Transient Crack Growth Behavior Research Articles

Fatigue crack growth in Ti-6Al-4V specimens produced by Laser Powder Bed Fusion and submitted to Hot Isostatic Pressing

The fatigue crack growth (FCGR) behaviour of titanium alloy Ti-6Al-4V submitted to Hot Isostatic Pressing through CT specimens manufactured by Laser Powder Bed Fusion (LPBF) is analysed. Both the crack growth under constant amplitude loading and the transient crack growth behaviour after the application of overloads or underloads were studied. The effect of the mean stress and the transient delay behaviour were studied using the crack closure parameter. The HIP treatment showed an increment in the FCGR strength increasing the fatigue threshold, the critical stress intensity factor and the FCGR resistance. Crack closure induced by plasticity for the stress ratio R = 0 was observed and for R = 0.4 no crack closure was detected. The applied overloads caused a FCGR retardation due to the increase of crack closure effect induced by plasticity resulting from the overloads while for underloads application no change in the FCGR behaviour and crack closure levels was observed. The overloads applied with 100% intensity for R = 0.4 achieved a high level of plasticity in the crack tip leading to higher crack retardation and superior transient period due to a higher zone affected by the overloads.

Overloading effect on transient fatigue crack growth of Ti-6Al-4V parts produced by Laser Powder Bed Fusion

The fatigue crack growth (FCGR) behavior of titanium alloy Ti-6Al-4V submitted to Hot Isostatic Pressing (HIP) through CT specimens manufactured by Laser Powder Bed Fusion (LPBF) is analyzed. Both the crack growth under constant amplitude loading and the transient crack growth behavior after the application of overloads were studied. Crack closure induced by plasticity for the stress ratio R=0 was observed at the Paris regime. The applied overloads for OLR=2 at R=0 caused a FCGR retardation due to the increase of crack closure effect induced by plasticity resulting from the overloads. Rapid transient periods were registered due to small zone affected by the overloads.

Fatigue crack growth behaviour in Ti6Al4V alloy specimens produced by selective laser melting

The current study presents the fatigue crack growth behaviour of titanium alloy Ti6Al4V parts manufactured by selective laser melting (SLM), obtained as standard 6 mm thick compact specimens (CT). Both the crack propagation under constant amplitude loading and the transient crack growth behaviour after the application of overloads were studied. The effect of the mean stress and the transient retardation behaviour were analysed using the crack closure parameter, obtained both by compliance and digital image correlation techniques. A reduced crack closure level for the stress ratio $$\hbox {R}=0$$ was detected and for $$\hbox {R}=0.4$$ no crack closure was observed. The digital image correlation technique showed better results in the Paris regime and during the transient retardation behaviour. The overload application produced crack growth retardation due to the increase of the crack closure effect. The failure surfaces showed a transgranular crack growth in $$\upbeta $$ phase contouring the martensitic $$\upalpha $$ phase.

A study on fatigue crack growth behavior subjected to a single tensile overload: Part II. Transfer of stress concentration and its role in overload-induced transient crack growth

The combined effects of overload-induced enlarged compressive residual stresses and crack tip blunting with secondary cracks are suggested to be responsible for the observed changes in the crack opening load and resultant post-overload transient crack growth behavior [Lee SY, Liaw PK, Choo H, Rogge RB, Acta Mater 2010;59:485–94]. In this article, in situ neutron diffraction experiments were performed to quantify the influence of the combined effects by investigating the internal-stress evolution at various locations away from the crack tip. In the overload-retardation period, stress concentration occurs in the crack blunting region (an overload point) until a maximum crack arrest load is reached. The stress concentration is then transferred from the blunting region to the propagating crack tip (following the overload), requiring a higher applied load, as the closed crack is gradually opened. The transfer phenomena of the stress concentration associated with a crack opening process account for the nonlinearity of strain response in the vicinity of the crack tip. The delaying action of stress concentration at the crack tip is understood in conjunction with the concept of a critical stress (i.e. the stress required to open the closed crack behind the crack tip). A linear relationship between Δ ε eff and Δ K eff provides experimental support for the hypothesis that Δ K eff can be considered as the fatigue crack tip driving force.

A study on fatigue crack growth behavior subjected to a single tensile overload: Part I. An overload-induced transient crack growth micromechanism

Neutron diffraction and electric potential experiments were carried out to investigate the growth behavior of a fatigue crack subjected to a single tensile overload. The specific objectives were to (i) probe the crack tip deformation and fracture behaviors under applied loads; (ii) examine the overload-induced transient crack growth micromechanism; (iii) validate the effective stress intensity factor range based on the crack closure approach as the fatigue crack tip driving force; and (iv) establish a quantitative relationship between the crack tip driving force and crack growth behavior. Immediately after a single tensile overload was introduced and then unloaded, the crack tip became blunt and enlarged compressive residual stresses in both magnitude and zone size were observed around the crack tip. The results show that the combined contributions of the overload-induced enlarged compressive residual stresses and crack tip blunting with secondary cracks are responsible for the observed changes in the crack opening load and the resultant post-overload transient crack growth behavior.

