Fatigue Crack Propagation Data Research Articles

Finite element modelling of crack closure during simulated fatigue threshold testing: McClung, R.C. Int. J. Fract.52 2 (15 Nov 1991) 145–157

Several fatigue cracks growth laws have been suggested over the past 50 years. Recently, a fatigue crack propagation law was proposed by Castillo-Canteli-Siegele (CCS model) based on the assumption that fatigue crack growth takes the form of a Gumbel cumulative distribution function. Besides many physical aspects, the fatigue crack propagation laws need to account for fatigue crack opening and closure effects. Thus, in this paper a modification of the CCS fatigue crack growth law is proposed to account for the crack opening and closure effects, as well as the stress R-ratio effect, Rσ. The fatigue crack opening and closure effects are taken into account using a plasticity-induced crack-closure model. Other fatigue crack closure models can also be used in the proposed modification of the CCS crack growth model through the quantitative parameter U = ΔKeff/ΔK. This modified CCS crack propagation model using the effective stress intensity factor range, ΔKeff, is a new version of an explicit fatigue crack propagation model, supported by mathematical and physical assumptions. In this paper, the proposed model is applied using the fatigue crack growth data and mechanical properties that were collected for the 2219-T851 aluminium alloy. Based on the plasticity-induced crack-closure model, which was first formulated by Newman, and in this paper modified with a boundary correction factor, Fw, the crack opening stress intensity factor, Kop, and quantitative parameter U are determined. The results showed a good agreement between the proposed modification of the CCS fatigue crack propagation model taking into account the plasticity-induced crack-closure model with the boundary correction factor and experimental results of the fatigue crack propagation data for the 2219-T851 aluminium alloy.

The effects of microstructure and fracture surface roughness on near-threshold fatigue crack propagation characteristics of a two-phase cast stainless steel: Murakami, R.-I., Kim, Y.H. and Ferguson, W.G. Fatigue Fract. Eng. Mater. Struct. July 1991 14, (7), 741–748

Fatigue crack propagation in Duplex Stainless Steels (DSSs) is strongly affected by microstructure and therefore by the choice of the steel grade or by the heat treatment conditions. In the present work fatigue behaviour of 22 Cr 5 Ni DSS has been investigated both after solution annealing and after different embrittling thermal cycles. Fatigue tests have been done in air considering the influence of both the stress ratio and the loading direction. Roughness analysis has been performed to point out a relationship between fracture morphology and fatigue crack propagation data and it shows that fracture surface roughness influence on fatigue crack advancing depends on loading conditions. A scanning electron microscope (SEM) fracture surface analysis has been performed to investigate the fatigue crack propagation micro mechanisms in 22 Cr 5 Ni duplex stainless steel.

Fatigue Crack Propagation in Metals and Polymers with a Unified Formulation

Abstract As the traditional ΔK-based fatigue crack propagation (FCP) law has recently been elucidated by Chow and his group [1–7] to be inadequate for a complete description of fatigue cracking, we examine in this investigation the applicability of a unified formulation for characterizing FCP of both metals and nonmetals. We start by defining a new cyclic J-integral, ΔJ, which, under the confines of linear elastic fracture mechanics (LEFM), is equivalent to the range of elastic strain energy release rate, ΔG. ΔJ (or ΔG) is used as a proper criterion for FCP so long as the crack does not extend during the unloading portion of one load reversal. ΔJ (or ΔG) as such is also interpreted as the source supplier for the energy flow rate dissipated on crack-tip cyclic plastic deformations. It is then assumed that, under fixed test conditions, the minimum specific work of fracture required for fatigue crack initiation, ΔJth, is a material property independent of the load ratio R. Fatigue threshold values thus predicted are correlated favorably with experimental measurements. For fatigue crack propagation, a unified formulation is subsequently derived from the thermodynamic theory of irreversible processes with the cracked surface area taken as an internal variable together with the assumption that the rate of energy dissipated on FCP depends on ΔJ only. It is noted that, in addition to ΔJ, the role played by (Jc − Jmax) is also required in the unified FCP law. Applicability of the law for producing a master FCP diagram is finally examined using FCP data of both metals and polymers, including mild steel, aluminium alloys, PMMA, PVC, short fiber composites, and adhesives.

