AMTS STANDARD WORKSHOP PRACTICE. Bond Design

Transcription

1 AMTS STANDARD WORKSHOP PRACTICE Reference Number: AMTS_SWP_0027_2008 Date: December 2008 Version: A 1

2 Contents 1 Technical Terms Scope Primary References Basic Typical joint types Single lap joint Tapered lap joint Butt joint Scarf joint Strap joint Double strap joint Tapered strap joint Double lap joint Stepped lap joint Joint strength Peel strength Water absorption Starved joints Thermal expansion Surface preparation of materials used in bonding Application procedure Compatibility of adhesive with adherents Optimum Optimum bond design for single lap joints Collection of physical data Determining the bond overlap (Redux method) Inclusion of a Safety Factor into the designing prosess Adhesive Testing Specimen design

3 1 Technical Terms Adhesion: Adhesion between surfaces in which the adhesive holds the parts together by interlocking action. Compression strain: Deforming of a material when subjected to a compressive force. Peel: The average load per unit width of bond line required to separate progressively, one member from the other over the adherent surface. Shear strength: The ability of a material to with stand a force parallel to the area to which it is subjected. Strain: Deforming of a material as the result of an applied force. Tensile strength: The ability of a material to with stand a force perpendicular to the area. 2 Scope The purpose of this document is to discuss different methods for the designing of bonds and covers the following: Basic bond design Optimum Bond design Adhesives testing 3 Primary References [1] FM van den Bergh (B. Ing.) Investigation into the effect of bond thickness on bonding properties of laminating epoxy derived adhesives. (School of Mechanical Engineering, North- West University) [2] Hexcel Composites Redux Bonding Technology. December 1997, Publication No. RGU 034b 4 Basic Adhesive bonding is a widely used industrial process in which a polymeric material (the adhesive) is used to join two separate pieces (the adherents). Bonded joints may be preferred if thin composite sections are to be joined and stresses in bolted joints would be too high. In 3

4 general, thin structures with well-defined load paths are good candidates for adhesive bonding, while thicker structures with complex load paths are better candidates for mechanical fastening. Joints are designed to handle a wide variety of stresses but do not necessarily behave equally well under each. As a result, these factors must be taken into consideration when designing a specific joint. Figure 4-1: From left to right: tension, shear, compression, peel and cleavage. Fig. 4-1 shows the different loadings which can be applied to joints. Adhesives behave well when subjected to tension, shear and compressive stresses, but may fail when these loadings are combined with cleavage and peel stresses. The most effective bond will be achieved when: The loading is in the same direction as that of the adhesive s greatest strength. The bond is not subjected to any peel- or cleavage loadings. 4.1 Typical joint types Single lap joint Figure 4-2: Single lap joint. As shown in fig. 4-2 the single lap joint is constructed by overlapping two adherents with the adhesive sandwiched in between. Even though single lap joints are commonly used, they cannot be used for every application. Sometimes a more stable joint is required and at 4

5 other times the type of loads that the joint is subjected to will predict the necessary geometry of the joint Tapered lap joint Figure 4-3: Tapered lap joint. By tapering the square edges of a single lap joint, the tapered lap joint is formed. As seen in fig. 4-3 above, the edges are bonded together and only the part of the adherent adjacent the joint is tapered. The benefits of this type of joint include: Less peeling effect due to strain, as a result of the tapering. Can be loaded in shear as well as in tension. When using tapered lap joints it is important to note that the bending moment that occurs in single lap joints is still present Butt joint Figure 4-4: Simple butt joint. The butt joint is a type of joint in which the faces of the adherents are at right angles and directly opposite to each other. Benefits of this type of joint are: No bending moment present as is the case with single lap joints. (Bending moments will however occur when there is an impurity present in the joint). This type of joint can be loaded in shear and tension. 5

6 4.1.4 Scarf joint Figure 4-5: Scarf joint. This type of joint is formed by joining two scarfed adherents together as shown in fig Benefits of this type of joint are: The bending moment that forms in this joint is less than in single lap joints. When compared to double lap and single lap joints through the finite element method, it is seen that the scarf joint is the most efficient for carrying shear loads. The joint can be loaded in shear and tension. Note: Although the other joints carry higher peak shear loads the scarf joint has a more uniform shear distribution through the adhesive bond Strap joint Figure 4-6: Strap joint. As seen in fig. 4-6, the strap joint is formed by combing a butt joint with a lap joint. Benefits of this type of joint are: The strap joint can be loaded in shear and in tension. The added strap lends more adhesive area to the joint, providing more support and thus stabilizes the joint. 6

