STUDY ON FATIGUE SHORT CRACK GROWTH LAW AND FATIGUE LIFE FOR MEDIUM CARBON STEELS

The fatigue crack initiation from notch root and the short-crack growth laws of two medium carbon alloying structural steels—35CrMo and 42CrMo are investigated under the different stress ratios R =0.1, 0.3) and three-point bending condition. The relationships between the maximum stress range at the notch root Δ σ max and the number of cycles before fatigue crack initiation N i are determined. The threshold stresses of fatigue crack initiation (Δ σ max ) th are got, and the smallcrack growth laws are obtained for these steels. An effective and convenient method is proposed for predicting the fatigue life of the notch specimens.

GROWTH OF SHORT CRACKS OF A537CLI STEEL UNDER FATIGUE LOADING IN 3.5% NaCl SOLUTION

The effects of environmental media,both in atmosphere and in 3.5% NaCl aqueous solution, on the growth of short cracks in comparison with the long ones have been investigated under fatigue loading on the specimens of single-edge crack about 0.05—0.15 mm for A537CLI steel.The growth rate of short cracks is faster than those of long ones around the threshoht stress intensity.The threshold stress intensity for short crack growth decreases with de- creasing crack length.The acceleration effect of the salt water,as compared with atmosphere environment,on the growth of short cracks under fatigue is much greater than that on the growth of long cracks.

Theoretical study on Manson-Coffin equation for physically short cracks and lifetime prediction

The Manson-Coffin equation has been widely applied to the prediction of fatigue lifetime.But this equation does not explicitly express the relation between the fatigue lifetime and the crack length.The present paper proposes that the grain size can be re-placed by the maximum non-damaging crack length.Thus,the growth rate will decrease to zero when the crack reaches this size.Combining with the theory of the fatigue short crack propagation,we derived the relations between physically short crack's initiation-propagation rates and material's mechanical properties,as well as crack length,stress and strain.With the derived relations,fatigue lifetime of short cracks can be successfully predicted by basic mechanical properties.Similar to the format of Manson-Coffin equation,our relations uncover the essence of Manson-Coffin equation which may reveal the short crack's initiation-propagation mechanism.Predictions of fatigue lifetime using our relations were compared with the results of well-known experiments.Good agreement is found in many aspects,such as coefficients,exponents,as well as fatigue life-times,especially for short cracks around 10 micrometers.Predictions on the short crack propagation rates are also compared for 16 types of carbon steels.Satisfactory consistency shows that our relations have wide applicability.

Effects of stick-slip on stress intensity factors for subsurface short cracks in rolling contact

This paper theoretically investigates the effects of stick-slip in roiling contact zone on stress intensity factors （SIFs） for sub- surface short cracks. New mathematical models for SIFs including stick-slip ratio are deduced in two cases. One is a subsur- face short crack parallel to surface, and the numerical analysis shows that the value of Kn increases with the increase of stick-slip ratio; the other is a subsurface short crack perpendicular to the surface, and the numerical analysis indicates that the positive value of KI decreases with the increase of stick-slip ratio. As AKI and AKI are necessary to evaluate the fa- tigue crack propagation rate or fatigue lifetime, the influences of stick-slip ratio on them are then discussed. It is found that the maximum variations of AK1 and AKu are both around 3.0% due to stick-slip ratio variation.

Density Evolution of the Surface Short Fatigue Cracks of 1Cr18Ni9Ti Pipe-Weld Metal

The evolutionary density and the scatter of densities of the short fatigue cracks on the surface of 1Cr18Ni9Ti pipe-weld metal were observed by local and overall viewpoints, respectively. The local viewpoint, which is in accordance with a so-called "effectively short fatigue crack criterion", paid attention to the dominant effective short fatigue crack (DESFC) initiation zone and the zones ahead of the DESFC tips. The overall viewpoint focused on the whole test piece of specimen. The results revealed that the density and scatter evolution exhibited a significant character of microstructural short crack and physical short crack stages. The evolutionary behavior by the local viewpoint was sensitive to the increase of DESFC size and tip location. The mechanism of the short crack growth associated with the general test observations that the DESFC acted gradually as a long crack and the scatter of DESFC growth rates tended gradually to that of a long crack was well revealed. Intrinsic causes of the random cyclic strain-life relations and stress-strain responses are appropriately given. In contrast, the evolutionary behavior by the overall viewpoint was non-sensitive and violated the general test observations. Therefore, the intrinsic localization and randomization of material evolutionary fatigue damage should be more appropriately revealed from the observations by the local viewpoint.

