A comparison of different approaches for imaging cracks in composites by X-ray microtomography.

X-ray computed tomography (CT) has emerged as a key imaging tool in the characterization of materials, allowing three-dimensional visualization of an object non-destructively as well as enabling the monitoring of damage accumulation over time through time-lapse imaging. However, small defects and cracks can be difficult to detect, particularly in composite materials where low-contrast, plate-like geometries of large area can compromise detectability. Here, we investigate a number of strategies aimed at increasing the capability of X-ray CT to detect composite damage such as transverse ply cracking and delamination, looking specifically at a woven glass fibre-reinforced three-dimensional composite. High-resolution region of interest (ROI) scanning, in situ loading, phase contrast and contrast agents are examined systematically as strategies for improving the defect detectability. Spatial resolution, contrast, signal-to-noise ratio, full width at half maximum, user friendliness and measurement time are all considered. Taken together, the results suggest that high-resolution ROI scanning combined with the increased contrast resulting from staining give the highest defect detectability. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'.

Modelling of stiffness degradation due to cracking in laminates subjected to multi-axial loading.

The paper presents an analytical approach to predicting the effect of intra- and interlaminar cracking on residual stiffness properties of the laminate, which can be used in the post-initial failure analysis, taking full account of damage mode interaction. The approach is based on a two-dimensional shear lag stress analysis and the equivalent constraint model of the laminate with multiple damaged plies. The application of the approach to predicting degraded stiffness properties of multidirectional laminates under multi-axial loading is demonstrated on cross-ply glass/epoxy and carbon/epoxy laminates with transverse and longitudinal matrix cracks and crack-induced transverse and longitudinal delaminations. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'.

Internal instability as a possible failure mechanism for layered composites.

This paper revisits a three-dimensional analytical approach to study internal instability in layered composites, when the behaviour of each component of the material is described by the three-dimensional equations of solid mechanics. It shows the development of a unified computational procedure for numerical realization of the three-dimensional analytical method as applied to various constitutive equations of the layers and fibres, and different loading schemes (uniaxial or biaxial loading). The paper also contains many examples of calculation of critical controlled parameters for particular composites as well as analysis of different buckling modes. The results of this method can be used as a benchmark for simplified models. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'.

Dynamic damage in carbon-fibre composites.

The Taylor test is used to determine damage evolution in carbon-fibre composites across a range of strain rates. The hierarchy of damage across the scales is key in determining the suite of operating mechanisms and high-speed diagnostics are used to determine states during dynamic loading. Experiments record the test response as a function of the orientation of the cylinder cut from the engineered multi-ply composite with high-speed photography and post-mortem target examination. The ensuing damage occurs during the shock compression phase but three other tensile loading modes operate during the test and these are explored. Experiment has shown that ply orientations respond to two components of release; longitudinal and radial as well as the hoop stresses generated in inelastic flow at the impact surface. The test is a discriminant not only of damage thresholds but of local failure modes and their kinetics. This article is part of the themed issue 'Multiscale modelling of the structural integrity of composite materials'.

Factors which may increase stresses at the pin-bone interface in external fixation: a finite element analysis study.

The pin-bone interface of the external fixation system has been studied using a finite element mode. The maximum stresses were observed in the near 'cortex' adjacent to the site of entry of the pin. This location coincides with the area where loosening is usually first observed radiologically. Other results indicate that the stress values are significantly increased by using deep threads and by using stainless steel instead of titanium.

Monitoring fracture healing with strain gauges: the effect on strain values of strain gauge location on the pin circumference.

In an attempt to establish some principles for the use of strain gauges in monitoring fracture healing, four strain gauges were fastened onto the most proximal pin of a Shearer external fixator applied to a hardwood fracture model. Compression and bending loads were applied and the strain data were obtained. The results show that the location of the strain gauge on the circumference of the pin has a significant effect on strain values.

An investigation of the bending stiffness of and the plane stresses generated by a flanged external fixator pin.

In an attempt to reduce pin loosening, a flanged external fixator pin has been designed and its bending stiffness has been compared with that of a standard pin. The pins were inserted into pilot holes previously drilled into a piece of teak hardwood. Loads of different magnitudes were applied at a fixed moment arm and force-deflection curves were obtained. Thereafter, percentage stiffness increase was calculated for each pilot hole size. The results show that the addition of a collar to the external fixator pin increases its stiffness and its ability to resist bending forces. In a parallel study, the stresses generated at the pin-bone interface by this pin and a standard pin were compared using finite element analysis techniques. The results show that the flange significantly reduced the stresses generated at the pin-bone interface. In addition, stresses were dissipated over a wider area.

External fixation of upper limb fractures: the effect of pin offset on fixator stability.

A Shearer fracture fixator was applied to a hardwood model using various pin configurations and load-displacement curves were obtained from compressive and bending loads. The results show that offsetting the pins by 60 degrees increases rigidity by a factor of 4-5.

Comparison of the stresses generated at the pin-bone interface by standard and conical external fixator pins.

The stresses generated at the pin-bone interface by external fixator pins have been compared using finite element analysis. The results show that higher stresses are generated by conical pins compared with standard pins of the same diameter. This raises the possibility that the rate of pin loosening may be higher for conical pins.

Bending stiffness of conical and standard external fixator pins.

The bending stiffnesses of a conical and a standard external fixator pin have been compared. The pins were inserted into pilot holes in a piece of teak hardwood and loads of different magnitudes were applied at a fixed moment arm. Force-deflection curves were obtained for each pin, and stiffness (newtons per metre) and percentage stiffness reduction were calculated for each pilot hole size. The results show that deflection increased (i.e. stiffness decreased) with increasing force or diameter of pilot hole. This loss of stiffness was linear for the standard pin but was bimodal for the conical pin.

Pin-hole shear stresses generated by conical and standard external fixation pins.

Pin-hole shear stresses generated by tapered (conical) and untapered (standard) external fixator pins were compared using a photoelastic technique. The results show that shear stresses progressively increase as a conical pin is inserted but remain the same for the standard pin irrespective of the depth of insertion.

A finite element analysis of the effect of pin distribution on the rigidity of a unilateral external fixation system.

The effect of pin distribution on the rigidity of a unilateral external fixation system has been investigated using finite element techniques. The results show that spreading the pins out along the entire length of the bone is more stable than grouping the pins in clusters as is customary.