Near-threshold fatigue crack growth behaviour of a ferritic stainless steel at elevated temperatures

Near-threshold fatigue crack propagation (FCP) behaviour has been studied in an 18%CrNb stabilized ferritic stainless steel at temperatures ranging from room temperature to 700 °C. At a stress ratio of 0.1 increasing the test temperature from room temperature to 500 °C resulted in an increase of the growth rates in the midrange growth regime and a sharply defined threshold at a δK level higher than the room temperature threshold, giving rise to a crossover type of behaviour. At temperatures higher than 500 °C increased crack tip plasticity predominates and the fatigue crack growth rates decrease smoothly with a decreasing value of ΔK to thresholds lower than the room temperature value. Crack closure measurements suggest that asperity-induced closure dominates at room temperature but transitions to plasticity-induced closure dominate at 500 °C. A constant- K max, increasing R-ratio (CKIR) test procedure was utilized at room temperature and at 500 °C in an attempt to identify near-threshold FCP data in the absence of crack closure. However, the type of crossover behaviour identified with constant R-ratio tests at room temperature and 500 °C was also observed in the CKIR tests. This is attributed to a change in the closure mechanism from a roughness-induced one to one involving crack tip plasticity.

Characterization of Fatigue Crack Propagation in Short Fiber Reinforced Thermoplastics—A Unified Approach

The paper presents a study conducted to examme the applicability of a unified approach of fatigue crack propagation (FCP) to include a short fiber composite (SFC), which is subjected to tensile cyclic stressing, undergoing generalized plasticity. The proposed approach expressed in the form of da/dN = C( ΔG) m/( Gc - Gmax) has recently been shown capable of characterizing a wide range of materials including mild steel, aluminium alloys, PMMA and PVC with a master FCP diagram. For plasticially loaded cracked members, the above formulation is generalized to be as da/dN = C(Δ J) m/( Jc - Jmax) and the difference between the J-integral range, Δ J, and the con ventional cyclic J-integral, Δ Jcycl, are elucidated The unified formulation is also shown to be derivable from the thermodynamic theory of irreversible processes, had the cracked surface area been taken as an internal state variable. The measured FCP rates for SFC together with two other polymeric materials, PMMA and PVC, are analyzed using the unified formulation and its validity for producing a master diagram is verified when all FCP data are successfully placed in one single line yielding a satisfactory coefficient of correlation of 0.987.

Free bending tests on large spiral strands: Raoof, M. Proc. Inst. Civ. Eng. Dec. 1989 2, (87), 605–626

This paper presents the methodology and results of an effort to analyze and model a large set of fatigue crack propagation data on A508 Class 2 and Class 3 and A533B pressure vessel steel in light water reactor (LWR) environments. The data were from a variety of laboratories worldwide, in most cases contributed to the EPRI Database on Environmentally Assisted Cracking (EDEAC). The data were analyzed in a consistent manner using the computer code FATDAC, which minimizes the scatter arising from numerical differentiation during fatigue data reduction. The models were developed in the time domain, then converted to the more conventional da/dN versus ΔK form. Two modes of corrosion fatigue crack growth behavior were identified and modeled, one about a factor of two faster than the rates in air and independent of loading rate and the other up to two orders of magnitude faster and strongly dependent on loading rate. Variables such as material sulfur content, sulfide inclusion morphology, water chemistry, R-ratio, load rise time, stress intensity range, temperature, electrochemical potential, and flow velocity affect both the probability of observing the highly enhanced crack growth rates and the rates themselves. Representative crack propagation models are developed and presented in this paper together with supporting data.

