Fatigue crack growth retardation in plane stress and plane strain

The fatigue crack growth retardation phenomenon following a single peak overload applied tothick andthin SEN bend specimens has been studied for a low carbon structural steel. Fatigue tests were performed under a constant load ratio of 0.6, constant stress intensity range of 10 MPam^1/2 and a constant overload ratio of 2.5. An immediate increase followed by a transient retardation in the fatigue crack growth rate, due to the applied overload, was observed for thethick specimen but complete crack arrest was obtained for thethin specimen. The immediate increase in the fatigue crack growth rate following the single peak overload in thethick specimen was attributed to the coincidence of monotonic fracture modes.     

The retardation phenomenon of fatigue crack growth in HT80 steel

The fatigue crack growth behavior resulting from single and multiple applications of overload was investigated for HT80 steel. A peak load was found to cause retardation of the crack growth rate, which becomes stronger with increasing the peak/baseline stress ratio or with decreasing baseline stress intensity. Multiple overloads resulted in additional retardation. These experimental data were explained according to a new model based on crack closure conception, being correlated with the residual plastic zone size ahead of crack tip induced by the peak overload(s). The proposed formulation of retardation was expressed simply as a function of peak/baseline stress ratio, $\text{r}$, and two material parameters, $\text{m}$ and β. $\text{m}$ the exponent parameter in Paris equation and β is the ratio of crack distance at the maximum retardation to the residual plastic zone size. The retardation was predicted to increase with increasing $\text{r}$ and $\text{m}$ and with decreasing β. It was suggested that the parameter, β, reflects the change in the morphology of crack tip resulted from the application of overload, which determines the shape of the curve for retarded crack growth rate vs crack distance.     

Comparison of retardation behaviour of 2024-T3 and 7075-T6 Al alloys

Retardation in fatigue crack growth rate following the application of single and periodic tensile overloads was studied for 2024‐T3 and 7075‐T6 aluminium alloys. Tests were performed at constant stress and at constant stress intensity factor ranges, at a load ratio of R= 0.1, at a baseline ΔK in the 10–20 MPa√m range which corresponds to the Paris regime. Overload ratios of 1.3–1.65 were studied with overload spacing, n, varying from 20 to 10 000 cycles. 2024‐T3 displayed an order of magnitude higher retardation, N_d, due to single tensile overloads compared to 7075‐T6. Periodic overloads induced maximum retardation when n/N_d≈ 0.5 for both alloys, the magnitude being only 15% higher for 2024‐T3.     

An experimental investigation of fatigue crack growth of stainless steel 304L

A series of fatigue crack growth experiments were conducted using round compact tension specimens of AISI 304L stainless steel under Mode I loading. The influences of the R-ratio (the ratio of the minimum load to the maximum applied load in a cycle), notch size, the tensile and compressive overloads, and the loading sequence on crack growth were studied. The results show that the material displays sensitivity to the R-ratio. The application of a tensile overload results in a short period of acceleration in the crack growth rate followed by a significant retardation in the crack growth rate. A compressive overload (underload) produces a short period of acceleration in crack growth and the magnitude of such an acceleration depends on the value of the loading amplitude of the constant-amplitude loading. Results from the two-step high-low loading sequence reveal a period of crack growth retardation at the beginning of the lower amplitude step, an effect similar to that of a single overload. Two existing crack growth models which are based on the stress intensity factor concept are evaluated using the experimental results. A two-parameter crack driving force approach together with a modified Wheeler’s model is found to correlate well the crack growth experiments.     

On the development of crack closure at high R levels after an overload

ABSTRACT In a 1999 paper it was asserted that crack closure cannot be of major importance in the mechanism of crack retardation following an overload, particularly since the authors found no evidence for crack closure at high R‐values, although crack retardation was observed. In the present work, overload experiments were carried out at R = 0.5 and crack closure was observed. In addition, the rate of fatigue crack growth in both constant amplitude and overload tests was found to be a function of ΔK_eff. It is concluded that crack closure is an important part of the retardation mechanism.     

