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Fractographic analysis of tensile failures of aerospace grade composites
Kumar, Masa Suresh;Raghavendra, Kesarabandi;Venkataswamy, Magalapalaya Anjanappa;Ramachandra, Honnudike Venkateshwararao;
Materials Research , 2012, DOI: 10.1590/S1516-14392012005000141
Abstract: this paper describes fractographic features observed in aerospace composites failed under tensile loads. unidirectional carbon fibre reinforced plastic (ud cfrp) and unidirectional glass fibre reinforced plastic (ud gfrp) composite specimens were fabricated and tested in tension. the morphology of fractured surfaces was studied at various locations to identify failure mechanism and characteristic fractographic features. cfrp composites displayed transverse crack propagation and the fracture surface showed three distinct regions, viz., crack origin, propagation and final failure. significant variations in the fractographic features were noticed in crack propagation and final failure regions. crack propagation region exhibited brittle fracture with chevron lines emanating from the crack origin. the entire crack propagation region exhibited radial marks on the individual fibre broken ends. on the other hand, the final fracture region revealed longitudinal matrix splitting and radial marks in majority of locations, and chop marks at some locations. the change in fracture mode in the final fracture was attributed to superimposition of bending loads. gfrp composites exhibited broom like fracture with extensive longitudinal splitting with radial marks present on individual fibre broken ends. transverse fracture was observed at a few locations. these fracture features were analyzed and correlated with the loading conditions.
Fractographic aspects of crack branching instability using a phase field model  [PDF]
H. Henry,M. Adda-Bedia
Physics , 2014, DOI: 10.1103/PhysRevE.88.060401
Abstract: A phase field model of a crack front propagating in a three dimensional brittle material is used to study the fractographic patterns induced by the branching instability. The numerical results of this model give rise to crack surfaces that are similar to those obtained in various experimental situations. Depending on applied loading configurations and initial conditions, we show that the branching instability is either restricted to a portion of the crack front or revealed through quasi two dimensional branches. For the former, the crack front leaves on the main broken surface either aligned or disordered parabolic marks. For the latter, fractography reveals the so called {\em \'echelons} cracks showing that branching instability can also induce crack front fragmentation.
Deterministic Approach to Predicting the Fatigue Crack Growth in the 2024-T3 Aluminum Alloy Under Variable Amplitude Loading
Dorota Kocańda , Janusz Torzewski
Fatigue of Aircraft Structures , 2009, DOI: 10.2478/v10164-010-0010-1
Abstract: The paper presents the attempt to predict fatigue crack growth rate in a component subjected to variable amplitude loading containing overload-underload cycles. For this goal in a deterministic approach the modified Willenborg retardation model was applied. To provide experimental data the research into fatigue crack growth for 2024-T3 aluminum alloy sheet CCT specimens under LHL type block program loading with multiple overload-underload cycles was developed. The microfractographic analysis of fatigue fractures with the use of the transmission electron microscope (TEM) made it possible to trace the effect of block program loading on the crack growth rate. The knowledge of the affection of a particular overload-underload cycle or a block of these cycles on crack rate on the basis of microfractographic analysis was utilized for assessing the equivalent loading for the LHL block program. The diagrams that presented the crack growth rate both on the surface and inside the aluminum alloy sheet was performed. The crack growth rate inside the sheet was estimated on the basis of the striation spacing analysis.
