CN112710566B - Method for testing critical energy release rate of interface II type crack - Google Patents

Method for testing critical energy release rate of interface II type crack Download PDF

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CN112710566B
CN112710566B CN202011491149.6A CN202011491149A CN112710566B CN 112710566 B CN112710566 B CN 112710566B CN 202011491149 A CN202011491149 A CN 202011491149A CN 112710566 B CN112710566 B CN 112710566B
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displacement
release rate
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energy release
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CN112710566A (en
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黄培彦
李稳
陈展标
郭馨艳
郑小红
杨怡
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South China University of Technology SCUT
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    • G—PHYSICS
    • G01—MEASURING; TESTING
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    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/24—Investigating strength properties of solid materials by application of mechanical stress by applying steady shearing forces
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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    • G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
    • G01N1/36—Embedding or analogous mounting of samples
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N1/00—Sampling; Preparing specimens for investigation
    • G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N1/00—Sampling; Preparing specimens for investigation
    • G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
    • G01N1/36—Embedding or analogous mounting of samples
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    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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Abstract

The invention discloses a critical energy release rate testing method for interface II type cracks, which comprises the steps of preparing a plurality of groups of shear test pieces with different prefabricated interface crack lengths, carrying out a shear experiment on each group of shear test pieces to obtain a load-displacement curve of the shear test pieces, carrying out normalization processing on the load-displacement curve of each group of test pieces, establishing a load-displacement curve equation, determining critical displacement or critical load, calculating the energy release rate of the interface II type cracks according to the definition of the energy release rate, and determining the critical energy release rate. The invention can provide an effective test method for the critical energy release rate of various interface II-type cracks, and lays a foundation for formulating the critical energy release rate test standard/rule of the interface II-type cracks, establishing the fracture criterion of the interface crack problems and evaluating the interface peeling failure behaviors of various composite structures.

Description

Method for testing critical energy release rate of interface II type crack
Technical Field
The invention relates to a fracture mechanics testing technology, in particular to a critical energy release rate testing method for an interface II type crack.
Background
Interfacial peel failure from interfacial type II crack propagation is a typical failure mode that often occurs in composite structures, such as fiber reinforced composite (FRP) Reinforced Concrete (RC) structures, FRP reinforced steel structures. In order to evaluate the reinforcing effect of FRP on RC structural members or steel structural members, the fracture mechanics theory is adopted to evaluate the bonding performance of FRP-concrete/steel structural interface with II-type crack-shaped defects by using destructive behavior, and the method is an effective method.
The energy release rate and the fracture energy during crack propagation can be used as the mechanical property parameters for evaluating the interface peeling damage. At present, the interfacial fracture energy is widely applied to an average bonding strength calculation formula and a bonding-slippage model of various interfaces, but has the following defects: the fracture energy of the interface II type crack is generally directly defined as the area surrounded by the bonding shear stress and the slip amount in the bonding-slip curve, but the discreteness of the bonding-slip curve obtained through experiments is very large, and the bonding-slip model has no unified form, so that the calculation result of the fracture energy is also very discrete. Although numerous scholars have developed various empirical formulas for fracture energy, such as: and respectively taking the concrete compressive strength and the concrete tensile strength as main empirical parameters. However, the results of calculations for different empirical models tend to be quite different.
With regard to the energy release rate of the interface crack, a single-test-piece indirect test method is currently employed. The method is based on
Figure GDA0003264960250000012
And (4) calculating the energy release rate of the II-type interface crack through the bearing capacity of the test piece. But in
Figure GDA0003264960250000013
In the basic assumption of the model, the deformation of the adhesive layer is ignored, and the interface damage occurring in the adhesive layer is obviously inconsistent with the assumption, such as the damage of an FRP-steel plate interface and the damage of an FRP-concrete interface in a high-temperature and high-humidity environment. Furthermore, it is possible to provide a liquid crystal display device,
Figure GDA0003264960250000011
the model only calculates the release rate of the linear elastic strain energy, and does not consider the influence of plastic work. Therefore, the effectiveness of the critical energy release rate of interfacial cracking obtained by this method remains to be examined.
