Crack deflection analyses of different peel stopper designs for sandwich structures

This paper relates to a newly developed peel stopper concept for sandwich structures. The proposed concept is a specially designed core insert, which has the ability to confine face sheet debonding/delamination (peeling) by deflecting a delamination crack front away from the face/core interface into the bulk of the sandwich core, and thereby constraining the debonding/delamination to a limited prescribed area. In this paper various peel stopper designs are analysed for their ability to deflect cracks away from propagating along a face–core interface. The crack deflection ability of the studied peel stopper designs leads to design guidelines, which describes the minimum requirements regarding the relation between the two interface toughnesses. The analysis further reveals that compliant peel stopper wedges are preferred because they lead to the lowest interface toughness ratio requirement. This has been confirmed through an experiment with a sandwich beam subjected to three-point bending loading. The experiment has shown that the ability of a peel stopper to deflect cracks is highly dependent on the stiffness of the wedge.     

Fatigue Performance of Sandwich Beams With Peel Stoppers

Abstract:  This paper concerns a newly developed peel stopper for sandwich structures, which may be embedded as a core insert or an edge stiffener. The major purpose of the peel stopper is to prevent large debonds/delaminations between face sheets and core in sandwich structures in the case of failure. Experimental investigations of conventional sandwich beams and beams furnished with peel stoppers, under static and fatigue loading conditions, and with temperature monitoring, were conducted. The experimental programme included investigation of crack initiation and propagation, as well as of fatigue endurance of conventional and modified sandwich beams. The results showed that although the peel stoppers did not significantly influence the fatigue life of the sandwich beams, they were exceptionally effective in re-routing the crack propagation away from the face–core interface. Moreover, one of the two peel stopper designs presented prevented face–core debonding/delamination and total failure of the sandwich beams.     

Compression strength of sandwich panels with sub-interface damage in the foam core

This paper addresses the effect of a local quasi-static indentation or a low-velocity impact on the residual strength of foam core sandwich panels subjected to edgewise compression. The damage is characterized by a local zone of crushed core accompanied by a residual dent in the face sheet. Experimental studies show that such damage can significantly alter the compressive strength. Theoretical analysis of the face sheet local bending is performed for two typical damage modes (with or without a face–core debonding). The solutions allow estimation of the onset of (a) an unstable dent growth (local buckling) or (b) a compressive failure in the face sheet. The theoretical results are in agreement with the test data for two considered sandwich configurations.     

Compression and bending performances of carbon fiber reinforced lattice-core sandwich composites

To restrict debonding, carbon fiber reinforced lattice-core sandwich composites with compliant skins were designed and manufactured. Compression behaviors of the lattice composites and sandwich columns with different skin thicknesses were tested. Bending performances of the sandwich panels were explored by three-point bending experiments. Two typical failure mechanisms of the lattice-core sandwich structures, delaminating and local buckling were revealed by the experiments. Failure criteria were suggested and gave consistent analytical predictions. For panels with stiff skins, delamination is the dominant failure style. Cell dimensions, fracture toughness of the adhesives and the strength of the sandwich skin decide the critical load capacity of the lattice-core sandwich structure. The mono-cell buckling and the succeeding local buckling are dominant for the sandwich structures with more compliant skin sheets. Debonding is restricted within one cell in bending and two cells in compression for lattice-core sandwich panels with compliant face sheets and softer lattice cores.     

Moisture diffusion in composite sandwich structures

The mechanical properties of polymer core materials in sandwich structures are often degraded by moisture that is absorbed during storage. To date, there is no reliable model to predict the amount of moisture that is present in these sandwich core materials. A multi-layer diffusion model applicable to these sandwich structures is described in this report. Inputs to this model are: (1) diffusivities of core and face sheet materials as functions of temperature, (2) moisture saturation data as a function of relative humidity, and (3) sandwich structure exposure history. The output is a prediction of the amounts of moisture in the core material and face sheets as a function of time.

