Fatigue and fatigue crack growth behavior of tool steel (original) (raw)
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Determining of the Fatigue Crack Growth Rate of HSLA Steel at Room Temperature
Advanced technologies and materials, 2022
Welded joint is a critical region of a welded structure and fracture mechanics analysis is inevitable in the structural integrity assessment of all welded structures. This paper shows the determining of parameters of the fatigue crack for constituents of welded joints produced of high strength low alloyed steel. The applied methodology refers to the Paris relation where the link was established between the variable load quantity or the corresponding stress intensity factor range and crack growth per cycle. Results have shown that the position of the notch and crack initiation affect the values of the stress intensity range of fatigue threshold ΔKth and parameters in the Paris' equation. This is mostly expressed when determining growth parameters of the fatigue crack in heat affected zone of HSLA steel, where different changes of growth speed of the fatigue crack clearly express differences in structure of the crack pass.
Fatigue crack growth mechanisms in steels
International Journal of Fatigue, 2003
Fatigue crack growth behavior of structural steels is examined by using the Unified Approach developed by the authors. In this approach, fatigue requires two-load parameters involving maximum stress intensity, K max , and stress intensity amplitude, ̅K. For a fatigue crack to grow, both K max and ̅K must exceed their respective threshold values. Similarly, for any other crack growth rate, two limiting values, K max * and ̅K * are required to enforce the growth rate. The variation of these two critical values forms the crack growth trajectory map, which is defined by plotting ̅K * vs. K max * as a function of crack growth rate. In this trajectory map, the line defined by ̅K * = K max * represents pure fatigue crack growth behavior induced by cyclic strains. It is shown that this line provides a reference norm for defining deviations in the trajectory resulting from environmental and/or monotonic fracture modes superimposed on fatigue. Using this approach, we have examined the crack growth behavior of many structural steels. The trajectory maps of these steels show deviations due to superimposed environmental effects. These effects vary with grain size, yield strength, microstructure, and chemistry. In addition, for a given material, changes in the trajectory paths occur because of changing crack growth mechanisms. Analysis of material behavior using trajectory maps provides a clear understanding of the relative magnitude of cyclic and environmental damage and how the microstructure, chemistry, and crack tip plasticity affect fatigue crack growth behavior of steels.
Experimental investigations of fatigue cracks nucleation, growth and coalescence in structural steel
International Journal of Fatigue, 2003
A new method for experimental investigation of fatigue crack nucleation, growth and coalescence in structural steel is proposed. This method is based on the observations of the inelastic strain during the fatigue life of smooth specimens subject to an oscillating, fully reversible load. Changes of inelastic strain range during the stress-controlled, constant amplitude tests are considered to be a manifestation of the material damage process. Analysis of the data obtained allowed the division of the fatigue life of the investigated structural steel into three periods separated by crack nucleation and small crack coalescence (formation of dominant crack) moments. As a result, relations between crack nucleation or coalescence time and fatigue life were found for the range of applied stress amplitudes. Additionally, an endurance limit was found on the basis of the data analysis as the stress amplitude corresponding to zero value of small cracks growth rate. Damaging stress for the investigated alloy was also found to be the value of stress amplitude corresponding to crack nucleation in the first cycle of loading. A method of fatigue life prediction is proposed. This method is based on the analysis of the experimental data presented in this paper.
Influence of stress ratio on fatigue crack growth in mild steel
Engineering Fracture Mechanics, 1995
Crack propagation experiments were performed on a mild steel side edge notched specimen for various load ranges and stress ratios at constant maximum loads. The life of the specimen increased as the load ratio increased. The crack growth data were analysed in terms of AKeff as a function of stress ratio R. Good results were determined for U = 0.7 + 0.15R(2 + R) in both cases. Two crack growth rate equations were also developed.
Fatigue fracture toughness and crack propagation rate
International Journal of Fracture, 1987
Fatigue crack growth rate, da/dN, of two high strength steels were examined in a laboratory air at different stress ratios, covering almost the entire range of stress intensity, AK, from nearly threshold value, AKth, to final fracture. The fatigue fracture toughness, AKfc, corresponding to the final fracture in fatigue, was also determined. The lower the AKfc, the higher da/dN and reduced AKth are revealed. This correlation was analyzed quantitatively based on the four parameter Weibull function. And the stress ratio dependency of the fatigue crack propagation curve can be cleared in a successful manner. The fatigue characteristic stress intensities, Ke and Kv, are proposed to define the transition behaviour in fatigue crack growth curve, from so called region 1 to 2, and from region 2 to 3, respectively. Especially the Kv value can be specified to be the 0.63Kfc.
