《纺织复合材料》课程教学资源（书籍文献）Microstructural Design of Fiber Composites

Microstructure-performance relationships 23 reduction in interlaminar normal stress is achieved with hybrid laminates.The experimentally measured delamination initiation stress and failure stress are 324.3 MPa and 800.4 MPa,respectively. The corresponding stresses for the hybrid laminate are 800.4 MPa and 883.2 MPa,respectively.Thus,the addition of the glass/epoxy plies significantly improves the delamination stress.The gain in failure stress is not as significant since the 0 plies in both laminates dominate the ultimate strength. Reinforcements in the thickness direction can suppress inter- laminar failure.Figure 1.15 shows the X-ray radiographs of [+45/02/90Js laminates under uniaxial tension.The specimen with through-the-thickness stitches along the free edges experiences much less delamination than the specimen without stitches. Besides relying on textile performing techniques such as stitching, weaving and braiding,delamination in brittle resin matrix compos- ites can be remedied by adding a ductile matrix in the form of thin adhesive layers.The resulting composite has a hybridized matrix.It has been demonstrated in [0/90/45/-45ls carbon/epoxy lamin- ates that by reducing the free-edge effect the laminate strength can be greatly improved.Furthermore,the laminate strength becomes an isotropic property which can be predicted by the classical failure theory.The use of adhesive layers in laminates subject to low- velocity impact also proves to be effective in suppressing the development of matrix cracking and delamination. Fig.1.14.Free-edge interlaminar normal stresses on the mid-surface in carbon/epoxy and carbon/glass/epoxy laminates.(After Sun 1989). 0.25 Carbon ---Hybrid 0.125 0.0 8 12 16 20 24 Distance from free edge/ply thickness

Microstructure-performance relationships 23 reduction in interlaminar normal stress is achieved with hybrid laminates. The experimentally measured delamination initiation stress and failure stress are 324.3 MPa and 800.4 MPa, respectively. The corresponding stresses for the hybrid laminate are 800.4 MPa and 883.2 MPa, respectively. Thus, the addition of the glass/epoxy plies significantly improves the delamination stress. The gain in failure stress is not as significant since the 0° plies in both laminates dominate the ultimate strength. Reinforcements in the thickness direction can suppress inter￾laminar failure. Figure 1.15 shows the X-ray radiographs of [±45°/05/90°]s laminates under uniaxial tension. The specimen with through-the-thickness stitches along the free edges experiences much less delamination than the specimen without stitches. Besides relying on textile performing techniques such as stitching, weaving and braiding, delamination in brittle resin matrix compos￾ites can be remedied by adding a ductile matrix in the form of thin adhesive layers. The resulting composite has a hybridized matrix. It has been demonstrated in [07907457 —45°]s carbon/epoxy lamin￾ates that by reducing the free-edge effect the laminate strength can be greatly improved. Furthermore, the laminate strength becomes an isotropic property which can be predicted by the classical failure theory. The use of adhesive layers in laminates subject to low￾velocity impact also proves to be effective in suppressing the development of matrix cracking and delamination. Fig. 1.14. Free-edge interlaminar normal stresses on the mid-surface in carbon/epoxy and carbon/glass/epoxy laminates. (After Sun 1989). 0.25 iS 0.125 Carbon Hybrid J_ J_ J_ J_ 0 4 8 12 16 20 Distance from free edge/ply thickness 24

