Fractographic Observations Research Articles

Failure analysis of AISI 440C strand dies used in twin screw extruders during polymer compounding

In view of excellent wear and corrosion resistance, AISI 440C Martensitic Stainless steel finds wide use as Dies in Twin Screw Extruders in Polymer, Food and Pharma applications, especially when either additives, which could be corrosive, are added to aid compounding of polymers or moisture and/or free radicals are present in the processing material. Two AISI 440C Die Plates part of Strand Die Assembly in a φ40 mm size Twin Screw Extruder abruptly cracked during operation in succession disrupting regular production with the second failure occurring after about two years of the Extruder in service. While visual examination of the failed Die Plates showed presence of surface / sub-surface cracks and marks of corrosion, the sectioned Die Plate revealed multiple internal cracks. The hardness survey on the failed Die Plates confirmed values between 30 and 36 HRC, which are about 20–25 HRC lower than the hardness measured in as-supplied condition. Macro-examination of the sectioned Die Plate revealed multiple micro-crack origin locations together with strong evidence of corrosion. The microstructural examination showed carbide network along grain boundaries in a matrix consisting of tempered martensite and revealed intergranular fracture. Fractographic observations using Scanning Electron Microscopy on the cracked Die Plate clearly revealed segregation of fine carbides along crack boundaries. Based on detailed analyses, which included site data collection and investigation, the failure of Die Plates was attributed to Stress Corrosion Cracking. Finally, recommendations were proposed to prevent recurrence of such failures in service.

Mechanical Properties, Structure and Fracture Behavior of Additive Manufactured FFF-ABS Specimens

The Fused Filament Fabrication (FFF) method is one of the most important additive manufacturing (AM) technologies. This technology is used today with various kinds of thermoplastic materials, including ABS. The present study deals with the flexural strength and axial deflection of ABS specimens versus relative density, to observe the influence of build-orientations, build model and microscopic level defects of these properties. In this study, the mechanical and structural characterization of AM-FFF ABS material was studied by CAD modelling of different orientations, three point bending mechanical testing, visual testing, and multifocal light microscopy observation, including fractography analysis. To that end, three different standard building orientations (Flat, On Edge and Upright) were printed, and each was built in two different angle orientations (-45o/+45o and 0°/90o). Based on the three point bending testing results, it was found that the specimen with the highest flexural strength was not necessarily the one with the highest deflection. It was also observed that On Edge 0/+90o orientations showed a relatively larger flexural strength difference in comparison to other building orientations (Flat and Upright). When the mechanical properties achieved from a bending test next to the building platform were compared to the properties far from the building platform, only a slight difference was found, which means that the flexural strength difference results from the building strategy and it is not related to the specific bending surface. Based on fractography observation, there is a major difference in the mechanical properties and fracture surface appearance, when the samples are bent between the layers (Upright orientation) or when the samples are bent through the layers (Flat and On Edge orientation).

Failure analysis of self-healing epoxy resins using microencapsulated 5E2N and carbon nanotubes

Investigation on self-healing properties of epoxy containing microcapsules of pure 5-ethylidene-2-norbornene (5E2N) and mixed multi-walled carbon nanotube (MWCNT) suspension is presented in this work using lap shear and mode I fracture tests. While both systems show significant self-healing functionalities between 39% and 97%, those repaired with MWCNT/5E2N microcapsules are found to have mechanical recovery efficiencies two times higher than those containing 5E2N alone, under different types of loading. MWCNT/5E2N microcapsules are also found to improve the physical strength of the epoxy matrix much higher than microcapsules containing only 5E2N. Compared to self-healing epoxy systems made of 5E2N microvessels, the dispersion of 0.10 wt.% of only MWCNTs inside the encapsulated monomer increases the lap shear strengths of the pristine materials and the self-healing materials tested after one fracture/curing cycle, by about 50% and 250%, respectively. Using fractographic observations, Raman spectroscopy and scanning electron microscopy, we observe the occurrence of a ring opening metathesis polymerization reaction and the contribution of MWCNTs to strengthening the material. The present work indicates that the MWCNT/5E2N microcapsule when incorporated into an appropriate epoxy formulation can serve as efficient self-healing matrix material for laminated composites, as well as self-healing structural adhesives for composite bonded joints, two features that make this system highly relevant for applications in aerospace engineering.