In order to accurately test the critical energy release rate of the interface II type crack, the critical energy release rate of the interface II type crack needs to be directly measured according to the definition of the critical energy release rate, but the test method is not reported yet.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a method for testing the critical energy release rate of an interface II type crack. The method can measure the critical energy release rate of various interface II-type cracks, establish the fracture criterion of the interfaces, evaluate the interface destruction behavior of the composite structure, lay a foundation for establishing the energy release rate test standard/procedure of the interface II-type cracks, and verify the existing experience model of interface fracture.
The purpose of the invention is realized by the following technical scheme: the critical energy release rate testing method for the interface II type crack comprises the following steps:
s1, preparing a plurality of groups of prefabricated interface crack shear test pieces with different lengths;
s2, performing shearing experiments on all shearing test pieces, and obtaining load-displacement curves of all groups of shearing test pieces based on experimental data of the shearing experiments;
s3, carrying out data processing on the experimental data of the shearing experiment, and establishing a corresponding load-displacement curve equation;
s4, carrying out data processing on the experimental data of the shearing experiment, and determining the critical displacement or the critical load of each shearing test piece;
s5, calculating potential energy variation under unit width when the crack in the shear test piece expands according to a load-displacement curve equation to obtain the energy release rate of the interface II type crack by calculation, and obtaining a relation curve between the energy release rate and the load and a relation curve between the energy release rate and the loading displacement;
and S6, determining the corresponding energy release rate according to the critical displacement or the critical load determined by S4 and the relation curve between the energy release rate and the loading displacement or the relation curve between the energy release rate and the load determined by S5, wherein the energy release rate is the critical energy release rate of the interface II type crack.
Preferably, step S1 includes the steps of:
s1-1, designing and manufacturing a concrete test block mold;
s1-2, enabling the threaded steel bars used for loading and centering to penetrate through the through holes at the end part of the concrete test block mould, and fixing the threaded steel bars on the concrete test block mould;
s1-3, pouring the well-stirred concrete into a concrete test block mould, tamping the concrete by using a vibrating rod, curing for a certain time to solidify the concrete in the concrete test block mould into a concrete test block, and wrapping a rod body of the threaded steel bar with the concrete test block;
s1-4, curing the concrete test block according to a curing standard, and polishing the surface of the cured concrete test block to which the continuous fiber sheet is adhered;
s1-5, connecting one end of the twisted steel bar with a fastening cap, closely attaching the fastening cap to the free end face of the concrete test block, and bonding the upper and lower surfaces of the concrete test block with the continuous fiber sheet through bonding glue;
and S1-6, pre-cracking the interface between the continuous fiber sheet and the concrete to prepare shearing test pieces, wherein the pre-cracking length increment of two adjacent groups of test pieces is delta a, and the delta a is a constant.
Preferably, step S2 includes the steps of:
s2-1, spraying speckles on the surface of the continuous fiber sheet, clamping the whole shear test piece by using a clamping device, and connecting the other end of the twisted steel bar with a testing machine through a combined loading head;
s2-2, installing a DIC testing device, wherein a data acquisition part of the DIC testing device is aligned to the surface of the plate coated with speckles;
s2-3, loading the shear test piece by the testing machine in a displacement control mode to complete a shear experiment;
and S2-4, obtaining the load-displacement curve of each shearing test piece based on the recorded data of the test machine control system.
Preferably, in step S2-3, when the shear specimen is loaded by the testing machine in the displacement control mode, the loading speed is 0.005 mm/S.
Preferably, step S3 includes the steps of:
s3-1, deleting data after a crack initiation point of a load-displacement curve of each shear test piece, wherein the load and the displacement corresponding to the initiation point are respectively the critical load and the critical displacement of each shear test piece;
and S3-2, fitting the data of each group of different shearing test pieces by adopting a polynomial to obtain a load-displacement equation.
