Crack Propagation Phenomena Research Articles

Characteristics of dynamic fracture of a cantilever with a crack subjected to impact bending.

Subsequent to previous reports, this paper is concerned with the dynamic fracture criterion and dynamic fracture behavior of a cracked cantilever, which is subjected to a transversal impact. Experiments were performed on four kinds of specimens of different sizes. By means of the dynamic photoelastic method using a high speed image converter camera, the change of the dynamic stress intensity factor and the crack propagation phenomenon were analysed. Moreover, the dynamic finite element method was used to analyse the dynamic stress intensity factor and the dynamic fracture behavior. The results showed that there was a trend toward a larger time delay for the dynamic stress intensity factor with an increasing crack length, compared with that for the incident stress. Also, they showed that experimental stresses for initiating crack propagation could not be explained simply from the static similar laws.

Improved fatigue-life prediction by acoustic emission

Low cycle fatigue tests were conducted by tension-tension until rupture, on an Alclad 7075-T6 aluminum sheet alloy. Extremal peak Acoustic Emission amplitudes were continuously monitored. They were shown to be extremally distributed. The prediction of the number of cycles left until failure is made possible, and improved within a fair practical interval and at a good confidence level, very early in the fatigue damage accumulation process. It has been shown that only a few percent of the elastic energy associated with the crack propagation phenomena is converted into detectable acoustic energy.

Studies on the Fatigue Crack Growth under Random Loading (1st Report)

An efficient and simplified calculating method for estimation of crack opening displacement was proposed, and the method was applied to the analysis of fatigue crack closure mechanism and effective stress range ratio during crack growth process under cyclic loading.Crack opening displacement, crack closure stress and effective stress range ratio were calculated by the method and these values were compared with those obtained by Dugdale model analysis, finite-element method and various experiments, and good agreement was found among them, further more various factors affecting on the effective stress range ratio were discussed.From the above results, the proposed method is considered to be a very useful tool for the calculation of crack opening displacement, analysis of fatigue crack closure and propagation phenomenon.

The crack tip opening angle (CTOA) of the plane stress moving crack

Recently, Crack Tip Opening Angle (CTOA) was proposed by C.F. Shih et al. to describe the instability criterion of ductile crack propagation during plane strain (flat crack) conditions, and was derived by J. R. Rice analytically by means of the slip line field theory and the incremental theory of plasticity. CTOA appears to be applicable in (some or most) cases, but does not accurately describe the plane stress growing crack (slant crack). Unstable ductile crack propagation of the plane stress crack is widely studied for the safe design of highly pressurized gas pipelines. The impact absorption energy of the Charpy test is well correlated to the fracture arresting properties of the structures, but the mechanics of the fracture are not yet well established. In this paper, CTOA of the plane stress growing crack is derived from the plane stress plasticity of perfectly plastic materials by Sokolovsky's approach. Our proposed modification of CTOA expressed as follows: CTOA = (α/δ 0)( dJ/ dl) + β(δ 0/E) ln(eR/r) where β = 1.40 under the plane stress conditions. CTOA in the Dugdale model is also defined and compared with the results of laboratory test. The results show that α = 0.5, and β = 1.27 for plane stress crack growth. These analyses give similar results to those obtained by Rice et al. for CTOA under plane strain conditions, that is, α = 0.65 from the experimental results and β = 5.08 from the slip line theory. The CTOA obtained for plane stress ductile crack growth is applied to the wide plate tensile crack growth test. The results of the present analysis coincide well with those of the plane stress finite element method (FEM) computed by T. Kanazawa et al. The phenomena of plane stress ductile crack propagation are also explained by the CTOA criterion under plane stress conditions.

