Fractographic Investigation Research Articles

Crack branching in polymers

One of the little-studied problems of modern mechanics and the physics of fracture is the crack branching in materials of a different nature. For its research, an analysis of the criteria and mechanisms for crack branching. The investigations of crack branching in polymers and metals, fractographic investigation of the fracture surface, criteria for crack branching as a dynamic stress intensity factor, crack speed are considered. Experimental studies of crack branching in a brittle polymer, measuring of crack velocity by breaking the conductive strips with rectilinear propagation and branching of the crack have been carried out. A measurement installation was created on the basis of a precision signal converter of resistance thermometers and thermocouples “Tercon” connected to a computer. It is established that the crack at branching in brittle polymers reaches the limiting propagation speed 500–800 m/s. A hypothesis is proposed for the physical mechanism of crack branching.

The effect of mean axial and torsional stresses on the fatigue strength of 34CrNiMo6 high strength steel

The present study consists of a theoretical, experimental and fractographic investigation of the effect of superimposed static axial and shear stresses on the high cycle fatigue behavior of a 34CrNiMo6 high strength steel in quenched and tempered condition (UTS = 1210 MPa), commonly employed in highly stressed mechanical components. The Haigh diagrams for the axial and torsional cases under different values of mean stress were obtained. In both cases, experimental results showed that increasing the mean stress gradually reduces the stress amplitude that the material can withstand without failure. The results of the present tests are compared with the theoretical predictions from Findley, based on the maximum damage critical plane; and the methods of Marin and Froustey, which are energetic based criterions. Froustey’s method shows the best agreement with experimental results for torsional fatigue with mean shear stresses, showing a non-conservative behaviour for the axial fatigue loading case. Macro-analyses and micro-analyses of specimen fracture appearance were conducted in order to obtain the fracture characteristics for different mean shear stress values under torsion fatigue loading.

The Influence of Temperature and Stress Cycle Asymmetry at Fatigue Cracks Initiation and Propagation in Cylindrical Specimens of Ni-based Alloy EP741NP

Using the scanning electron microscope, information on the initiation and kinetics of the fatigue cracks development in the powder Ni-based superalloy EP741NP was obtained. This alloy is used for the manufacture of GTE disks. The low-cycle fatigue test of cylindrical specimens was carried out in the wide range of temperatures from 20 °C to 750 °C under strain control (fixed strain range) and different asymmetry ratios. The fractographic investigation of broken specimens made it possible to determine the kinetics of the fatigue crack growth and durability, which also made it possible to determine the period of crack initiation. After the fractographic analysis, the mathematical modeling was carried out using the ANSYS software, which made it possible to determine the Paris parameters which are responsible for the growth of fatigue crack in this material. The comparison of obtained parameters with the Paris factors derived with standard procedure during the tests of CT specimens revealed the mismatch at high temperatures. The obtained information will enable to predict the durability of parts manufactured of this material.

Characteristics of spark plasma sintered ZrB2-SiC-SCFs composites

The effects of short carbon fibers on sinterability, flexural strength, fracture toughness and thermal shock resistance of spark plasma sintered ZrB2-SiC composites were investigated. Four samples containing 10, 20, 30 and 40 vol% of short carbon fibers (SCFs) were consolidated by spark plasma sintering at 1800 °C for 6 min under an external pressure of 40 MPa. Mechanical properties of the samples including flexural strength and fracture toughness were measured via three-point bending strength test and indentation method, respectively. Fractographical investigations were carried out via scanning electron microscopy. Thermal shock resistance of the samples was also studied via quenching method. Obtained results showed that a higher fraction of SCFs (10 vol% SCFs to 40 vol% SCFs) leads to lower relative density (90.8–89.1%) and higher amount of open porosity (7.8 vol% to 8.9%). It is also indicated that SCFs ascent from 10 to 40 vol% improve the fracture toughness from 4.1 to 5.1 MPa m1/2 by promoting toughening mechanisms such as crack deflection, branching, and pull-out. Finally, it was cleared that by SCFs ascent from 10 to 40 vol%, ΔTC increases from 530 °C up to 600 °C.