Micromechanisms of fatigue crack growth retardation following overloads

New mechanistic interpretations to rationalize fatigue crack growth retardation due to load excursions are presented. It is reasoned that crack closure arising from residual tensile displacements is not the primary mechanism for growth attenuation following a peak tensile overload. A new mechanism for retardation is discussed in terms of a “micro-roughness” model. Quantitative analyses are provided to estimate the extent of reductions in effective driving force in the retarded growth region due to possible crack branching and fracture face micro-roughness. It is argued that the retarded crack advance is effectively governed by the micromechanisms of Stage I growth although nominally Stage II conditions exist in the post-overload zone. The implications of the present arguments are shown to be consistent with a number of typical post-overload phenomena cited in the literature.     

RETARDATION AND ARREST OF FATIGUE CRACK GROWTH IN AISI 4340 STEEL BY INTRODUCING REST PERIODS AND OVERLOADS

Abstract— The aim of this study is to investigate the effects of an intermittent rest period at 300°C, an overload in tension, and the combination of an overload and a subsequent rest period, on fatigue crack growth in AISI 4340 steel. The intermittent rest period was found to stop fatigue crack growth completely near the threshold level of Δ K. The alleviation effect of a rest period on crack growth was more distinct at lower levels of Δ K. With overload, the higher overload ratio caused a greater effect on crack growth rate. The reduced stress intensity factor caused by crack branching and an enhanced roughness of crack surfaces probably contribute to the retardation and arrest of fatigue crack growth. The most distinct retardation of fatigue crack growth was found after the combined treatment of an overload and a subsequent rest period. Compressive residual stresses following an overload and strain-age hardening during the rest period at 300°C are suggested as assisting the arrest or retardation of fatigue crack propagation.     

INTERACTION OF HIGH CYCLE FATIGUE AND CREEP IN 9%Cr-1%Mo STEEL AT ELEVATED TEMPERATURE

High-cycle-fatigue/creep experiments were performed on a 9%Cr-1%Mo temperered marten-site ferritic steel at 873 K in air. The stress ratio R=σ_min/σ_max ranged from-1 (“pure” fatigue) to 1 (“pure” creep). The maximum stress σ_max was kept constant at 240 MPa. The lifetime depends on the stress ratio R in a non-monotonic way. In the stress ratio interval 0.6 < R < 1.0 both the creep strain rate and the lifetime are controlled by mean stress σ_massof the stress cycle. In the stress ratio interval — 1 < R < 0.2 the lifetime is controlled by the stress amplitude na. The fatigue/creep interaction occurs in between these intervals. The fatigue/creep loading induces transformation of the tempered martensite ferritic structure into an equiaxed subgrain structure. The resulting subgrain size depends strongly on the stress ratio.     

Fatigue of shot peened 7075-T7351 SENB specimen – A 3-D analysis

As‐received or shot peened 7075‐T7351 single‐edged notch bend (SENB) specimens, 8.1‐mm thick, were fatigued at a constant maximum load and at stress ratios of R= 0.1 and 0.8 to predetermined numbers of fatigue cycles or to failure. The SENB specimens were then fractured by overload and the tunnelling crack profiles were recorded. The crack‐growth rate, da/dN, after crack initiation at the notch was determined by crack‐profile measurement and fractography at various fatigue cycles. The shot peened surface topography and roughness was also evaluated by three‐dimensional (3‐D) laser scanning microscopy. Residual stresses in the as‐received specimens and those generated by shot peening at Almen scales of 0.004A, 0.008A, 0.012A and 0.016A, were measured by an X‐ray diffraction stress analyser with an X‐ray target, CrK, every 0.1 mm to a depth of 1 mm. The 3‐D stress intensity factor of the curved crack front was determined by the superposition of the 3‐D finite element solutions of the stress intensity factor of the loaded SENB specimen without the residual stress and the stress intensity factor of the unloaded SENB specimen with a prescribed residual stress distribution. da/dN versus the resultant stress intensity factor amplitude, ΔK_I, plots showed that while the residual stress locally retarded the crack‐growth rate it had no effect on the overall crack‐propagation rate.     

