Beach Marks Research Articles

A fracture mechanics approach to interior fatigue crack growth in the very high cycle regime

Growth rates of optically dark areas (ODA) and fish-eyes (FE) were quantified in kHz-ultrasonic fatigue tests on SUJ2 and 17-4PH steels at constant and repeated two-step amplitudes. Sizes of ODAs and FEs depended on the stress intensity factor (SIF) range, and interior fatigue crack growth rates (FCGR) were slower than those of “long” cracks in air, suggesting vacuum as ODA growth environment. Repeated two-step tests on SUJ2 steel served to form beach marks so that, a quantification of ODA sizes, interior FCGRs and SIFs became possible. Additional FCGR measurements of long cracks in vacuum and comparable fracture morphologies allowed estimating the growth rates of ODAs and FEs in 17-4PH steel.

A failure study of the railway rail serviced for heavy cargo trains

In this case study, a failed railway rail which was used for heavy cargo trains was investigated in order to find out its root cause. The macroscopic beach marks and microscopic fatigue striations were not observed by macro and microscopic observations. The chevron patterns were observed by macro observations. The crack origin was at the tip of chevron patterns. The fan-shaped patterns, cleavage step and the river patterns were observed at the crack origin, which demonstrated the feature of cleavage fracture. The metallurgical structures at the crack origin were pearlite and ferrite networks. The crack is supposed to be initiated from the weaker ferrite networks. Given all of that, the failed railway rail is considered to be caused by overload. It is of great importance to improve the welding technology, and control the load of train in order to prevent similar failure in future.

Visualization of internal small fatigue crack growth

Internal small fatigue crack growth caused by gigacycle fatigue of high-strength steel was successfully visualized using beach marks created by a repeated two-step fatigue test. The created beach marks were clear enough to visualize the initial stage of the internal fatigue crack growth. The beach marks indicated that the internal crack shape just after initiation from an inclusion was not a circle but a half-ellipse, suggesting that the early stage of the internal crack growth requires three-dimensional modeling, and therefore, an extremely slow crack propagation rate, smaller than the lattice length, is expected in two-dimensional modeling. Moreover, it was found that the crack propagation life exceeded 1.42×107 cycles. This meant that the minimum crack propagation rate per cycle in two-dimensional modeling was smaller than the lattice length.

An investigation of the repetitive failure in an aircraft engine cylinder head

Cylinder head (CH) failures in aircraft piston engine may have serious or fatal consequences to the safety of the crew and the aircraft. Moreover, when failure becomes undoubtedly repetitive and critical resulting in loss of aircraft, destruction of properties, and first and foremost loss of human lives, the cause of the failure requires to be investigated using a scientific approach. Therefore, the aim of this study is to investigate and identify the root cause of a repetitive premature failure in an aircraft engine CH.The piston engine of the training aircraft Utva-75 has malfunctioned during the flight due to the cracking of its aluminum cast CH. It has been the second engine failure of this type of aircraft due to the cracking in the CH in a very short span of time. From the visual examination of the mating fracture surfaces, it has been possible to observe typical beach and ratchet marks indicating the occurrence of fatigue failure. The crack has initiated from multiple origins located on the inner flange fillet on the exhaust side of the CH. Further examinations by using scanning electron microscopy as well as energy dispersive spectroscopy and metallography have shown that the fatigue had promoted from pre-existing material defect due to a high concentration of shrinkage pores at the initiation crack site and can be most likely associated with the manufacturing process of casting. The stress analysis of the cylinder assembly, carried out by means of finite element analysis, has also confirmed that the crack origin was located at the most stressed area of the cylinder assembly i.e. on the inner flange fillet of the exhaust side of the CH.This case study, together with the other recently reported, has definitely confirmed the repetitive and therefore systematic problems with the CH of air cooled, horizontally opposed, aircraft piston engines.

