Damage evaluation of concrete plates by high-velocity impact

This paper describes the evaluation of the local damage of concrete plates by the impact of high-velocity rigid projectiles. A new launching system of mushroom-shaped projectiles has been developed. Impact tests for concrete plates have been conducted by using the system to examine failure modes of the local damage of concrete plates. The damage or failure behavior has been discussed on the basis of the failure process captured by a high speed video camera and the strain histories obtained by strain gauges on the concrete plate. Numerical simulations have been also carried out in order to explain the mechanism of the local damage observed by the experiment. A reasonable numerical model has been discussed in terms of a constitutive model and strain rate effect of concrete material. Mechanism of the local damage of concrete plates has been illustrated schematically.     

低速冲击下RC梁的动态响应和破坏机理研究

考虑材料非线性和应变率相关性等因素的影响,运用LS_DYNA非线性有限元软件对RC梁横向低速冲击试验进行了数值模拟,从动态损伤扩展、冲击能量转化等方面研究了RC梁的冲击响应过程和破坏机理。结果表明:RC梁的冲击响应过程可分为局部响应、整体响应和回弹变形三个阶段;RC梁的损伤主要发生在局部响应阶段,梁体变形主要发生在整体响应阶段;局部响应阶段的冲击力完全由梁体惯性力平衡,整体响应阶段的变形模式和截面弯矩分布与刚塑性模型基本相同;受拉钢筋变形耗能是RC梁最主要的冲击耗能机制。     

Impact damage in hybrid braided twill composites

Impact damage to composite plates is significantly reduced by replacing some of the high-strength fibres with more ductile glass or synthetic fibres. Hybrid composites reduce impact damage by distributing more widely the deformations and strain in the contact region. This investigation focussed only on hybrid textile composites with individual tows composed of either carbon or glass which are braided together in a twill textile. At a similar level of impact energy, low and high-speed impact tests resulted in different failure mechanisms dominated, respectively, by quasi-static and flexural wave deformations. The damage severity was evaluated in terms of damage area (C-scan) and absorbed energy.     

大桥受船撞击灾害的计算机评价方法

本文提出了一种对撞桥灾害进行评价的计算机方法。首先用非线性有限元法计算由于船撞引起的桥体局部损伤分布,局部损伤用材料损伤在该处的密度表示,材料损伤是塑性应变和塑性应变率的函数,在桥体材料模型中加以反映,通过对有限元计算出的桥体局部损伤数据、现场测量数据和大桥以往累积损伤数据进行分析,利用专家知识库确定大桥目前整体损伤指标,用整体损伤指标表示大桥当前的安全状态,如果整体损伤指标超过给定临界值,则大桥退役或需要进行大的安全防护措施,否则,大桥可以继续服役,但需对其在未来如船撞、地震等可能灾害作用下的安全性能进行预测,如果预测出的安全性偏低,则需要对大桥提前进行维修。用所提出的方法对清江高坝大桥船撞灾害的实例进行了应用,验证了方法的有效性。     

复合材料加筋板低速冲击损伤及剩余压缩强度试验研究

采用落锤法对复合材料加筋板进行了低速冲击损伤（LVI）试验,根据复合材料加筋板构型,设计了冲击支持支架,研究了支持支架的间距对冲击结果的影响;用相同的冲击能量对复合材料加筋板结构中3处典型位置进行冲击,得到不同位置的损伤形貌;分别对完好件和损伤试验件进行压缩试验,将试验结果进行对比,分析不同位置的冲击损伤对结构压缩性能的影响。试验结果表明：在相同的冲击能量下,支持支架间距越小,所造成的冲击损伤越严重;在50 J冲击能量下,筋条区蒙皮处的冲击所造成的损伤不易观察,筋条间蒙皮处的冲击所造成的损伤最为明显,而筋条边缘蒙皮处的冲击可以导致筋条边缘的脱粘;冲击损伤会使加筋板屈曲载荷轻微下降,筋条间蒙皮和筋条区蒙皮冲击损伤对压缩结果影响相对较小,筋条边缘处的冲击会引起损伤处蒙皮的子层屈曲,并影响结构破坏形式,使结构压缩承载能力有较为明显的下降。     

Hypervelocity impact damage prediction in composites: Part II—experimental investigations and simulations

The extension of damage in composites during hypervelocity impact (HVI) of space debris is controlled by failure thresholds and subsequent energy consumption during damage growth. Characterisation and modelling of the material under partially and fully damaged states is essential for the prediction of HVI effects on fibre-composite structures. Improved experimental and numerical analysis techniques have been developed and are summarised in an accompanying paper. The present paper deals with the establishment of two precise damage experiments under HVI conditions as a validation basis for numerical simulations: The first type consists of space debris impact configurations optimised for damage evaluation and the second experiments reproduce HVI strain rates and compressions in plate impact. Coupling of damage analysis techniques (visual, ultrasonic, residual strength) to quantify different aspects of failure has been achieved. Numerical simulations using the commercial hydrocode AUTODYN in mesh-based and SPH formulations are presented using the material model and data described in the accompanying paper.     

