February** Dynamic Incentive Program
The most common way to evaluate bond quality is to determine bond strength. Bonding strength is an important indicator in adhesive technology, which is of guiding significance for the selection of adhesives, the development of new adhesives, the design of joints, the improvement of bonding processes, and the correct application of adhesive structures.
Bond strength definition
Adhesion strength refers to the stress required to damage the interface between the adhesive in the adhesive and the adherent or its vicinity under the action of external force, and the adhesion strength is also called the adhesive bonding strength.
The bonding strength is the stress required when the adhesive system is damaged, and its magnitude depends not only on the adhesion, the mechanical properties of the adhesive, the properties of the adherent, and the bonding process, but also on the joint form, force (type, size, direction, frequency), environmental factors (temperature, humidity, pressure, medium), test conditions, experimental technology, etc. It can be seen that adhesion is only one of the important factors that determine the strength of the bond, so the adhesion strength and the adhesion force are two completely different concepts and must not be confused.
The form of force on the bonding joint
The force on the adhesive layer under the action of external force can be summarized into four forms: shear, tensile, uneven tearing and peeling.
1) Shear. The external force is equal in magnitude and opposite in direction, basically parallel to the bonding surface, and evenly distributed on the entire bonding surface.
2) Stretching. Also known as uniform tearing, it is subjected to the action of pulling in the opposite direction, perpendicular to the adhesive surface, and evenly distributed throughout the adhesive surface.
3) Uneven tearing off. It is also called splitting, and although the direction of the external force is also perpendicular to the bonding surface, it is unevenly distributed.
4) Stripping. The direction of the external force is at a certain angle to the bonding surface, and it is basically distributed in a straight line of the bonding surface.
Classification of bond strength
According to the different forces of the adhesive joints, the bonding strength can be divided into shear strength, tensile strength, uneven tear off strength, peel strength, compressive strength, impact strength, bending strength, torsional strength, fatigue strength, creep resistance, etc.
(1) Shear strength
Shear strength refers to the shear force that can be experienced per unit of adhesive surface when the adhesive part is broken, and its unit is expressed in megapascals (MPa). Shear strength is divided into tensile shear, compressive shear, torsional shear and bending shear strength according to the force mode of the test.
Adhesives with different properties have different shear strengths, and in general, tough adhesives have higher shear strength than flexible adhesives. A large number of tests have shown that the thinner the bond line, the higher the shear strength.
The most important influence of the test conditions is the ambient temperature and the test speed, the shear strength decreases with the increase of temperature, and decreases with the slowdown of the test speed, which indicates that the temperature and speed have an equivalent relationship, that is, increasing the test temperature is equivalent to reducing the loading speed.
(2) Tensile strength
Tensile strength, also known as uniform tear-off strength and forward tensile strength, refers to the tensile force per unit area when the viscous force is broken, and the unit is expressed in megapascals (MPa). Because the tensile force is much more uniform than the shear force, the tensile strength of the general adhesive is much higher than the shear strength. In the actual measurement, under the action of external force, the deformation of the adhesive is larger than that of the adherent, and the different axiality of the external force is likely to produce shear and transverse compression, so it may break at the same time when it is torn. If the length of the specimen can be increased and the bonding area can be reduced, the effect of peeling during tearing can be reduced, and the stress action can be distributed more evenly. The effects of elastic modulus, adhesive line thickness, test temperature and loading speed on tensile strength are basically similar to those of shear strength.
(3) Peel strength
The peel strength is the maximum load that can be carried per unit width of the adhesive when separating under the specified peel conditions, and its unit is expressed in kn m. There are various forms of peeling, which can generally be divided into L-type peeling, U-type peeling, T-shaped peeling and surface peeling, as shown in the figure below.
As the peel angle changes, so does the peel form. When the peel angle is less than or equal to 90°, it is L-shaped peeling, and when it is greater than 90° or equal to 180°, it is U-shaped peeling. These two forms are suitable for the peeling of rigid and flexible material bonds. T-stripping is used for peeling when bonding two flexible materials. The peel strength is affected by the width and thickness of the specimen, the thickness of the adhesive layer, the peel strength, the peel angle and other factors.
