The matrix compression parameters in continuum damage mechanics models are often calculated from assumptions requiring detailed knowledge of fracture and frictional behavior of the material. A direct method for experimentally determining the matrix compression damage parameters would simplify the process and potentially increase accuracy of parameters. In this study, specimen geometry is determined and validated to directly evaluate the energy dissipated by matrix compression damage experimentally. Initially, candidate specimens were determined from literature on fiber compression testing including compact compression (CC), center notched compression (CNC), and four-point bending (4PB) specimens. These specimens were modeled using the finite element program ABAQUS. Geometry and boundary conditions of the specimens were varied to test different specimen geometries and fixture types. The matrix compression damage isolation of the specimens was evaluated by measuring the region of matrix compression damage without other damage modes present in the material. 4PB specimens showed damage primarily at the loading points. Both CNC and CC specimens showed fairly good isolation of matrix compression damage with the former showing a tendency to split off-axis depending on the fixture used and the latter showing tensile splitting after significant compressive damage growth. CC specimens were selected because they require less complex loading fixtures and are less dependent on the fixtures for damage isolation than CNC specimens. The geometry was varied on the CC specimens to increase the compression damage isolation. Specimens were manufactured for experimental validation. A layup with the fiber direction of all plies parallel to the notch tip was used to isolate the loading to the matrix. It was determined that 20 plies was sufficiently thick to prevent bucking. The specimens showed good isolation of compression damage at the notch tip. This is due to the stress concentration at the notch tip that lowers the load required to cause compression damage to below the global buckling load. Ultimate failure was due to tensile splitting opposite of the notch, but only after sufficient compressive damage growth. The size of the damage zone was able to be tracked visually from video of the tests. Load-displacement data and the damage zone size were used to calculate the strain energy release rate using the basic compliance calibration method. This method is limited because it is based in fracture mechanics principals and may not be accurate for the damage modes present in the material, but is sufficient for initial validation of the CC specimen. The strain energy release rate for the material was measured to be 35 in-lbs./in2. CC specimens show promise for measuring the energy dissipation of matrix compression damage for use in continuum damage mechanics models due to their ability to isolate compressive damage modes without buckling. Refined data collection methods can be implemented to increase the accuracy and generality of the strain energy release rate measured.
With the increasing use of composite materials in multiple industries, especially aerospace, better understanding of damage mechanisms is needed. Damage in composites can be categorized into four types: fiber tension, fiber compression, matrix tension, and matrix compression. Experimental methods for classifying damage and propagation have been thoroughly studied for the first three categories but matrix compression has received little attention. A previous study showed that compact compression (CC) specimens, modified from standard ASTM compact tension specimens, can be used to determine the behavior of matrix compression damage in carbon fiber reinforced polymers (CFRP), however CC specimens are not as effective as needed at identifying initiation conditions [1]. This paper presents a specimen to determine the strain energy dissipation rate at crack initiation and the primary failure mechanism of a selected CFRP that is small, has simple geometry, and requires a simple loading fixture. The simple geometry of the UC specimens allows for the stress-displacement behavior to be measured in a more direct manner than the CC specimens providing an opportunity for examination and classification of the material response. Small 15 mm × 15 mm × 3 mm rectangular cuboid uniform compression (UC) specimens were manufactured and tested to compare experimental results with previously tested CC specimen results. UC specimens were loaded in compression until fracture using two flat plates on the thickness face with fibers oriented at 90° from the loading face. The results indicate CC and UC specimen agreement between the strain energy dissipation rate at crack initiation for comparable crack angles, with a range of values between 33.6 kJ/m2 and 45.7 kJ/m2. The primary failure mechanism for both specimens was observed to be shear cracks through the thickness of the laminate with an angle between 47° and 73° measured from the plane normal to the loading direction. UC specimen results also indicate an inverse relationship between the strain energy dissipation and the fracture angle. The stress-displacement results suggest behavior can be split into three distinct response regions: elastic, plastic, and visible damage progression. These results indicate that small, simple UC specimens can be used to directly measure the stress-displacement behavior, determine the strain energy dissipation rate at crack initiation, and determine the primary failure mechanism under compressive loads. Further studies need to be conducted to fully understand the relationship between crack angle and strain energy dissipation at crack initiation.