Experimental-numerical study of laser-shock-peening-induced retardation of fatigue crack propagation in Ti-17 titanium alloy

Residual stresses induced by laser shock peening in Ti-17 titanium specimens were experimentally and numerically investigated to identify the mechanisms and generation conditions of the retardation of fatigue crack propagation (FCP). The retardation was experimentally observed with fatigue life prolonged by 150%. A multi-step simulation strategy for fatigue life prediction is applied, which successfully predicts the experimentally observed FCP behavior. The fractographic observations and numerical simulation indicate that crack closure, as opposed to other microstructural influences, is the dominant effect on retardation. The studies of multi-FCP aspects show that significant retardation occurs in specimens at high values of residual stresses, small peening gap distances, and lower externally applied loads.

The influence of temperature and moisture on the mode I fracture toughness and associated fracture morphology of a highly toughened aerospace CFRP

This paper addresses the characterisation of the mode I interlaminar fracture toughness of a carbon fibre/epoxy composite material, toughened with thermoplastic particles in the ply interlayers. The characterisation is undertaken at −55 °C, 19 °C, and 90 °C, on both dry and fully moisture saturated coupons. Fractographic observations of the delamination surfaces allows identification of the failure mechanisms. The mode I propagation fracture toughness tested at wet/90 °C exhibits a 176% increase compared to the dry/19 °C specimens, due to enhanced plastic deformation of the interlayers and more prominent fibre bridging. Moisture-saturated coupons tested at −55 °C suffered a 57% reduction of mode I fracture toughness compared to those under dry/19 °C conditions. This is due to the dis-bond and consequent plucking of the thermoplastic particles from the surrounding matrix. This observation points to the fact that wet/cold conditions may represent the worst-case scenario for the interlaminar fracture performance of composite systems toughened with thermoplastic interleaves.

Load Controlled Fatigue Behaviour of Microplasma Arc Welded Thin Titanium Grade 5 (6Al-4V) Sheets.

The current study investigates the load controlled fatigue properties of the microplasma arc welded thin titanium Grade 5 (6Al-4V) sheets. In order to explore the effect of weld geometry on the fatigue, two different welded joints were used in the experimental studies. Load controlled fatigue test results were evaluated to present an outlook on the behaviour of microplasma welded titanium alloy Grade 5 sheets under cyclic loading. Even though the previously published monotonic tests showed successful use of microplasma arc welding to join thin titanium Grade 5 sheets with mechanical properties comparable to the base metal, fatigue life of the welded joints was lower than the lives of samples without welds. In particular, the fatigue performance of overlap joints was very poor. This was presumed to be due to the changed material properties of the heat affected zone which was formed by the excess heat of the welding process as fractures often occurred at such locations. Based on experimental findings and fractographic observations, a clear adverse effect of welding process in material behaviour was discovered. Despite the concentrated heat of microplasma arc welding, post-weld heat treatment of the weld area is recommended to improve the mechanical behaviour of the welded joints.

Strengthening characteristics in TC17 titanium alloy treated during LSP

The objective was to address the strengthening characteristics in TC17 titanium alloy treated during laser shock processing (LSP) by simulation and experiment. A simulation model with the size of 4 mm × 4 mm×2 mm during LSP was established by ANSYS/LS-DYNA. The propagation characteristics of stress waves and residual stresses were simulated, and the residual stresses were also measured by an X-ray stress. The influence rules of different spot overlap ratios on the residual stresses were analyzed, and the distribution laws of residual stresses on the model surface were obtained in uniaxial direction. Further, the impact effect was discussed when the spot overlap ratio was 60 % by experimental test and fractographic observation. Simulation and experimental results explained that the plastic deformation layer on the model surface became more even with the increase of spot overlap ratio, and impact unevenness was greatly reduced. Especially, the residual compressive stress was greatly increased when adopting 60 % overlap ratio, the maximum residual compressive stress could be reached 691 MPa, and a stable plastic deformation layer was formed on the model surface.