Effects of thermal cycling on physical and tensile properties of injection moulded kenaf/carbon nanotubes/polypropylene hybrid composites

Polymer as well as polymer composites are inclined to premature due to effect on mechanical and physical properties during service time. In this paper, the effect of thermal cycling (−30 to 80 °C) on the physical and tensile properties of Polypropylene (PP), Kenaf/PP, Kenaf/PP/CNTs and Kenaf/CNTs/MAPP/PP hybrid composites for the injection moulded car battery tray was investigated. Although this study had found no physical changes on the samples in terms of colour or cracking, the samples, however, had experienced warpage and dimensional changes. The dimension of samples containing CNT had increased by 20%, while the PP and PP/Kenaf samples had decreased by 3.5% and 87%, respectively. Although the process of thermal cycling was found to have rapidly decreased the hardness of Pure PP and Kenaf/PP composites, it had, however, gradually increased the hardness of composites with CNT fillers. The tensile strength, Young's modulus and elongation of the composites had also been examined in this research. Furthermore, according to the fractographic investigations using scanning electron microscope, it is perceived that the fracture morphology of the samples is strongly influensed by thermal cycling.

Microscopic characterization on low cycle fatigue behavior at room temperature of Zircaloy-4 alloy with recrystallized microstructure

Low cycle fatigue (LCF) tests, with the total strain amplitude (ΔƐt/2) in the range of ± (0.5–3.0%), were employed at room temperature on Zircaloy-4 (Zr-4) alloy with recrystallized microstructure, aiming to investigate its fatigue life, fatigue crack initiation and propagation mechanisms. Results indicated that the fatigue behavior of Zr-4 alloy followed Coffin-Mason relation with fatigue ductility exponent of 0.2426. Fractographic investigation of all mechanically tested specimens revealed that the fatigue specimen surface is the preferential crack initiation place. SEM observation on the crack propagation regions of fatigue specimens showed that a large number of fatigue striations distributed in the crack propagation regions. Further detailed investigation revealed that micro-crack growth rates can be quantitatively evaluated in terms of strain amplitude.

Hygrothermal Effect on Composites Under In-Plane Fatigue at Stress Ratios of R = −1 and R = 0.1: An Analysis of Quasi-Isotropic Stitched Carbon Fibers

The aeronautic structures normally operate under high levels of hygroscopic moisture from the surrounding environment at different temperature ranges while in service. Under such conditions, the behavior of laminate composite submitted to cyclic or static loadings can change drastically. In order to understand those effects in stitched fabrics, fatigue tests with open-hole specimens were carried out with a stress ratio of R = −1 and R = 0.1. The specimens were fatigue-tested as provided (environmental conditions) and after exposed to hygrothermal weathering conditions. Based on evidences from recent studies available in the open literature, e.g., effect of water diffusion on epoxy matrix, the overall results indicated a significant reduction in stiffness after the specimens are exposed to hygrothermal effects. The reduction in matrix stiffness, in this case, enhanced the fatigue strength in tension–tension load (R = 0.1) when compared to the specimens in normal conditions. The opposite occurs for the specimens loaded with stress ratio of R = −1, in which the delamination mechanisms changed during the loading reversion from tension to compression that promoted early delamination. Therefore, this process reduced the fatigue life of the specimens under hygrothermal condition. Then, by fractographic investigation, it was verified fracture patterns that regard to mode II damage in R = −1, in which mode II fracture toughness is known to decrease in the presence of water molecules.

