Origin Of Crack Initiation Research Articles

Assessment on structural integrity of aluminium alloy AA6061 water tank used in satellite launch vehicles

AA 6061 is an age-hardenable aluminium alloy with 1wt% magnesium and 0.6wt% silicon as major alloying constituents and is used for manufacture of toroidal water tanks of earth storable liquid rocket engines of ISRO’s launch vehicles. These water tanks are used for storing water to generate steam in gas generator as well as for cooling various components of liquid rocket engine. As a part of the developmental efforts of a new source for fabrication of water tanks, a qualification test procedure was laid down, wherein a quarter portion of the water tank, namely the test article, is designed, fabricated and subjected to series of acceptance and qualification tests, including proof pressure test, cyclic pressure tests with intermediate non-destructive tests. These were finally followed by burst testing of the test article for conforming structural design margins and quality standards adopted for realizing the hardware. A burst test is a destructive test, performed to determine the overload capacity of the pressure vessels depicting margin of safety with respect to its mechanical strength limit. This work highlights the detailed manufacturing processes viz. forming, machining and welding including heat treatment of shells, metallurgical analysis carried out on the parent and weld specimens as well as results from proof pressure and pressure cyclic tests. Further, fractographic studies were carried out on the burst tested hardware, wherein specimens collected from different locations of the burst tested hardware were analyzed using optical scanning electron microscopy, to locate the origin of crack initiation and nature of the failure.

Influence of Silica Specific Surface Area on the Viscoelastic and Fatigue Behaviors of Silica-Filled SBR Composites.

This work aimed at studying the effect of a silica specific surface area (SSA), as determined by the nitrogen adsorption method, on the viscoelastic and fatigue behaviors of silica-filled styrene–butadiene rubber (SBR) composites. In particular, silica fillers with an SSA of 125 m2/g, 165 m2/g, and 200 m2/g were selected. Micro-computed X-ray tomography (µCT) was utilized to analyze the 3D morphology of the fillers within an SBR matrix prior to mechanical testing. It was found with this technique that the volume density of the agglomerates drastically decreased with decreasing silica SSA, indicating an increase in the silica dispersion state. The viscoelastic behavior was evaluated by dynamic mechanical analysis (DMA) and hysteresis loss experiments. The fatigue behavior was studied by cyclic tensile loading until rupture enabled the generation of Wöhler curves. Digital image correlation (DIC) was used to evaluate the volume strain upon deformation, whereas µCT was used to evaluate the volume fraction of the fatigue-induced cracks. Last, scanning electron microscopy (SEM) was used to characterize, in detail, crack mechanisms. The main results indicate that fatigue life increased with decreasing silica SSA, which was also accompanied by a decrease in hysteresis loss and storage modulus. SEM investigations showed that filler–matrix debonding and filler fracture were the mechanisms at the origin of crack initiation. Both the volume fraction of the cracks obtained by µCT and the volume strain acquired from the DIC increased with increasing SSA of silica. The results are discussed based on the prominent role of the filler network on the viscoelastic and fatigue damage behaviors of SBR composites.

Effects of building direction and defect sensitivity on the fatigue behavior of additively manufactured H13 tool steel

Selective laser melting (SLM) is a laser powder-bed fusion process, which produces near-net shaped components with layered microstructures leading to anisotropy in the mechanical properties. The intrinsic process-induced defects, particularly microporosity and lack of fusion, as well as their size and relative orientation with respect to the building direction, affect the fatigue performance of the SLM-processed components. Herein, the influence of building direction and defect sensitivity on the fatigue behavior of SLM-fabricated AISI H13 steel is investigated. Samples are tested after tempering and turning to find a compromise between tensile strength and toughness and minimize the effect of geometrical distortion. The fatigue strength is assessed by employing the Murakami model, and short-crack effect is evaluated by calculating the El Haddad–Smith–Topper length parameter. An inferior fatigue performance of the SLM parts is observed in comparison to that of the conventionally processed materials, owing to the presence of defects that act as origins of crack initiation. The fatigue strength is significantly influenced by the defect size, shape, and the distance from surface, which in turn are affected by the building direction. A slightly higher fatigue resistance is observed for the 90°-oriented samples, owing to the smaller defect size and the lower stress concentration factor. The data for the three building directions show a high scatter, attributed to the process-defect-induced stress concentration factors.