Preferably, step S4 includes the steps of:
s4-1, determining the critical displacement or critical load of the shear test piece corresponding to each effective shear test piece load-displacement experimental curve according to each effective shear test piece load-displacement experimental curve;
s4-2, critical displacement delta of each test piececAnd the pre-crack length a0Experimental data or critical load F ofcAnd the pre-crack length a0Fitting the experimental data to obtain deltac～a0Equation of the curve or Fc～a0Curve equation to determine critical displacement delta of shearing specimen interfacecOr critical load Fc。
Preferably, in step S5, the potential energy variation δ Π per unit width of the shear specimen during crack propagation is calculated according to the load-displacement curve equation, and the following two conditions are provided:
(1) when the load is not changed:
Figure GDA0003264960250000041
the formula is shown as (1), wherein delta is the displacement difference value corresponding to different pre-crack lengths under the same load condition;
(2) when the displacement is not changed:
Figure GDA0003264960250000042
this is equation (2) where δ F is the difference in load for different pre-crack lengths under the same displacement conditions.
Preferably, the bonding width of the continuous fiber sheet and the concrete is w, the propagation length of the pre-crack after the shearing experiment is finished is δ a, and the relationship between the potential energy variation δ Π and the energy release rate G is as follows:
- δ Π ═ Gw δ a, this being formula (3),
then, the following formula (1) and formula (3) can be used:
Figure GDA0003264960250000043
this is formula (4);
from formulas (2) and (3):
Figure GDA0003264960250000044
this is formula (5);
based on the experimental data in step S2, a relationship curve between the energy release rate G and the load F can be obtained from equation (4), and a relationship curve between the energy release rate G and the load displacement Δ can be obtained from equation (5).
Compared with the prior art, the invention has the following advantages:
(1) the critical energy release rate of the interface II type crack can be directly tested, and unnecessary errors in indirect measurement and calculation results based on hypothesis are avoided;
(2) the method can be suitable for testing the critical energy release rate of the II-type crack in the component interface consisting of any two different materials, and has good universality;
(3) by utilizing the measured critical energy release rate of the II-type crack (the shear fracture toughness of the interface material), the fracture criterion of the interface II-type crack problem can be conveniently established, and the interface destruction behavior of the composite structure can be conveniently evaluated;
(4) the method can lay a foundation for formulating the critical energy release rate test standard/rule of the interface II type crack.
Drawings
Fig. 1 is a schematic view of a concrete test block mold in this embodiment.
Fig. 2 is a schematic diagram of a full-adhesion FRP-concrete interface shear specimen with a prefabricated crack and a loading scheme in this embodiment. The arrow in the figure indicates the direction of the load F.
FIG. 3 is a graph of load versus displacement (F- Δ) for shear specimens of different pre-crack lengths according to this example. In which the experimental curves of load-displacement (F-delta) of the shear specimen of each set of FIGS. 3(a) to 3(d) are shown.
FIG. 4 is a load-displacement (F-Delta) curve equation of four test pieces with different pre-crack lengths in this example.
FIG. 5 shows the critical displacement of the shear specimen in this exampleΔCWith length a of the pre-crack0The relationship between them.
Fig. 6 is a relationship between the potential energy variation of the unit pasting width interface and the length of the pre-crack under different loading displacements in this embodiment.
FIG. 7 is a graph showing the relationship between the energy release rate G and the load displacement Δ of the FRP-concrete interface in this example.
Wherein, 1 is the concrete test block mould, 2 is the bottom plate, 3 is the curb plate, 4 is the end plate, 5 is the baffle, 6 is the moulding chamber, 7 is the through-hole, 8 is the twisted steel, 9 is the concrete test block, 10 is the fastening cap, 11 is the continuous fibers sheet, 12 is the bonding glue, 13 is clamping device.
Detailed Description
The invention is further illustrated by the following figures and examples.
A critical energy release rate test method for interface II type cracks comprises the following steps:
s1, preparing a plurality of groups of shear test pieces with prefabricated interface cracks of different lengths; the step S1 includes the steps of:
s1-1, designing and manufacturing a concrete test block mould as shown in figure 1.
S1-2, enabling the threaded steel bars used for loading and centering to penetrate through the through holes at the end part of the concrete test block mould, and fixing the threaded steel bars on the concrete test block mould; as shown in fig. 1, the concrete test block mold comprises a bottom plate, 2 side plates, 2 end plates and 1 partition plate, wherein the 2 side plates are respectively fixed on two sides of the bottom plate, the 2 end plates are respectively fixed on two ends of the bottom plate, the side plates and the end plates form an accommodating cavity, and the partition plate is installed in the middle of the accommodating cavity to divide the accommodating cavity into 2 molding cavities with equal size; the 2 end plates are also provided with through holes through which the thread steel bars pass.