Rail squats: progress in understanding the Australian experience

Rail squats or studs, being a sub-surface crack in a track below a depression of the rail surface, have been studied in the Australian CRC for Rail Innovation project R3.105, from a number of perspectives. Examination of squats on track by the Rail Corporation of NSW has revealed statistics about their distribution. Examination with optical and scanning electron microscopy at the University of Queensland has shown the frequent presence of a brittle white etching layer (WEL) on the rail surface. This has led to successful adaptation of eddy currents to detect WEL rather than cracks. Tests at the University of Queensland have investigated whether the WEL can form below the normal 720℃ needed to form austenite. Examination of crack surfaces shows beach marks indicative of growth in modes II and III. Neutron diffraction testing of Australian rail has shown residual stresses in a railhead with a WEL, similar to those reported elsewhere, but with a broad surface layer in compression, not a narrow running band. Elasto-plastic finite element simulation has shown residual compression transforms into residual shear at a crack tip, which increases that due to plastic deformation from successive wheel passages, tending to encourage the crack to continue in a direction of sub-surface growth. The rate of growth of squats measured on-track in Sydney shows crack growth that corresponds to a power law with a low exponent, implying that the stress intensity at the crack tip is not a strong function of crack length. This is partly due to the localised nature of contact stresses, which allows the crack to grow beyond the region loaded. However, accounting for this effect leads to the prediction of a smaller reduction in growth rate than that observed. It is likely the low exponent reflects a smaller effect of water on crack growth, as the crack enlarges. There is also a redistribution of contact loading that occurs as a crack grows.

항공기 조향장치 파손원인 사례연구

An aircraft made an emergency landing through the loss of capability controlling steering. A torsion link which is a part of steering apparatus has been adrift from the steering system and the bolt connected to the steering link has fractured. At the same time, the FLIR(Forward Looking Infrared Radar) mounted in front of the steering link has been also damaged. In the early of this investigation, we considered the failure of the FLIR had occurred first, that FLIR hit the steering link and finally the bolt fractured. The fractured section of the bolt has shown a beach mark and a dimple mark as well. The outside of the bolt has shown a large deformation by a heavy load. As a result, we have found out what the cause of the heavy load and the fractures for bolt, link and FLIR have occurred in what order.

Análisis de falla del disco de embrague de un camión de 2 ton

Este artículo muestra el análisis de la falla presentada en el disco de embrague de un camión para transporte de 2 Ton. El elemento que presentó la fractura estaba fabricado de acero para resortes de alto carbono. Aunque la zona de falla era muy delgada y de difícil interpretación, el examen de la superficie de fractura reveló marcas chevron y presencia mínima de marcas de playa muy tenues. La parte de falla (cushioning plate), fabricada de acero al carbono 1070 templado, estaba sometida a esfuerzos alternantes cortantes y flexión. La falla final fue causada por carga de impacto.

Fatigue tests on steel plates with longitudinal weld attachment strengthened by ultra high modulus carbon fibre reinforced polymer plate

ABSTRACTWelded steel connections of infrastructures are susceptible to fatigue failure. Advanced carbon fibre reinforced polymer (CFRP) has been demonstrated promising for fatigue strengthening of steel structures. Limited research was conducted on CFRP strengthening of welded connections. This paper focuses on the application of ultra high modulus (UHM) CFRP plates with a modulus of 460 GPa to strengthen steel plates with longitudinal fillet weld attachment using five CFRP strengthening configurations. A series of fatigue tension tests were carried out with constant amplitude fatigue loading. Beach marking technique was adopted to record the crack propagation process. Effects of CFRP bond length, bond width and bond locations on fatigue performance of welded steel joints were investigated. The experimental results showed that UHM CFRP plates could generally increase the fatigue life of the welded steel joints. It seems better to apply CFRP on the welding side of the specimen to achieve longer fatigue life. Then, the effects of weld and weld attachment on the CFRP strengthening efficiency was further studied by comparing experimental results of non‐welded steel plates with single side UHM CFRP plate strengthening. Finally, the classification method was adopted to assess the strengthening efficiency of the UHM CFRP plate to the steel plates with longitudinal weld attachment.

