Engineering topics
Publications and source records attributed to Martin, Roderick H..
Alternative method proposed for characterizing mode-I fatigue delamination by use of double-cantilever-beam (DCB). In new method, one monitors number of cycles, N, to onset of delamination from end of thin insert. Fiber bridging avoided. Results presented easily in form of G-vs.-N plot.
This paper describes a technique to determine interlaminar tensile strength, sigma(sub 3c), of a fiber reinforced composite material using a curved beam. The specimen was a unidirectional curved beam, bent 90 deg, with straight arms. Attached to each arm was a hinged loading mechanism that was held by the grips of a tension testing machine. Geometry effects of the specimen, including the effects of loading arm length, inner radius, thickness, and width, were studied. The data sets fell into two categories: low strength corresponding to a macroscopic flaw related failure and high strength corresponding to a microscopic flaw related failure. From the data available, the specimen width and loading arm length had little effect on sigma(sub 3c). The inner radius was not expected to have a significant effect on sigma(sub 3c), but this conclusion could not be confirmed because of differences in laminate quality for each curve geometry. The thicker specimens had the lowest value of sigma(sub 3c) because of poor laminate quality.
The results are summarized of several interlaboratory 'round robin' test programs for measuring the mode 1 interlaminar fracture toughness of advanced fiber reinforced composite materials. Double Cantilever Beam (DCB) tests were conducted by participants in ASTM committee D30 on High Modulus Fibers and their Composites and by representatives of the European Group on Fracture (EGF) and the Japanese Industrial Standards Group (JIS). DCB tests were performed on three AS4 carbon fiber reinforced composite materials: AS4/3501-6 with a brittle epoxy matrix; AS4/BP907 with a tough epoxy matrix; and AS4/PEEK with a tough thermoplastic matrix. Difficulties encountered in manufacturing panels, as well as conducting the tests are discussed. Critical issues that developed during the course of the testing are highlighted. Results of the round robin testing used to determine the precision of the ASTM DCB test standard are summarized.
Isothermal aging was conducted on two composite systems being considered as possible candidates for the next generation supersonic transport. The composite systems were IM7/5260, a carbon/thermoset, and IM7/8320, a carbon/amorphous thermoplastic. The materials were isothermally aged for a total of 5000 hours at 125 C and 175 C. These temperatures are approximately equivalent to the upper skin temperatures of an aircraft flying at Mach 2.0 and Mach 2.4, respectively. The variations of the following properties were determined as a function of aging time: weight loss, moduli, glass transition temperature, microcracking, and modulus and strength of a +/- 45 laminate. The difficulties and accuracy of strain measurements are also discussed.
This paper describes a technique to determine interlaminar tension strength, sigma(sub 3c) of a fiber reinforced composite material using a curved beam. The specimen was a unidirectional curved beam, bent 90 degrees, with straight arms. Attached to each arm was a hinged loading mechanism which was held by the grips of a tensile testing machine. Geometry effects of the specimen, including the effects of loading arm length, inner radius, thickness, and width, were studied. The data sets fell into two categories: low strength corresponding to a macroscopic flaw related failure and high strength corresponding to a microscopic flaw related failure. From the data available, the loading arm length had no effect on sigma(sub 3c). The inner radius was not expected to have a significant effect on sigma(sub 3c), but this conclusion could not be confirmed because of differences in laminate quality for each curve geometry. The thicker specimens had the lowest value of sigma(sub 3c) because of poor laminate quality. Width was found to affect the value of sigma(sub 3c) only slightly. The wider specimens generally had a slightly lower strength since more material was under high stress, and hence, had a larger probability of containing a significant flaw.
Delamination failure in a unidirectional curved composite laminate was investigated. The curved laminate failed unstably by delaminations developing around the curved region of the laminate at different depths through the thickness until virtually all bending stiffness was lost. Delamination was assumed to initiate at the location of the highest radial stress in the curved region. A closed form curved beam elasticity solution and a 2D finite element analysis (FEA) were conducted to determine this location. The variation in the strain energy release rate, G, with delamination growth was then determined using the FEA. A strength-based failure criteria adequately predicted the interlaminar tension failure which caused initial delamination onset. Using the G analysis the delamination was predicted to extend into the arm and leg of the laminate, predominantly in mode I. As the initial delamination grew around the curved region, the maximum radial stress in the newly formed inner sublaminate increased to a level sufficient to cause a new delamination to initiate in the sublaminate with no increase in applied load. This failure progression was observed experimentally.
