Threshold Stress Intensity For Crack Growth Research Articles

High-R low growth rate fatigue crack propagation at elevated temperatures

While fatigue cracking is widely considered as the primary cracking mode under cyclic loading conditions, this is not the case for high-R (Kmin/Kmax) low growth rate fatigue crack propagation at elevated temperatures. Creep cracks can sometimes be observed in such situations when the stress intensity factor range ΔK is around the fatigue crack growth threshold stress intensity factor ΔKth. A Time Dependent Failure Assessment Diagram (TDFAD) has been adopted in this study for predicting creep cracking in high-R cyclic loading conditions at elevated temperatures. In addition, attention is also paid to oxide-induced crack closure which is greatly influential at high temperatures. Three power plant steels with different chromium contents, namely 1%Cr, 9%Cr and 18%Cr steels, have been investigated at temperatures between 550°C and 650 °C. The experiments were performed by reducing ΔK continuously to ΔKth. Good accordance has been found between fracture surface observations and the TDFAD approach predictions of creep crack initiation. Apart from creep cracking, high-R ΔKth values have also been found to be influenced by oxide-induced crack closure at elevated temperatures. The enhancement is studied through the relationship between the crack tip opening displacement and stress intensity factor.

Oxidation and creep interactions during high temperature high-R fatigue crack growth threshold determination

While knowledge relating to the determination and practical application of fatigue crack growth threshold stress intensity factors (ΔKth) for defect assessment is relatively well established for many circumstances, this is not the case for materials and conditions which are sensitive to time dependent mechanisms. In particular, current testing standards do not address determination of the property for very high R (Kmin/Kmax) ratios, and those conditions when the environment can be influential and ΔKth is sensitive to the adopted da/dN(ΔK) criterion. The incidence of creep crack growth in association with very slowly growing high-R high cycle fatigue cracks is not always discernable by the direct examination of near-to-threshold da/dN(ΔK) test records, but can be predicted by means of a time dependent failure assessment diagram analysis. In addition to a general state of knowledge review, particular attention is paid to circumstances relating to high-R ΔKth in power plant steels at high temperatures for which oxide induced closure and creep cracking can be influential. Evidence for a low alloy 1%Cr steel, a martensitic 9%Cr steel and an austenitic 18%Cr steel is considered, and the practical implications are examined.

The effects of laser peening and shot peening on fretting fatigue in Ti–6Al–4V coupons

This paper describes testing of Ti–6Al–4V coupons in fretting fatigue and compares the effects of mechanical surface treatments on performance. Fretting fatigue tests were performed using a proving ring for fretting load, bridge-type fretting pads, and applied tension–tension cyclic fatigue stress. As-machined (AM), shot peened (SP), and laser peened (LP) coupons were evaluated, and data generated to compare residual stress, surface condition, lifetime, and fractographic detail encountered for each. Near-surface residual stress in SP and LP coupons was similar. The layer of compressive residual stress was far deeper in LP coupons than in SP coupons and, consequently, subsurface tensile residual stress was significantly greater in LP coupons than in SP coupons. SP coupons exhibited a rough surface and had the greatest volume of fretting-induced wear. LP coupons exhibited a wavy surface and had a small volume of wear localized at wave peaks. SP coupons had the greatest fretting fatigue lifetime, with significant improvement over AM coupons. Lifetimes of LP coupons were similar to those for SP coupons at high fatigue stress, but fell between AM and SP coupons at lower fatigue stress. Fractographic evaluation showed that fractures of AM samples were preceded by initiation of fretting-induced cracks, transition of a lead fretting crack to mode-I fatigue crack growth, and crack growth to failure. SP and LP samples exhibited behavior similar to AM samples at high fatigue stress, but in coupons tested at low stress the lead crack initiated subsurface, near the measured depth of maximum tensile residual stress, despite the presence of fretting-induced cracks. The level of fatigue stress above which lead cracks were initiated by fretting was higher for LP than for SP, and was predicted with good accuracy using an analysis based on linear elastic fracture mechanics, the fatigue crack growth threshold stress intensity factor range, and superposition of measured residual stress and applied fatigue stress.

Fatigue crack propagation along polymer-metal interfaces in microelectronic packages

In this study, a fracture mechanics-based technique was used for characterizing fatigue crack propagation (FCP) at polymer-metal interfaces. Sandwich double-cantilever beam (DCB) specimens were fabricated using nickel and copper-coated copper substrates bonded with a thin layer of silica-filled polymer encapsulant. Under cyclic loading, crack propagation was found to occur at the polymer-metal interface. The interfacial failure mode was verified by scanning electron microscopy (SEM) analysis of the fatigue fracture surfaces. The crack growth rate was found to have a power-law dependence on the strain energy release rate range, and exhibited a crack growth threshold, much like the fatigue crack growth threshold stress intensity factor range for monolithic bulk metals, polymers, and ceramics. Interfacial FCP data for three candidate encapsulants predicted cracking resistances that were well correlated with package-level reliability tests. By varying the surface roughness of the copper and nickel plating, it was shown that interfacial FCP resistance increased with increasing roughness. The observed increases in FCP resistance were attributed to a reduction in the effective driving force for fatigue fracture along the rougher interfaces, and could be accounted for by a crack-deflection model.

Mechanical behavior of cast SiC p-reinforced Al-4.5%Cu-1.5%Mg alloy

SiC p-reinforced Al-4.5%Cu-1.5%Mg composite specimens were processed by vigorous stirring of the carbide in a semi-solid alloy slurry, followed by remelting and casting (stir-casting). The tensile and fatigue properties were evaluated in the as-cast and in the heat-treated conditions. In monotonic tensile testing, reinforcement with SiC p produced a substantial increase in the work hardening of the material. This increase became more significant with increasing volume fraction of carbide. The yield and ultimate tensile strength, and the elastic modulus of the material, increased with heat-treatment and volume fraction of carbide at the expense of ductility. These properties are inferior to those of other reinforced, more complex aluminum alloys processed by other methods. In stress-controlled fatigue tests under fully reversed ( R = −1) bending conditions, the fatigue life of the composite was longer than that of the unreinforced specimen at intermediate and lower stress levels. At higher stress levels the improvement was negligible. In heat-treated reinforced alloy specimens the fatigue strength at 1 × 10 7 cycles decreased with increasing carbide particle size. With solid solution and precipitation strengthening, as well as carbide dispersion strengthening of the alloy, the crack growth threshold stress intensity factor K th, increased, as did the crack initiation time and the crack growth rate.