An accelerated method for determining the long-term tensile strength of reconstructed composite structural wood

Abstract

This invention discloses an accelerated method for determining the long-term tensile strength of reconstituted composite timber, relating to the field of green wood building materials technology. The method includes: determining the long-term tensile mechanical properties of the reconstituted composite timber to be tested; applying a stepped loading method to the tensile specimen; and obtaining the strength ratio q of the tensile specimen. i And the failure time, and convert the failure time into the failure time T under the first-order step stress. i The transformed destruction time T i As an independent variable, the intensity ratio q i Regression analysis was performed to obtain the fitted parameter values ​​A and B: q i =A+Blog(T) i Based on the fitted parameter values ​​A and B, and the results of short-term mechanical tests, the long-term tensile strength of reconstituted composite timber is calculated. This method can significantly shorten the determination period for the long-term tensile strength of reconstituted composite timber, accelerating its transformation from non-structural applications such as walkways and furniture to structural applications in building construction.

Description

Technical Field

[0001] The invention relates to the technical field of green wood building materials, in particular to an accelerated determination method for the long-term tensile strength of reconstituted composite structural wood. Background Art

[0002] Reconstructed composite structural timber is a new type of wood composite material formed by rolling, debonding, drying, impregnation, cold pressing and heat curing of peeled veneer. It has good dimensional stability and mechanical strength and has broad application prospects in the field of green building materials. As a biomass material, under long-term load, internal damage continues to accumulate, resulting in a gradual decrease in strength with the increase in load time. Determining the long-term strength of reconstructed composite structural timber is a prerequisite for its application in the field of building structures. However, there is currently a lack of methods for determining the long-term strength of reconstructed composite structural timber, and it is impossible to calculate and evaluate the strength of reconstructed composite structural timber components of different sizes, resulting in its current main application in non-structural fields such as plank roads and furniture, with low added value. Proposing a method for determining the long-term strength of reconstructed composite structural timber is of great significance for accurately and quantitatively evaluating the strength of reconstructed composite structural timber and promoting the application of reconstructed composite structural timber in the field of building structures.

[0003] According to the search of relevant literature and materials, there is currently no method for determining the long-term tensile strength of reconstructed composite structural wood. Document 1 Invention Patent CN109696355B discloses a method for determining the long-term tensile strength of reconstructed composite structural wood. First, reconstructed composite structural bamboo tensile specimens for short-term mechanical tests and long-term mechanical tests are processed according to the sample matching method; the stress level to be applied in the long-term mechanical test is determined through the short-term mechanical test; the failure time of the bamboo tensile specimen is obtained through the long-term mechanical test; the strength ratio of the reconstructed composite structural bamboo tensile specimen is used as an independent variable, and the failure time of the reconstructed composite structural bamboo tensile specimen is subjected to regression analysis to obtain the fitting parameter value; according to the actual designed service life of the reconstructed composite structural bamboo component, the long-term tensile strength of the reconstructed composite structural bamboo corresponding to the service life is calculated.

[0004] The inventors used this method to study the long-term tensile strength of reconstructed bamboo. According to the public document 2 (Construction and Building Materials, 2022, Issue 338, Article No. 127659, "Long-term mechanical properties of bamboo scrimber"), the inventors conducted two groups of long-term tensile tests, each with 48 specimens. After 15 months of long-term loading, 29 and 31 specimens were destroyed, respectively, and the long-term tensile strength of the reconstructed bamboo was determined accordingly.

[0005] The shortcomings of the above technologies are mainly manifested in:

[0006] (1) The test time is long and uncontrollable. Due to the variability of material strength, such as the tensile strength of reconstructed bamboo in Reference 2, which is between 80MPa and 180MPa, the test specimens under long-term loads vary greatly. Generally speaking, the test needs to be loaded for more than one year, which is a very long time. In addition, in order to obtain sufficient data to evaluate the long-term strength of the material, the test can only be completed after enough specimens are destroyed after loading, and the specimen destruction time is unknown, so the test time is uncontrollable.