Tests for corrosion crack propagation behavior of aeronautical aluminum alloys

- , 2017, DOI: 10.13700/j.bh.1001-5965.2016.0360
Abstract: 摘要 腐蚀环境下的裂纹扩展性能是航空金属结构损伤容限设计的重要前提,为此,试验测定了3种航空铝合金材料(即2E12-T3、2E12-T42和7050-T7451)在2种腐蚀环境(3.5wt% NaCl溶液和油箱积水)下的裂纹扩展性能,在试验数据的基础上进行性能对比,并对试样断口进行SEM分析,研究了腐蚀和载荷联合作用对裂纹扩展的影响机理,研究结果表明:油箱积水环境对航空铝合金材料裂纹扩展的影响比3.5wt% NaCl溶液严重,铝合金2E12-T3和2E12-T42的腐蚀裂纹扩展性能优于铝合金7050-T7451,腐蚀环境下的氢脆效应和阳极溶解机制是造成腐蚀裂纹扩展加速的主要原因。
Abstract:Fatigue crack propagation behavior in typical corrosion environments is the precondition of damage tolerance design for metallic structures in aircraft; therefore, in order to determine corrosion fatigue crack propagation behavior, fatigue tests were performed on three categories of aluminum alloys (i.e., 2E12-T3, 2E12-T42 and 7050-T7451) in two kinds of corrosion environments (3.5wt% NaCl solution and fuel tank ponding) under constant amplitude loading. Corrosion fatigue crack propagation properties in different corrosion environments were analyzed and compared with each other, and the interaction mechanisms between corrosion and fatigue were deduced from fractographic studies by using SEM analysis. It is showed that the effect of fuel tank ponding on fatigue crack propagation behavior is more significant than that of 3.5wt% NaCl solution, and aluminum alloys 2E12-T3 and 2E12-T42 hold superiority over aluminum alloy 7050-T7451 in corrosion fatigue crack propagation behavior. Corrosion crack propagation is enhanced due to the hydrogen embrittlement effect and anodic dissolution mechanism.
Temperature effect on crack propagation properties of aluminum alloys in aircraft

- , 2017, DOI: 10.13700/j.bh.1001-5965.2016.0244
Abstract: 摘要 高低温裂纹扩展性能是航空金属结构损伤容限设计的前提,为此,试验测定了3种系列的6种航空铝合金材料(2024-T351、2397-T8、6061-T651、7050-T7451、7050-T7452和7475-T761)在5种温度环境(-70、-54、25、125 和150℃)下的裂纹扩展性能,观测了试验现象,并进行了性能对比分析和疲劳断口扫描电子显微镜(SEM)分析,研究了温度对航空铝合金材料裂纹扩展性能的影响机制,获得了具有工程参考价值的结果与结论:与25℃相比,低温下裂纹扩展阻力系数的对数值降低7%~15%,而高温下却增大5%~23%;低温下裂纹扩展指数增大7%~21%,而高温下却减少5%~34%;氢脆效应和高温氧化作用是导致裂纹扩展速率随温度升高而加快的主要原因。
Abstract:Fatigue properties on crack propagation rate at low and elevated temperatures are the precondition of damage tolerance design for aviation metallic structures. Therefore, in order to determine fatigue properties on crack propagation rate, fatigue tests were performed on six categories of aluminum alloys (i.e. 2024-T351, 2397-T8, 6061-T651, 7050-T7451, 7050-T7452, 7475-T761) at five kinds of temperatures (-70℃, -54℃, 25℃, 125℃, 150℃) under constant amplitude loading, and fatigue crack propagation properties were determined and the comparisons between fatigue properties on crack propagation rate at different temperatures were carried out. The mechanisms of temperature effect on crack propagation rate were deduced from fractographic studies by using scanning election microscope (SEM). The results show that,compared with the situation at 25℃, logarithmic crack propagation resistance coefficient decreases by 7% to 15% at low temperature, but increases by 5% to 23% at elevated temperature; crack propagation exponent increases by 7% to 21% at low temperature, but decreases by 5% to 34% at elevated temperature, compared with the situation at 25℃; hydrogen embrittlement and oxidation effects are likely to be the main reasons for more rapid crack propagation with the increasing temperature.
On the dependency of the parameters of fatigue crack growth from the fractal dimension of rough crack profiles  [PDF]
Marco Paggi,Oleg Plekhov
Physics , 2014, DOI: 10.1177/0954406213515643
Abstract: A theoretical study based on dimensional analysis and fractal geometry of crack profiles is proposed to establish the relation between their fractal dimension D (1

OUYANG Hui,WANG Yansheng,

金属学报 , 1983,
Abstract: The studies were carried out of the test of fatigue crack growth rate and relative to the microscopic morphology of fracture surface in certain high-strength steels. It was found that the results measured by both macro-and microscopic examinations are identical. The fatigue striation of the subcritical crack propagation is quantitatively related to the mean stress. In the same range of stress intensity factors the crack growth rate increases with the mean stress. The fractographic observations of the fatigue crack propagation on specimens of 18CrMn2MoBA steel reveal that the microstructural particle phase constituent with martensite and austenite can be responsible for the reduction of the fatigue crack growth rate.
