Quasi-brittle fracture during structural impact of AA7075-T651 aluminium plates

The stress–strain behaviour of the aluminium alloy 7075 in T651 temper is characterized by tension and compression tests. The material was delivered as rolled plates of thickness 20 mm. Quasi-static tension tests are carried out in three in-plane directions to characterize the plastic anisotropy of the material, while the quasi-static compression tests are done in the through-thickness direction. Dynamic tensile tests are performed in a split Hopkinson tension bar to evaluate the strain-rate sensitivity of the material. Notched tensile tests are conducted to study the influence of stress triaxiality on the ductility of the material. Based on the material tests, a thermoelastic–thermoviscoplastic constitutive model and a ductile fracture criterion are determined for AA7075-T651. Plate impact tests using 20 mm diameter, 197 g mass hardened steel projectiles with blunt and ogival nose shapes are carried out in a compressed gas-gun to reveal the alloy's resistance to ballistic impact, and both the ballistic limit velocities and the initial versus residual velocity curves are obtained. It is found that the alloy is rather brittle during impact, and severe fragmentation and delamination of the target in the impact zone are detected. All impact tests are analysed using the explicit solver of the non-linear finite element code LS-DYNA. Simulations are run with both axisymmetric and solid elements. The failure modes are seen to be reasonably well captured in the simulations, while some deviations occur between the numerical and experimental ballistic limit velocities. The latter is ascribed to the observed fragmentation and delamination of the target which are difficult to model accurately in the finite element simulations.     

Failure of aluminium self-piercing rivets: An experimental and numerical study

The present paper investigates the fracture mechanisms of AA7278-T6 aluminium self-piercing rivets under compression during the riveting process. First, a microstructure investigation was conducted to disclose the grain structure and the particle distribution of the extruded aluminium alloy. Transmission electron micrographs revealed precipitate free zones along grain boundaries. Uniaxial tensile tests in three different directions with respect to the extrusion direction revealed anisotropy of the alloy in strength and ductility and a change in fracture mode with tensile direction. The behaviour of the alloy under compression was studied experimentally using upsetting tests and self-piercing riveting tests. Micrographs of the deformed specimens provided insight into the influence of the microstructure on the deformation and fracture of the alloy under compression. Second, numerical analyses were carried out using a 2-D axisymmetric model in LS–DYNA in an attempt to investigate the role of different physical variables on the final failure of the rivet. The numerical results revealed that constituent particles, precipitate free zones, and friction between the rivet and plates are important for strain localisation and fracture in the rivet.     

Influence of the thermomechanical processing on the fracture mechanisms of high strength aluminium/pure aluminium multilayer laminate materials

The microstructure and mechanical properties, with emphasis in the impact fracture toughness behaviour, of two multilayer laminate materials have been investigated. The multilayer materials are constituted by alternated sheets of pure aluminium (Al 1200 or Al 1050) and high strength Al 7075 alloy. Stacked layers of these alloys have been successfully joined using two processing routes with different total hot rolling strains. Both laminates have been tested at room temperature under impact Charpy tests, three-point bend tests and shear tests on the interfaces. Both laminates exhibited more than eight times improvement in impact fracture toughness over the monolithic Al 7075-T6. The toughness increase in the higher rolling strained laminate is almost entirely due to crack blunting mechanism, while in the lower strained laminate, crack deflection by delamination and crack renucleation processes were active.     

Investigation of fracture mechanism of 6063 aluminum alloy under different stress states

Fracture mechanisms in a 6063 aluminum alloy were investigated and analyzed carefully by in-situ tensile tests in SEM with a vacuum chamber. Specimens used were designed to produce different stress states. Studies indicated that with stress triaxiality (σ _m/σ _e) decreasing, the fracture modes changed from normal fracture to shear fracture and the fracture surfaces changed from the dimples and intragranular dominated fracture mode to the shear dominated fracture mode. The grain boundaries of the 6063 aluminum alloy were the weakest positions. In the case of high stress triaxiality, the grain boundary cracks were produced by normal stress or by the incompatibility of deformation between neighboring grains, and the normal stress dominated the crack propagation. In the case of low stress triaxiality, the boundary cracks were produced by the relative slipping of grains against neighboring grains, and the shear stress dominated the crack propagation. The final fracture of the specimens occurred by connections of cracks through transgranular cracking of the ligaments among these cracks.     

