Bearing damage evolution of a pinned joint in CFRP laminates under repeated tensile loading

A mechanical pinned joint in the CFRP laminates such as [0/±45/90]_3S, [90/±45/0]_3S, [0/±45/90]_2S and [90/±45/0]_2s is loaded statically and cyclically to finally obtain the critical condition for fatigue. It is derived that in the static loading, the critical damage that yields shear matrix crack is kink and the critical condition to the final failure is the appearance of kink in every inner 0° layer and that in the fatigue loading within the moderate load, the critical damage that yields shear matrix crack is almost always kink-like damage along the collapse front and at high load it is rather kink. Next, the non-elastic elongation of a joint at the maximum load subtracted by the one at 10th cycle is focused on and its capability is figured out for various stacking sequences. The critical value U_NE,F^* for the elongation rate change to the final fatigue failure is around 50–65 μm in the present material. The critical condition to the final fatigue failure and corresponding to U_NE,F^* is roughly the appearance of mostly kink-like damage in every inner 0° layer.     

铸造Ti46A18Nb合金拉伸过程中的损伤

研究了热等静压处理后铸态Ti46A18Nb合金拉伸过程中的损伤．结果表明：1）消除了疏松后，Ti46A18Nb合金拉伸强度的数据分散性仍然很大，表明疏松不是导致数据分散的唯一原因．在拉伸过程中不同强度试样的声发射行为有很大差异，说明其具有不同的损伤特征．断口分析和预拉伸着色实验结果证明，拉伸强度的数据分散性是合金晶粒粗大及其片层结构拉伸行为明显的各向异性所致；2）将预拉伸后样品表面去除约300μm，在随后的第二次拉伸过程中声发射Kaiser效应不再出现，说明在拉伸加载初期（低应力下）该合金的损伤主要集中于近表面．     

On the symmetric dynamic fracture of a beam under tensile loading

The dynamic fracture response of a long beam of brittle elastic material under tensile loading is studied by means of two different one-dimensional models. If the magnitude of the applied loading is increased quasi-statically to a critical value, two coplanar edge cracks are assumed to propagate across the beam's cross section. The first model parallels that of [6] with the crack length, crack speed and the loading on the fracturing section being determined as functions of time after fracture initiation. The second model is derived by means of energy considerations in the vicinity of the fracturing section. The results obtained from both models are similar except during the final phase of the fracture process.     

Crack initiation in VHCF regime on forged titanium alloy under tensile and torsion loading modes

This paper is focused on the VHCF behavior of aeronautical titanium alloy under tensile and torsion fatigue loadings. Tensile tests were carried out with two different stress ratios: R = −1 and R = 0.1. Both surface and subsurface crack initiations were observed. In the case of subsurface crack initiation several fatigue life controlling mechanisms of crack initiation were found under fully-reversed loading conditions: initiation from (1) strong defects; (2) ‘macro-zone’ borders; (3) quasi-smooth facets and (4) smooth facets. Tests with stress ratio R = 0.1, have shown that initiation from the borders of ‘macro-zones’ becomes the dominant crack initiation mechanism in presence of positive mean stress. Like for the tensile results, surface and subsurface crack initiations were observed under ultrasonic torsion in spite of the maximum shear stress location on the specimen surface. But the real reason for the subsurface crack initiation under torsion was not found.     

