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Nondestructive evaluation of damage accumulation processes in composite laminates
Composites Science and Technology 25 (1986) 103-118
Nondestructive Evaluation of Damage Accumulation Processes in Composite Laminates*
Wayne W. Stinchcomb Materials Response Group, Engineering Science and Mechanics Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 (USA)
S UMMA R Y This paper presents results from several interdisciplinary studies of the mechanical response of composite materials, in which mechanics and materials science were used in concert with nondestructive evaluation (NDE) to investigate the damage accumulation process. Specific examples are cited for the initiation and growth of damage in notched and unnotched laminates subjected to cyclic loading histories. The process of damage accumulation in composite materials involves a number of damage modes, including matrix cracking, delamination, and fiber fracture, which initiate, grow, and interact to form a complex network of damage details called the damage state. Nondestructive techniques, including replication, light and electron microscopy, X-ray radiography, ultrasonics, stiffness change, and thermography, are used to detect and monitor the progressive development of damage throughout the fatigue life and to evaluate its effect on fatigue response of the laminates.
C O N C E P T S OF D A M A G E IN C O M P O S I T E M A T E R I A L S Damage in composite materials has been the subject of numerous research and development programs. The understanding of damage in composite materials and its effect on the response of composite structures * Paper presented at the International Symposium 'Composites: Materials and Engineering', Universityof Delaware, Newark, Delaware, USA, September 24-28, 1984. 103 Composites Science and Technology 0266-3538/86/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
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has matured to a level where several composite materials are qualified and certified materials in many engineering structures. As new material systems are introduced and more severe design requirements are imposed on composites, the understanding of the process of damage accumulation and the attendant behavior of composite structures must be increased to ensure that the materials are used reliably and efficiently. To achieve this goal in a multidisciplinary field such as composite materials requires the concerted efforts of several groups of people, including those with expertise in mechanics, materials science, and nondestructive evaluation (NDE). The general process of damage accumulation in composite materials consists of a number of damage modes, including matrix cracking, delamination, and fiber fracture, which affect material response. Throughout the loading history the progressive development and interaction of various damage modes in the damage state (the collective, cumulative form of damage) changes the structure of the composite and redistributes the stresses in the laminate. The specific details of the damage process in composite materials will depend on the material system, the geometry of the material/structure, and the loading history.
THE D A M A G E PROCESS
Surface damage As an example of the damage development process, Fig. 1 shows damage modes and their progressive development during the fatigue life of an unnotched laminate with several off-axis plies. The scenario is for graphite-epoxy laminates, ~ although similar scenarios can be constructed for other composites, including metal-matrix composites. The damage modes for notched laminates are the same as for unnotched laminates; however, the damage modes are more localized at the notch.: Laminates can be inspected nondestructively in order to detect matrix cracks and delaminations. In situations where the matrix cracks intersect the free edge of the laminate and couple along the edge to form interfacial debonds the cracks can be detected by direct observation using a light microscope (magnification of 50-100 × ), or by recording the image of the surface on a strip of cellulose acetate tape (the replication technique) and viewing the tape through a microscope or a microfiche reader.
NDE of damage accumulation processes in composite laminates
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DAMAGE MODES DURING FATIGUE LIFE I- Matrix Cracking
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Schematic representation o f the damage accumulation process in composite laminates.
Microscopy and replication can also be used to detect the presence of edge-related delaminations. It is convenient to study the progressive development of edge-related damage using the replication technique. By permanently recording the details of the damage state on tapes at various stages of life, without removing the laminate from the testing machine, the chronology of the edge damage history can be documented. Replicas are very sensitive to even the micro-details of damage. Figure 2(a) is a scanning electron micrograph of a replica of the edge of a [0, +45]2 s graphite-epoxy laminate after 50 cycles of tension-tension fatigue at a maximum tensile stress of 86 % of the tensile strength. 3 Matrix cracks in the + 45 ° and - 4 5 ° plies are quite evident and are already uniformly spaced at this early stage of fatigue life, forming a characteristic damage state. 4 There are also indications of damage in the 0 ° plies which are shown more clearly in Fig. 2(b). The high magnification micrograph shows that the damage in the 0 ° plies consists of broken fibers with the fractures banded along a zone across the thickness of the 0 ° plies. Returning to Fig. 2(a), each band of broken fibers is closely associated with the matrix cracks in the adjacent + 45 ° and - 45 ° plies. Fiber breaks will be discussed in more detail in a later section.
(a)
(b)
Fig. 2.
Scanning electron micrographs of a replica showing (a) matrix cracks and (b) broken fibers in a graphite-epoxy laminate.
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The portion of the damage development process revealed by the replication method affects the response of the laminate in several significant ways. First, the initiation of the individual edge damage modes is not independent of the state of damage in the material. The initiation of matrix cracks is not a single, instantaneous event, but is a series of sequential events dependent on the local distribution of stress and on local variations of strength throughout the laminate. The progressive initiation of matrix cracks forms a regularly spaced pattern of cracks in the off-axis plies, which is called the characteristic damage state and which is a laminate property. Matrix cracks also change the local stress field in the cracked ply, causing the interlaminar stresses along the edge to be redistributed along the ply interfaces. The redistributed stresses initiate interfacial debonds (crack coupling) which produce delamination along the length of the free edge. The matrix cracks also change the local stress field in the adjacent plies to produce a stress concentration near the crack tip. In the laminate shown in Fig. 2 the 0 ° plies are adjacent to the cracked, off-axis plies, and a high density of broken fibers is observed in the high stress zones. The second major point to be made concerns the early occurrence of fiber fracture in the fatigue life. While a large number of fiber fractures would be expected in the advanced stages of damage late in the fatigue life, fiber breaks (on the surface and in the interior) occur throughout the entire fatigue life. 5 Certainly some of the breaks are in statistically weak fibers; but, more importantly, a large percentage of the breaks are associated with other damage modes and the attendant stress redistributions, as will be discussed later.