Vertical Short Crack Initiation in Medium Carbon Bainitic Steel Under Mild Tractive Rolling Contact

To improve the current grinding procedure of the back-up roll of CVC hot rolling mills so that the back-up roll service life can be extended, the crack initiation and propagation behavior of medium carbon bainitic back-up roll steel was investigated, a kind of asperity-scale, surface originated vertical short cracks occurred at 5 × 10^2 -1 × 10^4 cycles. Theoretical analysis indicated that the maximum tensile stress occurring at the back edge of the contact of asperities keeps at above 1 347. 97 MPa, and ratcheting and cyclic plastic deformation take place at such sites within 1 × 10^4 cycles. The early initiation of the vertical short cracks is caused by the asperity contact. According to the crack initiation mechanism, short crack behavior and preventive grinding strategy, steel consumption can be reduced considerably by decreasing the surface roughness and removing the asperity influenced surface thin layer at about 70%-80% of the surface distress life.

A STATISTICAL INVESTIGATION OF THE BEHAVIOR OF SHORT FATIGUE CRACK IN MEDIUM CARBON STEELS

Rotary bending fatigue tests were carried out by using smoothspecimens of medium car- bon steel with two ferrite grain sizes. Inorder to clarify the physical background of the scatter in fa- tiguebehavior, the statistical characteristics of the initiation andgrowth of short fatigue crack were in- vestigated. Results show thatthe short crack initiation and growth life distribution can beexpressed by two-or three-parameter Weibull distribution. The Weibulldistribution of the mixed type is well fitted to the crack lengthdistribution for both steels. The knee values of Weibull plots arerelated to the grain size.

AN INVESTIGATION OF CHEMICAL SHORT CRACK CHARACTERISTICS IN GC－4 HIGH－STRENGTH STEEL

A method of constant △K value is applied experimentally to investigatechemical short crack characteristics of GC-4 high-strength steel in 3.5%NaClsolution. Experimental results show that there is a critical point of crack geometry aon da / dN-a curve of the crack propagation at constant △K value. When the cracklength a < a , chemical short crack effect appears. The value of a is almost not af-fected by the loading frequency, stress ratio and △K level. The ratio of characteristiccrack propagation rate of chemical short cracks to that of long cracks is a weak func-tion of the loading frequency, stress ratio and △K level. Based on the analysis, the rela-tion between the short crack propagation rate and crack size is suggested.

A Statistical Study on the Initiation and Growth Lives of Short Fatigue Cracks

In order to clarify the physical background of the scatter in fatigue behaviour, rotary bending fatigue tests are carried out using smooth speciments of a medium carbon steel with two kinds of grain sizes. The statistical characteristics of short carck initiation and growth lives are investigated by a new definition of short fatigue crack initiation. Detailed analysis reveals that the distribution of short crack initiation life can be expressed by two or threeparameter Weibull distribution, and the threeparameter Weibull distribution is well fitted to the distribution of short crack growth life, and the grain size and stress level have a great influence on the statistical characteristics of crack initiation and growth life.