S1-3, pouring the well-stirred concrete into a concrete test block mould, tamping the concrete by using a vibrating rod, curing for a certain time to solidify the concrete poured in the concrete test block mould into a concrete test block, and wrapping a rod body of the twisted steel by the concrete test block; in order to ensure the solidification effect of the concrete, the vibrating spear is adopted to maintain the concrete for one day, then the concrete test block mould is opened, and the concrete test block with the thread reinforcing steel bar is taken out.
S1-4, curing the concrete test block according to the curing standard, and polishing the surface of the cured concrete test block to which the FRP sheet is adhered; in the step, curing is carried out for 28 days according to curing standards to ensure the mechanical properties such as strength and the like of the concrete test block, then two side surfaces (namely the surfaces adhered to the FRP thin plate) of the concrete test block are polished until coarse aggregates are exposed, and the polished two side surfaces are cleaned by acetone to ensure the adhesion effect.
S1-5, connecting one end of the twisted steel bar with a fastening cap, closely attaching the fastening cap to the free end face of the concrete test block, and bonding the upper and lower surfaces of the concrete test block with the continuous fiber sheet through bonding glue;
and S1-6, pre-cracking the interface between the continuous fiber sheet and the concrete to prepare shearing test pieces, wherein the pre-cracking length increment of two adjacent groups of test pieces is delta a, and the delta a is a constant. Specifically, 2 cracks are prefabricated on the interface between FRP (fiber reinforced plastic) and concrete of 1 concrete test block, and the prefabricated crack length increment of two adjacent test pieces is delta a (delta a is a constant) so as to manufacture a plurality of groups of shearing test pieces with different interface prefabricated crack lengths. In this embodiment, the continuous fiber sheet is an FRP sheet, that is, a carbon fiber sheet (CFL). As shown in fig. 2, CFLs are adhered to the upper and lower surfaces of 2 concrete test pieces to form shear test pieces, and 2 interfacial cracks are preformed at the CFL-concrete interface of the shear test pieces, and the increment of the preformed crack length of two adjacent groups of shear test pieces is Δ a (Δ a is a constant). In the embodiment where the reinforced material (concrete) is a heterogeneous material, Δ a may be 2 times the maximum coarse aggregate diameter in the concrete sample.
Specifically, as shown in fig. 2, the prepared full-adhesion FRP sheet-concrete interface double shear test piece containing type II prefabricated cracks includes: two CFLs, bonding glue, two concrete test blocks, two twisted steel bars and two fastening caps. Wherein each CFL has a length L1430mm and a width w1The calculated thickness was 0.23mm, 100 mm. Length of each concrete block in shear test pieceDegree L2200mm, width w2100mm and 100mm thickness h. The bonding width between the CFL and the concrete test block is w3100mm, namely, full paste. The type of the adhesive is
Figure GDA0003264960250000071
the-131A/B carbon board adhesive is a two-component modified epoxy resin structural adhesive and is produced by Nanjing Haitou composite material, LLC. The length of the twisted steel bar is L2-250 mm, the strength grade is 12.9 grade, and the external diameter is M116 mm. In this embodiment, 4 sets of shear test pieces are used, and the initial pre-crack lengths of the 4 sets of shear test pieces are respectively: a is0＝0mm，a1＝20mm，a2＝40mm，a360mm, all crack widths are w to ensure that the interfacial cracks are through cracks4＝100mm。
S2, performing shearing experiments on all shearing test pieces, and obtaining load-displacement curves of all groups of shearing test pieces based on experimental data of the shearing experiments; step S2 includes the following steps:
s2-1, spraying speckles on the surface of the CFL, clamping the whole shear test piece by using a clamping device, and connecting the other end of the twisted steel bar with a testing machine through a combined loading head; the clamping device is provided with 3 pairs, the clamping device mainly comprises a bolt and a clamping plate, wherein the two pairs are respectively positioned at the end parts of two ends of the shearing test piece, and the other 1 pair is positioned at the free end part of 1 concrete test block without prefabricated interface cracks.
S2-2, installing a DIC testing device, wherein a data acquisition part of the DIC testing device is aligned to the surface of the CFL coated with speckles; and measuring the displacement data of the same section or line in real time by utilizing a DIC testing device to finish the acquisition of the displacement data.