Fatigue life prediction under variable loading based on a new damage model

To examine the performance of nonlinear models proposed in the estimation of fatigue damage and fatigue life of components under random loading, a batch of specimens made of 6082 T 6 aluminium alloy has been studied and some of the results are reported in the present paper. The paper describes an algorithm and suggests a fatigue cumulative damage model, especially when random loading is considered. This paper contains the results of mono-axial random load fatigue tests with different mean and amplitude values performed on 6082 T 6 aluminium alloy specimens. Cycles were counted with rainflow algorithm and damage was cumulated with a new model proposed in this paper and with the Palmgren–Miner model. The proposed model has been formulated to take into account the damage evolution at different load levels and it allows the effect of the loading sequence to be included by means of a recurrence formula derived for multilevel loading, considering complex load sequences. It is concluded that a ‘damaged stress interaction damage rule’ proposed here allows a better fatigue damage prediction than the widely used Palmgren–Miner rule, and a formula derived in random fatigue could be used to predict the fatigue damage and fatigue lifetime very easily. The results obtained by the model are compared with the experimental results and those calculated by the most fatigue damage model used in fatigue (Miner’s model). The comparison shows that the proposed model, presents a good estimation of the experimental results. Moreover, the error is minimized in comparison to the Miner’s model.     

Dynamic fracture of a beam or plate under tensile loading

The dynamic fracture response of a long beam of brittle elastic material under tensile loading is studied. If the magnitude of the applied loading is increased to a critical value, a crack is assumed to propagate across the beam cross section. In a parallel analysis to [t] the crack length and applied loading at the fracture face are determined as functions of time measured from fracture initiation. The results of the analysis are shown in graphs of crack length, crack tip speed and fracturing section tensile loading vs time. As found in [1], the crack tip accelerates very quickly to a speed near the characteristic terminal speed for the material, travels at this speed through most of the beam thickness, and then decelerates rapidly in the final stage of the process. Finally, by appropriate change of the elastic modulus, the results may be applied to plane strain fracture of a plate under pure tensile loading.     

Optimization of a composite scarf repair patch under tensile loading

Mechanics of the composite repair under tensile loading with and without overlay plies was examined for nontraditional patch ply orientations. Three-dimensional nonlinear analysis was performed for repair failure prediction and good baseline comparison for open hole scarfed panels and panels repaired by using standard ply-by-ply replacement patch composition was achieved. Multidimensional optimization was performed to calculate the repair patch ply orientations which minimize the von Mises stresses in the adhesive. These optimal stacking sequences achieved significant reduction of the stress levels and resulted in predicted up to 85% and 90% strength restoration for flush and single ply thickness over-ply repair. These results are intended to illustrate additional design variables available for efficient composite repair design, namely the composition of the repair patch.     

Testing of anchorages in concrete under dynamic tensile loading

The tensile dynamic behaviour of anchors in concrete has been experimentally investigated by employing Hopkinson bar techniques. Dynamic pull-out tests with two anchor size diameters have been performed, concrete cone breakout failure has been predominantly induced, and full force-displacement curves have been obtained. The test results show that properly post-installed anchors can achieve the same load bearing capacity as the cast-in-place ones. It has also been verified that the three different types of anchors tested (with chemical adhesive, undercut, and headed studs) can develop comparable failure loads under similar embedment depths. Test results have demonstrated that force-displacement diagrams for dynamic loading tend in general to lie well above the corresponding static ones, i.e. additional ductility is available. Comparisons with predictions from design formulae have been conducted.     

Mechanical characterization by dynamical tensile loading of 2017 aluminium alloy joints welded by diffusion bonding

The mechanical properties of diffusion-welded joints of 2017 aluminium-copper alloys have been studied under dynamic loading with the help of a Hopkinson bar linear assembly. With the aim of comparing with previous results obtained under static loading, the strength, failure elongation and failure energy were evaluated in the welding temperature range from 475 to 600 °C. Measurements of the above mechanical properties were also performed on treated specimens which were base-material specimens subjected to the same thermal cycle as the welded samples. It was found that for a welding temperature above 525 °C the strength of the welded joints could reach values of the same order of magnitude as those of the treated samples. In contrast, the failure elongation and failure energy remained clearly lower, whatever the welding temperature was. For a given value of the welding pressure, the welding temperature dependence rate of the dynamic strength was shown to be increased with increasing welding time. With regard to the failure elongation or the relatively weak influence of the welding pressure on the joint strength, the spread in the experimental results (probably arising from the brittle nature of the sample failure mode) does not allow a clear conclusion. In spite of this restriction, it can be concluded that differences in the mechanical properties of the diffusion-welded joints could be evidenced by dynamic loading tests, even if no significant differences could be observed under static loading.     