Dynamic Tensile Fracture Behaviours of Selected Aluminum Alloys under Various Loading Conditions

Experiments investigating dynamic tensile fracture were performed on the extruded rods of 2024‐T4 and 7075‐T6 aluminum alloys under varying loading conditions. The initial yield stress and fracture strain of 7075‐T6 alloy obtained in spilt Hopkinson tension bar tests are higher than that of 2024‐T4 alloy. But the initiation fracture toughness and spall strength of 2024‐T4 alloy are higher than those of 7075‐T6 alloy in three‐point bending and plate impact experiments, which indicates that 2024‐T4 alloy has better crack initiation tolerance and stronger spall failure resistance. Based on metallurgical investigations by using optical and scanning electron microscopes, it is revealed that the microstructure has a profound effect on the dynamic tensile fracture mechanism of each aluminum alloy. The 2024‐T4 alloy is relatively brittle due to voids or cracks nucleated at many coherent CuMgAl₂ precipitate phases in the grain interiors, and the fracture mode is predominantly transgranular. The 7075‐T6 alloy exhibits relatively ductile fracture because voids or cracks growth is partly intergranular along the grain boundaries and partly transgranular by void formation around coarse intermetallic particles. The obvious differences of damage distribution and void coalescence mechanisms for 2024‐T4 and 7075‐T6 alloys under plate impact are also discussed.     

玻璃纤维-不锈钢网混杂增强环氧树脂层合板对球形弹斜冲击响应特性实验研究

为了研究玻璃纤维-不锈钢网混杂增强环氧树脂层合板在球形弹高速斜冲击下的损伤特性,利用一级气炮对2 mm厚度的玻璃纤维增强环氧树脂复合材料层合板和含一层、三层304不锈钢网的玻璃纤维-不锈钢网混杂增强环氧树脂层合板进行倾角为30°的冲击实验,以揭示304不锈钢网对层合板弹道极限和能量吸收的影响规律,并分析层合板损伤特征及其机理。通过实验发现,含有三层不锈钢网层合板的弹道极限最高,而不含不锈钢网层合板和含一层不锈钢网层合板的弹道极限速度接近。层合板吸收的能量随着弹体速度增加呈现出先增加后趋于平稳,然后急剧上升的趋势。层合板损伤模式为基体开裂和破碎、分层、不锈钢丝拉伸断裂、纤维拉伸断裂和剪切断裂。层合板分层损伤面积随弹体速度增大先增大后减小,最后趋于稳定。当弹体速度较低时,层合板主要发生纤维拉伸断裂、基体开裂、层间有分层损伤产生。随着弹体速度的增大,层合板正面纤维逐渐发生压剪断裂、基体破碎,背面纤维发生严重的拉伸撕裂。      

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.     

Fracture mechanism of AZ31 magnesium alloy processed by equal channel angular pressing comparing three point bending test and tensile test

A commercial magnesium alloy, AZ31 in hot-rolled condition, has been processed by equal channel angular pressing (ECAP) to get microstructure modified. Uniaxial tensile tests were conducted along the rolling/extrusion direction for as-received AZ31 alloy and ECAPed AZ31 alloy. Then, three point bending fracture tests were conducted for specimens with a pre-crack perpendicular to the extruded direction. Digital image correlation (DIC) technique was adopted to determine the deformation field around the crack tip. The fracture surfaces of the failed specimens after tensile tests and fracture tests were observed by Scanning Electron Microscope (SEM). To explore the deformation mechanism, the microstructure and texture of different regions on the deformed specimens were examined through electron backscatter diffraction (EBSD). The results show ECAP process improves both the tensile elongation and fracture toughness of AZ31 alloy. Different from the slip dominated deformation mechanism in the tensile test, deformation twinning presents in the deformation zone adjacent to the crack tip in the three point bending fracture tests. The fracture surface is characterized by co-occurrence of dimple and cleavage features.     