S2-3, loading the shear test piece by the testing machine in a displacement control mode, wherein the loading speed is 0.005mm/S, so as to complete the shear test; the testing machine in the embodiment adopts an electro-hydraulic servo dynamic and static testing machine.
And S2-4, obtaining the load-displacement curve of each shearing test piece based on the recorded data of the test machine control system. As shown in fig. 3. Wherein, 5 are shown in FIG. 3(a)The load-displacement curve of the shear test piece without the prefabricated crack is that the length of the prefabricated crack is 0; FIG. 3(b) is a load-displacement curve of a shear specimen having 2 pre-cracks of 20mm in length; FIG. 3(c) is a load-displacement curve of 2 shear specimens with a pre-crack length of 40 mm; FIG. 3(d) is a load-displacement curve of 3 test pieces having a pre-crack length of 60 mm. As can be seen from fig. 3(a) to 3(d), the load of the shear test pieces of different pre-crack lengths increases with the displacement, and both exhibit a typical brittle failure mode. Moreover, obvious turning sections exist in most of the experimental curves of load-displacement, and the turning points are caused by local peeling of the interface, and the load at the turning points is defined as the initiation load Fin。
The shear test can ensure that the interface crack is in a pure shear stress state and the failure mode of the interface is stripping failure in the test process; meanwhile, the shearing experiment can adopt a displacement control mode and a load control mode, is suitable for a general material testing machine, and has a wide application range.
S3, carrying out data processing on the experimental data of the shearing experiment, and establishing a load-displacement curve equation corresponding to each shearing test piece; step S3 includes the following steps:
s3-1, deleting data after the starting point (or the expansion point) of the load-displacement curve of each shear test piece;
and S3-2, fitting the data of each group of different shearing test pieces by adopting a polynomial to obtain a load-displacement (F-delta) equation.
And (4) deleting the data after cracking and fitting by using the load and displacement data obtained in the step S2 to respectively obtain a fitting equation of the load-displacement curve ascending section of the 4 groups of shearing test pieces:
F0＝-0.829Δ3-0.667Δ2+29.7 Δ, r _ square ═ 0.983, this is formula (6),
F20＝-0.571Δ3+0.388Δ2+25.4 Δ, r _ square ═ 0.998, this is formula (7),
F40＝-2.36Δ3+6.11Δ2+20.9Δ,r_square＝0.990, this is formula (8),
F60＝-0.864Δ3+2.44Δ2+20.4 Δ, r _ square ═ 0.990, this is formula (9),
in formulae (6) to (9), F0～F60And delta respectively represent the load and the displacement of the test piece when the length of the pre-crack of each group of the shearing test piece is 0, 20mm, 40mm and 60 mm. The equation of the load-displacement curve of each group of test pieces is shown in fig. 4. E-mail
S4, carrying out data processing on the experimental data of the shearing experiment, and determining the critical displacement or the critical load of each shearing test piece;
this example determines the critical displacement Δ for an interfacial shear test piecec. As shown in fig. 3, since the critical displacements of the test pieces with different pre-crack lengths are different, in this embodiment, the critical displacement test values of the test pieces with different pre-crack lengths are considered, and linear fitting is performed on the test values, as shown in fig. 5, so that the critical displacement Δ can be obtainedcThe fitted linear equation of (1):
Δc＝2.61+6.10×10-4a0this is represented by the formula (10),
by using the formula (10) and considering that the peeling failure of the FRP-concrete interface belongs to a quasi-brittle failure mode, the displacement when the crack just begins to expand is taken as the calculation critical displacement, namely the intercept between the fitting curve and the ordinate axis in FIG. 5 is taken as the calculation critical displacement, and deltac＝2.61mm。
S5, calculating potential energy variation under unit width when the crack in the shear test piece expands according to a load-displacement curve equation to obtain the energy release rate of the interface II type crack by calculation, and obtaining a relation curve between the energy release rate and the load and a relation curve between the energy release rate and the loading displacement;
in step S5, according to the load-displacement curve equation, the potential energy variation δ Π in unit width when the crack propagates in the shear specimen is calculated, which has the following two conditions:
(1) when the load is not changed:
Figure GDA0003264960250000091
the formula is shown as (1), wherein delta is the displacement difference value corresponding to different pre-crack lengths under the same load condition;
(2) when the displacement is not changed:
Figure GDA0003264960250000092
this is equation (2) where δ F is the difference in load for different pre-crack lengths under the same displacement conditions.