On the controlling parameters for fatigue-crack threshold at low homologous temperatures

Fatigue crack propagation phenomena near the threshold stress intensity level ..delta..K /SUB TH/ , has been a vigorously studied topic in recent years. Near threshold the crack propagates rather slowly, thus giving enough time for various physical and chemical reactions to take place. Room air, which is the most commonly encountered environment, can still supply various ingredients such as oxygen, water vapor (and thus hydrogen) to support these reactions. Much effort had been directed toward the environmental aspects of near threshold fatigue crack growth. By conducting tests under vacuum, Suresh and coworkers found that the crack propagation rate in a 2-1/4 Cr-1Mo steel was higher in vacuum than in air. An oxide induced closure, which served to reduce the effective stress intensity at the crack tip, seems to furnish a good explanation. Neumann and coworkers proposed that during the fatigue process, extrusion-intrusion pairs can develop as a consequence of reversed slip around the crack tip when the crack was propagated near threshold stress intensity. Beevers demonstrated that fatigue fracture surfaces contact each other during unloading even under tension-tension cycling. Kanninen and Atkinson also reached the conclusion that the compressive stress acting at the crack tip due to residual plasticity canmore » induce closure. Microstructural effects have also been cited as important factors in near threshold crack growth. It is generally accepted that coarser grains have a beneficial effect on the resistance to the near threshold crack propagation.« less

Dynamic compliance measurement?A proposed new and efficient method to investigate crack nucleation and propagation phenomena

Dynamic compliance measurement?A proposed new and efficient method to investigate crack nucleation and propagation phenomena

Wetting phenomena during alternating heating in O 2 and H 2 of supported metal crystallites

Specimens of Pd crystallites (20 to 200 Å in size) supported on thin alumina films have been heated alternately in O 2 and H 2. The changes in size, shape, and position of each crystallite after each heat treatment have been followed by examining the same areas of the specimens with a transmission electron microscope. During heating in O 2, most of the crystallites extended by spreading on the alumina surface and became irregular in shape. Pits and cavities developed on many of the crystallites. During heating in H 2, most of the crystallites which extended during the previous heating in O 2 contracted to a more circular form. Some large crystallites, however, grew in size. Some small crystallites disappeared during heating in O 2 or H 2. The extension and recontraction are explained in terms of wetting. Pit formation is explained as either a surface tension gradient driven phenomenon enhanced by a spreading-produced stress, and/or as a crack propagation phenomenon induced by the spreading-generated tensile stress and enhanced by the oxidation of the freshly exposed atoms at the tip of the crack.

Fatigue Crack Propagation Model with consideration for Crack Opening/Closing Phenomena

It is well known that fatigue cracks remain closed during part of the load cycle under constant- and variable-amplitude loading. It seems that Elber's effective stress intensity factor range produced by a crack opening stress is in successfully good agreement with experimental fatigue crack propagation behaviour under various loading including constant-, block-, and spike-loading from a huge amount of tests. Recently, J. C. Newman introduced the interesting calculation model of the crack opening stress which was based on the Dugdale model, but was modified to leave plastically-deformed material along the crack surfaces as the crack advances.However in his model, he neglected the stress re-distribution in the region of crack ahead in the calculation of crack opening load which leads to the occurrence of the re-distribution. Moreover, he also neglected the elastic deformation for the materials along the crack surfaces.In this paper, Newman's calculation model is modified to solve the above problems. By the way, from the physical point of view, the idea that crack-opening stress is a limit stress for a fatigue crack propagation proposed by Elber comes to question. We considered that a fatigue crack does not propagate if a local part in the vicinity of a crack tip remains elastic, i. e., a plastic strain does not accumulate at a crack tip. This limit loading stress is named “consuming stress Scs” which was confirmed to have nearly the same value as the crack opening stress by our calculation results. Various kind of case studies including constant-amplitude, spike and step down (for the threshold phenomenon) loading were carried out by using the model. These calculation results suggest the following phenomena.(1) Scs value increases rapidly just after fatigue crack initiates from a natural sharp defect. This result supports the phenomenon that fatigue crack can easily initiate from a sharp crack even if the length is small because of a large effective stress range.(2) Scs value reaches constant value approximately as a crack advances under constant-amplitude loading.(3) Scs value changes in accordance with the behaviour of acceleration and delayed retardation of a crack propagation after a spike over-loading.Scs(4) Phenomenon of threshold crack propagation seems to be able to be explained by the Scs value under step down loading.