The dependency of the crack growth rate on the loading pattern and temperature in stress corrosion cracking of strain-hardened 316L stainless steels in a simulated BWR environment

The stress corrosion cracking (SCC) growth rate of a warm-rolled (WR) 316L stainless steel contoured double cantilever (CDCB) specimen was measured in high purity water at various temperatures and under various loading patterns. An alternating current potential drop (ACPD) technique was used to monitor the crack growth kinetics throughout the tests. The fracture surface exhibited typical intergranular SCC characteristics. Depending on the test conditions, three kinds of crack growth kinetics, i.e., increasing with time then becoming steady, being constant during the whole period, decreasing with time then becoming steady, were identified and are described. The steady state crack growth rate (CGR) values are used to quantify the effects of the loading pattern and the environmental temperature. A moderate increase in the crack growth rate was encountered by employing periods of unloading and reloading to form a trapezoidal loading pattern and the enhancement factor was found to depend on the holding time and the times for unloading and reloading. It was found that the crack growth is thermally activated; however, the apparent activation energy is not constant but seems to be greater at higher temperatures. Several types of temperature-dependent crack growth kinetics are proposed based on the rate-determining step for the crack growth. The present experimental results can be rationalized by considering multiple element processes such as aqueous mass transport and solid-state mass transport in the crack growth. The cracking mode, the temperature dependence of the crack growth rate, and the transient crack growth behavior for WR 316L SS after changing the environmental temperature are quite similar to those for a cold-worked(CW) 316L SS tested in the same environment, despite their different absolute crack growth rate values. The effect of yield strength on CGR is more significant at lower temperatures and the apparent activation energy for the crack growth rate seems to be lower in the material with a higher yield strength. Time-lag crack growth behavior was found at points during several test steps on WR 316L SS, for example, just after in situ pre-cracking and after increasing or decreasing the temperature, which is quite consistent with the results obtained with CW 316L SS. The importance of in situ monitoring of the crack growth for obtaining steady state crack growth rates is emphasized, especially for those steps for which a nonlinear crack growth period occurs after changing the test condition.

Fatigue crack growth and crack closure in an AlMgSi alloy

This study reports an experimental investigation of fatigue crack propagation in AlMgSi1‐T6 aluminium alloy using both constant and variable load amplitudes. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. For the constant amplitude tests four different stress ratios were analysed. The crack closure parameter U was calculated and related with ΔK and the stress ratio, R. The threshold of the stress intensity factor range, ΔKth , was also obtained. Fatigue crack propagation tests with single tensile peak overloads have been performed at constant load amplitude conditions. The observed transient post overload behaviour is discussed in terms of the overload ratio, ΔK baseline level and R. The crack closure parameter U trends are compared with the crack growth transients. Experimental support is given for the hypothesis that crack closure is the main factor determining the transient crack growth behaviour following overloads on AlMgSi1‐T6 alloy for plane stress conditions.

Transient crack growth behavior in aluminum alloys C415-T8 and 2519-T87

This research focused on the creep crack growth characteristics of two creep-brittle aluminum–copper alloys in sheet form, 2519-T87 (2.921 mm) and its chemical variant C415-T8 (2.286 mm), at 135 and 150°C. Transient crack growth behavior during which the creep crack growth rate does not correlate with K was observed in both aluminum alloys. By utilizing a transition time, t g, concept, the data obtained under transient crack growth conditions were separated from the quasi-steady-state crack growth data. In the quasi-steady-state regime, the creep crack growth rates were shown to best correlate with the stress intensity factor, K, for both alloys and test temperatures. The window for steady-state crack growth in creep-brittle materials is demonstrated to be very narrow, and the proposed condition for steady-state crack growth behavior is that V ̇ c / V ̇ ≤0.50 and that V ̇ c / V ̇ changes by less than 100% over a crack extension of 0.5 mm. K/ r q c is investigated as a crack tip parameter applicable to both transient and steady-state crack growth conditions; however, the experimental difficulties in determining the creep zone size were encountered and are discussed in the paper.