Let the bonding width of CFL and concrete be w, where w is 2w3When the propagation length of the pre-crack after the shearing experiment is finished is δ a, the relationship between the potential energy variation δ Π and the energy release rate G is as follows:
- δ Π ═ Gw δ a, this being formula (3),
then, the following formula (1) and formula (3) can be used:
Figure GDA0003264960250000101
this is formula (4);
from formulas (2) and (3):
Figure GDA0003264960250000102
this is formula (5);
based on the experimental data in step S2, a relationship curve between the energy release rate G and the load F can be obtained from equation (4), and a relationship curve between the energy release rate G and the load displacement Δ can be obtained from equation (5).
Specifically, when the shear test piece with the pre-crack is subjected to a load F, the displacement is delta, and the crack length is a0And the load-displacement curve is OM (FIG. 4), the elastoplastic strain energy is the triangular area SOCMPotential energy is quadrilateral area SOCMQ. Thus, the total potential energy is:
Π(a0)＝-SOQMthis is represented by the formula (11),
when the crack propagates to a0+δa0And then, the total potential energy is:
Π(a0+δa0)＝-SOQRthis is the formula (12)
Since this example is an interface shear experiment performed in the displacement control mode, it is selected to calculate the energy release rate G with the same displacement.
The amount of change in potential energy (corresponding to S in fig. 4) can be obtained from equation (2)OMH)：
Figure GDA0003264960250000103
This is the formula (13)
The sticking width of the concrete interface shearing test piece fully stuck with CFL is 2w1The formula (4) is as follows:
Figure GDA0003264960250000104
this is formula (14).
By using the equation (14), the potential energy variation of the interface under the unit pasting width can be obtained according to the load-displacement curve equation obtained in step S3, and as shown in fig. 6, the slope of the curve in the graph is-G under the corresponding displacement.
The energy release rate G of the interface crack under different displacement Δ is calculated, and the variation relationship of G with displacement Δ, i.e., the G- Δ curve, can be obtained, as shown in fig. 7.
And S6, determining the corresponding energy release rate according to the critical displacement or the critical load determined in the step S4 and the relation curve between the energy release rate and the load or the relation curve between the energy release rate and the loading displacement, wherein the energy release rate is the critical energy release rate of the interface II type crack.
Specifically, the critical displacement Δ determined from fig. 5 and equation (10) obtained in step S4cFrom the G-Delta curve shown in FIG. 7, the energy release rate corresponding thereto, i.e., the critical energy release rate G of the interfacial II type crack, can be determinedC(GC1.10 ± 0.09N/mm), also referred to as the shear fracture toughness of the interface material.
The above-mentioned embodiments are preferred embodiments of the present invention, and the present invention is not limited thereto, and any other modifications or equivalent substitutions that do not depart from the technical spirit of the present invention are included in the scope of the present invention.