Internal damage Microscopy and replication provide very valuable information about the accumulation of damage on an exposed surface of a laminate, such as an edge or the surface of a hole. However, these techniques do not give complete information on the global extent and distribution of matrix cracks, delaminations, and fiber breaks throughout the laminate. At this time there is no single N D E method which can detect all of the major damage modes in composites in a global measurement and relate that information, in a quantitative way, to the strength, stiffness, and life of the material or structure. X-ray radiography is one of the N D E methods frequently used to inspect the interior of a composite laminate. The damage modes that can
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be detected using this technique are dependent on the composite material system being investigated. Many of the commonly used composite materials are transparent to X-rays. For example, X-ray radiography can detect broken boron fibers in a boron-epoxy composite 6 (actually the Xrays detect the breaks in the tungsten core of the boron fibers) but cannot detect broken graphite fibers in a graphite-epoxy laminate. Matrix cracks in epoxy matrix composites are easily detected if the cracks are filled with a medium opaque to X-rays, such as zinc iodide, 7 or if very soft X-rays are used. s Delaminations are most easily observed if an opaque medium fills the debonded interfacial region. Although X-ray radiography will detect matrix cracks and delaminations if an opaque medium is used, there are two important conditions that must be remembered when interpreting Xray data. First, the opaque medium, or penetrant, may not infiltrate all of the matrix cracks and delaminations in the laminate. The penetrant must be applied on the surface of the composite and allowed to flow into the damage zone. The penetrant solution should contain an additive to reduce the surface tension of the fluid in order to increase penetration along the cracks. When possible, the penetrant should be applied to the composite under load to 'open up' the damage and increase flow. Penetration can be increased if the load can be cycled a few times to 'pump' the solution into the damaged regions. However, the penetrant may not be able to fill all damage zones because some of the internal damage sites may not be connected to an external surface. Such damage will usually not be detected in X-ray transparent materials such as graphite-epoxy. The second condition concerns damage, usually large delaminations, which may not be completely resolved even though the penetrant can wet the debonded region. When the delaminated surfaces open, capillary action draws the penetrant to the boundary of the delamination, leaving a large debonded region void of penetrant. The radiograph will show only the boundary of the delaminated region, and the nature and size of the damage could be misinterpreted. Figure 3(a) is a radiograph showing tension-tension fatigue damage in a [0/902] Sgraphite-epoxy laminate. The radiograph is made with the Xray beam perpendicular to the plane of the laminate. Regularly spaced matrix cracks in the 90 ° plies form the characteristic damage state. Matrix cracks in the 0 ° plies, caused by cyclic tensile stresses perpendicular to the fibers, are also evident, as are several small, shaded regions around the crossing matrix cracks in the adjacent 0 ° and 90 o plies. Two of these regions are indicated by the arrows in Fig. 3(a). A portion of
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(b)
Fig. 3. Zinc iodide enhanced radiographs of fatigue damage in a graphite-epoxy laminate: (a) matrix cracks in 0 ° and 90 ° plies; (b) internal delaminations initiating at points of crack intersections. 1
the radiograph is enlarged in Fig. 3(b) to show the shaded regions more clearly. The regions have been identified as internal delaminations on the 0/90 interface 1,9 and can be identified using the stereoradiograph technique. 7, l o The planes of the matrix cracks in the adjacent 0 o and 90 o plies intersect at a point on the 0/90 interface, producing a region of tensile interlaminar normal stress around the point of intersection.l'x Under conditions of cyclic tensile loads, delaminations nucleate at the crack intersection point and grow on the 0/90 interface. Much attention
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has been given to the free edge delamination problem (delaminations along the length of a straight-sided test specimen, for example). However, it is important to recognize that the complex network of internal matrix cracks creates internal surfaces which act as micro-free edges. The attendant internal stress field produces internal delminations in the same manner as that in which the stress field at the free, external surface of a laminate produces edge delamination. Because X-ray radiography provides so much information on the state of damage in a composite material or structure (matrix cracks, delaminations, and, in some cases, fiber breaks), it is a very valuable N D E method. However, it must be remembered that a radiograph is a twodimensional, plan view of a three-dimensional damage field. The radiograph shown in Fig. 4a shows matrix cracks and delaminations in a center-notched, quasi-isotropic graphite-epoxy laminate which had been subjected to tension-tension cyclic loads.12 The plies containing matrix cracks can be identified by the orientation of the cracks, although the identification of specific plies becomes increasingly difficult as the thickness and number of repeated plies increases. Even in the eight-ply laminate shown in the radiograph, it is not possible to identify the interfaces which have delaminated nor the size of the individual
Fig. 4a.
Fatigue damage in a [45/90/- 45/0]2 graphite-epoxy laminate with center hole after 10'* cycles at am.x=0"85tru~t+ R = O ' l . 12
N D E o f damage accumulation processes in composite laminates
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Fig. 4b. Delaminations on several interfaces of a center-notched [45/90/-45/0]~ graphite-epoxy laminate after cyclic loading.~2 delaminations. The stereoradiography technique creates a threedimensional image of the damage field and aids in the identification of interfaces and size ofdelaminations. The internal delaminations shown in Fig. 3 were identified and studied using stereoradiography. However, it is difficult to resolve individual details and specific locations in severely damaged, thick laminates using the stereo method. Figure 4b shows a schematic of the delaminations on the interfaces of the eight-ply, quasiisotropic laminate shown in Fig. 4a. The delaminated regions were identified by separating the plies using the de-ply method 13 (a destructive technique) and viewing the de-plied surfaces through a light microscope. Ultrasonic techniques are often used to detect and map internal damage fields. The conventional ultrasonic C-scan is less sensitive than radiography to all the details of damage and is very useful for scanning a component for a suspected delamination and mapping the damaged region. The C-scan is particularly well suited for detecting internal delaminations because the delaminated region does not have to be infiltrated by an opaque medium through an external surface. However, the conventional C-scan technique is not as well suited as radiography for detecting matrix cracks. The ultrasonic beam is sensitive to the presence of matrix cracks, but the resolution capability is less than that of radiography. Enhanced image techniques 14 and back-scattering techniques 15 are being used to improve the resolution of ultrasonic inspection methods and make them more general-purpose techniques.