A fracture mechanics damage methodology to predict edge delamination is described. The methodology accounts for residual thermal stresses, cyclic thermal stresses, and cyclic mechanical stresses. The modeling is based on the classical lamination theory and a sublaminate theory. The prediction methodology determines the strain energy release rate, G, at the edge of a laminate and compares it with the fatigue and fracture toughness of the composite. To verify the methodology, isothermal static tests at 23, 125, and 175 C and tension-tension fatigue tests at 23 and 175 C were conducted on laminates. The material system used was a carbon/bismaleimide, IM7/5260. Two quasi-isotropic layups were used. Also, 24 ply unidirectional double cantilever beam specimens were tested to determine the fatigue and fracture toughness of the composite at different temperatures. Raising the temperature had the effect of increasing the value of G at the edge for these layups and also to lower the fatigue and fracture toughness of the composite. The static stress to edge delamination was not affected by temperature but the number of cycles to edge delamination decreased.
Interlaminar fracture characterization has been investigated for several years. Only now is it well enough understood for standardization organizations to attempt to write standard test methods. A review of the current philosophies in characterizing interlaminar fracture is presented. First, the mode 1, double cantilever beam test (DCB) for measuring G(sub 1c) and the end notched flexure (ENF) test for measuring G(sub 2c) are discussed. These tests have undergone the most extensive research throughout the years and are furthest towards standardization. In addition, the mode 2, end loaded split (ELS) specimen is discussed. Mixed mode fracture is also discussed and the recently developed mixed mode bending (MMB) test is detailed. Then, tests for evaluating mode 3 fracture toughness, including the split cantilever beam (SCB), are reviewed. Finally, the work done on interlaminar fracture characterization in fatigue is reviewed.
Analytical and experimental work is detailed which is required to predict delamination onset and growth in a curved cross plied composite laminate subjected to static and fatigue loads. The composite used was AS4/3501/6, graphite/epoxy. Analytically, a closed form stress analysis and 2-D and 3-D finite element analyses were conducted to determine the stress distribution in an undamaged curved laminate. The finite element analysis was also used to determine values of strain energy release rate at a delamination emanating from a matrix crack in a 90 deg ply. Experimentally, transverse tensile strength and fatigue life were determined from flat 90 deg coupons. The interlaminar tensile strength and fatigue life were determined from double cantilevered beam specimens. Cross plied curved laminates were tested statically and in fatigue to give a comparison to the analytical predictions. A comparison of the fracture mechanics life prediction technique and the strength based prediction technique is given.
Static and fatigue double-cantilever beam (DCB) and end-notch flexure (ENF) tests were conducted to determine the effect of the simulated initial delamination in interlaminar fracture toughness, G(sub c), and fatigue fracture threshold, G(sub th). Unidirectional, 24-ply specimens of S2/SP250 glass/epoxy were tested using Kapton inserts of four different thickness - 13, 25, 75, and 130 microns, at the midplane at one end, or with tension or shear precracks, to simulate an initial delamination. To determine G(sub c), the fatigue fracture threshold below which no delamination growth would occur in less than 1 x 10(exp 6) cycles, fatigue tests were conducted by cyclically loading specimens until delamination growth was detected. Consistent values of model 1 fracture toughness, G(sub Ic), were measured from DCB specimens with inserts of thickness 75 microns or thinner, or with shear precracks. The fatigue DCB tests gave similar values of G(sub Ith) for the 13, 25, and 75 microns specimens. Results for the shear precracked specimens were significantly lower that for specimens without precracks. Results for both the static and fatigue ENF tests showed that measured G(IIc) and G(IIth) values decreased with decreasing insert thickness, so that no limiting thickness could be determined. Results for specimens with inserts of 75 microns or thicker were significantly higher than the results for precracked specimens or specimens with 13 or 25 microns inserts.
Delamination onset and growth from a matrix crack in a curved composite laminate are studied. Finite element analysis is used to determine values of G which can be compared to the interlaminar fracture toughness of the composite for delamination predictions. The delamination is assumed to grow from the matrix crack and to remain straight and perpendicular to the edge. G is larger at the free edge than in the center and increases with delamination growth. Thus, once delamination begins from the matrix crack it should continue around the curved section with no increase in applied load. In the local vicinity of the matrix crack the rate of change of G decreases initially and then begins to increase with a point of inflection in between. Measured failure loads and the interlaminar fracture toughness of the composite are used to show that the point of inflection corresponds to the critical value of G.
Two techniques for characterizing fatigue delamination in double cantilever beams, the delamination growth onset method and the delamination growth and arrest method, are evaluated. Two glass/epoxy systems, S2/SP250 and S2/CE9000, were tested at R = 0.1 and R = 0.5. The values of the strain energy release rate threshold Delta G(Ith) for no onset of delamination growth until one million cycles were compared with the values of Delta G(I) at delamination arrest. The values at arrest were consistently higher because fiber bridging occurred in the tests. Normalizing Delta G(I) by the mode I value of interlaminar fracture toughness G(Ir) at the corresponding delta A gave values of Delta G(I)/G(Ir) at delamination arrest which were closer to the threshold values obtained from the Delta G-N technique. However, this normalization did not represent a generic materials characterization technique and did not allow for delamination onset predictions.
