Key Engineering Materials Vols. 525-526

Abstract: A series of hypervelocity impact tests on stainless steel mesh/aluminum plate multi-shock shield were practiced with a two-stage light gas gun facility. Impact velocity was approximately 4km/s. The diameter of projectiles was 6.4mm. The impact angle was 0°. The fragmentation and dispersal of hypervelocity particle against stainless steel mesh bumper varying with mesh opening size and the wire diameter were investigated. It was found that the mesh wall position, diameter of wire, separation distance arrangement and mesh opening had high influence on the hypervelocity impact characteristic of stainless steel mesh/aluminum plate multi-shock shields. When the stainless steel mesh wall was located in the first wall site of the bumper it did not help comminuting and decelerating projectile. When the stainless steel mesh wall was located in the last wall site of the bumper, it could help dispersing debris clouds, reducing the damage of the rear wall. Optimized design idea of stainless steel mesh/aluminum plate multi-shock shields was suggested.

Abstract: In order to study the fragmentation of projectile and ejection of debris clouds caused by hypervelocity impacting mesh bumper, simulation of aluminum sphere projectile hypervelocity normal impacting aluminum mesh bumper was practiced with SPH arithmetic of LS-DYNA soft. The diameter of projectile was 4mm. Impact velocities of aluminum spheres were varied between 2.2km/s and 6.2km/s. The impact angle was 0°. The relationship between the debris clouds characteristic of projectile and the impact position on aluminum mesh bumper was studied. The effect on fragmentation of projectile from different combination mode of aluminum mesh bumper was analyzed. The results showed that the morphologies of the debris cloud varied with the impact position when a projectile impacted the mesh bumper. The debris clouds as palpus was found, and some local kinetic energy concentrated appeared in the debris clouds. Debris clouds distribution was more uniform when projectile impacted wire across point on the mesh bumper. Debris clouds had more diffuse area and less residual kinetic energy when mesh bumper was combined with interleaving mode. Mesh bumper combined with interleaving mode was helpful in enhancing the protection performance of shields.

Abstract: Ellipse crack tip can be passivated when pulse current is perpendicular to crack surface of metal structure which can achieve the purpose of crack. In practice, it is hard that the loading current direction perpendicular to the crack surface because of the crack existence of different position and orientation. Theory and numerical simulation analyze the effect of crack arrest by electromagnetic heating based on metal structure with oblique-elliptical embedding crack. And current density and temperature distribution of crack tip are derived. The result shows that due to heat concentration around the oblique-elliptical embedding crack tip, the crack tip temperature exceeds the melting point of materials and small welded joints are formed by metal melting. Thermal compressive stress which is generated near the crack tip can prevent the crack propagation effectively. Thus, electromagnetic heating is proved to be an effective method of preventing general crack propagation.

Abstract: Delamination is a particularly dangerous damage mode of high performance laminated composites. In order to describe the composites ductile cracking and its progressive evolution accurately, the adjusted exponential cohesive zone model (CZM) is adopted, which correlates the tensile traction with the corresponding interfacial separation along the fracturing interfacial zone. At first the adjusted exponential CZM is used to simulate the mode I delamination of the standard double cantilever beam (DCB). The simulated results are in good agreement with the corrected beam theory and the corresponding experimental results. Then in order to research how the interfacial properties influence the mode I fracture, the interfacial strength and the critical energy release rate are studied. The main results are obtained as follows. The interfacial strength plays a crucial role in the laminated composites delamination onset, and it affects the peak load significantly if there is not a pre-crack. Once the delamination propagation begins to occur in the laminated composites, the responses of the load-displacement plots are relatively insensitive to the interfacial strength, and only the critical energy release rate is of critical importance. Furthermore, the peak load increases with the increase of the critical energy release rate and interfacial strength.

Abstract: This paper discusses the combination of element enrichment by mesh refinement controlled by density function with the extended finite element method and its application in fracture mechanics. Extended finite element method (XFEM) is an effective numerical method for solving discontinuity problems in the finite element work frame. A numerical example of fracture mechanics is analyzed at the end of this paper to show the application of the above method.

Abstract: Surface roughness characterizes the micro-geometric appearance variation and significantly affects the fatigue properties of machined specimen. Low cycle fatigue (LCF) tests of alloy steel specimens with different surface roughness were carried out in this paper to investigate the effect of roughness on fatigue life. The dumbbell plate specimens were tested in uniaxial stress-controlled mode on the hydraulic servo machine at room temperature. Obviously discrepant lifetime results corresponding to different surface roughness implied that the greater the roughness was, the lower the fatigue life was. An approximate power function relationship between the roughness and the fatigue life was established through the fitting of the experimental data.

Abstract: Alternating temperature accelerated aging test was designed to ensure that accelerated aging could preferably simulate real world storage of HTPB composite propellant. Mechanical properties of HTPB propellant aged for three different periods were measured and analyzed. The results indicate that, the tensile strength increased and the elongation decreased after accelerated aging, showing the same trend of real world storage; the mechanical properties of HTPB propellant is strongly influenced by the rate of temperature change; alternating temperature accelerated aging test is consistent well with real world storage. The results can be very helpful for solid rocket motor life prediction.

Abstract: Crack tip opening displacement (CTOD) tests were carried out on different positions of X70, X80 and X100 pipe by three-point bend testing method. The results show that the crack propagation resistance and the tear modulus of base metal, heat affected zone (HAZ), and weld of X70 and X80 pipe are larger than that of X100 pipe. At the same time,The initial CTOD values and the CTOD values withmaximum load of X70 and X80 pipe are larger than that of X100 pipe.The test results suggest that the fracture toughness, crack initiation and propagation resistance of theX70 and X80 pipe are greater than that of X100 pipe, and the mechanismis analyzed finally.

Abstract: Based on integral nonlinear viscoelastic constitutive relationship, stress-strain field was analyzed of composite propellant under cyclic temperature loads to ensure the stress-strain distribution and damageable locality. The simulation conditions were designed in the temperature range from-10 to 60 at three different rate-temperature. The results indicate that the stress and strain have the same trend of changing. Adhesive interphase is prone to failure because of the alternating stress.

Abstract: The use of ultrasonic guided waves for damage detection suffers from the multi-modes and dispersion. Much attention has been paid to transducer design and excitation frequency chosen to suppress the multiple modes and dispersion. However, little attention has been paid to complex signal processing. In this paper, the dispersive propagation of the guided waves are firstly reviewed. And then the matching pursuit method is introduced as a feature extraction algorithm. In order to present well the characteristic of the guided waves signal, a dispersive dictionary is designed based on the guided waves propagation. A two-stage pursuit method consisted of coarse and fine matching is used. At last, the proposed method is verified by finite element simulation and successfully extracted damage related dispersive pulses from measured noisy signal.