RU2515337C1 - Method to determine strength of adhesive connection of rubber-like coating with base - Google Patents

Abstract

FIELD: testing equipment.

SUBSTANCE: proposed method to determine strength of an adhesive connection of a rubber-like coating with a base from solid propellant includes using two stiff elements, providing for application of a stretching load, one of which is put in contact with a coating by means of glue, adhesion of which to the coating is deliberately more than the adhesion of the investigated adhesive connection of the coating and the base, and the second one is exposed to interaction with the base. At the same time in the edge area of the adhesive connection of the rubber-like coating with the stiff element they perform circular break-out, the width of which makes Δ=(0.6-1.5)·δ, where δ - coating thickness. Besides, the circular break-out is made by mechanical method after completion of the process of hardening of the adhesive connection of the coating with the base or before realisation of the process of hardening by means of usage of the circular insert from the material having anti-adhesion properties to a stiff element and a coating.

EFFECT: increased validity of test results in part of obtaining experimental information on a higher level of real resources of strength of fixation of a coating and a base.

3 cl, 15 dwg, 1 tbl

Description

The invention relates to the field of testing to assess the strength of the adhesive bonding of materials in rocket technology and can be used in a comparative assessment of the strength of various types of adhesive bonding of solid fuels (base) with a rubber-like polymer (such as heat-shielding) coating, as well as for quantitative assessment of the quality of bonding of a fuel cell solid propellant rocket engine (RTTT) with heat-shielding coating (TZP) of the body.

In the known methods for determining the strength of the adhesive bonding 1 of the coating 2 with the base 3, various designs of samples are used to ensure the implementation of tensile conditions in the zone of the boundary of the coating with the base. In most cases, the base 3 is made in the form of a cylinder with parallel ends (figure 1, figure 2, figure 3), a cylinder with cone-shaped ends (figure 4) or two cylinders coaxially conjugated with a fillet (figure 5) and glued to the test glue on one of the ends of the coating 2. To the other end of the coating 2 is glued with a strong (for example, epoxy) glue a rigid element 4 (grip), pivotally connected to the movable traverse of the test setup. The second rigid element 5 is connected to the base 3 using strong glue (figure 2, figure 3, figure 4), or brought into direct contact with the base 3 and is made in the form of stops (figure 1, figure 5), providing axial tension of the base.

When testing these samples, a force (F) is recorded, with which the base material and the border of the bond of the base with the coating resist the movement of the rigid elements (grips) of the test setup (not shown).

The realized level of adhesion characteristics σ _a is determined by the ratio of the load (max F) during the destruction of the sample to the contact area of the substrate with the coating fastened by the test adhesive with one of the rigid elements (grips)

$${\sigma }_{\mathrm{but}}=\frac{\max F}{S}\text{, (one)}$$

where max F is the force during the destruction of the sample;

S is the area of bonding of the coating with the base.

The use of relation (1) implies the validity of the assumption of a uniform distribution of contact stresses in the zone of the boundary between the coating and the base upon failure. The use of such methods for testing and processing the results of the experiment is expedient for a comparative assessment of the adhesive strength characteristics of the specified types of contacted materials bonded with various adhesive compositions, or for monitoring the reproducibility of the manufacturing process of the manufacture of similar multilayer products.

The source [GOST 209-75] (clause 4.3) indicates that the results obtained from testing various types of samples are not comparable. Indeed, experiments show (Fig. 6) that, with the same coating materials, fixing composition and base, tensile diagrams I and II of different types of samples (Figs. 3 and 5) differ in both the level of stiffness and ultimate strength characteristics during fracture in the border zone, the coating is the base. This is due to the different nature of the heterogeneity of the stress distribution in the zone of the fracture boundary for different types of samples.

In solving the problem of quantifying the real resource of the strength working capacity of the adhesive joint of the fuel cell with the solid propellant fuel cell, the rational and most representative method for the experimental assessment of strength adhesive characteristics should be focused on minimizing the effects of stress inhomogeneity in the area of the contact boundary of the materials under study. The presence of local stress concentration zones at the contact boundary under study is a factor initiating premature failure of the specimen under loading, a factor distorting (underestimating) the real level of ultimate strength adhesion characteristics σ _a . The experimental determination of the indicated distribution of contact stresses during fracture is not possible due to the lack of the necessary measuring instruments for the distribution patterns σ _a (x) along the radial coordinate x in the area of the coating-base contact boundary.

