Electronic Shearography for Detecting Disbonds in Lattice/Skin Structures

Aluminum structures with an integral lattice (rib system) and skin currently are used for a variety of large space structures (Titan and Delta series) due to their structural efficiency. Various components of space structures will be manufactured out of graphite/epoxy composites in the future. Interstages of rockets, decks of small satellites (MightySat program), payload shrouds, solar cell substrates, rocket motor casings, etc., are being fabricated at the Air Force Phillips Laboratory. Tests have shown that the principal mode of initial failure in these structures involves separation between the skin and the ribs, during fabrication or loading. The redundancy of the rib system in these structures leads to alternate load paths that make it difficult to detect such defects. The fabrication, integration, and testing often are carried out in different parts of the country. It is therefore important to have a method of detecting and possibly quantifying the intergrity of the rib/skin interface. The objective of this paper is to demonstrate that electronic shearography (ES) can be used to detect disbonds in rib/skin ``Isogrid' structures. This laser-based interferometry technique provides fringe patterns that represent full-field displacement gradients. The ruggedness and portability of the system make it a prime candidate for in-service inspection of large structures. The observed fringe patterns change dramatically for disbonded ribs that form a basis for rapidly detecting disbonds over a large area. The expected fringe patterns can be quantified and compared with results from finite-element (FEM) analyses of the structure. A commercial FEM code was used with orthotropic material properties that are representative of the composites used.     

Multiphysics modelling of the coupled behaviour of precision-guided projectiles subjected to intense shock loads

Precision-guided projectiles (PGPs) typically deployed in smart munitions are operated and guided by highly sophisticated embedded electronic systems (EES). These PGPs are subjected to severe shock loads resulting from the ignition of the propellant during their launch. These shock loads, which are typically characterised by high intensity, short duration and wave reflections at varied frequencies, often lead to the failure of the EES. It is the objective of this work to conduct a comprehensive multiphysics analysis of the launch process of PGPs accounting for coupling and interaction effects between the different media (propellant, PGP, confined volume and free space). Specifically, we investigated the entire launch process that include the ignition of the propellant to observe local and global features of the setback, set forward pressure and acceleration histories using explicit axisymmetric Lagrangian–Eulerian finite element simulations. In this work, we also examine the severity and frequency of the reflected waves as well as the springback of the PGPs resulting from these local oscillations as they exit the muzzle. In addition, the flight state transition due to muzzle exit in terms of pressure and flow velocity is also discussed. Our results reveal the complex phenomena associated with the dynamic response of the PGPs and pressurization process resulting from the ignition of propellant during launch that are characterized by high oscillatory pressure profiles and projectile springback.     

Dynamic responses of a kinetic energy projectile under transverse loadings

We report on the effects of launch tube nonstraightness and asymmetric loading on the accuracy performance of a kinetic energy projectile. Modeling the projectile as a rigid body within the launch tube, we obtain and solve the equations of rotary motion to calculate the orientation of the projectile relative to the tube as a function of time. Three launch tube geometries are modeled; curiously, the most severe environment does not produce the most deviant projectile orientations during in-bore travel or at muzzle exit. To determine the effects of asymmetric loading, we model the rod as a nonuniform two-dimensional beam, subject to a transverse blast load. Determined experimentally, the sabot equivalent stiffness is bounded between 10⁶ and 10⁷ N/m. These bounds are used in an elastic boundary condition to the rod finite element model. The ANSYS transient vibration analyses predict a peak transverse displacement of 20 mm and a peak transverse velocity of 75mm/s at muzzle exit. We conclude that: (1) base pressures asymmetry induces transverse vibrations in the projectile, and these vibrations are affected by sabot stiffness; and (2) launch tube profile nonstraightness induces rigid body rotations in the projectile, and these rotations may or may not increase with launch tube severity.     

A review of explosive accelerators for hypervelocity impact

A technical overview of experimental methods using high explosive techniques for conducting hypervelocity impact studies is presented. The explosive techniques use the explosive detonation fronts as means of accelerating the projectile, or as means of compressing a light gas which is then used to launch the projectile.