24 Introduction The transport properties,e.g.electrical conductivity,thermal conductivity,dielectric constants,magnetic permeability and diffusion coefficients of composites,are also sensitive to the microstructure of the reinforcements.McCullough (1985)has demonstrated the importance of structural features that promote transport along the preferred path,i.e.percolative mechanisms. Consider,for instance,the electrical behavior of metal-filled poly- mers.The effective resistivity changes sharply from non-conducting to conducting behavior upon crossing a 'percolation threshold'. Figure 1.16 illustrates such a transition for a composite containing conductive fillers (pr=10-cm)in an insulating polymer matrix (pm=10162cm).The decrease in resistivity with the increase in filler volume fraction is attributed to the enhancement in probability of particle-particle contact.McCullough has concluded that these contacts promote the formation of continuous conduction paths that mimic the behavior of conducting fibers. 1.5.3 Intelligent composites Traditionally,fiber composites have been designed and manufactured with the purpose of serving very specific functional Fig.1.15.X-ray radiographs showing delamination in unstitched(left)and stitched (right)[+45/03/90J laminates under uniaxial tension.(After Mignery,Tan and Sun 1985.) 551 MPa (80 ksi) 641 MPa (93 ksi) 689 MPa (100 ksi)

24 Introduction The transport properties, e.g. electrical conductivity, thermal conductivity, dielectric constants, magnetic permeability and diffusion coefficients of composites, are also sensitive to the microstructure of the reinforcements. McCullough (1985) has demonstrated the importance of structural features that promote transport along the preferred path, i.e. percolative mechanisms. Consider, for instance, the electrical behavior of metal-filled poly￾mers. The effective resistivity changes sharply from non-conducting to conducting behavior upon crossing a 'percolation threshold'. Figure 1.16 illustrates such a transition for a composite containing conductive fillers (pf = 10~6 Q cm) in an insulating polymer matrix (pm = 1016 Qcm). The decrease in resistivity with the increase in filler volume fraction is attributed to the enhancement in probability of particle-particle contact. McCullough has concluded that these contacts promote the formation of continuous conduction paths that mimic the behavior of conducting fibers. 1.5.3 Intelligent composites Traditionally, fiber composites have been designed and manufactured with the purpose of serving very specific functional Fig. 1.15. X-ray radiographs showing delamination in unstitched (left) and stitched (right) [±45°/02/90°]s laminates under uniaxial tension. (After Mignery, Tan and Sun 1985.) 551 MPa(80ksi) 641 MPa(93ksi)

Microstructure-performance relationships 25 goals.Such goals and considerations may include stiffness,fracture toughness,fatigue life,impact resistance,electromagnetic shielding, corrosion resistance,and biocompatibility,just naming a few.With the expansion in available material systems for composites,advance- ments in fabrication technologies,and improvements in analysis and design techniques,it becomes increasingly feasible for develop- ing multi-functional fiber composites for which a number of functional goals are satisfied simultaneously,and the performance can be optimized. A new breed of multi-functional composites is dubbed 'smart composites'or 'intelligent composites'.Takagi (1989)has defined intelligent materials as 'those which can manifest their own func- tions intelligently depending on environmental changes'.Thus, intelligent composites can react to the thermal,electrical,magnetic, chemical or mechanical environment and adjust their performance accordingly.It should be borne in mind that intelligent composites are made possible only through the design of their microstructures. There are two basic requirements for intelligent composites to 'think'for themselves.First,the ability to detect the change in the environment,such as pressure,strain,temperature,and electro- Fig.1.16.Illustration of chain formation in a particulate filled composite. Open circles and closed circles indicate,respectively,isolated particles and contacting particles participating in chain formation.p and V denote resistivity and filler volume fraction,respectively.(After McCullough 1985.) 0 1016 0 Filler volume fraction.Vr

Microstructure-performance relationships 25 goals. Such goals and considerations may include stiffness, fracture toughness, fatigue life, impact resistance, electromagnetic shielding, corrosion resistance, and biocompatibility, just naming a few. With the expansion in available material systems for composites, advance￾ments in fabrication technologies, and improvements in analysis and design techniques, it becomes increasingly feasible for develop￾ing multi-functional fiber composites for which a number of functional goals are satisfied simultaneously, and the performance can be optimized. A new breed of multi-functional composites is dubbed 'smart composites' or 'intelligent composites'. Takagi (1989) has defined intelligent materials as 'those which can manifest their own func￾tions intelligently depending on environmental changes'. Thus, intelligent composites can react to the thermal, electrical, magnetic, chemical or mechanical environment and adjust their performance accordingly. It should be borne in mind that intelligent composites are made possible only through the design of their microstructures. There are two basic requirements for intelligent composites to 'think' for themselves. First, the ability to detect the change in the environment, such as pressure, strain, temperature, and electro￾Fig. 1.16. Illustration of chain formation in a participate filled composite. Open circles and closed circles indicate, respectively, isolated particles and contacting particles participating in chain formation, p and V{ denote resistivity and filler volume fraction, respectively. (After McCullough 1985.) 1016 o C3 Filler volume fraction, Kf