Interaction of an interfacial crack with linear micro-defects under out-of-plane shear loading  [PDF]
Gennady Mishuris,Alexander Movchan,Natasha Movchan,Andrea Piccolroaz
Physics , 2010,
Abstract: The interaction of an interfacial crack with small impurities is analysed on the basis of an asymptotic formula derived by the authors. The interaction between the main crack and the defects (e.g. small cracks or inclusions) is described asymptotically by analysing the dipole fields and the corresponding dipole matrices of the defects in question. The method is generic, and it serves interfacial cracks with general distributed loading on the crack faces, taking into account possible asymmetry in the boundary conditions, and in a particular configuration for a crack in a homogeneous medium results agree with those obtained earlier by Gong (1995). Shielding and amplification effects of the defects on the propagation of the main crack along the interface are investigated. Numerical computations based on the explicit analytical formulae show potential applications in the design of composite and fiber reinforced materials.
Simulation of Fatigue Crack Growth in Integrally Stiffened Panels Under the Constant Amplitude and Spectrum Loadin
Petr Augustin
Fatigue of Aircraft Structures , 2009, DOI: 10.2478/v10164-010-0001-2
Abstract: The paper describes methodology of numerical simulation of fatigue crack growth and its application on integrally stiffened panels made of 2024-T351 aluminium alloy using high speed cutting technique. Presented approach for crack growth simulation starts by the calculation of stress intensity factor function from finite element results obtained using MSC. Patran/Nastran. Subsequent crack growth analysis is done in NASGRO and uses description of crack growth rates either by the Forman-Newman-de Koning relationship or by the table lookup form. Three crack growth models were applied for spectrum loading: non-interaction, Willenborg and Strip Yield model. Relatively large experimental program comprising both the constant amplitude and spectrum tests on integral panels and CCT specimens was undertaken at the Institute of Aerospace Engineering laboratory in order to acquire crack growth rate data and enable verification of simulations. First analyses and verification tests of panels were performed under the constant amplitude loading. For predictions of crack growth using the spectrum loading a load sequence representing service loading of the transport airplane wing was prepared. Applied load spectrum was measured on B737 airplane within the joint FAA/NASA collection program. The load sequence is composed of 10 flight types with different severity analogous to the standardized load sequence TWIST. Before application on the stiffened panels a calculation of crack growth under the spectrum loading was performed for simple CCT specimen geometry. The paper finally presents comparison of simulations of fatigue crack propagation in two-stringer stiffened panel under the spectrum loading with verification test carried out in the IAE lab. The work was performed within the scope of the 6th Framework Programme project DaToN - Innovative Fatigue and Damage Tolerance Methods for the Application of New Structural Concepts.
Crack Initiation and Propagation of Bonded Metallic Joints under Mixed-Mode Cyclic Loading
TA Hafiz
International Journal of Engineering Innovations and Research , 2013,
Abstract: Crack Initiation and Propagation of Bonded Metallic Joints under Mixed-Mode Cyclic LoadingDouble Cantilever Beam (DCB) specimens have been used to carry out Fatigue tests to investigate mixed mode crack growth behaviour in bonded joints. Equal thickness mild steel DCB substrates bonded with FM73 adhesive were used. The bonded joints were tested under pure mode I and a range of fatigue mixed-mode conditions using a relatively simple, variable-mode loading fixture as described in [1and 2]. The fatigue testing was carried out at a load ratio (R) of 0.1, in displacement control such that the initial maximum fatigue load was 70 0of the corresponding quasi-static fracture load. The fatigue load decreased as the fatigue crack grew and this load was recorded. Crack growth was monitored and measured using a video microscope. The strain energy release rate components were determined using corresponding values of fatigue crack length and fatigue load. The fatigue crack growth rates were characterised using a Paris Law approach and a significant influence of mode II component on the crack growth rate has been observed.
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