Effect of Reinforcement Clustering on Crack Initiation Mechanism in a Cast Hybrid Metal Matrix Composite during Low Cycle Fatigue

The reinforcement distribution of metal matrix composites (MMCs) plays an important role in low cycle fatigue. Thus, it is essential to study the effect of reinforcement clustering on the crack initiation mechanism of MMCs. In this study, the effect of reinforcement clustering on the microcrack initiation mechanism in a cast hybrid MMC reinforced with SiC particles and Al2O3 whiskers was investigated experimentally and numerically. Experimental results showed that microcracks always initiated in the particle-matrix interface, located in the cluster of the reinforcements. The interface debonding occurred in the fracture which created additional secondary microcracks due to continued fatigue cycling. The microcrack coalesced with other nearby microcracks caused the final fracture. To validate the experimental results on the microcrack initiation, three dimensional unit cell models using finite element method (FEM) were developed. The stress distribution in both the reinforcement clustering and non-clustering regions was analyzed. The numerical analysis showed that high stresses were developed on the reinforcements located in the clustering region and stress concentration occurred on the particle-matrix interface. The high volume fraction reinforced hybrid clustering region experienced greater stresses than that of the SiC particulate reinforced clustering region and low volume fraction reinforced hybrid clustering region. Besides, the stresses developed on the non-clustering region with particle-whisker series orientation were reasonably higher than that of the non-clustering region with particle-whisker parallel orientation. The high volume fraction reinforced hybrid clustering region is found to be highly vulnerable to initiate crack in cast hybrid MMC during low cycle fatigue.

INVESTIGATION OF THE LOW-CYCLE FATIGUE AND FATIGUE CRACK GROWTH BEHAVIORS OF P91 BASE METAL AND WELD JOINTS

Low cycle fatigue tests and crack growth propagations tests on P91 pipe base metal and its weld joints were conducted at three different temperatures: room temperature, 550℃ and 575℃. The strain-life was analyzed, and the changes in fatigue life behavior and fatigue growth rates with increasing temperature were discussed. The different properties of the base metal and its weld joint have been analyzed.

Very High Cycle Fatigue Behaviors and Surface Crack Growth Mechanism of Hydrogen-Embrittled AISI 304 Stainless Steels

The influence of hydrogen embrittlement on the fatigue behaviors of AISI 304 stainless steel is investigated. The fatigue endurance limits of the untreated and hydrogen-embrittled materials were almost the same at 400 MPa, and hydrogen embrittlement had little influence even though the sample contained about 8.1 times more hydrogen. Thus, the sensitivity of hydrogen gas in this material is very low. A surface crack initiation, growth, coalescence, and micro ridge model is proposed in this study. Slip line formation?⇒microcrack formation?⇒increases in the crack width, and blunting of the crack tip as it grows?⇒formation of many slip lines because of deformation in the shear direction?⇒growth of the crack in the shear direction, forming micro ridges, coalescence with adjacent cracks ⇒?continuous initiation, growth, coalescence, and ridge formation of surface cracks and specimen breakage.

Effect of long-period stacking ordered structure on very high cycle fatigue properties of Mg-Gd-Y-Zn-Zr alloys

Magnesium alloys with a long-period stacking ordered(LPSO)structure usually possess excellent static strength,but their fatigue behaviors are poorly understood.This work presents the effect of the LPSO structure on the crack behaviors of Mg alloys in a very high cycle fatigue(VHCF)regime.The LPSO lamellas lead to a facet-like cracking process along the basal planes at the crack initiation site and strongly prohibit the early crack propagation by deflecting the growth direction.The stress intensity factor at the periphery of the faceted area is much higher than the conventional LPSO-free Mg alloys,contributing higher fatigue crack propagation threshold of LPSO-containing Mg alloys.Microstructure observation at the facets reveals a layer of ultrafine grains at the fracture surface due to the cyclic contact of the crack surface,which supports the numerous cyclic pressing model describing the VHCF crack initiation behavior.

Effect of main inclusions on crack initiation in bearing steel in the very high cycle fatigue regime

This work aims to investigate the effect of main inclusions on crack initiation in bearing steel in the very high cycle fatigue（VHCF） regime. The size and type of inclusions in the steel were quantitatively analyzed, and VHCF tests were performed. Some fatigue cracks were found to be initiated in the gaps between inclusions（Al2 O3, Mg O-Al2 O3） and the matrix, while other cracks originated from the interior of inclusions（Ti N, Mn S）. To explain the related mechanism, the tessellated stresses between inclusions and the matrix were calculated and compared with the yield stress of the matrix. Results revealed that the inclusions could be classified into two types under VHCF; of these two, only one type could be regarded as holes. Findings in this research provide a better understanding of how inclusions affect the high cycle fatigue properties of bearing steel.