In order to validate this model, moisture diffusion experiments were performed on a sandwich material consisting of graphite–epoxy face sheets and a core of Rohacell^® polymethacrylimide 200WF foam. Samples of this material were dried, and then hydrated at either 32 °C or 65 °C at either 83% or 100% relative humidity. The face sheets were separated from the core and each component was weighed, dried, and weighed again in order to determine the moisture distribution in the sandwich structure. The results were then compared with the model predictions.     

Experimental and numerical evaluation of the mechanical behaviour of GFRP sandwich panels

This work deals with the analysis of the mechanical behaviour of a class of sandwich structures widely employed in marine constructions, constituted by fiber-glass laminate skins over PVC foam or polyester mat cores. In detail, a systematic experimental study and numerical simulations have shown that the theoretical prediction of the strength and the actual failure mechanism of these sandwich structures can be affected by significant errors, specially in the presence of prevalent shear loading. Moreover, because of the low shear stiffness and the elastic constants mismatch of the skins and core material, failure modes and strength are strongly influenced by eventual stresses orthogonal to the middle plane of the sandwich. In particular, for the sandwich structures with a PVC foam core, such a stress interaction leads to early skin–core delamination failure, whereas for those with a polyester core it leads to core shear-cohesive failure. By means of accurate non-linear simulations, accurate failure criteria, that can be used at the design stage in the presence of complex loading, have also been developed.     

缝合泡沫夹芯复合材料低速冲击的多尺度数值方法

为了发展缝合泡沫夹芯复合材料低速冲击损伤的多尺度分析方法, 建立了缝合泡沫简化力学模型, 将缝合泡沫等效为缝线树脂柱增强的正交各向异性芯材, 其材料参数由各组分性能及所占体积分数根据均一化理论计算得出; 同时, 建立冲击试验有限元模型, 通过界面元模拟面板与芯材之间的层间分层。采用GENOA渐进损伤分析模块对缝合结构冲击动态响应过程进行数值模拟, 并将计算结果与试验记录进行对比分析。结果表明: 缝合可以减小面板破坏面积, 抑制面板与泡沫分层的扩展; 但缝纫会对结构造成初始损伤, 较高的缝合密度使芯材刚度增加, 不利于泡沫结构的缓冲吸能。数值模拟结果与试验记录吻合良好, 验证了多尺度分析方法的正确性。     

Effects of local damage on vibration characteristics of composite pyramidal truss core sandwich structure

The effects of local damage on the natural frequencies and the corresponding vibration modes of composite pyramidal truss core sandwich structures are studied in the present paper. Hot press molding method is used to fabricate intact and damaged pyramidal truss core sandwich structures, and modal testing is carried out to obtain their natural frequencies. A FEM model is also constructed to investigate their vibration characteristics numerically. It is found that the calculated natural frequencies are in relatively good agreement with the measured results. By using the experimentally validated FEM model, a series of numerical analyses are conducted to further explore the effects of damage extent, damage location, damage form on the vibration characteristics of composite pyramidal truss core sandwich structures as well as the influence of boundary conditions. The conclusion derived from this study is expected to be useful for analyzing practical problems related to structural health monitoring of composite lattice sandwich structures.     

Impact response of three-dimensional stitched sandwich composite

The paper aims at evaluating the damage resistance of sandwich structures composed of stitched foam core and glass facesheets subjected to low-velocity impact. To obtain a suitable baseline comparison, the equivalent set of properties was measured for an equivalent unstitched sandwich.Based on the force and energy histories, parameters have been introduced as following: load at incipient damage, maximum load, penetration depth at maximum load, total energy absorbed during impact and impact damage area. The impact resistance of the sandwich structure is greatly improved by the presence of the stitches. Skin/core delamination is limited and initial energy is used to degrade core’s stitches. Moreover the global behavior under impact is influenced by the stitching geometrical parameters.     