Preparation and characterization of Ge epitaxially grown on nano-structured periodic Si pillars and bars on Si(001) substrate

The selective epitaxial growth of germanium on nano-structured periodic silicon pillars and bars with 360 nm periodicity on Si(001) substrate is studied to evaluate the applicability of nano-heteroepitaxy on the Ge-Si system for different fields of application. It is found that SiO₂ used as masking material plays the key role to influence the strain situation in the Si nano-islands. To analyze this in detail, X-ray diffraction techniques in combination with theoretical simulations based on the kinematical X-ray scattering from laterally strained nano-structures and finite element method (FEM) calculations of the strain field are applied. The oxide related strain in the Si scales about linearly with the thickness of the SiO₂ mask, but FEM simulations supposing a homogeneous stress distribution in the oxide are not sufficient to describe the local strain distribution in the nano-structures. It is demonstrated that the Ge lattice relaxes completely during growth on the Si nano-islands by generation of misfit dislocations at the interface, but a high structural quality of Ge can be achieved by suited growth conditions.     

The damage tolerance of GRP laminates under biaxial prestress

The damage tolerance of E-Glass reinforced/polyester laminated plates subjected to a low velocity impact while under an in-plane prestress is investigated. Prior to test, the plates are subjected to either uniaxial or biaxial loading by means of a specially designed test rig which enables independent tension/tension, tension/compression or compression/ compression testing in all stress/strain quadrants. Impact tests are carried out for a single strike energy for a comprehensive range of pre-strains. Absorbed energy, damage area, peak impact loads and the maximum permanent indentation depth are assessed in characterising the resulting damage.

It is shown that the shape, orientation and size of the damage zone is strongly influenced by the nature and magnitude of the pre-strain. Impact specimens subject to shear loading give rise to the largest increase in damage area when compared to unstressed plates.     

A combined experimental and numerical approach to study ballistic impact response of S2-glass fiber/toughened epoxy composite beams

A combined experimental and 3D dynamic nonlinear finite element (FE) approach was adopted to study damage in composite beams subject to ballistic impact using a high-speed gas gun. The time-histories of dynamic strains induced during impact were recorded using strain gages mounted on the front of the composite beam specimen. During ballistic impact tests, the impact velocity was also measured. The commercially available 3D dynamic nonlinear FE code, LS-DYNA, modified with a proposed user-defined nonlinear-orthotropic damage model, was then used to simulate the experimental results. In addition, LS-DYNA with the Chang–Chang linear-orthotropic damage model was also used for comparison. Good agreement between experimental and FE results was found from the comparisons of dynamic strain and damage patterns. Once the proposed nonlinear-orthotropic damage model was verified by experimental results, further FE simulations were conducted to predict the ballistic limit velocity (V₅₀) using either the number of damaged layer approach or a numerically established relation between the projectile impact velocity versus residual velocity or energy similar to the classical Lambert–Jonas equation for metals.     

Crack patterns and strain energy distributions in model brittle systems in a random environment

Monte Carlo simulations of crack growth are performed using simple two-dimensional model systems for brittle materials. Crack growth is modelled as a series of processes of release and transfer of strain energies on the system of square grains. The simulations are concerned with crack growth in a random environment. Considerable attention is paid to the correlation between crack patterns and strain energy distributions. It is shown that the model system settles into a stationary state. In this state, the features of crack pattern have a close relation to the characteristics of strain energy distribution. Some implications of the results are discussed in regard to crack patterns caused by heating, radiation, or random mechanical loading on the surface of real brittle materials.     

