2024铝合金喷丸试件疲劳寿命试验及仿真研究

现有的喷丸材料疲劳性能研究扩展有限元模型没有考虑残余应力对裂纹扩展的影响。对2024铝合金的喷丸与未喷丸试样进行三弯疲劳试验,以明确喷丸工艺对试件疲劳寿命的强化作用。通过ABAQUS建立试件的二维平面应力模型,导入残余应力并利用扩展有限元法模拟循环载荷下裂纹的萌生与扩展,对比试验结果来验证该扩展有限元数值模型的正确性。最后基于该数值模型,改变载荷工况,研究不同载荷工况下残余应力对疲劳寿命的影响,得到喷丸残余应力强化作用与载荷工况的关系。结果表明:喷丸引入的残余应力可以有效地增强试件的疲劳寿命;过大的循环载荷可能造成喷丸残余应力发生松弛;在最大载荷不变的前提下,应力比越小,试件疲劳寿命越短;应力比越大,残余应力对疲劳寿命强化效果越明显。     

喷丸强化对2XXX铝合金疲劳寿命的影响

研究喷丸对2XXX铝合金拉-拉疲劳性能的影响。对未喷丸试样和喷丸强化试样的微观组织、显微硬度、残余应力和拉-拉疲劳性能进行对比分析。结果表明:喷丸处理后,试样的组织和微结构未发现明显变化,但其粗糙度、残余压应力和显微硬度有所提高,分别是未喷丸试样的6.25倍,3.85倍和1.12倍;拉-拉疲劳性能显著提高,其中值疲劳寿命是未喷丸的1.67倍。在99.9%存活率下,喷丸试样的安全寿命是未喷丸试样的1.45倍。且表面喷丸强化后疲劳裂纹源由多个变为一个。     

喷丸强化对材料旋转弯曲疲劳强度影响的定量研究

以往的工作已经提出了金属表面及内部疲劳极限的新概念，成功地分析了喷丸对三点弯曲（应力比Ｒ＝０．０５）条件下材料疲劳强度的影响。本文采用３００Ｍ钢研究了喷丸强化对旋转弯曲疲劳强度的影响。结果表明，经适当表面强化后，疲劳裂纹萌生于试样的次表层，萌生疲劳裂纹的；陆界应力（称内部疲劳极限）为未经喷丸强化试样疲劳极限（称表面疲劳极限）的１．３９倍，表明内部疲劳极限理论在旋转弯曲条件下仍然有效。     

再次喷丸周期对TC18钛合金疲劳寿命的影响

研究了再次喷丸对于经过预先喷丸的TC18钛合金残余应力和室温疲劳寿命的影响.使用X射线衍射仪和旋转弯曲疲劳机测定了再次喷丸后的合金的表面残余应力以及总疲劳寿命.结果表明,疲劳试验会使预先喷丸的TC18钛合金疲劳试样表面残余压应力松弛30％～50％,而再次喷丸可以使由于疲劳而松弛的表面残余压应力回复到疲劳试验前试样的50％～70％.此外,相比未经过再次喷丸的试样,选择合适的再次喷丸周期可使TC18钛合金的总疲劳服役寿命提高了75％.     

GH909合金喷丸强化残余应力场的研究

研究了GH909合金不同喷丸强化工艺下的残余应力场和残余应力在550℃的松驰情况，总结了喷丸强化所获得的残余应力场特性。结果表明，GH909合金经适当喷丸强化后获得的有益残余压应力场可改善其疲劳性能。     

喷丸强化对OCr13Ni8Mo2Al钢疲劳性能的影响

研究了表面喷丸强化后表面残余应力、表面粗糙度和表面层残余压应力场对0Cr13Ni8Mo2Al钢疲劳性能的影响.结果表明:0Cr13Ni8Mo2Al钢经喷丸强化后,在表面层残余压应力场的作用下疲劳裂纹源由表面被"驱赶"到表面强化层下,疲劳寿命得到显著提高.     

