Incipient Crack Research Articles

Interface cracking phenomena in constrained metal layers

The deformation in thin ductile metal layers bonding elastic adherends is constrained. This constraint produces stress distribution with a large component of hydrostatic tension, such that the normal stress on the interfaces can greatly exceed the tensile flow strength of the layer material. The interaction of such stress fields with small incipient interface cracks is studied in this paper. Three models are analyzed: (a) pre-existing stationary cracks, (b) cracks which “nucleate” on a pre-loaded interface, and (c) cracks which grow along the pre-stressed interface, shielded by a thin plasticity-free region. A striking feature is a highly selective size dependence of the cracking process. A crack having a diameter roughly one half the layer thickness experiences significantly higher loading intensity than either larger or smaller cracks. This feature is related to recent experimental observations on interface debonding at thin ductile layers.

A MODEL FOR FATIGUE DEFECT NUCLEATION IN THERMITE RAIL WELDS

Abstract— A fatigue model for thermite rail welds is presented in this paper. The aim of the model is to provide a means for predicting the types of rail‐head fatigue defects that might form under a given set of railway operating conditions and to provide an estimate of the life consumed during stage I growth of incipient fatigue cracks. Eshelby's equivalent inclusion method is incorporated in the model so that an assessment of the effects of metallurgical discontinuities can be made. Findley's critical‐plane fatigue parameter is used as the criterion for computing fatigue damage. Results from using this model indicate that pores are the more damaging of the discontinuities typically present in thermite rail welds which also contain alumina inclusions. The shape and orientation of pores is predicted to have a strong influence on rail‐head fatigue defect formation in thkwermite rail welds.

Ｎｉ基超合金単結晶におけるクリープ‐疲労き裂の初生成長挙動

The fatigue and creep-fatigue crack initiation and growth behaviors of Ni-base single crystal superalloy TMS 12 were investigated at room temperature and 920°C using double edge notched specimens. At room temperature the fatigue incipient crack growth from the notch occurred by failure on the (111) plane, and the crack growth was not dependent on the number of cycles. At 920°C the crack growth behavior did not exhibit anisotropic fracture, which is caused from multiple slips. Under creep-fatigue and creep, rhombic cavities were observed at the crack tip vicinity. Alpha wolfram precipitates were observed at the sites of cavities. Cracks initiated from the interface rafted γ' and γ at alpha wolfram precipitates in (100) plane, which formed square-type cavities.

Thermal mounding in high speed dry sliders: experiment, theory and comparison

Selected results (both new and old) from our research involving severe wear via thermal mounding in dry sliders (carbon graphite on copper) is presented. Highlighted will be the comparison of theory (simulations of the evolution of thermal, stress and fracture fields within a thermal amount mound that predict contact area size, wear particle size and shape, and estimate wear rate) with experiment (wear tests that plot wear rate (micrograms per second) vs. rotor speed and recently developed photoelastic techniques that have visualized the sliding interface and permitted direct measurement of contact areas and thermal mound lifetimes). Comparison of theory with experiment was based on Bryant's particle ejection theory established for wear associated with thermal mounding on carbon graphite sliders. Contact between a slider and rotor initiates a thermal mound cycle in which extreme temperatures evolving inside the slider concentrates the contact into a thermal mound; during simulations, stresses intensify until material strengths are exceeded and stress failure creates a microcrack. Simulations thereafter follow propagation of this incipient crack; fracture mechanics crack propagation studies provide data that predict wear particle size and shape. Using Bryant's particle ejection theory, theoretically and experimentally derived thermal mound lifetimes, wear particle sizes and shapes, and contact area sizes were compared. Theory and experiment agree well.

Influence of Softened Heat-affected Zone on Stress Oriented Hydrogen Induced Cracking of a High Strength Line Pipe Steel.