Influence of Rare Earth Element on Short Fatigue Crack Threshold for Al－Li Alloy 2090

Curves of △Kth vs.△ath and △Kcl·th vs.△ath for aluminum-lithium alloy 2090 have been determined and compared with those for traditional aluminum alloy 2024.The growth resistance of short fatigue cracks for aluminum-lithium alloy 2090 is quantitatively evaluated.The essential reason resulting in stronger short cracks effect has been ascertained.Influence of cerium on the growth behavior of short fatigue cracks for aluminum-lithium alloy 2090 was investigated.The results show that by adding cerium into this alloy a significant improvement in △Ki can be achieved and at the same time △Kcl·th is increased.Consequently,the essential factor resulting in lower growth resistance of short fatigue cracks for this alloy is removed and the growth resistance of long and short fatigue cracks is increased.

EXPERIMENTAL AND THEORETICAL STUDY ON NUMERICAL DENSITY EVOLUTION OF SHORT FATIGUE CRACKS

Fatigue testing was performed using a kind of triangular shaped specimen to obtain the characteristics of numerical density evolution for short cracks at the primary stage of fatigue damage. The material concerned is a structural alloy steel. The experimental results show that the numerical density of short cracks reaches the maximum value when crack length is slightly less than the average grain diameter, indicating grain boundary is the main barrier for short crack extension. Based on the experimental observations and related theory, the expressions for growth velocity and nucleation rate of short cracks have been proposed. With the solution to phase space conservation equation, the theoretical results of numerical density evolution for short cracks were obtained, which were in agreement with our experimental measurements.

SHORT FATIGUE CRACK BEHAVIOUR IN ISO-STRESS SPECIMENS

Fatigue tests were made on a vibration machine using specially designed specimens to investi- gate the initiation and propagation features of short fatigue cracks in a weld metal.The spec- imens were triangular in shape so that when loaded as a cantilever beam the surface tensile stress is constant.Test results show that short fatigue cracks mainly originate from slip bands within ferrite grains.The development of short fatigue cracks is marked by a gradual increase in crack density with fatigue cycles.Coalescence of short cracks leads to crack propagation and the crack path is predominantly transgranular.

GROWTH BEHAVIOR OF SHORT FATIGUE CRACKS FOR ALUMINUM- LITHIUM ALLOY 8090

The growth behaviors of short through cracks (0.2 < △a < 2.2mm) and long cracks are compared using CT type specimens in aluminum-lithium alloy 8090 T651. It is found that the short cracks grow much more than long ones and are observed to grow at the stress intensity ranges far below the long crack threshold. The distinction of growth bahavior between short and long cracks is attributed to the difference of their crack closure effect. The growth behavior of short cracks can be rationalized with that of long ones in terms of effective stress intensity ranges. The upper demarcation value of short through cracks for aluminum-lithium alloy 8090 is presented.

Optimization of Preventive Grinding of Backup Roll against Contact Fatigue Cracking

In order to optimize the current grinding procedure of the backup roll of 2050 continuously variable crown (CVC) mills, the behavior of rolling contact fatigue (RCF) cracking was investigated. Two RCF short cracks, including vertical short crack and ratcheting short crack initiated from ratcheting, were observed. The behavior of both RCF cracks was analyzed in detail. Then a modified grinding procedure was proposed according to the behavior of RCF cracks and the preventive grinding strategy.

Influence Mechanism of Cerium on the Threshold of Short Fatigue Crack for Aluminum-lithium Alloy 2090

The thresholds of short fatigue cracks for aluminum-lithium alloys 2090 and 2090+Ce are quantitatively evaluated. The essential reason resulting in stronger short crack effect has been ascertained. Influence of cerium on the threshold of short fatigue cracks for aluminum-lithium alloy 2090 was investigated. The results show that, by adding Ce into this alloy, DeltaK(i) and DeltaK(Cl.th) are increased. The influence mechanism of Ce on the threshold of short fatigue crack for alloy 2090 was explored fi om the bound energy, T, phase, the energy of anti-phase boundary, the energy of super-lattice intrinsic stacking fault and the electron bonds. By adding Ce into alloy 2090, the bound energy of Cu atom in this alloy is increased; the effect of thinning and dispersing T, phase is obtained; the effect of increasing the energy of anti-phase boundary and decreasing the energy of super-lattice intrinsic stacking fault for delta ' phase can be achieved.

