多损伤复合材料加筋壁板高周疲劳特性及剩余压缩强度

为研究常用于飞机垂尾的复合材料加筋壁板的冲击疲劳特性,设计了该型加筋壁板多点冲击试验、高周疲劳试验及剩余压缩强度试验。讨论了不同冲击能量对筋条边缘冲击损伤的影响,及施加低应力水平的疲劳载荷后各冲击损伤区域的扩展情况,对比分析了疲劳对冲击后剩余压缩强度的影响。结果表明:40J能量冲击后的损伤面积和凹坑深度较大,C扫描损伤形貌很不规则。100万次低应力疲劳后主损伤区附近衍生出新损伤,导致压缩破坏时产生向上、下夹具扩展的裂痕。该型加筋壁板疲劳后破坏载荷保持率为95.6%,有较好的抗冲击疲劳能力,为加筋壁板耐久性及后屈曲设计提供了思路。     

A progressive damage simulation algorithm for GFRP composites under cyclic loading. Part II: FE implementation and model validation

The FE implementation of FADAS, a material constitutive model capable of simulating the mechanical behaviour of GFRP composites under variable amplitude multiaxial cyclic loading, was presented. The discretization of the problem domain by means of FE is necessary for predicting the damage progression in real structures, as failure initiates at the vicinity of a stress concentrator, causing stress redistribution and the gradual spread of damage until the global failure of the structure. The implementation of the stiffness and strength degradation models in the principal material directions of the unidirectional ply was thoroughly discussed. Details were also presented on the FE models developed, the computational effort needed and the definition of final failure considered. Numerical predictions were corroborated satisfactorily by experimental data from constant amplitude uniaxial fatigue of multidirectional glass/epoxy laminates under various stress ratios. The validation of predictions included fatigue strength, stiffness degradation and residual static strength after cyclic loading.     

Evaluation of impact assessment methodologies. Part II: Experimental validation

The present paper is the second part of an evaluation study of the impact simulation tools CODAC and IDAT. While in Part I the methodologies for stress analysis, failure detection, material degradation and time integration were presented and discussed, Part II focuses on the experimental validation of the applied methodologies. Therefore, computational results in terms of impact damage and residual compression strength are compared to the results of an impact test program with monolithic composite panels. The evaluation of the tools is based on the accuracy and efficiency of the impact and residual strength analyses.     

Compression-after-Impact Strength of Sandwich Panels with Core Crushing Damage

Compression-after-impact (CAI) strength of foam-cored sandwich panels with composite face sheets is investigated experimentally. The low-velocity impact by a semi-spherical (blunt) projectile is considered, producing a damage mainly in a form of core crushing accompanied by a permanent indentation (residual dent) in the face sheet. Instrumentation of the panels by strain gauges and digital speckle photography analysis are used to study the effect of damage on failure mechanisms in the panel. Residual dent growth inwards toward the mid-plane of a sandwich panel followed by a complete separation of the face sheet is identified as the failure mode. CAI strength of sandwich panels is shown to decrease with increasing impact damage size. Destructive sectioning of sandwich panels is used to characterise damage parameters and morphology for implementation in a finite element model. The finite element model that accounts for relevant details of impact damage morphology is developed and proposed for failure analysis and CAI strength predictions of damaged panels demonstrating a good correlation with experimental results.     

An engineering approach for predicting residual strength of carbon/epoxy laminates after impact and hygrothermal cycling

A semi-empirical analysis on residual compressive strength (RCS) of carbon/epoxy woven composite laminate was developed which included the damage effects caused by impact and hygrothermal cycling. Impact damage is modelled as a soft inclusion with an exponentially reduced stiffness and the stiffness is further reduced due to hygrothermal cycling. A complex variable method was used to determine the in-plane stress distribution near the impact-induced damage and point stress failure criterion is then used to predict the failure load. Based on the semi-empirical model, the RCS can be related to damage width, damage intensity, undamaged strength and a degradation factor due to hygrothermal cycling. The results from the analysis coincide reasonably well with the experimental data for the plain-woven fabric laminates.     