The susceptibility to hydrogen induced cracking under applied stress was examined for a large diameter high strength sour service line pipe steel produced by employing the thermo-mechanical control process, i.e. controlled rolling followed by accelerated cooling. The low carbon content aimed at high toughness and weldability resulted in softening in the grain-refined and inter-critical heat-affected zone by thermal cycles of longitudinal sub-merged arc welding. The cracking susceptibility of weldment was transversely evaluated by TM0177-90 method A and compared with the parent metal.The parent metal indicated high resistance to cracking. Nonethelss, the weldment indicated the decrease in threshold stress, since the weldment ruptured preferably along the softened heat-affected zone. The cracking morphology in the softened heat-affected zone was recognized as stress oriented hydrogen induced cracking. Also performed was a large scale test to reveal the internal incipient cracks in the softened heat-affected zone. The observation confirmed that the initiation site was a small cluster of non-metallic inclusions which had probably no adverse effect on the resistance of the parent metal. He stress analysis of the weldment proved that the tension through thickness was generated at mid-thickness under the deformation constraint from surrounding harder parts. The enhanced tri-axiality of stress in company with the through thickness tension facilitates the onset of cracking caused by the hydrogen pressure mechanism. The crack morphology was consistently associated with the stress distribution.

Forging limits for an aluminum matrix composite: Part I. Experimental results

Forging limits in a discontinuously reinforced aluminum (DRA) matrix composite, 2014 Al/15 vol pct A1-2O-3, were determined by compressing samples of various cylindrical geometries under different conditions of temperature, strain rate, and lubrication and measuring the limit strains attained prior to incipient crack formation. In some cases, circumferential grids were machined on the sample surface to obtain the local fracture strain states. Crack formation was caused by the secondary tensile stresses; however, crack propagation was relatively slow and somewhat more severe at 300 °C than at 400 °C. The forging limit of the composite was found to be higher at 400 °C than at 300 °C and also higher at slower strain rates. The plane-strain forging limit of the composite at 300 °C and a strain rate of 0.5 s”1 was less than 0.05, while that of the matrix was higher than 0.5. It was found that the forging limits can be influenced by the depth of the circumferential grids and can be lower than those for the smooth surface samples.

Mixed-mode I/II fracture behaviour of an aluminium alloy

Mixed-mode fracture in aluminium 7075-T6 has been investigated by means of experimental and numerical considerations. Efforts were focused on incipient crack propagation where fractographical examinations revealed the presence of two different types of growth. In the near mode II range initiation occurred by straight, macroscopically stable, shear separation. For the remaining part, crack growth initiation took place in a similar way as in pure mode I where extensive shearing was observed to occur in a zig-zag pattern. The shift in initiation behaviour seemed to result in an increase of fracture toughness. In the numerical study where crack growth was assumed to occur as a consequence of expired work-hardening capacity, the location and orientation of incipient shear band development was investigated in a comparative manner, which indeed showed some resemblance to the experimentally observed features.

Brittle fracture initiation in low carbon steels at the ductile-brittle transition temperature region

The fracture process that determines the temperature dependence of toughness at the ductile-brittle transition was investigated for low carbon steels. It was revealed that the resistance against the stretch zone at the crack tip and the stable crack extensions is temperature independent while it depends on the carbon content. Applicability of elastic-plastic analyses of the crack tip fields was examined in terms of the relationship between J integral value normalized by the flow stress and crack opening displacement. Temperature dependence of toughness is determined primarily by the brittle fracture initiation. Locations of the initiation sites were revealed to coincide with the maximum stress triaxiality rather than the maximum tensile stress. It was suggested that the brittle fracture is caused by the deformation-induced initiation and triaxial stress assisted growth of an incipient crack.

The design philosophy of fatigue life on a cylinder wall with surface cracks

The design philosophy and the assessment method for fatigue life or fatigue-creep life on a cylinder wall with surface cracks are presented in this paper. The criterion for cyclic behaviour is dependent upon the total increment of a depth at which a crack was closed for fatigue failure or fatigue-creep failure, modulus of elasticity and the cyclic stress of an incipient crack.