Nanoscale heterogeneity promotes energy dissipation in bone

Nanomechanical heterogeneity is expected to influence elasticity, damage, fracture and remodelling of bone. Here, the spatial distribution of nanomechanical properties of bone is quantified at the length scale of individual collagen fibrils. Our results show elaborate patterns of stiffness ranging from approximately 2 to 30 GPa, which do not correlate directly with topographical features and hence are attributed to underlying local structural and compositional variations. We propose a new energy-dissipation mechanism arising from nanomechanical heterogeneity, which offers a means for ductility enhancement, damage evolution and toughening. This hypothesis is supported by computational simulations that incorporate the nanoscale experimental results. These simulations predict that non-uniform inelastic deformation over larger areas and increased energy dissipation arising from nanoscale heterogeneity lead to markedly different biomechanical properties compared with a uniform material. The fundamental concepts discovered here are applicable to a broad class of biological materials and may serve as a design consideration for biologically inspired materials technologies.     

Impact response of fiber metal laminate sandwich composite structure with polypropylene honeycomb core

Fiber metal laminates (FMLs) were used as skin on polypropylene honeycomb core to form a sandwich structure. Impact response was measured by conducting a series of low-velocity impact test. Impact force and the force time history were recorded and analyzed. It was found that the maximum impact load increased up to a threshold value at which it plateaus while the energy absorption in the structure increased with increasing impact energy. Post-impact optical image showed a change in damage area with increasing impact energy. The impact damage threshold energy for the sandwich structure was clearly shown in the range of impact energy between 7.84 J and 11.76 J where damages including delamination of the skins and global bending of the structure were observed.     

A model for deformation and fragmentation in crushable brittle solids

A unified framework of continuum elasticity, inelasticity, damage mechanics, and fragmentation in crushable solid materials is presented. A free energy function accounts for thermodynamics of elastic deformation and damage, and thermodynamically admissible kinetic relations are given for inelastic rates (i.e., irreversible strain and damage evolution). The model is further specialized to study concrete subjected to ballistic loading. Numerical implementation proceeds within a finite element context in which standard continuum elements represent the intact solid and particle methods capture eroded material. The impact of a metallic, spherical projectile upon a planar concrete target and the subsequent motion of the resulting cloud of concrete debris are simulated. Favorable quantitative comparisons are made between the results of simulations and experiments regarding residual velocity of the penetrator, mass of destroyed material, and crater and hole sizes in the target. The model qualitatively predicts aspects of the fragment cloud observed in high-speed photographs of the impact experiment, including features of the size and velocity distributions of the fragments. Additionally, two distinct methods are evaluated for quantitatively characterizing the mass and velocity distributions of the debris field, with one method based upon a local energy balance and the second based upon global entropy maximization. Finally, the model is used to predict distributions of fragment masses produced during impact crushing of a concrete sphere, with modest quantitative agreement observed between results of simulation and experiment.     

Impact failure of beams using damage mechanics: Part I—Analytical model

A simple theoretical method, which is based on ductile damage mechanics and which retains strain rate effects, is presented for predicting the failure of beams made from a perfectly plastic material and subjected to impact loads. For this class of materials, the strains can be estimated by defining a hinge length. The definition adopted here leads to reasonable predictions for the plastic strains and the strain rate, as shown by comparing the results with numerical calculations and experimental data. The equivalent strain and the strain rate can be used in the damage model to predict the failure of beams, as shown in a companion paper (Alves, Jones, Int J Impact Eng 2002;27(8):863–90).     

A modified energy-balance model to predict low-velocity impact response for sandwich composites

An energy-balance model is often used to analyze impact dynamics for composite structures. However this model tends to overestimate the peak impact load after the onset of damage since it does not account for damage initiation and propagation. In this paper, the energy-balance model is coupled with the law of conservation of momentum to extend its validity beyond the elastic response regime for a composite sandwich structure subjected to low-velocity impact. Closed-form solutions were derived for the plate’s elastic structural stiffness and the critical load at the onset of damage. The critical load was theoretically predictable by accounting for the elastic energies absorbed by the plate up to core failure. Impact test results also showed that the relative loss of the plate’s transverse stiffness after damage was directly related to the energy absorbed by the plate, which could be calculated given the damage initiation energy. The stiffnesses and the critical load were then used in the modified energy-balance model to predict transient load and deflection histories. Predicted results were comparable with test data, in terms of critical and peak loads, as well as the overall behaviour. This impact model is an efficient design tool which can complement detailed FE simulations.     

