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Damage in composite materials: Basic mechanisms, accumulation, tolerance, and characterisation
validity of the double-grooved specimen tensile-shear testing method for evaluating the interlaminar shear strength for composite laminates. The method was found to depend on the shear surface length. A modification to Markham's formula, which included an allowance for the stress concentration at the root of the groove was proposed. Hirai et al discussed the development of thermal residual stresses in antisymmetrically laminated composites. Laminates of the form [ + 0,/-0,] were modelled using the finite element method, and compared with experimental results obtained from observations of laminates allowed to cool from Tg to room temperature. Moire fringe topography was employed to monitor the out-of-plane deflections. Large interfacial stresses were generated at and near the free edges, the magnitude depending upon the laminate stacking sequence. Fattuhi investigated the toughness of thin mortar slabs under flexural and impact loading. Reinforcements of monofilament polypropylene, hooked steel fibres, fibrillated polypropylene networks and deformed steel bars were used. The measurement of interfacial shear strength in graphite fibre/aluminium matrix composites. A described by Provenzano et a l . The method of critical fibre length was used. This involves the measurement of aspects ratios of the broken fibres in specimens fractured in tension. This was followed by a paper by Tolf describing a theoretical investigation of the mechanical behaviour of short fibre composites. The model of the stress field in a two-dimensional composite predicted that small deviations from loading along the fibre axis cause a rapid decrease in the strength of the composite.
Bradley and Cohen gave a paper on delamination and transverse fracture in graphite/epoxy composites. A number of different resin and fibre systems were investigated; the fracture mode being dependent on the ductility of the matrix. In brittle systems, mixed mode I/mode II loading was found to increase the total energy dissipated in fracture. Nicolaides gave a presentation on the mechanical properties of energetic polymeric materials. Gilbert et al presented a paper on the direct observation of the micromechanisms of fracture in polymeric solids. The presentation consisted almost entirely of a video recording of crack propagation in various thermoplastics. The materials investigated were polystyrene, PMMA, high impact polystyrene and toughened PMMA. The cracks were produced by loading compact tension and double torsion specimens inside a scanning electron microscope. Visual presentations such as these help to illustrate crack propagation mechanisms, especially as in this case of toughened and untoughened materials. E1-Soaly discussed the generalization of Mindlin's kinematical plate theory for anisotropic materials. The conference was generally well run, though the reviewer noted some disappointment at the lack of a question period after the invited speakers' lectures. The session chairmen were fairly good timekeepers-essential when a large number of papers are presented. There were a few who continued for long after their allotted time, the invited speakers not being entirely blameless in this respect. 'The continuation of this series seems assured; the decision to hold ICM5 in Beijing, China in 1987 has already been taken by the ICM board.
R.P. Harrison
Book r e v i e w Damage in Composite Materials: Basic Mechanisms, Accumulation, Tolerance, and Characterisation Edited by: K.L. Reifsnider ASTM Special Technical Publication 775 (£28.00) This rather unwieldy title describes a publication of great potential value to scientists and engineers who work with or who use fibre composite materials. It represents the proceedings of a symposium, sponsored jointly by two major ASTM committees: E-7 on Nondestructive Testing, and E-9 on Fatigue, that was held at Bal Habour, Florida, USA, in November 1980. In his Introduction, the Editor, himself a major contributor to the proceedings, points out that the material chosen for presentation at the meeting was selected to serve three groups of people: materials scientists and non-destructive evaluation practitioners; researchers on the fatigue of composite materials; and designers concerned with the applications of composites. The importance of the symposium lies not least in the fact that the papers and discussions deal with the subject in a multidisciplinary fashion, and the
72
editor stresses his conviction that much is to be gained in this field from a broad-based approach of this kind. The value of composites to designers is well-publicized, usually in terms of their superior strength and stiffness advantages, together with a concomitant low density, by comparison with those of conventional materials. From the designer's point of view, however, the fatigue and fracture behaviour of composites is rather less easy to cope with because of the complex nature of the accumulation of damage and the effect of that damage on the residual strength or life of the material in question. The microfailure events that constitute this damage, and which reduce the load-bearing ability of the composite, are of many different kinds, are complex in nature, and may interact with each other in unpredictable ways. They give rise to a wide range of failure modes under different circumstances and in
COMPOSITES. JANUARY 1984
