The following paper summarises a number of international research projects being undertaken to understand the mechanical properties of natural cellulose fibres and composite materials. In particular the use of novel techniques, such as Raman spectroscopy, synchrotron x-ray and half-fringe photoelastic methods of measuring the physical and micromechanical properties of cellulose fibres is reported. Current single fibre testing procedures are also reviewed with emphasis on the end-use in papermaking. The techniques involved in chemically modifying fibres to improve interfacial adhesion in composites are also reviewed, and the use of novel fibre sources such as bacterial and animal cellulose. It is found that there is overlap in current international research into this area, and that there are complementary approaches and therefore further combining of these may make further progress possible. In particular a need to measure locally the adhesion properties and deformation processes of fibres in composites, with different chemical treatments, ought to be a focus of future research.

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Review: Current international research into cellulosic fibres and composites

The design of a automated system for photoelastic analysis of complex components is described, and an outline of the theory used in its operation is given. The potential of the system for providing detailed data over the full field of view is demonstrated by the analysis of a slice from a model of a bolt.

Towards full field automated photoelastic analysis of complex components

Mixed-mode fracture of orthotropic functionally graded materials using finite elements and the modified crack closure method

The factors affecting the mechanical and environmental durability (or stability), and performance of the adhesively bonded joints in various adherends including metallic alloys, polymers and composite materials are studied in detail. The primary function of a joint is to transfer load from one structural member to another. In most bonded joints the load transfer takes place through interfacial shear. At present, the use of adhesive bonded joints are largely applied to secondary non-critical structures. Whereas the use of adhesive bonding in primary structural applications has been somewhat limited because of the difficulty in defining and predicting joint strength, and designing the joint geometry to optimize strength and reliability. The determination of adhesive joint strength is complicated primarily by the nature of the polymeric material itself. Since these problems are mainly mechanical in nature, stress analysis is required to understand how the force loads are distributed along the adherends and adhesive layer. Most structural engineers consider the durability or stability of a joint to be fatigue related. This is only partly true for adhesive bonds as most durability issues are driven by environmental resistance rather than fatigue loads. The environmental resistance of an adhesive bond is determined by the chemical bonds formed during cure of the adhesive and the resistance of the chemical bonds to environmental degradation. Environmental resistance is fundamental to the durability of a bonded joint or repair. Most in-service failures are caused by environmental degradation of the interface between the bonding surface and the adhesive. Although the use of adhesive bonding is increasing rapidly, there are still important issues which need to be addressed in joint analysis, design, durability, and performance considerations. Therefore, the study of joints usually involves consideration of (a) joint geometries, (b) materials (i.e., adhesives and adherends), (c) loading conditions (i.e., static and dynamic loadings), (d) failure modes (i.e., cohesive, adhesive or mixed failure modes), and (e) temperature and moisture or environmental effects (humidity, solvents, corrosion, temperature extremes, thermal cyling etc.). Therefore, in the present paper the adhesive joints are critically assessed in terms of these factors which affect the durability and performance of them. There are two basic mathematical approaches for the analysis of adhesively bonded joints: (a) closed-form or analytical model and (b) numerical solutions (i.e., finite element analysis, FEA). In the closed-form approach, a set of differential equations and boundary conditions is formulated. The solutions of these equations are analytical expressions which give values of stresses at any point of joint. The analytical approach for the solution of complex stress distributions in the joints has been progressively refined until recent times. In the second approach, solutions of differential equations are obtained by numerical methods or the continuum is represented by a discrete model at the outset. The solution of these equations gives displacements at the determined points from which strains and stresses can be obtained for any point within the model. Among the numerical methods, finite element analysis (FEA) has been extensively used with success. The two- and three-dimensional finite element analyses approaches have been extensively applied by many workers to analyse the adhesive joints considering the linear and geometric nonlinearities.

Adhesively-bonded joints in metallic alloys, polymers and composite materials: Mechanical and environmental durability performance

In this paper a new full-field method for the automatic analysis of isochromatic fringes in white light is presented. The method, named RGB photoelasticity, eliminates the typical drawbacks of the classical approach to photoelasticity in white light which requires a subjective analysis of colors and an experienced analyst to acquire and interpret the results.

Towards RGB photoelasticity: Full-field automated photoelasticity in white light

This paper presents a new method for whole-field stress analysis based on a symbiosis of two techniques—classical photoelasticity and modern digital image analysis. The resulting method is called ‘half-fringe photoelasticity (HFP)’. Classical photoelasticity demands materials with high birefringence, which leads to extensive use of plastics as model materials. Since the behavior of these materials is often different from that of the prototype materials, their use distorts the similitude relationships. In many contemporary problems this distortion is untenable. HFP offers a way out of this dilemma. It permits materials and loads to be chosen so that no more than one half of a fringe order appears in the area of interest. Thus, for example, glass, which behaves linearly up to high stress levels and over a wide range of temperatures, could be used as model material. Alternatively, models from polymeric materials could be used under very low load in order to stay within the linear part of the stress-strain diagram and to prevent large deformations. The half-fringe-photoelasticity system, which is described here, utilizes the resulting low levels of birefringence for effective stress analysis. This paper describes the system. It outlines a calibration routine and illustrates its application to two simple problems using glass models.

Half-fringe photoelasticity: A new approach to whole-field stress analysis

Dynamic photoelasticity was used to study the stresses when an impulse wave propagates through a single lap joint. An explosive was detonated in one of the adherends. The resulting stress wave propagated down the adherend and through the joint area. At the junction between the adherends, the stress wave interacted with the ends of the adherends and significant changes in the stress wave occurred. The stress wave then propagated through the joint and, in doing so, interacted with the two square corners at the adhesive/adherend interface. It was found that a significant biaxial tensile stress occurs at one of the corners. With only the usual black and white photographs, the ordering and interpretation of the photoelastic-fringe orders was difficult. The paper indicates how color photographs were used to simplify identification of the fringes.

Dynamic wave propagation in a single lap joint

A new method for analyzing low-order moirefringe patterns of displacement fields is presented. This method adapts the techniques of half-fringe photoelasticity to moire and extracts continuous displacement information in the regions between integral fringes. The effectiveness of the technique is illustrated with three examples: a uniform uniaxial field, a tapered specimen in tension, and a disk in diametral compression.

Fractional moiré strain analysis using digital imaging techniques

The history of developments in photoplasticity can be traced from the work of Filon at the beginning of this century to the most recent attempts to model large-scale flow in metal-forming processes. The results of some of the studies on strain fields in upset forging and hot rolling are presented.

C. P. Burger

Papers

Half-fringe photoelasticity: A new approach to whole-field stress analysis

Dynamic wave propagation in a single lap joint

Fractional moiré strain analysis using digital imaging techniques

Nonlinear photomechanics: A review of progress in photoplasticity and its recent applications to studies of metal-forming processes

Stress intensity factors for interacting cracks