International Journal of Fatigue
The fatigue crack growth rate (FCGR) curve of metallic alloys is usually divided into three regions. Region II is often referred to as the Paris regime and is usually modelled with a power law relationship with a single exponent. Regions I and III are located at the beginning and end of the FCGR curve, respectively, and are frequently modelled with asymptotic relationships. In this paper we hypothesize that fatigue crack growth is governed by power law behaviour at all crack lengths and all stress intensity factor ranges (ΔK). To accommodate for the changes in the FCGR slope at regions I-III mathematical pivot points are introduced in the Paris equation. Power law behaviour with the presence of pivot points enables direct fitting of the crack length vs. cycles (a-N) curve to obtain the FCGR as a function of ΔK. This novel approach is applicable to small and long crack growth curves and results in accurate multilinear FCGR curves that are suitable for reconstruction of the measured aN curves. The method is subsequently applied to i) different alloys to show local changes in the FCGR curve for changes in alloy composition and heat treatments, ii) naturally increasing ΔK testing of microstructurally small cracks to obtain accurate small crack FCGR data. The comparison with accurate long crack data shows that small cracks are faster, but the transition from region I to region II occurs at a specific fatigue crack growth rate which results in an apparent shift in ΔK at the transition. iii) Long cracks, which show that the FCGR increases with maximum stress for a given ΔK and stress ratio when the maximum stress approaches the yield stress. The maximum stress phenomenon becomes important in the case of fatigue testing, where the initial crack lengths are usually small and maximum stresses are high. It is concluded that for long cracks the phenomenon explains why the Paris equation is applicable rather at low maximum stress, while the Frost-Dugdale equation is more applicable at high maximum stress.
Fatigue crack growth resistance of S960QL high strength steel
Quenched and tempered steels belong to the highest strength categories of structural steels. Their weldability is more complicated than mild steels due to the risk of cold cracking and the decrease of toughness and strength properties in heat affected zones. Since the out-standing strength properties can be primarily exploited in mobile structures where signifi-cant energy saving can be achieved with their application, cyclic loading can often occur among the various loading conditions. Due to the above mentioned reasons the analysis of fatigue crack growth is relevant in terms of this steel group. There are different prescrip-tions containing fatigue crack propagation limit curves and rules for the prediction of the crack growth. The research work aimed to determine fatigue crack propagation limit curves for S960QL and its welded joint, based on the Paris-Erdogan law. Experiments were performed on specimens cut from gas metal arc welded joints and the propagating cracks in the specimens represent the different possible locations of the real cracks in the structural elements. Fatigue crack growth tests were executed by ΔK-decreasing and con-stant load amplitude methods. The evaluation process consists of six steps, and by means of the selected values a statistical method can be proposed for determination of the fatigue crack propagation limit curves.
Modelling of Fatigue in Some Steels and Non-Ferrous Alloys
Ecf17 Brno 2008, 2013
For most engineering alloys the plot of short fatigue crack growth rate against crack length can be modeled by parabolic-linear system of equations. This model is illustrated on two low-carbon steels subjected to tension-tension loading. At elastic-plastic fracture mechanics conditions, the fatigue-data presentation including short fatigue crack growth rate against J-integral range takes place for some steels and non-ferrous alloys based on titanium and copper. An alternative method of short fatigue crack data presentation is proposed that involves a specific energy fatigue-function expressed by an almost straight line termed fatigue tendency of the material at a given stress-range. A comparative analysis between the standard and new presentations shows that at the same number of crack-size measurements, the precision of the latter is significantly higher. This result suggests a possible decrease of fatigue measurements and is approved for another 20 materials.
Influence of High Strength Steel Microstructure on Fatigue Crack Growth Rate
This study examines the effect of high strength steel microstructure morphology on fatigue crack growth rate (FCGR). To achieve this aim, three different heat treatment methods (normalizing, austempering quenching and tempering) were considered and all the steel specimens were initially heated to 950 0 C austenization temperature for ninety minutes and then processed via the different heat treatment methods before viewing the resultant microstructures under light optical microscope (LOM). Fatigue crack growth rate tests were conducted on the resultant microstructures with compact tension specimens at room temperature as prescribed by American standard testing method E647. Results of FCGR tests showed normalized microstructure has the lowest FCGR (6.2698E-06), followed by quenched and tempered (7.9519E-06), as-received (8.15E-06) and austempered (9.6667E-06) microstructure considering a low stress intensity factor range. The trend of results showed insignificant effect of microstructure over the Paris regime growth indicating fatigue crack growth rate is not a reliable parameter for correlating rate of crack propagation to microstructure.