Effect of long period stacking ordered phase on the fatigue properties of extruded Mg-Y-Zn alloys

The fatigue properties of two Mg-Y-Zn alloys with different amount of long-period stacking order (LPSO) phase, Mg97Zn1Y2 and Mg89Zn4Y7, were investigated. Low-cycle fatigue experiments were conducted under strain-controlled conditions. Both the monotonic and cyclic yield strength increased with the LPSO volume fraction. Cleavage-like facets were found to be at the origin of crack initiation in both alloys. High-cycle fatigue experiments were conducted under microscopy monitoring. Increasing the LPSO-phase volume fraction decreased the fatigue life but improved the endurance limit. Cracks initiated from α-Mg grains in both materials. Crack growth rates were found to be relatively similar.

Determination of the Root Causes for Cracking in a Large-Size Cast Ingot of AISI 4317 Steel Using Microstructural Analysis

The objective of the present work is to study the root causes for the cracking and material detachment from the external surface of a 10 metric tons high-strength low-alloy steel ingot after solidification and annealing processes. Failure analysis investigations were conducted on different samples from the cracked areas as well as from non-cracked ones. A combination of optical and electron microscopies, x-ray diffraction, and microhardness techniques were used to study and analyze the nature, morphology, and composition of the different microstructural components in the cracked regions. Thermodynamic software, Thermo-Calc® was used to corroborate and interpret the experimental findings with the existing theories. The results indicated a strong intergranular character of the cracks accompanied by the presence of second-phase particles at prior austenite grain boundaries. Segregation of chromium and manganese was demonstrated through local chemical analysis of the grain boundaries. It was concluded that a rapid cooling rate combined with the weakening of grain boundaries was at the origin of crack initiation.

Revealing the deformation mechanism of amorphous polyethylene subjected to cycle loading via molecular dynamics simulations.

Despite tremendous efforts being devoted to the study of the deformation behavior of polyethylene, the deformation mechanism of an amorphous polyethylene polymer under cycle shear-loading remains largely unknown. Here, we report the cycle shear deformation mechanism of an amorphous polyethylene polymer using molecular dynamics (MD) simulations. The stress–strain behaviors, including the elastic, yield, strain hardening, and strain softening regions, are qualitatively in agreement with the previous results. The values of the yield stress, Young's modulus and ultimate strength obtained from MD simulations are consistent with the previous data. The effects of the shear strain rate, temperature, and cycle shear-loading number on the stress–strain behaviors are investigated. Higher strain rate and a lower temperature result in a higher strength in the amorphous polyethylene polymer, attributed to the agglomeration of high local strains. With the increase of the cycle shear-loading number, the high strain region gradually expands from the upper and lower surface to the interior of the polyethylene polymer matrix, which provides the origin of crack initiation. The energy contributions are used in elucidating the inherent deformation mechanisms within the elastic, yielding, strain hardening, and strain softening regions, and the variation trend of energy is consistent with the stress–strain response.

Fractography of self-glazed zirconia with improved reliability

The fractography of a new grade of zirconia ceramics, known as self-glazed zirconia, was investigated. The as-sintered intact top surface was made with superior smoothness that mimicked the optical appearances of the natural teeth enamel. The beneath surface opposite to this was made hierarchically rough with microscopic pits of the size up to 60μm together with grain-level roughness of about 2μm. The three-point bending test of the samples made with the hierarchically rough surface being tensile one demonstrated an average bending strength of 1120±70MPa and a Weibull modulus of as high as 18 ascribed to the improved structural homogeneity. Surface topography was found the main origins of crack initiation leading to fracture. The observed unusually predominant transgranular fracture mode of submicron-sized grains disclosed a possible toughening mechanism of disassembling of mesocrystalline grains that differs significantly from the commonly quoted phase transformation toughening of this category of ceramics.