Effect of Sour Environment pH on Crack Morphology in Ultra-High Strength Drilling Steel Under Cyclic Stress

Catastrophic failure of a pre-cracked compact tension UD-165 sample after a cyclic stress versus frequency experiment in a 5% NaCl solution buffered with CO2/NaHCO3 and in contact with 0.83 MPa H2S at 20°C, pH=7, occurred in air. Only fatigue crack propagation was observed in a similar sample after a similar test in the pH=12 solution. The log fatigue crack rate measured as a function of log frequency for UD-165 in the pH=7 solution was linear with a gradient of -0.492 while in the pH=12 solution exhibited a plateau indicating different mechanisms based on fracture mechanics theory. The microscopic investigations of the sample after testing at pH=7 revealed the presence of fatigue cracks, initiated by sample pre-cracking. Also, on the surface of the catastrophically fractured base metal, pits and other secondary cracks were observed. The latter propagated along the beach marks. The surface analysis of UD-165 after testing in pH=12 solutions showed corrosion products covering the crack but not crack tip. Intergranular cracks were found on the surface of the fractured base metal in liquid nitrogen. The microscopic surface investigations corroborate the fracture mechanics results.

Fractographic investigations of the failure of L-1 low pressure steam turbine blade

This paper deals with fracture investigations of low pressure (LP) steam turbine blade of a 110MW thermal power plant in Punjab in India. These blades were made from chrome alloy steel X20Cr13 (Tempered martensitic stainless steel). The main focus of the paper is the fracture analysis, which took place at the airfoil region at 203mm from the root of the blade. One of the fractured blades was retrieved to identify the cause of failure. The investigations included the visual examination, SEM fractography, chemical analysis, hardness measurement, and micro-structural characterization. While preparing the test specimen, silicon rich phases were detected on the blade along with oxide scale present inside the corrosion pits and fractured region. Transgranular cleavage fractures along with beach marks are observed. The cause of failure is possibly the corrosion-fatigue. These causes of failure are also underlined in this paper.

Failure analysis of an aircraft engine cylinder head

The piston engine of the training aircraft malfunctioned during the flight due to the cracking of its cylinder head (CH), which is manufactured from an aluminum casting alloy. Based on the fractographic examination of the mating fracture surfaces, the characteristic ratchet and beach marks were observed indicating the occurrence of fatigue failure. The crack was initiated from multiple origins located on the inner flange fillet on the exhaust side of the CH. The metallography examination has shown that the fatigue was promoted from pre-existing material defect due to an elevated presence of shrinkage pores at the crack initiation zone and was most likely associated with the manufacturing process of casting. The finite element (FE) method, utilized to determine the stress state of the CH subjected to gas pressure, also confirmed that the crack origin was located at the most stress area.

Failure of a crankshaft of an aeroengine: A contribution for an accident investigation

The objective of this work is to determine the main cause of failure of a crankshaft from an ULM airplane aeroengine. The core of this paper is focused in the analysis of the damage mechanisms which were in the base of a catastrophic failure of the crankpin journal. Based on a preliminary observation of the fracture surface, there are clear evidences of a fatigue process as both beach and striation marks have been identified. The judicious characterization of the failing mechanism, including the identification of the crack initiation site and the assessment of the crack propagation rate, is of paramount importance to support the investigation of this aircraft accident. In this context, an exhaustive observation of the fracture surface by means of optical and electronic microscopic techniques, in parallel with microstructural examinations, were carried out as determinant information for the correct assessment of the contributive causes to this accident.

Fatigue Tests of Cracked Steel Plates Strengthened with UHM CFRP Plates

Carbon fibre reinforced polymer ( CFRP) has shown promise for improving the fatigue performance of steel structures. Previous studies have indicated that increasing the Young's modulus of CFRP can be beneficial for decreasing the stress intensity factor at the fatigue crack tip. In this project, ultra high modulus (UHM) CFRP plates with Young's modulus of 460 GPa were adopted to study their fatigue repair effectiveness. A series of fatigue tension tests was carried out on steel plates with an initial crack in the centre. Five strengthening configurations were used and a constant amplitude fatigue loading was applied to all the specimens. The beach marking technique was utilized to record the fatigue crack propagation. The effects of CFRP bond length, bond width and bond locations on the fatigue performance of cracked steel plates were also studied. The experimental results show that UHM CFRP plates can greatly increase the fatigue life of cracked steel plates by a factor ranging from 3.26 to 7.47. When CFRP plates cover the whole crack surface, the fatigue crack of the steel plate is arrested. The strengthening effectiveness of UHM CFRP plates is also compared with those using high Young's modulus CFRP sheeting and normal Young's modulus CFRP plates with or without prestressing.

