Crack Growth Rate Range Research Articles

The influence of residual stress on fatigue crack growth rates of additively manufactured Type 304L stainless steel

To examine the influence of residual stress on mechanical performance, specifically fatigue crack growth resistance, additively manufactured (AM) Type 304L stainless steel produced by directed energy deposition (DED) was evaluated and compared to conventional wrought Type 304/304L stainless steel. Increasing and decreasing alternating stress intensity factor (ΔK) tests were used to assess fatigue crack growth behavior over a range of crack growth rates in the near threshold regime (<10-8 m/cycle). Bulk residual stress and residual stress intensity factor (Kres) profiles of a fatigue specimen were measured using the incremental slitting method. Tensile residual stress at the edges of the DED materials led to positive values of Kres and faster fatigue crack growth rates in the DED material as compared to wrought material at the same applied ΔK. Correcting for the effects of Kres and crack closure in DED Type 304L and commercially available wrought Type 304/304L stainless steel shows that fatigue crack growth rates are similar at values of ΔK lower than 6 MPa·m0.5.

Implications of ΔK‐Rratio in vacuum

AbstractIn this article we discuss the high vacuum implications on the role of positive load ratios (R = Pmin/Pmax) on the threshold ΔKthand fatigue crack growth (FCG) behavior in commercial alloys. It is experimentally observed that the ΔKthis independent ofRin vacuum. Also, for a given applied ΔK, FCG rate is weakly dependent ofRover the entire range of crack growth rates for several alloys. In addition, limited data on both short and long crack FCG in vacuum results fall within the same experimental scatter suggesting that a typical small crack FCG effect observed in lab air was insignificant or diminished in vacuum. A marginalR‐ratio effects on ΔKthin vacuum are seen for materials with different microstructures, yield strengths, elastic modulus, work hardening properties, and slip modes. The provided analysis and examples seem to imply that FCG in vacuum be primarily dependent on applied ΔK that comes from mechanical driving force. Possible mechanistic explanations are given.

Comparison of crack propagation rates in selected structural components made from AISI 304 grades: Three-point bending test

This study focuses on crack propagation rates of AISI 304 stainless-steel specimens made from selected structural components experimentally tested in three-point bending. Components of two productions loaded with constant force with selected stress ratio were submitted to cyclic loading in order to measure crack opening displacement. Experimentally obtained data were evaluated using 3D modelling in finite element modeling software intending to determine fracture mechanical parameters. The tool to describe fatigue behavior was Paris’ law, used to describe crack propagation in intermediate crack growth rate range. Paris–Erdogan equation was evaluated as a fitting of linear part of the graphical plot of crack growth rate with respect to the stress intensity range and material constants were gained from numerical description of the relation. Comparison of fatigue crack behavior in AISI 304 steel components of two productions was studied and evaluated.

Verification of stress-intensity factor and compliance relations for a single edge notch specimen under tension-compression loading

Stress-intensity factor and compliance solutions developed for a single edge notch (SEN) specimen has been validated and verified for tension-compression loading. Fatigue crack growth tests were conducted on the SEN and a surface-crack specimen at stress ratios, R of −1.0 and −2.0 using IN718 forged nickel base superalloy material. (The surface-crack specimen is referred to as the KB-bar specimen in the literature.) Experimentally determined compliance values for the SEN specimen compared well with the analytically predicted values. In addition, crack lengths measured using the compliance equation compared well with crack lengths measured using potential drop and optical techniques. Finally, agreement was found between the SEN and KB specimens in the intermediate crack growth rate range.

Effect of Overload on the Near-Threshold Fatigue Crack Growth Rate in a 2024-T3 Aluminum Alloy: II. Fatigue Crack Growth Simulation for Calculating the Fatigue Life under Alternating Loading

The results of overloading and underloading tests of aluminum specimens in near-threshold crack growth rate range are considered. A combined model is proposed to take into account crack closure and the influence of the local stresses at the crack mouth. The model is based on the Neuber and Ramberg–Osgood equations and can be used to estimate the fatigue life of crack growth in the near-threshold range. This model is grounded from a physical standpoint.