Fractographic observations on delamination growth and the subsequent migration through the laminate

This paper describes fractographic observations from the detailed examination of delamination fracture surfaces and offers an interpretation of the key growth mechanisms. Firstly, the relationship between toughness, delamination failure criteria and fracture morphology is presented and the influence of cusp formation and deformation on toughness is discussed. Observations regarding delaminations migrating through the lamina at multidirectional ply interfaces are then discussed. It is demonstrated how this migration process can be avoided in fracture toughness coupons and consequently the toughness of multidirectional ply interfaces can be characterised. The influence of migration on delamination growth from embedded defects in laminates under compression is presented, and these results are extended to demonstrate how migration influences damage growth in structures. The paper concludes by making recommendations for realistic modelling of migration, and suggests how it can be exploited in damage tolerant structural design.     

Crack deflection by core junctions in sandwich structures

The paper treats the problem of crack propagation in sandwich panels with interior core junctions. When a face-core interface crack approaches a tri-material wedge, as it may happen at a sandwich core junction, two options exist for further crack advance; one is for the interface crack to penetrate the wedge along the face-core interface, and the second is deflection along the core junction interface. Crack deflection is highly relevant and a requirement for the functionality of a newly developed peel stopper for sandwich structures. The physical model presented in this paper enables the quantitative prediction of the ratio of the toughnesses of the two wedge interfaces required to control the crack propagation, and the derived results can be applied directly in future designs of sandwich structures. The solution strategy is based on finite element analysis (FEA), and a realistic engineering practice example of a tri-material composition (face and core materials) is presented.     

Elastic–plastic response of structural foams subjected to localized static loads

Structural foams are increasingly used in engineering applications where high strength and low weight are important. They are used also as energy absorbers. Sandwich structures are a typical area for application of structural foams (as core materials). In a sandwich structure, the core transfers the transverse forces as shear stresses and supports the face sheets against buckling and wrinkling. The structural foams are notoriously sensitive to failure by the application of localized surface loads. Thus, the proper design requires an understanding of the mechanical response of the foam materials to localized external loads.In this paper, the elastic–plastic behavior of closed-cell cellular foams subjected to point and line loads is investigated both experimentally and numerically. Two types of Divinicell foam (H60 and H100) are studied. A finite element modeling procedure is developed using the ABAQUS package. Both plane and axisymmetric formulations for local indentations by rigid bodies are considered. The plastic behavior is described using the *CRUSHABLE FOAM HARDENING material model. This model is calibrated using experimental curves from uniaxial compression tests. Geometrical non-linearity is also taken into account. Both indentation and unloading phases are modeled. Static indentation tests of foam panels and beams are performed using spherical and cylindrical indentors, respectively. A comparison of indentation response obtained from the numerical analysis and from the tests is carried out. A good agreement between the modeling and the experimental data is achieved. In perspective view, the present investigation can contribute towards the development of a damage tolerance methodology for rigid foams.     

Modelling complex progressive failure in notched composite laminates with varying sizes and stacking sequences

The emergence of advanced computational methods and theoretical models for damage progression in composites has heralded the promise of virtual testing of composite structures with orthotropic lay-ups, complex geometries and multiple material systems. Recent studies have revealed that specimen size and material orthotropy has a major effect on the open hole tension (OHT) strength of composite laminates. The aim of this investigation is develop a progressive failure model for orthotropic composite laminates, employing stepwise discretization of the traction–separation relationship, to predict the effect of specimen size and laminate orthotropy on the OHT strength. The results show that a significant interaction exists between delamination and in-plane damage, so that models without considering delamination would over-predict strength. Furthermore, it is found that the increase in fracture toughness of blocked plies must be incorporated in the model to achieve good correlation with experimental results.     

