Showing papers on "Paris' law published in 2013"

Effect of Hydrogen on Fatigue Crack Growth of Metals

Abstract: Abstract The present paper shows several important phenomena obtained by investigations of the effect of hydrogen on fatigue crack growth behaviour, including the measurement of the hydrogen content in various materials such as low-carbon, Cr–Mo and stainless steels. Particularly important phenomena are the localization of fatigue slip bands, strain-induced martensite in Types 304, 316 and even 316L, and also strong frequency effects on fatigue crack growth rates. For example, with a decrease in frequency of fatigue loading down to the level of 0.2 Hz, the fatigue crack growth rate of a Cr–Mo steel is accelerated by 10–30 times. The same phenomenon also occurs even in austenitic stainless steels at the frequency of the level of 0.001 Hz. Striation morphology is also influenced by hydrogen. It has been revealed by re-analysing the results of the authors’ separately published reports that this basic hydrogen embrittlement mechanism is essentially the same throughout all the materials, i.e. low-carbon, Cr–Mo and stainless steels. Thus, the coupled effects of hydrogen content, hydrogen diffusion coefficient (for BCC or FCC), load frequency, localization of fatigue slip bands and strain-induced martensite must be always considered in fatigue test and analysis of hydrogen embrittlement.

Fatigue crack growth simulations of interfacial cracks in bi-layered FGMs using XFEM

Abstract: An investigation of fatigue crack growth of interfacial cracks in bi-layered materials using the extended finite element method is presented. The bi-material consists of two layers of dissimilar materials. The bottom layer is made of aluminium alloy while the upper one is made of functionally graded material (FGM). The FGM layer consists of 100 % aluminium alloy on the left side and 100 % ceramic (alumina) on the right side. The gradation in material property of the FGM layer is assumed to be exponential from the alloy side to the ceramic side. The domain based interaction integral approach is extended to obtain the stress intensity factors for an interfacial crack under thermo-mechanical load. The edge and centre cracks are taken at the interface of bi-layered material. The fatigue life of the interface crack plate is obtained using the Paris law of fatigue crack growth under cyclic mode-I, mixed-mode and thermal loads. This study reveals that the crack propagates into the FGM layer under all types of loads.

A critical review of the influence of hydrogen on the mechanical properties of medium-strength steels

Abstract: As medium-strength steels are promising candidates for the hydrogen economy, it is important to understand their interaction with hydrogen. However, there are only a limited number of investigations on the behavior of medium-strength steels in hydrogen. The existing literature indicates that the influences of hydrogen on the tensile properties of medium-strength steels are mainly the following: (i) the steel can be hardened by hydrogen, as demonstrated by an increase in the yield stress or ultimate tensile stress; (ii) some steels can be embrittled by hydrogen, as revealed by lower yield stress or ultimate tensile stress; (iii) in most cases, these steels may experience hydrogen embrittlement (HE), as indicated by a reduction in ductility. The degree of HE mainly depends on the test conditions and the steel. The embrittlement can lead to catastrophic brittle fracture in service. The influence of hydrogen on the fatigue properties of medium-strength steels is dependent on many factors such as the stress ratio, temperature, yield stress of the steel, and test frequency. Generally, the hydrogen influence on fatigue limit is small, whereas hydrogen can accelerate the fatigue crack growth rate, leading to a shorter fatigue life. Inclusions are an important factor influencing the properties of medium-strength steels in the presence of hydrogen. However, it is not possible to predict the influence of hydrogen for any particular steel that has not been experimentally evaluated or to predict service performance. It is not known why similar steels can have different behavior, ranging from good resistance to significant embrittlement. A better understanding of the microstructural characteristics is needed.