[0007] (2) Low data utilization efficiency. In a test, only some specimens fail during long-term loading. For example, in Reference 2, only about 60% of the specimens fail, and nearly 40% of the specimens fail despite being loaded for 15 months. Therefore, they do not provide useful information for determining long-term tensile strength. Summary of the invention

[0008] In view of this, an embodiment of the present invention provides an accelerated determination method for the long-term tensile strength of reconstructed composite structural timber, so as to significantly shorten the determination period of the long-term tensile strength of the reconstructed composite structural timber, and accelerate the transformation of the reconstructed composite structural timber from non-structural application fields such as plank roads and furniture to building structure application fields.

[0009] A method for accelerating the determination of the long-term tensile strength of reconstructed composite structural wood, comprising:

[0010] Step 1: Process the reconstructed composite structural wood to be tested into 2n tensile specimens, use n tensile specimens among the 2n specimens for short-term mechanical tests, and use the other n tensile specimens among the 2n specimens for long-term mechanical tests;

[0011] Step 2: Determine the stress that should be applied to the reconstructed composite structural wood in the long-term mechanical test through short-term mechanical test;

[0012] Step 3: Determine the long-term load tensile mechanical properties of the reconstructed composite structural wood through long-term mechanical tests, wherein the tensile specimen is subjected to step loading to obtain the strength ratio q of the tensile specimen. i and failure time, and convert the failure time into stress as the failure time T under the first level step stress i ;

[0013] Step 4: Convert the destruction time T in step 3 i As an independent variable, the intensity ratio q in step 3 i Perform regression analysis to obtain fitting parameter values ​​A and B: q i =A+Blog(T i );

[0014] Step 5: According to the fitting parameter values ​​A and B and the short-term mechanical test results in step 2, the long-term tensile strength of the reconstructed composite structural timber is calculated.

[0015] Furthermore, in step 1, n is an even number not less than 40;

[0016] And / or, in step 1, each tensile test piece is in the shape of a dumbbell, and the cross-sectional dimensions of two ends of the dumbbell-shaped tensile test piece are larger than the central cross-sectional dimension of the tensile test piece.

[0017] Furthermore, the step 2 comprises:

[0018] Step 21: Test the n tensile specimens used for the short-term mechanical test in step 1 respectively. The tensile specimens are destroyed within 3-5 minutes. The load at the time of destruction of the tensile specimens is recorded. According to the load at the time of destruction, the short-term strength of all tensile specimens is calculated respectively. The average value of the short-term strength of all tensile specimens is recorded as m. The short-term strength of all tensile specimens is sorted from small to large, and the sorted short-term strength is recorded as f. i , will be related to the short-term strength f i The corresponding cumulative damage frequency is recorded as C i , C i =(i+1) / (n+1);

[0019] Step 22: Based on the short-term strength average value m obtained in step 21, m / N is used as the step stress S to be applied when conducting a long-term mechanical test on the reconstructed composite structural wood to be tested, where S=m / N.

[0020] Furthermore, the step 3 comprises:

[0021] Step 31: According to step 22, the loads to be applied step by step during the long-term mechanical test are obtained, and the long-term mechanical test is performed on the other n tensile test pieces of step 1;

[0022] Step 32: Calculate the strength ratio q of the remaining undamaged tensile specimens except the tensile specimens damaged during the first step stress S loading process in step 31 i ;

[0023] Step 33: Use the damage accumulation-based failure time equivalent method to convert the failure time under multi-level loads into the failure time T under stress S i .

[0024] Further, the step 31 includes:

[0025] The first step stress S was applied to all specimens. After loading was completed, the load was maintained for a time of D. The number of specimens destroyed during the loading process g and the destruction time of the destroyed specimens during the loading and holding stage of the first step stress S were recorded; the undestroyed specimens were loaded with the second step stress 2S. After loading was completed, the load was maintained for a time of D. The destruction time of the destroyed specimens during the loading and holding stage was recorded; then the third step stress 3S was applied to the undestroyed specimens. After loading was completed, the load was maintained for a time of D. The destruction time of the destroyed specimens during the loading and holding stage was recorded. The above process was repeated until all specimens were destroyed. The destruction time of the specimens under the first step stress was recorded as T 1i , the failure time of the specimens under the second level step stress is T 2i , the failure time of the specimens under the third level step stress is T 3i , and so on.

[0026] Further, the step 32 includes:

[0027] For the specimens that failed during the first step stress maintenance period, q i =S / f i , for the specimens that failed during the second step stress maintenance period, q i =2S / f i , for the specimens that failed during the third step stress maintenance period, q i =3S / f i , and so on.

