Tension failure

The principal failure modes of bolted joints are (1) bearing failure of the material as in the elongated bolt hole of Figure 7-44, (2) tension failure of the material in the reduced cross section through the bolt hole, (3) shear-out or cleavage failure of the material (actually transverse tension failure of the material), and (4) bolt failures (mainly shear failures). Of course, combinations of these failures do occur. 

Net-tension failures can be avoided or delayed by increased joint flexibility to spread the load transfer over several lines of bolts. Composite materials are generally more brittle than conventional metals, so loads are not easily redistributed around a stress concentration such as a bolt hole. Simultaneously, shear-lag effects caused by discontinuous fibers lead to difficult design problems around bolt holes. A possible solution is to put a relatively ductile composite material such as S-glass-epoxy in a strip of several times the bolt diameter in line with the bolt rows. This approach is called the softening-strip concept, and was addressed in Section 6.4. 

Bonded-bolted joints generally have better performance than either bonded or bolted joints. The bonding results in reduction of the usual tendency of a bolted joint to shear out. The bolting decreases the likelihood of a bonded joint debonding in an interfacial shear mode. The usual mode of failure for a bonded-bolted joint is either a tension failure through a section including a fastener or an interlaminar shear failure in the composite material or a combination of both. 

Compression/tension failure in the short beam shear test. 

A first parameter to be studied is the applied potential difference between anode and cathode. This potential is not necessarily equal to the actual potential difference between the electrodes because ohmic drop contributions decrease the tension applied between the electrodes. Examples are anode polarisation, tension failure, IR-drop or ohmic-drop effects of the electrolyte solution and the specific electrical resistance of the fibres and yarns. This means that relatively high potential differences should be applied (a few volts) in order to obtain an optimal potential difference over the anode and cathode. Figure 11.6 shows the evolution of the measured electrical current between anode and cathode as a function of time for several applied potential differences in three electrolyte solutions. It can be seen that for applied potential differences of less than 6V, an increase in the electrical current is detected for potentials great than 6-8 V, first an increase, followed by a decrease, is observed. The increase in current at low applied potentials (<6V) is caused by the electrodeposition of Ni(II) at the fibre surface, resulting in an increase of its conductive properties therefore more electrical current can pass the cable per time unit. After approximately 15 min, it reaches a constant value at that moment, the surface is fully covered (confirmed with X-ray photo/electron spectroscopy (XPS) analysis) with Ni. Further deposition continues but no longer affects the conductive properties of the deposited layer. 

Fig. 30a and b. Rod model of a cellular structure (a) and tension-failure diagram (b) of this model for rigid foamed polymers... 

Figure 11.3. Rotary veneer lathe (Feihl and Godin, 1967). (a) Knife and roller nosebar mounted on a single carriage advance towards the chucks as the log rotates. The insert shows tension failures that are liable to form as the veneer is bent to pass between the nosebar and knife the worst of the lathe checks are inhibited by slightly compressing the veneer, (b) Close-up of a fixed nosebar and knife. The gap between the two determines the degree of compression of the veneer, while the nominal thickness of the veneer is a function of the rate of advance of the knife carriage and the speed of rotation of the bolt, (c) Close-up of roller nosebar. (d) Microsharpening of the knife increases its resistance to damage from hard knots.

Refractory failures are categorized as either tension or compression failures. These failures can result from bending or pure tension/compression loads. In a tension failure the crack is initiated and grows. A "cold joint is the preferred fix for a tension failure. 

Figure 113 Failure modes in composite bolted joints (a) bearing failure, (b) net-tension failure, (c) shear-out failure, (d) cleavage failure, (e) fastener puU-through, (f) bolt failure.

Tension failure - tensile rupture of the FRP prior to concrete crushing. 

Compression failure is the most desirable of the above failure modes. This failure mode is less abrupt than tension failure, and is similar to the failure of an over-reinforced section when using steel reinforcement. Tension failure is less desirable, since tensile rupture of FRP reinforcement will occur with less warning. Tension failure will occur when the reinforcement ratio is below the balanced reinforcement ratio for the section. This failure mode is permissible with certain safeguards. 

A second paradox is the use of the word shear to describe the fracture. It is evident from the calculation used to obtain Equation (15.5) that shear is not mentioned. The joint peels but does not slide or shear (Fig. 15.11). Only tension forces and displacements are needed to explain the failure of the joint. In fact, it would be far more logical to describe this failure as a tension failure, just as the Griffith equation describes tension failure. Of course, shear stresses exist around the crack tip, as in every crack geometry known, but the energy associated with these stresses remains constant as the crack moves and therefore cannot drive the crack. 

Carlson G.A., Henry K.W. Technique for studying tension failure in application to glycerol. J.Appl.Phys. V.42 5(1973) 2201-2206. 

P(3) Fastener failure is a simple tension failure (see Figure 5.24). 

Characteristic distance from the fastener hole for predicting netsection (tension) failure... 

Characteristic dimensions (net-section failure) Tension failure in x-direction (mm) di.k.x 2.8... 

A series of simply supported beams are provided with notches of different depth and location. The mode of cracking depends on the depth a of the notch and on its location defined by factor 7. The value of 7 = 0 corresponds to pure Mode I and the other values of 7 induce either a mixed mode at the notch tip or again pure Mode I when tension failure occurs at the midspan. Which of these possibilities is actually realized depends on both values 7 and a. The results obtained were compared with calculations by finite element method assuming LFFM solutions for Modes I and II. The tests were executed under static and impact loading and the test and calculation results are shown in Figure 10.35. As the notch was moved away from the centre... 

The ultimate strengths are similar to those of polyesters (Table 4.1). The impact strength and strain to failure indicate that these are tough resins. In-tension failure is initiated by yielding rather than brittle fracture. 826HT is a low shrinkage resin developed especially for pultrusion. 

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