Cracking of tungsten carbide rolls during hot rolling of branded rebars in a bar mill

Premature failure of Tungsten Carbide (WC) rolls, due to cracking during hot rolling of branded rebars was found to raise the cost of rolling due to unscheduled stoppage of the mill. Hence, a detailed investigation was conducted to reduce above problem using non-destructive analytical technique such as Ultrasonic velocity measurement, X-ray diffraction (XRD). Finite Element Analysis (FEA) was carried out to know the rolling stress and its adverse effects. Optical technique Scanning Electron Microscopy (SEM) and fractography also carried out to know the root cause of the cracking problem. XRD results indicated high tensile residual stresses at branded area (420.5MPa) and unbranded area(279.2MPa). FEA results revealed that the rolling stress was much lower than the yield stress of WC roll. SEM and Fractography revealed beach and striation marks which confirmed fatigue failure. Whereas ultrasonic velocity measurements indicated density variations of 0.49g/cm3 which is very high in comparison to the desired permissible variation of 0.05g/cm3. The rolling stress together with the impact stresses developed in the branded area due to surface unevenness and causes variation in compactness (density) which in turn causes fatigue failure. This phenomenon was also confirmed with SEM and fractography.

Determination of the Paris law constants in round bars from beach marks on fracture surfaces

In this article, a 3-step reverse engineering technique capable of determining the Paris law constants from the analysis of final fracture surfaces of fatigued round bars is proposed. Initially, at least two crack shapes and the number of cycles between them are obtained experimentally. Then, a 3D-FE automatic fatigue crack growth technique, able to predict the crack shape evolution and the number of fatigue cycles, is employed. Finally, a comparison between numerical and experimental results in terms of crack shapes and fatigue lives is carried out in order to find the Paris law constants that best fit the numerical predictions to the experimental results. This technique has been successfully applied to circular cross-section specimens made of S45 steel. Differences between experimental and predicted C and m constants less than 5% and 3%, respectively, were found.

Vibration analysis for detecting failure of compressor blade

In a 210MWe gas turbine, compressor blades in stage 3 failed and caused large scale collateral damage to the plant. The plant outage resulted in heavy loss to the generation company. The blade fractured at root of the airfoil due to high cycle fatigue as evidenced by beach marks and microscopic striations on the leading edge of the blade. Corrosion related pitting was also noticed on the leading edge fillet region on many blades of stage 3 and also on many blades of other stages. The Campbell diagram indicated possibility of high vibratory stresses at the leading edge due to excitation of the fundamental mode of the blade. Due to large number of startups caused due to technical reasons, it was estimated that the response of the blade could have initiated crack in the pitted region. It was further estimated that after the startup, crossing of the other fundamental modes at the synchronous speed could have propagated the crack to failure under normal operating stress.On the two other working units of similar make and capacity, it was necessary to assess the health of the compressors blades and take necessary action if warranted. The paper provides failure analysis of the failed blade and a new approach based on casing vibration analysis for assessing health of compressor blades in two operating plants. The innovative approach identified abnormalities in both the operating units. In the immediate plant outage, the compressor blades were examined by ultrasonic technique and cracks were detected in several blades at the root. The unit was thus saved from yet another major mishap.