A single edge notch specimen for fatigue, creep-fatigue and thermo-mechanical fatigue crack growth testing

The importance of capturing crack growth behavior under creep-fatigue and thermo-mechanical-fatigue is well known. Such service load environments are often simulated by performing tests under load and displacement control conditions where the specimen may be subjected to both tensile and compressive loads. A single edge notch specimen was developed to accommodate tension and compression loading in fatigue, creep-fatigue and thermo-mechanical fatigue crack growth experiments. The stress intensity factor and compliance solutions were developed for the single edge notch specimen using a 3D fracture code (FRANC3D) and 2D boundary element code (FADD2D). Both codes agreed well with each other. It is shown in this work that using simple approximations in a 2D code such as FADD2D, accurate stress-intensity and compliance solutions can be obtained for specimen geometry with varying cross sections. An importance of using correct boundary condition is emphasized by displaying the difference in stress intensity solutions for rectangular edge-cracked plate under uniform-stress, and uniform-displacement cases. Also, it is shown that stress intensity solution for a threaded edge-cracked specimen considered in this paper is different from the solution for rectangular edge-cracked plate under displacement control. Asa validation of the developed stress-intensity and compliance equations, fatigue crack growth tests were conducted using the single edge notch, compact type and a surface crack specimen. Tests were conducted under constant amplitude load control of 0.1 stress ratio, R for IN738 nickel base super alloy material. Agreement was found between the different specimen types in the intermediate crack growth rate range.

OS0744 金属薄膜法を用いたアルミナセラミックスの繰返し疲労寿命予測(構造用材料の疲労挙動と寿命評価,オーガナイズドセッション)

In order to predict the cyclic fatigue life of alumina ceramics, a new method was proposed. First, to monitor the small crack growth behavior during static fatigue, a thin metal film method was proposed. Second, based on the crack growth data, crack growth characteristics and R-curve behavior were evaluated. Then, cyclic fatigue life was predicted, and predicted results were compared with cyclic fatigue test results. As the results, it is found that crack growth behavior can be monitored over wide range of crack growth rate by using the thin metal film method. And it is confirmed that the cyclic fatigue life of ceramics can be predicted by using proposed method.

Crack Growth Characteristics Evaluation Method in Ceramics Based on DT Technique

A new method to evaluate the relationship between stress intensity factor KI and crack growth rate V of ceramics which is based on Double-Torsion (DT) technique was proposed. Materials were soda-lime glass, glass ceramics and alumina ceramics. Plate type DT specimens were prepared, and rectangular section guide groove was introduced in the upper surface of specimen. As a crack propagation test, a constant loading rate test was performed. Furthermore, a crack growth behavior was monitored by a compliance method. As the results, it was found that the clack growth behavior can be detected with high accuracy by using the proposed method. And it was confirmed that crack growth characteristic over wide range of crack growth rate can be evaluated by single experiment. Furthermore, since the method is based on a simple measurement system, it is considered that the method has a good availability for KI-V evaluation in ceramics.

Crack closure stress estimation by decodable marker banding techniques and comparison of results with electron fractographic technique

A specially programmed marker load sequence was designed that permits unambiguous measurement of crack opening and crack closure stress from marker band spacing using both optical and scanning electron fractography. Crack growth tests using programmed marker loading sequence were carried out on single edge notch tension specimens made out of Al-Cu alloy 2014-T3 (BS: L73 equivalent) from crack initiation to failure and the fracture surface examined under optical and electron microscope. Reproducible marker band patterns on the fatigue fracture surface could be obtained using this method. Measured crack closure/opening stress values are observed to be consistent with known material behaviour. The marker banding technique can be applied over wider crack growth rate ranges compared to striation measurement technique, which is restricted to a small range of crack growth rate and in select materials. Quantitative fractography of the marker bands reveals a degree of load interaction under variable amplitude loading that requires further examination.