Fatigue strength of hybrid welded 22MnB5 overlap joints

Abstract In order to reduce the weight of automotive parts, lightweight materials like the press hardened AlSi coated high strength steel 22MnB5 is used. This steel enables a reduction of wall thicknesses and simultaneously ensures a high stiffness of the part, e. g., the B pillar. The usage of these steels requires suitable joining technologies. Therefore, a laser GMA hybrid welding process for fillet welds in overlap configuration was designed. This process provides high welding velocities and process tolerances against gaps and misalignments. In this case a single-mode fiber laser was used in order to reduce the thermal impact. Due to the fact that the laser focus spot has only a small diameter of 25 µm, a beam oscillation was used in order to distribute the laser energy over the melt pool width. This paper is focused on the cyclic load tests which were carried out and analyzed by the staircase method. The influences of laser power and oscillation width are dominating, whereas welding velocity and wire velocity have only a minor influence. By analyzing the failure location and crack propagation of specimens after the cyclic load test, the influence of the AlSi coating can be determined. Coating agglomerations at the fusion line of the melt pool are origins of crack initiation. This is attributed to the formation of brittle intermetallic phases.

Role of Metallographic Analysis in the Identification of Location of Crack Initiation in a Burst Tested AA 2219 Propellant Tank

Owing to its excellent combination of weldability and strength at low temperatures (up to 4 K), aluminum alloy AA 2219 is the material of choice for the design of propellant tanks for cryogenic engines of satellite launch vehicles. In order to assess the design margins available, one propellant tank has been subjected to burst test. This paper highlights the role of microstructural analysis conducted on the burst tested propellant tank in locating the origin of crack initiation. Specimens collected from different locations of the burst tested tank have been subjected to optical microscopy and scanning electron microscopy to locate the origin of crack initiation.

Experimental Study on Dry Sliding Wear Behaviour of Sintered Fe-C-W P/M Low Alloy Steels

Powder metallurgy (P/M) manufacturing process is one of the rapidly emerging fields and has extended the applications in aerospace, automotive, manufacturing industries replacing all traditional methods of metal forming operations because of its less energy consumption, maximum material utilization, low relative material wastage, low capital cost. Unlike wrought materials, wear behaviour of porous materials is different and is influenced by the pores present in the materials. The presence of pores leads to weakness and also acts as origin of crack initiation in sintered materials. The present investigation is made to study the dry sliding wear characteristics of P/M Fe-1% C-1%W low alloy steel with different densities (85%, 90%, 95%), as they find several applications in manufacturing industries, particularly in automobile industries. The wear behavior of the as-sintered preforms were studied under dry conditions on pin-on disc arrangement (ASTM G99) against EN 38 steel disc of Hardness HRC 60 with a sliding speed of 2 m/s and at a normal loads of 30, 50, 70N respectively. Wear regimes of P/M steels has been characterized using optical microscopy. From the microstructure of wear pattern it is evident that, presence of carbides of the carbide forming elements W offers greater wear resistance. The elements gets embedded as hard particles in a soft matrix of Fe- C, resulting in non-uniform wear pattern in higher density preforms (95%), whereas uniform wear was observed in low density preforms (85%) even at higher loads during the test. Delamination is a common wear mechanism found in both P/M alloy steels with an increase in applied load, but oxidation wear was observed in all the preforms at low levels of applied load.

Metallurgical optimisation of PM superalloy N19

Microstructures of the new PM superalloy N19 have been investigated for various heat treatments in order to reach the best compromise between static strength and cyclic resistance. One subsolvus and several supersolvus heat treatments were applied to produce fine (7 μm) and medium (25 μm) grain sizes, respectively. The alloy is shown to be quite sensitive to the cooling conditions after solutioning as the γ ′ hardening precipitates, both secondary and tertiary, have a direct influence on mechanical properties. Two cooling conditions after solutioning produce a high crack propagation resistance at 650 °C with dwell time cycles, which is one of the basic requirements. The low cycle fatigue behaviour appears to be correlated to the grain size, which determines the origin of crack initiation (from ceramic inclusions or not). The other mechanical properties (tensile, creep) remain above target levels. Despite the medium size grain microstructure in the supersolvus condition, a high level of mechanical strength is observed in N19 at elevated temperature. It is understood that further improvement in properties can be achieved by developing coarse grain microstructures.