The Crack Tip Opening Angle (CTOA) of the Plane Stress Moving Crack

Recently, Crack Tip Opening Angle (CTOA) was proposed by C. F. Shih et al to describe the instability criterion of ductile crack propagation during plane strain (fiat crack) condilions, and was derived by J. R. Rice analytically by means of the slip line field theory and the incremental theory of plasticity. CTOA appears to be applicable in (some or most) cases, but does not accurately describe the plane stress growing crack (slant crack).Unstable ductile crack propagation of the plane stress crack is widely studied for the safe design of highly pressurized gas pipelines. The impact absorption energy of the Charpy test is well correlated to the fracture arresting properties of the structures, but the mechanics of the fracture are not yet well established.In this paper, CTOA of the plane stress growing crack is derived from the plane stress plasticity of perfectly plastic materials by Sokolovsky's approach. Our proposed modification of CTOA expressed as follows :CTOA= (α/σ0) (dJ/dl) +β (σ0/E) ln (eR/r)where, β=1.40 under the plane stress conditions.CTOA in the Dugdale model is also defined and compared with the results of lat, oratory test. The results show that α=0.5, and β=1.27 for plane stress crack growth. These analyses give similar results to those obtained by Rice et al for CTOA under plane strain conditions, that is, α=0.65 from the experimental results and β=5.08 from the slip line theory.The CTOA obtained for plane stress ductile crack growth is applied to the wide plate tensile crack growth test. The results of the present analysis coincide well with those of the plane stress finite element method (FEM) computed by T. Kanazawa et al. The phenomena of plane stress ductile crack propagation are also explained by the CTOA criterion under plane stress conditions.

Studies on the Fatigue Crack Propagation Rate (2nd Report)

In this paper, phenomenon of fatigue crack propagation is considered as continuous fatigue crack initiation at crack tip. It is assumed that fatigue crack initiation life is used up when damage caused by cyclic strain near crack tip is accumulated and magnitude of the damage is reached to a certain critical value at ΔN strain cycles and size of the crack length Δa is average grain size of material.Using the above assumptions and proposed cumulative damage law, crack propagation rate da/dN under fully reversed loading is expressed by the following equation.da/dN=Δa/ΔN=d/ef-2e0Δeαpwhere Δep : cyclic plastic strain range at a point situated Δa apart from crack-tip ef : monotonic true fracture strain e0 : maximum strain at 1 st cycle d : average grain size α : exponent in equation Δeαp·Nc=constant The above equation can be transformed in to the following form.da/dN=d/ef-2e0 [ΔK/E√2πd {(ΔK/2σY'√2πd-1) Z- (ΔK/2σY'√2πd-1) -Z}] αwhere ΔK : range of stress intensity factor E : Young's modulus Z : (1-n) / (1+n) n : exponent in equation of cyclic stress-strain curve with strain hardening effect σY' : yield stress in the cyclic stress-strain curve Further, the above equation may be expressed by the well known power low formula approximately in the range where ΔK is 50 kg·mm-3/2500 kgmm-3/2.da/dN=C (ΔK) mC=d/ef {1/E√2πd (1/2σY'√2πd) Z} αm= (1+Z) αkg·mm-3/2There is the following relation between coefficients C and m.logC=A·m+BA=1/1+Z·log1/E√2πd (1/2σY'√2πd) ZB=log (d/ef) Crack propagation rates calculated by these equations agree with experimental ones of mild steel quite well, and then these equation may be used for estimation of crack propagation life with good accuracy.

