A numerical procedure is described for predicting the viscoelastic response of general laminates. A nonlinear compliance model is used to predict the creep response of the individual laminae. A biaxial delayed failure model predicts ply failure. The numerical procedure, based on lamination theory, increases by increments through time to predict creep compliance and delayed failures in laminates. Numerical stability problems and experimental verification are discussed. Although the program has been quite successful in predicting creep of general laminates, the assumptions associated with lamination theory have resulted in erroneous bounds on the predicted material response. Delayed failure predictions have been conservative. Several improvements are suggested to increase the accuracy of the procedure.
An experimental evaluation of the dynamic fracture properties of an automotive epoxy is presented. Pronounced stick-slip behavior was observed in both quasi-static and impact tests of aluminum and composite adherends bonded with this adhesive. An experimental technique for conducting low speed impact of adhesively bonded automotive composite joints is presented. Based on the use of a modified drop tower, mode I, II, and mixed mode values for critical energy release rate were determined to create a fracture envelope for the composite/epoxy system. Because load measurements are erratic and unreliable at higher test rates, displacement-based relationships were used to quantify these energy release rates. Displacement data were collected with an imaging system that utilizes edge detection to determine displacement profiles, end displacements, and opening displacements where applicable. Because of the resolution of the image-based approach being used, determining crack length experimentally is difficult. As a result, numerical methods based on edge detection algorithms were developed to objectively determine the crack length based on the available experimental data in mode I, II, and mixed mode I/II configurations.
Thermomechanically bonded nonwoven fabrics contain discrete bonds that are formed from melted and fused fibers. In these fabrics, the fibers are loosely organized although they lie predominantly in the machine direction (MD). Through a custom-built biaxial testing device and simultaneous image capture, the mechanical response of individual bonds in thermomechanically bonded nonwoven fabrics made of polyethylene/polypropylene sheath–core fibers was studied. Toward this end, cruciform specimens ( n = 20) with bonds in the gauge areas and arms aligned in the MD and the cross-direction (CD) were subjected to displacement-controlled equi-biaxial tests. The biaxial force–displacement curves along the two loading directions were found to be different. The average maximum force and average stiffness were significantly higher in the MD than in the CD ( p < 0.05). This difference was determined by the amount and orientation of fibers and size of the bonds in the two directions. By analyzing the images captured during equi-biaxial testing, the bonds were always observed to disintegrate into their constituent fibers. Digital image correlation was used to measure the local and average Eulerian strains of the bonds before their breakage initiated. The average axial strain experienced by the bond in the MD was always monotonically increasing with the axial load. The average axial strain in the CD, however, varied among bonds: it was monotonically increasing, monotonically decreasing, and increasing and decreasing with the axial load. Strain maps demonstrated the inhomogeneity in strain experienced by the bonds. These findings can guide the design and development of thermomechanically bonded nonwoven fabrics for applications in automotive, medical, consumer products, and civil engineering industries.
Although graphite fibers behave in an essentially clastic manner, the polymeric matrix of graphite/epoxy composites is a viscoelastic material which exhibits creep and delayed failures. The creep process is quite slow at room temperature, but may be accelerated by higher temperatures, moisture absorption, and other factors. Techniques are being studied to predict long-term behavior of general laminates based on short-term observations of the unidirectional material at elevated temperatures. A preliminary numerical procedure based on lamination theory is developed for predicting creep and delayed failures in laminated composites. A modification of the Findley nonlinear power law is used to model the constitutive behavior of a lamina. An adaptation of the Tsai-Hill failure criterion is used to predict the time-dependent strength of a lamina. Predicted creep and delayed failure results are compared with typical experimental data.
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