Numerical studies (by the finite element method) of the stress state in the bonding zone of the base with the coating showed that during tensile testing, the stress distribution σ _a (x) along the x coordinate (distance of the point under consideration from the axis of the sample) is characterized by significant heterogeneity (Fig. 7, diagram A , B - samples of Figs. 3 and 5, respectively, Δl is the movement of the movable rigid element, mm). In the boundary zone where the coating is bonded to the substrate, stress concentration effects are realized that inevitably lead to premature failure initiation in the concentrator zone and its subsequent propagation along the studied boundary of the adhesive bonding between the substrate and the coating. The results of such tests underestimate the real level of adhesive strength characteristics.

The technical solution proposed in the utility model of the Russian Federation No. 7506 (publ. 08.16.98), in which the method for determining the strength of the adhesive bonding of a rubber-like coating with a polymer base is implemented, is aimed at solving the problem of reducing the stress concentration in the bonding zone of the base with the coating. material, including the use of two rigid elements providing the application of a tensile load, one of which is brought into contact with the coating by means of glue, the adhesion of which to the coating is known to be greater than adhesion of the studied adhesive joints of the coating to the base, and the second is subjected to interaction with the base.

The stress concentration in the boundary zone of bonding the coating to the base of the prototype sample shown in FIG. 5 actually decreased to K ^σ = 2.6 (curve B in FIG. 7) compared to the sample shown in FIG. 3 (curve A on Fig.7, K ^σ = 3,4). This allowed us to recommend the use of the sample (figure 5) for testing materials with high deformability.

Nevertheless, the stress concentration coefficient remained significant, which still leads to premature failure of the specimen in the edge bonding zone, underestimating the real life of the adhesive characteristics of the coated substrate. To increase the reliability of the experimental assessment of the adhesive strength characteristics of the studied type of adhesive joints, a more effective decrease in stress concentration parameters is necessary.

The objective of the present invention is to develop a method for determining the strength of the adhesive bonding of a rubber-like coating with a solid rocket fuel base, which allows to increase the reliability of the test results in terms of obtaining experimental information about a higher level of real life of the bond strength of the coating with the base by creating conditions that provide a local decrease in stress concentration in the boundary zone of bonding of the coating with the base, and excluding cohesive destruction of the base to destruction of its adhesive bonded coating.

At the same time, the possibility of unreasonable rejection of workable products is significantly reduced and the reliability indicators of quality control of fastening the fuel cell with the solid propellant body are increased.

The problem is solved by the proposed method for determining the strength of the adhesive bonding of a rubber-like coating with a base of solid rocket fuel, including the use of two rigid elements providing tensile loads, one of which is brought into contact with the coating by means of glue, the adhesion of which to the coating is known to be greater than the adhesion of the studied adhesive coatings to the base, and the second is subjected to interaction with the base. The peculiarity lies in the fact that in the edge zone of the adhesive connection of the rubber-like coating with the rigid element, annular fastening is performed, the width of which is Δ = (0.6-1.5) · δ, where δ is the thickness of the coating.

In particular, ring loosening is performed mechanically after completion of the curing process of the adhesive bonding of the coating to the base or before the curing process is carried out by using an annular liner made of a material having release properties to the rigid element and the coating.

In particular, when using a base made in the form of a monoblock of two cylinders of different diameters coaxially paired with a fillet, the second rigid element interacting with the base is made in the form of a glass and is brought into interaction with the cylindrical part of the base of a larger diameter by adhesive bonding.

The prior art does not know the technical solution to the problem in which the proposed combination of features would take place.

The proposed method is practically implemented for all known samples (Figs. 1-5) and is illustrated by examples of two samples with a base 3 made in the form of a monoblock of two cylinders of different diameters coaxially conjugated with a fillet, in each of which ring fastening 6 is made in the edge zone of the adhesive compounds 1 rubber-like coating 2 with a rigid element 4, which should not be interpreted as limiting the scope of the invention:

Fig. 8 is a sample in which the rigid element 5 interacting with the base is made mountainous and is brought into direct interaction with the surface of the fillet;

Fig.9 is a sample in which the rigid element 5, interacting with the base, is made in the form of a glass and is brought into interaction with the cylindrical part of the larger diameter base sample by adhesive bonding.