A novel damage variable to characterize evolution of microstructure with plastic deformation for ductile metal materials under tensile loading

Damage and fracture of ductile metal materials are greatly associated with evolution of their microstructure under loading, which meso-dimensionally corresponds to grain deformation as well as nucleation, growth, and coalescence of microvoids in later local deformation. The evolution behavior of microstructure is important to realize ductile damage and fracture mechanism of materials. In the present work, a novel damage variable of shape factor of grains was put forward to quantitatively describe the character of microstructure; its evolution with the plastic deformation was built-up by microanalytical and mechanical experiments for Armco iron and mild steel tensile bars. The evolution rule based on the damage variable of shape factor is possible to be extended into all of ductile metal materials.     

Dynamic compressive behaviour of Ti-6Al-4V alloy processed by electron beam melting under high strain rate loading

This paper documents an investigation into the compressive deformation behaviour of electron beam melting(EBM) processing titanium alloy(Ti-6A1-4V) parts under high strain loading conditions.The dynamic compression tests were carried out at a high strain rate of over1×10~3/s using the split Hopkinson pressure bar(SHPB)test system and for comparison the quasi-static tests were performed at a low strain rate of 1×10~3/s using a numerically controlled hydraulic materials test system(MTS) testing machine at an ambient temperature.Furthermore,microstructure analysis was carried out to study the failure mechanisms on the deformed samples.The Vickers micro-hardness values of the samples were measured before and after the compression tests.The microstructures of the compressed samples were also characterized using optical microscopy.The particle size distribution and chemical composition of powder material,which might affect the mechanical properties of the specimens,were investigated.In addition,the numerical simulation using commercial explicit finite element software was employed to verify the experimental results from SHPB test system.     

Microstructure and properties of magnesium alloy processed by a new severe plastic deformation method

A new severe plastic deformation (SPD) method called C shape equal channel reciprocating extrusion (CECRE) was developed to fabricate fine grained AZ31 Mg alloys. The results show that homogeneous microstructure with mean grain size of 3.6 μm is obtained as the accumulated true strain is increased to 11. Strain localization leading to dynamic recrystallizaion (DRX) occurring is the main reason for grain refinement during CECRE process. At the same time, the hardness of AZ31 alloy increases from 62.6 of as-extruded to 74.6 of CECRE 4 passes.     

Onset of free-edge delamination in composite laminates under tensile loading

Edge delamination onset on composite laminates has been investigated for a carbon/epoxy T800/914 composite material. On the edge of laminates, out-of-plane stresses arise, even up to material's failure. Layer thickness is also known as well to influence delamination onset stress. Making use of a conventional model (that is to say assuming plies homogeneity, elastic linear behaviour, plane interlaminar surface and interface's infinite stiffness) and of a local stress tensor correction near the edge, allows, thanks to an asymptotic method, an efficient calculation of the full stress tensor. A stress criterion has then been studied. Criterion parameters assessment, from test results, has been focused on, based on conjugate gradient method and experimental thickness effect. Edge Delamination Tests have been performed on several specimens of various layups. Interlaminar shear, tension, as well as shear and tension combination have been investigated. Acoustic emission was used to detect delamination onset. As expected, these tests have exhibited layer thickness influence on onset stress. Shear parameter assessment shows good agreement between theory and experimental results. A single set of parameters is necessary to predict delamination for different layups. But experimental testing for both tensile and mixed mode has shown that failure may not be interlaminar, as expected, but intralaminar.     