Effect of low cycle fatigue pre-damage on very high cycle fatigue

Carbon-manganese steel is often applied in components of pipes in nuclear plant. Ultrasonic fatigue tests following low cycle fatigue （LCF） cycles damaged are used to study the strength of very high cycle fatigure （VHCF）. The comparison of test results of simple VHCF and cumulative fatigue （LCF plus VHCF） shows that LCF load influences the following VHCF strength. Continuum damage mechanics model is extended to VHCF region.

Low cycle fatigue behavior of high strength gun steels

The low cycle fatigue (LCF) behavior of two high strength steels, withnominal chemical compositions (mass fraction, %) of 0.40C-1.5Cr-3Ni-0.4Mo-0.2V (PCrNi3MoV) and0.25C-3Cr-3Mo-0.8Ni-0.1Nb (25Cr3Mo3NiNb), was investigated by using the smooth bar specimenssubjected to strained-controlled push-pull loading. It is found that both steels show cyclicsoftening, but 25Cr3Mo3NiNb steel has a lower tendency to cyclic softening. 25Cr3Mo3NiNb steel hashigher fatigue ductility, and its transition fatigue life is almost three times that of PCrNi3MoV.25Cr3Mo3NiNb steel also shows higher LCF life either at a given total strain amplitude above 0.5% orat any given plastic strain amplitude, despite its lower monotonic tensile strength than that ofPCrNi3MoV. It also means that 25Cr3Mo3NiNb steel can endure higher total strain amplitude andplastic strain amplitude at a given number of reversals to failure within 10~4. 25Cr3Mo3NiNb steelis expected to be a good gun steel with high LCF properties because only several thousand firingsare required for gun barrel in most cases.

Effects of recrystallization on the low cycle fatigue behavior of directionally solidified superalloy DZ40M

The effects of recrystallization on low cycle fatigue behavior were investigated on directionally solidified Co-base superalloy DZ40M. Optical microscopy and SEM were used to examine the microstructure and fracture surface of the specimens. The mechanical testing results demonstrated that the low cycle fatigue property of DZ40M significantly decreased with the partial recrystallization. Fatigue cracks initiate near the carbides and the grain boundaries with slip-bands. Both the fatigue crack initiation and propagation can be accelerated with the occurrences of recrystallized grain boundaries.

A two-parameter model to predict fatigue life of high-strength steels in a very high cycle fatigue regime

In this paper, ultrasonic （20 kHz） fatigue tests were performed on specimens of a high-strength steel in very high cycle fatigue （VHCF） regime. Experimental results showed that for most tested specimens failed in a VHCF regime, a fatigue crack originated from the interior of specimen with a fish-eye pattern, which contained a fine granular area （FGA） centered by an inclusion as the crack origin. Then, a two-parameter model is proposed to predict the fatigue life of high-strength steels with fish-eye mode failure in a VHCF regime, which takes into account the inclusion size and the FGA size. The model was verified by the data of present experiments and those in the literature. Furthermore, an analytic formula was obtained for estimating the equivalent crack growth rate within the FGA. The results also indicated that the stress intensity factor range at the front of the FGA varies within a small range, which is irrespective of stress amplitude and fatigue life.

Low cycle fatigue estimation of low yield point steel

With the development of technology for earthquake resistant,the research of the low yield point(LYP) steel which used for the fabrication of energy dissipation damper were paid more and more attention.The common studies of the low yield point steel is mainly about the performance with constant amplitude and constant frequency.The low cycle fatigue properties of low yield piont steel were studied by series of test with continuous varying amplitude and varying frequency with the materials testing system by us.The test results showed that low yield point steel of Baosteel have excellent low cycle fatigue properties,which meet the requirement for steel used for the fabrication of energy dissipation damper completely.The low cycle fatigue performance of low yield point steel of Baosteel mainly depended on the amplitude in test.And the effect of varying frequency for the low yield point steel was more less than varying amplitude.