Application of the small punch test to evaluate the integrity of a cold spray titanium coating

Metal Cold Spray (MCS) is currently under evaluation for its suitability for aerospace applications. However, before this technology can be implemented into the jet engine, the mechanical performance and structural integrity of this novel process must be fully understood. Limited data is currently available to determine key materials properties given the discrete and transient nature of a MCS component. Furthermore, it is extremely challenging to produce uniaxial test coupons that are truly representative of the in-service geometry. As such, the small punch (SP) test offers an attractive alternative, since miniature disc SP specimens can be extracted from localised discrete locations. This paper will report the findings from an experimental collaborative programme of work currently being undertaken by Swansea University, Rolls-Royce Singapore and Nanyang Technological University Corp Lab to understand the contrasting modes of failure in a Ti-6Al-4V coating sprayed on to a Ti-6Al-4V substrate, which is expected to have properties akin to a forged variant. This will include a series of SP tests to assess the integrity and performance across the substrate, bond line and coating. Results will be supported by additional microstructural and fractographic investigations.

Specific Features of the Fatigue Fracture of Welded Joints of 34KhN2MA Steel Formed by Electrodes with Different Phase Compositions

We study the cyclic crack resistance of the metal of welded joints of 34KhN2MA thick-plate steel obtained with the help of ferritic and austenitic electrodes. It is shown that the weld metal has a higher crack resistance than the base metal only within the second part of the kinetic diagrams of fatigue fracture. In the first and third parts, the characteristics of the weld metal are worse. The results of fractographic investigations demonstrate that the decrease in the fatigue crack-growth resistance in pure austenitic welds is explained by the presence of hot cracks in the zone between the near-root metal of the welded joint and the base metal. As for the mixed ferrite-austenitic welds, the decrease in the fatigue crack-growth resistance is explained by inadmissibly large slag inclusions creating additional stress concentration and deteriorating the characteristics of cyclic crack-growth resistance.

Optimal synergy between micro and nano scale: Hierarchical all carbon composite fibers for enhanced stiffness, interfacial shear strength and Raman strain sensing

Multifunctional hierarchical reinforcements are fabricated by growing multi walled carbon nanotubes (CNTs) onto carbon fibers (CFs) via chemical vapor deposition. In contrast to typical hybrid multi scale reinforced composites, the biomimetic approach of hierarchical interconnection between CFs and CNTs is followed in order to provide the dimensional confinement required for the effective synergy between the nanophase i.e. the CNTs and the micron phase i.e. the CFs. Compared to the reference CF: (i) ASTM single fiber tensile tests reveal up to 50% increase in tensile modulus together with the expected decrease in tensile strength, (ii) Single Fiber Fragmentation Tests (SFFT) reveal up to 134% enhancement for Interfacial Shear Strength (IFSS), (iii) the frequency of the Raman 2D graphitic vibrational mode with strain shows a strain sensitivity enhancement up to 87.4% and (iv) fractographic investigation shows bridging of the CNTs only for specific growth conditions, which correspond to the optimal IFSS. Furthermore, a direct correlation between the Raman strain sensitivity with the young moduli of the CF and the hierarchical CF-CNT is found, proving the efficient stress transfer from the nano to micron scale in a “composite” fiber. Overall, an optimal synergy between the reinforcing graphitic phases is achieved, attaining for the first time an equivalent stiffness for the CNT reinforcement close to theoretically obtained values. Thus, biomimetic hierarchical reinforcements provide the roadmap for the full exploitation of the unique properties of the nanophase in advanced structural composites.

Structure and Crack Resistance of N-A-XTRA-70 Steel Joints Manufactured by Hybrid Laser-Arc Welding

Crack resistance of welded joints is one of the most important indices of service reliability of critical designation structures. The purpose of the research is the establishment of the influence of technological modes, hybrid laser-arc welding of high strength steel NA-XTRA-70 on the formation of structure in the metal welded joints, as well as analysis of the influence of this structure on their crack resistances. For the experiments the laboratory setup has been developed on which the experiments were carried out in the technological scheme that provides the location of the consumable electrode arc at a certain distance in front of the laser beam in the welding direction. Nd: YAG-laser DY 044 of company ROFIN (Germany) was a laser radiation source. Power for consumable electrode arc was carried out by welding generator PSG-500. Investigations of structural-phase changes in weld metal and HAZ were carried out by means of optical microscopy (optical microscope Versamet-2 and Neophot-32), microhardness was measured using M-400 gage of Leco company at 0.98N loading. Fractographic investigations were carried out by scanning electron microscopy (scanning electron microscope SEM-515 of PHILIPS company, Netherlands) on fractured specimens of welded joints, made as a result of impact bend tests. An analysis of crack resistance of the welded joints was carried out under different conditions of test temperatures. It was based on experimental data, received by a quantitative fractography method. It is shown that metal of a welded joint produced by hybrid laser-arc welding at Vw = 72m/h rate has mainly failure of ductile nature after external loading. This indicates sufficient level of crack resistance and further working capacity under operation conditions.