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Fiber fracture
One of the most important damage modes, and the most difficult to detect nondestructively, is fiber fracture. Two situations where broken fibers can be detected nondestructively have been mentioned previously: fibers, such as boron on a tungsten filament which is opaque to X-rays, and fibers on an external surface which can be detected using microscopy. Other nondestructive inspection methods being considered for the detection of broken fibers are eddy current and acoustic emission; however, more developmental work needs to be done before these methods can be used reproducibly and reliably. Our knowledge of fiber failure is based on a number of fiber fracture models 16-18 and a limited amount of experimental data obtained from destructive testing and inspection. C o m m o n points of agreement between various models and experimental data include the concentration of stress and the redistribution of stress due to broken fibers. The strength of the fibers in a given ply is distributed throughout the volume of the ply. Some fibers fail early in the life of the laminate, as represented schematically in Fig. 1. The load carried by the broken fiber is distributed through the matrix and neighboring fibers. The local changes in geometry and discontinuities introduced by broken fibers also introduce stress concentrations and raise the stress in the adjacent, unbroken plies. The combined effects of the redistributed stress and the concentrated stress promote additional fiber breaks throughout the life of the composite, with a large percentage of the breaks occurring in the first half of the life. 5 Stress concentrations and stress redistribution in fibers are produced by sources other than broken fibers alone. Other damage modes, such as matrix cracking in adjacent plies and delamination of an adjacent interface, cause local adjustments in the stress field to satisfy equilibrium conditions. Figure 5 is a scanning electron photomicrograph of a portion of a 0 ° ply, adjacent to a matrix crack in a 90 ° ply in a [ 0 / 9 0 2 ] s graphite-epoxy laminate. A number of broken fibers can be seen. Figure 6 shows the damage picture on a larger scale. The broken fibers shown in Fig. 5 are concentrated in the region of the matrix crack and are caused by the locally high stresses associated with the crack. While it is useful to develop models of the process of fiber failure and to identify the mechanisms of the process, a critical and fundamental question remains unanswered. What are the important details of the fiber fracture process that control the strength and life of composite materials ?
NDE of damage accumulation processes in composite laminates
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113
Scanning electron micrograph of broken fibers in a 0 ° ply of a [0/902L graphite-epoxy laminate after cyclic loading.l
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Schematic of fatigue damage in a graphite-epoxy laminate showing the association between fiber fracture and matrix cracks.
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Wayne W. Stinehcomb
A second, related question is: Can those details be measured nondestructively in a way that would give early, reliable warnings of fracture? The answers to these questions will become increasingly important as the design strain levels are raised in future generation composite structures.
REAL-TIME, N O N D E S T R U C T I V E MEASURES OF R E S P O N S E Establishing relationships between the state of damage and the subsequent response of a material or structure is an interdisciplinary task incorporating nondestructive evaluation, mechanics, and materials science. The nondestructive methods for inspecting composite materials and structures which have been presented provide details and descriptions of the damage accumulation process in composites. For completeness damage must be evaluated in terms of changes in the state of the material and their consequences on the engineering response of the material or structure. As an example consider matrix cracks and their collective effect on composite material behavior. Detection of a crack in a metallic structure causes great concern. In contrast, a single matrix crack or complete saturation of matrix cracks causes relatively little concern about the degradation of strength of a composite structure. It is true that matrix cracks do not have a direct consequence on the strength and life of composite laminates of engineering significance; however, matrix cracks act as catalysts to spawn other damage modes which do degrade strength and life, particularly in long life situations. As previously shown, matrix cracks on the edge of a laminate couple along the interface to form edge delaminations; crossing matrix cracks in adjacent plies create a stress field which promotes internal delaminations; and matrix cracks also create locally high stresses in adjacent plies which cause fiber fracture. The relationships between damage and response are not necessarily straightforward. Figure 7 shows the change in response of a [45/90/-45/0L graphite-epoxy laminate with a center hole caused by tension-tension fatigue when cycled at a maximum stress of 85 ~o of the monotonic tensile strength. The damage process is nondestructively recorded throughout the loading history. Matrix cracks and delaminations accumulate to form a damage pattern similar to that shown in Fig. 4a. The combined effect of the damage reduces axial stiffness (increases compliance), measured
NDE of damage accumulation processes in composite laminates r Stren t~h. |o-85%test level 140 I- • -low test level
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across the hole in real-time. However, the residual tensile strength of nominally identical laminates, subjected to the same loading history, increases during the early part of the fatigue life (damage initiation around the hole) and decreases during the later part of life (damage growth away from the hole). The developing damage field associated with the hole alters the distribution of strain around the hole (changes the mechanics problem), thereby reducing the strain concentration due to the hole, as shown by Moir6 interferometry, 12 and increasing the tensile strength of the notched laminate. As damage continues to accumulate and grows throughout the cyclic loading, the strength of the ligaments of material on each side of the hole degrades, causing fatigue failure. Stiffness, or change in stiffness, is well suited as a sentinel of damage in composite materials because it is quantitative, it is directionally sensitive, and it is the material property which relates stress and strain. Therefore it is a viable parameter to use in mechanics-based formulations of strength and life. Acousto-ultrasonic techniques currently being developed and studied19,2 0 also show promise as real-time damage monitors. Controlled ultrasonic signals introduced into a composite interact with, and are scattered by, various defects, before being detected by a downstream
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Wayne W. Stincheomb
acoustic emission transducer. The technique has the potential of detecting and quantifying damage and of identifying the fracture location. A third real-time, quantitative measure of damage is thermography. Used in an active mode thermography uses low frequency-high amplitude or high frequency-low amplitude mechanical vibrations to activate internal sources of heat, such as the rubbing or clapping together of two delaminated surfaces in a composite panel. 21 The frequency at which a particular type of defect becomes thermally active is a characteristic of the defect type, size, and location. For the case of internal delamination the measured frequency for thermal activity corresponds to the resonant frequency of two flat plates bonded top to bottom along their edges and debonded in the center to simulate a delamination, z2 CONCLUSIONS The process of damage accumulation in composite materials involves a number of damage modes, which interact to form a complex network of damage details called the damage state. A complete and thorough description of all the details within the damage state is a challenging, if not impracticable, task. The nondestructive evaluation of damage in a composite material or structure involves a knowledge of which damage modes are expected for a particular loading history, the detection and identification of the damage modes in their collective form, and an understanding of the relationships between the state of the material or structure and the subsequent response for a specific loading condition. The following set of guidelines is suggested to assist in the evaluation: 1. 2.