7 4.1.6 Double strap joint Figure 4-7: Double strap joint. Fig. 4-7 shows how the double strap joint is formed by adding an extra strap to the single strap joint. This increases the bonding surface even more and further stabilizes the joint. Benefits of this type of joint are: Increased stability The joint can be loaded in shear and in tension. Although bending moments do occur, they are greatly reduced by the addition of the extra strap which spreads the load over a larger surface Tapered strap joint Figure 4-8: Tapered strap joint. This joint is constructed in the same way as the double strap joint with the addition of tapering on both of the straps. Benefits of this type of joint are: Peeling is minimized as a result of the tapering towards the ends of the straps. The added straps on both sides stabilize the joint. This type of joint can be loaded in shear and tension. 7

8 4.1.8 Double lap joint Figure 4-9: Double lap joint. The double lap joint seen in fig. 4-9 above is a variation on the single lap joint discussed previously. The adherent and the adhesive are sandwiched between two plates. Benefits of this type of joint are: This joint can be loaded in shear and tension. The bending moment found in single lap joints is greatly reduced Stepped lap joint Figure 4-10: Stepped lap joint. As seen in fig the stepped lap joint is constructed by stepping both of the adherents. The joint consists of the same area as the butt joint and the extra area resulting from the step in the adherents. Benefits of this type of joint are: This joint can be loaded in shear and tension. The bending moment is less than that of a lap joint. The step results in a larger bonding surface, improving the quality of the bond. 4.2 Joint strength The different forces that work on a joint have different impacts on the strength of the joint, yet applied forces are not the only factors making an impact on the strength of the joint. 8

9 The secondary factors affecting the strength of joints are: Peel strength Water absorption Starvation of joints (too little adhesive) Thermal expansion Surface preparation methods of adherents Application procedure Compatibility of adhesive with the adherents Peel strength Figure 4-11: Induced peeling as a result of the bending moment in the material. Peel is defined as: the average load per unit width of bond line required to separate progressively, one member from the other over the adherent surface. The peel strength is expressed in [N/mm] of width. When loads are applied to the joint the material will begin to deform under the applied strain. This deformation will not be uniformly distributed through the material resulting in the ends of the material peeling away from the bond. The combined tensile loading and induced peeling will weaken the joint and ultimately lead to failure of the adhesive. Joints such as the tapered strap joint and scarf joint behave well under these loading conditions, because they counteract induced peeling near the ends of the joint Water absorption Water absorption is a means of describing the ratio of water weight absorbed by a material to the dry weight of the material. When the bonding areas absorb moisture, the water molecules penetrate the bond structure, partially changing the bonding forces from cohesive to adhesive and as result reduces the bond strength. Important: When bonds are designed it is important to insure that they operate in a dry environment or are protected from the adverse effects of moisture. 9

10 4.2.3 Starved joints Starved joints occur when the joint does not contain enough adhesive to produce a joint capable of holding two adherents together. This may be because the adherent was applied too thin or that it was not left to cure for long enough. There are other factors that may play a role, such as using an incorrect adhesive, penetration or the application of too much pressure when curing. These factors may differ in each situation. When a joint is starved the bond strength will be significantly weaker than intended and the desired factor of safety will not be achieved Thermal expansion Thermal expansion plays a big part in the strength of a structure, especially if the structure is exposed to the harsh outdoor sun. The thermal expansion of an adherent and adhesive may not be the same and will cause thermal stresses within structure, if subjected to heat fluctuations. These stresses can cause failure in some cases. This phenomenon occurs frequently in hot-melt adhesives as well as adhesives which are cured for long periods of time. As the adhesive cools, thermal stresses are pre-formed in the joint. Care should be taken in the design of a bond to ensure it can withstand the effects of thermal expansion Surface preparation of materials used in bonding A bond is only as strong as its weakest link. If the adhesive does not bond to the surface of the components, the bond will be significantly weakened. Refer to SWP 12 on Adhesive Bonding for details on the preparation of a surface for adhesive bonding Application procedure See SWP 12 on Adhesive Bonding for detailed methods of the application of adhesives to a bonding surface Compatibility of adhesive with adherents An adhesive must be compatible with the adherents being bonded and should be able to maintain its required strength when exposed to in-service stresses and environmental conditions. 10