For these two types of samples, the result of a calculated (FEM) assessment of the effect of the width Δ of ring freeing 6 (Δ / δ) on the nature of the distribution of the normalized stress K ^σ in the coating – base interface is shown in FIGS. 10 and 11, respectively. At the level of Δ / δ≤0.2 ÷ 0.3, the introduction of ring freeing 6 insignificantly (up to K ^σ = 2.1 ÷ 2.2) reduces the effects of stress concentration in the edge zone of the coating-base bonding. An increase in the width of unfastening 6 to Δ / δ = 0.6 ÷ 1.5 allows one to reduce the concentration level of maximum stresses in the edge zone of the coating-base by 1.5 ÷ 2.0 times (to K ^σ = 1.1 ÷ 1.3) . This leads to an increase in the level of breaking load (max F) during tensile testing of the specimen, which makes it possible to increase the reliability of assessing the real strength adhesive characteristics (σ _a ) in the bonding zone of the substrate with the coating. An increase in the width of freeing over Δ / δ = 0.6 ÷ 1.5 is inappropriate due to the increase in the effects of inhomogeneity σ (x) due to the formation of a high level of max σ (x) in the areas adjacent to the axis of the sample.

Experimental confirmation of theoretical calculations was obtained by tensile testing of the samples shown in Figs. 8, 9. The recipe and process parameters of the coating, adhesive, and base material in all tests remained unchanged. SURELL type rubber was used in the adhesive. When constructing tensile diagrams, the stress value σ was determined in accordance with relation (1), in which the parameter “S” for all types of samples was also assumed to be the same and equal to the area of contact of the coating with the substrate. An analysis of the experimental tensile diagrams (Fig. 12 for the specimen shown in Fig. 8 and Fig. 13 for the specimen shown in Fig. 9, where Δl denotes the movement of the movable rigid element) shows that an increase in the width of the annular groove Δ leads to a significant an increase in the force max F at failure and the adhesion strength of the coating-base interface determined by relation (1) in σ _a = 1.0–1.6 times. The experimentally obtained effect of increasing max F and σ _a upon failure, confirming the conclusions of theoretical studies, remains unchanged for both types of samples with annular loosening (Figs. 8 and 9).

The dependences of the normalized value of the strength of the adhesive joint K ^σa coating-base on the width Δ of the ring fastening 6 are shown for samples (Figs. 8 and 9) in Figs. 14 and 15, respectively.

An additional series of similar tests was carried out on these samples (Fig. 8, Fig. 9) for another type of adhesive composition (PDI-3A rubber was used). The coating and base material did not change. The obtained results also confirmed the effect of an increase of ~ 1.5 times the adhesion strength σ _and the coating-base interface with the introduction of ring fastening Δ / δ = 1.2 ÷ 1.5 in the boundary zone of bonding of the coating with a grip.

In addition, it should be noted that experience with prototype samples showed that tensile failure was repeatedly realized not in the region of the coating-base interface, but in the contact zone of the toroidal rigid element with the base (Fig. 5) at a distance of 3-5 mm from the border coating-base. In these cases, the toroidal rigid element was an additional initiator of premature failure, which did not allow a quantitative assessment of the real level of adhesive strength characteristics in the zone of the coating-base interface. The introduction of ring loosening in accordance with the present invention increases the reliability of the results, but not enough.

Therefore, in relation to the particular case of using a base made in the form of a monoblock of two cylinders of different diameters coaxially conjugated with a fillet, it is advisable, in addition to annular unfastening, to perform a second rigid element interacting with the base in the form of a cup, which eliminates the additional stress concentrator in the region K of the fillet (Fig. 5) and simulate in the fracture zone of the specimen the stress state corresponding to the most loaded zone of bonding the full-size fuel cell to the body RDGT. At the same time, the cohesive (base) fracture zone is stably shifted to the coating-base border zone as in other known samples shown in Figs. 1-4 with the changes provided by the present invention. When performing the second hard element of the sample of Fig. 5 in the form of a glass, the measure of achieving a technical result realized according to claims 1 and 2 of the claims increases.

A comparative analysis of the type of stress state in various types of samples and full-scale products (the zone of fastening the fuel cell with the housing) is given in the Table.