3.
4.
Determine what information is needed about the material, the loading conditions, and the expected damage modes. Determine how the information can be obtained (e.g. determine which NDE methods should be used to identify the damage modes). Determine how the composite material behavior will change owing to the accumulation of damage, and which failure modes are likely. Determine how to relate the state of damage in the material to residual strength or remaining life.
The fourth guideline is certainly the most difficult and represents the frontier of understanding of the response of composite materials.
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REFERENCES 1. R. D. Jamison and K. L. Reifsnider, Advanced Fatigue Damage Development in Graphite Epoxy Laminates, AFWAL-TR-82-3103, Air Force Wright Aeronautical Laboratories, December 1982. 2. F. H. Chang, D. E. Gordon and A. H. Gardner, A study of fatigue damage in composites by nondestructive testing techniques, Fatigue of Filamentary Composite Materials, ASTM STP 636,~1977, pp. 57-72. 3. K. Schulte and W. W. Stinchcomb, Damage development near the edges of a composite specimen during quasi-static and fatigue loading, Composites Technology Review, 6 (1984) pp. 3-9. 4. K. L. Reifsnider, E. G. Henneke and W. W. Stinchcomb, Defect-Property Relationships in Composite Materials, Final Report AFML-Tr-76-81, Part IV, Air Force Materials Laboratory, June 1979. 5. K. L. Reifsnider, E. G. Henneke, W. W. Stinchcomb and J. C. Duke, Fatigue Damage-Strength Relationships in Composite Laminates, AFWAL-TR-833084, Vol. I, Air Force Wright Aeronautical Laboratories, September 1983. 6. G. L. Roderick and J. D. Whitcomb, Fatigue damage of notched boron/epoxy laminates under constant-amplitude loading, Fatigue of Filamentary Composite Materials, ASTM STP 636, 1977, pp. 73-88. 7. W. D. Rummel, T. Tedrow and H. D. Brinkerhoff, Enhanced X-Ray Stereoscopic NDE of Composite Materials, AFWAL-TR-80-3053, Air Force Wright Aeronautical Laboratories, June 1980. 8. P. C. Yeung, W. W. Stinchcomb and K. L. Reifsnider, Characterization of constraint effects on flaw growth, Nondestructive Evaluation and Flaw Criticality./or Composite Materials, ASTM STP 696, 1979, pp. 316-38. 9. J. E. Bailey, P. T. Curtis and A. Parvizi, On the transverse cracking and longitudinal splitting behavior of glass and carbon fibre reinforced epoxy cross-ply laminates and the effect of Poisson and thermally generated strain, Proc. Roy. Soc. London, A-366 (1979) pp. 599-623. 10. G. P. Sendeckyj, G. E. Maddux and E. Porter, Damage documentation in composites by stereo radiography, Damage in Composite Materials, ASTM STP 775, 1982, pp. 16-26. 11. A. S. D. Wang, N. N. Kishore and C. A. Li, Crack development in graphiteepoxy cross-ply laminates under uniaxial tension, Composites Science and Technology, 24 (1985) pp. 33-46. 12. G. R. Kress and W. W. Stinchcomb, Fatigue response of notched graphite epoxy laminates, Recent Advances in Composites in the United States and Japan, ASTM STP 864, 1985. 13. S. M. Freeman, Damage Progression in Graphite-Epoxy by a Deplying Technique, AFWAL-TR-81-3157, Air Force Wright Aeronautical Laboratories, December 1981. 14. R. A. Blake, Digital nondestructive evaluation of composite materials (ultrasonic inspection), in: International Advances in Nondestructive Testing (ed. W. J. McGonnagle), Gordon and Breach,, New'York, 1983,.pp. 227 48. 15. Y. Bar-Cohen and R. L. Crane, Acoustic-backscattering imaging of subcritical flaws in composites, Materials Evaluation, 40 (1982), pp. 970-5.
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16. B. W. Rosen, Tensile failure of fibrous composites, A I A A J., 2 (1964) pp. 1985-91. 17. C. Zweben, Tensile failure of fiber composites, A1AA J., 6 (1968) pp. 2325-31. 18. S. B. Batdorf, Tensile strength of unidirectionally reinforced composites I, J. Reinforced Plastics and Composites, 1 (1982) pp. 153-64. 19. A. Vary and K. J. Bowles, Ultrasonic evaluation of the strength of unidirectional graphite-polymide composites, in: Proceedings Eleventh Symposium on Nondestructive Evaluation (eds R. E. Beissner and H. 1. Hoffman), Southwest Research Institute, San Antonio, Texas, 1977, pp. 242-58. 20. R. Talreja, A. Govada and E. G. Henneke, Quantitative assessment of damage in graphite-epoxy laminates by acousto-ultrasonic measurements, in: Review of'Progress on Quantitative Nondestructive Evaluation (eds D. O. Thompson and D. E. Chimenti), Plenum Press, New York, 1984, pp. 1099-106. 21. K.. L. Reifsnider, E. G. Henneke and W. W. Stinchcomb, The mechanics of vibrothermography, in: Mechanics oINondestructive Testing (ed. W. W. Stinchcomb), Plenum Press, New York, 1980, pp. 249 76. 22. S. S. Russell and E. G. Henneke, Dynamic effects furing vibrothermographic NDE of composites, N D T International, 17 (1984) pp. 19-25.