[0028] Further, the step 33 includes:

[0029] For the first step load, the failure time T i =T 1i , for the second level step load, the failure time Similarly, for the pth step load, the failure time And so on.

[0030] Further, the step 33 includes:

[0031] The value of s is based on the strength ratio q of the specimen under the first step stress. i and the destruction time T 1i , press T 1i =e sqi Regression analysis was performed to determine.

[0032] Further, the step 5 comprises:

[0033] Set a design service life L of the reconstructed composite structural wood, use the fitting parameter values ​​A and B to calculate the long-term tensile strength ratio Q of the reconstructed composite structural wood corresponding to the design service life L: Q = A + Blog (L), and then according to the average value m of the short-term strength of all tensile specimens obtained in step 21, the long-term tensile strength f of the reconstructed composite structural wood is obtained. L , f L =m×Q.

[0034] The embodiment of the present invention provides an accelerated determination method for the long-term tensile strength of a reconstructed composite structural timber. The reconstructed composite structural timber to be tested is processed into 2n tensile test pieces, n tensile test pieces among the 2n test pieces are used for short-term mechanical tests, and the other n tensile test pieces among the 2n test pieces are used for long-term mechanical tests; the stress to be applied when the reconstructed composite structural timber to be tested is subjected to a long-term mechanical test is determined through the short-term mechanical test; the long-term load tensile mechanical properties of the reconstructed composite structural timber to be tested are determined through the long-term mechanical test, wherein the tensile test piece is subjected to step loading to obtain a strength ratio q of the tensile test piece. i and failure time, and convert the failure time into stress as the failure time T under the first level step stress i ; The destruction time T after conversion in the above step i As an independent variable, the intensity ratio q in the above steps i Perform regression analysis to obtain fitting parameter values ​​A and B: q i =A+Blog(T i ); According to the fitting parameter values ​​A, B, and the short-term mechanical test results in the above steps, the long-term tensile strength of the reconstructed composite structural timber is calculated. Compared with the prior art, the accelerated determination method of the long-term tensile strength of the reconstructed composite structural timber provided by the embodiment of the present invention can significantly shorten the determination period of the long-term tensile strength of the reconstructed composite structural timber, and accelerate the transformation of the reconstructed composite structural timber from non-structural application fields such as plank roads and furniture to building structure application fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0036] Figure 1 It is a schematic diagram of the process of the present invention;

[0037] Figure 2Schematic diagram of the structure of the reconstructed composite structural wood tensile test piece used in the method of the present invention, wherein (a) is a front view, (b) is a view of (a) along the AA direction, and (c) is a view of (a) along the BB direction;

[0038] Figure 3 It is a schematic diagram of the cumulative failure frequency of the reconstructed composite structural wood tensile test piece involved in the method of the present invention. DETAILED DESCRIPTION

[0039] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0040] It should be clear that the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] The embodiment of the present invention provides a method for accelerating the determination of the long-term tensile strength of a reconstructed composite structural wood. Figure 1 As shown, including:

[0042] Step 1: Process the reconstructed composite structural wood to be tested into 2n tensile specimens, use n tensile specimens among the 2n specimens for short-term mechanical tests, and use the other n tensile specimens among the 2n specimens for long-term mechanical tests;

[0043] As an optional embodiment, in step 1, n is an even number not less than 40;

[0044] As another optional embodiment, in step 1, each tensile test piece is in the shape of a dumbbell, and the cross-sectional dimensions of two ends of the dumbbell-shaped tensile test piece are larger than the central cross-sectional dimension of the tensile test piece.

[0045] Step 2: Determine the stress that should be applied to the reconstructed composite structural wood in the long-term mechanical test through short-term mechanical test;

[0046] As an optional embodiment, the step 2 includes:

[0047] Step 21: Test the n tensile specimens used for the short-term mechanical test in step 1 respectively. The tensile specimens are destroyed within 3-5 minutes. The load at the time of destruction of the tensile specimens is recorded. According to the load at the time of destruction, the short-term strength of all tensile specimens is calculated respectively. The average value of the short-term strength of all tensile specimens is recorded as m. The short-term strength of all tensile specimens is sorted from small to large, and the sorted short-term strength is recorded as f. i , i = 1, 2…, n, n is an integer, which will be related to the short-term intensity f i The corresponding cumulative damage frequency is recorded as C i, i=1,2…,n,C i =(i+1) / (n+1);

[0048] Step 22: Based on the short-term strength average value m obtained in step 21, m / N is used as the step stress S to be applied when conducting a long-term mechanical test on the reconstructed composite structural wood to be tested, where S=m / N.