Table An object Load σ _and / σ ₀ Note Sample Figure 5 Sprain 0.002 σ _and , σ ₀ - the first and second invariant of the stress tensor Fig.9 Sprain 0.20 RDTT fuel cell (bonding area with housing) Temperature, Δ, T 0.19 σ _and / σ ₀ - parameter of the type of stress state

The results of a theoretical analysis presented in the Table by numerical methods of mechanics showed that the type of stress state in the bonding zone of a solid fuel solid fuel element with a body differs significantly from the stress state in the studied bonding zone of the coating with the base of the sample shown in Fig. 5 (prototype). It becomes apparent that the proposed in claim 3 of the formula of the present invention, the modernization of methods for determining the strength of the adhesive characteristics in the bonding zone of a rubber-like coating with a fuel base.

In the sample shown in Fig. 9, bonding with a glass prevents deformation of the fuel (base) in the transverse direction, which makes it possible to realize a type of stress state (σ _and / σ ₀ = 0.20), simulating the bonding zone with the body of the full-scale product (Table) . This is an additional factor ensuring the representativeness of the sample (Fig. 9) for solving the problem of predicting and evaluating the adhesion characteristics in the bonding zone of the fuel cell with the solid propellant body.

The implementation of the proposed method for determining the strength of the adhesive bonding of a rubber-like coating with a base when using samples known from the prior art (Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5 without using a glass) equipped with ring fastening, or similar that can be obtained without departing from the essence and scope of the invention, is carried out in a one-step, and the sample shown in Fig. 5 using a glass, as shown in Fig. 9, in a two-stage scheme.

At the first stage, a fuel base 3 is made with a rigid element 4 and a rubber-like coating 2 glued to it, onto which the studied uncured adhesive composition is preliminarily applied by spraying or brushing. The fuel base is produced by casting (or molding under pressure) the fuel mass into a cylindrical press form, with the possibility of using, if necessary, an insert, for example, from caprolactan, which allows to simultaneously obtain 3 samples. After filling, the mold enters the heat chamber, where the curing process of the molded sample is carried out.

For the manufacture of the sample shown in Fig. 9, it is necessary to implement the second stage, during which the obtained fuel sample is glued with a cylindrical part of a larger base 3 with epoxy or other glue into a rigid element 5 made in the form of a glass, for example, steel, fiberglass, etc. .P. material.

An annular fastening 6 along the boundary of the coating - the rigid element is formed after the curing process of the adhesive connection 1 of the coating 2 with the base 3 (using a mechanical device, for example, a thin knife), or before the curing process is carried out using an annular liner made of a material having release properties to the rigid element 4 and the coating 2, for example, of fluoroplastic or other material, the use of which is obvious to a person skilled in the art.

Moreover, the thickness of the mechanical type device or insert used in accordance with the present need is comparable to the thickness of the adhesive joint.

The tensile test of the fabricated specimen is carried out using standard test devices (for example, supplied by Instron 5655), allowing registration of the tensile load before fracture of the specimen. Quantitative indicators of the strength of the adhesive bond are determined using the relation (1).

The calculations and experiments confirmed the feasibility of the proposed method.

The relevance of the proposed method is due to the ability to identify previously unused resource strength performance and the ability to increase the reliability parameters of rocketry products. For a number of promising energy-intensive materials, the application of the proposed method is the only way to create highly efficient solid propellant rocket motors.

Claims (3)

1. A method for determining the strength of the adhesive joint of a rubber-like coating with a solid rocket fuel base, comprising the use of two rigid elements providing a tensile load, one of which is brought into contact with the coating by means of glue, the adhesion of which to the coating is known to be greater than the adhesion of the studied adhesive joint of the coating to basis, and the second is subjected to interaction with the basis, characterized in that in the edge zone of the adhesive connection of the rubber-like coating with a rigid element Ring loosening is performed, the width of which is Δ = (0.6-1.5) · δ, where δ is the coating thickness. 2. The method according to claim 1, characterized in that the ring fastening is performed mechanically after the curing process of the adhesive bonding of the coating to the substrate or before the curing process is carried out by using an annular liner made of a material having release properties to the rigid element and the coating. 3. The method according to claim 1 or 2, characterized in that when using a base made in the form of a monoblock of two cylinders of different diameters coaxially conjugated with a fillet, the second rigid element interacting with the base is made in the form of a glass and is brought into interaction with the cylindrical part of the base of a larger diameter by adhesive bonding.

Images