The explosive launchers are in six subdivisions: high explosive pellet accelerators, flyer plate accelerators, shaped charges, explosive-formed projectiles, fragment and microparticle accelerators, and explosive gas guns. Each one of the subdivisions presents the various techniques, their advantages and disadvantages, the range of mass and velocity capable of being accelerated, and whether the technique can be scaled for larger or smaller masses.     

Experimentation and modeling of inclined ballistic impact in thick polycarbonate plates

The penetration and perforation of a thick polycarbonate (PC) plate (one and 3 stacked) by an armor piercing 7.62 mm projectile is investigated experimentally and numerically. The characteristic structure of the projectile’s trajectory in the PC plates is studied. It is observed that the trajectory consist of a cavity and a circumferential cracked zone attached to it, which is fully embedded within a cylindrical plastic zone. The size of the plastic zone is approximately twice that of the cavity zone and can be clearly observed due to the change of the refractive properties of the material. Strong local recovery of the PC is shown as well.A 3D transient non-linear adiabatic finite element simulation is performed using the commercial software Abaqus 6.9-EF1. The numerical analyses include two combined failure criteria: “Ductile failure with damage evolution”, and tensile failure. The material properties are strain rate and temperature dependent. The numerical simulations are tested by comparing the numerical trajectory prediction to actual trajectories of inclined impacts of projectiles. It is found that the projectile perforates the plate at angles of inclinations of 30° and higher. The observed agreement between experiments and numerical modeling indicates that the combined effect of the two failure criteria (tensile vs. ductile failure) can reasonably well predict the projectile’s trajectory within a thick PC plate.The numerical analyses are further used to study the effect of the projectile impact velocity on the depth of penetration (DOP). It is found that the DOP scales slightly non-linear with the impact velocity. The core velocity during the penetration process is also slightly non-linear. The deceleration during penetration is almost a linear function of the penetration velocity and it is higher for higher penetration velocities.     

An investigation on failure mechanisms of ceramic/metal armour subjected to the impact of tungsten projectile

The failure mechanism of AD95 ceramic/4340 steel armour subjected to the penetration of the tungsten projectile was investigated at the nominal velocity of 820 m/s. Typical failure modes of the targets with various boundary conditions were presented. The effect of cover plate and confinement on failure mechanisms of the target was analyzed. The results showed that the cover plate can effectively reduce the damage of the target due to it can force the ejected ceramic fragments to decelerate the projectile, and a ceramic powders column was formed beneath the eroded projectile in the confined ceramic sandwiched between cover and support plate during penetration. Compared to the confinement, cover plate is more effective to reduce the damage of the support plate. Based on the experimental results, the failure mechanisms and penetration process of different target configuration have been discussed.     

弹体头部形状对碳纤维层合板抗冲击性能影响实验研究

为研究弹体头部形状对碳纤维层合板抗冲击性能的影响,利用一级气炮发射卵形头弹、半球形头弹和平头弹,对2 mm厚碳纤维层合板进行了冲击实验。利用公式拟合处理实验数据,揭示弹体头部形状对靶板弹道极限与能量吸收的影响,并且分析靶板冲击损伤形貌及机理特征。研究结果表明:平头弹弹道极限最高,半球形头弹次之,卵形头弹最低。弹体在低速度冲击时,弹体头部形状对靶板能量吸收率的影响更为显著。平头弹冲击时,靶板迎弹面受到均匀分布的环向剪切力,纤维同时被剪切,基体发生大面积剪切破坏。半球形头弹冲击时,靶板迎弹面受到非均匀分布的剪切力和挤压作用,纤维发生剪切断裂和拉伸断裂,基体发生剪切破坏和挤压破碎。卵形头弹冲击时,纤维发生单一的拉伸断裂,而基体则发生挤压破碎。弹体头部形状对靶板损伤的影响主要集中在迎弹面和中部纤维层。     