TRANSIENT FATIGUE CRACK GROWTH BEHAVIOUR FOLLOWING SINGLE OVERLOADS AT HIGH STRESS RATIOS

Abstract—Fatigue crack growth tests with constant amplitude loading and single overload have been performed on a long mode I crack in 2017‐T3 aluminium alloy at various stress ratios from 0 to 0.7. Two crack tip parameters of σop and σtt were evaluated using a finite element analysis for a growing crack under these loading conditions. The former is the crack opening stress and the latter is the applied stress level at which the stress at the crack tip becomes tensile. It was found that transient crack growth behaviour following single overloads at high stress ratios was significantly different from that at a low stress ratio: at higher stress ratios, following the application of the overload, there was a rapid retardation which was followed by an acceleration in growth rate and then a faster return to the steady state level at baseline loading. The experimentally observed transient post‐overload behaviour is discussed in terms of the two effective stress range ratios of Uop and Utt, which are determined from σop and σtt, respectively. For the stress ratios and overload ratios studied, the results indicate that the changes in Utt with crack extension after the overload are reasonably consistent with the crack growth rate trends. The stress distribution at minimum applied stress would account for the transient changes in Utt.

Transient subcritical crack-growth behavior in transformation-toughened ceramics

Transient subcritical crack-growth behavior following abrupt changes in the applied load are studied in transformation-toughened ceramics. A mechanics analysis is developed to model the transient nature of transformation shielding of the crack tip K s, with subcritical crack extension following the applied load change. Conditions for continued crack, growth crack growth followed by arrest, and no crack growth after the load change, are considered and related to the magnitude and sign of the applied load change and to materials properties such as the critical transformation stress. The analysis is found to provide similar trends in K s compared to values calculated from experimentally measured transformation zones in a transformation-toughened Mg-PSZ. In addition, accurate prediction of the post load-change transient crack-growth behavior is obtained using experimentally derived steady-state subcritical crack-growth relationships for cyclic fatigue in the same material.

Transient and intermittent crack growth during embrittlement of 7075-T651 aluminum by mercury

This paper presents some results which show intermittent and transient crack growth behavior of a high strength aluminum alloy embrittled by liquid mercury. Run-arrest-run type of cracking behavior resulted in ''n'' and ''m''-shaped curves on the crack velocity (da/dt) vs. stress intensity (K) diagram. While variations in oxygen activity in the crack tip environment may play a role in producing non-unique /nu/-K behavior, the results reported here suggest that formation and rupture of ligaments during crack extension may also be at work in a way that causes the stress intensity associated with the field local to the crack tip (K/sub tip/) to vary stochastically as the crack extends.

A mathematical description of transient crack growth behaviour in glass

Transient crack growth behaviour resulting from time-dependent changes in crack-tip radius can occur near the fatigue limit. In the present work, mathematical expressions describing this transient behaviour are developed assuming that a dissolution reaction is responsible for changes in crack geometry. An elliptical crack is analysed because of its mathematical simplicity. The theoretical model slightly underestimates the extent of crack-tip blunting occurring below the fatigue limit. However, the predicted transient behaviour associated with the crack-tip sharpening processes which take place above the fatigue limit compares favourably with experimental data for glass.

Influence of overloads and block loading sequences on Mode III fatigue crack propagation in A469 rotor steel

A study has been made of the influence of variable amplitude loading on Mode III (anti-plane shear) fatigue crack propagation in circumferentially-notched cylindrical specimens of ASTM A469 rotor steel (yield strength 621 MN/m 2), subjected to cyclic torsional loading. Specifically, transient crack growth behavior has been examined following spike and fully-reversed single overloads and for low-high and high-low block loading sequences, and the results compared to equivalent tests for Mode I (tensile opening) fatigue crack growth. It is found that the transient growth rate response following such loading histories is markedly different for the Mode III and Mode I cracks. Whereas Mode I cracks show a pronounced transient retardation following single overloads (in excess of 50% of the baseline stress intensity), Mode III cracks show a corresponding acceleration. Furthermore, following high-low block loading sequences, the transient velocity of Mode I cracks is found to be less than the steady-state velocity corresponding to the lower (current) load level, whereas for Mode III cracks this transient velocity is higher. Such differences are attributed to the fact that during variable amplitude loading histories. Mode III cracks are not subjected to mechanisms such as crack tip blunting/branching and fatigue crack closure, which markedly influence the behavior of Mode I cracks. The effect of arbitrary loading sequences on anti-plane shear crack extension can thus be analyzed simply in terms of the damage accumulated within the reversed plastic zones for each individual load reversal. Based on a micro-mechanical model for cyclic Mode III crack advance, where the crack is considered to propagate via a mechanism of Mode II shear (along the main crack front) of voids initiated at inclusion close to the crack tip, models relying on Coffin-Manson damage accumulation are developed which permit estimation of the cumulative damage, and hence the crack growth rates, for arbitrary loading histories. Such models are found to closely predict the experimental post-overload behavior of Mode III cracks, provided that the damage is confined to the immediate vicinity of the crack tip, a notion which is consistent with fractographic analysis of Mode III fracture surfaces.