7A65铝合金喷丸强化表面完整性及疲劳性能

目的 以7A65高强度铝合金为研究对象,研究喷丸强度、弹丸介质(铸钢丸和陶瓷丸)对靶材疲劳性能的影响规律。方法 利用扫描电镜、激光共聚焦显微镜、X射线衍射仪等仪器表征喷丸强化7A65铝合金表面完整性和疲劳失效断口,分析喷丸工艺参数与疲劳性能、断裂模式的相关性。结果 喷丸强化后铝合金表面粗糙化严重,表面粗糙度从初始0.622 μm增加至4.736 μm(铸钢丸、喷丸强度为0.22 mmA),并出现褶皱损伤;在相同喷丸强度下陶瓷丸喷丸表面粗糙度较低,无褶皱损伤。2种弹丸在金属表面引入的残余应力场基本相同,残余压应力层深约300 μm,最大残余压应力值为-480.6 MPa,其产生位置为距离表面75 μm处(喷丸强度为0.22 mmA)。铝合金疲劳性能对铸钢丸介质敏感性较高,当喷丸强度较低(0.11 mmA)时喷丸强化效果最佳,疲劳寿命是原始寿命的5倍多,疲劳源从表面转移至次表面(500 μm);当喷丸强度增至0.22 mmA时,裂纹源向表面靠近,疲劳寿命为原始寿命的2倍。铝合金疲劳性能对陶瓷丸介质敏感性较低,在喷丸强度为0.11～0.22 mmA时疲劳寿命较为稳定,在喷丸强度为0.11 m...     

喷丸强化对膜片簧疲劳性能的影响

本文研究了膜片簧经喷丸强化和疲劳试验之后残余应力的分布以及残余应力对其疲劳强度的影响。     

孔强化对TC18钛合金疲劳寿命的影响

为提高TC18钛合金带孔零件的疲劳寿命,使用基体和焊缝上开孔的TC18钛合金试样,研究孔挤压和孔喷丸强化前后的表面残余应力,孔强化工艺对试样疲劳寿命的影响以及试样疲劳断口.研究表明,对基体和焊缝上的孔进行喷丸强化处理后,孔表面残余压应力值达到-300MPa以上,由于残余压应力和表面完整性的作用,孔喷丸强化效果比挤压强化...     

金属板料激光喷丸成形新技术

激光喷丸塑性成形金属板料是从激光强化发展起来的新技术.当激光诱导冲击波的峰值超过材料的动态屈服极限时,板料冲击区的表层发生微观塑性变形,导致了板料厚度方向上不均匀应力分布,使板料发生宏观变形.不同的激光脉冲参数在板料内部产生不同的残余应力,不同的残余应力使板料发生不同的变形,通过控制喷丸的工艺参数来控制板料内部应力场的精确分布,就可实现板料的精确变形.其显著特点是成形精确,且表层留有有益的残余压应力.     

Mechanism modelling of shot peening effect on fatigue life prediction

A new mechanism modelling is proposed in this paper to explain the shot peening effect on fatigue life predictions of mechanical components. The proposed methodology is based on the crack growth analysis of shot peened specimens, which are affected by the interaction of surface roughness and residual stress produced during the shot peening process. An asymptotic stress intensity factor solution is used to include the surface roughness effect and a time‐varying residual stress function is used to change the crack tip stress ratio during the crack propagation. Parametric studies are performed to investigate the effects of surface roughness and the residual stress relaxation rate. Following this, a simplified effective residual stress model is proposed based on the developed mechanism modelling. A wide range of experimental data is used to validate the proposed mechanism modelling. Very good agreement is observed between experimental data and model predictions.     