Spark plasma sintered tungsten – mechanical properties, irradiation effects and thermal shock performance

Tungsten-based materials are the most prospective candidates for the plasma facing components for future fusion devices, such as DEMO. In order to improve their properties, various modifications are being developed, including composites, alloys, and different processing routes. Spark plasma sintering (SPS) is among the prospective preparation technologies; thanks to the relatively low temperatures and short processing times, it enables the preservation of fine grain structure, beneficial for radiation resistance. In a previous study, SPS W has shown promising mechanical properties at moderate temperatures, however, the irradiation effects were yet to be investigated.Fine-grained W was prepared by spark plasma sintering. Together with other W-based materials, the samples were neutron-irradiated at the BR2 reactor at 600 and 1000 °C up to 0.24 and 0.7 dpa, respectively. Mechanical testing - including tensile test and fracture toughness tests - was performed in irradiated and un-irradiated states in the 200–600 °C temperature range. Fractographic observations were performed to help in understanding the impact of the irradiation effects on the fracture mechanism. For the SPS W, a shift of DBTT from ~300 °C to ~600 °C due to irradiation was observed. High heat flux testing was carried out in repeated thermal shock mode at the PSI-2 device at room temperature, 400 and 1000 °C and fluxes up to 1.6 GW/m2. The results showed rather promising resistance to cracking under these conditions. In these tests, the SPS tungsten showed comparable or better performance than reference, ITER-qualified tungsten material.

The Effect of Electrolytic Hydrogenation on Mechanical Properties of T92 Steel Weldments under Different PWHT Conditions.

In the present work, the effects of electrolytic hydrogen charging of T92 steel weldments on their room-temperature tensile properties were investigated. Two circumferential weldments between the T92 grade tubes were produced by gas tungsten arc welding using the matching Thermanit MTS 616 filler material. The produced weldments were individually subjected to considerably differing post-welding heat treatment (PWHT) procedures. The first-produced weldment was conventionally tempered (i.e., short-term annealed below the Ac1 critical transformation temperature of the T92 steel), whereas the second one was subjected to its full renormalization (i.e., appropriate reaustenitization well above the T92 steel Ac3 critical transformation temperature and subsequent air cooling), followed by its conventional subcritical tempering. From both weldments, cylindrical tensile specimens of cross-weld configuration were machined. The room-temperature tensile tests were performed for the individual welds’ PWHT states in both hydrogen-free and electrolytically hydrogen-charged conditions. The results indicated higher hydrogen embrittlement susceptibility for the renormalized-and-tempered weldments, compared to the conventionally tempered ones. The obtained findings were correlated with performed microstructural and fractographic observations.

Low-cycle fatigue behavior and damage progression of a fiber reinforced titanium matrix composite

The low-cycle fatigue behavior and damage progression of a SiC fiber reinforced Ti–6Al–2Sn–4Zr–2Mo-0.1Si matrix composite was investigated systematically. The fatigue life increased from 4 to 52388 cycles when the maximum stress decreased from 1500 MPa to 1100 MPa. The modulus calculated from the stress-strain response exhibited little variation for all the specimens during the fatigue process. Fractographic observations showed that the fatigue cracks initiated internally and propagated preferentially in the internal sputtered matrix instead of the unreinforced matrix in the external layer for the cylindrical specimen. The proportion of the fatigue region in the fracture surface increased as the applied stress level decreased. The internally-progressed damage, especially breaks of bridging fibers, was well characterized in situ by the acoustic emission system.

Numerical simulation and experimental investigation on a steam turbine blade fractured from the lacing hole

Failure analysis of a 17th stage blade of a steam turbine fractured in the vicinity of the lacing hole is discussed in this study. For this purpose, visual inspection, chemical and structural analyses, hardness and uniaxial tensile tests, macro and micro-fractography, fracture mechanics calculations, as well as computer simulations have been carried out. According to the results of the hardness and tensile tests and a comparison with the properties of AISI 410 martensitic stainless steel, it can be claimed that the blade did not experience any deterioration of mechanical properties during the service. Fractographic observations revealed the high cyclic fatigue as the main failure mechanism of the blade and showed that the fatigue crack is initiated and propagated adjacent to the lacing hole, from the brazing interface between the rod and the blade due to a lack of proper filler attachment as well as the presence of micro cracks within the brazing metal. In addition, estimation of the failure stresses based of fractographic observations, fracture mechanics and computer-aided calculations indicated that the failure of the blades has not been due to any abnormal stresses. Finally, a computer simulation of the fatigue crack growth indicated that after the crack initiation, about 3.75 h should be enough for the crack to reach the critical crack dimensions.

Critical ductile fracture criterion based on first principal stress and stress triaxiality

A series of quasi-static fracture tests on ten types of specimens for six types of materials were performed to investigate the ductile fracture failure behavior of materials. The full-range equivalent stress-strain relations of materials were obtained by using finite-element-analysis aided testing method. The critical failure behavior was discussed in detail by the analysis of the local stress and strain distributions on the cross-sections of specimens and fractographic observation of specimens. The results show that the first principal stress is the dominant factor of ductile fracture failure. A novel critical fracture criterion of ductile materials represented by the critical first principal stress and critical stress triaxiality is proposed, and the validity of the proposed criterion has been successfully examined.