The role of primary carbides in fatigue crack propagation in aeroengine bearing steels

Fatigue crack propagation, tensile and fracture toughness data for four aeroengine bearing steels are reported. The steels involved are the through-hardened tool steels 18-4-1 (T1) and M50, and two similar carburized steels, RBD and Volvic. Crack growth data have been obtained at 20 °C and 280 °C to cover the range of oil temperatures experienced in aeroengine bearing operations. At 20 °C threshold ΔK values ( ΔK th) ranged between 3.5 and 4.5 MPa √m with Paris exponents ( m) of between 2.0 and 3.5. The lowest m-values were seen in the carburizing steels, which also exhibited lower Paris regime crack growth rates than M50 and 18-4-1. For all the steels, growth rates were higher at 280 °C,than 20 °C, although there was a slight tendency for ΔK th to increase, probably associated with oxide-induced closure at 280 °C. The effects of primary carbides, strength and toughness on fatigue crack growth behaviour are discussed, in relation to the importance of static-mode cracking.

Crack growth rates of structural steel in air and aqueous environments: Yazdani, N. and Albrecht, P. Eng. Fract. Mech. 1989 32, (6), 997–1007

Fatigue crack propagation (FCP) rates were obtained by R-constant (ASTM E647) and Kmax-constant test methods for a spectrum of R-ratios and Kmax values on the Ni-base alloy Nicrofer 5219 Nb (annealed) and AI 7075-T7351. Furthermore, non-propagation conditions ΔKth as a limit for the FCP regime, were determined for these materials. The partitioning point Kop dividing ΔK into ΔKeff had been experimentally determined for Nicrofer 5219 Nb (annealed) and Al 7475-T7351 [1]. The Kop was expressed as a polynomial equation of R-ratio and Kmax. These Kop data were applied to the FCP data of Nicrofer 5219 Nb (annealed) and Al 7075-T7351. It is shown that the ΔKeff concept is a unique and complete description of FCP. Even the wedging action of the wake zone associated with a fatigue crack, which frequently occurs when testing at low R-ratios, is easily accounted for by the ΔKeff concept.

Effect of stress ratio on the rate of growth of fatigue cracks in 1100 Al-alloy

Axial load fatigue crack propagation tests were conducted on notched sheet specimens made of 1100 aluminum alloy. These tests were made at stress ratios R (ratio of the minimum stress to the maximum stress) ranging from −0.5 to 0.5 in order to study the effects of stress ratio on fatigue crack growth. Experimental fatigue crack propagation data were analysed using Paris-Erdoǧan stress intensity analysis. The data from the tests at different stress ratios showed that two regions, namely, region I and region II were present.

A semi-Markovian model for low-cycle elastic-plastic fatigue crack growth

Elastic-plastic fatigue crack growth is described by a continuous time semi-Markovian stochastic process. The load on the member is considered to be deterministic and the resistance is assumed to be random. Crack growth resistance is described by an equation employing the J-integral. Quantities of interest that can be obtained from this model include emergency statistics and probability distributions on the time to reach a critical crack length. The model is tested with fatigue crack propagation data for A533B pressure vessel steel. The results indicate that the model underpredicts the mean first-passage time at the start of a test but predicts it well in the mid-ranges and towards the end of the test; the variance of the process is better predicted for specimens that undergo large plasticity than those with low plasticity.

FATIGUE AND CORROSION FATIGUE OF 8090 Al-Li-Cu-Mg ALLOY

The fatigue resistance of metals can be profoundly affected by the environmental reactions that change crack initiation and propagation processes. These corrosion fatigue processes can occur in high strength aluminium alloys in aircraft, which frequently encounter salt spray and high humidity environments. For this reason, the fatigue behaviour of 8090-T6 alloy in the form of 25mm thick plate has been investigated in both air and an aerated 2.5% NaCl solution. Results are compared to a conventional high strength aluminium alloy, 2014-T6. Fatigue crack initiation and propagation data are presented for different crack orientations and mean stress levels. The slightly lower fatigue strength of 8090-T6 in air is attributed to earlier crack initiation. The fatigue crack propagation behaviour of 8090-T6 is shown to be superior to that of 2014-T6 at low to intermediate levels of ƊK, under both environmental conditions. The mechanisms associated with fatigue crack initiation and propagation, and environmental effects, are discussed.