(4) NopeUniform tear-off strength
The uneven tear-off strength indicates the maximum load that the adhesive joint can bear when subjected to the uneven pull-off force, because the load is mostly concentrated on two edges or one edge of the adhesive line, so it is a unit length rather than a unit area of the force, and the unit is kn m2.
(5) Impact strength
Impact strength refers to the maximum work consumed per unit of bonding area when the adhesive is damaged by impact load, and the unit is kj m2. According to the different forms of joints and stress modes, impact strength is divided into flexural impact, compressive shear impact, tensile shear impact, torsional shear impact and T-shaped peel impact strength.
The impact strength is affected by the toughness of the adhesive, the thickness of the adhesive layer, the type of adhesive, the size of the specimen, the impact angle, the ambient humidity, the test temperature, etc. The tougher the adhesive, the higher the impact strength. When the modulus of the adhesive is low, the impact strength increases with the thickness of the adhesive line.
(6) Long-lasting strength
The long-lasting strength is the maximum load that the adhesive can bear per unit of bonding area after being subjected to static load for a long time, and the unit is expressed in megapascals (MPa). The enduring strength is affected by the loading stress and the test temperature, and decreases with the increase of the loading stress and temperature.
(7) Fatigue strength
Fatigue strength refers to the maximum stress of a bonded joint that is repeatedly applied to a certain load to a specified number of times without causing failure. Generally, the fatigue strength at 10 times is called the fatigue strength limit. Generally speaking, adhesives with high shear strength always have lower peeling, bending, impact and other strengths; The adhesive with high peel strength has higher impact and bending strength. Different types of adhesives have a wide range of strength characteristics.
The following is a brief introduction to the methods for measuring tensile strength and shear impact strength.
Method for determining tensile strength
a.Determination of tensile strength of metal bonds
The most commonly used specimens for determining the tensile strength of metal bonds are shown on the left side of the figure below.
The diameter of the two cylinders of the specimen should be the same, and the coaxiality should be 01mm, the parallelism of the two bonding planes is 02 mm, machining roughness of 50μm。The specimen is bonded according to the process requirements, and in order to ensure the consistency of the adhesive layer, 0The copper wire of about 1 (2 3) mm is placed in the adhesive layer before lamination, and is positioned and cured by a special device (see the right picture above).
The diameter d of the cylinder is measured from both sides of the adhesive line (accurate to 1 10-6 m) before the measurement. During the measurement, the specimen is installed on the fixture of the tensile testing machine, the force centerline is adjusted to be consistent with the axis of the specimen, and the loading speed is stretched at (10 20) mm min, and the failure load is recorded when the tension is broken, and the tensile strength is calculated according to the following formula, and the unit is mpa.
f A: f - the load when the specimen is damaged;
A - the adhesive area of the specimen, a = d2 4.
Each group of adhesive specimens should not be less than 5, and the arithmetic average value shall be taken according to the allowable deviation of 15%, and 3 significant digits shall be retained. If it is necessary to measure the tensile strength at high and low temperatures, the specimen and the fixture should be put together in a heating or cooling device, and kept at the required temperature for (40 60) min, and then measured.
b.Determination of tensile strength of non-metal-to-metal bonds
The determination of the tensile strength of non-metallic and metallic bonding adopts the method of sandwiching a layer of non-metal between two metals. Here, we will introduce a method for measuring the tearing strength of rubber-to-metal bonding. The rubber thickness is (2 0.).3) mm, the size of the bonded specimen is shown in the figure below.
The specimen is bonded according to the requirements of the process conditions, and the dislocation of the bonding surface should not be greater than 02 mm。During the test, the specimen is mounted on the fixture, the position is adjusted so that the direction of force is perpendicular to the bonding surface, stretched at the loading speed of (50 5) mm min, the maximum load at the time of failure is recorded, and the tearing strength is calculated according to the following formula, and the unit is MPa.
c=f A: f - the load of the specimen when it is broken;
a – bonding area, a = d2 4.