Features of the microstructural and mechanical degradation of long term operated mild steel

PurposeThe purpose of this paper is to assess the technical state of old and repair steels of Shukhov’s tower elements after operation during ~ 110 and 70 years of the water tower in Nikolaev, basing on their mechanical tests, metallography and fractography investigations.Design/methodology/approachFor their certification, the fractographic and structural features and mechanical properties (hardness, strength, plasticity and impact toughness) were analyzed. Both the steels under consideration were characterized by low values of hardness and brittle fracture resistance. The mechanical characteristics of the old steel are lower compared with the repair one. It cannot be only explained by the quality of metal rolling. Moreover, the plasticity characteristics of both steels, defined in synthetic acid rain environment, are lower than in the air. Using fractography investigation, the operational damages in the bulk metal in the form of the elements of cleavage fracture in the central part of the fracture surfaces of specimens tested at the hydrogenation condition by synthetic acid rain environment were revealed.FindingsThe results of this study suggested a degradation of steels’ characteristics caused by the development of scattering damages during their operation. Higher relative elongation of the old steel at lower hardness and impact toughness were also evidenced in that. The metallography and fractography investigations also supported this finding.Originality/valueThis original study aimed at characterizing the microstructural and mechanical degradation of mild steels that was collected from Shukhov’s tower structural elements.

Stress corrosion cracking of biodegradable Mg-4Zn alloy in simulated body fluid at different strain rates – A fractographic investigation

Degradable magnesium based implants are predisposed to failure by stress corrosion cracking (SCC) when subject to tensile loads in the human body. Slow strain rate (SSR) tests are used in SCC analysis and the applied strain rate is a critical factor to determine if the material failed by stress corrosion or mechanical overload. In this paper, the SCC susceptibility of Mg-4Zn alloy in simulated body fluid (SBF) is investigated at different strain rates. It was found that elongation to failure (ε) and ultimate tensile strength (UTS) decreased by 80% and 58% respectively when extension rate in SBF decreased from 3.6 × 10−4 s−1 to 3.6 × 10−6 s−1. When the Mg-4Zn alloy is strained in SBF, multiple surface cracks appear on the gauge, initiated at the corrosion pits that induce mechanical overload and failure at higher strain rates. As the strain rate decreases, the increased contact time between the surface and SBF results in hydrogen embrittlement, micro crack formation and progressive transgranular failure that is evident in the fractography. The reduction in UTS and elongation even at the highest strain rate tested indicate that the critical strain rate for stress corrosion cracking of Mg-4Zn alloy in simulated body fluid could be as high as 3.6 × 10−4 s−1.

Instability of Plastic Deformation in Low-Alloy Copper Alloys

The purpose of this paper is to determine the influence of temperature of plastic deformation on the structure and mechanical properties of copper alloys of the types CuCo1NiBe (CCNB), CuCo2Be (CB4), CuNi2SiCr (CNCS), CuNi1P (CNP) and CuCr1Zr (CW106C) applied on electrodesduring a tensile test. Tensile tests were carried out on polycrystalline samples of above mentioned alloys, which confirmed the presence of inhomogeneous plastic deformation in specified temperature ranges for each alloy. The tensile test of the investigated copper alloys were realized in the temperature range of 20÷800 °C with strain rate of 1.2•10-3s–1 on the universal testing machine. Metallographic observations of the structure were carried out on a light microscope and the fractographic investigation of fracture on an electron scanning microscope. Performed experimental studies have proven that analyzed structural factors, in a range of investigated strain conditions at elevated temperature, significantly influence the phenomenon of the Portevin Le Chatelier (PLC) type instability of plastic strain, revealed in low-alloy copper alloys. Moreover, it was found that the impact of examined factors on the PLC effect should be considered comprehensively, taking into account their synergic interactions.