Nondestructive Evaluation of Damage Accumulation Processes in Composite Laminates*
Wayne W. Stinchcomb Materials Response Group, Engineering Science and Mechanics Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 (USA)
S UMMA R Y This paper presents results from several interdisciplinary studies of the mechanical response of composite materials, in which mechanics and materials science were used in concert with nondestructive evaluation (NDE) to investigate the damage accumulation process. Specific examples are cited for the initiation and growth of damage in notched and unnotched laminates subjected to cyclic loading histories. The process of damage accumulation in composite materials involves a number of damage modes, including matrix cracking, delamination, and fiber fracture, which initiate, grow, and interact to form a complex network of damage details called the damage state. Nondestructive techniques, including replication, light and electron microscopy, X-ray radiography, ultrasonics, stiffness change, and thermography, are used to detect and monitor the progressive development of damage throughout the fatigue life and to evaluate its effect on fatigue response of the laminates.
C O N C E P T S OF D A M A G E IN C O M P O S I T E M A T E R I A L S Damage in composite materials has been the subject of numerous research and development programs. The understanding of damage in composite materials and its effect on the response of composite structures * Paper presented at the International Symposium 'Composites: Materials and Engineering', Universityof Delaware, Newark, Delaware, USA, September 24-28, 1984. 103 Composites Science and Technology 0266-3538/86/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain
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Wayne W. Stinchcomb
has matured to a level where several composite materials are qualified and certified materials in many engineering structures. As new material systems are introduced and more severe design requirements are imposed on composites, the understanding of the process of damage accumulation and the attendant behavior of composite structures must be increased to ensure that the materials are used reliably and efficiently. To achieve this goal in a multidisciplinary field such as composite materials requires the concerted efforts of several groups of people, including those with expertise in mechanics, materials science, and nondestructive evaluation (NDE). The general process of damage accumulation in composite materials consists of a number of damage modes, including matrix cracking, delamination, and fiber fracture, which affect material response. Throughout the loading history the progressive development and interaction of various damage modes in the damage state (the collective, cumulative form of damage) changes the structure of the composite and redistributes the stresses in the laminate. The specific details of the damage process in composite materials will depend on the material system, the geometry of the material/structure, and the loading history.
THE D A M A G E PROCESS
Surface damage As an example of the damage development process, Fig. 1 shows damage modes and their progressive development during the fatigue life of an unnotched laminate with several off-axis plies. The scenario is for graphite-epoxy laminates, ~ although similar scenarios can be constructed for other composites, including metal-matrix composites. The damage modes for notched laminates are the same as for unnotched laminates; however, the damage modes are more localized at the notch.: Laminates can be inspected nondestructively in order to detect matrix cracks and delaminations. In situations where the matrix cracks intersect the free edge of the laminate and couple along the edge to form interfacial debonds the cracks can be detected by direct observation using a light microscope (magnification of 50-100 × ), or by recording the image of the surface on a strip of cellulose acetate tape (the replication technique) and viewing the tape through a microscope or a microfiche reader.
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DAMAGE MODES DURING FATIGUE LIFE I- Matrix Cracking
3 -Delaminotion Fiber Breaking
Fiber Breaking
5- Fracture
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Schematic representation o f the damage accumulation process in composite laminates.
Microscopy and replication can also be used to detect the presence of edge-related delaminations. It is convenient to study the progressive development of edge-related damage using the replication technique. By permanently recording the details of the damage state on tapes at various stages of life, without removing the laminate from the testing machine, the chronology of the edge damage history can be documented. Replicas are very sensitive to even the micro-details of damage. Figure 2(a) is a scanning electron micrograph of a replica of the edge of a [0, +45]2 s graphite-epoxy laminate after 50 cycles of tension-tension fatigue at a maximum tensile stress of 86 % of the tensile strength. 3 Matrix cracks in the + 45 ° and - 4 5 ° plies are quite evident and are already uniformly spaced at this early stage of fatigue life, forming a characteristic damage state. 4 There are also indications of damage in the 0 ° plies which are shown more clearly in Fig. 2(b). The high magnification micrograph shows that the damage in the 0 ° plies consists of broken fibers with the fractures banded along a zone across the thickness of the 0 ° plies. Returning to Fig. 2(a), each band of broken fibers is closely associated with the matrix cracks in the adjacent + 45 ° and - 45 ° plies. Fiber breaks will be discussed in more detail in a later section.
(a)
(b)
Fig. 2.
Scanning electron micrographs of a replica showing (a) matrix cracks and (b) broken fibers in a graphite-epoxy laminate.
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The portion of the damage development process revealed by the replication method affects the response of the laminate in several significant ways. First, the initiation of the individual edge damage modes is not independent of the state of damage in the material. The initiation of matrix cracks is not a single, instantaneous event, but is a series of sequential events dependent on the local distribution of stress and on local variations of strength throughout the laminate. The progressive initiation of matrix cracks forms a regularly spaced pattern of cracks in the off-axis plies, which is called the characteristic damage state and which is a laminate property. Matrix cracks also change the local stress field in the cracked ply, causing the interlaminar stresses along the edge to be redistributed along the ply interfaces. The redistributed stresses initiate interfacial debonds (crack coupling) which produce delamination along the length of the free edge. The matrix cracks also change the local stress field in the adjacent plies to produce a stress concentration near the crack tip. In the laminate shown in Fig. 2 the 0 ° plies are adjacent to the cracked, off-axis plies, and a high density of broken fibers is observed in the high stress zones. The second major point to be made concerns the early occurrence of fiber fracture in the fatigue life. While a large number of fiber fractures would be expected in the advanced stages of damage late in the fatigue life, fiber breaks (on the surface and in the interior) occur throughout the entire fatigue life. 5 Certainly some of the breaks are in statistically weak fibers; but, more importantly, a large percentage of the breaks are associated with other damage modes and the attendant stress redistributions, as will be discussed later.