Acoustic emission monitoring of incipient crack propagation and its growth rate in EN-24 steel under fatigue : Journal of Acoustic Emission, Vol. 8, Nos. 1–2, pp. 5122–5125 (Jan. - Jun. 1989). Special Supplement — Extended Summaries of Papers to be presented at the World Meeting on Acoustic Emission, Charlotte, North Carolina, USA, 20–23 Mar. 1989

Acoustic emission monitoring of incipient crack propagation and its growth rate in EN-24 steel under fatigue : Journal of Acoustic Emission, Vol. 8, Nos. 1–2, pp. 5122–5125 (Jan. - Jun. 1989). Special Supplement — Extended Summaries of Papers to be presented at the World Meeting on Acoustic Emission, Charlotte, North Carolina, USA, 20–23 Mar. 1989

Crack Growth in a ductile-phase-toughened in situ intermetallic composite under monotonic and cyclic loading

One approach to improving the ductility and toughness of brittle solids is to incorporate a ductile phase into the brittle matrix in order to impede the extension of incipient cracks. In this regard, recent word has shown that niobium can provide significant ductile-phase toughening in several intermetallic composites via crack bridging, plastic stretching and interfacial debonding mechanisms; however, under cyclic loading the role of the ductile phase appears to be less effective. Accordingly, the purpose of the current study is to examine the effect of the addition of ductile Nb phase on fracture and fatigue behavior in an equiaxed Nb/Nb[sub 3]Al in situ composite; results are compared with behavior in unreinforced Nb[sub 3]Al and Nb to determine possible mechanisms of crack propagation in Nb-toughened Nb[sub 3]Al intermetallic composites.

Reactor pressure vessel functionality issues and the development of a low temperature overpressurization transient limit for precluding crack initiation

Attainment of the design service life of the reactor pressure vessel (RPV) has become a difficult goal for some pressurized water reactors (PWRs) and consequently life extension may be precluded as an option. The difficulty has emerged due to the embrittlement of the pressure vessel steel from irradiation, and affects both reactor vessel integrity and functionality issues. Since the functional life of the reactor vessel is dependent upon the ability to adhere to the RPV pressure-temperature ( P-T) limits and the low temperature overpressure protection requirements, these normal operation constraints become quite important and control the reactor coolant system (RCS) operating window. This window is anticipated to close for many PWRs during their design life, preventing effective operation of the RCS and the RPV. To address this problem, the margin to incipient crack initiation associated with ASME Code Section III, Appendix G, fracture mechanics procedures has been evaluated and alternate brittle fracture limits for low temperature overpressure transients have been developed. These limits permit higher pressures for transient conditions than those associated with the normal operation P-T limits of Appendix G. The P-T limits for low temperature transient conditions have been developed, using the principles of linear elastic fracture mechanics. The stress intensity factors, K I have been developed through the use of a superposition technique with influence coefficients. The low temperature transient P-T limits preclude crack initiation for a range of postulated defect sizes and account for actual mechanical and thermal loadings, including those neglected in the simplified ASME Code procedures. This paper chronicles the RPV functionality issue along with the development of a low temperature overpressurization transient P-T limit for resolution of this operational problem.

Cuticle cracking in tomato fruit

The effects of fruit pruning and deleafing practices on fruit cuticle cracking were investigated in greenhouse tomato plants. Three treatments were established: (1) a standard practice of fruit pruning to three or four fruit per truss, with deleafing up to the ripening truss, (2) increased fruit pruning to one or two fruit per truss and deleafed to the ripening truss or (3) increased deleafing up to the fifth leaf above the ripening truss with fruit pruning to three or four fruit per truss. Fruit pruning substantially increased the plant leaf/ fruit ratio and significantly increased the incidence of cracking. The increased cracking with fruit pruning was not attributable to the increase in fruit size and growth rate observed in that treatment. Increased deleafing slightly reduced the plant leaf/fruit ratio and did not significantly affect cracking incidence. Further analysis of the individual treatments revealed a relationship between fruit size and cracking only in the deleafed plants. No correlations were found between the degree of cracking and the fruit relative growth rate at the time of incipient cracking, approximately six weeks after anthesis, or at two earlier times (two and four weeks after anthesis). Cracking was significantly and positively correlated with a seasonal change in weekly average irradiance in the standard and deleafed treatments. The data suggests that assimilate supply influences cracking but does not act through changes in fruit growth rate or size.