刚性尖头弹扩孔贯穿金属靶板理论模型的讨论

针对延性扩孔破坏模式,讨论了刚性尖头弹贯穿韧性金属靶板的已有六个理论模型（F-W、C-L、JZG、WHM、S-W和JBL）对于靶板厚度和弹头形状的适用范围,统一了各模型参数的选取准则,分别给出了JZG模型尖锥头形和尖卵头形弹体半锥角和无量纲曲率半径（CRH）的适用范围。基于12组冲击速度为200~1600m/s,厚径比（靶体厚度与弹身直径之比H/d）为0.605~9.17的多种弹靶材料的穿甲实验,得出：对于尖锥头形弹体贯穿靶板后的残余速度,S-W和WHM、JZG、F-W模型分别对于较薄靶板、中等厚度靶板和较厚靶板的预测效果较好;而对于尖卵头形弹体,WHM和JBL模型预测效果较好。同时,各模型对于弹道极限预测效果的结论和残余速度一致。分析结论可为坦克、舰船等单、多层金属装甲防护结构设计与计算提供参考和依据。     

Mg/Al波纹复合板抗冲击性能研究

目的 比对波纹轧制结构和平面复合结构的Mg/Al复合板抗冲击性能与吸能机制.方法 采用波纹辊轧制工艺制备Mg/Al复合板,使用半球形铝合金弹丸对传统平面复合板与波纹复合板进行不同速度下的冲击试验研究,并对比分析2种复合板的损伤机理,探明波纹结构对复合板抗冲击性能的影响.结果 Mg/Al平面复合板抗半球形弹丸冲击的吸能机制主要是通过靶板的塑性变形、剪切破坏、拉伸断裂、分层破坏和弹丸与靶板间摩擦等形式来吸收能量.波纹复合板对冲击能量的吸收主要依赖靶板的局部塑性变形、沿着波纹方向的开裂、结合界面的分层以及弹丸与靶板间的摩擦耗能.结论 当冲击速度低于弹道极限速度时,波纹复合板的抗冲击性能优于平面复合板,高于弹道极限速度时,2种复合板的抗冲击性能和耗能程度相当.     

Failure Analysis of a Launch Vehicle Umbilical Shutter Mechanism During Vibration Qualification

Payload fairing (PLF) of a launch vehicle is exposed to harsh vibration environments due to jet noise during liftoff and in-flight aerodynamic noise. Accordingly, the systems mounted on the payload fairing are to be qualified for the vibration levels, predicted corresponding to the envelope of acoustic spectrums at critical instants of atmospheric flight. This paper presents a detailed study of a failure observed on the payload cooling umbilical system, mounted on the cylindrical portion of the PLF structure, during its design qualification vibration testing. The umbilical shutter inadvertently opened during the test. The vibration responses on the shutter, the dynamic behavior of the system, and the forces and moments on the mechanism are analyzed, and the physics of failure is understood. The design marginality is identified, and the shutter locking mechanism reconfigured to achieve the desired level of robustness in the system.     

A nonlocal model for plug formation in plates

By means of a nonlocal viscous fluid model, an investigation is carried out of the problem of penetration of a cylindrical projectile into a plate leading to a failure of the plate by a plug formation. The effect of impact is represented by a uniform initial velocity distribution over a circular region on the surface of the plate. The behavior of this plate material is assumed to be viscous and spatially nonlocal, and only the effects of vertical shearing stress are considered. The expression of stress, velocity and displacement are obtained and the calculated displacement profiles are compared with some existing experimental profiles.     

考虑身管柔性的坦克行进间发射动力学研究

针对弹丸起始扰动会影响坦克行进间射击密集度问题,建立身管柔性的坦克行进间发射动力学模型;考虑弹丸动不平衡及质量偏心、弹炮相互作用、弹炮间隙,建立坦克行进间射击弹丸膛内运动方程;编制行进间射击的坦克发射动力学仿真程序,获得某坦克行进间射击弹丸膛内运动规律及千米立靶密集度,并试验验证仿真结果。该结果可为提高坦克行进间射击精度提供理论基础与仿真手段。     

Design and testing of the Minotaur advanced grid-stiffened fairing

A composite grid-stiffened structure concept was selected for the payload fairing of the Minotaur launch vehicle. Compared to sandwich structures, this concept has an advantage of smaller manufacturing costs and lighter weight. To reduce weight the skin pockets are allowed to buckle visibly up to about 0.5 cm peak displacement.

Various failure modes were examined for the composite grid-stiffened structure. The controlling criterion for this design was a joint failure in tension between the ribs and skin of the structure. The identification of this failure mechanism and the assessment of bounding strains required to control it required extensive test and analysis effort. Increasing skin thickness to control skin buckling resulted in reduced strains between the skin and ribs.