Internal damage Microscopy and replication provide very valuable information about the accumulation of damage on an exposed surface of a laminate, such as an edge or the surface of a hole. However, these techniques do not give complete information on the global extent and distribution of matrix cracks, delaminations, and fiber breaks throughout the laminate. At this time there is no single N D E method which can detect all of the major damage modes in composites in a global measurement and relate that information, in a quantitative way, to the strength, stiffness, and life of the material or structure. X-ray radiography is one of the N D E methods frequently used to inspect the interior of a composite laminate. The damage modes that can
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be detected using this technique are dependent on the composite material system being investigated. Many of the commonly used composite materials are transparent to X-rays. For example, X-ray radiography can detect broken boron fibers in a boron-epoxy composite 6 (actually the Xrays detect the breaks in the tungsten core of the boron fibers) but cannot detect broken graphite fibers in a graphite-epoxy laminate. Matrix cracks in epoxy matrix composites are easily detected if the cracks are filled with a medium opaque to X-rays, such as zinc iodide, 7 or if very soft X-rays are used. s Delaminations are most easily observed if an opaque medium fills the debonded interfacial region. Although X-ray radiography will detect matrix cracks and delaminations if an opaque medium is used, there are two important conditions that must be remembered when interpreting Xray data. First, the opaque medium, or penetrant, may not infiltrate all of the matrix cracks and delaminations in the laminate. The penetrant must be applied on the surface of the composite and allowed to flow into the damage zone. The penetrant solution should contain an additive to reduce the surface tension of the fluid in order to increase penetration along the cracks. When possible, the penetrant should be applied to the composite under load to 'open up' the damage and increase flow. Penetration can be increased if the load can be cycled a few times to 'pump' the solution into the damaged regions. However, the penetrant may not be able to fill all damage zones because some of the internal damage sites may not be connected to an external surface. Such damage will usually not be detected in X-ray transparent materials such as graphite-epoxy. The second condition concerns damage, usually large delaminations, which may not be completely resolved even though the penetrant can wet the debonded region. When the delaminated surfaces open, capillary action draws the penetrant to the boundary of the delamination, leaving a large debonded region void of penetrant. The radiograph will show only the boundary of the delaminated region, and the nature and size of the damage could be misinterpreted. Figure 3(a) is a radiograph showing tension-tension fatigue damage in a [0/902] Sgraphite-epoxy laminate. The radiograph is made with the Xray beam perpendicular to the plane of the laminate. Regularly spaced matrix cracks in the 90 ° plies form the characteristic damage state. Matrix cracks in the 0 ° plies, caused by cyclic tensile stresses perpendicular to the fibers, are also evident, as are several small, shaded regions around the crossing matrix cracks in the adjacent 0 ° and 90 o plies. Two of these regions are indicated by the arrows in Fig. 3(a). A portion of
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(b)
Fig. 3. Zinc iodide enhanced radiographs of fatigue damage in a graphite-epoxy laminate: (a) matrix cracks in 0 ° and 90 ° plies; (b) internal delaminations initiating at points of crack intersections. 1
the radiograph is enlarged in Fig. 3(b) to show the shaded regions more clearly. The regions have been identified as internal delaminations on the 0/90 interface 1,9 and can be identified using the stereoradiograph technique. 7, l o The planes of the matrix cracks in the adjacent 0 o and 90 o plies intersect at a point on the 0/90 interface, producing a region of tensile interlaminar normal stress around the point of intersection.l'x Under conditions of cyclic tensile loads, delaminations nucleate at the crack intersection point and grow on the 0/90 interface. Much attention
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has been given to the free edge delamination problem (delaminations along the length of a straight-sided test specimen, for example). However, it is important to recognize that the complex network of internal matrix cracks creates internal surfaces which act as micro-free edges. The attendant internal stress field produces internal delminations in the same manner as that in which the stress field at the free, external surface of a laminate produces edge delamination. Because X-ray radiography provides so much information on the state of damage in a composite material or structure (matrix cracks, delaminations, and, in some cases, fiber breaks), it is a very valuable N D E method. However, it must be remembered that a radiograph is a twodimensional, plan view of a three-dimensional damage field. The radiograph shown in Fig. 4a shows matrix cracks and delaminations in a center-notched, quasi-isotropic graphite-epoxy laminate which had been subjected to tension-tension cyclic loads.12 The plies containing matrix cracks can be identified by the orientation of the cracks, although the identification of specific plies becomes increasingly difficult as the thickness and number of repeated plies increases. Even in the eight-ply laminate shown in the radiograph, it is not possible to identify the interfaces which have delaminated nor the size of the individual
Fig. 4a.
Fatigue damage in a [45/90/- 45/0]2 graphite-epoxy laminate with center hole after 10'* cycles at am.x=0"85tru~t+ R = O ' l . 12
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Fig. 4b. Delaminations on several interfaces of a center-notched [45/90/-45/0]~ graphite-epoxy laminate after cyclic loading.~2 delaminations. The stereoradiography technique creates a threedimensional image of the damage field and aids in the identification of interfaces and size ofdelaminations. The internal delaminations shown in Fig. 3 were identified and studied using stereoradiography. However, it is difficult to resolve individual details and specific locations in severely damaged, thick laminates using the stereo method. Figure 4b shows a schematic of the delaminations on the interfaces of the eight-ply, quasiisotropic laminate shown in Fig. 4a. The delaminated regions were identified by separating the plies using the de-ply method 13 (a destructive technique) and viewing the de-plied surfaces through a light microscope. Ultrasonic techniques are often used to detect and map internal damage fields. The conventional ultrasonic C-scan is less sensitive than radiography to all the details of damage and is very useful for scanning a component for a suspected delamination and mapping the damaged region. The C-scan is particularly well suited for detecting internal delaminations because the delaminated region does not have to be infiltrated by an opaque medium through an external surface. However, the conventional C-scan technique is not as well suited as radiography for detecting matrix cracks. The ultrasonic beam is sensitive to the presence of matrix cracks, but the resolution capability is less than that of radiography. Enhanced image techniques 14 and back-scattering techniques 15 are being used to improve the resolution of ultrasonic inspection methods and make them more general-purpose techniques.