Experimental studies on mixed‐mode dynamic fracture behaviour of aluminium alloy plates with narrow U‐notch

Mixed‐mode dynamic fracture behaviour of cast aluminium alloy ZL205A thin plates with narrow U‐notch was studied by split Hopkinson tensile bar apparatus. Specimens with different loading angles were designed to realize different fracture modes. The same loading condition was maintained during the tests. Recovery specimens show that crack propagates along the notch direction. Force–elongation relations show that with the loading angle increasing, the fracture force increases while the final elongation decreases. Deformation and fracture process was observed by a high‐speed camera. Displacement distribution around the crack was calculated through digital image correlation technique. Based on the photos and displacement results, initiation time of the crack was derived. Besides, two stress components (normal stress and shear stress) applied on the fracture surface were investigated. Results show that crack initiation stresses at different loading angles satisfy the ellipse equation. Pure mode I and II fracture stresses are 425.3 and 236.7 MPa, respectively. Furthermore, specific fracture energy of different specimens was calculated. The energy data vary with loading angle and located on an approximate upward parabolic curve. From the curve, the minimum specific fracture energy of the thin plate specimen is 42.0 kJ/m² under loading angle of 76.3°.     

Using local out-of-plane compression (LOPC) to study the effects of residual stress on apparent fracture toughness

To study and understand the effects of residual stresses on fracture behaviour, it is necessary to introduce well characterised and reproducible residual stresses into laboratory fracture specimens. One technique capable of providing such residual stresses is local compression, where the local compression is applied to the sides of a test specimen. In this paper, the technique is used to create a residual stress field in compact tension, C(T), specimens. The specimens are used subsequently to study the effects of residual stress on fracture. Finite element studies show that significant changes to the distribution of the residual stresses occur when the position of the compression tools is changed relative to the crack tip. It is also revealed that both a single and double pair of compression tools can generate both tensile and compressive residual stresses in the vicinity of the crack tip depending upon the location of the tools relative to the crack tip. The impact of local compression is illustrated by experimental results from room temperature fracture tests performed on two aluminium alloys, Al2650 and Al2024. Tensile residual stresses, created by the application of a single pair of compression tools, reduced the initiation fracture toughness of Al2650 by about one half. The ductile tearing resistance of Al2024 decreases when a double pair of tools introduces tensile residual stresses. Conversely, the tearing resistance increases when compressive residual stresses are created through local compression.     

铁素体管线钢的分层裂纹及其对断裂的影响

通过对针状铁素体管线钢缺口根部三维应力状态的有限元分析和不同形式的断裂实验,研究了管线钢分层裂纹产生的条件及其对断裂性能的影响.结果表明裂纹或缺口根部的三维应力状态是产生分层裂纹的必要条件,材料的强度分布影响分层裂纹的形式和方向.分层裂纹均为主裂纹扩展前材料中的弱界面在垂直该弱界面的拉应力作用下产生的,其数量和方向受裂纹端部三维应力场和材料的强度分布状态控制.分层裂纹面上的应力为零,分层裂纹有一定的间距.在断裂过程中产生的分层裂纹使裂纹或缺口根部的构形发生改变,从而对裂尖的应力状态和材料的断裂性能产生巨大的影响.穿透裂纹体的分层裂纹使其有效厚度减小,表面裂纹体的分层裂纹与裂纹扩展方向垂直.在断裂过程中产生分层裂纹需要消耗更多的能量、降低裂端三维应力约束、有效厚度降低或裂尖钝化.这些因素均使断裂扩展更加困难,而使材料韧性得到提高.     