Fracture analysis in directed energy deposition (DED) manufactured 316L stainless steel using a phase-field approach

Experimental and numerical study regarding fracture in laser-processed steel components is addressed in the present work. Samples of stainless steel (SS) 316L were obtained by an additive manufacturing process, the directed energy deposition (DED), using different deposition orientations, and tested experimentally until fracture. Microstructural investigations, prior and after fracture, were performed by observing micro-cavities and porosities and fractographic images of the fracture surfaces.A numerical approach based on the phase-field diffusive model was utilised in a micromechanical pressure-dependent plasticity context using Rousselier damage criterion and implemented within the finite element framework. The ability to predict the material failure induced by the porosity evolution through the micro-void growth mechanism is considered as a key feature of the proposed material model. The performance of the numerical model is assessed via material deformation analysis, including initiation and propagation of cracks, which are found to be in good agreement with the experimental and fractographic observations from the fabricated tensile test samples.

Versatile fabrication of bulk metallic glass composites reinforced by dissimilar secondary phase

Bulk metallic glass (BMG) matrix composites have attracted increasing attentions in the past decades, for combining the superior strength of BMGs with the excellent malleability of crystalline alloys. Insofar, increasing methods have been developed for the fabrication of BMG composites. However, versatile methods capable of fabricating high-performance BMG composites reinforced by diverse secondary phases remain lack. Here, the versatile fabrication of BMG composites reinforced by dissimilar crystalline alloy via ultrasonic vibration-assisted thermoplastic forming was reported. Wire gauze made of 304 stainless steel and Zr-based BMG were chosen as the secondary phase and the matrix, respectively. The as-prepared BMG composites exhibited metallurgical bonding at the interface between BMG and stainless steel, in contrast to the reference sample prepared by normal thermoplastic forming which exhibited merely mechanical bonding. The formation of metallurgical bonding was attributed to the self-cleaning effect induced by ultrasonic vibration which disposed the surface oxidation layer, and the enhanced atomic interdiffusion between BMG and stainless steel by ultrasonic vibration. The fracture toughness of the as-prepared BMG composites exhibited an improvement from 65 to 132 MPa m1/2. Based on fractography observation, the improved fracture toughness was attributed to the deflection of crack at the interfaces between BMG and stainless steel. These results prove the versatility of ultrasonic vibration-assisted thermoplastic forming in the fabrication of BMG composites.

Static and dynamic shear-compression response of additively manufactured Ti6Al4V specimens with embedded voids

The shear-compression response of additively manufactured Ti6Al4V specimens containing discrete artificial voids is investigated under quasi-static and dynamic loadings. Specimens containing spherical voids are designed with one or three voids with variable spacing between them. Specimens containing spheroidal prolate voids are designed with different void orientations with respect to the shear direction, while the total volume fraction of all void configurations is kept constant in the whole study. It is found that the presence of the void(s) reduces the displacement to failure, compared to the dense specimens, in both quasi-static and dynamic regimes. The shape and the number of voids have a noticeable effect on results only in the quasi-static regime. However, changing the distance between the spherical voids or changing the orientation of the prolate voids does not affect significantly the load-displacement curves in both strain rate regimes. Fractographic observations show that shear-compression specimens fail dominantly by shear, whereas previously investigated shear-tension specimens fail dominantly by tension. These observations are supported by preliminary numerical simulations.

Influence of dynamic-strain aging due to excess Mg on fatigue crack growth rate scatter in Al6061-T6 alloy

Fatigue failure results in high industrial costs, and its mechanism requires close examination; however, current methods are costly and time-consuming due to the need for a large number of test specimens. The purposes of this study are to investigate the influence of dynamic strain aging on fatigue crack growth rate (FCGR) scatter in Al 6061-T6 alloys and to present a new approach to evaluate the scatter of FCGR using a limited number of the test specimen. Rotating bending fatigue tests of 6061-T6-based Al alloys with added Zr and excess Mg were performed under constant amplitude loading using smooth specimens. The scatter behavior of FCGR is investigated by examining the fatigue crack growth on the specimen surface and fractographic observation on the fracture surface. The accounting for the interaction effects of multiple surface cracks and fractographic examination on striation formation from previous findings revealed that excess Mg promoted small scatter in Mode I fatigue crack growth. This study showed that local plastic deformation affected the FCGR scatter of microstructurally-large fatigue cracks. These findings suggest that dynamic strain aging of Mg induces stable Mode I crack growth due to pinning of dislocation movement on slip planes during the crack growth process.