Following the identification of the relevant failure criteria, a final design for the fairing was generated. The resulting 6 m tall fairing was constructed of a tow-placed carbon fiber composite grid structure that was over-wrapped to create a laminated skin. Upon completion of curing and machining, the fairing was cut in half to create the classic “clam-shell” fairing. Static qualification testing demonstrated the structural integrity of the fairing, thereby proving the design and manufacturing process. Loads were applied incrementally in a static loading scenario. The applied load envelope exceeded worst-case dynamic flight conditions with an added safety factor of 25%. At peak load the fairing maintained structural integrity while remaining within the required displacement envelope for payload safety.

Data were collected during the test from a variety of sensors including traditional displacement transducers and strain gages. In addition, full field displacement was monitored at critically loaded fairing sections by means of digital photogrammetry. This paper summarizes the test results, presents the overall performance of the fairing under the test loads, correlates test response and analysis, and identifies lessons learned.

Work continues at the Air Force Research Laboratory (AFRL) and Boeing to identify means of further controlling tensile failure of the un-reinforced polymer bonded joint between the ribs and skin. Stiffening of skin adjacent to the joints and introduction of lightweight foam jackets at the interior of the fairing both show promise of delaying joint failure to higher loads.     

Initiation of adiabatic shear failure in a clamped circular plate struck by a blunt projectile

This paper presents an analytical model for studying the material failure in shear hinges formed during the dynamic plastic response of a circular plate under projectile impact. Analytic solutions for the ballistic perforation performance of a fully clamped circular plate struck by a blunt projectile are obtained. It takes into account both the global response (plate bending) and the localized shear. Based on the estimations of the shear strain and the size of shear hinge, condition for the initiation of an adiabatic shear band is formulated. The effectiveness of the present model is demonstrated by reasonable agreement between the theoretical predictions and the available experimental data for the perforation of Weldox 460E steel plates.     

Numerical multi-scale modeling for textile woven fabric against ballistic impact

In this study, a FEM analysis has been carried out to find out pertinent multi-scale model for an investigation of a ballistic impact on 2D KM2® plain-woven fabrics. Multi-scale models are a combination between macroscopic and mesoscopic models. This study aims at testing a multi-scale model in order to minimize the computing time. Three configurations were analyzed by varying the ratio of macroscopic and mesoscopic areas: 75.3–24.7%, 65.5–34.5%, 56.3–43.7% with two impact velocities 60 m/s and 245 m/s. In these multi-scale models, the continuity in macroscopic–mesoscopic interfaces is ensured by checking the evolution of global displacements of the fabric during impact. The effect of the macroscopic area of multi-scale models on the ballistic performance of the fabric is also investigated. The optimal multi-scale model was validated by comparison with results obtained from a mesoscopic model in terms of the evolutions of the projectile velocity, energy forms, the overall behavior of the fabric during impact and the force applied on the projectile. The failure criterion Forming Limited Diagram (FLD) is suggested for bundle failure. The observed damage mechanisms of the fabric during penetration time of the projectile are discussed and compared among numerical models.     

Fracture mechanisms of aluminium alloy AA7075-T651 under various loading conditions

The fracture behaviour of the aluminium alloy AA7075-T651 is investigated for quasi-static and dynamic loading conditions and different stress states. The fracture surfaces obtained in tensile tests on smooth and notched axisymmetric specimens and compression tests on cylindrical specimens are compared to the fracture surfaces that occur when a projectile, having either a blunt or an ogival nose shape, strikes a 20 mm thick plate of the aluminium alloy. The stress state in the impact tests is much more complex and the strain rate significantly higher than in the tensile and compression tests. Optical and scanning electron microscopes are used in the investigation. The fracture surface obtained in tests with smooth axisymmetric specimens indicates that the crack growth is partly intergranular along the grain boundaries or precipitation free zones and partly transgranular by void formation around fine and coarse intermetallic particles. When the stress triaxiality is increased through the introduction of a notch in the tensile specimen, delamination along the grain boundaries in the rolling plane is observed perpendicular to the primary crack. In through-thickness compression tests, the crack propagates within an intense shear band that has orientation about 45° with respect to the load axis. The primary failure modes of the target plate during impact were adiabatic shear banding when struck by a blunt projectile and ductile hole-enlargement when struck by an ogival projectile. Delamination and fragmentation of the plates occurred for both loading cases, but was stronger for the ogival projectile. The delamination in the rolling plane was attributed to intergranular fracture caused by tensile stresses occurring during the penetration event.     