OS11W0316 On the formation of striations during fatigue crack growth

A study of the fatigue crack growth process in a low carbon steel has been carried out over a wide range of crack growth rates in air and vacuum at R=0.05 using the techniques of stereo-microscopy and scanning-transmission electron microscopy(STEM). Examination of the profiles of striations indicated that they are of the stair-case type, as originally proposed by Schivje. In this steel the striations were not resolvable below a growth rate of 10^<-4> mm/cycle. In the crack growth rate range from 10^<-4>mm/cycle down to 10^<-8>mm/cycle a dislocation cell structure developed whose dimension in the direction of fatigue crack growth was approximately 1.5 x 10^<-4> mm/cycle, independent of the ΔK level. The constancy of this dimension has been interpreted to indicate that the fatigue crack growth process at low crack growth rates is discontinuous rather than cycle-by-cycle in nature. However the evidence gathered in this investigation supports the view that the fatigue crack growth process occurs on a cycle-by-cycle basis even at low fatigue crack growth rates. As is known, striations are difficult to find on the fracture surfaces of specimens tested in vacuum. The use of the STEM technique provides evidence that a striation is created in vacuum on unloading, but that due to the greater extent of crack tip blunting in vacuum as compared to in air, in the next loading cycle the previously created striation is swept into the flanks of the blunted crack and largely eliminated. For comparison purposes an aluminum alloy, 2519-T87, was also tested.

An experimental and theoretical approach for an estimation of ΔKth

The existence of a fatigue threshold value may affect the design process when a damage‐tolerant design is considered that uses non‐destructive techniques for evaluating the shape and dimensions of the defects inside materials. Obviously it should be possible to estimate the stress field surrounding these defects and this is not generally a problem with modern numerical methods.Many factors are involved in determining the growth rate of a fatigue crack. Some of these are highly significant and it is possible to obtain the coefficients of a correlation function. Some others are not well defined and the only effect is to expand the scatter of experimental data.Consider the sigmoidal curve we obtain when plotting the crack growth rate versus the applied ΔKI . A very difficult parameter to measure but very useful for fatigue design is the ΔKIth value, because below this value a crack may be forming, hence, here ΔKIth is defined by the transition between a normal (e.g. 10−10 m/cycle) and a very low range of crack growth rate (&lt;10−10 m/cycle).The ΔKIth value is very difficult to obtain by experimental methods because the growth rate is of the order or less than the atomic lattice span (3 × 10−10 m/cycle), but we can correlate the transition value with the cyclic crack tip plastic zone size and other structural parameters of metallic materials.The aim of this work is to offer a contribution about the parameters which influence ΔKIth in stainless steels and welded joints based on the crack tip plastic zone radius.

Fatigue Crack Growth Mechanisms Of Cast Ti-48Al (at.%) Alloy

Substantial cost reduction for cast TiAl automotive parts could be achieved by eliminating hot isostatic pressing (HIPing) procedure during the material processing. In this respect, a cast Ti-48Al (at %) intermetallic alloy is evaluated in the non-HIPed condition with the objective to establish the fatigue crack growth response and identify the fracture mechanisms. For this purpose, a series of fatigue crack growth tests were performed on the alloy with three different microstructures, with a load ratio, R=0.1, in both air and vacuum environment. Environment effect is manifested in the threshold I”K levels of 9.6 and 13 MPaâˆsm for crack growth response in air and vacuum, respectively. In the intermediate range of crack growth rates between 10 -5 to 10-3 mm/cycle, crack bridging by shear ligament formation describes the crack growth process, with I”K varying from 13 to 35 MPaâˆsm. Extensive crack bridging effect in the duplex microstructure is reflected in the crack growth retardation which reaches the lowest minimum crack growth rate at 2A—10-6 mm/cycle in each of the three crack bridging stages observed. Other fatigue damage mechanisms observed are crack deflection, crack branching, stepped crack growth and microcracks initiation in the crack tip plastic zone, particularly at near threshold crack growth.