The number of specimens shall not be less than 5, and the number shall not be less than 60% of the original number after rounding, and the arithmetic average value shall be taken, and the allowable deviation shall be 10%.
Determination of shear impact strength of adhesives
Shear impact strength refers to the work consumed per unit of adhesive area when the sample is damaged by shear impact load at a certain speed, and its unit is expressed in j m2. The shear impact strength of the adhesive is determined according to GB T6328-1986 standard.
1.Principle
The specimen composed of two test blocks is glued together, so that the glue joint surface is subjected to a shear impact load at a certain speed, the work consumed when the specimen is damaged, and the shear impact damage force borne by the unit glue area is calculated.
Test block - a block-like adherent with a specified shape, size, and accuracy.
Specimen - the upper and lower test blocks are glued together under certain process conditions.
Hit height - when the pendulum blade hits the above test block, the distance from the blade to the upper surface of the lower test block is represented by h, as shown in the figure below.
2.Instruments and equipment
1) Testing machine. The adhesive shear impact testing machine should use a pendulum impact testing machine. The speed of its pendulum is 335m/s。The damage work of the specimen should be selected within (15 85)% of the capacity of the measuring plate of the testing machine.
2) Fixtures. The fixtures used should be able to ensure that the impact height of the specimen is in (0.).8～1.0)mm, and keep the impact surface of the specimen and the upper surface of the lower test block parallel to the pendulum blade.
3) Gage. The minimum graduation value of the gage used is 005 mm。
3.Block and specimen preparation
1) Test blocks
Block material. The test block can be made of metal materials such as steel, aluminum, copper and its alloys, and non-metallic materials such as wood and plastic. However, the wood test block needs to use a bulk density greater than 055 g cm3 of birch or equivalent straight wood grain species. The bulk density of the upper and lower test blocks should be approximately the same. Wood with knots, spots, decay, and abnormal color cannot be used to process test blocks. The moisture content of the wood is maintained at (12 15)% (based on the total dry mass).
Block size. The size of the upper test block is: length (25 0.).5) mm, width (25 0..)5) mm, thickness (10 0..)5)mm;The size of the lower test block is: length (45 0.).5) mm, width (25 0..)5) mm, thickness (25 0..)5)mm。
When processing non-metallic test blocks, care should be taken not to damage the test blocks due to overheating.
2) Specimen preparation
The pretreatment method of the adhesive surface of the test block, the adhesive coating and the sample preparation process should be determined according to the process regulations of the product.
When the wood test block is glued, the wood grain direction of the upper and lower test blocks should be consistent.
In the absence of special requirements, 10 metal samples are generally taken, and 12 non-metal samples are generally taken.
4.Test procedure
1) Place the samples parked under normal conditions in the test environment (temperature 23, relative humidity 50%) for more than 30min.
2) Before starting the testing machine, measure the length and width of the glue joint at 3 points with a measuring tool, accurate to 01mm。The arithmetic mean value is taken to calculate the glued area.
3) Install the specimen on the fixture as required.
4) Start the testing machine, make the pendulum fall to hit the sample, and record the damage work of the sample W1.
5) The upper test block that will be knocked out is overlapped with the lower test block, and the operation (4) is repeated 1 time to record the inertial work w0 of the sample.
6) Record the failure type of each sample, such as: interface failure, adhesive layer cohesion failure, mixed failure and test block deformation state.
5.Test results
The shear impact strength is calculated according to the following formula in J m.
is=(w1-w0)/a
where: w1 - the impact damage work of the specimen;
w0 – the work of inertia of the specimen;
a - gluing area.
The test results are expressed as the arithmetic mean of the shear impact strength, taking 3 significant figures.
