Characterizing compressive and fracture strengths of fiber reinforced composites using off-axis specimens

Qida Bing, Purdue University

Abstract

The present report focuses on testing and modeling strain rate dependent compressive and fracture strengths of unidirectional fiber reinforced composites by using off-axis specimens. Experimental results in split Hopkinson pressure bar (SHPB) tests with AS4/3501-6 off-axis specimens showed large bending wave existing in the incident bar and transmission bar because of the strong interface friction induced by the direct contacts between the stiff fiber ends and loading surfaces, thus the extension-shear coupling was prevented fully developing in the off-axis specimens under compression and the one-dimensional wave propagation assumption in the SHPB analysis was violated. Through the vapor deposition technique a thin titanium layer is coated on both specimen ends, smooth sliding between loading surfaces and specimen ends assures the pressure is the only interaction. Stress-strain curves are generated at different strain rate using this type of specimens with various off-axis angles under strain controlled testing mode. From these curves a rate-dependent viscoplasticity constitutive model is developed to predict the stress-strain relations at various strain rates and a compressive strength model is established from the viscoplasticity model for AS4/3501-6 carbon/epoxy composite. Comparison between the model predictions and experimental results shows good agreement and indicates that the compressive strength model obtained by using low strain rate test data is valid for high strain rate too. From the established rate dependent compressive strength model, the longitudinal compressive strengths at various strain rates are predicted. Through an extrapolation technique proposed in the work, rate dependent 0° compressive strengths of unidirectional AS4/3501-6 composite are obtained from off-axis test data and compared to model predictions. By utilizing the advantage of off-axis specimens that a combined stress state can be obtained from a simple uniaxial loading, a new method to perform mode II fracture tests is presented which involves using off-axis specimen by properly introducing a pre-crack in the specimen along the fiber direction. Static off-axis fracture tests are conducted with S2/8552 glass/epoxy composite at various off-axis angles and finite element (FE) method is used to evaluate the critical mode II energy release rate (fracture toughness). Note that the crack surface is not frictionless and existence of pressure on the crack surfaces, crack surface friction is considered in the FE analysis and it is found that the virtual crack closure method is still valid in the presence of crack surface friction. Different crack surface frictions are obtained by varying the off-axis angle and friction effect on mode II fracture toughness of S2/8552 glass/epoxy composite is found to increase as off-axis angle increases. Moreover, the transverse compressive stress is found to have significant effect on mode II energy release rate from FE analysis. The small size of block off-axis specimen makes it possible to perform dynamic fracture tests on a SHPB setup and high crack propagation speed is obtained with this type of specimen. By defining a time rate of mode II energy release rate [special characters omitted] to represent the load rate or using average strain rate of the off-axis specimen, it is found that the dynamic initiation mode II fracture toughness is larger than the static value of mode II fracture toughness and a load rate effect is observed. Specimen size effects in off-axis compression tests are studied as well by conducting experiments with small block off-axis specimens of low modulus S2 glass fiber reinforced composites and high modulus AS4 carbon fiber reinforced composites. It was found that the off-axis compressive strength of the glass/epoxy composite decreased by a small amount (<5%) when increasing either specimen width or thickness. However, an appreciable reduction in off-axis compressive strength of the high modulus carbon/epoxy composite was observed as specimen width or thickness increased with lapped specimens. But when a thin layer of titanium was applied on both contact ends of the specimen, the contact friction was significantly reduced, leading to much smaller reductions in off-axis compressive strength as the specimen width or thickness increases. FEA were carried out to explain the size effect on the specimen size-dependent behavior.

Degree

Ph.D.

Advisors

Sun, Purdue University.

Subject Area

Aerospace materials|Mechanical engineering

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