Laminate testing Mechanical Properties

Mechanical Properties. The performance of various polyester resin compositions can be distinguished by comparing the mechanical properties of thin castings (3 mm) of the neat resin defined in ASTM testing procedures (15). This technique is used widely to characterize subtle changes in flexural, tensile, and compressive properties that are generally overshadowed in highly filled or reinforced laminates. 

Mechanical Property Testing. Mechanical tests were performed on both unirradiated and irradiated materials at -157°C, 24°C, and 121°C. Specimens were kept dry prior to testing in an environmental chamber mounted in a tensile testing machine. Tensile test specimens of [0]4, [10]4, [45]4, and [90]4 laminates were cut from 4-ply composite panels. All specimens were straight-sided coupons. For tension and shear tests the length/width aspect ratio was 8. For the compression tests the aspect ratio was 0.25 and the unsupported length was 0.64 cm. The [0]4 laminates were used to measure the ultimate tension and compression strength, Xit the axial... 

Thermal residual stresses are inherent to fibre reinforced composites due to the heterogeneity of the thermo-mechanical properties of their two constituents. Such stresses build up when composite structures are cooled down from the processing temperature to the test temperature. Residual stresses will be present on both a fibre-matrix scale (micro-scale), and on a ply-to-ply scale (macro-scale) in laminates built up from layers with different orientations. It is recognised that these stresses should be taken into account in any stress analysis. 

The resin content in both composites is about 37% by weight. The porosity of the composites was characterized by ultrasonic C-scans. The test specimens of no measurable porosity were used. The resin specimens and the composite laminates were cured in a hydraulic press at 250 F and 75 psl for one hour, and subsequently postcured at the same temperature in the absence of pressure for another two hours. Additional curing for up to 16 hours in the case of HX-205 and F-185 resins showed no measurable changes in dynamic mechanical properties. 

Tranj-laminar fracture of composites with a certain amount of fibres in the throughthickness direction will lead to fibre breaking with significant effects on delamination resistance (see e.g. Refs [24,37] for details). Woven fibre mats, 3D fibre performs or additional 3D reinforcement (pins and stitching) of FRP composites with fibres ahgned in one plane (see e.g. Refs [84,85]) have been developed and can be tested for their fracture mechanics properties. The typical approach for testing these is to apply a standard test method developed for unidirectionally reinforced FRP composites and to assess the difference in delamination resistance compared to the standard laminate. So far, that approach has yielded (nominal) numbers, but their interpretation is not... 

The flexibilized and unreinforced resin was also tested at 20°C. Moisture absorption in the unreinforced resin and in the Navy GRP was consistent with Pick s law. The Navy GRP showed the least total takeup of water. The epoxy laminates deviated from ideal Fickian behaviour. At 20°C, both the glass and polyester fibre laminates reached a peak mass and then decreased, suggesting that some material was being leached out into the water. The mechanical properties were determined by dynamic mechanical thermal analysis (DMTA). All the laminates experienced a reduction in the effec-... 

The above-mentioned thermomechanical models only consider the elastic behavior of materials. Boyd et al. [13] reported on compression creep rapture tests performed on unidirectional laminates of E-glass/vinylester composites subjected to a combined compressive load and one-sided heating. Models were developed to describe the thermoviscoelasticity of the material as a function of time and temperature. In their work, the temperature-dependent mechanical properties were determined by fitting the Ramberg-Osgood equations and the temperature profiles were estimated by a transient 2D thermal analysis in ANSYS 9.0. 

Tests dealing with the mechanical properties of the mat or fabric may be required by the manufacturer of the FRP product. However, they will generally be more appropriate as part of the testing associated with the development of a new laminate or a new manufacturing process (see 7.3). 

The test programme was necessary because of lack of a reliable joint data. For example, there are very limited data on the behaviour of laminated joints. Also, adhesive manufacturers are generally not able to provide those adhesive mechanical properties required in the design process. 

The results of laminate material tests are presented in Table 4. Mechanical properties of A300 WR were not tested as it had the same materials, ply orientations, and number of plies as the A300 HL laminate. The fibre volume fraction presented in Table 4 was... 

As a conclusion, it must be noticed that, in fact, all the tests performed on laminates are recommended only when the level of adhesion established between fibre and matrix is already known. The main problem involved in these types of tests is to control precisely the degree of alignment of fibres in the given direction. For example, a recent work [5] has shown that the mechanical responses of unidirectional laminates are very sensitive to the misorientation of fibres, even when this latter does not exceed 1 to 2%. Obviously, bridges or entanglements between fibres tend to increase greatly the measured mechanical properties and, thus, caution should be exercised when interpreting the results. 

Morye and Wool [82] used symmetric and non-symmetric stacking sequences with glass/flax hybrid composites and also varied the glass/flax fiber ratio, namely, 100/0, 80/20, 60/40, 40/60, and 0/10. For non-symmetric composites, flexural and impact tests were performed with the top face of the composite being either glass fibers or flax fibers. The mechanical properties of the composites were found to depend on the fiber layer arrangement in the composite, and the non-symmetrical laminates with flax at the loaded top face presented superior performance under flexure or impact. 

Some difficulties arise when the material requirements, or the material test methods, must be specified for balsa wood because its physical properties are uniquely complex. Structural components typically are fabricated from a multiplicity of laminated strips or planks. These are selected and combined in such a manner that the mechanical properties averaged over the full size component will meet or exceed certain specified values. The quality assurance and quality control (QA/QG) procedures for making sure that this is the case are not simple, however. It will be shown later that conventional test coupons complying with recommended ASTM Standard Methods of Test are likely to exhibit mechanical properties that are far from being representative of the whole laminate. Conventional methods of material testing. 

In this paper, the thermal and mechanical characteristics of balsa wood and balsa wood laminates are reviewed, and it is shown that "composite" mechanics that have been developed for the class of synthetic fiber-reinforced plastic (SFRP) materials may be useful for describing the density and direction—dependent mechanical properties of balsa wood in bulk or laminated form. It may be asked whether such advanced analytical methods, perhaps combined with specially developed methods of test, could be used effectively towards developing more applicable QA/QC procedures that will clearly qualify balsa wood as a structural material in applications where strictest code compliance is a necessity. This question has prompted the following review and discussion. 

T1-T6 tests concern the physical properties of composites. T1-T4 tests can only be applied to dry fabrics, whereas T5-T6 tests can be applied for bofli types of composites (pultruded or onsite laminated). T7-T8 tests concern the mechanical properties and the bonding of the adhesive used. 

Mechanical characterization of composite materials is a complex scenario to deal with, either because of the infinite number of combinations of fiber and matrix that can be used, or because of the enormous variety of spatial arrangements of the fibers and their volume content. The foundation of the testing methods for the measurement of mechanical properties is the classical lamination theory this theory was developed during the nineteenth century for homogeneous isotropic materials and only later extended to accommodate features enhanced by... 

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