[0049] In this step, the value of N is usually 2, 3 or 5, corresponding to the average tensile strength being achieved through 2, 3 or 5 steps of loading, respectively.

[0050] Step 3: Determine the long-term load tensile mechanical properties of the reconstructed composite structural wood through long-term mechanical tests, wherein the tensile specimen is subjected to step loading to obtain the strength ratio q of the tensile specimen. i and failure time, and convert the failure time into stress as the failure time T under the first level step stress i ;

[0051] As an optional embodiment, step 3 includes:

[0052] Step 31: According to step 22, the loads to be applied step by step during the long-term mechanical test are obtained, and the long-term mechanical test is performed on the other n tensile test pieces of step 1;

[0053] Further, the step 31 includes:

[0054] First, the first-level step stress S is applied to all specimens. After loading is completed, the load is maintained for a time of D. The number of specimens g destroyed during the loading process and the destruction time of the destroyed specimens during the loading and holding stage of the first-level step stress S are recorded; then, the undestroyed specimens are loaded with the second-level step stress 2S. After loading is completed, the load is maintained for a time of D. The destruction time of the destroyed specimens during the loading and holding stage is recorded; then, the third-level step stress 3S is applied to the undestroyed specimens. After loading is completed, the load is maintained for a time of D. The destruction time of the destroyed specimens during the loading and holding stage is recorded. The above process is repeated until all specimens are destroyed. The destruction time of the specimens under the first-level step stress is recorded as T 1i , the failure time of the specimens under the second level step stress is T 2i , the failure time of the specimens under the third level step stress is T 3i , and so on, i=1, 2,…, n.

[0055] Step 32: Calculate the strength ratio q of the remaining undamaged tensile specimens except the tensile specimens damaged during the first step stress S loading process in step 31 i ;

[0056] In this step, the number of remaining undamaged tensile specimens is ng.

[0057] Further, the step 32 includes:

[0058] For the specimens that failed during the first step stress maintenance period, q i =S / f i , for the specimens that failed during the second step stress maintenance period, q i =2S / f i , for the specimens that failed during the third step stress maintenance period, q i =3S / f i , and so on.

[0059] Step 33: Use the damage accumulation-based failure time equivalent method to convert the failure time under multi-level loads into the failure time T under stress S i .

[0060] In the present invention, after adopting step loading, the failure time of the specimen can be accelerated, thereby shortening the long-term time, but the difficulty lies in how to process the test data obtained after step loading. To evaluate the long-term tensile strength of the material, it is necessary to have statistical data on the failure time of the specimen under a certain constant load level, and what is obtained under step loading / load is the statistical data on the failure time of the specimen under step loading. At this time, it is necessary to equate the failure time under different step loadings to the failure time under a certain constant loading level (usually the first-order loading) in order to determine the law of change of the strength of the specimen over time, that is, how to equate the failure time under a high load level to the failure time under the first-order stress (that is, a low load level). For this reason, this step proposes a failure time equivalence method based on damage accumulation, so that step loading can be applied to the long-term tensile strength determination of reconstructed composite structural wood. The specific method is as follows:

[0061] Under the long-term action of the load, the specimen is damaged. The damage factor α is defined to describe the degree of damage to the specimen, and the value range is 0-1. Under the continuous action of the load, the damage factor gradually accumulates. When the damage factor increases to 1, the specimen is destroyed. From t0 to t, the specimen is subjected to a strength ratio of q i When the constant load is applied, the increase of the damage factor of the specimen is:

[0062]

[0063] Where s is the material parameter, which needs to be fitted according to the test results.