Claims (6)

1. A critical energy release rate testing method for an interface II type crack is characterized by comprising the following steps:
s1, preparing a plurality of groups of shear test pieces with different lengths of prefabricated interface cracks;
s2, performing shearing experiments on all shearing test pieces, and obtaining load-displacement curves of all groups of shearing test pieces based on experimental data of the shearing experiments;
s3, carrying out data processing on the experimental data of the shearing experiment, and establishing a corresponding load-displacement curve equation;
s4, carrying out data processing on the experimental data of the shearing experiment, and determining the critical displacement or the critical load of each shearing test piece;
s5, calculating potential energy variation under unit width when the crack in the shear test piece expands according to a load-displacement curve equation to obtain the energy release rate of the interface II type crack by calculation, and obtaining a relation curve between the energy release rate and the load and a relation curve between the energy release rate and the loading displacement;
s6, determining a corresponding energy release rate according to the critical displacement or the critical load determined by S4 and the relation curve between the energy release rate and the loading displacement or the relation curve between the energy release rate and the load determined by S5, wherein the energy release rate is the critical energy release rate of the interface II type crack;
in step S5, according to the load-displacement curve equation, the potential energy variation δ Π in unit width when the crack propagates in the shear specimen is calculated, which has the following two conditions:
(1) when the load is not changed:
Figure FDA0003264960240000011
the formula is shown as (1), wherein delta is the displacement difference value corresponding to different pre-crack lengths under the same load condition;
(2) when the displacement is not changed:
Figure FDA0003264960240000012
the formula is shown as (2), wherein deltaF is the load difference value corresponding to different pre-crack lengths under the same displacement condition;
let the bonding width of the continuous fiber sheet and the concrete be w, and the propagation length of the pre-fabricated crack after the shearing experiment be δ a, the relationship between the potential energy variation δ Π and the energy release rate G is:
- δ Π ═ Gw δ a, this being formula (3),
then, the following formula (1) and formula (3) can be used:
Figure FDA0003264960240000021
this is formula (4);
from formulas (2) and (3):
Figure FDA0003264960240000022
this is formula (5);
based on the experimental data in step S2, a relationship curve between the energy release rate G and the load F can be obtained from equation (4), and a relationship curve between the energy release rate G and the load displacement Δ can be obtained from equation (5).
2. The critical energy release rate test method for interfacial type II cracks of claim 1, wherein step S1 comprises the steps of:
s1-1, designing and manufacturing a concrete test block mold;
s1-2, enabling the threaded steel bars used for loading and centering to penetrate through the through holes at the end part of the concrete test block mould, and fixing the threaded steel bars on the concrete test block mould;
s1-3, pouring the well-stirred concrete into a concrete test block mould, tamping the concrete by using a vibrating rod, curing for a certain time to solidify the concrete in the concrete test block mould into a concrete test block, and wrapping a rod body of the threaded steel bar with the concrete test block;
s1-4, curing the concrete test block according to a curing standard, and polishing the surface of the cured concrete test block to which the continuous fiber sheet is adhered;
s1-5, connecting one end of the twisted steel bar with a fastening cap, closely attaching the fastening cap to the free end face of the concrete test block, and bonding the upper and lower surfaces of the concrete test block with the continuous fiber sheet through bonding glue;
and S1-6, pre-cracking the interface between the continuous fiber sheet and the concrete to prepare shearing test pieces, wherein the pre-cracking length increment of two adjacent groups of test pieces is delta a, and the delta a is a constant.
3. The critical energy release rate test method for interfacial type II cracks of claim 1, wherein step S2 comprises the steps of:
s2-1, spraying speckles on the surface of the continuous fiber sheet, clamping the whole shear test piece by using a clamping device, and connecting the other end of the twisted steel bar with a testing machine through a combined loading head;
s2-2, installing a DIC testing device, wherein a data acquisition part of the DIC testing device is aligned to the surface of the plate coated with speckles;
s2-3, loading the shear test piece by the testing machine in a displacement control mode to complete a shear experiment;
and S2-4, obtaining the load-displacement curve of each shearing test piece based on the recorded data of the test machine control system.
4. The critical energy release rate test method for interfacial type II cracks of claim 3, wherein: in step S2-3, when the test machine adopts the displacement control mode to load the shearing test piece, the loading speed is 0.005 mm/S.
5. The critical energy release rate test method for interfacial type II cracks of claim 1, wherein: step S3 includes the following steps:
s3-1, deleting data after a crack initiation point of a load-displacement curve of each shear test piece, wherein the load and the displacement corresponding to the initiation point are respectively the critical load and the critical displacement of each shear test piece;
and S3-2, fitting the data of each group of different shearing test pieces by adopting a polynomial to obtain a load-displacement equation.
6. The critical energy release rate test method for interfacial type II cracks of claim 1, wherein: step S4 includes the following steps:
s4-1, determining the critical displacement or critical load of the shear test piece corresponding to each effective shear test piece load-displacement experimental curve according to each effective shear test piece load-displacement experimental curve;
s4-2, critical displacement delta of each shearing test piececAnd the pre-crack length a0Experimental data or critical load F ofcAnd the pre-crack length a0Fitting the experimental data to obtain deltac～a0Equation of the curve or Fc～a0Curve equation to determine critical displacement delta of shearing specimen interfacecOr critical load Fc。
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