Finite element modelling of impact on preloaded composite panels

Composite aircraft structures are susceptible to impact damage during manufacture, maintenance and in-flight. Low energy impact damage is often internal and invisible, but can significantly reduce the stiffness and strength or cause catastrophic failure when the structure is under load during the impact event. This paper describes the development and application of an explicit finite element (FE) model, incorporating a bi-phase material degradation model, to predict the behaviour of loaded carbon/epoxy panels when impacted over a range of low energy levels. Overall, the trends predicted in the FE simulations were consistent with experimental data, although quantitatively the FE results were generally conservative. However, the model greatly underestimated the catastrophic failure boundary. The model was used to investigate the effect of various parameters including magnitude of preload, impact velocity and specimen geometry on the amount of damage and the residual strength of carbon/epoxy panels.     

Assessment of damage tolerance in composites

Damage tolerance to processing and normal service damage has been assessed for carbon epoxy coupons and built-up panels in uniaxial loading. In coupons, the low velocity impact damage is more severe than damage in holes, delaminations or porosity and it causes compression strength loss from 58% at the barely visible threshold to 73% at the easily visible threshold. Damaged coupon fatigue ($\text{R}\text{= 10}$) S-N curves are relatively flat with 67% strength loss at 10⁶ cycles for impact damage. In built-up panels representing multispar (M-S) and multirib (M-R) wing designs, again impact damage is more severe than delaminations and the impacted M-R design is stronger than the impacted M-S design. Damage in two adjacent M-S midbays and at the edge of the M-R stringer flange are the critical locations for impact, but damage area strongly depends on the panel configuration. Impact damage grows under constant amplitude fatigue ($\text{R}\text{= 10}$) with peak load at 65% of damaged static strength for both M-S and M-R designs. Built-up configuration of the panels provides a significant increase in impact damage tolerance over that of coupons.     

复合材料层板低速冲击后剩余压缩强度

对两种材料体系和铺层的复合材料层合板进行低速冲击后压缩强度试验 , 以研究低速冲击后层合板的压缩破坏机理。讨论了表面凹坑深度、 背面基体裂纹长度、 损伤面积以及剩余压缩强度与冲击能量的关系。在试验研究的基础上 , 建立了复合材料低速冲击后剩余强度估算的一种椭圆形弹性核模型。该模型将冲击损伤等效为一刚度折减的椭圆形弹性核 , 采用含任意椭圆核各向异性板杂交应力有限元分析含损伤层合板的应力应变状态 ,并应用点应力判据预测层板的压缩(或压、 剪)剩余强度。理论分析与试验结果对比表明 , 该模型简单有效。      

Residual stress and damage effect on integrity of ground silicon nitride

Ceramic workpiece integrity and residual surface stresses generated by single pass diamond grinding were evaluated for three flaring cup wheels and four machine-loop stiffnesses. Stresses in silicon nitride bars ground on one face were characterized by X-ray diffraction, strength by four-point bending, and grinding damage depth by scanning electron microscopy. A custom-built workpiece holder was used to tune the grinding machine-loop stiffness. Electrolytic in-process dressing was applied to one of the wheels to provide stable cutting conditions. The experimental results indicate machine stiffness does not have significant influence on flexural strength, but rather affects the depth of cut. All ground surfaces have some degree of damage and residual stress, and differences are revealed between wheel bonds and grit sizes. The competing phenomena of strength enhancement due to residual stress and strength degradation due to damage are discussed.     

Damage tolerance of impacted curved panels

The final aim of this study is to evaluate the influence of impact damage on the residual strength of carbon/epoxy vessels stressed by internal pressure. An intermediate stage determined the residual behaviour of pre-impacted curved panels loaded in tension. Curved panels were impacted, reproducing the damage types observed in impacted vessels filled with propellant. Delamination damage was assessed by ultrasonics and optical microscopy used to observe intra-laminar mechanisms. Tension after impact (TAI) tests quantified the residual behaviour. An experimental design was used as an alternative to the complex analytical modelling of dynamic damage mechanisms. With this original technique, empirical relationships were established, linking impact parameters to residual properties. The force to failure was found to vary in a bi-linear manner with impact energy. Below a specific level of impact energy corresponding to failure in 4/7 of the plies, there is no significant reduction in the residual strength. The composite Young's modulus decreased linearly with impact energy.     

