The Influence of Compressive Loads on Fatigue Crack Propagation in Metals

A B S T R A C T The maximum crack opening displacement is introduced to investigate the effect of compressive loads on crack opening stress in tension-compression loading cycles. Based on elastic-plastic finite element analysis of centre cracked finite plate and accounting for the effects of crack geometry size, Young's modulus, yield stress and strain hardening, the explicit expression of crack opening stress versus maximum crack opening displacement is presented. This model considers the effect of compressive loads on crack opening stress and avoids adopting fracture parameters around crack tip. Besides, it could be applied in a wide range of materials and load conditions. Further studies show that experimental results of da/dN À ΔK curves with negative stress ratios could be condensed to a single curve using this crack opening stress model.Keywords compressive load effect; crack opening stress; finite element analysis; maximum crack opening displacement; tension-compression loading. N O M E N C L A T U R Ea = initial half crack length E = Young's modulus k = slope K max = maximum stress intensity factor K op = crack opening stress intensity factor L = half-length of plate L e = the smallest element size in crack tip n = strain-hardening exponent of material R = stress ratio, R = σ min /σ max r y = crack-tip plastic zone size r p = crack-tip forward plastic zone size u max = maximum crack opening displacement corresponding to σ max (MCOD) u min = maxmum crack opening displacement corresponding to σ min W = half-width of plate ΔK eff = effective stress intensity factor range ΔK = stress intensity factor range Δu = variable quantity of maximum crack opening displacement (VMCOD) σ op = crack opening stress (COS) σ s = yield stress σ max = maximum stress in the cyclic tensile load, which is positive σ min = minimum stress in the cyclic tensile load, which is negative σ op_0 = crack opening stress when the remote applied minimum stress is zero ν = Poisson's ratioCorrespondence: Y. Huang.

Section: Finite Element Analysissupporting

confidence: 90%

A new expression for crack opening stress determined based on maximum crack opening displacement under tension–compression cyclic loading

Chen

You

Huang

2017

“…This discrepancy may be related to the fact that the mode I crack closure level and the equivalent crack surface build ups may be overestimated if p res is defined at the first emergence of CSI‐induced strains 23 ,. 24 The value of θ is close to the average of facet angles for randomly orientated polycrystals, and the value of α is within the possible range of friction angles for mild steel 25 . With the use of these acceptable coefficients, the modelling predictions and the experimental results are quite compatible.…”

Section: Discussionsupporting

confidence: 55%

Mixed‐mode crack surface interference under cyclic shear loads

Abel

2000

A previous modelling analysis predicted that crack surface interference under cyclic shear loads is a combination of cyclic shear attenuation and cyclic wedge‐opening. In the present study, experimental evidence is provided on notched thin‐walled tubular specimens to evaluate the modelling predictions. Tests were carried out with varied static tensions superimposed on fully reversed (Rτ = −1) or pulsating (Rτ = 0) cyclic shear loads. The crack surface interference was measured by near‐tip strain gauge methods. Based on the single and dual strain gauge readings, the strains induced by the mode I and II interference are separately identified so that the cyclic wedge‐opening behaviour was noted as a companion of the cyclic shear attenuation. The crack surface interference under cyclic shear loads is compared with the influence from varied static tensions and shear stress ratios. A comparison of the mode I crack surface interference is also made between the conditions with cyclic shear loads and cyclic tensile loads. Finally, the characteristics of crack surface displacements are discussed, and the experimental results of effective mode I and II stress intensity ranges are compared with the modelling predictions.

“…However, Δ K CF is not an effective SIF range promoting fatigue crack growth. As Lang and Huang 31 stated that Δ K CF was only the part of applied amplitude experienced by the crack tip, and the material around the crack tip was still under the residual compressive stress. The crack can propagate only when the stress exceeds the compressive stress.…”

Section: Differences Of Several Parameter δKeffmentioning

confidence: 99%

Examination of fatigue crack driving force parameter

Xiong

Katsuta

Kawano

et al. 2008

A B S T R A C T Most of the previous parameters that utilized as a crack driving force were established in modifying the parameter K op in Elber's effective SIF range K eff ( = K max − K op ). However, the parameters that replaced the traditional parameter K op were based on different measurements or theoretical calculations, so it is difficult to distinguish their differences. This paper focuses on the physical meaning of compliance changes caused by plastic deformation at the crack tip; the tests were carried out under different amplitude loading for structural steel. Based on these test results, differences of several parameter K eff in literature are analysed and an improved two-parameter driving force K drive ( =(K max ) n ( K ∧ ) 1−n ) has been proposed. Experimental data for several different types of materials taken from literature were used in the analyses. Presented results indicate that the K drive parameter was equally effective or better than K(=K max − K min ), K eff (=K max − K op ) and K * ( = (K max ) α ( K + ) 1−α ) in correlating and predicting the R-ratio effects on fatigue crack growth rate. BCAL = block constant amplitude loading BFS = back face strain CAL = constant amplitude loading CPLM = crack growth load measurement K * = two-parameter crack driving force k = unified two-parameter crack driving force K + = tensile part of the stress intensity range K = applied SIF range K appl = applied stress intensity range K CF = stress intensity factor range corresponding to P CF ( = P max − P CF ) K CF , P CF = stress intensity factor corresponding to P CF , crack flanks contact load K cl , P cl = closure stress intensity factor, closure load K drive = improved two-parameter crack driving force K eff,th , K th = the effective or threshold stress intensity factor range K max,appl = applied maximum stress intensity factor K max,tot = resultant maximum stress intensity factor K max , P max = maximum stress intensity factor, maximum load K min,act = actual stress intensity transmitted to the crack tip at externally applied stress 754