Impact of Holes on the Reduction of the Strength of Composite Specimens with Different Laying of Fibers

The methods of fastening structural elements made of fiber-reinforced composites to other structural elements are almost the same as those used for metals. With the most common bolting, a hole is drilled in the element, i.e., a part of the material is removed, thus resulting in a stress concentration near the hole. Certain difficulties can thus arise in designing and calculating the bolted joints of metal elements in critical structures. The use of composites is associated with more serious problems. In this case, both the structural element and the material for this element are manufactured simultaneously. Subsequent machining is quite undesirable, since it violates the integrity of the element and, consequently, the bearing capacity of the entire structure. The efficiency of using composites depends on the technology of manufacturing the material of the element by 90%. We present the results of experimental and theoretical study of the effect of holes made by different technologies on the strength and bearing capacity of fiberglass specimens with different fiber laying patterns. The stress concentration factor is compared with the strength reduction factors for the holes made by drilling and by expanding the fibers without their damage. The results of tensile tests of specially made composite specimens with different reinforcement structures ([0], [0/90], [0/±45/90]) and with central holes are presented. The strength reduction factors calculated from the results of the experiment turned out to be significantly lower than the stress concentration factors. The reasons for this effect are considered and an estimation of the characteristic radius of curvature after “blunting” of a hole in a composite specimen is given. The obtained results of the study made it possible to draw conclusions about the effectiveness of the technology for production of holes in composite specimens without breaking fibers, as well as about the effect of the number of fiber families on the strength reduction factor near the holes.