INFLUENCE OF AN OVERLOAD ON THE FATIGUE CRACK GROWTH IN STEELS

Abstract— This study is concerned with the influence of a single-peak overload and the overload ratio on the subsequent rate of growth of a fatigue crack in steels. Retardation increases with increasing overload ratio.
The crack opening load was also measured during all tests. It is shown that the Elber's crack closure concept is not able to explain the effect of overloads. The importance of the material yield stress was evaluated by testing steels of different strength. It seems that the residual stress state induced by the overload is the major factor causing retardation. Two models are analyzed.     

FATIGUE CRACK GROWTH BEHAVIOUR IN MOLTEN-METAL PROCESSED Sic PARTICLE-REINFORCED ALUMINIUM ALLOYS

Fatigue crack initiation and subsequent short crack growth behaviour of 2014-5wt%SiC aluminium alloy composites has been examined in 4-point bend loading using smooth bar specimens. The growth rates of long fatigue cracks have also been measured at different stress ratios using pre-cracked specimens. The distributions of Sic particles and of coarse constituent particles in the matrix (which arise as a result of the molten-metal processing and relatively slow cooling rate) have been investigated. Preferential crack initiation sites were found to be Sic-matrix interfaces, Sic particles associated with constituent particles and the coarse constituent particles themselves. For microstructurally short cracks the dispersed SiC particles also act as temporary crack arresters. In the long crack growth tests, higher fatigue crack growth rates were obtained than for monolithic alloys. This effect is attributed to the contribution of void formation, due to the decohesion of Sic particles, to the fatigue crack growth process in the composite. Above crack depths of about 200 μm “short” crack growth rates were in good agreement with the long crack data, showing a Paris exponent, m= 4 in both cases. For the long crack and short crack growth tests little effect of specimen orientation and grain size was observed on fatigue crack growth rates, but, specimen orientation affected the toughness. No effect of stress ratio in the range R=0.2-0.5 was seen for long crack data in the Paris region.     

Experimental evaluation of shielding effect on growing fatigue cracks under overloads using ESPI

The effect of single cycle overloads, ranging in size from 10% to 50%, on fatigue crack growth behaviour in compact tension specimens subject to constant amplitude loading with an R-ratio of 0.6 has been investigated using electronic speckle pattern interferometry. The resultant displacement fields have been used to evaluate crack opening and closing loads and effective stress intensity factors using the Christopher–James–Patterson model that takes explicit account of the effect of crack tip and wake plasticity on the singularity-dominated elastic fields surrounding the crack. The results show that the crack opening loads and effective stress intensity factors at the overload event are proportional to the magnitude of the overload and that period of post-overload retardation of crack growth is also proportional to the magnitude of the overload. These findings demonstrate the usefulness of the CJP model and should help enhance understanding of plasticity-induce closure phenomena.     

The role of air in fatigue load interaction

Natural fatigue crack formation and growth were studied in notched Al–Cu alloy coupons through high‐resolution SEM fractography. The experiments were conducted under programmed loading conditions designed to induce microscopic marking of the crack formation and growth process under varying stress ratio and closure‐free crack tip conditions. Control experiments were performed by switching between an air and vacuum environment. In air, varying the stress ratio from 0.74 down to 0.64 retards crack growth by up to a factor of five. This ‘closure‐free’ stress ratio history effect totally disappears in vacuum, suggesting a significant environmental influence on stress ratio and its history. Crack‐tip stress state appears to moderate environmental action, revealing a potential mechanism sensitive to residual stress. Consequently, crack closure, residual stress and crack front and plane orientation are identified as major load interaction mechanisms whose synergistic action controls fatigue under variable amplitude loading. The study also appears to suggest that as a consequence of the crack seeking the path of least resistance, load‐sequence sensitive crack plane and front orientation may only induce retardation effects.     

Fracture toughness and growth of short and long fatigue cracks in ductile cast iron EN‐GJS‐400‐18‐LT

Heavy components of ductile cast iron frequently exhibit metallurgical defects that behave like cracks under cyclic loading. Thus, in order to decide whether a given defect is permissible, it is important to establish the fatigue crack growth properties of the material. In this paper, results from a comprehensive study of ductile cast iron EN‐GJS‐400‐18‐LT have been reported. Growth rates of fatigue cracks ranging from a few tenths of a millimetre (‘short’ cracks) to several millimetres (‘long’ cracks) have been measured for load ratios R=?1, R= 0 and R= 0.5 using a highly sensitive potential‐drop technique. Short cracks were observed to grow faster than long cracks. The threshold stress intensity range, ΔK_th, as a function of the load ratio was fitted to a simple crack closure model. Fatigue crack growth data were compared with data from other laboratories. Single plain fatigue tests at R=?1 and R= 0 were also carried out. Fracture toughness was measured at temperatures ranging from ?40 °C to room temperature.     