Examination of the failure of sandwich beams with core junctions subjected to in-plane tensile loading

The paper concerns local effects occurring in the vicinity of junctions between different cores in sandwich beams subjected to tensile in-plane loading. It is known from analytical and numerical modelling that these effects display themselves by an increase of the bending stresses in the faces as well as the core shear and transverse normal stresses at the junction. The local effects have been studied experimentally to assess the influence on the failure behaviour both under quasi-static and fatigue loading conditions. Typical sandwich beam configurations with aluminium and glass-fibre reinforced plastic (GFRP) face sheets and core junctions between polymer foams of different densities and rigid plywood or aluminium were investigated. Depending on the material configuration of the sandwich beam, premature failure accumulating at the core junction was observed for quasi-static and/or fatigue loading conditions. Using Aluminium face sheets, quasi-static loading caused failure at the core junction, whereas no significance of the junction was observed for fatigue loading. Using GFRP faces, a shift of the failure mode from premature core failure in quasi-static tests to face failure at the core junction in fatigue tests was observed. In addition to the failure tests, the sandwich configurations have been analysed using finite element modelling (FEM) to elaborate on the experimental results with respect to failure prediction. Both linear modelling and nonlinear modelling including nonlinear material behaviour (plasticity) was used. Comparing the results from finite element modelling with the failure behaviour observed in the quasi-static tests, it was found that a combination of linear finite element modelling and a point stress criterion to evaluate the stresses at the core junction can be used for brittle core material constituents. However, this is generally not sufficient to predict the failure modes and failure loads properly. Using nonlinear material properties in the modelling and a point strain criterion improves the failure prediction especially for ductile materials, but this has to be examined further along with other failure criteria.     

十字嵌锁型格栅夹芯结构设计及低速冲击性能

针对传统复合材料格栅夹芯结构极限承载能力较低、单胞封闭易造成水汽凝结的问题,在分析管胞微观结构和功能性的基础上,提出一种新型十字嵌锁型格栅夹芯结构。首先选取最小体积(最小质量)和最小变形(最大刚度)为优化目标,利用第二代非支配遗传算法(NSGA-Ⅱ)完成多目标优化,采用三维Hashin失效准则和改进的刚度退化方法建立格栅夹芯板的冲击渐进损伤有限元分析模型,研究多种低速冲击载荷对不同相对密度夹芯结构的不同位置的破坏机制及力学响应。结果表明：新型格栅夹芯结构表现出良好的低速冲击阻抗,其随芯子的空间分布存在差异,格栅间隙处的抗冲击性能较弱,芯子密度的提高不能有效增强该位置处的冲击强度,夹芯结构所受到的破坏远远大于冲击器撞击格栅交点处的情况;受不同冲击位置和冲击速度的影响,载荷-时间和位移-时间曲线呈现出不同的典型模式,芯子出现屈曲、分层、粘接剥离、折弯变形等失效形式,复合材料上面板发生混合损伤,随着冲击速度的增加,芯子和面板的损伤程度也愈严重。     

Fatigue failure of sandwich beams with face sheet wrinkle defects

This paper presents experimental fatigue results for GFRP face sheet/balsa core sandwich beams with face sheet wrinkle defects, subjected to fully reversed in-plane fatigue loading. An estimate of the fatigue design limit is presented, based on static test results, finite element analyses and application of the Northwestern University failure criteria. The presence of a wrinkle defect reduced the fatigue life by approximately 66%, compared to that of an unnotched reference laminate. Furthermore, the results from the fatigue tests revealed that the design limit was initially overestimated, as the specimens loaded close to the predicted design limit typically failed before reaching the target life, or reached test run-out with visible face sheet damage indicating imminent final failure in the worst case. It was found that specimens would reach target life with no visible or otherwise detectable damage by lowering the fatigue load amplitude below 80% of the predicted design limit. By extrapolating the test results it appears that the undamaged specimens would reach a fatigue life of 10⁷–10⁸ load cycles and would thus be safe for design of wind turbine blades.     