[0064] After loading to p-level step load, the stress level is pS, time T pi At this moment, the damage factor of the specimen is:

[0065]

[0066] If the specimen is subjected to the first step stress Load until T i At the moment, the specimen reaches the same damage, and the damage factor of the specimen is:

[0067]

[0068] Let the damage factor α(T pi ) and the damage factor α(T i ) is the same, the failure time T under multi-level step load can be established pi and the failure time T under the first step load i The equivalent relationship is:

[0069]

[0070] According to the above method, the failure time under multi-level load is converted into the failure time under stress S. For the first level of step load, the failure time of the specimen is T i =T 1i ; For the second level step load, the failure time Similarly, for the pth step load, the failure time And so on.

[0071] The value of s can be calculated based on the strength ratio q of the specimen under the first step stress. i and the destruction time T 1i , press T 1i =e sqi Regression analysis was performed to determine.

[0072] Step 4: Convert the destruction time T in step 3 i As an independent variable, the intensity ratio q in step 3 i Perform regression analysis to obtain fitting parameter values ​​A and B: q i =A+Blog(T i );

[0073] Step 5: According to the fitting parameter values ​​A and B and the short-term mechanical test results in step 2, the long-term tensile strength of the reconstructed composite structural timber is calculated.

[0074] Further, the step 5 comprises:

[0075] Set a design service life L of the reconstructed composite structural wood, use the fitting parameter values ​​A and B to calculate the long-term tensile strength ratio Q of the reconstructed composite structural wood corresponding to the design service life L: Q = A + Blog (L), and then according to the average value m of the short-term strength of all tensile specimens obtained in step 21, the long-term tensile strength f of the reconstructed composite structural wood is obtained. L , f L =m×Q.

[0076] The embodiment of the present invention provides an accelerated determination method for the long-term tensile strength of a reconstructed composite structural timber. The reconstructed composite structural timber to be tested is processed into 2n tensile test pieces, n tensile test pieces among the 2n test pieces are used for short-term mechanical tests, and the other n tensile test pieces among the 2n test pieces are used for long-term mechanical tests; the stress to be applied when the reconstructed composite structural timber to be tested is subjected to a long-term mechanical test is determined through the short-term mechanical test; the long-term load tensile mechanical properties of the reconstructed composite structural timber to be tested are determined through the long-term mechanical test, wherein the tensile test piece is subjected to step loading to obtain a strength ratio q of the tensile test piece. i and failure time, and convert the failure time into stress as the failure time T under the first level step stress i ; The destruction time T after conversion in the above step i As an independent variable, the intensity ratio q in the above steps i Perform regression analysis to obtain fitting parameter values ​​A and B: q i =A+Blog(T i ); According to the fitting parameter values ​​A, B, and the short-term mechanical test results in the above steps, the long-term tensile strength of the reconstructed composite structural timber is calculated. Compared with the prior art, the accelerated determination method of the long-term tensile strength of the reconstructed composite structural timber provided by the embodiment of the present invention can significantly shorten the determination period of the long-term tensile strength of the reconstructed composite structural timber, and accelerate the transformation of the reconstructed composite structural timber from non-structural application fields such as plank roads and furniture to building structure application fields.

[0077] The technical solution of the present invention is described in detail below with reference to specific examples.

[0078] This example includes the following steps:

[0079] (1) The reconstructed composite structural wood to be tested was processed into 144 tensile test pieces, each of which was in the shape of a dumbbell (see Figure 2 11), the cross-sections of the two ends of the dumbbell-shaped tensile specimen (see Figure 2 The size of the middle label number 12 is 50mm×20mm, and the center cross section (see Figure 2The size of the specimens is 5 mm × 20 mm. All specimens are divided into 3 groups, each with 48 specimens. Group A is used for short-term mechanical tests; Group B is used for long-term mechanical tests using the method proposed in the present invention to accelerate the determination of the long-term tensile strength of reconstructed composite structural wood; Group C uses the traditional constant loading method for long-term tensile performance tests.