Effects of TiN and WC coating on the fatigue characteristics of dental implant

This study examined the effects of TiN (titanium nitride) and WC (tungsten carbide) coatings on the fatigue characteristics of dental implants using a sinusoidal cyclic loading machine under load between 42–420N and 58–580N according to the ISO 14801. In this study, an abutment screw was coated with a TiN and WC film by EB-PVD and sputtering, respectively. For the fatigue test, the implant fixture and abutment were tightened to a torque 32Ncm using a digital torque gauge with 0.1Ncm accuracy. Field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) were used to observe the coated surface and fracture surface of the abutment screw.The uncoated abutment screws contained scratches caused by the mechanical process, whereas the TiN and WC coated screws showed a smooth surface at the screw thread and screw valley. The mean fatigue life of the uncoated, TiN coated, and WC coated abutment screw at 420N was 5.8×105, 8.2×105 and 106cycles, respectively. At 580N, the mean fatigue life of the uncoated, TiN coated and WC coated abutment screw was 2.1×104, 4.1×104 and 2.3×104cycles, respectively. The uncoated abutment screw showed the beach mark and fatigue striation. On the other hand, in the case of TiN and WC coated abutment screws, the semi-cleavage fracture and ductile fracture appeared at 420N. Semi-cleavage fracture and massive fatigue striation with a step of 0.5μm/cycle were observed in TiN coated abutment screw, whereas a minute slip band was noted and the interval of fatigue striation increased to several μm/cycle in the WC coated screw at 580N.

스톰 하중을 받는 용접 구조물의 피로 수명 평가에 대한 실험 연구

In this paper, fatigue tests are conducted for the specimens with longitudinal and transverse attachment under variable amplitude axial loading based on storm model. Considered loadings include repeated single storm, 6 or 8 storms randomly, and storms including calm sea condition while the mean stress and the maximum stress of loadings are changed. The effect of three variables are investigated; root mean square(RMS) value of stress amplitude, mean stress shift and maximum stress, which can characterize storm loading on fatigue life. In addition, experiments including calm sea loading are also carried out to investigate the effect of calm sea state. Test results are evaluated and compared with DNV-CN2005 and Matsuoka's method for the estimation of crack initiation and propagation life. To verify the validity of the criteria, the measured crack initiation lifes are compared with the specific crack length 15mm, which are calculated with beach marks.

Low Cycle Fatigue Damage Developed on the Surface and Interior of Steels

In this study, low cycle fatigue tests were conducted for carbon steel, STS410, low alloy steel, SFVQ1A and austenitic stainless steel, SUS316NG, in order to investigate surface and internal fatigue crack growth characteristics. The tests were conducted under repeated two-step variable amplitude straining conditions to appear beach marks of cracks on the fracture surface. In these tests, periodical surface removal was conducted to suppress surface fatigue cracks to be a main crack which led final fracture. STS410 specimen under surface removal test was not broken until its diameter became too small. There was a defect observed on the section of the specimen cut longitudinally. Cracks initiated from internal inclusion for SFVQ1A and SUS316NG and those fatigue lives were slightly larger under surface removal test compared to those under no surface removal tests. Internal crack growth rates calculated from beach marks were similar to those of surface cracks under the same crack driving force or J integral range, ΔJ.

Fatigue Strength and Internal Crack Growth Behavior of High Strength Steel under Variable Amplitude Stressing in Very High Cycle Regime

Ultrasonic fatigue tests were carried out for high carbon chromium bearing steel, SUJ2, under two-step variable amplitude stressing at a frequency of 20 kHz in order to investigate the growth of an interior crack. Since it is important to clarify the damage mechanisms under variable amplitude stressing for practical applications, fatigue tests were conducted under repeated two-step variable amplitude stressing. An inclusion was observed at the initiation site of fracture. Optically dark area (ODA) and fish-eye were observed around the inclusion. Sizes of ODA and fish-eye depended on the stress intensity factor range. Interior crack growth rate was slower than the surface one due to vacuum environment. Cumulative damage values calculated by Miner's rule were relevant to the size of inclusions under repeated two-step variable amplitude stressing. Characteristics of interior fatigue crack growth were determined by the fracture surface observations for beach marks and the ODA sizes. Based on the results, the formation mechanism of ODA was discussed in conjunction with the reports by the other researchers.