Long-term hygrothermal effects on damage tolerance of hybrid composite sandwich panels

A sandwich construction, composed of hybrid carbon-glass fibre-reinforced plastic skins and a syntactic foam core, was selected as the design concept for a wind tunnel compressor blade application, where high damage tolerance and durability are of major importance. Beam specimens were prepared from open-edge and encapsulated sandwich panels which had previously been immersed in water at different temperatures for periods of up to about two years in the extreme case. Moisture absorption and strength characteristics, as related to time of exposure to hygrothermal conditions, were evaluated for the sandwich specimens and their constituents (skins and foam). After different exposure periods, low-velocity impact damage was inflicted on most sandwich specimens and damage characteristics were related to impact energy. Eventually, the residual compressive strengths of the damaged (and undamaged) beams were determined flexurally. Test results show that exposure to hygrothermal conditions leads to significant strength reductions for foam specimens and open-edge sandwich panels, compared with reference specimens stored at room temperature. In the case of skin specimens and for beams prepared from encapsulated sandwich panels that had previously been exposed to hygrothermal conditions, moisture absorption was found to improve strength as related to the reference case. The beneficial effect of moisture on skin performance was, however, limited to moisture contents below 1% (at 50°C and lower temperatures). Above this moisture level and at higher temperatures, strength degradation of the skin seems to prevail.     

复合材料层压板剩余刚度剩余强度关联模型

基于剩余强度和剩余刚度取决于同一损伤状态的假设,给出了基于剩余刚度的损伤定义和基于剩余强度的损伤定义之间的关系,建立了剩余刚度剩余强度关联模型。用3种不同铺层形式的层压板试验数据对本文中提出的剩余刚度模型及剩余强度模型进行了验证,结果表明:本文中提出的剩余刚度和剩余强度模型能很好地描述复合材料层压板疲劳过程中的剩余刚度和剩余强度退化规律;通过关联模型,可以在已知剩余刚度退化规律的前提下,用少量剩余强度试验确定剩余强度退化规律;与剩余刚度关联的剩余强度模型中的参数可以被认为是材料常数。      

考虑温度和纤维含量影响的单向层合板材料的退化模型

根据复合材料的疲劳损伤机理,重新定义了疲劳损伤因子.根据这个疲劳损伤因子,提出了一种考虑纤维的含量和温度影响的单向纤维增强复合材料剩余刚度和剩余强度的模型;进而根据室温剩余刚度-剩余强度关联模型引入温度修正参数得到了一定温度下的剩余刚度-剩余强度关联模型,并进一步得到了与剩余刚度相关的剩余强度模型.于是,在建立剩余强度...     

层板复合材料疲劳性能的实验研究

对层板复合材料在拉伸 -拉伸疲劳载荷作用下的初始静刚度、初始静强度、剩余刚度、剩余强度、疲劳寿命进行了实验研究 ,取得了大量的有意义的实验数据 ,分析了层板复合材料的初始静刚度、初始静强度和疲劳寿命的概率分布 ,讨论了层板复合材料在不同应力水平下剩余刚度随疲劳循环周次的衰减变化及损伤破坏的形式 ,得到了一些有意义的结论     

复合材料层合板低速冲击后压-压疲劳试验研究及寿命预报

进行了复合材料层合板低速冲击和冲击后压-压疲劳试验。在疲劳试验过程中详细测量了损伤扩展情况,获得了损伤扩展规律。将冲击损伤等效为一圆形开孔,应用含椭圆形夹杂的杂交应力单元分析含圆孔有限大板的应力分布,采用特征曲线和点应力判据相结合的方式并通过引入损伤扩展规律建立了含低速冲击损伤复合材料层板压-压疲劳寿命预测模型。通过与试验数据的对比,证明了该模型的有效性。同时,该模型还可预报在疲劳载荷下含冲击损伤层板的剩余压缩强度。     

Low velocity impact and compression after impact characterization of woven carbon/vinylester at dry and water saturated conditions

The low velocity impact response and compression after impact strength of dry and water saturated plain weave carbon/vinylester composites have been determined. The composites employed T700 carbon fibers and vinylester 510A and 8084 resins. Quasi-static impact tests were conducted on dry C/VE510A and C/VE8084 to estimate the threshold impact force required to initiate damage in the composites. Falling-weight impact tests were conducted on the composites over a range of impact energies from 6.7 to 47 J. Destructive inspection of damaged panels revealed damage in the form of matrix cracks as well as delamination between fiber bundles. The quasi-static estimation of the threshold impact force was in reasonable agreement with that measured in the impact test. To examine structural degradation due to impact loading, impacted panels were tested in compression (CAI). The CAI strength decreased with increasing impact energy. Absorbed moisture caused further reductions of the CAI strength.     

