Creep testing, bonded joints

Creep tests on structural adhesives can be divided into tests on bulk hardened adhesive specimens and tests on adhesively bonded joints. The former provides information on the mechanical properties of the adhesive rather than the joints made from them. Fig. 2.28 displays the change in creep modulus with time for a range of cold-cure epoxy adhesives(26). These curves were derived from four point bend tests on adhesive prisms loaded in accordance with Fig. 2.16 using extreme fibre stresses ranging from 0.25 to 2.0 N/mm. The curves represent the stability of the adhesive with time under... 

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... 

In instrumented creep tests taken to failure, one learns not only how long specimens last but also how deformation increases throughout the creep process. For lap joints, delay times have been seen in creep tests, probably due to the increasing uniformity of the shear stress state, as predicted by the shear lag model as the creep compliance of the adhesive increases with time. In other situations, no such delay time is seen. A schematic illustration of a creep curve for an adhesive bond consisting of a butt joint bonded with a pressure sensitive foam tape is shown in Fig. 2, exhibiting classical primary, secondary and tertiary regions of creep behaviour. 

Keywords Adhesive modulus Adhesys expert system Co-axial joints Compression Concealed joints Creep Elastic limit Epoxy Epoxy composite Einite element analysis Glue line thickness Goland and Reissner Hart-Smith Heat exchanger Hooke s Law Joint designs Joint thickness Lap shear strength Peel Plastic behaviour Polyurethane Pipe bonding Shear stresses Shear modulus Stress distribution Thick adherend shear test Tubular joints Volkersen equation Young s modulus... 

The creep test is performed as follows (Fig. 41) bonded parts are submitted to a dead load W, at the required temperature (in a chamber at this temperature) and the creep (or deformation of the joint) is measured, with a microscope, every hour or day or month, until failure when the bottom part falls (a system records that moment, for instance, by breaking or opening an electric circuit, starting an alarm). An operator will record the dead load used, the creep in mm per day, and the time to failure, and also the temperature of testing. 

The DB-procedure was optimised in respect with the kinetic requirements and the high-temperature mechanical properties of the Ni-superalloy. From the kinetic point of view, the bonding temperature should be over 1000°C when alumina and transition metals are directly bonded [6]. The bonding procedure was always carried out in high vacuum, better than 2-10 mbar (0.2 mPa). The typical thermal and axial compression cycles are presented in Fig.la. It was experimentally found that the ambient bonding temperature is 1100"C or less due to the fast creep of the superalloy beyond this. The compression for the tests was selected as 10 MPa in ceramic-metal joints and 20 MPa in ceramic-ceramic joints [6]. 

Further information is given in articles on Pre-treatment of metals prior to bonding, Durabiiity creep rupture, Durabiiity subcritical debonding. Weathering of adhesive joints and Weathering tests. 

This chapter will discuss the testing, analysis, and design of structural adhesive joints. Adhesive bond test techniques to be considered include tensile, shear, peel, impact, creep, and fatigue. Some considerations will also be given to the effect of environment and test rate. A continuum approach to the analysis of adhesive joints will discuss tensile, shear, and peel stresses which arise in various joint geometries. Classical theories by Volkersen, Goland and Reissner, and others will be included. References to finite element analysis will be made where appropriate throughout the chapter. 

The results of creep rupture tests on single-lap joints yielded the reduction factor, /l, for an adhesive bond subjected to constant static loading (Fig. 30). The strength of the adhesive decreases with increasing exposure. In constant-load tests of this kind, particularly at higher temperatures, creep strain is observed in the adhesive layer where a certain initial load is exceeded. 

For bonding, the important mechanical properties are principally E-modulus, elongation at break and viscoelastic behaviour. E-modulus governs the stiffness of the joint. Elongation at break is relevant for impact resistance and is measured after durability tests. Viscoelastic behaviour determines if creeping is expected in the application. 

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. 

---