Nonlinear compressive stiffness in impacted composite laminates determined by an inverse method

This paper presents use of an inverse method and non-contact optical measurements for determining the apparent compressive stiffness reduction in impact damage zones in composite laminates. The tensile stiffness distribution and nonlinearity is also briefly covered. The method is based on iterative updating of the material properties in a finite element model with the objective to match the predicted displacement fields to those measured optically in impacted specimens under load. To examine the effect of the damage on initial imperfections, strain and buckling, the displacement fields obtained experimentally by digital image correlation are demonstrated and discussed. Finally, the method is applied to the obtained full-field measurements and the influence of applied strain on the nonlinear tensile stiffness and apparent compressive stiffness of real impact damage zones is evaluated. Material nonlinearity in tension is found to increase towards the damage centre where fibre damage is more severe. Stiffness in compression can only be represented by a uniform apparent material nonlinearity, which is strongly linked to local buckling.     

Data science assisted cohesive finite element modelling of impact behaviour of AP-HTPB crystal binder composite

In this work, the response of an ammonium perchlorate (AP)-hydroxyl-terminated polybutadiene (HTPB) composite material under impact loading is presented, utilizing computational cohesive finite element method (CFEM) simulations that are validated with drop hammer experiments. This study examined the impact behaviour of AP crystal sizes between 200 and 400 μm by varying impact velocities between 3 and 10 m/s. Based on the outcome of CFEM simulations, analysis of variance (ANOVA) tests and a response surface method (RSM) were utilized to construct a mathematical model approximating the relationships between simulation inputs and outcomes. Both computational and experimental results show that the local strain rate has a considerable positive correlation with crystal size, and the rate of temperature change has positive correlations with both crystal size and impact velocity. Further, it was observed that stiffness and compression energy are the primary factors to variances in local strain rate and rate of change of temperature. RSM has been found to be an effective tool for modelling impact responses of materials under varying experimental conditions.     

Compressive strength of composite laminates following free edge impact

Barely Visible Impact Damage (BVID) can occur when laminated composite material is subject to free edge impact loads in the plane of the laminate and can result in a significant reduction in compressive strength caused by buckle-driven delamination. This paper will report on a semi-analytical fracture mechanics model that predicts the Compression After Impact (CAI) strength of composite laminates subject to in-plane free edge impact. Compression testing has been undertaken on three impacted coupons in order to validate the theoretical results. Analytical results are shown to be on average within 10% of experimental values for the strain at which propagation of damage occurs.     

Characterization and Simulation of Tensile Deformation of Non‐Uniform Cellular Aluminium Until Damage

The uniaxial tensile modulus and strength of Alulight® foams are measured and simulated taking into account the non‐uniform mass density distribution characterized non‐destructively by X‐ray computer tomography. The density mapping method is employed for the reconstruction of the hard and soft regions in the samples investigated. A finite element (FE)‐model is introduced for simulations of the deformation of a continuum composed by domains of different local densities. Existing constitutive laws for cellular structures are incorporated for the numerical simulation of tensile deformation and the variance of the material parameters is determined with the aid of a scaling relationship. The experimental results for the stiffness, the ultimate strength, and the corresponding strain agree with the developed 3D FE simulations and are compared with the estimations according to scaling laws for uniform cellular structures. The non‐uniformity of the material distribution affects the strength and the ductility significantly. Simulations taking this into account provide conservative property predictions. The calculated positions of local strain concentration correspond with the observed locations of crack initiation. The material modelling and the simulation of the elasto‐plastic deformation up to damage are suggested for application to macroscopic components made of non‐uniform cellular metals.     

Experimental and numerical study of the impact behavior of SMC plates

Steel components absorb impact energy by plastic deformation whilst composite materials absorbing it by damage mechanisms such as fiber debonding, fiber fracture, and matrix cracking. Therefore, in order to properly substitute metal components with composite ones in industrial applications, the impact property of composite materials must be well known. In this study, the impact behavior of sheet molding compounds (SMC), which is widely used in automobile industry due to its relatively low cost and high productivity, was examined both experimentally and numerically. In order to investigate the impact behavior of SMC, an experimental study was carried out by setting up a drop weight impact test system. Using this system, the dissipated impact energies of SMC flat plates were measured to investigate the influence of the mass and shape of impactor, initial velocity, and specimen thickness on the impact behavior.

For numerical predictions, a modified damage model for SMC was developed and adopted in the user defined material subroutine of the commercial simulation program LS-DYNA3D. For the sake of improving efficiency of impact simulations, the SMC material property was determined in consideration of the local differences of the fiber volume fractions. The dissipated impact energies under various conditions and the reliability of the developed impact simulation process were examined through comparisons of the predicted data with the experimental results.

From this comparison, it was found that, in the scope of current study, the specimen thickness is the most important parameter that should be considered in the design of SMC components for the aspect of impact behavior.