EXPERIMENTAL STUDY OF THE CRACK BRANCHING SPEED IN POLYMERS

One of the little-studied problems of modern mechanics and the physics of fracture is the branching of the crack, which is observed in materials of a different nature. For his research, an analysis of the criteria and mechanisms for crack branching. The investigations of crack branching in polymers and steel, fractographic investigation of the fracture surface are considered; criteria for crack branching as a dynamic stress intensity factor, crack speed. It is established that the crack at branching in brittle plastics reaches the limiting propagation speed V * = 500 – 800 m/sec. Tensile tests of flat samples from polymethylmethacrylate (PMMA) at temperatures of +20 and –60°C were carried out, with measurement of crack speed by the method of rupture of conducting strips. A measuring device based on a precision converter of signals of resistance thermometers and thermocouples «TERCON», connected to a computer, was created. Measured crack speed for rectilinear crack propagation in the mirror, matte, and feather zones of fracture surface of the PMMA; with single branching of the crack; with multiple branching of cracks with parallel movement of the front of several cracks. A hypothesis is proposed for the physical mechanism of fracture branching.

Mechanical investigation of weak regions in a wound oxide-oxide ceramic matrix composite

The main aim of the investigation was to quantify the influence of production-related cross-lines on static mechanical properties (tensile, flexural and shear) of an oxide-oxide CMC as a comparison between specimens with cross-lines and specimens without cross-lines in tested regions. Investigated material was a weak-matrix oxide-oxide CMC (WHIPOX™) made of Nextel™ 610 fibers (3000 denier) and alumina matrix with a special winding pattern. Mechanical tests at room temperature revealed that cross-lines were local weak regions in a wound component. Spatial separation of the cross-line within the composite (2 mm shift from layer to layer) did not improve the negative influence of the cross-lines on mechanical properties. Fractographic investigations revealed that cross-lines acted as a trigger of material failure.

Special Features of Metal Lamination in Hydrogen Sulfide Absorber of Catalytic Reforming Unit and Hydraulic Cleaning

The paper analyzes the technical state of the hydrogen sulfide absorber of the catalytic reforming unit and hydraulic cleaning containing hydrogen and hydrogen sulfide with its operation time of 168,200 h (21 yr). 17G1S manganese-silicon steel is used in the absorber structure. In operation, in the vessel shell, a cone-shaped region of subsurface metal lamination is developed across the entire periphery. The region is under constant observation using the methods of nondestructive testing. In case the region of the damaged metal intersects the longitudinal welded joint, the vessel shell is being repaired. A low-temperature (to the boiling point) hydrogen-related fracture of metal in the petroleum refining due to hydrogen saturation and its accompanying cracking are considered to be the most critical types of corrosion of oilfield equipment. In view of this, a set of laboratory tests using specimens of metal templates cut out of the damaged shell region has been performed to evaluate the serviceability of the absorber, as well as to determine the effect of hydrogen sulfide on the structure and mechanical characteristics of the metal. It is found that the process of lamination is detected inside the metal intersection and is caused by the macroinhomogeneity of rolled metal, i.e., the axial segregation zone, which is the most critical region of the deformed metal. X-ray phase analysis lends credence to the metallurgical nature of lamination of the shell plate. The results of metallographic examination imply that the subsurface hydrogen-induced cracking is caused by aggregation of brittle refined nonmetallic inclusions, mainly of MnS, and it propagates chiefly over sulfide films along the rolled metal direction. The results of experimental investigations on the specimens are provided. From the fractographic investigations on the fractured surface of specimens after mechanical testing it is established that the process of fracture is ductile occurring via microvoids coalescence in the fracture of bridges between them. The key mechanism of microvoids formation is the inhomogeneity of the plastic strain in the micro-volumes around the nonmetallic inclusions.

