Accelerated vibratory fatigue test by ultrasonic frequency at cryogenic temperature

The fatigue test is a time-consuming experiment. The accelerated fatigue testing technique is a dream for all researchers. In our laboratory, a vibratory fatigue testing machine was built, which works at 20 kHz for R = −1 and permits to carry out tests at cryogenic temperature. This is an automatic machine controlled by a PC computer. By using this apparatus, it is easy to save time by 400 to 500 times and a lot of liquid air (nitrogen or helium).

A titanium alloy, Ti6A14V (TA6V PQ french mark), was tested in liquid nitrogen (77 K) with this machine for S---N curve. At the same time, other fatigue tests were performed in SEP (Société Européenne de Propulsion) in conventional fatigue for the same material and the same conditions except the frequency. The comparison shows that the results are coherent.     

A comprehensive study of plastic deformation mechanism of the metallic materials by ‘X-ray’ computed tomography (‘X-ray’ CT)

The study of microstructure and texture deformation of the metallic materials necessitates detailed information of physical evidence about the plastic deformation mechanism, which involves a direct relationship between mechanical properties and their behaviours under the working conditions. Generally, the mechanical properties of materials are essentially the function of their structure and their compositions. So, the study of texture deformation of mechanical parts with an efficient way in manufacturing engineering is of considerable practical interest. The present paper entails the study of the deformation mechanism in microscopic scale—in situ observation of microstructure and texture deformation by using ‘X-ray’ computed tomography (CT) Medical Scanner installed in the CNAM-Paris, Industrial Materials Laboratory, for evaluating the plastic deformation mechanism. A tomographic inner-health analysis will be presented from 2D slices of the examined parts in the laboratory scale on the as-received and heat-treated aluminium specimens.     

TiAl 合金高周弯曲疲劳研究

应用压电超声疲劳试验技术,开发20 kHz频率下的三点弯曲疲劳试验系统,完成室温下TiAl合金超高周弯曲疲劳试验.结果表明,在疲劳循环大于107周时,试样仍会发生疲劳断裂.S-N曲线显示,当应力比R=0.7时,在107周后会出现疲劳极限,表面粗糙度对疲劳性能没有明显影响.当R=0.1 及R=0.5时,在105～1010周之间疲劳强度随循环次数的增加连续下降,S-N曲线未出现水平.光学显微观察发现,TiAl合金由α2-Ti3Al 和γ-TiAl组成(γ α2钛铝合金),具有全片层状显微结构.疲劳破坏主要起源于试件表面承受应力最大处,层状晶体内层间裂纹萌生是 TiAl合金主要的裂纹萌生方式,疲劳裂纹主要以穿晶方式扩展.在超高周循环条件下, 疲劳裂纹也会从试样近表层处的铸造缩孔萌生.     

FATIGUE BEHAVIOUR OF Al2O3 SHORT FIBRE REINFORCED ALUMINIUM ALLOY

The room temperature fatigue strengths of A1-7Si-0.6Mg and A1-SSi-3Cu-1Mg composites randomly reinforced with A1₂O₃ short fibres were determined using a stress ratio of 0.1, a frequency of 50 Hz. The fatigue strength of the composites under these testing conditions can be expressed by a single curve-fitting equation. The effect of surface roughness on fatigue strength was also studied and the results show that these composites are not sensitive to surface roughness. The orthogonal growth of cracks is an important fatigue fracture mechanism in these composites. The residual strength decreased as the fatigue load increased.     

Subsurface crack initiation and propagation mechanisms in gigacycle fatigue

In the very high cycle regime (N_f > 10⁷ cycles) cracks can nucleate on inclusions, “supergrains” and pores, which leads to fish-eye propagation around the defect. The initiation from an inclusion or other defect is almost equal to the total crack growth lifetime, perhaps much more than 99% of this lifetime in many cases. Integration of the Paris law allows one to predict the number of cycles to crack initiation. A cyclic plastic zone around the crack exists, and recording the surface temperature of the sample during the test may allow one to follow crack propagation and determine the number of cycles to crack initiation. A thermo-mechanical model has been developed. In this study several fish-eyes from various materials have been observed by scanning electron microscopy, and the fractographic results analyzed as they related to the mechanical and thermo-mechanical models.     

Deformation behaviour of elastomeric matrix composites under static loading conditions

Damage mechanisms of elastomeric matrix composites (EMCs), propose a complex interplay between material properties and service conditions. The occurrence of defects such as the cavitations in EMCs specimens in using conditions is an important problem. This situation requires the well understanding of the damage mechanisms of EMCs used in automotive and aeronautical fields.Elastomeric matrix composites subjected to static and fluctuating loads basically fail due to the initiation and growth of defects (cracks, cavities, etc.). In fact, high hydrostatic pressures influence mechanical behaviours of EMCs. This paper reviews the damage mechanism of EMCs under static loading. In order to evaluate mechanical properties and fracture behaviour of EMCs, in situ observations were made by using X-rays computed tomography (CT). Two types of specimens are investigated in this work; Natural rubber, NR vulcanised and reinforced by carbon black, and synthetic rubber (styrene-butadiene-rubber), SBR. A detailed study was carried out by scanning electron microscopy (SEM) for a better understanding the damage mechanisms and confirming the CT results.     

On the giga cycle fatigue behaviour of two-phase (α2+γ) TiAl alloy

Fatigue crack initiation behaviour is investigated at room temperature in the (α₂-Ti₃Al and γ-TiAl) alloy. High cycle fatigue tests ranging up to 10¹⁰ cycles are carried out on the powder metallurgy (P/M) bar specimens under different loading conditions with a stress ratio of R=0.1 and R=0.5. Microstructural characterization and fracture surface analysis are also investigated by optical (OM) and scanning electron microscopy (SEM). Ti–Al alloy studied here shows two phases in microstructure (nearly refined lamellar thickness) composed of α₂-Ti₃Al and γ-TiAl (hereafter called γ+α₂ alloys) and fracture mechanism is explained with different plastic incompatibilities between the two phases.     

Effect of inclusion on subsurface crack initiation and gigacycle fatigue strength

The effect of inclusions on crack initiation and propagation in gigacycle fatigue was investigated experimentally and analytically in six high strength low alloy steels. Fatigue testing was performed at very high numbers of cycles through ultrasonic fatigue tests at 20 kHz. Inclusions at subsurface are common sites for fatigue crack nucleation in these alloys when cycles to failure was >10⁷ cycles. A significant change in the slope of the S–N curve was observed accompanying the transition from surface to subsurface crack initiation. A deterministic model has been developed to predict the total fatigue life, i.e. crack initiation life and crack propagation life, from the measured inclusion sizes. The predicted fatigue strength agreed reasonably well with the experimental results. It is a tendency that smaller inclusions are associated with longer fatigue life. The results demonstrated that the portions of life attributed to subsurface crack initiation between 10⁷ and 10⁹ cycles are >99%.     

There is no infinite fatigue life in metallic materials

Generally, the shape of the S – N curve beyond 107 cycles is unknown except in some statistical approaches, and this is also true for the fatigue limit. In the case of a statistical approach, the standard deviation applied to the average fatigue limit is certainly not the best way to reduce the risk of rupture in fatigue. Only the exploration of the life range between 106 and 10¹⁰ cycles will create a safer basis for modelling.
Today, some piezoelectric fatigue machines are very reliable, capable of producing 10¹⁰ cycles in less than 1 week. We based our research on accelerated fatigue tests which were performed at 20  kHz in the gigacyclic fatigue regime in order to study several typical alloys from the aeronautical and space industries.