Microstructural heterogeneities and fatigue anisotropy of forged steels

In this study, various experimental methods are employed to determine the anisotropic fatigue behavior of a 25MnCrSiVB6 forged steel (Metasco MC). This material has a bainitic microstructure and contains many elongated non-metallic inclusions in the rolled direction, which are grouped into clusters. Specimens with different orientations relative to the rolling direction have been extracted from a hot rolled bar and the ability of certain experimental techniques to capture the fatigue anisotropy has been tested. Results obtained from monotonic tensile tests and Charpy impact tests show that the material has isotropic fracture strength and anisotropic ductility. The influence of the inclusion clusters is clearly demonstrated via observation of the fracture surfaces. Concerning the fatigue behavior, results from a classical staircase experimental procedure are compared to results from self-heating fatigue tests. For specimens orientated at 0° relative to the rolled direction, microcrack initiation is controlled by the material matrix and the prediction of the fatigue strength with the self-heating method has been observed to be correct. For specimens orientated at 45° and 90°, the elongated manganese sulfide inclusion clusters are the origin of crack initiation and the fatigue strength drops significantly. For this case, it appears that the self-heating method has difficulty predicting the fatigue behavior.

Modelling the role of non-metallic inclusions on the anisotropic fatigue behaviour of forged steel

Forged components are known to show high cyclic and monotonic mechanical properties. This is mainly due to a better compactness and a finer microstructure introduced by the forming process. However, this good mechanical behaviour is sometimes a source of anisotropy especially when the microstructural heterogeneities are not randomly distributed and/or oriented. This study aims at describing the high cycle fatigue response of a forged bainitic steel. This material contains a lot of elongated manganese sulphide (MnS) inclusions, oriented as a function of the rolling or forging direction. Specimens with different orientations relative to the rolling direction are tested in fatigue under push–pull uniaxial and torsion loads. The influence of “inclusion clusters” is clearly demonstrated via the observation of the failure surfaces. Experiments show that the anisotropic fatigue behaviour is due to a change in the crack initiation mechanism. At 0°, when the inclusions are parallel to the applied stress, micro-crack initiation is controlled by the material matrix. At 45° and 90°, elongated manganese sulphide inclusion clusters are the origin of crack initiation and the fatigue strength drops significantly. A statistical approach based on the competition between two different crack initiation mechanisms is proposed. One mechanism is modelled by local elastic shakedown concepts and the other by linear elastic fracture mechanics. This approach leads to a Kitagawa type diagram and explains the anisotropy in the material. The approach developed in this study demonstrates a framework using both the elastic shakedown concept and the weakest link theory to account for the loading mode, loading path and data scatter in high cycle fatigue.

Failures analysis of compressor blades of aeroengines due to service

In spite of the high levels of reliability of modern aeroengine components resulting from rigid standards and practices, failures of compressor and turbine blades during normal operational environments are common situations which compromise the flight safety. The investigation of real failures affecting these components allows gaining a deeper knowledge concerning the mechanisms of crack initiation and propagation which, in turn, can be used in order to prevent future incidents or accidents. This paper presents the analysis of two in service failures involving blades breakage belonging to different compressor stages. Crack growth mechanisms were evaluated based on the visual inspection of the affected components and with both macroscopic and microscopic observations of the fracture surfaces. The origins of crack initiation were evaluated through the examination of crack path and beach marks on the fracture surface. Mechanical analyses were carried out to identify the possible causes of the failures by examining anomalies in the mechanical behaviour of the materials such as hardness tests, chemical composition and surface coating analysis through SEM observations. The analysis of the different fracture surfaces shows that crack propagation is mainly related with fatigue mechanisms whilst crack initiation can be attributed to distinct causes, either the presence of defects in the surface of the blade due to impact of debris or intrinsic material defects or some degrading mechanisms affecting the internal microstructure of the material. In some cases the blades were in service after having being object of an overhaul procedure, which can justify some crack initiation cases that caused in service failures.

