carbon content, as a result of the decreased fatigue strength of shaft material. The fracture occurred at the fillet root of flange connected to the propeller-shaft, where high stress concentration was caused by geometry design. Deep circumferential machining grooves were present on the fillet root of flange for additive stress concentration. The inhomogeneities acted as potential sites for fatigue crack initiation. The low fatigue strength of shaft material promoted initiation and propagation of fatigue cracks in the weaker region under the reversed torsion loading. The static strength analysis of intermediate shaft was performed to assess the design implications. The measured average ultimate strength of failed intermediate shaft was 12% lower than the lower limit in Rule for Classification of Sea-going Steel Ships issued by China Classification Society. The safety factor of the failed intermediate shaft is lower than the permitted one.

Microstructure, Residual Stresses, and Strain-Rate-Dependent Deformation and Fracture Behavior of AISI 304L Joints Brazed with NiCrSiB Filler Metals

The knowledge of alloy–process–structure–property relationships is of particular interest for several safety-critical brazed components and requires a detailed characterization. Thus, three different nickel-based brazing filler metals were produced with varying chromium and molybdenum content and were used to braze butt joints of the austenitic stainless steel AISI 304L under vacuum. Two holding times were used to evaluate diffusion-related differences, resulting in six specimen variations. Significant microstructural changes due to the formation and location of borides and silicides were demonstrated. Using X-ray diffraction, alloy-dependent residual stress gradients from the brazing seam to the base material were determined and the thermal-induced residual stresses were shown through simulations. For mechanical characterization, impact tests were carried out to determine the impact toughness, as well as tensile tests at low and high strain rates to evaluate the strain-rate-dependent tensile strength of the brazed joints. Further thermal, electrical, and magnetic measurements enabled an understanding of the deformation mechanisms. The negative influence of brittle phases in the seam center could be quantified and showed the most significant effects under impact loading. Fractographic investigations subsequently enabled an enhanced understanding of the fracture mechanisms.

The mechanical behavior and microstructure of additively manufactured AlSi10Mg for different material states and loading conditions

Abstract The mechanical behavior of AlSi10Mg produced by selective laser melting (SLM) and heat treated to T5 and T6 was investigated under uniaxial compressive, tensile and combined shear-compression loading for strain rates up to 2·103s−1 and different temperatures (23 °C/400 °C). AlSi10Mg-T5 possess lower strength but higher elongation to fracture, higher specific absorbed mechanical energy and higher strain rate sensitivity as the T6 condition. In addition, the material in the T6 state was found to be more prone to biaxial loading and elevated temperature than the T5 state. The microstructural and fractographical investigation reveal that these differences are mainly related to different sizes and species of precipitates. The study is complemented by a constitutive description of the flow stress as function of strain and strain rate for the whole range of strain rates. This model is based on the Orowan equation including dislocation drag mechanism in combination with the well-known Kocks-Meckin-Esrin approach. With this modelling it was shown that typical length scales, mobile/total dislocation densities and dislocation generation/annihilation significantly differ as function of strain.

Fractured Metallic Tracheostomy Tube: Report of a Fractographic Study

Objective: To investigate the cause of a broken metallic tracheostomy tube.Material and Methods: In this study, we performed a fractographic investigation of a broken tracheostomy tube from an elderly patient using surface visualization, scanning electron microscopy, energy dispersive X-ray spectroscopy, and chemical composition analysis using X-ray fluorescent and metallographic testing.Results: Surface visualization revealed multiple corrosive pits which were confirmed by liquid penetrant testing. Scanning electron microscopy and energy dispersive X-ray spectroscopy revealed the chemical composition of the tube to be an austenitic chromium-nickel-manganese stainless steel alloy. Metallographic analysis suggested that the fracture site originated from the inner surface from intergranular corrosion.Conclusion: The evidence suggests that the corrosion resistance properties of this material might not be suitable for long term use in the human trachea. Higher grade stainless steel or more frequent device change is recommended.

Variability in Fatigue Life of Near-α Titanium Alloy IMI 834

Most aero-engine components and structures are subjected to life critical fatigue loads during service. Near-α titanium alloy IMI 834 is one candidate aero-engine material that is used as high-pressure compressor discs due to its superior fatigue strength and creep resistance. The effect of heat treatment on the microstructure and high-cycle fatigue behaviour of the material has been studied and reported here. The alloy was solution-heat-treated at 1060 °C and subsequently quenched in different media. A strong effect of quench media (cooling rate) on high-cycle fatigue life has been observed. Fractographic investigations were performed to correlate the fracture micro-mechanism with heat treatment. Further, a generalized stress-life model has been deduced from the fatigue data and integrated with finite element analysis to develop a fatigue model for the alloy.

Structural integrity and mechanical properties of the functionally graded material based on 316L/IN718 processed by DED technology

Additive manufacturing is a one of the most promising technology nowadays that offers the advantages not only in building products of complex shapes but also of complex materials. A complex structure is characteristic for Functionally Graded Composites, which basics, principles and applicability have been widely investigated over the last years. The present study is focused on the detailed investigation of mechanical and structural properties of FGC consisting of stainless steel 316L and Inconel 718 processed by Blown Powder Directed Energy Deposition system. Mechanical properties within single layers and over layers transitions were investigated with the use of tensile tests and fracture toughness tests. Metallographic and fractographic investigations were carried out. Metallographic investigation revealed the differences in the interfaces between single material layers, nucleation processes and subsequent growth of the grains of the used materials. It has been shown that the formation of transition region between deposited single material layers is dependent on the order of material deposition since different deposition parameters are used for certain material. Evaluation of the tensile properties showed that the mechanical properties of a single material layers are in very good agreement regardless of the deposition height. However, the types of interfaces considering to the results of fractographic observations affect the tensile performance of the Functionally Graded Composite. The fracture toughness test results demonstrate changes in the mechanism of crack propagation at the interface between materials with respect to the type of transition. Furthermore, the material layers interfaces turned out to be the weakest points of the Functionally Graded Composite. • A great continuation of the Inconel's grains is observed when deposited on 316 L. • 316 L grains show rare crystallographic traceability when deposited on the IN718. • Tensile properties are in very good agreement regardless of the deposition height. • Material layers interfaces are the weakest points of the functionally graded material. • The crack propagation mechanism in FGC is dependent on interface type.