Durability: creep rupture

For example, single-lap shear joints can have a known static load applied, then inserted into a specified environment, and the time-to-failnie of joints noted as a function of variables such as adhesive type, pre-treatment and stress (see Shear tests). Static loads can also be applied to other joint configurations. Many studies have shown that such tests exhibit good discrimination between different surface pre-treatments. The creep performance of adhesives can also be measured by means of static load testing (see Durability creep rupture and Durability sub-critical debonding). 

This short article begins by describing what is meant by creep and outlining typical creep behaviour and the role played by temperature. Supplementary articles include Durability creep rupture, Durability sub-critical debonding and Viscoelastidty. Various material laws that can be used to model such behaviour are then briefly presented. This article then addresses various aspects of creep in adhesively bonded stractures and finishes with the interaction between creep, fracture and fatigue. 

A further discussion of the topic can be found in Engineering design with adhesives and other relevant considerations in Durability creep rupture and Durability fatigue. 

The usual way in which the life of differently made bonds can be compared is to prepare types of test pieces and then place them in a jig, which will allow a strain to be placed upon the rubber and the bond edge (see Durability creep rupture). The test pieces are then subjected to contact with a hostile environment such as hot water or salt fog (see ASTM B I 17). The resultant failures generally depend upon the type of surface preparation used and may be used to monitor these procedures. 

Adhesive joints may be subjected to a variety of adverse service conditions, including elevated temperature, organic solvents, water and stress (see Durability fundamentals. Durability creep rupture). Solvent-based, emulsion and melt adhesives are normally based on thermoplastics with fairly low softening temperatures. If a loaded joint is subjected to elevated temperature, failure may occur because of Creep unless the adhesive is cross-linked. Likewise, attack by organic solvents can be minimized by cross-linking. Solvent-based, emulsion and hot melt systems are available that cross-Unk after the initial bonding has been carried out. These systems provide improved in-service performance. 

Energy loss through viscoelastic dissipation usually contributes to the fracture energy of an adhesive bond sometimes that contribution is dominant. Adhesives are often designed to operate in the regions where viscoelastic losses (tan 5) are high. Further discussion can be found under Adhesion - fundamental and practical. Peel tests and Tensile tests see also Creep and Durability creep rupture. 

Durability creep rupture D A DILLARD Creep under sustained stress Durability fatigue D A DILLARD Effect of cyclic loads... 

Stress distribution shear lag solution D A DILLARD Stresses in shear joints Creep A D CROCOMBE Occurrence protection against creep failure Durability creep rupture D A DILLARD Creep under sustained stress Durability fatigue D A DILLARD Effect of cyclic loads... 

Long-term durability of adhesively bonded joints may require resistance to a number of individual or combined degradation modes, including environmental attack, fatigue and time-dependent failures. Time-dependent failure mechanisms are often characterized nsing either a strength approach, involving creep and creep-rupture tests, or a fracture approach, in which debond rate is determined. In creep-rupture tests, adhesive joints are subjected to... 

Significant scatter is often evident in time to failure data obtained from stress rupture tests conducted on either neat materials or on bonded joints. This scatter may obscure trends and frustrate the user. Results are typically plotted as load level versus the time to failure, a form that is analogous to S-N plots used in fatigue tests (see Durability Fatigue). In keeping with the principles of polymer physics, the time to failure axis should be plotted on a log scale, as illustrated in Fig. 1. Many creep-rupture models for homogeneous materials are based on forms like... 

Further consideration relating mechanics to joint design are given in Joint design strength and fracture perspectives, Durabiiity creep rupture. Durability subcritical debonding and Durability fatigue. 

The bonded joints between typical thin structural elements have an extensive capability to tolerate the load redistribution that is caused by local flaws and porosity with no loss whatever in strength or durability. This derives from the same minimum overlaps needed to provide resistance to creep rupture that were discussed earlier. Figs. 26-29, taken from ref. [19], address this issue in the context of the longitudinal skin splices in the PABST forward fuselage, where the thickness was 0.050 inch of 2024-T3 aluminum alloy. Fig. 27 shows the adhesive stress distribution at room temperature for a load of 1000 Ibs./in., which corresponds with a 1.3 x P proof pressure load. Significantly, this load does not even exceed the elastic capability of the adhesive for this environment. 

The lower frequency creep or creep-rupture tests are time dependant. In these tests joints are subjected to a nominally constant load. The tests may proceed for a chosen time period, or may be continued until complete rupture occurs. For example, the durability of thermoplastic adhesives for nonstructural wood applications is classified in European standards EN 204 2001 and EN 205 2003 according to their ability to withstand various water treatments and relatively rapidly applied loads. However, an additional characteristic that can be specified is resistance to static load, which can be determined using the method described in EN 14256 2007. This method is used to determine the ability of a test piece bonded with a thermoplastic adhesive, to support a given load for up to 21 days without fracture or excessive distortion, and specifies a mean survival time of 14 days. 

Tressler, R.E., and J.A. DiCarlo. 1995. Creep and rupture of advanced ceramic fiber reinforcements. Pp. 141-155 in High Temperature Ceramic Matrix Composites 1, Design, Durability, and Performance. Vol. 57 in Ceramic Transactions, A.G. Evans and R. Naslain (eds.). Westerville, Ohio American Ceramic Society. 

---