Compression-deflection curve

Introduction. Rigid urethane foams are hard (high ratio of load bearing/density) foams having very low flexibility. They show permanent deformation, i.e., no complete recovery after compression. In other words, the compression deflection curves of rigid urethane foams exhibit yield points. 

Compressive Behavior. The most kiformative data ki characterising the compressive behavior of a flexible foam are derived from the entire load-deflection curve of 0—75% deflection and its return to 0% deflection at the speed experienced ki the anticipated appHcation. Various methods have been reported (3,161,169—172) for relating the properties of flexible foams to desked behavior ki comfort cushioning. Other methods to characterize package cushioning have been reported. The most important variables affecting compressive behavior are polymer composition, density, and cell stmcture and size. 

The CFD (compression force deflection) as well as IFD (indentation force deflection) curves of these foams are relatively linear in comparison with slabstock foams, as shown in Figures 10 and 11. 

Quasi-static and dynamic tests were conducted at the DLR Institute of Stmctores and Design, Stuttgart. The quasi-static test was conducted in a Zwick 1494 servo-hydraulic uniaxial loading frame (max. loads 500 kN, max. crosshead displacement 850 mm) with the stmcture test setup as shown in Figure 10.14(a). Vertical compression loads were applied by the test machine crosshead via a load cell to the test stmcture through the I-beam and measured at the load cell and on the load platform. The test was performed initially at a crosshead velocity of 5 mm/min for the first 20 mm of crosshead displacement, then increased and maintained at 20 mm/min until final collapse at 62 mm crosshead displacement. From the measured load-deflection curve the absorbed energy at failure can be calculated, which was measured to be 6.3 kJ at 62 mm displacement. This energy represents... 

There is an empirical relationship between the clamping pressure on the gasket and the hydraulic pressure retained in the system. This relationship is derived from the load/deflection curve of a printed bead under compression. 

When minimum movement capability is required, the arch is sometimes filled with soft rubber using a suitable adhesive. The maximum amount of movement (axial extension and compression, lateral deflection and angular rotation) that an expansion joint is capable of absorbing is called the rated movement. This rating depends on various factors, such as the size of the expansion joints, the thickness of the tube, arch or convolution, and the type and properties of rubber compound and fabric used in construction. Rated movements are established by manufacturers of expansion joints theoretically, or are based on actual load deflection curves of each size of joint. Rubber expansion joints are generally subjected to hydraulic and vacuum tests at 1.5 times the operating pressure. No internationally accepted standard technical specification for rubber expansion bellows is available, since they are mostly custom built to specific operational requirements. The Expansion Joint Manufacturers Association in New York has laid down standards for rubber expansion joints, which are called EJMA standards [2]. 

Flexible polyurethane foams are open-cell structures which are usually produced with densities in the range 1.5—3 Ib/ft The major interest in flexible foams is for upholstery applications and thus the load-compression characteristics are of importance. Typical load-deflection curves for polyether and polyester foams are shown in Figure 14.2. The most obvious difference between polyether and polyester foams is the lower resilience of the polyester materials. This feature has led to a preference for polyether foams in cushioning applications. Compared to polyether foams, polyester foams have higher tensile strength, elongation at break and hardness consequently polyester foams are preferred in such applications as textile laminates and coat shoulder pads. 

Carman et al. [ 18] developed a test called the meso-indentation test which used a hard spherical ball indenter to apply a compressive force to a surface of the composite perpendicular to the fiber axis. The indenter was much larger than the diameter of a single fiber therefore, when the ball was forced into the end of the composite, it made a permanent depression in the material. From the size of the depression and the force-deflection curve, they calculated a mean hardness pressure as a function of strain in the coupon. Qualitative differences have been reported in tests conducted on carbon fiber-epoxy composites where the fiber-matrix adhesion had been varied systematically. 

Flexible polyurethane foams are open-cell structures which are usually produced with densities in the range 24-48 kg/m (1.5-3 Ib/ft ). The major interest in flexible foams is for upholstery applications and thus the load-compression characteristics are of importance. Typical load-deflection curves... 

The resulting load vs. deflection curves may then be analysed according to JSCE-SF5. A compressive toughness factor, T, is defined as ... 

The use of an appropriate FRC matrix can increase the ultimate moment and ultimate deflection of conventionally reinforced beams [6,7] the higher the tensile stress carried by the FRC, the higher the ultimate moment. However, it has also been shown [6] that if compression steel is also used, the beneficial effects of the fibres are reduced. Since the role of the fibres is primarily to provide tensile capacity in the bottom portion (tension side) of a beam, it has been suggested that, it may not be necessary to provide fibres throughout the full depth of a reinforced concrete beam. For reasons of economy, it may be sufficient to add fibres only in the bottom half of a beam. However, Bentur and Mindess [8] showed that, with a steel fibre volume of 1.5%, partial fibre reinforcement (to 1/2 of the beam depth) increased the ultimate load by 32%, while full depth fibre reinforcement increased the ultimate load by about 55%. Thus, there are benefits to having fibres even in the compression half of a beam. Typical load vs. deflection curves for these tests are shown in Figure 14.1. 

The simply supported beam has a load applied centrally. The upper skin go into compression while the lower one goes into tension, and a uniform bending curve will develop. However, this happens only if the shear rigidity or shear modulus of the cellular core is sufficiently high. If this is not the case, both skins will deflect as independent members, thus eliminating the load-bearing capability of the plastic composite structure. 

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