Fatigue crack growth behavior of D6AC space shuttle steel

Fatigue crack propagation rate properties of a D6AC steel were developed, covering a wide range of crack growth rates from approximately 10 −3 m/cycle (4 × 10 −2 in./cycle) to 10 −10m/cycle (4 × 10 −9 in./cycle). The low crack propagation rates were obtained using a decreasing ΔK test method, while the fast crack growth rates were generated by the J- controlled test technique. At high ΔK levels [> 122 MPa √ m (111 ksi√ in.)], dimpled fracture was the dominant fracture mode, while at ΔK levels between 122 MPa √m (111ksi √in.) and 176.6 MPa√m (16 ksi√in.), the transgranular fracture mode was found. Below ΔK levels of 17.6 MPa√m (16 ksi√in.), some intergranular fracture mixed with transgranular fracture was observed. At near-threshold levels, oxide deposits were present on fracture surfaces. Oxide thickness was measured, and was found to be much less than crack tip opening displacement. Thus, oxide-induced crack closure was negligible during near-threshold crack growth testing.

極低温用構造材料の疲労き裂進展速度

The fatigue crack growth rates of material A (iron base alloy A286), material M (austenitic stainless steel 12Cr-12Ni-10Mn-5Mo) and material S (austenitic stainless steel SUS304L) were investigated experimentally. The fatigue tests were conducted with compact specimens at cryogenic temperatures (4 and 77K) and room temperature.The results obtained were as follows.(1) For materials A, M and S, the crack growth rates at 4 and 77K were lower than that at room temperature. However, the temperature dependence of the crack growth rate was different among the three materials and no clear tendency was observed.(2) At 4K, the crack growth rate of material A was lower than that of material M or S.(3) The crack growth rates at 4K of materials A, M, S, titanium alloy and aluminum alloy were related to the strain intensity factor range ΔK/E.(4) For materials A and M, a good correspondence between the striation spacing and the crack growth rate was observed in the crack growth rate range of 10-4-10-3mm/cycle, irrespective of test temperatures.

A STUDY OF FATIGUE FRACTURE SURFACES OF A CAST NICKEL‐BASE SUPERALLOY

AbstractMorphological features of fatigue fracture surfaces of a cast nickel‐base super‐alloy were investigated over the range of crack growth rates, 10‐10 ‐10‐6m/cycle. The cleavage facet growth in stage I and the transition of crack propagation from state I to stage II were studied by a combined study of surface crack growth rate and fracture surface examinations. A correlation between fatigue crack growth and the fracture surface appearance plus microstructural aspects has been established. It is suggested that fatigue‐induced vacancies might play a significant part in cleavage facet growth at lower crack growth rates.

Acoustic emission from stress corrosion cracks in aligned GRP

Acoustic emission (AE) produced by the propagation of stress corrosion cracks in an aligned glass fibre/polyester resin composite material has been recorded. Tests have been carried out over a range of crack growth rates and the variation of AE with crack velocity/applied stress intensity has been examined. The main source of AE is fibre fracture and there is a one-to-one relationship between the number of fibre fractures and the number of high-amplitude AE signals. This enables crack growth to be monitored directly from acoustic emission. The amplitude of AE signals produced by fibre failure appears to be proportional to the fracture stress of the fibres, although further analysis requires a greater understanding of the generation, transmission and detection of AE signals. This work demonstrates that stress corrosion cracking is an ideal source for the study of AE produced by fibre fracture without complications caused by interface effects, such as fibre debonding or pullout.