SMALL FATIGUE CRACK GROWTH IN A LOW CARBON STEEL UNDER TENSION–COMPRESSION AND PULSATING-TENSION LOADING

The growth behaviour of small fatigue cracks has been investigated in a low carbon steel under axial loading at the stress ratios R of –1 (tension-compression) and 0 (pulsating-tension). Crack closure was measured to evaluate the effects of stress ratio and stress level on small crack growth. Except for the accelerated growth at stress levels close to the yield stress of the material, at R=–1 small cracks grow faster than large cracks below a certain crack length, but at R= 0 the crack growth rates for small cracks are coincident with those for large cracks in the whole region of crack length investigated. The critical crack length, 2c_c, above which the growth behaviour of small cracks is similar to that of large cracks depends on stress ratio, being 1–2 mm at R=–1 and smaller than 0.7 mm at R=0. The 2c_c value at R=–1 agrees with that obtained under rotating bending (R=–1). The small crack data are closely correlated with large crack growth rates in terms of the effective stress intensity range, ΔK_eff; thus ΔK_eff is found to be a characterizing parameter for small crack growth including the growth at the higher stress levels.     

Mapping and load response of overload strain fields: Synchrotron X-ray measurements

High energy synchrotron X-ray diffraction measurements have been performed to provide quantitative microscopic guidance for modeling of fatigue crack growth. Specifically we report local strain mapping, along with in situ loading strain response, results on 4140 steel fatigue specimens exhibiting the crack growth retardation “overload effect”. Detailed, 2D, ε_yy-strain field mapping shows that a single overload (OL) cycle creates a compressive strain field extending millimeters above and below the crack plane. The OL strain field structures are shown to persist after the crack tip has grown well beyond the OL position. The specimen exhibiting the maximal crack growth rate retardation following overload exhibits a tensile residual strain region at the crack tip. Strain field results, on in situ tensile loaded specimens, show a striking critical threshold load, F_c, phenomenon in their strain response. At loads below F_c the strain response is dominated by a rapid suppression of the compressive OL feature with modest response at the crack tip. At loads above F_c the strain response at the OL position terminates and the response at the crack tip becomes large. This threshold load response behavior is shown to exhibit lower F_c values, and dramatically enhanced rates of strain change with load as the crack tip propagates farther beyond the OL position. The OL strain feature behind the crack tip also is shown to be suppressed by removing the opposing crack faces via an electron discharge cut passing through the crack tip. Finally unique 2D strain field mapping (imaging) results, through the depth of the specimen, of the fatigue crack front and the OL feature in the wake are also presented.     

Effects of an overload event on crack closure in 3-D small-scale yielding: finite element studies

This paper describes the effects of a single overload event, within otherwise constant amplitude cycles, on the plasticity‐induced closure process for mode I fatigue crack growth in the small‐scale yielding (SSY) regime. The 3‐D finite element (FE) analyses extend the initially straight, through‐thickness crack front by a fixed amount in each load cycle, using a node release procedure. Crack closure during reversed loading occurs when nodes behind the growing crack impinge on a frictionless, rigid plane. A bilinear, purely kinematic hardening model describes the constitutive response of the elastic–plastic material. Extensive crack growth in the analyses, both before and after the overload, allows the crack to grow out of the initial and the post‐overload transient phases, respectively. The work presented here shows that the large plastic deformation in the overload cycle reduces the crack driving force through enhanced closure. Further, the residual plastic deformations due to the overload cause a disconnected pattern of closure in the wake long after the crack front passes through the overload plastic zone. The computational studies demonstrate that the 3‐D scaling relationship for crack opening loads established in our earlier work for constant amplitude cycling (with and without a T‐stress) also holds before, during and after the overload event. For a specified ratio of overload‐to‐constant amplitude loading (R_OL=K^OL_max/K_max) , the normalized opening load (K_op/K_max) at each location along the crack front remains unchanged when the constant amplitude peak load (K_max) , thickness (B) and material flow stress (σ₀) all vary to maintain a fixed value of . The paper concludes with a comparison of the post‐overload response predicted by the 3‐D analyses and by the conventional Wheeler model.     