Fracture surface characterization of laser welding processed Ti alloy to stainless steel joints

The laser welding of Ti6Al4V alloy to 304 stainless steel using a 1-mm-thick Cu interlayer was developed by changing the laser power. The fracture characteristics of joints were analyzed by SEM, EDS, and XRD. The fracture examinations indicated that both the joint strength and the fracture occurrence location in the joints depend on the laser power settings. The optimal value for the maximized tensile strength of the joints, up to 300 MPa, was obtained at a power of 4 kW. Increasing the laser power by 4 kW increased the tensile strength; it decreased as the laser power exceeded 4 kW. In the 4-kW samples, the fracture characterization indicated that the higher remaining Cu deposited on the Ti sheet that caused the fracture path tended to make the fracture propagate toward the Cu interlayer. The fractography investigations illustrated that the fracture surface of a 4-kW joint had a greater rough area than that of either the 3- or the 6-kW joints. The XRD results demonstrated that the most of solid solution phases, such as αTi, (V), and (Ni) coexisting with various types of Al-intermetallics could occur on the fracture surface of a 4-kW weldment.

Experimental and Numerical Study of Notch Geometry Effects on Fatigue Life Under Mixed Mode I and III Loading

Fatigue is one of the main reasons of fracture in mechanical components. Generally performing experiments is a useful and effective method for determination of fatigue life. In this paper, results of different notch geometry effects on the fatigue life under mixed mode I and III were studied experimentally and numerically. For this purpose, fatigue specimens were manufactured according to the ASTM-A370 standard and all the specimens were quenched and tempered. Two types of notch geometries, V-shape and U-shape were considered in this study. The fatigue life was determined experimentally for each notch geometry and results were verified by finite element method simulation. Fractographic investigation was done broadly to determine cause of failure in the specimens. Our results showed that the notch geometry had significant effect on the fatigue life. Fatigue crack growth in the V-shape notch specimens happened quickly and the maximum fatigue life reduction occurred for that.

Effect of chemical surface treatment of titanium on its bond with dental ceramics

Statement of problemAirborne-particle abrasion of titanium is a clinically accepted method of surface preparation. As a side effect of airborne-particle abrasion, particles of the abrasive material get embedded into the surface. How particle presence or removal from the titanium surface affects the strength of the titanium-ceramic bond is unclear. PurposeThe purpose of this in vitro study was to determine the effect of removing Al2O3 particles embedded into the surface by means of chemical surface treatment on the strength of the titanium-ceramic bond. Material and methodsTitanium (TritanCpTi 1, Dentaurum, 99.5% Ti) disks were airborne-particle abraded with 110 μm Al2O3 at a pressure of 0.4 MPa and an angle of approximately 45 degrees. The surface was etched chemically using 1 of 8 reagents, and the veneering ceramic applied and fired. The strength of the metal-ceramic bond was determined using the shear strength test. Further, the effect of thermal fatigue on the bond strength was evaluated. The results were analyzed with 2-way ANOVA and the Tukey honest significant difference (HSD) test (α=.05). Fractographic investigations and microscopic tests were also performed to determine the quality of the titanium-ceramic bond. ResultsEffective etching of the titanium surface and removal of Al2O3 particles included a 30% water solution of HNO3 + 3% HF, a mixture of HNO3 + HF + glycerin, a 4% solution of HF in H2O2, and a 4% solution of HF in H2O. A statistically significant difference (of about 50%) in bond strength was found between the groups subjected to chemical etching and the control group (P<.05). Additionally, a statistically significant difference (about 25%) was found after thermocycling (P<.05). ConclusionsRemoving the Al2O3 particles embedded into the titanium surface after airborne-particle abrasion lowers the strength of the titanium-ceramic bond (P<.05). Thermocycling also weakens the strength of the titanium-ceramic bond, regardless of the surface preparation (P<.05).