materials of different characteristics. An understanding of the nature of these microfailure events and the ways in which they affect the macroscopic failure of the composite is extremely important if composites are to be used safely and economically. Hence the value of a symposium of this nature. The symposium consists of fourteen papers and a comprehensive summary by the Editor. The papers are divided into two main groups, the first dealing with accumulation of damage and its non-destructive assessment, and the second with tolerance and characterization of damage. It is clear, however, that a divorce of this kind is somewhat artificial and there is inevitably a good deal of overlap.Although the principles at issue are general to the whole field of application of composites, it happens that the emphasis is on the behaviour of reinforced plastics laminates, and only one paper deals specifically with metal matrix composites. The first section begins with a paper emphasizing the general philosophy of the multidisciplinary approach to the study of damage in composites, which is followed by short papers on the powerful technique of stereo-radiography as a means of studying the threedimensional character and distribution of damage, and a discussion of the more familiar fractographic methods, perhaps still the most common means of characterizing damage. Then follows a comprehensive discussion of the accumulation of damage in quasiisotropic laminates, based on a careful edge replication and X-radiographic study, showing the effect of ply stacking sequence on the development of different types of damage. The results are discussed in terms of the now-familiar VPI model of a characteristic damage state, perhaps one of the more important of modern concepts in this field. The authors, Masters and Reifsnider, show, in particular, that the saturation crack spacing in the off-axis plies is characteristic of the ply orientation and is the same whether the laminate is loaded monotonically or cyclically. They also show that the characteristic crack spacing for the different plies depends upon the ply stacking sequence. The final paper in the first section deals with the effects of moisture and residual thermal stress on damage development in carbon/epoxy laminates. Moisture appears to alter the pattern of initial damage development in quasi-isotropic laminates significantly,
but the fully-developed crack patterns in wet and dry laminates are similar, under both cyclic and monotonic loading conditions; there is, consequently, little effect of moisture on either the strength or fatigue behaviour. The first paper in the second section discusses the factors influencing accumulation of damage in boron/ aluminium composites, and provides an interesting antithesis to the behaviour of resin-based materials. Specific reference is made to saturation damage states, and to the effects of ply stacking sequences and loading histories. Seven of the remaining papers deal, variously, with the correlation between stiffness reduction and the development of a transverse ply crack pattern (Highsmith and Reifsnider), with the effect of ply thickness on transverse cracking and delamination (Crossman and Wang), with other aspects of delamination and compression damage in laminates of various kinds, and with the relationship between damage mechanisms and life prediction. In this last case, Badaliance and Dill develop a model in which the summing of strain energy densities from the individual plies allows the correlation of data, from different laminates tested at different stress ratios, to provide a life prediction model that gives reasonable agreement with experimental results. The concluding paper by Fong is an attempt to make an assessment of the current state of understanding of the subject. This offers some thought-provoking insights, but in drawing too extensively on data relating to fatigue of metallic materials, the author detracts somewhat from the power of his arguments. It appears from this paper and from the Editor's summary that there is, at present, no available definition of a damage parameter that can be easily measured and that can be used directly to calculate the remaining strength or life of a specimen or component. Neither, it seems, are we within sight of having such a parameter. Such a pessimistic conclusion detracts the reader's attention somewhat from an appreciation of what has, in fact, been achieved in this field. This is a well-produced and well-edited publication, such as one has come to expect of the ASTM's Special Technical Publications. One hopes that the field will be re-reviewed, at an appropriate time, in the next two or three years.
Bryan Harris
Reprints of all articles in this journal are available in quantities of 100 or more. Reprints are essential - for the company that wants to distribute impartial comment on its activities to potential customers and clients. for the company that wants to up-date its technical staff on new techniques and new technologies.
For full details of prices and availabilityof reprints, please write to:
The Reprint Department, PO Box 63, Westbury House, B u r y S t r e e t , G u i l d f o r d , S u r r e y , GU2" 5 B H .
COMPOSITES. JANUARY 1984
73
Bradley and Cohen gave a paper on delamination and transverse fracture in graphite/epoxy composites. A number of different resin and fibre systems were investigated; the fracture mode being dependent on the ductility of the matrix. In brittle systems, mixed mode I/mode II loading was found to increase the total energy dissipated in fracture. Nicolaides gave a presentation on the mechanical properties of energetic polymeric materials. Gilbert et al presented a paper on the direct observation of the micromechanisms of fracture in polymeric solids. The presentation consisted almost entirely of a video recording of crack propagation in various thermoplastics. The materials investigated were polystyrene, PMMA, high impact polystyrene and toughened PMMA. The cracks were produced by loading compact tension and double torsion specimens inside a scanning electron microscope. Visual presentations such as these help to illustrate crack propagation mechanisms, especially as in this case of toughened and untoughened materials. E1-Soaly discussed the generalization of Mindlin's kinematical plate theory for anisotropic materials. The conference was generally well run, though the reviewer noted some disappointment at the lack of a question period after the invited speakers' lectures. The session chairmen were fairly good timekeepers-essential when a large number of papers are presented. There were a few who continued for long after their allotted time, the invited speakers not being entirely blameless in this respect. 'The continuation of this series seems assured; the decision to hold ICM5 in Beijing, China in 1987 has already been taken by the ICM board.