Time derivative equations for mode I fatigue crack growth in metals

Predicting fatigue crack growth in metals remains a difficult task since the available models based on the Paris law are cycle-derivative equations (da/dN), while service loads are often far from being cyclic. This imposes a cycle-reconstruction of the load sequence, which significantly modifies the load history in the signal. The main objective of this paper is therefore to propose a set of time-derivative equations for fatigue crack growth in order to avoid any cycle reconstruction. The model is based on the thermodynamics of dissipative processes. Its main originality lies in the introduction of a supplementary state variable for the crack, which allows describing continuously the state of the crack throughout any complex load sequence. The state of the crack is considered to be fully characterized at the global scale by its length a, its plastic blunting ρ, and its elastic opening. In the equations, special attention is paid to the elastic energy stored inside the crack tip plastic zone, since, in practice, residual stresses at the crack tip are known to considerably influence fatigue crack growth. The model consists finally in two laws: a crack propagation law, which is a relationship between dρ/dt and da/dt and which observes the inequality stemming from the inequality of Clausius Duhem, and an elastic–plastic constitutive behaviour for the cracked structure, which provides dρ/dt versus load and which stems from the energy balance equation. The model was implemented and tested. It successfully reproduces the main features of fatigue crack growth as reported in the literature, such as the Paris law, the stress ratio effect, and the overload retardation effect.     

The effect of microstructures on fatigue crack growth in Q345 steel welded joint

It is a traditional that the fatigue crack growth behavior is sensitive to microstructure in threshold regime, while it is sensitive to R‐ratio in Paris regime. Fatigue test is carried out for welded joints of a Q345 steel where the compact tension specimens with 3.8 and 12.5 mm thickness are used, and comparisons of fatigue crack growth behavior between base metal and a few different locations in the welded joint are considered in Paris regime. Welding residual stresses are removed by heat treatment to focus the study on the microstructural effect. It is shown that fatigue crack growth rate (FCGR) in the base metal is not sensitive to R‐ratio, but the FCGR increases in the overheated zone, the fusion zone and the weld metal zone with R‐ratio increasing. To the low R‐ratio, FCGR in the three zones is smaller than that in the base metal, but they approximate the same with base metal under the high R‐ratio. The mechanism of fatigue crack growth is analyzed through crack path in microstructures and SEM fractograph. The coarse‐grained ferrite in the base metal is of benefit to relaxation of the average stress at the crack tip, and the fatigue crack growth predicts branching and deflection within above different locations in the welded joint. These tortuous crack paths with crack branching and deflection will promote crack closure as well as crack‐tip stress shielding and then resulted in higher crack growth resistance.     

CRACK FACE INTERACTIONS AND NEAR-THRESHOLD FATIGUE CRACK GROWTH

Rough fracture surfaces usually influence substantially the fatigue growth properties of materials in the regime of low growth rates. Friction, abrasion, interlocking of fracture surface asperites and fretting debris reduce the applied load amplitude to a smaller effective value at the crack tip (“sliding crack closure”, or “crack surface interaction” or “crack surface interference”). The influence of these phenomena on the fatigue crack growth properties of structural steel is discussed and compared for the two kinds of mixed mode loading employed in this work. Mixed mode loading was performed by (A): cyclic mode III + superimposed static mode I and (B): cyclic mode I + superimposed static mode III loading. Such loading cases frequently occur in rotating load-transmission devices. Several differences are typical for these two mixed-mode loading cases. A superimposed static mode I load increases the crack propagation rate under cyclic mode III loading whereas cyclic mode I fatigue crack propagation is retarded when a static mode III load is superimposed. Increase of the R -ratio (of the cyclic mode III load) leads to an insignificant increase of fracture surface interaction and subsequently to a small decrease of the crack growth rate for cyclic mode III loading, whereas higher R -values during cyclic mode I+ superimposed static mode III loading lead to a significant reduction of the crack growth rates.