A reliability evaluation method for phased-mission system considering multi-mechanism coupling within phases

Due to the difference of environmental stress and system configuration, as well as the coupling between phases, the phased-mission system (PMS) has significant disparity in the failure modes and mechanisms of components in different phases. From the perspective of failure mechanism, an initial independent one will affect or be affected by other failure mechanisms during a task, and eventually leads to the failure of system. Therefore, the coupling of failure mechanisms in PMS is considered in this paper, and the recursive quantitative analysis of coupling from failure mechanism level to system level and phase level are accomplished by means of the basic rules of reliability physics. Furthermore, taking a core circuit from an aircraft electronic system as an example, the reliability modeling and analysis are performed. The results show that after considering the coupling relationship between failure mechanisms within each phase, the reliability and lifetime of components are reduced compared with the mechanism- independence situation, however, the cumulative failure probability of the system is increased. It reveals that it is very important to involve the coupling relationship between failure mechanisms in the reliability analysis of PMS, which can improve the accuracy of the prediction.     

A model for predicting the evolution of multiple cracks on multiple length scales in viscoelastic composites

A model is presented herein for predicting the evolution of numerous cracks on multiple length scales, the objective of such a model being to develop the capability to predict failure of structural components to perform their intended tasks. Such a capability would then be useful as a predictive tool for designing structural components so as not to fail, but rather to succeed in performing their intended tasks. The model developed herein is somewhat involved, being based in continuum mechanics and thermodynamics, but is nevertheless expected to be cost effective (wherever sufficient accuracy permits) when compared to more costly experimental means of determining component life. An essential ingredient within the context of the model is that cracks must develop on widely differing length scales. Where this is observed to occur in nature, which is surprisingly often, there are potential simplifications over more generally described but practically untenable approaches, that can lead to (at least partly) computational multiscale algorithms capable of assimilating failure due to multiple cracking with a high degree of accuracy. The model presented herein will be briefly described within a mathematical framework, and an example problem will be presented that is representative of certain currently relevant technologies.     

FEM prediction of buckling distortion induced by welding in thin plate panel structures

Welding distortion generated during assembly process has a strongly nonlinear feature, which includes material nonlinearity, geometric nonlinearity, and contact nonlinearity. In order to obtain a precise prediction of welding distortion, these nonlinear phenomena should be carefully considered. In this study, firstly, a prediction method of welding distortion, which combines thermo-elastic-plastic finite element method (FEM) and large deformation elastic FEM based on inherent strain theory and interface element method, was developed. Secondly, the inherent deformations of two typical weld joints involved in a large thin plate panel structure were calculated using the thermo-elastic-plastic FEM and their characteristics were also examined. Thirdly, using the developed elastic FEM and the inherent deformations, the usefulness of the proposed elastic FEM was demonstrated through the prediction of welding distortion in the large thin plate panel structures. Finally, the influences of heat input, welding procedure, welding sequence, thickness of plate, and spacing between the stiffeners on buckling propensity were investigated. The numerical simulation method developed in this study not only can be used to predict welding distortion in manufacturing stage but also can be employed in design or planning stage.     

Critical response of shielded plates subjected to hypervelocity impact

A ballistic limit equation for hypervelocity impact on thin plates is derived analytically. This equation applies to cases of impulsive impact on a plate that is protected by a multi-shock shield, and is valid in the range of velocity above 6 km/s. Experimental tests were conducted at the NASA Johnson Space Center on square aluminum plates. Comparing the center deflections of these plates with the theoritical deflections of a rigid-plastic plate subjected to a blast load, one determines the dynamic yield strength of the plate material. The analysis is based on a theory for the expansion of the fragmented projectile and on a simple failure criterion. Curves are presented for the critical projectile radius verus the projectile velocity, and for the critical plate thickness versus the velocity. These curves are in good agreement with curves that have been generated empirically.