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Fiber fracture
One of the most important damage modes, and the most difficult to detect nondestructively, is fiber fracture. Two situations where broken fibers can be detected nondestructively have been mentioned previously: fibers, such as boron on a tungsten filament which is opaque to X-rays, and fibers on an external surface which can be detected using microscopy. Other nondestructive inspection methods being considered for the detection of broken fibers are eddy current and acoustic emission; however, more developmental work needs to be done before these methods can be used reproducibly and reliably. Our knowledge of fiber failure is based on a number of fiber fracture models 16-18 and a limited amount of experimental data obtained from destructive testing and inspection. C o m m o n points of agreement between various models and experimental data include the concentration of stress and the redistribution of stress due to broken fibers. The strength of the fibers in a given ply is distributed throughout the volume of the ply. Some fibers fail early in the life of the laminate, as represented schematically in Fig. 1. The load carried by the broken fiber is distributed through the matrix and neighboring fibers. The local changes in geometry and discontinuities introduced by broken fibers also introduce stress concentrations and raise the stress in the adjacent, unbroken plies. The combined effects of the redistributed stress and the concentrated stress promote additional fiber breaks throughout the life of the composite, with a large percentage of the breaks occurring in the first half of the life. 5 Stress concentrations and stress redistribution in fibers are produced by sources other than broken fibers alone. Other damage modes, such as matrix cracking in adjacent plies and delamination of an adjacent interface, cause local adjustments in the stress field to satisfy equilibrium conditions. Figure 5 is a scanning electron photomicrograph of a portion of a 0 ° ply, adjacent to a matrix crack in a 90 ° ply in a [ 0 / 9 0 2 ] s graphite-epoxy laminate. A number of broken fibers can be seen. Figure 6 shows the damage picture on a larger scale. The broken fibers shown in Fig. 5 are concentrated in the region of the matrix crack and are caused by the locally high stresses associated with the crack. While it is useful to develop models of the process of fiber failure and to identify the mechanisms of the process, a critical and fundamental question remains unanswered. What are the important details of the fiber fracture process that control the strength and life of composite materials ?
NDE of damage accumulation processes in composite laminates
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113
Scanning electron micrograph of broken fibers in a 0 ° ply of a [0/902L graphite-epoxy laminate after cyclic loading.l
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A second, related question is: Can those details be measured nondestructively in a way that would give early, reliable warnings of fracture? The answers to these questions will become increasingly important as the design strain levels are raised in future generation composite structures.
REAL-TIME, N O N D E S T R U C T I V E MEASURES OF R E S P O N S E Establishing relationships between the state of damage and the subsequent response of a material or structure is an interdisciplinary task incorporating nondestructive evaluation, mechanics, and materials science. The nondestructive methods for inspecting composite materials and structures which have been presented provide details and descriptions of the damage accumulation process in composites. For completeness damage must be evaluated in terms of changes in the state of the material and their consequences on the engineering response of the material or structure. As an example consider matrix cracks and their collective effect on composite material behavior. Detection of a crack in a metallic structure causes great concern. In contrast, a single matrix crack or complete saturation of matrix cracks causes relatively little concern about the degradation of strength of a composite structure. It is true that matrix cracks do not have a direct consequence on the strength and life of composite laminates of engineering significance; however, matrix cracks act as catalysts to spawn other damage modes which do degrade strength and life, particularly in long life situations. As previously shown, matrix cracks on the edge of a laminate couple along the interface to form edge delaminations; crossing matrix cracks in adjacent plies create a stress field which promotes internal delaminations; and matrix cracks also create locally high stresses in adjacent plies which cause fiber fracture. The relationships between damage and response are not necessarily straightforward. Figure 7 shows the change in response of a [45/90/-45/0L graphite-epoxy laminate with a center hole caused by tension-tension fatigue when cycled at a maximum stress of 85 ~o of the monotonic tensile strength. The damage process is nondestructively recorded throughout the loading history. Matrix cracks and delaminations accumulate to form a damage pattern similar to that shown in Fig. 4a. The combined effect of the damage reduces axial stiffness (increases compliance), measured
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across the hole in real-time. However, the residual tensile strength of nominally identical laminates, subjected to the same loading history, increases during the early part of the fatigue life (damage initiation around the hole) and decreases during the later part of life (damage growth away from the hole). The developing damage field associated with the hole alters the distribution of strain around the hole (changes the mechanics problem), thereby reducing the strain concentration due to the hole, as shown by Moir6 interferometry, 12 and increasing the tensile strength of the notched laminate. As damage continues to accumulate and grows throughout the cyclic loading, the strength of the ligaments of material on each side of the hole degrades, causing fatigue failure. Stiffness, or change in stiffness, is well suited as a sentinel of damage in composite materials because it is quantitative, it is directionally sensitive, and it is the material property which relates stress and strain. Therefore it is a viable parameter to use in mechanics-based formulations of strength and life. Acousto-ultrasonic techniques currently being developed and studied19,2 0 also show promise as real-time damage monitors. Controlled ultrasonic signals introduced into a composite interact with, and are scattered by, various defects, before being detected by a downstream
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Wayne W. Stincheomb
acoustic emission transducer. The technique has the potential of detecting and quantifying damage and of identifying the fracture location. A third real-time, quantitative measure of damage is thermography. Used in an active mode thermography uses low frequency-high amplitude or high frequency-low amplitude mechanical vibrations to activate internal sources of heat, such as the rubbing or clapping together of two delaminated surfaces in a composite panel. 21 The frequency at which a particular type of defect becomes thermally active is a characteristic of the defect type, size, and location. For the case of internal delamination the measured frequency for thermal activity corresponds to the resonant frequency of two flat plates bonded top to bottom along their edges and debonded in the center to simulate a delamination, z2 CONCLUSIONS The process of damage accumulation in composite materials involves a number of damage modes, which interact to form a complex network of damage details called the damage state. A complete and thorough description of all the details within the damage state is a challenging, if not impracticable, task. The nondestructive evaluation of damage in a composite material or structure involves a knowledge of which damage modes are expected for a particular loading history, the detection and identification of the damage modes in their collective form, and an understanding of the relationships between the state of the material or structure and the subsequent response for a specific loading condition. The following set of guidelines is suggested to assist in the evaluation: 1. 2.