[0080] (2) Through short-term mechanical tests, determine the stress that should be applied to the reconstructed composite structural wood during long-term mechanical tests. The specific process is as follows:

[0081] (2-1) The 48 tensile specimens in group A used for the short-term mechanical test are tested separately. The tensile specimens are destroyed in 3-5 minutes. The load when the tensile specimens are destroyed is recorded. The short-term strength of all tensile specimens is calculated according to the load when they are destroyed. The average value of the short-term strength of all tensile specimens is recorded as m, m=85.57MPa. The short-term strength of all tensile specimens is sorted from small to large, and the sorted short-term strength is recorded as f i , i = 1, 2…, n. will be related to the short-term strength f i The corresponding cumulative damage frequency is recorded as C i , i=1,2…,n,C i =(i+1) / (n+1);

[0082] (2-2) For the specimens in group B, based on the short-term strength average value m obtained above, m / N is used as the step stress S that should be applied when conducting long-term mechanical tests on the reconstructed composite structural timber to be tested, S = m / N; N is taken as 3, and S is taken as 28.52 MPa.

[0083] (3) The long-term load tensile mechanical properties of the reconstructed composite structural timber of group B are determined through long-term mechanical tests. The specific process is as follows:

[0084] (3-1) First, a first-level step stress of 28.52MPa was applied to all specimens. After loading was completed, the load was maintained for a time of D, D = 7 days. Two specimens were destroyed during the first-level step stress S maintenance stage. Then, the undestroyed specimens were loaded with a second-level step stress of 56.48MPa. After loading was completed, the load was maintained for 7 days. Four specimens were destroyed during the second-level step stress S maintenance stage. The above process was repeated. Four specimens were destroyed under the third-level step stress of 85.57MPa, 14 specimens were destroyed under the fourth-level step stress of 113.80MPa, 16 specimens were destroyed under the fifth-level step stress of 142.05MPa, and 8 specimens were destroyed under the sixth-level step stress of 171.14MPa. Record the destruction time T of each specimen. pi The last specimen was destroyed at 39.5 days, that is, the test period was 39.5 days.

[0085] (3-2) Calculate the strength ratio q of each specimen according to the sequence number corresponding to the short-term test. i , that is, the ratio of the step stress at the final failure of the specimen to the corresponding short-term tensile strength.

[0086] (3-3) According to the strength ratio q of the specimen under the first step stress i and the destruction time T 1i , press T 1i =e sqi Regression analysis was performed to determine that the s value was 5.76. Then, the failure time T of the specimen at each step stress level was pi according to Transformed into the failure time T under the first step stress i .

[0087] (4) The destruction time T after conversion in the above step i As an independent variable, the intensity ratio q in the above steps i Perform regression analysis to obtain fitting parameter values ​​A and B: q i =A+Blog(T i ); fitting parameter values ​​A = 0.5615, B = -0.0599.

[0088] (5) According to the actual design life of the reconstructed composite structural timber L = 50, it is substituted into the regression equation in the above step to obtain the strength ratio q = 0.306 corresponding to the design life L = 50 years. Then, according to the average value of the short-term strength of all tensile specimens obtained in the above step of 85.57 MPa, the long-term tensile strength f of the reconstructed composite structural timber is obtained. L , f L =m×q=85.57×0.306=26.18MPa.

[0089] (6) For the tensile specimens of group C, a constant load of 40% of the short-term strength was applied to test the long-term tensile properties of the reconstructed composite structural timber. The applied stress level was S = 34.23 MPa.

[0090] (7) The long-term load tensile mechanical properties of the C group reconstructed composite structural timber are determined through long-term mechanical tests. The specific process is as follows:

[0091] (7-1) A long-term mechanical test was conducted on the 48 tensile specimens in group C under constant load. After 429.1 days, the 24th specimen was damaged by tension and the test ended, i.e., the test period was 429.1 days. The damage time of the 24 tensile specimens was obtained, and the damage time of the n tensile specimens was sorted from small to large. The damage time after sorting from small to large was recorded as Ti , i=1,2…,n;

[0092] (7-2) From the above T i The total number of tensile specimens with a failure time of less than 5 minutes is 2, and the failure time T is calculated. i The strength ratio q of 22 tensile test pieces with a duration greater than 5 minutes i ,q i =S / f i , denoted as q i (i=3, ..., 24);

[0093] (8) Using the destruction time T in the above step i (i=3, ..., 24) are independent variables, and the intensity ratio q in the above steps is i (i=3, ..., 24) to perform regression analysis and obtain the fitting parameter values ​​A and B: q i =A+Blog(T i ); fitting parameter values ​​A = 0.5728, B = -0.0603;

[0094] (9) According to the actual design life of the reconstructed composite structural timber L = 50, it is substituted into the regression equation in the above step to obtain the strength ratio q = 0.316 corresponding to the design life L = 50 years. Then, according to the average value of the short-term strength of all tensile specimens obtained in the above step of 85.57 MPa, the long-term tensile strength f of the reconstructed composite structural timber is obtained. L , f L =m×q=85.57×0.316=27.04MPa.