Acoustic emission during fatigue of Ti-6Al-4V: incipient fatigue crack detection limits and generalized data analysis methodology

The fundamentals associated with acoustic emission monitoring of fatigue crack initiation and propagation of Ti-6Al-4V were studied. Acoustic emission can detect and locate incipient fatigue crack extensions of approximately 10 μm. The technique therefore can serve as a sensitive warning to material failure. There are three distinct stages during which acoustic emission is generated. These stages are: crack initiation, slow crack propagation and rapid crack propagation. The distinction between the stages is based primarily on the rate of acoustic emission event accumulation. These three stages of acoustic emission correspond to the three stages of the failure process that occurs during fatigue loading. That is, changes in acoustic emission event rate correspond to changes in crack extension rate. Acoustic emission event intensities are greater during crack initiation than during slow crack propagation and greatest during rapid crack propagation. In a given fatigue cycle, event intensities increase with increasing stress and most high-intensity events occur near or at the maximum stress. Acoustic emission may therefore be used with confidence to detect, monitor and anticipate failure, in real-time.

Acoustic emission monitoring of incipient crack propagation and its growth rate in EN-24 steel under fatigue: Journal of Acoustic Emission, Vol. 8, Nos. 1–2, pp. 5122–5125 (Jan.–Jun. 1989). Special Supplement - Extended Summaries of Papers to be presented at the World Meeting on Acoustic Emission, Charlotte, North Carolina, USA, 20–23 Mar. 1989

Acoustic emission monitoring of incipient crack propagation and its growth rate in EN-24 steel under fatigue: Journal of Acoustic Emission, Vol. 8, Nos. 1–2, pp. 5122–5125 (Jan.–Jun. 1989). Special Supplement - Extended Summaries of Papers to be presented at the World Meeting on Acoustic Emission, Charlotte, North Carolina, USA, 20–23 Mar. 1989

Do We Need to Vacuum Mix or Centrifuge Cement?

In total hip surgery, the goal of porosity reduction techniques in the preparation of acrylic bone cement is to provide a stronger, more fatigue resistant material between the implant and bone. Conventional mixing of polymethylmethacrylate bone cement produces porosity of 5% to 16%, whereas vacuum mixing or centrifugation reduces the porosity to a range of 0.1% to 3.4%. Multiple studies have demonstrated that this results in a cement that has a significant increase in static and dynamic testing to failure. Fracture of the cement mantle has been found as a part of the failure pattern in many total hip prostheses requiring revision for loosening. Vacuum mixing or centrifugation produces a stronger cement to resist the component of loosening caused by fracture of the cement mantle. Where failure occurs at the bone-cement interface, as in cemented acetabular migration, no improvements from porosity reduction would be expected. Along with enhanced femoral designs, improvements in cement technique with modern methods of bone preparation and administration of the cement have resulted in a marked improvement in clinical and roentgenographic loosening rates in cemented femoral components at medium-term follow-up periods of five to ten years. Intact total hip prostheses, retrieved for reasons other than loosening, at longer-term follow-up periods, have shown intact bone-cement interfaces. However, these specimens have also shown incipient cracks in the acrylic cement that emanate from and connect defects in the cement mantle and at the metal-cement interface. The use of a void-free, structurally stronger material is expected to improve the stability and longevity of the cement supporting femoral implants.