3.
4.
Determine what information is needed about the material, the loading conditions, and the expected damage modes. Determine how the information can be obtained (e.g. determine which NDE methods should be used to identify the damage modes). Determine how the composite material behavior will change owing to the accumulation of damage, and which failure modes are likely. Determine how to relate the state of damage in the material to residual strength or remaining life.
The fourth guideline is certainly the most difficult and represents the frontier of understanding of the response of composite materials.
NDE of damage accumulation processes in composite laminates
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REFERENCES 1. R. D. Jamison and K. L. Reifsnider, Advanced Fatigue Damage Development in Graphite Epoxy Laminates, AFWAL-TR-82-3103, Air Force Wright Aeronautical Laboratories, December 1982. 2. F. H. Chang, D. E. Gordon and A. H. Gardner, A study of fatigue damage in composites by nondestructive testing techniques, Fatigue of Filamentary Composite Materials, ASTM STP 636,~1977, pp. 57-72. 3. K. Schulte and W. W. Stinchcomb, Damage development near the edges of a composite specimen during quasi-static and fatigue loading, Composites Technology Review, 6 (1984) pp. 3-9. 4. K. L. Reifsnider, E. G. Henneke and W. W. Stinchcomb, Defect-Property Relationships in Composite Materials, Final Report AFML-Tr-76-81, Part IV, Air Force Materials Laboratory, June 1979. 5. K. L. Reifsnider, E. G. Henneke, W. W. Stinchcomb and J. C. Duke, Fatigue Damage-Strength Relationships in Composite Laminates, AFWAL-TR-833084, Vol. I, Air Force Wright Aeronautical Laboratories, September 1983. 6. G. L. Roderick and J. D. Whitcomb, Fatigue damage of notched boron/epoxy laminates under constant-amplitude loading, Fatigue of Filamentary Composite Materials, ASTM STP 636, 1977, pp. 73-88. 7. W. D. Rummel, T. Tedrow and H. D. Brinkerhoff, Enhanced X-Ray Stereoscopic NDE of Composite Materials, AFWAL-TR-80-3053, Air Force Wright Aeronautical Laboratories, June 1980. 8. P. C. Yeung, W. W. Stinchcomb and K. L. Reifsnider, Characterization of constraint effects on flaw growth, Nondestructive Evaluation and Flaw Criticality./or Composite Materials, ASTM STP 696, 1979, pp. 316-38. 9. J. E. Bailey, P. T. Curtis and A. Parvizi, On the transverse cracking and longitudinal splitting behavior of glass and carbon fibre reinforced epoxy cross-ply laminates and the effect of Poisson and thermally generated strain, Proc. Roy. Soc. London, A-366 (1979) pp. 599-623. 10. G. P. Sendeckyj, G. E. Maddux and E. Porter, Damage documentation in composites by stereo radiography, Damage in Composite Materials, ASTM STP 775, 1982, pp. 16-26. 11. A. S. D. Wang, N. N. Kishore and C. A. Li, Crack development in graphiteepoxy cross-ply laminates under uniaxial tension, Composites Science and Technology, 24 (1985) pp. 33-46. 12. G. R. Kress and W. W. Stinchcomb, Fatigue response of notched graphite epoxy laminates, Recent Advances in Composites in the United States and Japan, ASTM STP 864, 1985. 13. S. M. Freeman, Damage Progression in Graphite-Epoxy by a Deplying Technique, AFWAL-TR-81-3157, Air Force Wright Aeronautical Laboratories, December 1981. 14. R. A. Blake, Digital nondestructive evaluation of composite materials (ultrasonic inspection), in: International Advances in Nondestructive Testing (ed. W. J. McGonnagle), Gordon and Breach,, New'York, 1983,.pp. 227 48. 15. Y. Bar-Cohen and R. L. Crane, Acoustic-backscattering imaging of subcritical flaws in composites, Materials Evaluation, 40 (1982), pp. 970-5.
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16. B. W. Rosen, Tensile failure of fibrous composites, A I A A J., 2 (1964) pp. 1985-91. 17. C. Zweben, Tensile failure of fiber composites, A1AA J., 6 (1968) pp. 2325-31. 18. S. B. Batdorf, Tensile strength of unidirectionally reinforced composites I, J. Reinforced Plastics and Composites, 1 (1982) pp. 153-64. 19. A. Vary and K. J. Bowles, Ultrasonic evaluation of the strength of unidirectional graphite-polymide composites, in: Proceedings Eleventh Symposium on Nondestructive Evaluation (eds R. E. Beissner and H. 1. Hoffman), Southwest Research Institute, San Antonio, Texas, 1977, pp. 242-58. 20. R. Talreja, A. Govada and E. G. Henneke, Quantitative assessment of damage in graphite-epoxy laminates by acousto-ultrasonic measurements, in: Review of'Progress on Quantitative Nondestructive Evaluation (eds D. O. Thompson and D. E. Chimenti), Plenum Press, New York, 1984, pp. 1099-106. 21. K.. L. Reifsnider, E. G. Henneke and W. W. Stinchcomb, The mechanics of vibrothermography, in: Mechanics oINondestructive Testing (ed. W. W. Stinchcomb), Plenum Press, New York, 1980, pp. 249 76. 22. S. S. Russell and E. G. Henneke, Dynamic effects furing vibrothermographic NDE of composites, N D T International, 17 (1984) pp. 19-25.