Application of the crack layer theory of fatigue crack propagation in PMMA

Abstract The crack layer theory has been well established as a good description of the energy dependence of fatigue crack propagation in thermoplastics associated with multiple crazes or large deformation preceding the crack tip. Since PMMA usually displays crack propagation preceded by a single craze, the applicability of the theory in such extreme cases needs to be tested. Fatigue crack propagation data obtained in a previous interferometric investigation in PMMA have been utilized here. Two different approaches are attempted for estimation of the energy dissipated in craze formation and growth. Using both approaches, the crack layer formalism provides a reasonably good description of the crack propagation behavior in PMMA.

Initiation and propagation of fatigue crack in pure aluminium single crystals

The fatigue crack initiation and propagation in the pure aluminum single crystals with different orientations are investigated. Acoustic Emission (AE) technique is used to monitor the fatigue crack initiation and propagation data. The results show that the acoustic emission technique could be used for detecting fatigue crack initiation and propagation. The total counts of AE are plotted as a function of the number of fatigue cycles, and change in slope of the curve corresponded to the initiation of the fatigue crack. The fatigue crack initiation and propagation are related to the specimen crystalline orientation. Specifically, they depend on the ratio K of the Schmid factor of the secondary slip system, to that of the primary one. The greater the ratio K, the easier the fatigue crack initiates and the faster the fatigue crack propagates. When the ratio K is equal or close to one, the fatigue crack grows either along a cleavage plane which intersects both the primary plane and the secondary one or along the primary slip plane and the secondary one. Growth along alternating planes can also occur giving rise to a zigzag path. When the ratio K is much less than one, the fatigue crack grows along the primary slip plane only. The results are explained with a model for the faceted mode of fatigue crack growth.

A simple viscoelastic model for fatigue crack propagation in polymers as a function of molecular weight

A simple theory is presented to explain the strong influence of molecular weight ( M) on rates of fatigue crack propagation (FCP) in amorphous polymers. It is proposed that the equation describing FCP rates may be expressed as the product of two functions, one involving the stress intensity factor ( ΔK), and the other characterizing the relaxation process occurring in the plastic zone. To provide a physical network in the plastic zone that can sustain fatigue loading, it is proposed that one needs a sufficient fraction of molecular fibrils per unit area ( W∗) whose lengths are greater than M c, the critical value of M required for entanglement. This effect can be summarized as a generalized rate process (confined at the plastic zone) expressed by A exp ( Bσ) where σ is a stress and A and B are constants ( B including the volume of activation). It is deduced that M influences the activation volume through the values of W∗ and W, the weight fraction of molecules whose M> M c. Using the equation developed it was possible to correlate FCP data of PVC and PMMA as a function of M with a high degree of confidence. Also, the value of activation volumes obtained compared favourably with those in the literature for static tests. The complementary value of W∗ for these polymers was also seen to approximate closely to the void fraction in a craze. Extension to other cases such as semi-crystalline materials also seems possible.

Craze development in poly(methyl methacrylate) during stable fatigue crack propagation

AbstractIn order to obtain a more complete understanding of failure mechanisms in glassy polymers subjected to fatigue loading conditions, craze zone dimensions (i.e., length and thickness at the crack tip) were measured simultaneously with fatigue crack propagation data in poly(methyl methacrylate) (PMMA) by optical interferometry. Since the craze shape was observed to assume a wedge‐shaped configuration similar to the one described by the Dugdale plastic strip model, crazing stresses were inferred on the basis of this model. When varying the stress ratio (R = minimum load/maximum load) of the applied cyclic load in the range from 0.1 to 0.7, it was found that both craze length and craze thickness are essentially independent of the R‐ratio and can be correlated in terms of the maximum stress intensity factor only. On the other hand, significant variations in craze dimensions with test frequency occurred over the range from 0.1 to 250 Hz. The results are discussed in terms of the viscoelastic nature of the material and a competition between the effects of strain rate and hysteretic heating.