Stability Theory of Cohesive Crack Model

A mathematical formulation is proposed to describe equilibrium and stability behavior of the cohesive‐crack model (mode 1 cracking). Under the condition that no unloading occurs, total potential energy of the system can be defined, including energy dissipation in the process zone. Displacement and crack propagation can be formally solved from the potential energy. The properties of stable crack propagation are discussed using second‐order potential‐energy variation. The slope of load‐deflection curve is determined as a function of the second‐order variations of potential energy. Particularly, the second‐order variation of the potential energy with respect to displacement is found to be critical at incipient crack instability, a feature very useful in calculating critical quantities such as peak load. Based on the proposed stability theory, an analytic model is derived for three‐point beams to demonstrate the validity and usefulness of the formulation. The predictions of the analytical model compared excellently with numerical‐simulation results. Comparison with experimental data is not attempted, since the cohesive crack model was already validated experimentally by Hillerborg (1976) and Hillerborg et al. (1983) and many other authors.

Residual stress and surface roughness in fretting fatigue

Fretting involves contact between surfaces undergoing small cyclic relative tangential motion. The resultant wear and initiation of fatigue cracks are strongly influenced by the nature of the surface. Shot-peening is an established surface treatment which produces both surface roughening and the development of compressive stresses in the surface. If the material is susceptible to work hardening it also produces surface hardening, since local plastic deformation occurs. Experiments have been devised to separate these three effects in a study on the behaviour of an Al-4Cu-1Mg age-hardening alloy. The results on the fretting - fatigue behaviour of the alloy show that the residual compressive stress has the greatest effect on retarding the propagation of fatigue cracks initiated by fretting. The present investigation is designed to assess the effect of the roughening produced by different levels of shot-peening on the fatigue and fretting fatigue of an age-hardened aluminium alloy after removal of the residual compressive surface stress. On removal of the stress by heat treatment, the surface damage resulting from shot-peening is more damaging in plain fatigue than the increase in roughness would suggest, due to the presence of incipient cracks. These are less damaging in fretting fatigue because they are nearly parallel to the surface and are closed up by pressure from the fretting pad.

Acoustic emission during fatigue of Ti-6Al-4V: Incipient fatigue crack detection limits and generalized data analysis methodology

The fundamentals associated with acoustic emission monitoring of fatigue crack initiation and propagation of Ti-6Al-4V were studied. Acoustic emission can detect and locate incipient fatigue crack extensions of approximately 10 μm. The technique therefore can serve as a sensitive warning to material failure. There are three distinct stages during which acoustic emission is generated. These stages are: crack initiation, slow crack propagation and rapid crack propagation. The distinction between the stages is based primarily on the rate of acoustic emission event accumulation. These three stages of acoustic emission correspond to the three stages of the failure process that occurs during fatigue loading. That is, changes in acoustic emission event rate correspond to changes in crack extension rate. Acoustic emission event intensities are greater during crack initiation than during slow crack propagation and greatest during rapid crack propagation. In a given fatigue cycle, event intensities increase with increasing stress and most high-intensity events occur near or at the maximum stress. Acoustic emission may therefore be used with confidence to detect, monitor and anticipate failure, in real-time.

Propagation mechanism and metallurgical characterization of first bond brittle heel cracks in AlSi wire

Bottom die heel cracks in 1.25-mil Al-1%Si wires were found to cause opens and shorts, which were attributed to brittle-looking fatigue die heel breaks. This failure mechanism has found to be directly caused by the 60-kHz ultrasonic energy vibrating the wire behind the tool during the first bond. In order to increase the frequency of these brittle cracks, hard AlSi wire was deliberately produced for this work. The frequency of the bottom cracks increased from one in a thousand to eight out of ten wires, when normal wire (19-g tensile breaking force) was replaced with hard wire (35-g tensile breaking force). The wire bonding sequence was partitioned to determine the onset of the crack and its propagation. Results show that the ultrasonic vibration during first bond produces an incipient crack that further propagates during the remainder of the wire bond cycle. Full metallographic characterization of the wire, which included optical and scanning electron microscopy, was performed in order to investigate the matrix/particle involvement in the failed specimens. The possible fracture formation and propagation characteristics are described, and the specific contribution of the AlSi alloy system is discussed. >