Studies on the Fatigue Crack Propagation Rate (1 st Report)

In this paper, phenomenon of fatigue crack propagation is considered as continuous fatigue crack initiation at crack tip. It is assumed that fatigue crack initiation life is used up when damage caused by cyclic strain near crack tip is accumulated and magnitude of the damage is reached to a certain critical value at ΔN strain cycles and size of the crack length Δa is average grain size of material.Using the above assumptions and proposed cumulative damage law, crack propagation rate da/dN under fully reversed loading is expressed by the following equation.da/dN=Δa/ΔN=d/εf-2ε0Δεαpwhere Δεp : cyclic plastic strain range at a point situated Δa apart from crack-tip εf : monotonic true fracture strain ε0 : maximum strain at 1 st cycle d : average grain size α : exponent in equation Δεαp·Nc=constant The above equation can be transformed in to the following form.da/dN=d/εf-2ε0 [ΔK/E√2πd {(ΔK/2σY'√2πd-1) Z- (ΔK/2σY'√2πd-1) -Z}] αwhere ΔK : range of stress intensity factor E : Young's modulus Z : (1-n) / (1+n) n : exponent in equation of cyclic stress-strain curve with strain hardening effect σY' : yield stress in the cyclic stress-strain curve Further, the above equation may be expressed by the well known power low formula approximately in the range where ΔK is 50 kg·mm-3/2500 kgmm-3/2.da/dN=C (ΔK) mC=d/εf {1/E√2πd (1/2σY'√2πd) Z} αm= (1+Z) αkg·mm-3/2There is the following relation between coefficients C and m.logC=A·m+BA=1/1+Z·log1/E√2πd (1/2σY'√2πd) ZB=log (d/εf) Crack propagation rates calculated by these equations agree with experimental ones of mild steel quite well, and then these equation may be used for estimation of crack propagation life with good accuracy.

A Study on Fatigue Crack Propagation Rule and ΔKth

The phenomenon of ΔKth and fatigue crack propagation in the ΔKth region is investigated theoretically.When a difference between the binding energy of atoms and the strain energy of crystal lattice becomes nearly equal to that of thermal fluctuation, the separation of atoms may be regarded as a stochastic process. The ΔKth is defined by the K value which gives such stochastic condition at a crack tip. Through this concept, the author proposed that a binding energy of atoms or Young's modulus is a most influensive physical property in determining ΔKth.Moreover, standing on a view point that the crack tip opening displacement has a great influence on the crack propagation rate, he derived a second power rule for fatigue crack propagation in the region of slow fatigue crack growth. Consequently, it was concluded that the crack propagation rate is inversely proportional to the second power of Young's modulus.It is found that both theories proposed by the author yielded fair agreement with experimental results reported hitherto.

Fatigue Strength of Steel Members with Welded Details (T.R. Higgins Award)

Early studies on the fatigue of welded steel structures and components revealed the influencing factors such as stress magnitudes and geometry of structural details. Approximate design relationships were developed in the 1940s and, overall, very few problems occurred with steel bridges and structures as a result of fatigue. The fatigue cracking of coverplated steel beam bridges of the AASHO Road Test3 in the early 60s led to further examinations of the controlling factors and more extensive studies. A substantial amount of experimental data has been developed on steel beams since 1967 from a study sponsored by the National Cooperative Highway Research Program (Project 12-7) and carried out at Fritz Laboratory, Lehigh University. These studies have shown that the most important factors which govern the fatigue strength are the stress range and the type of details. For each type of welded structural detail, regardless of the strength of the steel, the applied stress range was found to cause propagation of fatigue cracks from initial discontinuities at the details. The fatigue life of a detail was nearly exhausted when the crack had propagated through the thickness of the flange plate. These findings were observed to be applicable to rolled beams, welded beams, beams with groove-welded flange splices, coverplated beams, and to beams with welded stiffeners and attachments. Comprehensive specifications based on stress range alone have been developed for bridge and building structures 6-8 utilizing these beam test data and other available information in the literature. This paper reviews very briefly the governing parameter of stress range, the experimental results on welded stiffeners and attachments, the phenomenon of crack propagation, and the data basis for the specifications. In addition, some secondary considerations which have caused fatigue cracking are examined and possible ways of retrofitting cracked structural details are suggested.