Fatigue Crack Initiation Potential from Defects in terms of Local Stress Analysis

The competition of surface and subsurface crack initiation induced failure is critical to understand very high cycle fatigue（VHCF） behavior, which necessitates the elucidation of the underlying mechanisms for the transition of crack initiation from surface to interior defects. Crack initiation potential in materials containing defects is investigated numerically by focusing on defect types, size, shape, location, and residual stress influences. Results show that the crack initiation potency is higher in case of serious property mismatching between matrix and defects, and higher strength materials are more sensitive to soft inclusions（elastic modulus lower than the matrix）. The stress localization around inclusions are correlated to interior crack initiation mechanisms in the VHCF regime such as inclusion-matrix debonding at soft inclusions and inclusion-cracking for hard inclusions（elastic modulus higher than the matrix）. It is easier to emanate cracks from the subsurface pores with the depth 0.7 times as large as their diameter. There exists an inclusion size independent region for crack incubation, outside which crack initiation will transfer from the subsurface soft inclusion to the interior larger one. As for elliptical inclusions, reducing the short-axis length can decrease the crack nucleation potential and promote the interior crack formation, whereas the long-axis length controls the site of peak stress concentration. The compressive residual stress at surface is helpful to shift crack initiation from surface to interior inclusions. Some relaxation of residual stress can not change the inherent crack initiation from interior inclusions in the VHCF regime. The work reveals the crack initiation potential and the transition among various defects under the influences of both intrinsic and extrinsic factors in the VHCF regime, and is helpful to understand the failure mechanism of materials containing defects under long-term cyclic loadings.

Masing Behavior and Microstructural Change of Quenched and Tempered High-Strength Steel Under Low Cycle Fatigue

Low cycle fatigue behavior of a quenched and tempered high-strength steel(Q960 E) was studied in the strain amplitude ranging from ± 0.5% to ± 1.2% at room temperature. As a result of fatigue loading, the dislocation structural evolution and fracture mechanism were examined and studied by transmission electron microscopy and scanning electron microscopy(SEM). The results showed that this Q960 E steel showed cyclic softening at different strain amplitudes, and the softening tendency was more apparent at strain amplitude of ±(0.6–1.2)% than that at ± 0.5%. The reduction in dislocation density with increasing strain amplitude is responsible for the softening tendency of cyclic stress with the strain amplitude. The material illustrates near-Masing behavior at strain amplitude ranging from ± 0.6% to ± 1.2%. The near-Masing behavior of Q960 E high-strength steel can be the result of stability of martensite lath at different strain amplitudes. Partial transformation from martensite laths to dislocation cells is responsible for the derivation from ideal Masing behavior. In the SEM examination of fracture surfaces, transgranular cracks initiate on the sample surface. Striations can be found during the crack propagation stage.

In vitro corrosion-fatigue behavior of biodegradable Mg/HA composite in simulated body fluid

Magnesium and its composites as biodegradable materials offer especial capabilities to be used as bio-absorbable implants.However,their poor corrosion and fatigue properties in the physiological environment can restrict their applications.In this study,corrosion-fatigue tests have been performed on the extruded magnesium and magnesium/hydroxyapatite(Mg/HA)composites in a high cycle regime.To produce the composites,pure magnesium was reinforced by 2.5 wt.%and 5 wt.%of hydroxyapatite submicron particles using an electromagneticmechanical stirring method and hot extrusion process.The experimental density measurement exhibits that the porosity increases in the extruded samples with more hydroxyapatite particles.To investigate the corrosion and corrosion-fatigue behavior of the specimens,simulated body fluid(SBF)was used during in vitro tests.The results of the potentiodynamic polarization corrosion test show that the composite with 2.5 wt.%of hydroxyapatite(Mg/2.5 wt.%HA)and the pure magnesium specimen exhibit the highest and the lowest corrosion resistance,respectively.Regarding the elemental mapping analysis of the corroded samples,this behavior could be due to the formation of strong phosphorus-calcium based layers on the composite surface.The results obtained from the mechanical tests indicate that Mg/5 wt.%HA offered the highest tensile and compressive yield strengths,as well as the most promising high cycle fatigue behavior.During the corrosionfatigue test,the simultaneous effects of fatigue and corrosion damages led to a similar corrosion-fatigue behavior in both composites.The fracture surfaces of the corrosion-fatigue samples suggest that the cracks are initiated in the corroded regions of the samples surface,which reduces the crack initiation step and subsequently decreases the fatigue life.In comparison with the pure magnesium,both composites exhibit more promising corrosion and corrosion-fatigue behaviors with a significant fatigue life improvement in the physiological environment.

Effect of laser shock peening on combined low- and high-cycle fatigue life of casting and forging turbine blades

Laser shock peening （LSP） is a novel effective surface treatment method to improve the fatigue performance of turbine blades. To study the effect of LSP on combined low- and high-cycle fatigue （CCF） life of turbine blades, the CCF tests were conducted at elevated temperatures on two types of full-scale turbine blades, which were made of K403 by casting and GH4133B by forging. Probabilistic analysis was conducted to find out the effect of LSP on fatigue life of those two kinds of blades. The results indicated that LSP extended the CCF life of both casting blades and forging blades obviously, and the effect of LSP on casting blades was more evident; besides, a threshold vibration stress existed for both casting blades and forging blades, and the CCF life tended to be extended by LSP only when the vibration stress was below the threshold vibra- tion stress. Further study of fractography was also conducted, indicating that due to the presence of compressive residual stress and refined grains induced by LSP, the crack initiation sources in LSP blades were obviously less, and the life of LSP blades was also longer; since the compressive residual stress was released by plastic deformation, LSP had no effect or adverse effect on CCF life of blade when the vibration stress of blade was above the threshold vibration stress.