Investigation of sub-critical fatigue crack growth in FRP/concrete cohesive interface using digital image analysis

Abstract: The cohesive stress transfer during the sub-critical crack growth associated with the debonding of FRP from concrete under fatigue loading is experimentally investigated using the direct shear test set-up. The study focused on high-amplitude/low-cycle fatigue. The fatigue sub-critical crack growth occurs at a load that is smaller than the static bond capacity of the interface, obtained from monotonic quasi-static loading, and is also associated with a smaller value of the interfacial fracture energy. The strain distribution during debonding is obtained using digital image correlation. The results indicate that the strain distribution along the FRP during fatigue is similar to the strain distribution during debonding under monotonic quasi-static loading. The cohesive crack model and the shape of the strain distribution adopted for quasi-static monotonic loading is indirectly proven to be adequate to describe the stress transfer during fatigue loading. The length of the stress transfer zone during fatigue is observed to be smaller than the cohesive zone of the interfacial crack under quasi-static monotonic loading. The strain distribution across the width of the FRP sheet is not altered during and by fatigue loading. A new formulation to predict the debonding crack growth during fatigue is proposed.

Advanced Methods of Fatigue Assessment

Abstract: Generalised Neuber Concept of Fictitious Notch Rounding.- Extended Stress Intensity Factor Concepts.- Local Strain Energy Density Concept.- Elastic-Plastic Fatigue Crack Growth.

Real-time, high-resolution study of nanocrystallization and fatigue cracking in a cyclically strained metallic glass.

Abstract: Metallic glasses (MGs) exhibit greater elastic limit and stronger resistance to plastic deformation than their crystalline metal counterparts. Their capacity to withstand plastic straining is further enhanced at submicrometer length scales. For a range of microelectromechanical applications, the resistance of MGs to damage and cracking from thermal and mechanical stress or strain cycling under partial or complete constraint is of considerable scientific and technological interest. However, to our knowledge, no real-time, high-resolution transmission electron microscopy observations are available of crystallization, damage, and failure from the controlled imposition of cyclic strains or displacements in any metallic glass. Here we present the results of a unique in situ study, inside a high-resolution transmission electron microscope, of glass-to-crystal formation and fatigue of an Al-based MG. We demonstrate that cyclic straining progressively leads to nanoscale surface roughening in the highly deformed region of the starter notch, causing crack nucleation and formation of nanocrystals. The growth of these nanograins during cyclic straining impedes subsequent crack growth by bridging the crack. In distinct contrast to this fatigue behavior, only distributed nucleation of smaller nanocrystals is observed with no surface roughening under monotonic deformation. We further show through molecular dynamics simulation that these findings can be rationalized by the accumulation of strain-induced nonaffine atomic rearrangements that effectively enhances diffusion through random walk during repeated strain cycling. The present results thus provide unique insights into fundamental mechanisms of fatigue of MGs that would help shape strategies for material design and engineering applications.

Historical-Log Recording System for Crack Opening and Growth Based on Mechanoluminescent Flexible Sensor

Abstract: Recently, there are innovative mechanoluminescent (ML) particles made available, each of which repeatedly emits light in response to small applied stresses even in elastic region When dispersedly coated onto a structure, each particle acts as a sensitive mechanical sensor, while the two-dimensional emission pattern of the whole assembly reflects the dynamical stress distribution inside the structure and the mechanical information around the crack and defect To use the remarkable advantage of the ML sensor in flexibility, electricity/lead-free, low-cost, and so forth, and to answer social needs for historical-log of stress/damage accumulation on social infra-structure, we investigate historical-log recording system for crack opening and fatigue crack growth, and finally succeed to record it with responding position and intensity reflecting the trace of propagating crack tip and stress intensity factor around the tip Furthermore, crack mouse opening displacement accompanied by general traffic of bridge in use is successfully detected

Fatigue crack growth behaviors in hot-rolled low carbon steels: A comparison between ferrite–pearlite and ferrite–bainite microstructures