Laser peening and shot peening effects on fatigue life and surface roughness of friction stir welded 7075-T7351 aluminum

The effects of laser peening, shot peening, and a combination of both on the fatigue life of Friction Stir Welds (FSW) was investigated. The fatigue samples consisted of dog bone specimens and the loading was applied in a direction perpendicular to the weld direction. Several laser peening (LP) conditions with different intensities, durations, and peening orders were tested in order to obtain the optimum peening parameters. The surface roughness resulting from various peening techniques was assessed and characterized. The results indicate a significant increase in fatigue life using LP compared to shot peening when tested on their native welded specimens.     

Effect of shot peening on fatigue limit of surface flawed samples

The effects of shot peening on the fatigue limit of specimens having a semicircular notch of varied surface length, 2a , are investigated. In the case of un-peened specimens, the fatigue limit of specimens having a notch of a = 0.05 mm was equal to that of the un-notched specimens. However, the fatigue limit of a = 0.3 mm was 46% smaller than that of the un-notched specimens. On the contrary, in the case of peened specimens, the fatigue limit of a = 0.2 mm was equal to that of the un-notched specimens and furthermore, that of a = 0.3 mm was only 5% smaller than that of the un-notched specimens. Multiple non-propagating cracks were observed in peened specimens after fatigue testing. The stress intensity factor of the maximum non-propagating crack size corresponded to that of a = 0.2 mm notch. These results indicate that shot peening increases fatigue limit and decreases the likelihood that a surface flaw will result in failure.     

Influence of shot peening parameters on high‐cycle fatigue strength of steel produced by powder metallurgy process

In this study, the effect of shot peening parameters on fatigue strength of steel manufactured by powder metallurgy (PM) was investigated. Steel material obtained from Höganas ASC 100.29 in chemical composition of Fe–0.5% C–2% Cu was produced by using a single action press PM process. To determine the effect of shot peening parameters on fatigue performance, fatigue tests were performed on 20 unpeened and 80 shot‐peened samples, which were machined from sintered steel. Furthermore, shot‐peened samples were peened at different peening intensities, 100% and 200% saturation and full coverage conditions. Fatigue performance of steel, produced by PM process, was improved by surface peening process. For the studied PM steel, the best fatigue performance was obtained with the samples that were shot peened at 20 Almen intensity and 100% saturation. Fatigue strength and limit of the samples, however, were reduced after a certain cold work level. Higher intensity and saturation levels of peening process thus deteriorated the beneficial effect on fatigue strength and limit.     

Effects of laser peening on the fatigue strength and defect tolerance of aluminum alloy

The effects of laser peening (LP) on the bending fatigue strength of the 7075‐T651 aluminum alloy were investigated. Accordingly, the defect tolerance of the aluminum alloy subjected to LP is discussed on the basis of fracture mechanics. The results indicate that a deeper compressive residual stress was induced by LP compared with the case of shot peening (SP). The fatigue strengths increased when both peening types were used. Semicircular slits with depths less than 0.4 and 0.1 mm were rendered harmless on the basis of the applications of LP and SP, respectively. The apparent threshold stress intensity factor range ΔK_th,ap increased by approximately five and two times owing to LP and SP, respectively. The increase of the ΔK_th,ap was caused by the compressive residual stress induced by the peening. The Kitagawa‐Takahashi diagram of the laser‐peened specimens shows that the defect tolerance of the aluminum alloy was improved by LP.     

Recovery of fatigue life using laser peening on 2024‐T351 aluminium sheet containing scratch damage: The role of residual stress

The aim of the current work was to study the effect of laser shock peening (LSP) when applied to 2‐mm thick 2024‐T351 aluminium samples containing scratch‐like defects in the form of V‐shaped scribes 50 to 150 μm deep. The scribes decreased fatigue life to 5% of that of the pristine material. The effect of laser peening on fatigue life was dependent on the specifics of the peen treatment, ranging from further reductions in life to restoration of the fatigue life to 61% of pristine material. Fatigue life was markedly sensitive to near‐surface tensile residual stress, even if a compressive residual stress field was present at greater depth. Fatigue life after peening was also dependent on sample distortion generated during the peening process. Sample distortion modified local stresses generated by externally applied loads, producing additional life changes. Models based on residual stress intensity and crack closure concepts were successfully applied to predict fatigue life recovery.     

