Composite Strips with a Circular Stress Concentration under Tension

A series of tensile experiments were performed on S2 glass/toughened epoxy composite strips with a center hole or a pin joint at various temperatures within the range of −60C and 125C. Four different lay-up configurations each of 24 layers: [0°], [0°/90°], [45°/−45°] and [0°/45°/90°/−45°] and four different hole diameters: 1/8”, 1/4”, 3/8”, 1/2” were used in both types of specimens. For certain configurations, strain gages were placed at locations around the hole to measure local strains. A loading-displacement curve was constructed for each configuration. The results indicate that the failure of the joints is significantly affected by stacking sequence, temperature, and hole size. The initial slope of the loading-displacement curve, i.e. the stiffness of the joint, increases with decreasing hole diameter or temperature. As expected, the [0°] specimens have the highest average stiffness and strength, whereas the [45°/-45°] specimens the lowest with the remaining configurations fall somewhere in between. The failure mode, on the other hand, depends only on the fiber orientation in an individual layer. Failure in [0°] layers is predominantly in shear-out mode whereas tension and shear failures are the main modes in [90°] and [±45°] layer, respectively. For the joint specimens, these average stiffness and strength also increase as the distance between the center of the pin joint and the free end of the strip decrease. Finally, a finite element failure model based on the Tsai-Wu tensor theory and progressive damage evolution was also developed. The results indicate that the failure model simulates the experimental data quite well. INTRODUCTION Use of composite materials is becoming widespread in U.S. Army applications. The future U.S. Army is required to be more mobile and lightweight. Thus, composite materials appear to be one of the best candidates to accomplish this goal. Several analyses have been developed for predicting failure strength and failure mode of composites with hole and joints [1-8]. The current project was designed to study the damage behavior of S2 glass/toughened epoxy composite strips and joints at room, elevated and low temperatures. In the earlier work [9-10], tensile and 3-point bending tests were conducted on composite specimens of various stacking sequences. In this study, the testing was extended to composite strips with a hole and joints. It was found that the temperature, stacking sequence and the size of the hole significantly affect the failure stress, stiffness and damage behavior of the composite. Also, it was determined that with the proper finite element model one can predict the location of failure initiation and damage progression. In this project S2 glass/toughened epoxy composite strips with a hole and joints were tested at room, elevated (up to 125C) and low (up to −60C) temperatures in an MTS 810 Universal Testing Machine to determine their failure behavior. The composite strips and joints had various lay-up configurations, hole sizes, and hole locations. To determine the local strain field at critical locations (e.g. near hole boundary), strain gages were mounted to the composite strip specimens. The testing program was extensive. First, 24-ply composite strips with a hole were tested at room temperature. Four different hole sizes were used: .125”, 0.25”, 0.375” and 0.5”. The testing was repeated on 4 lay-up configurations: [0]24-ply, [02/902]3s, [+452/−452]3s and [03/+453/903/−453]s. On these specimens strain gages at strategic sites (around the hole) were mounted to measure the local strain. The tests were repeated at higher (75C, 125C) and lower (−20C, −60C) temperatures. Next the same tests were repeated for composite joints, on all configurations (except 0.125” hole size) and again at high and low temperatures. Here another variable, namely the distance of the hole from the edge (H) was added. Two values of H were used: 0.75” 1.5”. A vast amount of data was obtained during this study; only limited results are presented in this paper. Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 07 JUN 2004 2. REPORT TYPE Journal Article 3. DATES COVERED 19-02-2004 to 13-05-2004 4. TITLE AND SUBTITLE COMPOSITE STRIPS WITH A CIRCULAR STRESS CONCENTRATION UNDER TENSION 5a. CONTRACT NUMBER