26 Introduction magnetic radiation is necessary.Next,the ability in feedback and control is also needed so corrective actions can be taken. An example of intelligent composites under consideration by researchers is the skin of an aircraft wing (see Port,King and Hawkins 1988).The resin-based composite skin in this case has built-in optic-fiber sensors which through the pulses of laser light can detect internal defects and damages,the weight of ice or incoming electromagnetic radar waves.Signals from the sensors would be analyzed by patches of chips mounted on a flexible printed circuit board bonded over the skin. It has been suggested that implanting monolithic microwave integrated circuit chips around an airplane's surface would produce a huge,omnidirectional antenna that would be far more effective than the small forward looking units now mounted on its nose. Other applications of intelligent composites have been envisioned for the purpose of in-flight damage assessment capability on airplanes and orbiting spacecrafts,prelaunch checks for leaks and structural integrity of the casing around rockets,altering the stiffness of sporting equipment such as golf club and fishing rods in response to the changing operating conditions,and monitoring the sway of high-rise buildings induced by hurricane winds or earthquakes so measures to compensate such deformations can be activated (Port,King and Hawkins,1988).Some of the issues of intelligent structures have been discussed by Rogers(1988). In summary,the challenges of intelligent composites are mani- fested by the following factors:(a)development of sensing, feedback and control systems as well as the technologies for fabricating composites imbedded with such devices,(b)implemen- tation of the required changes in the shapes of the structural components,for example the change of the angle and shape of an airplane's wing,and (c)perhaps the most challenging task,the ability of a material to change its performance,for example the stiffness or transport properties. The combination of the structural and non-structural roles of a composite in an integrated manner will undoubtedly change the performance of fiber composites in a way not envisioned in the past. 1.6 Concluding remarks Having examined the evolution of engineering materials, and the role of fiber composites in materials technology,it is perhaps useful to put in perspective the research and economic opportunities of advanced composites

26 Introduction magnetic radiation is necessary. Next, the ability in feedback and control is also needed so corrective actions can be taken. An example of intelligent composites under consideration by researchers is the skin of an aircraft wing (see Port, King and Hawkins 1988). The resin-based composite skin in this case has built-in optic-fiber sensors which through the pulses of laser light can detect internal defects and damages, the weight of ice or incoming electromagnetic radar waves. Signals from the sensors would be analyzed by patches of chips mounted on a flexible printed circuit board bonded over the skin. It has been suggested that implanting monolithic microwave integrated circuit chips around an airplane's surface would produce a huge, omnidirectional antenna that would be far more effective than the small forward looking units now mounted on its nose. Other applications of intelligent composites have been envisioned for the purpose of in-flight damage assessment capability on airplanes and orbiting spacecrafts, prelaunch checks for leaks and structural integrity of the casing around rockets, altering the stiffness of sporting equipment such as golf club and fishing rods in response to the changing operating conditions, and monitoring the sway of high-rise buildings induced by hurricane winds or earthquakes so measures to compensate such deformations can be activated (Port, King and Hawkins, 1988). Some of the issues of intelligent structures have been discussed by Rogers (1988). In summary, the challenges of intelligent composites are mani￾fested by the following factors: (a) development of sensing, feedback and control systems as well as the technologies for fabricating composites imbedded with such devices, (b) implemen￾tation of the required changes in the shapes of the structural components, for example the change of the angle and shape of an airplane's wing, and (c) perhaps the most challenging task, the ability of a material to change its performance, for example the stiffness or transport properties. The combination of the structural and non-structural roles of a composite in an integrated manner will undoubtedly change the performance of fiber composites in a way not envisioned in the past. 1.6 Concluding remarks Having examined the evolution of engineering materials, and the role of fiber composites in materials technology, it is perhaps useful to put in perspective the research and economic opportunities of advanced composites