THRESHOLD OF SHORT FATIGUE CRACK FOR Al-Li ALLOY 8090

Threshold of fatigue for short through cracks for Al-Li alloy 8090 was evaluated to be lower than that of long cracks,and to decrease with the shortening of the crack length.The correla- tion among the threshold,crack wake,and crack tip shielding of the short fatigue cracks was discussed.The super demarcation length and lowest theoretical threshold of short through fa- tigue cracks were presented.

Size Evolution of the Surface Short Fatigue Cracks of 1Cr18Ni9Ti Weld Metal

Size evolution of the surface short fatigue cracks of lCrl8Ni'9Ti weld metal was investigated. A local viewpoint is applied to be agreement with a so-called 'effectively short fatigue crack criterion'. Attention was paid to the dominant effectively short fatigue crack (DESFC) initiation zone and the zones ahead of the DESFC tips. The results revealed that the evolutionary size shows a significant character of microstructural short crack (MSC) and physical short crack (PSC) stages. In the MSC stage, fatigue damage is due to mainly the initiation and irregular growth of the effectively short fatigue cracks (ESFCs). In the PSC stage, the damage is conversely due to mainly the DESFC growth and partially, the growth of the ESFCs and the coalescence of the ESFCs themselves with the DESFC. The process involves from a non-ordered/chaotic state in the initiation of MSC stage, gradually to an independently random state at the transition point between the MSC and PSC stages and then, to an ordered/history-dependent random state. Interactive effect of the collective cracks is stronger and shows an increase in the MSC stage, it reaches a maximum value at the transition point and then, tends to a decrease in the PSC stage. The DESFC acts as a result of the interactive cracks and thus, is deemed suitable to describe the behaviour of collective cracks.

Cracking Behaviour of Al_2O_3 Short Fibre/Al-5.5Zn Composite in TEM

The micromechanical behaviour of Al2O3 short fibre reinforced Al-5.5Zn matrix composite has been in-situ studied by using TEM. The results show that in the composite the nucleation and propagation of cracks take place mainly in the matrix, as well as in the region near the fibre/matrix interface. It has been also observed that the microcracks can close during unloading.So the fracture property can be characterized by crack open displacement (COD). The cracking characteristic may result from the large strength difference between the matrix and fibre.

钢桥复杂构造疲劳裂纹跨尺度扩展与寿命预测

针对传统宏观唯像学方法无法描述钢桥疲劳损伤由微观到宏观的多尺度演化过程的问题,建立了可描述钢桥复杂构造疲劳裂纹跨尺度扩展与寿命预测方法.首先,结合微观晶体塑性本构模型,建立了以能量耗散理论为基础的微观短裂纹成核与扩展模型.同时,结合线弹性断裂力学,建立了宏观长裂纹扩展模型.在此基础上,以钢桥典型的纵肋与顶板焊接构造细节为研究对象,结合既有疲劳试验,模拟并验证了微观短裂纹的成核、扩展,以及宏观长裂纹扩展全过程.数值模拟结果与试验结果基本一致,表明所提出的疲劳裂纹跨尺度扩展与寿命预测方法对于钢桥复杂构造细节疲劳损伤演化与评估具有较好的适用性.

Nonequilibrium Statistical Modeling of Fatigue Crack Growth Behaviour

In order to study the influence of microstructural texture on the growth of short fatigue cracks in metals, the nonequilibrium statistical theory of fatigue fracture correlating a microscopic mechanism with the macroscopic properties is modified to take into consideration the microstructural features of a material, thereby allowing a rationalisation of the experimental data of short fatigue crack growth and long fatigue crack growth. The nonequilibrium statistical theory thus developed relates the growth of cracks with a dislocation mechanism to simulate short fatigue crack growth with the long fatigue crack growth behaviour and predicts the fatigue crack growth rates throughout the fatigue lifetime. The results is finally compared with that of other fatigue theories.