Puncture deformation

The films used for the mechanical tests were previously conditioned for 48 h at 20°C (climatized room) inside desiccators containing saturated saline solutions of MgCl2, NaOH, NaCl, KCl, and CUSO4 which provided relative humidity of 33.2, 65.5, 75.5, 85.5, and 97.3%, respectively. The mechanical properties (pimcture force and puncture deformation) were determined. 

The effect of relative humidity in the domain between 30 and 98% and of the film thickness from 0.13 to 0.32 mm on the puncture force and deformation can be observed in Figure 26.1. It can be seen that the increment of the relative humidity of conditioning provoked the reduction of the puncture force and the increase of puncture deformation following an exponential behavior, according to Equation 26.3 ... 

Puncture force (a) and puncture deformation (b) as function of the relative humidity of conditioning and film thickness in gelatin-based films. 

Parameters of Equation 26.3 Calculated by Nonlinear Regression of the Puncture Eorce (PE) and Puncture Deformation (PD) Data as Eunction of the Relative Humidity (RH) for Different Thicknesses (X)... 

Only few studies have been reported on the effect of the addition of cross-links on film properties. Chemical cross-linkers such as formaldehyde, glyoxal or glutaraldehyde were efficient in enhancing the maximum puncture force of films made from cotton seed proteins. Brault et al introduced cross-links between aromatic compounds (i.e. dityrosine) in caseinate films by exposing them to y-ionisation. Puncture strength and puncture deformation were both increased but this effect was very dependant of the ratio glycerol/protein. 

Resistance to puncture is another type of loading. It is of particular interest in applications involving sheet and film as well as thin-walled tubing or molding and other membrane type loaded structures. Hie surface skins of sandwich panels are another area where it is important. A localized force is applied by a relatively sharp object perpendicular to the plane of the sheet of material being stressed. If the material is thick compared to the area of application of the stress, it is effectively a localized compression stress with some shear effects as the material is deformed below the surface of the sheet. 

Figure 8.11 Effect of orientation on deformation zones observed during puncture testing, viewed from above ...

Figure H2.2.1 Force/deformation curves illustrating three puncture probe tests (50 mm/min deformation rate) of an apple specimen and a cone penetrometer test (10 mm/min deformation rate) of Cheddar cheese, all at room temperature.

Puncture probes are commonly used for fruits and vegetables, and allow for the determination of force at rupture of the cellular structure. The procedure outlined below is adapted from the method of Bourne (1979). Cone penetrometers are commonly employed for determining firmness and yield value for foods such as margarine and butter, which may be a reflection of the product s spreadability. Quite often it is desirable to use a testing system that provides a constant deformation rate. Additionally, a mechanical testing machine allows for production of a force/deformation curve to further analyze the data. 

Protection against wear or abrasion. In use 2 (1), if the "work" (steel plate, etc.) bangs the side of the tank and the brick were not there to protect the rubber, the "work" could cut or puncture the rubber sheet. In use 2 (3), traffic over an asphalt membrane can cut or deform it, but a brick floor over it will protect the membrane from such damage. 

The effect of the relative humidity of conditioning on the puncture force and deformation can be related to the increase of the matrix hydration considering that gelatin and glycerol are both hydrophilic. The consequent... 

However, it was observed that, in general, the increment of the film fhickness provoked an increase of ifs mechanical resistance, that is, increased the puncture force, and af fhe same time implied a reduction of fhe deformation capacity. However, these effects are dependent on the relative humidity of condifioning, cerfainly as a function of the possible structural alterations within the polymeric matrix. 

The properties of gelatin-based films are strongly influenced by the film thickness and the conditioning conditions. It was verified that an increment of the relative humidity provokes a reduction of the puncture force and an increase of the pimcture deformation. The temperature also affects the water vapor permeability of the films, but this effect depends on the film thickness. 

Throughout a puncture test, mechanical properties of samples with different moisture content were analyzed. Parameters obtained from these curves were the maximum force required to break the sample (Fmax)/ distance at this point (d) and the ratio force-distance at the break point. The ratio force-distance at the break point (Fmax/rf) is related to the product resistance to fracture or sample firmness (Prothon et al., 2001). The ratio Fmax/d has been represented vs. the moisture content in Figure 63.3, showing a sigmoid relationship, also typical in crispy products (Peleg, 1994). This behavior indicates a decrease from the resistance to deformation with the increment in moisture product content, as a consequence of the plasticizing effect of water. The ratio was adjusted to the Fermi s model... 

The primary textural property of fruits and vegetables is firmness (6). Three principles are used to measure firmness. 1) The puncture test measures the force required to push a probe into the product. 2) The extrusion test measures the force required to make the product flow through one or more slots or holes. 3) The deformation test measures the distance the product compresses under a small force. All three test principles are used on fresh produce, but only the first two (puncture and extrusion) are used on processed material (7). 

The most indicative deformation and strength characteristics of inhibiting films are the breaking tensile strength and relative elongation at rupture [1]. The methods for their determination also have respective standards [144-146]. Films intended for wrapping bulky or sharp-edged hardware have to meet the requirements on resistance to rupture [1,147-151] and puncture [1,152]. 

Figure 2.10.3 shows these resuits, it can be recognized that the iines are curved, simiiar to the initiai section of the generai force-deformation curve in Figure 2.10.2. Because the curves end at the defiection where puncture occurred, the reverse curve at higher forces and defiec-tions is not shown.

Fig. 4.60 Schematic representation of load (F) - deflection (/) diagrams resulting from different material behaviour with Fin - maximum load, /m - deflection at maximum load, - energy up to maximum load, Fp - puncture load corresponding to Fm/2, /p - deformation at puncture load and Fr - crack propagation energy.

This method is simpler than the sand-cone method. However, it is not suggested for very soft soil materials, which are deformed easily, and the walls of the hole are not stable. Additionally, this test method may not be suitable for soils containing crushed rock fragments or sharp edge materials that may puncture the rubber membrane. Regarding the restriction on maximum particle size contained in the soil material, the maximum shall not be more than 63 mm. 

Even without penetration, modem pistol bullets contain enough energy to cause blunt force trauma under the impact point. Vest specifications will typically include both penetration resistance requirements and limits on the amount of impact energy that is delivered to the body. Vests designed for bullets offer little protection against blows fi om sharp implements, such as knives, arrows or ice picks, or from bullets manufactured of non-deformable materials, that is, those containing a steel core instead of lead. This is because the impact force of these objects stays concentrated in a relatively small area, allowing them to puncture the fiber layers of most bullet-resistant fabrics. 

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