Cohesive layer models for predicting delamination growth and crack kinking in sandwich structures

There are potentially two types of fracture that sandwich structures with strong and stiff facing sheets and lightweight cores are liable to suffer. These are the delamination growth at the face-sheet core interface and crack kinking into the sandwich core, respectively. The paper proposes computational models to simulate these failure mechanisms. The models employ the cohesive layer concept and are so constructed as to ensure that the crack advance is controlled by the critical value of strain energy release rate in mode I fracture. Of these, the first model can treat only delamination along a predetermined plane and is designated as CLD (cohesive layer delamination model). The performance of this model is thoroughly investigated in the light of experimental results. The influence of the key parameters of the model, viz. the thickness of the cohesive layer and the strength and stiffness of the cohesive layer material, have been studied. It is found that the model, as developed in this study, is fairly robust and is not sensitive to changes in parameters other than the critical strain energy release rate. The second model can track crack growth which is not predetermined in its direction. This it does by identifying the element in which the maximum principal tensile stress exceeds a critical value; once a crack is nucleated, the stress across the crack is relieved so that the right amount of energy is released when the crack is fully developed - much in the same manner as in a cohesive layer model. This model is designated as CLDK (Cohesive Layer Delamination and Kinking) model as it deals with interfacial delamination and crack kinking- whichever is the preferred mode of fracture. Experimental results of three sandwich specimens, viz. bottom restrained beams with 0° and –10° tilt angle, respectively, and a compressed beam, were used for comparison. The results indicate that the both the models are able to capture the initiation and track the growth of the interfacial delamination. The CLDK model is capable in addition to track the crack kinking into the core, and its subsequent return to the face sheet-core interface.     

SANDWICH STRUCTURE DELAMINATION OF RESIN TRANSFER MOLDING

One of the apparent advantages of sandwich structures is that after the core is made, the sandwich is produced in one process by resin transfer molding (RTM) and no adhesive is used between the core and skins. The bond between the core and skins is therefore likely to depend upon the core material, the type of matrix and the core surface roughness. This is of great importance, because the stiffness of the sandwich structure is likely to be reduced by even partial delamination of the core and skins. The objective of this study was to ascertain the effects of manufacturing parameters such as injection pressure, mold temperature, core thickness and core surface roughness on skin/core adhesion using the direct tensile adhesion and peel test methods. Polyurethane foam was used as the core material throughout the work. The major objective was to examine different surface treatment methods by which the strength of the skin-core bond could be improved. The influence of the core surface roughness on the adhesive fracture energy and the delamination between core and skin were also measured. The fracture energy release rate equation was used as the basis for comparison and for measurements of the adhesion. For this purpose a double-cantilever beam was used to characterize the delamination. Critical energy release rate (G_IC) and fracture toughness (K_IC) were calculated using several alternative methods based on linear elastic fracture mechanics.     

Transverse impact damage and energy absorption of 3-D multi-structured knitted composite

Knitted composites have higher failure deformation and energy absorption capacity under impact than other textile structural composites because of the yarn loop structures in knitted performs. Here we report the transverse impact behavior of a new kind of 3-D multi-structured knitted composite both in experimental and finite element simulation. The knitted composite is composed of two knitted fabrics: biaxial warp knitted fabric and interlock knitted fabric. The transverse impact behaviors of the 3-D knitted composite were tested with a modified split Hopkinson pressure bar (SHPB) apparatus. The load–displacement curves and damage morphologies were obtained to analyze the energy absorptions and impact damage mechanisms of the composite under different impact velocities. A unit-cell model based on the microstructure of the 3-D knitted composite was established to determine the composite deformation and damage when the composite impacted by a hemisphere-ended steel rod. Incorporated with the unit-cell model, a elasto-plastic constitute equation of the 3-D knitted composite and the critical damage area (CDA) failure theory of composites have been implemented as a vectorized user defined material law (VUMAT) for ABAQUS/Explicit. The load–displacement curves, impact deformations and damages obtained from FEM are compared with those in experimental. The good agreements of the comparisons prove the validity of the unit-cell model and user-defined subroutine VUMAT. This manifests the applicability of the VUMAT to characterization and design of the 3-D multi-structured knitted composite structures under other impulsive loading conditions.