A three-dimensional progressive damage model for bolted joints in composite laminates subjected to tensile loading

A three-dimensional progressive damage model was developed to simulate the damage accumulation and predict the residual strength and final failure mode of bolted composite joints under in-plane tensile loading. The parametric study included stress analysis, failure analysis and material property degradation. Stress analysis of the three-dimensional geometry was performed numerically using the finite element code ANSYS with special attention given to the detailed modelling of the area around the bolt in order to account for all damage modes. Failure analysis and degradation of material properties were implemented using a set of stress-based Hashin-type failure criteria and a set of appropriate degradation rules, respectively. In order to validate the finite element model, a comparison of stress distributions with results from analytical models found in the literature was carried out and good agreement was obtained. A parametric study was performed to examine the effect of bolt position and friction upon damage accumulation and residual strength.     

Fracture behaviour of a TiAl alloy under various loading modes

Tensile tests, compression tests, in situ tensile tests, bending tests, tensile fatigue tests and bending fatigue tests were carried out for a TiAl alloy. Based on the global experimental results and microscopic observations of the fracture surfaces and cracking behaviour on the side surfaces of tested specimens, the fracture mechanisms of fully lamellar (FL) TiAl alloys under various loading modes are summarized as following: (1) Cracks initiate at grain boundaries and/or interfaces between lamellae. (2) When a crack extends to a critical length, which matches the fracture loading stress the crack propagates catastrophically through entire specimen. (3) The crack with the critical length can be produced promptly by the applied load in the tensile and bending test or be produced step-by-step by a much lower load in the fatigue tensile test. (4) For fatigue bending tests, the fatigue crack initiates and extends directly from the notch root, then extends step-by-step with increasing the fatigue bending loads. The fatigue crack maybe extends through entire specimen at a lower fatigue load or triggers the cleavage through the whole specimen at a higher load. (5) In compressive tests, cracks initiate and propagate in directions parallel or inclined to the compressive load after producing appreciable plastic strains. The specimen can be fractured by the propagation of cracks in both directions.     

Cycles of sub-critical tensile and shear alternating fracturing in diminishing dimensions,under tensile loading

A microscopic study reveals that when the curvature of striae that mark the fracture surface of PMMA glass with a chevron pattern increase beyond the critical angle, μc = 3° ± 2°, a breakdown into alternating tensile dark zones, and bright, ragged shear zones occurs. This breakdown was repeated in primary, secondary and tertiary cycles in diminishing scales. The secondary and tertiary breakdowns occurred exclusively in the shear zones. Similar breakdowns were found in chevron patterns on the fractured surface of a silicate glass ceramics. Due however to their different properties, certain differences were identified between the two materials in their breakdown characteristics, e. g. in the glass ceramic μc = 20° ± 2°. A similar primary breakdown was also identified on tensile fractures cutting rocks in geological outcrops. In the glass ceramic the interface angle , which the striae form with the fracture boundary, decreased from 32° ± 2°in the early stage of the striae growth at relatively low velocity, to 13° ± 2° during their advanced growth, at greater velocity, demonstrating that is a good tool for monitoring the change in fracture velocity in a given material. It was found that four interconnected factors determine the geometries and breakdown styles of the chevron pattern: (1) the curvatures of the fracture front and that of the striae which intersect each other orthogonally, (2) the influence of the fracture boundaries, (3) the material properties, such as stiffness, and (4) the fracture velocity in the material.     

铸造Ti46Al8Nb合金拉伸过程中的损伤

研究了热等静压处理后铸态Ti46Al8Nb合金拉伸过程中的损伤.结果表明:1)消除了疏松后,Ti46Al8Nb合金拉伸强度的数据分散性仍然很大,表明疏松不是导致数据分散的唯一原因.在拉伸过程中不同强度试样的声发射行为有很大差异,说明其具有不同的损伤特征.断口分析和预拉伸着色实验结果证明,拉伸强度的数据分散性是合金晶粒粗大及其片层结构拉伸行为明显的各向异性所致;2)将预拉伸后样品表面去除约300 μm,在随后的第二次拉伸过程中声发射Kaiser效应不再出现,说明在拉伸加载初期(低应力下)该合金的损伤主要集中于近表面.