Influence of Mean Stress on Crack Closure Phenomenon and Fatigue Crack Propagation

Measurements and observations on the closure of fatigue crack initiated from notch roots in an annealed and a quenched-and-tempered 0.54% carbon steel in tension-compression fatigue tests were made successively while increasing the stress level, in order to investigate the influence of mean stress on fatigue crack propagation and crack closure phenomenon. The following are the results obtained: (1) The crack closure phenomenon in tension-compression fatigue can be recognized more clearly in the annealed steel than in the tempered steel. (2) The crack propagation rate dl/dN can be expressed as [numerical formula] where Δσeff means the effective stress range under which the crack is fully open and l is the crack length. The exponent m is about 1.6 for the annealed steel and about 3 for the tempered steel. Mean stress affects Δσeff alone, therefore, the constant C is not related to the mean stress.

Influence of Mean Stress on Fatigue Crack Propagation and Crack Closure Phenomenon

Influence of Mean Stress on Fatigue Crack Propagation and Crack Closure Phenomenon

Influence of Mean Stress Intensity on Fatigue Crack Growth in an Aluminium Alloy

The effects of the mean stress-intensity factor, Km, and the range of the stress intensity, Δ K, on crack propagation phenomena in the Al-alloy RR 58 have been studied using contoured double-cantilever beam specimens providing a constant stress-intensity factor for all crack lengths. Based on the experimental data available, a relationship of the following form, between the cyclic crack growth rate, d a/d N, and the tensile loading levels, has been proposed: where Δ K = ( Kmax - Kmin); Kmax, Kmin and Km are the maximum, minimum and mean values of the stress-intensity factor; Δ Kth is the threshold value of Δ K for crack propagation; K1C is the critical fracture toughness in plane strain conditions; A and α arc constants. In tests at room temperature (21 °C) in laboratory air and at a loading frequency of 0.15 Hz, it was found that Δ Kth decreased with increasing values of K m, α was equal to 1.36 and A equalled 3.16 times 10-5(in/cycle).

Brittle crack propagation in gas filled pipelines?A model study using thin walled unplasticised PVC pipe

The phenomenon of brittle crack propagation in gas filled pipelines which results in sinusoidal crack paths was investigated by means of laboratory tests on thin walled uPVC pipe. Measurements of true crack tip speed and pressure distribution inside the fracturing pipe combined with high speed photographic records of crack propagation have enabled a greater understanding of the mechanism which causes this type of cracking to be obtained.

New aspects of fatigue and fracture mechanics

Recent Scanning Electron Microscope observations of the development of microstructural changes under reversed cyclic straining in soft, medium-strong and high strength metals have established the existence of two dominant fatigue mechanisms associated with, respectively, the development of glide fissures by reversed plastic slip in soft metals and of slip-less shear cracking in strong metals, as well as of combinations of various complexity of both in medium-strength metals. The stage of micro-crack multiplication and growth preceding the stage of propagation of the final macro-crack and following a very short stage of formation of point-defects, the significance of which has essentially been lost between the attempts to reduce fatigue either to a lattice defect phenomenon in physics or to a single crack-propagation phenomenon in mechanics represents the principal engineering aspect of fatigue except when, as the result of the pre-existence of a dominant local defect or stress-concentration, the fatigue life is completely determined by the propagating crack. Analytical treatment of this stage for consideration in fatigue life prediction and reliability assessment must await the development of suitable methods of statistical fracture mechanics. Some aspects of this approach are discussed.