Concluding remarks 27 First,from the viewpoint of materials research,it is important to recognize that the distinction between the three classes of materials, i.e.metals,ceramics and plastics,is disappearing.As observed by Kelly (1987a),there are now plastics as strong as metals which show some electrical conductivity.Metals are being made which are super-plastic and can be subjected to deformations in processing like conventional polymeric materials.Also the three classes of materials are beginning to show the same limits of strength and stiffness;fibers made from all three can attain stiffness and strength close to the theoretically predicted values.Furthermore,the pro- perties of all three classes of materials can be modified and improved by the use of surface coatings. As the distinction between the three classes of materials disap- pears,new possibilities and opportunities arise.One of these, according to Kelly,is the possibility of designing materials not so much for final properties but equally in terms of processability. These thoughts have profound implications for the future technol- ogy of fiber composites: (1)The commonality in processing shared by the three classes of materials,e.g.super-plastic forming of metal and poly- mers,injection molding of polymers and ceramic powders, will enable more extensive and effective transfer of know- how among the three basic disciplines and effect efficient processing technology for fiber composites. (2) The commonality in performance shared by the three classes of materials,e.g.stiffness,strength,thermal expan- sion,enables the material scientist to engineer composites with a broad spectrum of component materials.Conse- quently,hybridizations of materials,e.g.glass and low- melting-point metal,ceramics and thermoplastics,and polymer and metal in laminates or other interdispersed composite forms can be achieved and the properties op- timized (e.g.composites composed of metal and polymer components of nearly the same stiffness but different fatigue resistance,or thermal expansion coefficient). (3) The similarity in material property and behavior implies that analytical and design methodologies originally developed for a specific class of composites may be transferable to others.A notable example is the fracture and failure behavior of ceramic and polymer based composites

Concluding remarks 27 First, from the viewpoint of materials research, it is important to recognize that the distinction between the three classes of materials, i.e. metals, ceramics and plastics, is disappearing. As observed by Kelly (1987a), there are now plastics as strong as metals which show some electrical conductivity. Metals are being made which are super-plastic and can be subjected to deformations in processing like conventional polymeric materials. Also the three classes of materials are beginning to show the same limits of strength and stiffness; fibers made from all three can attain stiffness and strength close to the theoretically predicted values. Furthermore, the pro￾perties of all three classes of materials can be modified and improved by the use of surface coatings. As the distinction between the three classes of materials disap￾pears, new possibilities and opportunities arise. One of these, according to Kelly, is the possibility of designing materials not so much for final properties but equally in terms of processability. These thoughts have profound implications for the future technol￾ogy of fiber composites: (1) The commonality in processing shared by the three classes of materials, e.g. super-plastic forming of metal and poly￾mers, injection molding of polymers and ceramic powders, will enable more extensive and effective transfer of know￾how among the three basic disciplines and effect efficient processing technology for fiber composites. (2) The commonality in performance shared by the three classes of materials, e.g. stiffness, strength, thermal expan￾sion, enables the material scientist to engineer composites with a broad spectrum of component materials. Conse￾quently, hybridizations of materials, e.g. glass and low￾melting-point metal, ceramics and thermoplastics, and polymer and metal in laminates or other interdispersed composite forms can be achieved and the properties op￾timized (e.g. composites composed of metal and polymer components of nearly the same stiffness but different fatigue resistance, or thermal expansion coefficient). (3) The similarity in material property and behavior implies that analytical and design methodologies originally developed for a specific class of composites may be transferable to others. A notable example is the fracture and failure behavior of ceramic and polymer based composites