Interfacial fracture strength evaluation of Cu/SiN micro-components: applicability of the linear fracture mechanics criterion under a hydrogen environment

In this study, we have investigated microstructures of the delamination cracks observed from tensile and fracture toughness test specimens using an API X70 pipeline steel. It is found that the delaminations observed from both tensile and toughness specimens are intergranular fractures. At the occurrence, characteristics of brittle fracture were observed, but it was found not to be a brittle fracture as the delaminations were induced by plastic deformation. It is shown that severe plastic deformation produced strain concentration around particles located along grain boundaries and caused decohesion between adjacent grains, resulting in intergranular fracture.     

沉淀强化奥氏体合金的氢致断裂行为

研究了未充氢和热充氢沉淀强化奥氏体合金的拉伸断裂行为,分析了其氢脆敏感性与拉伸断裂行为间的联系,研究了氢对合金局部塑性变形及微裂纹形核的影响。结果表明:氢使沉淀强化合金由单一的韧窝断裂转变为韧窝断裂、沿晶断裂和滑移带开裂的混合断裂方式。其原因是:一方面,氢促进位错平面化滑移趋势、加剧局部塑性变形;另一方面,滑移带被晶界、孪晶界以及不同取向的滑移带所阻碍,引起了位错塞积和氢聚集。     

Stress corrosion cracking of Al-Mg-Zn-Cu alloys after precompression

Stress corrosion cracking (SCC) experiments have been carried out on double-cantilever-beam (DCB) specimens of 7017-T651 aluminium alloy. The specimens were first subjected to a known compressive load which caused plastic deformation at the notch tip. On unloading, this region developed a residual tensile stress field and on subsequent exposure to moist air at 40° C (95% relative humidity, r.h.), intergranular cracks formed. These cracks grew at a decelerating rate until they stopped. The final crack length increased with the value of the initial compressive preload, provided this was below the value for general yielding of the alloy. Electron fractography has been used to correlate changes in surface morphology with crack growth rate. It was found that ductile tearing of the notch tip may occur during unloading when the compression exceeds — 30kN. The practical importance of these results is outlined.     

Hydrogen induced cracking in high strength steel

Initiation of quasicleavage (QC) cracks in delayed fracture of hydrogen charged, quenched and tempered AISI 4340 steel was studied by means of a fractographic technique. The fatal crack leading to the delayed fracture of an unnotched specimen is found not to be caused by intergranular cracking as is commonly found in notched specimens, but by QC cracking. An SEM fractographic examination has revealed that QC cracks, which initiate around non-metallic inclusions and under the effect of hydrogen alone, grow mainly along the rolling direction of the specimen. Furthermore, the tensile component of stress causes a QC crack to grow in the plane of maximum tensile stress. The condition under which brittle fracture finally occurs in delayed fracture can be evaluated by the classical Griffith criterion from two parameters: the QC crack length and the nominal applied stress.

MST/3264     

FATIGUE CRACK GROWTH IN DUCTILE MATERIALS UNDER CYCLIC COMPRESSIVE LOADING

Abstract— Mode I fatigue crack growth has been studied in notched specimens of 7017-T651 aluminium alloy subjected to fully compressive cyclic loads. The specimens were first subjected to a deliberate compressive preload which causes plastic deformation at the notch tip. On unloading, this region developed a residual tensile stress field and on subsequent compressive cyclic loading in laboratory air, a fatigue crack was nucleated at the notch and grew at a diminishing rate until it stopped. The final crack length increased with an increase in the value of the initial compressive preload and with an increase in the negative value of the applied cyclic mean load. To gain a better understanding of crack growth in residual stress fields, the magnitude and extent of residual stress induced from compressive preloads have been analysed. This was achieved when extending the notch by cutting while recording the change in the back face strain. From residual strain models it was found that the fatigue crack growth was confined to a region of tensile cyclic stress within the residual stress field. The effective stress intensity range was investigated at selected mean loads and amplitudes, for correlating purposes, using both the compliance technique and by invoking the crack growth rate behaviour of the alloy. Finally, a brief discussion of the fracture morphology of cracks subjected to cyclic compression is presented.     