Non-destructive testing methods
At present, the most common application for determining bond strength is destructive testing, and the reliability of bond quality cannot be fully guaranteed due to sampling testing. With the increasing application of adhesive technology in high-tech fields such as aerospace, the requirements for bonding quality and reliability are becoming increasingly stringent, and non-destructive testing methods are urgently needed. Therefore, the study of non-destructive testing of bond strength is an important topic in the bonding process and practical use. Since the 60s of the 20th century, the relationship between the bond strength and certain physical properties of the adherent object has been used to determine the bond strength, such as the adhesive strength determination method based on the determination of the dynamic modulus of the adhesive by ultrasonic. In recent years, due to the application of new technologies and the continuous improvement of methods, the non-destructive testing of bond strength has developed from qualitative to quantitative, from manual data processing to computer intelligence, and non-destructive testing methods mainly use ultrasonic, acoustic and stress wave technologies.
1.Ultrasound technology
a.Polyvinylidene chloride piezoelectric probe uses metallized polyvinylidene chloride (PVDF) film as the probe for ultrasonic nondestructive testing, which has been successfully applied to the detection of ultrasonic echo, transmission and stress wave. Lightweight, flexible, ultra-thin and inexpensive, it has a faster response frequency than traditional ceramic piezoelectric probes and does not require any coupling agents.
b.Ultrasound coupling technology uses a rubber-lined probe without the use of liquid coupling agents, i.e., dry coupling technology. Inspecting the quality of bonded joints based on changes in sound energy within the material is ideal for rapid defect detection.
c.Planar Leakage DetectionPlane leakage (LLW) is a boundary-sensitive plane wave excited at the bonding joint level. The compensation phase in the LLW ineffective region is very sensitive to the interface condition of the adhesive line, and the lack of adhesive and the characteristics of the adhesive can significantly change the LLW response. When the plane wave is transmitted to the bonding surface, both compressive and shear stresses will be generated, which are affected differently by the interface properties, making this NDT have better detection results.
d.The signal measured by this method is the phase and radiation value of the monotone pulse reflected back from the bonding interface. According to the relationship between the propagation characteristics of the wave in the multilayer medium and the interfacial strength, the bonding quality parameters can be deduced, which has a good linear relationship with the tensile strength.
e.Ultrasonic spectrum testing uses ultrasonic spectrum technology to measure the thickness and modulus of the adhesive line, and the resonance frequency is sensitive to the change of the thickness and modulus of the adhesive line. Ultrasonic spectrum analysis is useful for the sensitivity of bonded joint properties and has great potential.
2.Acoustic technology
a.Acoustic emission
Acoustic emission is a dynamic non-destructive testing technology, which links the dynamic load of the sample with the deformation process, which can characterize the small deformation of the sample in the dynamic tester, and is a detection method to show the defect development process and the destructiveness of the defect.
b.Acousto-optic measurements
The micromechanical response of the material was analyzed by non-contact laser excitation by taking the bonded joint as a whole. The dynamic response parameters correlate well with the bond condition and can be used to easily and quickly test the bond quality.
3.Other non-destructive testing methods
a.Stress WaveStress wave is the product of the combination of acoustic emission and ultrasonic, which is a relatively new non-destructive testing technology, which absorbs the advantages of traditional ultrasonic and acoustic emission, and is still ultrasonic testing in essence. The stress wave method can show the comprehensive effect of defects and failure in the structure, can distinguish high bond strength from weak bond strength, can be used to monitor bond quality, and is very promising in controlling bond quality and ** bond strength.
b.Portable Holographic Interferometric Test System The portable holographic interferometric test system can detect the lack of glue and weak bond strength of the bonding joint, and provide a feasible integrity test device for the bonding site.
c.Thermal imaging technology simulates a series of factors affecting the heat exchange of the bonding part, calculates and analyzes the relationship between these factors and the type of bonding defect and the bonding condition, and the results show that there is an optimal heat transfer time during the inspection, and the maximum temperature difference of the detection is linearly related to the degumming width.
d.The eddy current method uses a new pulse frequency response technology, which adds electromagnetic waves to the sample to make it thermally vibrate, and then uses an eddy current probe to check the response characteristics of the test sample, and obtains a loss factor after calculation and analysis, which has a good correlation with the bonding defect and bonding strength.