复合材料T型加筋板筋条冲击损伤及冲击后压缩行为试验

本文研究了复合材料加筋板的筋条冲击损伤及冲击损伤对加筋板轴向压缩（CAI）行为的影响。针对T型单筋加筋板,通过落锤法从面板一侧对筋条进行5种能量水平的低速冲击。试验结果表明：冲击筋条产生的面板凹坑不易观察;当冲击能量低于筋条损伤门槛能量时,加筋板筋条无损伤出现,筋条-面板也不会发生脱粘;一旦冲击能量超过筋条损伤门槛能量,筋条的腹板会在弯曲拉伸应力作用下损伤,同时筋条-面板之间会出现严重脱粘。分别对完好和损伤试验件进行压缩试验,试验结果显示：低于门槛能量的冲击对加筋板的压缩屈曲载荷影响不大,同时只会略微降低失效载荷;而冲击造成筋条损伤后,筋条在压缩过程中会由于损伤扩展出现卸载;卸载后的筋条会对面板失去支撑,使面板的屈曲载荷降低,从而大幅地削弱加筋板的承载能力。     

Compression after impact test (CAI) on NOMEX™ honeycomb sandwich panels with thin aluminum skins

Sandwich panels are used in industrial fields where lightness and energy absorption capabilities are required. In order to increase their exploitation, a wide knowledge of their mechanical behavior also in severe loading conditions is crucial. Light structures such as the one studied in the present work, sandwich panels with aluminum skins and Nomex^™ honeycomb core, are exposed to a possible decrease of their structural integrity, resulting from a low velocity impact. In order to quantitatively describe the decrease of the sandwich mechanical performance after an impact, an experimental program of compression after impact tests (CAI) has been performed. Sandwich panel specimens have been damaged during a low velocity impact test phase, using an experimental apparatus based on a free fall mass tower. Different experimental impact energies have been tested. Damaged and undamaged specimens have been consequently tested adopting a compression after impact procedure. The relation between the residual strength of the panel and the possible relevant parameters has been statistically investigated. The results show a clear reduction of the residual strength of the damaged panels compared with undamaged ones. Nevertheless, a reduced dependency between the impact energy and the residual strength is found above a certain impact energy threshold.     

To replace or not to replace? — An investigation into the residual strength of damaged rock climbing safety equipment

This paper presents a study on the residual strength of carabiners which have sustained impact damage due to accidental dropping during lead climbing. The question answered here is whether damaged quickdraws can be reused for future climbing or whether they should be replaced. Well defined damages were introduced into the main body and the gate of three different types of quickdraw carabiners. The carabiners were visually inspected and tested for functionality before the residual strength was measured following procedures defined in mountaineering standards. Contrary to common perception no micro-cracks were found within the damaged carabiners. In general, the carabiners tested here showed good resistance to impact damage. Impact on the main body does not seem to affect the residual strength. Impact on the wire gate may result in failure of the gate. However, if the gate is still functional, the strength is not affected by an impact.     

Fracture due to damage from projectile impact

In the usual application, the methods of fracture mechanics are used to determine growth rates and critical load levels for sharp-tipped flaws such as fatigue cracks. By way of contrast to this classic application, another way of introducing fracture producing flaws is to expose a structure to impacts from penetrating projectiles. This paper presents recent test data and analytical techniques for predicting the fracture response of tensile panels impacted by small caliber bullets. Panels of 7075-T6, 2024-T81 and 6A1-4V were tested in thicknesses ranging from 0.032 to 0.375 in. A bracketing technique was used to establish threshold values of prestress for catastrophic fracture. For comparison, a number of impact damaged panels were statically tested for residual tensile strength. These results are compared with the impact fracture threshold values and with strength predictions using the plane-stress stress intensity factor for center-notched panels.