This paper reviews over fifty studies into the effect of through-the-thickness stitching on the in-plane mechanical properties of fibre-reinforced polymer composites. Reviewed are the in-plane tensile, compressive, flexure, interlaminar shear, creep, fracture and fatigue properties, although little work has been undertaken on the last three properties. When comparing studies it is apparent that many contradictions exist: some studies reveal that stitching does not affect or may improve slightly the in-plane properties while others find that the properties are degraded. In reviewing these studies it is demonstrated that predicting the influence of stitching on the in-plane properties is difficult because it is governed by a variety of factors, including the type of composite (eg. type of fibre, resin, lay-up configuration), the stitching conditions (eg. type of thread, stitch pattern, stitch density, stitch tension, thread diameter), and the loading condition. The implications of these findings for the use of stitching in lightweight engineering structures are discussed.

A review of the effect of stitching on the in-plane mechanical properties of fibre-reinforced polymer composites

New insights are presented into the mechanisms responsible for changes to the in-plane mechanical properties of polymer matrix laminates as a result of through-the-thickness stitching. A critical appraisal of a large amount of published mechanical property data reveals that stitching usually reduces the stiffness, strength and fatigue resistance of a laminate by not more than 10–20%, although in a few cases the properties remain unchanged or increase slightly. This range of changes is observed for loading in compression, tension, bending or shear. Softening and strengthening mechanisms are proposed to account for the changes in properties due to stitching. Areas of research that are needed to further the understanding of the relationships between mechanisms and properties are identified. These include more detailed reporting of the physical properties of the stitched and unstitched laminates (e.g. fiber content, fiber distortions), the stitching conditions (e.g. yarn tension) and more thorough examination of observed mechanisms.

A mechanistic approach to the properties of stitched laminates

Impact and after-impact properties of carbon fibre reinforced composites enhanced with multi-wall carbon nanotubes

A comparison of substantial published data for 3D woven, stitched and pinned composites quantifies the advantages and disadvantages of these different types of through-thickness reinforcement for in-plane mechanical properties. Stitching or 3D weaving can either improve or degrade the tension, compression, flexure and interlaminar shear properties, usually by less than 20%. Furthermore, the property changes are not strongly influenced by the volume content or diameter of the through-thickness reinforcement for these two processes. One implication of this result is that high levels of through-thickness reinforcement can be incorporated where needed to achieve high impact damage resistance. In contrast, pinning always degrades in-plane properties and fatigue performance, to a degree that increases monotonically with the volume content and diameter of the pins. Property trends are interpreted where possible in terms of known failure mechanisms and expectations from modelling. Some major gaps in data and mechanistic understanding are identified, with specific suggestions for new standards for recording data and new types of experiments.

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A mechanistic interpretation of the comparative in-plane mechanical properties of 3D woven, stitched and pinned composites

A status report on delamination resistance testing of polymer-matrix composites

Spherical-shaped ice simulating hailstones were projected onto woven carbon/epoxy composite panels to determine the damage resistance of thin-walled composite structures to ice impact, and to observe the resulting damage modes that occur over a wide range of velocity. To study the behavior of ice in isolation from the complex response of the composite panel targets, impacts onto a dynamic force measurement device were first conducted. These experiments show a linear relationship between the peak measured force and the projectile kinetic energy, regardless of projectile size. Composite panel impact experiments show a linear relationship between the kinetic energy at which failure initiates, referred to as the failure threshold energy (FTE), and the thickness of the panel. Impacts at kinetic energies greater than the FTE produced a multiplicity of damage modes. Glancing impact tests on composite panels show that the FTE can be accurately estimated using a simple trigonometric scaling relationship.

Experimental investigation of high velocity ice impacts on woven carbon/epoxy composite panels

Hail ice impacts are a realistic threat to exposed composite structures such as aircraft fuselage and wing skins, leading-edgeandcontrolsurfaces,enginenacelles,andfanblades.Toshedsomelightontothislittle-understood (and unavoidable ) threat, experimental, numerical, and analytical investigations have been conducted. Experiments in whichcarbon/epoxy compositepanelswereimpacted by icespheresathigh velocity (30‐200m/s)wereconducted to measure 1)the impact energy at which damage initiates, 2 ) the multiple failure modes exhibited by thin composite panels over a range of impact velocity, and 3 ) the elastic response of a composite panel resulting from impact. Subsequent numerical analyses of the last case were performed and were validated through correlation with experimental data. Insights gained from the numerical analyses were used to compose a mechanics-based formula that predicts the initiation of damage formation. This formula, derived using a global energy balance, provides a cost-effective (i.e., lower number of tests needed ) means by which the impact damage resistance of composite structures and material types can be established.

Modeling Hail Ice Impacts and Predicting Impact Damage Initiation in Composite Structures

A new composite structure impact performance assessment program

Recent research programs conducted on low-velocity impact events on composite structures have used force as the sole governing parameter and based their damage resistance and tolerance considerations on the peak recorded value. Understanding of other available parameters, such as contact duration and coefficient of restitution, which are related to the effective structural stiffness of the target, is fundamental in the design of a methodology for assessing impact performance and can offer greater insight in the interpretation of future research programs. An experimental database is gathered through drop tower impact testing by means of a rigid striker on clamped circular plates, for a particular polymer composite system. Several researchers have presented data showing that a critical value of the impact force for the onset of damage exists. Structural properties are hereby studied in both the sub- and supercritical regimes, which means for impact energy values below and above the damage threshold. A modified approach to the classic spring‐mass model, which employs the notions of damaged stiffness and dissipated energy, leads to the derivation of approximate formulas that describe the peak force-energy curve. In particular, the introduction of a dashpot to simulate the effect of damage greatly improves the accuracy of the model in the regime beyond the structural integrity threshold. A novel method to assess the residual performance of the damaged plate is developed, and it consists in low-energy, nondestructive impact testing, the results from which bear a striking resemblance with the curves obtained by compression after impact. MPACT tests can prove inherently difficult to understand because of the large number of parameters that play a key role in such events, particularly in the case of laminated composite structures due to their heterogeneous anisotropic nature and the complex failure modes that can occur. The current experimental investigation is conducted on square plates, supported over circular openings, having a quasi-isotropic [0/90/±45]4s stacking sequence. Such a configuration benefits from the axial symmetry of the circular geometry and the low degree of anisotropy of the laminate and thus facilitates the concentration on the mechanics of the impact event and the complex failure mechanisms. An extensive literature review has indicated that many questions still surround the impact response of composite plates. In particular, an ongoing debate exists on whether force or energy should be used to compare impact test results on different configurations, as well as whether a force- or energy-based criterion should be employed to predict the structural integrity threshold or uniquely and satisfactorily assess the state of damage in the plate. The aim of this paper is to show how governing parameters, such as force, energy, and structural stiffness vary between the subcritical and supercritical regimes and that peak force, although extremely valuable for predicting the damage threshold, cannot be used independently for investigating the impact performance of a composite structure. The use of an instrumented drop tower such as the General Re

http://www.lambolab.org/wp-content/uploads/03research/pub/01impact/2004-AIAAJrnl-LVI.pdf

Keith T. Kedward

Papers

Experimental investigation of high velocity ice impacts on woven carbon/epoxy composite panels

Modeling Hail Ice Impacts and Predicting Impact Damage Initiation in Composite Structures

A new composite structure impact performance assessment program

Enhanced Evaluation of the Low-Velocity Impact Response of Composite Plates

A method for modeling the local and global buckling of delaminated composite plates