The role of dislocation substructures in fatigue crack propagation in copper and alpha brass

The effect of dislocation substructures on fatigue crack propagation (FCP) behavior in copper and alpha brass was studied. Various dislocation substructures were obtained by prestraining in tension. Dislocation cells were formed by this prestraining in copper and 90/10 brass and when they formed the resistance to FCP at intermediate propagation rates (5×10−9 to ∼10−7 m/cycle) increased with increasing prestrain. Planar dislocation arrays were observed in 70/30 brass instead of cells, and the effect of prestraining on the FCP resistance was insignificant. From the FCP data for each material it was observed that, regardless of the difference in the dislocation substructures and grain sizes, the two constantsC andm in the Paris equation,da/dN=C(ΔK) m, were interrelated. Possible relations between the cyclic strain hardening exponent andm are discussed. The influence of both prestrain and grain size on threshold behavior was also studied.

Fatigue Life and Crack Propagation Analyses of Welded Components Containing Residual Stresses

AbstractThis study deals with the fatigue of welded components emphasizing the use of small specimen data, a general stress-strain expression, the fatigue notch factor, and the effect of residual stresses. A computer-based fatigue damage analysis for welded structures is investigated; although accurate fatigue predictions have been made for notched structures, this type of analysis has not previously been applied to welded components. Fatigue crack propagation data are presented emphasizing the mean stress effect on the rate of cracking. Crack measurements have been made for both butt and fillet welded specimens in which the cracks commence from the toe of the weld.For fillet welded attachments and butt welds with substantial reinforcement, a residual stress adjustment was made based on the assumption that the maximum stress at the notch root is equal to yield stress. A fatigue damage analysis for welded specimens subjected to block loading using a computer-based damage model gave results close to experimental data. With a proposed stress function as the controlling stress parameter for crack growth, crack propagation data were successfully correlated.

A new method of analysing fatigue crack propagation data

A new method of assessing the validity of fatigue crack propagation laws is described. The method is illustrated by correlating experimental data collected using compact tension specimens for several steels and several testing environments. At no stage in the analysis is it necessary to estimate crack growth rates from the crack length-cycles data, a procedure which is extremely difficult to carry out accurately in practice. Six different fatigue crack growth laws are considered and it is shown that a simple law derived from a damage accumulation theory consistently offers the best correlation for the limited sets of data so far tested. This fatigue crack growth law when used in conjunction with compact tension specimens could provide a very useful method of assessing by comparison the fatigue crack growth properties of metals and it could therefore be a valuable tool for use in materials selection procedures.

Fatigue crack propagation in iron and Fe-Mo solid solution alloys (77 to 296 K)

Fatigue crack propagation data were obtained for iron and Fe-Mo solid solution alloys through the ductile-brittle transition temperature (77 to 296 K). The fatigue crack growth rates were found to decrease with decreasing temperature for each alloy and to decrease with increasing solute content at 296 K but to increase with solute content at 77 K. These results are discussed in terms of the wide range of mechanical properties observed for the materials. It is shown that the fatigue crack growth rates are inversely proportional to a plastic work term and the square of a characteristic stress approximated by the yield or ultimate stress.

Transient effects in fatigue crack propagation

The transient fatigue crack propagation resulting from the sudden change of the stress intensity factor amplitude or from the change of the stress cycle asymmetry or from the application of the single overload cycle was measured on carbon steel specimens. To simplify the conditions and to increase the accuracy the shape of the specimens was chosen in such a way, that the stress intensity factor was independent of the crack length. It was shown that the transient effects can be qualitatively understood and quantitatively in the first approximation described solely on the basis of the steady state fatigue crack propagation data, provided that the threshold conditions of non-propagation are taken into account.