Abstract: The roles of microstructure types in fatigue crack growth behaviors in ferrite–pearlite steel and ferrite–bainite steel were investigated. The ferrite–bainite dual-phase microstructure was obtained by intermediate heat treatment, conducted on ferrite–pearlite hot-rolled low carbon steel. This paper presents the results from investigation using constant stress-controlled fatigue tests with in-situ scanning electron microscopy (SEM), fatigue crack growth (FCG) rate tests, and fatigue fractography analysis. Microscopy images arrested by in-situ SEM showed that the fatigue crack propagation in F–P steel could become unstable more ealier compared with that in F–B steel. The fatigue cracks in ferrite–pearlite were more tortuous and could propagate more freely than that in ferrite–bainite microstructures. However, frequent crack branching were observed in ferrite–bainite steel and it indicated that the second hard bainite phase effectively retarded the crack propagation. The variation of FCG rate ( da / dN ) with stress intensity factor range (Δ K ) for F–P and F–B steels was discussed within the Paris region. It was shown that FCG rate of F–P steel was higher than that of F–B steel. Moreover, the fatigue fracture surface analysis proved that grain boundaries could also play a role in the resistance of crack propagation.

Fatigue crack growth monitoring using high-frequency guided waves

Abstract: A common problem in aircraft maintenance is the development of fatigue cracks at fastener holes due to stress concentration. High-frequency guided ultrasonic waves allow for the structural health monitoring of critical areas of a structure and can be measured with high accuracy using a noncontact laser interferometer. The use of a specific type of high-frequency guided ultrasonic wave that has good sensitivity for the detection of small defects, excited using a standard Rayleigh wedge transducer and propagating along the structure, has been investigated. Fatigue crack growth at the side of a fastener hole in a tensile, aluminum specimen was induced by cyclic loading of the structure. The crack length was monitored optically and showed good correlation with fracture mechanics calculations of the expected growth rate. The changes in the guided wave signal due to the fatigue damage were monitored using a noncontact laser interferometer and quantified. The measurements show a good sensitivity for the early de...

Acoustic Emission Monitoring of Interlaminar Delamination Onset in Carbon Fibre Composites

Abstract: This article presents the development of an experimental methodology based on acoustic emission wave detection for determining delamination onset and propagation in carbon fibre composite materials under quasi-static and fatigue loading. Delamination was investigated in quasi-static interlaminar fracture testing over a wide range of mixed-mode ratios (GII/GT = 0, 0.3, 0.5 and 1) for unidirectional and woven samples. An acoustic emission wave detection method was developed to detect delamination onset, and the corresponding fracture toughness was computed. Interlaminar fracture toughness was also calculated by beam theory and from finite element analysis with the virtual crack closure technique. The mechanical testing results, acoustic emission monitoring and numerical model’s interlaminar fracture toughness were used to define delamination initiation criteria by drawing two-dimensional envelopes corresponding to GC = f(GII/GT). The acoustic emission wave detection method showed damage accumulation before ...

Mechanical properties and fatigue crack growth rates in friction stir welded nugget of 2198-T8 Al–Li alloy joints

Abstract: A 2.0 mm thick Al–Li alloy 2198-T8 plate is welded by friction stir welding in this work. Four dog-bone type samples are designed to use and measure stress–strain curves and mechanical properties. M(T) samples with transverse weld in the center are designed. Fatigue tests under different R ratio loads are performed on the welded samples with crack growing in the welded nugget and parent material samples. Fatigue crack growth rates in these welded nuggets are obtained and compared. It is shown that there is no peculiar yielding process in the stress–strain curve of the welded nugget compared with the parent material. Yield strength and tensile strength have a “U” shape through the weld zone and a lower value in the weld zone while the elongation is in reverse. Unlike the parent material, fatigue crack growth rates in the welded nugget are not sensitive to R ratios. Based on K res , finite element models are built and the crack closure method is used to calculate the effect of residual stress. Redistributions of residual stress with crack growth are taken into account. Fracture surfaces are investigated to summarize the fracture feature of the welded nugget under fatigue load.