The growth of lightweight components and need for non-destructive fastening techniques leads to the use of adhesives in many industries. Modeling the behavior of adhesives in fastening joints can help in the design process to make an optimized joint, with minimal waste. However, in available material properties provided by manufactures of adhesives there is a gap in what is sufficient to accurately model and predict the behavior of real-world adhesive conditions. An adhesive joint may be loaded in mode I, mode II, mode III, or a combination of these in service. In components with outdoor application the ambient temperature outside in many regions can vary to below freezing to over 40 °C. The environmental conditions at these temperatures may influence the adhesive material properties. This body of research presents the results of adhesive properties subject to temperature testing. The needed material properties to compose an accurate model have been shown to be the mode I cohesive strength, mode I cohesive toughness, mode II cohesive strength, and mode II cohesive toughness. These properties can be measured with a test specimen designed to isolate that loading mode and condition. The specimens used are the Dog Bone Tensile Specimen (DBTS), the Double Cantilever Beam (DCB), Shear Loaded Dual Cantilever Beam (SLDCB), and Double Lap Shear (DLS). The effect of temperature will be tested by testing each specimen at −30°C, 20°C, and 45°C. Triplicates of each specimen at the respective temperature were tested. These results will be used in a cohesive zone model that will be validated with additional testing. The results from the two tested adhesives, Plexus MA832 and Pliogrip 7779/220, indicate these temperature conditions can change the cohesive strength in mode I by −60 to −40 % and mode II by −13 to 2% when at high temperatures (HT). The cohesive toughness in mode I by −40 to −20% and mode II by −40 to −2% when at high temperatures. The cohesive strength in mode I by −50 to 15% and mode II by 8% to 60% when at low temperatures (LT). The cohesive toughness in mode I by −70 to −20% and mode II by 30 to 60% when at low temperatures. As compared with those tested at room temperature (RT). The ranges here represent the response for both adhesives.
Composite materials are becoming increasingly common in the aerospace industry. In order for simulation and modeling to accurately predict failure of composites, a material model based on observed damage mechanisms is required. Composites are commonly classified into four damage categories based on the composite constituents and their loading condition: fiber tension, fiber compression, matrix tension, and matrix compression. Previous work identified a compact compression (CC) specimen as a suitable option for isolating matrix compression damage. However upon continued testing, stable crack propagation in the specimen was limited to a relatively low material failure ratio (σCompressive/σTension). This paper presents specimen geometry that can isolate matrix compression damage in materials with a failure ratio greater than two, the limit of the compact compression specimens. Initial specimen selection used the compact compression specimens from previous research and added additional specimens based on commonly used compressions specimens for different materials. The added specimens included center notched compression (CNC), edge notch compression (ENC), and four-point bending (4PB). All specimens were evaluated experimentally with the success criteria of controlled propagation of a matrix compression crack. In addition to propagating a controlled matrix compression crack, specimens were required to have a visible region around the stress concentrator to allow for digital image correlation (DIC) image capture during the experiments. The specimens were manufactured from a carbon fiber reinforced polymer (CFRP) with a failure ratio greater than six. CC and 4PB specimens were unable to produce a compression crack before any other failure methods were present. CNC specimens produced an unstable compression crack that progressed from the notch to the edge of the specimen too rapidly to acquire meaningful crack propagation data. ENC specimens showed some ability to stably propagate a crack, however some tests resulted in an unstable crack propagation similar to the CNC specimens. In order to increase the test repeatability, a tapered thickness was added to the specimen around the notch tip. The resulting tapered ENC (TENC) produced repeatable controlled matrix compression crack propagation. Ultimately, the specimen fails when the crack has propagated through the entire width of the specimen. TENC specimens show promise for isolating matrix compression damage in materials with high failure ratios. Continued testing of CFRP with TENC specimens could be used to refine the material model for finite element analysis.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.