Technical note A study on stress, reflection and double shot peening to inrease compressive residual stress

The technique of shot peening is commonly used to increase the fatigue limit of a steel. However, there are many practical difficulties in applying it to very high HV steel and complicated components. To overcome these problems, the authors proposed two new methods: stress double shot peening and stress reflection double shot peening. Both techniques were applied to quench and tempered steel (QT steel) and induction-heated steel (IH steel). The main results were as follows: (a) by double shot peening, the compressive residual stress near the sample surface was increased considerably; (b) by stress shot peening, the maximum compressive residual stress ( σ _max ) and the surface compressive residual stress ( σ _s ) were greatly increased; (c) by stress double shot peening, very high compressive residual stresses ( σ _max = −1710 MPa and σ _s = −1320 MPa) were successfully introduced into a hard steel (HV = 700); (d) in the new method (stress reflection double shot peening), very high compressive residual stresses ( σ _max = −1760 MPa and σ _s = −1460 MPa) were successfully introduced into a hard steel (HV = 700).     

Laser shock peening on fatigue crack growth behaviour of aluminium alloy

The effect of laser shock peening (LPS) in the fatigue crack growth behaviour of a 2024‐T3 aluminium alloy with various notch geometries was investigated. LPS was performed under a ‘confined ablation mode’ using an Nd: glass laser at a laser power density of 5 GW cm^?2. A black paint coating layer and water layer was used as a sacrificial and plasma confinement layer, respectively. The shock wave propagates into the material, causing the surface layer to deform plastically, and thereby, develop a residual compressive stress at the surface. The residual compressive stress as a function of depth was measured by X‐ray diffraction technique. The fatigue crack initiation life and fatigue crack growth rates of an Al alloy with different preexisting notch configurations were characterized and compared with those of the unpeened material. The results clearly show that LSP is an effective surface treatment technique for suppressing the fatigue crack growth of Al alloys with various preexisting notch configurations.     

Influence of residual stresses from shot peening on fretting fatigue crack growth

One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental fretting life results from specimens in a Titanium alloy with and without shot peened surfaces were evaluated numerically. The residual stresses were measured at different depths below the fretting scar and compared to the corresponding residual stress profile of an unfretted surface. Thus, the amount of stress relaxation during fretting tests was estimated. Elastic–plastic finite element computations showed that stress relaxation was locally more significant than that captured in the measurements. Three different numerical fatigue crack growth models were compared. The best agreement between experimental and numerical fatigue lives for both peened and unpeened specimens was achieved with a parametric fatigue growth procedure that took into consideration the growth behaviour along the whole front of a semi‐elliptical surface crack. Furthermore, the improved fretting fatigue life from shot peening was explained by slower crack growth rates in the shallow surface layer with compressive residual stresses from shot peening. The successful life analyses hinged on three important issues: an accurate residual stress profile, a sufficiently small start crack and a valid crack growth model.     

The influence of partial surface shot peening on fatigue crack growth behaviour of a high‐strength ferritic steel

The effects of partial surface shot peening on the fatigue crack growth behaviour of a ferritic steel have been experimentally investigated in this paper. Dog‐bone specimens fabricated from Optim700QL were tested under tension‐tension fatigue loads. Three distinct extents of partial shot peening, with respect to the crack tip and specimen symmetry line, were tested. The fatigue crack growth results from these experiments have been compared with those obtained from the same specimen geometry but with no peening. The results show that the residual stress fields formed ahead of the initial notch tip due to the partial peening process play a significant role in the fatigue crack growth behaviour of the material and effectively result in accelerated crack propagation at the midwidth of the specimens. It has been shown in this study that partial peening can lead to a fatigue crack growth rate around twice as fast as that of the unpeened specimen.