COMBINED MODE FRACTURE OF A DUCTILE MATERIAL UNDER PLANE STRESS CONDITIONS

Abstract— On the basis of our experimental results on Ly12-CZ (similar to 2024-T3) aluminium alloy tensile sheet specimens and Tresca's theory, a fracture criterion for a combined mode crack in ductile materials under plane stress conditions is proposed.     

Fatigue fracture in plates in tension and out-of-plane shear

Straight cracks near a stiffening element, or curved cracks, in a pressurized shell can be subjected to out-of-plane tearing stresses in addition to normal tensile stresses due to the membrane stresses in the shell. To predict the rate of fatigue crack growth in such situations a theory and a crack growth rate correlation are needed. Such loadings are modelled as a superposition of plane stress tensile fracture (mode I) and Kirchhoff plate theory shearing fracture (mode 2). Finite element analyses using shell elements are used to compute the energy release rate and stress intensity factors associated with the loading. Three fatigue crack growth rate experiments were carried out on sheets of 2024-T3 aluminium alloy loaded in tension and torsion. The first set of experiments is constant amplitude fatigue crack growth tests. The second consists of experiments where crack closure is artificially eliminated to determine the rate of crack growth in the absence of crack face contact. The third is a set of constant stress intensity factor amplitude tests. The results all show that as the crack grows extensive crack face contact occurs, retarding crack growth. In the absence of crack face contact, however, the addition of out-of-plane shear loading increases the crack growth rate substantially.     

Influence of residual fatigue damage on the fracture toughness parameters of an AA6082-T6 aluminium alloy

The present investigation has been carried out in order to study the influence of the previous accumulated fatigue damage induced during high cycle fatigue (HCF), on the fracture toughness parameters of an AA6082-T6 aluminium alloy. The results show that previous fatigue damage accumulated in HCF does not affect the tensile static mechanical properties of the material, but gives rise to a significant debit of the toughness properties on this aluminium alloy. The fracture toughness results have shown that the crack opening displacement at a crack extension of 0.2 mm (COD_0.2) decreases in the range of ∼18 to 36% whereas the value of the non-linear fracture mechanics parameter  J _0.2, decreases in the range of ∼11 to 25% at applied maximum stresses of 200 and 275 MPa, respectively. Optical microscopy observations conducted on the surface of the specimens subjected to HCF damage indicate the existence of microcracks ∼15 to 25 μm long nucleated along the grain boundaries of the material. Also, the scanning electron microscopy (SEM) observations of the fracture surfaces after the tearing tests show the predominance of a ductile fracture mechanism for the material prior to residual fatigue damage, whereas a mixed ductile–brittle fracture mechanism and the presence of flat facets were observed on the fracture surfaces of the specimens with a fatigue damage of 0.70.     

Fatigue crack growth in 7249‐T76511 aluminium alloy under constant‐amplitude and spectrum loading

Fatigue crack growth tests were conducted on compact, C(T), specimens made of 7249‐T76511 aluminium alloy. These tests were conducted to generate crack growth rate data from threshold to near fracture over a wide range of load ratios (R). Four methods were used to generate near threshold data: (1) ASTM E‐647 load reduction (LR), (2) compression pre‐cracking constant‐amplitude (CPCA), (3) compression pre‐cracking LR, and (4) constant crack mouth opening displacement LR method. A crack closure analysis was used to develop an effective stress‐intensity factor range against rate relation using a constraint factor (α = 1.85). Simulated aircraft wing spectrum tests were conducted on middle crack tension, M(T), specimens using a modified full‐scale fatigue test spectrum. The tests were used to develop the constraint‐loss regime (plane strain to plane stress; α = 1.85 to 1.15) behaviour. Comparisons were made between the spectrum tests and calculations made with the FASTRAN life prediction code; and the calculated crack growth lives were generally with ±10% of the test results.