Mike I Jones

Size Effects in Interlaminar Tensile and Shear Strength of Unidirectional Glass Fibre/Epoxy

Curved beams tested in four point bending were used to measure interlaminar tensile strength, and straight beams loaded in three point bending to measure interlaminar shear strength. Results from specimens of 16, 32 and 64 plies with all dimensions scaled showed significant size effects. The average interlaminar tensile strength dropped from 109 MPa for the smallest specimens to 61 MPa for the largest, whilst the interlaminar shear strength was reduced from 97 MPa to 85 MPa.

Failure mechanisms in three and four point short beam bending tests of unidirectional glass/epoxy

Abstract Three and four point bending tests are compared both analytically and experimentally. In all the three point bending tests, damage was observed under the loading roller in addition to the interlaminar shear failure, while in the four point bending tests, only interlaminar shear failure was observed. Therefore, this four point bending test is valid for measuring the interlaminar shear strength. From the finite element analysis, it is found that the roller diameter is a critical parameter in determining the stress concentrations in short beam tests. In order to avoid damage under the roller and thus to make the short beam test a valid means for measuring the interlaminar shear strength, the appropriate roller diameters should be chosen. The damage under the loading roller in the three point bending test basically reduces the effective specimen thickness and thus this test underestimates the interlaminar shear strength. The interlaminar shear cracks in the short beam tests were found to be randomly distributed in a region between 30 percent and 70 percent through the thickness from the top surface. This is due to the non-linear shear response which means that the shear stress distribution is more uniform near the middle of the section. Also the maximum value of the shear stress is lower than the maximum value given by beam theory. A non-linear shear correction factor is suggested to account for this effect and for the glass/epoxy composite tested here, the actual interlaminar shear strength is only about 83 percent of the apparent value from classical beam theory. The interlaminar shear crack does not occur at the location of maximum shear stress. This may be because there is insufficient energy to propagate a crack at this location.

Reduction in compressive strain to failure with increasing specimen size in pin-ended buckling tests

Flexural tests were performed on unidirectional T800/924 carbon-fibre/epoxy composites in a pin-ended buckling rig producing consistent compressive failures. Three sets of tests were carried out with all dimensions geometrically scaled. The results showed a strong decrease in strain at failure with increasing specimen size. Three further sets of tests were conducted where the thickness was kept constant and the length and width of the specimens varied. These showed similar strains at failure, suggesting that the size effect observed with the scaled specimens is predominantly due to the reduction in strain gradient with increasing specimen thickness.

Effect of Through Thickness Tensile and Compressive Stresses on Delamination Propagation Fracture Energy

Strain energy release rate criteria are often used for predicting delamination. However, the fracture energy for delamination propagation is not a material constant, but varies depending on the through thickness normal stress. For tensile normal stresses this problem is usually overcome by using a mixed mode fracture criterion. However, the mixed mode approach fails to take into account compressive through thickness normal stresses. These can greatly increase the delamination stress and therefore cannot be simply ignored. A new approach is proposed in which it is assumed that the fracture energy for delamination propagation is a linear function of the average through thickness normal stress. Comparison with experimental results for unidirectional specimens with discontinuous plies shows excellent correlation, and accounts for a number of experimental observations that cannot be explained using the standard fracture mechanics approach.

Interlaminar failure of unidirectional glass/epoxy due to combined through thickness shear and tension

A test specimen in the form of a hoop with two 90° curved sections is described. When loaded in three point bending, both interlaminar tensile and shear stresses arise in the curved sections. Maximum tensile and shear stresses occur at similar locations, allowing the combined effect of these stress components to be investigated. Changing the geometry of the specimen alters the ratio between interlaminar shear and tension. Experimental results are presented for two sets of unidirectional glass fibre/epoxy specimens. It is shown that it is possible for specimens subjected to combined interlaminar tension and shear to withstand higher stresses than when subjected to only tension. This is believed to be due to the smaller volume subjected to the maximum tensile stress as a result of the stress gradient along the length of the specimen. An approach is presented to account for this effect based on Weibull statistical theory and a quadratic interaction equation between interlaminar tension and shear. Using stresses from finite element analysis, satisfactory predictions of the strength of the hoop specimens can be made.

Delamination Due to Interaction Between Curvature Induced Interlaminar Tension and Stresses at Terminating Plies

Curved unidirectional glass fibre-epoxy specimens with some of the plies on the tension side cut across the complete width have been tested in four point bending. Two sets of tests were carried out with the cut plies in different positions through the thickness to change the ratio between the overall interlaminar tensile stresses induced by the curvature and the local stresses at the cut. A failure envelope based on a linear interaction between overall and local stresses is shown to be a reasonable fit to the experimental data.

Edge delamination in curved (04/456)s glass-fibre/epoxy beams loaded in bending

An approach to predicting delamination due to the free-edge effect in a curved beam specimen loaded in bending is presented. Stress-based criteria do not give satisfactory results because of the stress singularity at the free edge. The conventional fracture-mechanics approach for free-edge delamination cannot be used because the strain-energy release rate does not reach an asymptotic value. An improved approach is developed on the basis of an assumed initial defect. The strain-energy release rate is calculated by using finite elements and this is then used to predict the failure load. Good correlation with the experimental results is obtained.

Delamination due to interaction between overall interlaminar shear and stresses at terminating plies

Abstract Constant thickness unidirectional glass fibre/epoxy specimens with some of the plies on the tension side cut across the complete width have been tested in three-point bending. Four sets of tests were carried out with the span and position of the cut varied to change the ratio between overall interlaminar shear stresses and local stresses at the cut. Four-point bending tests were also performed. Most of the specimens failed by delamination from the ends of the cut plies. The results show a linear interaction between overall interlaminar shear stresses and local stresses at the cut. A method of constructing a failure envelope is proposed which can be used as the basis for predicting delamination.

Delamination of unidirectional glass fibre-epoxy with cut plies loaded in four point bending

Constant thickness unidirectional specimens with plies cut across the complete width have been tested in four point bending. With the cut plies on both the tension and compression side, specimens failed by delamination initiating above and below the ends of the cut plies, and propagating along the interfaces between continuous and discontinuous plies. Finite element analysis has been carried out to determine the stress distributions and strain energy release rates, and an analytical expression for the strain energy release rate has also been derived. Predictions of the delamination stresses correlate well with measured values.