The impact behavior of composites and sandwich structures

For many years there has been considerable concern regarding the response of composite materials and lightweight sandwich structures to localized impact loading. Extensive testing has shown that very low impact energies are capable of generating significant damage over a large region. The first part of this paper reviews some of the key studies in this area, focusing primarily on experimental attempts to characterize damage initiation and propagation in these structures. The second section of this paper reviews the attempts to use numerical techniques to model the impact response of composites and sandwich structures

The analysis of the ultimate blast failure modes in fibre metal laminates

Finite element modelling has been applied to simulate various failure modes in fibre metal laminate (FML) panels under localized high intensity blast loading. A relatively simple material model, based on continuum damage mechanics, has been proposed to describe the constitutive response of the composite material in the FMLs. Simulations of the blast response of FMLs with various stacking configurations has been carried out, capturing both perforation and non-perforation failure modes. Blast loading was modelled by a pressure function applied on the front face of the FML panel. The definition of the pressure function accounts for both the temporal as well as the spatial distribution of the blast. The capability of the models has been assessed by comparing the predictions associated with both low and high intensity blast cases with published experimental data. Good qualitative and quantitative agreement has been observed for lay-ups with similar proportions of aluminium and composite. It is believed that the models can be employed for use in parametric studies that would facilitate the adoption of FMLs in wider engineering design

Six SNPs and a TTG indel in sheep desmoglein 4 gene are in complete linkage disequilibrium

Desmoglein 4 (DSG4) plays an important role in the regulation of growth and differentiation of hair follicles in mammals. In this study, a 755 bp long segment of DSG4 was screened in 544 sheep sampled from nine Chinese indigenous breeds and two Western breeds using PCR-SSCP assay with three different pairs of primers. Two of the three fragments showed polymorphisms with genotypes defined as AA, AB, BB and BC, and DD, DE, and EE, respectively. Interestingly, polymorphisms in these two fragments were in strong linkage disequilibrium. Only three haplotypes were found, of which haplotype AD determined by alleles A and D was the major one in all breeds, while haplotype BE was only found in Chinese breeds that possess divergent frequencies ranging from 0.02 to 0.43; haplotype CD was very rare and present in only one Chinese sheep. Sequences of the three haplotypes showed seven single nucleotide polymorphisms (SNPs) and a TTG insertion/deletion (indel), leading to five amino acid substitutions and a glycine indel. Our study provides valuable genetic markers in evaluating the impact of the DSG4 gene on wool traits in sheep.Key words: Sheep, DSG4 gene, single-strand conformational polymorphism (SSCP), variation, linkage disequilibrium

Damage initiation in composite materials under off-centre impact loading

© 2018 Elsevier Ltd The effect of off-centre impact loading on damage initiation in a woven glass fibre reinforced epoxy resin was studied experimentally. Low velocity impact tests were conducted, in which the incident impact energy was increased until damage was observed in the laminates. It was shown that multiple impacts, with increasing incident energy at the same location, did not greatly influence the critical force for damage initiation, Pcrit. Subsequent testing on a range of panel sizes showed that the critical force is highest for central impacts, decreasing slowly as the impact location moves towards the boundary. It was also shown that, for off-centre impact loading, Pcrit, follows a t3/2(t = laminate thickness) relationship that has previously been established for central impact. The slope of the plot of Pcritversus t3/2decreases as the impact location moves away from a central location, suggesting that the effective interlaminar shear stress also decreases with increasing offset. Tests at energies well above the damage threshold confirmed that off-centre impact is more serious than central impact loading. An energy-balance model was used to predict the off-centre impact response of the panels. Agreement between the energy-balance model and the measured impact response was good at energies that did not generate significant damage. Finally, it is suggested that the energy-balance model can also be used to predict a lower bound on the damage threshold energy in composite plates

An experimental and numerical study on scaling effects in the low velocity impact response of CFRP laminates

Scaling effects in the low velocity impact response of plain weave carbon-fibre-reinforced plastic (CFRP) panels have been investigated both experimentally and numerically. The experimental tests were undertaken using an instrumented drop-weight impact tower and the numerical simulations were conducted using the commercially-available finite element (FE) solver ABAQUS/Explicit. Here a rate-dependent damage model was implemented through the ABAQUS user-defined material interface, VUMAT, to describe the mechanical behaviour of the composite laminates. The experimental tests and numerical simulations both indicate that at energies above the damage threshold, damage does not obey a simple scaling law, becoming more severe as the scale size is increased. An examination of the damaged samples in the tests and numerical simulations indicated that, for a given scaled impact energy, fibre damage, in the form of large cracks extending in the warp and weft directions, was more severe in the larger samples. It is argued that the energy absorbed in fibre fracture scales with the square of the scale factor, i.e. n2, whereas the initial impact energy scales as n3. This discrepancy results in increased levels of energy needing to be absorbed in larger scale sizes, leading to greater levels of impact damage in the larger scale sizes

The low velocity impact response of curvilinear-core sandwich structures

The low velocity impact response of lightweight aluminium sandwich panels, based on a curvilinear aluminium alloy core, has been investigated to evaluate their energy-absorbing characteristics and to identify the associated failure mechanisms. Finite element models are then developed to predict the dynamic response of these lightweight structures. Here, an elasto-plastic model, capable of accounting for strain-hardening effects, material rate-dependence, as well as the relevant damage criteria, was employed to predict the dynamic response of the targets. The finite element models were then validated by comparing their predictions against the corresponding experimental results. Good agreement was obtained, indicating that the models are capable of predicting the dynamic behaviour of these all-metal sandwich structures under low velocity impact conditions. Once the finite element model had been validated, it was used to assess the effect of varying key test parameters, such as the projectile diameter, the material properties of the metal substrate as well as the angle of obliquity on the impact response. Here, it has been shown that the perforation energy increases as the impact angle is increased and also as the projectile diameter increases. An investigation of seven different all-metal sandwich structures has shown that an aluminium alloy offers the highest specific perforation resistance under conditions of low velocity impact loading