Delamination failure in a unidirectional curved composite laminate was investigated. The curved laminate failed unstably by delaminations developing around the curved region of the laminate at different depths through the thickness until virtually all bending stiffness was lost. Delamination was assumed to initiate at the location of the highest radial stress in the curved region. A closed form curved beam elasticity solution and a 2-D finite element analysis (FEA) were conducted to determine this location. The variation in the strain energy release rate, G, with delamination growth was then determined using the FEA. A strength-based failure criteria adequately predicted the interlaminar tension failure which caused initial delamination onset. Using the G analysis the delamination was predicted to extend into the arm and leg of the laminate, predominantly in mode I. As the initial delamination grew arould the curved region, the maximum radial stress in the newly formed inner sublaminate increased to a level sufficient to cause a new delamination to initiate in the sublaminate with no increase in applied load. This failure progression was observed experimentally.
Composite materials often fail by delamination. The onset and growth of delamination in AS4/PEEK, a tough thermoplastic matrix composite, was characterized for mode 1 and mode 2 loadings, using the Double Cantilever Beam (DCB) and the End Notched Flexure (ENF) test specimens. Delamination growth per fatigue cycle, da/dN, was related to strain energy release rate, G, by means of a power law. However, the exponents of these power laws were too large for them to be adequately used as a life prediction tool. A small error in the estimated applied loads could lead to large errors in the delamination growth rates. Hence strain energy release rate thresholds, G sub th, below which no delamination would occur were also measured. Mode 1 and 2 threshold G values for no delamination growth were found by monitoring the number of cycles to delamination onset in the DCB and ENF specimens. The maximum applied G for which no delamination growth had occurred until at least 1,000,000 cycles was considered the threshold strain energy release rate. Comments are given on how testing effects, facial interference or delamination front damage, may invalidate the experimental determination of the constants in the expression.
A test rig for testing a thick split cantilever beam for scissoring delamination (mode 3) fracture toughness was developed. A 3-D finite element analysis was conducted on the test specimen to determine the strain energy release rate, G, distribution along the delamination front. The virtual crack closure technique was used to calculate the G components resulting from interlaminar tension, GI, interlaminar sliding shear, GII, and interlaminar tearing shear, GIII. The finite element analysis showed that at the delamination front no GI component existed, but a GII component was present in addition to a GIII component. Furthermore, near the free edges, the GII component was significantly higher than the GIII component. The GII/GIII ratio was found to increase with delamination length but was insensitive to the beam depth. The presence of GII at the delamination front was verified experimentally by examination of the failure surfaces. At the center of the beam, where the failure was in mode 3, there was significant fiber bridging. However, at the edges of the beam where the failure was in mode 3, there was no fiber bridging and mode 2 shear hackles were observed. Therefore, it was concluded that the split cantilever beam configuration does not represent a pure mode 3 test. The experimental work showed that the mode 2 fracture toughness, GIIc, must be less than the mode 3 fracture toughness, GIIIc. Therefore, a conservative approach to characterizing mode 3 delamination is to equate GIIIc to GIIc.
Composite materials often fail by delamination. The onset and growth of delamination in AS4/PEEK, a tough thermoplastic matrix composite, was characterized for mode 1 and mode 2 loadings, using the Double Cantilever Beam (DCB) and the End Notched Flexure (ENF) test specimens. Delamination growth per fatigue cycle, da/dN, was related to strain energy release rate, G, by means of a power law. However, the exponents of these power laws were too large for them to be adequately used as a life prediction tool. A small error in the estimated applied loads could lead to large errors in the delamination growth rates. Hence strain energy release rate thresholds, G sub th, below which no delamination would occur were also measured. Mode 1 and 2 threshold G values for no delamination growth were found by monitoring the number of cycles to delamination onset in the DCB and ENF specimens. The maximum applied G for which no delamination growth had occurred until at least 1,000,000 cycles was considered the threshold strain energy release rate. Comments are given on how testing effects, facial interference or delamination front damage, may invalidate the experimental determination of the constants in the expression.
The effect of insert thickness and method of precracking on mode I interlaminar fracture toughness, GIc, and delamination fatigue threshold, GIth, values were determined for a glass/epoxy double cantilever beam specimen. The results of the static tests showed that precracking in tension would cause fiber bridging and thus may yield unconservative values of GIc and GIth. Precracking in shear yields suitable values of GIc but overly conservative values of GIth. For the glass/epoxy composite used, an insert thickness of 0.5 mil was most suitable for determining GIc and GIth values, although an insert thickness up to 3 mil was acceptable. Inserts thicker than 3 mil were not acceptable for determining GIc and GIth values.