Fatigue crack propagation in dual-phase steels: Effects of ferritic-martensitic microstructures on crack path morphology

microstructures with maximum resistance to fatigue crack extension while maintaining high strength levels. A wide range of crack growth rates has been examined, from ~10-8 to 10-3 mm per cycle, in a series of duplex microstructures of comparable yield strength and prior austenite grain size where intercritical heat treatments were used to vary the proportion, morphology, and distribution of the ferrite and martensite phases. Results of fatigue crack propagation tests, conducted on “long cracks” in room temperature moist air environments, revealed a very large influence of microstructure over the entire spectrum of growth rates at low load ratios. Similar trends were observed at high load ratio, although the extent of the microstructural effects on crack growth behavior was significantly less marked. Specifically, microstructures containing fine globular or coarse martensite in a coarse-grained ferritic matrix demonstrated exceptionally high resistance to crack growth without loss in strength properties. To our knowledge, these microstructures yielded the highest ambient temperature fatigue threshold stress intensity range ΔK0 values reported to date, and certainly the highest combination of strength and ΔK0 for steels (i.e., ΔK0 values above 19 MPa√m with yield strengths in excess of 600 MPa). Such unusually high crack growth resistance is attributed primarily to a tortuous morphology of crack path which results in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanisms. Quantitative metallography and experimental crack closure measurements, applied to currently available analytical models for the deflection and closure processes, are presented to substantiate such interpretations.

Micromechanisms of fatigue crack growth in aluminium alloys

The fatigue crack growth characteristics of high-strength aluminium alloys are discussed in terms of behaviour during mechanical testing and fracture surface appearance. For a wide range of crack growth rates, the crack extends both by the formation of ductile striations and by the coalescence of micro-voids. Dimples are observed at stress intensities very much less than the plane strain fracture toughness, and this is explained in terms of the probability of inclusions lying close to the crack tip. The striation formation process is described as a combination of environmentally-enhanced cleavage processes and plastic blunting of the crack tip.

Fatigue crack growth theory and experiment: A comparative analysis

A number of theoretical models have been proposed in the literature which explain the second or the fourth power dependence of fatigue crack growth rate on ..delta..K, the stress intensity factor range in the Paris-Erdogan relation da/dN = C ..delta..K /SUP m/ . All of these models pertain to the intermediate range of crack growth rates where the m values are relatively low in the range of 2 to 4. The values of m for many metals and alloys can be much larger than 4 at near threshold crack growth rates or at stress intensities close to the fast fracture, and in some cases throughout the range of ..delta..K when the faceted mode of crack growth occurs. For such cases, the models appear to have no relevance. In this report predictions of different theoretical models are critically examined in comparison to experimentally determined crack growth rates in a MA 956, oxide dispersion strengthened alloy. Cumulative damage models predict crack growth rates reasonably well except in the range where ductile striations are observed. Lack of agreement with any particular model in this range is related to the fact that at different regions across the specimen thickness different mechanisms, either plastic bluntingmore » or cumulative damage, control the crack growth.« less

Model of fatigue crack propagation by damage accumulation at the crack tip

Modelling fatigue crack propagation by damage accumulation at the crack tip originally proposed by McClintock is reworked out using metallurgical considerations. On a physical basis it is shown that the validity of such models is actually confined to the low crack growth rate range corresponding to stage I growth such as along crystallographic planes. At higher crack growth rates there is a transition to plastic stretching mechanisms which could be described by crack opening displacement models. An evaluation of two models has been carried out on a 33Co Ni alloy where extensive information was available: both are based on the strain singularity as computed by Tracey from a finite element analysis of plane strain small scale yielding and as adapted to cyclic loading under Rice's hypothesis and taking the grain as the critical element below which size continuum mechanics do no longer apply. It is pointed out that the use of this strain singularity which is not able to account for crack closure, renders models unable to predict experimental crack growth rates curves but only intrinsic curves relating the growth rate to the effective stress intensity amplitude as defined by Elber. The first model which assumes fatal cracking of a grain with an average uniform cyclic equivalent strain, underestimates the crack growth rates and in addition it yields results very sensitive to the shape and size of the grain. The second model which assumes progressive cracking in a grain with a cyclic equivalent strain gradient is able to predict the intrinsic crack growth rate curve in the 33Co Ni alloy and to yield predictions consistent with other experimental data.