R.P. Harrison
Book r e v i e w Damage in Composite Materials: Basic Mechanisms, Accumulation, Tolerance, and Characterisation Edited by: K.L. Reifsnider ASTM Special Technical Publication 775 (£28.00) This rather unwieldy title describes a publication of great potential value to scientists and engineers who work with or who use fibre composite materials. It represents the proceedings of a symposium, sponsored jointly by two major ASTM committees: E-7 on Nondestructive Testing, and E-9 on Fatigue, that was held at Bal Habour, Florida, USA, in November 1980. In his Introduction, the Editor, himself a major contributor to the proceedings, points out that the material chosen for presentation at the meeting was selected to serve three groups of people: materials scientists and non-destructive evaluation practitioners; researchers on the fatigue of composite materials; and designers concerned with the applications of composites. The importance of the symposium lies not least in the fact that the papers and discussions deal with the subject in a multidisciplinary fashion, and the
72
editor stresses his conviction that much is to be gained in this field from a broad-based approach of this kind. The value of composites to designers is well-publicized, usually in terms of their superior strength and stiffness advantages, together with a concomitant low density, by comparison with those of conventional materials. From the designer's point of view, however, the fatigue and fracture behaviour of composites is rather less easy to cope with because of the complex nature of the accumulation of damage and the effect of that damage on the residual strength or life of the material in question. The microfailure events that constitute this damage, and which reduce the load-bearing ability of the composite, are of many different kinds, are complex in nature, and may interact with each other in unpredictable ways. They give rise to a wide range of failure modes under different circumstances and in
COMPOSITES. JANUARY 1984
materials of different characteristics. An understanding of the nature of these microfailure events and the ways in which they affect the macroscopic failure of the composite is extremely important if composites are to be used safely and economically. Hence the value of a symposium of this nature. The symposium consists of fourteen papers and a comprehensive summary by the Editor. The papers are divided into two main groups, the first dealing with accumulation of damage and its non-destructive assessment, and the second with tolerance and characterization of damage. It is clear, however, that a divorce of this kind is somewhat artificial and there is inevitably a good deal of overlap.Although the principles at issue are general to the whole field of application of composites, it happens that the emphasis is on the behaviour of reinforced plastics laminates, and only one paper deals specifically with metal matrix composites. The first section begins with a paper emphasizing the general philosophy of the multidisciplinary approach to the study of damage in composites, which is followed by short papers on the powerful technique of stereo-radiography as a means of studying the threedimensional character and distribution of damage, and a discussion of the more familiar fractographic methods, perhaps still the most common means of characterizing damage. Then follows a comprehensive discussion of the accumulation of damage in quasiisotropic laminates, based on a careful edge replication and X-radiographic study, showing the effect of ply stacking sequence on the development of different types of damage. The results are discussed in terms of the now-familiar VPI model of a characteristic damage state, perhaps one of the more important of modern concepts in this field. The authors, Masters and Reifsnider, show, in particular, that the saturation crack spacing in the off-axis plies is characteristic of the ply orientation and is the same whether the laminate is loaded monotonically or cyclically. They also show that the characteristic crack spacing for the different plies depends upon the ply stacking sequence. The final paper in the first section deals with the effects of moisture and residual thermal stress on damage development in carbon/epoxy laminates. Moisture appears to alter the pattern of initial damage development in quasi-isotropic laminates significantly,
but the fully-developed crack patterns in wet and dry laminates are similar, under both cyclic and monotonic loading conditions; there is, consequently, little effect of moisture on either the strength or fatigue behaviour. The first paper in the second section discusses the factors influencing accumulation of damage in boron/ aluminium composites, and provides an interesting antithesis to the behaviour of resin-based materials. Specific reference is made to saturation damage states, and to the effects of ply stacking sequences and loading histories. Seven of the remaining papers deal, variously, with the correlation between stiffness reduction and the development of a transverse ply crack pattern (Highsmith and Reifsnider), with the effect of ply thickness on transverse cracking and delamination (Crossman and Wang), with other aspects of delamination and compression damage in laminates of various kinds, and with the relationship between damage mechanisms and life prediction. In this last case, Badaliance and Dill develop a model in which the summing of strain energy densities from the individual plies allows the correlation of data, from different laminates tested at different stress ratios, to provide a life prediction model that gives reasonable agreement with experimental results. The concluding paper by Fong is an attempt to make an assessment of the current state of understanding of the subject. This offers some thought-provoking insights, but in drawing too extensively on data relating to fatigue of metallic materials, the author detracts somewhat from the power of his arguments. It appears from this paper and from the Editor's summary that there is, at present, no available definition of a damage parameter that can be easily measured and that can be used directly to calculate the remaining strength or life of a specimen or component. Neither, it seems, are we within sight of having such a parameter. Such a pessimistic conclusion detracts the reader's attention somewhat from an appreciation of what has, in fact, been achieved in this field. This is a well-produced and well-edited publication, such as one has come to expect of the ASTM's Special Technical Publications. One hopes that the field will be re-reviewed, at an appropriate time, in the next two or three years.
Bryan Harris
Reprints of all articles in this journal are available in quantities of 100 or more. Reprints are essential - for the company that wants to distribute impartial comment on its activities to potential customers and clients. for the company that wants to up-date its technical staff on new techniques and new technologies.
For full details of prices and availabilityof reprints, please write to:
The Reprint Department, PO Box 63, Westbury House, B u r y S t r e e t , G u i l d f o r d , S u r r e y , GU2" 5 B H .
COMPOSITES. JANUARY 1984
73