Evaluation of Fatigue Strength in Very-long Life Regime of SNCM439 Steels

Nickel chromium molybdenum steels (SNCM439) were fatigued with the rotating bending fatigue testing machine and the ultrasonic fatigue-testing machine (20 kHz) in laboratory air. The duplex S-N curve could not be observed for the ultrasonic fatigue tests. The origins of crack initiation in the long life regime were internal inclusions. The fish-eyes were observed in the specimens broken from internal inclusions. When the number of stress cycles to failure was longer than 8.1×106, the facet region was observed around inclusions. The fracture surface could be divided into three regions. The first region surrounding inclusion was a facet area. The crack propagation in the facet region was Stage A and the crack propagation outside the facet region was Stage B. When the fatigue cracks reached the specimen surface, the fatigue cracks propagated as surface cracks. The crack propagation rate in each region was estimated to be a power function of the stress intensity range. Fatigue life was calculated by integrating with the crack propagation laws determined from the experimental data. The predicted fatigue life agrees with the experimental result.

Influence of casting defects on the fatigue limit of nodular cast iron

The high cycle fatigue behaviour of a nodular cast iron has been investigated under tension loading. Casting defects are at the origin of crack initiation for all samples tested. Critical defects (shrinkage) are located either at the surface or within the bulk. The role of the size and position of natural defect on the fatigue limit has been established. Results show that, for a given size, internal defects are less damaging than surface located ones. SEM observations revealed the existence of non-propagating surface cracks below the fatigue limit. By means of a marking technique, the 3D shape of these cracks initiated around the defects was fully described. In an attempt to simulate the evolution of the fatigue limit with defect size, some existing approaches are compared to experimental data; it is shown that 2D analysis of defects (crack or notch) is conservative.

GSW0442 Evaluation of long life fatigue strength at ultrasonic frequency

A C-Cr bearing steel (JIS SUJ2) was fatigued with the ultrasonic fatigue-testing machine (20kHz) in laboratory air. In the very long-life regime, the decline of fatigue limit was seen. The origins of crack initiation were mostly internal inclusions. However, even when the specimen broke in a long life region, some cracks were initiated from specimen surface. Fracture surface were observed with a scanning electron microscope (SEM). The fish-eyes were observed in the specimens broken from the internal inclusions. The fracture surface could be divided into three regions. The first region surrounding inclusion was a facet area. The crack propagation in the facet region was Stage A and the crack propagation outside the facet region was Stage B. When the fatigue cracks reached the specimen surface, the fatigue cracks propagated as surface cracks.The crack propagation rate in each region was estimated to be a power function of the stress intensity range. Fatigue life was calculated by integrating with the crack propagation laws determined from the data obtained for the rotating bending. The predicted fatigue life agrees with the experimental result.

715 高強度鋼の長寿命領域における疲労強度評価

A bearing steel (JIS SUJ2) and nickel chromium molybden steel (SNCM439) were fatigued with the ultrasonic fatigue-testing machine (20kHz) in laboratory air. In the very long-life regime, the decline of fatigue limit was seen. The origins of crack initiation were mostly internal inclusions. However, even when the specimen broke in a long life region, some cracks were initiated from specimen surface. Fracture surface were observed with a scanning electron microscope (SEM). The fish-eyes were observed in the specimens broken from the internal inclusions. The fracture surface could be divided into three regions. The first region surrounding inclusion was a facet area. The crack propagation in the facet region was Stage A and the crack propagation outside the facet region was Stage B. When the fatigue cracks reached the specimen surface, the fatigue cracks propagated as surface cracks. The crack propagation rate in each region was estimated to be a power function of the stress intensity range. Fatigue life was calculated by integrating with the crack propagation laws determined from the data obtained for the rotating bending. The predicted fatigue life agrees with the experimental result.