Plastic deformation and mixed-mode I/II fracture behavior of un-plasticized polyvinyl chloride

To further understand the mixed-mode I/II fracture behavior of Un-plasticized Polyvinyl Chloride (UPVC), a series of mixed-mode I/II experiments on analogy compact tension specimens have been conducted using an MTS 810 hydraulic test machine. The evolution of the displacement field during stable crack extension was monitored by two-dimensional digital image correlation technique. Results obtained from the mechanical testing demonstrate that the maximum load of mode II is about 100% higher than that of mode I, and the load-carrying capacity is dependent on orientation and load condition. Fractography observations indicate that the fracture surface can be divided into fatigue region with relatively smooth characters and stable crack extension region with randomly distribution of honeycomb-like voids. Furthermore, mode I failure (opening) should be the main failure mechanism of UPVC rather than mode II (shear failure). The results also suggest that the value of crack opening displacement (COD) tends to be nearly constant when the length of crack exceeds the thickness of the sample, which is independent on orientation and load condition. Therefore, COD criterion can be used to describe the overall severity of plastic deformation in the near-tip region of UPVC, and components of COD, which is perpendicular to the crack surface, should be the predominant controlling parameter of crack propagation.

The effect of temperature on fatigue strength of poly(ether‐imide)/multiwalled carbon nanotube/carbon fibers composites for aeronautical application

AbstractThis work concerns the fatigue behavior at three different temperature conditions (−40, 20, and 80°C) and the addition of multiwalled carbon nanotube (MWCNT) into a carbon‐fiber reinforced poly(ether‐imide) composite. The incorporation of MWCNT into the composite increased the tensile strength and Young's modulus by up 5 and 2%, respectively. At low temperature, the incorporation of the nanoparticles improved the fatigue strength of the laminates by 15%. The shear strength results obtained by interlaminar shear strength and compression shear test tests have shown an increase of about 16 and 58%, respectively, by the introduction of nanotubes into the laminates. Fractographic observations revealed that the surface of carbon nanotube laminate (PEI/MWCNT/CF) presented a ductile behavior, and differences in the fracture aspects of the material compared to the traditional PEI/CF laminate have been observed.

Shape Evolution of Unstable, Flexural Cracks in Brittle Materials

In this manuscript, the crack-shape evolution of unstable, flexural cracks was modeled using a continuum mechanics approach applied to dynamic, brittle, and elliptical cracks. The crack-shape evolution was first computed analytically for both flat and fractal cracks and subsequently compared to experimental values inferred from fractographic features present on silicate glass, gallium arsenide, and silicon single crystals fractured in bending. The trends predicted by the analytical model were consistent with the fractographic observations prior to the onset of crack-tip instabilities. It was determined that the effect of the surface roughness was to slow down the crack and to decrease the local crack-front radius of curvature which, in flexural cracks, led to crack shapes elongated along the free surface direction. This work represents the first attempt to predict the crack-shape evolution in rough, brittle samples fractured in bending and to understand the role played by the surface roughness during fast, unstable propagation.

High Temperature Micromechanical Behavior of a Pt-Modified Nickel Aluminide Bond-Coating and of Its Interdiffusion Zone with the Superalloy Substrate

The micromechanical properties of a $$\beta $$-(Ni,Pt)Al bond-coating was investigated between $$700\,^\circ \hbox {C}$$ and $$1000\,^\circ \hbox {C}$$ using ultrathin freestanding bond-coating specimens. Its brittle-to-ductile transition temperature was close to $$750\,^\circ \hbox {C}$$, with a significant ductility above $$800\,^\circ \hbox {C}$$ (up to 23 pct at $$1000\,^\circ \hbox {C}$$). The tensile strength decreased from 450 MPa at $$750\,^\circ \hbox {C}$$ down to 50 MPa at $$1000\,^\circ \hbox {C}$$. Fractographic observations evidenced the material brittleness at intermediate temperature with large cleaved grains and its ductility above $$750\,^\circ \hbox {C}$$ with important necking of individual grains.