Discussion on New Evaluation Technology of Non-Metallic Composite Continuous Pipe for Oil and Gas Field

In view of the serious lack and lag of the test and evaluation technology of non-metallic composite continuous pipe, and focusing on the characteristics of the application of non-metallic composite continuous pipe in oil field, this paper discusses a series of new full-scale test and evaluation technologies for accurately evaluating the product quality and practical application performance of non-metallic composite continuous pipe, which effectively solves the major technical problem that the new products of non-metallic pipe cannot be accurately evaluated. Based on the characteristics of the application of non-metallic composite continuous pipe in oil field, a series of new full-scale test evaluation technologies which can accurately evaluate the product quality and practical application performance of non-metallic pipe are designed through a large number of tests. The test and evaluation technology can accurately evaluate the key performance of high and low pressure cycle, high and low temperature cycle, gas permeability resistance, minimum bending radius etc. It provides a scientific evaluation basis for the standardized application of non-metallic continuous pipe and a reliable quality control method for the selection of products in oil field.

Investigation on high temperature strengthening and toughening of iron-base superalloy

A new modified A-286 (15Cr-28Ni-1.5Mo-lW-2Ti-Nb-Al) (mass fraction) designated as GH871 is characterized by high strengths but low ductility at 650℃ stress rupture and also high crack propagation rates at 650℃ creep and creep/fatigue interaction conditions. For improvement of ductility and crack propagation behaviour, a primary vacuum induction melting and followed electro-slag refining process (VIM+ESR) has been adopted instead of air melting and electro-slag refining process (AIM+ESR). Vacuum melted GH871 (VIM+ESR) can keep the high strength level of this alloy and improve the ductility and also decrease crack propagation rates by this alloy purification. It is a good combination of strengthening and toughening for the alloy improvement and development.

Predicting the Fatigue Life in Steel and Glass Fiber Reinforced Plastics Using Damage Models

Three cumulative damage models are examined for the case of cyclic loading of AISI 6150 steel, S2 glass fibre/epoxy and E glass fibre/epoxy composites. The Palmgren-Miner, Broutman-Sahu and Hashin-Rotem models are compared to determine which of the three gives the most accurate estimation of the fatigue life of the materials tested. In addition, comparison of the fatigue life of the materials shows the superiority of AISI 6150 steel and S2 glass fibre/epoxy at lower mean stresses, and that of steel to the composites at higher mean stresses.

30mm厚TC4钛合金电子束焊接头的高周疲劳与裂纹扩展速率

文中对比研究了30 mm厚TC4钛合金电子束焊接头和母材的高周疲劳性能,发现接头的高周疲劳性能与母材非常接近。在文中试验条件下,电子束焊接头在循环周次为2×10^(6)时的高周疲劳强度为474 MPa,达到母材的94.2%。观察电子束焊接头和母材的高周疲劳断口形貌,发现接头和母材的疲劳源均位于试样表面。疲劳裂纹扩展区都发现疲劳辉纹,相同载荷条件下,母材高周疲劳断口中辉纹的间距较小,约为1.49μm,电子束焊接头的相邻辉纹间距为2.14μm。在接头和母材的疲劳瞬断裂区还发现大量韧窝,呈现出典型的韧性断裂特征。文中还对比了母材、热影响区和焊缝区的疲劳裂纹扩展速率,发现热影响区的抗疲劳裂纹扩展性能最弱。分析认为,热影响区的高硬度是导致其抗疲劳裂纹扩展性能弱于其他区域的重要原因。

孔洞对疲劳裂纹扩展路径的影响

为了研究孔洞位置和尺寸对疲劳裂纹扩展路径的影响,利用ABAQUS有限元仿真软件建立改进CT试样的裂纹扩展仿真模型,采用扩展有限元法研究了在低周疲劳加载条件下Q235钢圆孔与裂纹尖端的角度、距离以及圆孔半径对裂纹扩展路径的影响,并通过引入裂纹偏转系数来定量描述不同参数对裂纹扩展路径偏转程度的影响。结果表明:当圆孔与裂纹尖端的夹角为45°时,裂纹偏转系数取得最大值;裂纹偏转系数随着孔洞半径的增加逐渐增大,随着圆孔与裂纹尖端距离的增大逐渐减小。