11 When designing bonds it is important that the material selection be made in such a way that the adherents will fail before the bond. Refer to SWP 12 on Adhesive Bonding for more on the compatibility between adhesives and adherents. 5 Optimum 5.1 Optimum bond design for single lap joints In this section the method for designing a single lap joint and the factors that influence bond strength will be discussed in detail Collection of physical data t t l Figure 5-1: Overlap dimensions The first step in the bond design process is the collection of physical data for use in the calculations. See SWP 4 to SWP 6 on Material Test Standards for detail on the collection of data. The test program must be completed in order to determine the mean shear strengths of various overlaps and material thicknesses. The tests should be sufficient to plot a curve of the measured shear strength vs. the factor. 11

12 Figure 5-2: Shear strength ( ) vs. for simple lap joints. Any particular point on the established curve represents the state of stress in a particular joint, and shows the relationship between the dimensions of the joint (x-axis), the mean stress in the adhesive (y-axis) and the mean tensile stress in the bonded material (the slope of a straight line drawn from the origin to the point on the curve). Important: It is imperative that all of the tests be done in a controlled environment in order to attain trustworthy results Determining the bond overlap (Redux method) Two methods may be followed to determine the dimensions of the bond: (With reference to [2]) 1. Calculate the optimum overlap, given a material thickness. 2. Calculate the material thickness, given an overlap length. 1. Determining the optimum overlap from the material thickness Use the diagram with the following formula: 12

13 With = the mean shear stress in the bond = tensile stress in the material t = the thickness of the material l = length of the bond P = load per unit length Use the following steps: 1. Using the given dimension t and the load per unit length P, determine from the equation: 2. Starting from the origin, draw a line with slope equal to the value of. 3. The value of is where the line and the graph intersects. 4. The overlap (l) can now be determined from the equation: 2. Determine material thickness (t) given an overlap (l): 1. Determine from load per unit length P and the overlap I using the definition of mean shear force: 2. The value of is found where a horizontal line with value intersects the graph. 3. Because the overlap l is given, t can be determined from Inclusion of a Safety Factor into the designing prosess The curve in fig. 5.2 represents the mean failure stresses for joints immediately after bonding. In practice, allowance should be made for a reduction in bonding strength due to the effect of weathering, sustained loading or high temperatures during service. In addition to environmental allowances a safety factor can be applied to the curve by lowering it by an amount equal to t divided by the safety factor. The factored curve is then used as prescribed above. 13

14 6 Adhesive Testing Adhesive bond strength is usually measured by the simple single lad shear test. The lap shear strength is reported as the failure stress in the adhesive, which is calculated by dividing the failure load by the bond area. Since the stress distribution in the adhesive is not uniform over the bond area (it peaks at the edges of the joint), the reported shear stress is lower than the true ultimate shear strength of the adhesive. While this specimen is relatively easy to fabricate and test, it does not give a true measure of the shear strength due to adherent bending and induced peel loads. However the single lap shear test is an effective screening process control test for evaluating adhesives, process control and surface preparations. Figure 6-1: Test specimen before the load is applied. Figure 6-2: Induced peeling as a result of the bending moment in the test specimen. Considerations when testing the characteristics of adhesives: All test conditions must be carefully controlled including the surface preparation, the adhesive as well as the bonding cycle. Tests should be run on actual joints that will be used in production. A thorough evaluation of the in-service conditions must be carried out, including temperature, moisture and any solvents or fluids that the adhesive will be exposed to during its service life. The failure modes for all specimens must be examined. Some acceptable and unacceptable failure modes are shown in the figures below: 14

15 Figure 6-3: Acceptable modes of failure. Figure 6-4: Unacceptable modes of failure. Unacceptable Failure Modes indicates a problem with the bonding process. For example: adhesion failure of the adhesive will indicate a problem with the surface preparation, as the adhesive is not bonding properly with the surface. 15

16 6.1 Specimen design The specimen is designed according to the specific needs of the test program. The test specimen must have ends that are long enough to be pulled on when fixed on the test bench. Generally the width of the specimen (d) is 10 mm to simplify the calculations and give sufficient results. A width of 10 mm is also sufficient enough to be gripped during the tension test. The thickness of the adherent (t) has been calculated with a safety factor of two, to prevent failure in the glass fiber plate before the adhesive fails. The thickness of the adherent must be calculated from the preliminary tests. 16