[0095] According to the above embodiment, the 50-year long-term tensile strength of the reconstructed composite structural timber measured by the present invention is 26.18 MPa, and the 50-year long-term tensile strength of the reconstructed composite structural timber measured by the conventional constant loading method is 27.04 MPa, and the difference between the two is 3.3%, that is, the measurement results of the two are basically the same, indicating that the present invention can be used as an alternative to the conventional solution (see Figure 3 ). However, the accelerated determination method of long-term tensile strength proposed by the present invention has a test period of 39.5 days, while the test period of the long-term tensile performance test using the traditional constant loading method is 429.1 days. The present invention can shorten the test period to about 10% of the traditional method. At the same time, all 50 specimens in the present invention were tensilely damaged, while only 24 specimens were tensilely damaged in the long-term tensile performance test using the traditional constant loading method. The data utilization efficiency of the present invention is about twice that of the traditional method.

[0096] In summary, the method for accelerating the determination of the long-term tensile strength of reconstructed composite structural timber of the present invention has the following advantages:

[0097] (1) The test time is significantly reduced, and the test cycle is controllable. Due to the use of step loading, each step increases the load S = m / N, and the duration of each load is D. Generally speaking, when the applied load reaches about 2 times the average value, all specimens will be tensilely damaged. Therefore, the entire test cycle is generally about 2ND. The entire test cycle can be controlled by controlling the values ​​of N and D. For example, if N is 3 and D is one week, the entire test cycle can be completed in about 6 weeks.

[0098] (2) Data utilization efficiency is significantly improved. The long-term test will end after all specimens are loaded and damaged in tension. In addition to the g specimens that were damaged during the first-level step stress S loading process, a total of (ng) groups of strength ratios q i and the destruction specimen T i Data for determining the long-term tensile strength of reconstituted composite structural timber.

[0099] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A method for accelerating the determination of the long-term tensile strength of reconstructed composite structural wood, characterized in that: include: Step 1: Process the reconstructed composite structural wood to be tested into 2n tensile specimens, use n tensile specimens among the 2n specimens for short-term mechanical tests, and use the other n tensile specimens among the 2n specimens for long-term mechanical tests; Step 2: Determine the stress that should be applied to the reconstructed composite structural wood in the long-term mechanical test through short-term mechanical test; Step 3: Determine the long-term load tensile mechanical properties of the reconstructed composite structural wood through long-term mechanical tests, wherein the tensile specimen is subjected to step loading to obtain the strength ratio q of the tensile specimen. i and failure time, and convert the failure time into stress as the failure time T under the first level step stress i ; Step 4: Convert the destruction time T in step 3 i As an independent variable, the intensity ratio q in step 3 i Perform regression analysis to obtain fitting parameter values ​​A and B: q i =A+Blog(T i ); Step 5: Calculate the long-term tensile strength of the reconstructed composite structural wood according to the fitting parameter values ​​A and B and the short-term mechanical test results in step 2; Wherein, the step 2 comprises: Step 21: Test the n tensile specimens used for the short-term mechanical test in step 1 respectively. The tensile specimens are destroyed within 3-5 minutes. The load at the time of destruction of the tensile specimens is recorded. According to the load at the time of destruction, the short-term strength of all tensile specimens is calculated respectively. The average value of the short-term strength of all tensile specimens is recorded as m. The short-term strength of all tensile specimens is sorted from small to large, and the sorted short-term strength is recorded as f. i , will be related to the short-term strength f i The corresponding cumulative damage frequency is recorded as C i , C i =(i+1) / (n+1), i=1, 2…, n, n is an integer; Step 22: Based on the short-term strength average m obtained in step 21, m / N is used as the step stress S to be applied when conducting a long-term mechanical test on the reconstructed composite structural wood to be tested, where S=m / N; Wherein, the step 3 comprises: Step 31: According to step 22, the loads to be applied step by step during the long-term mechanical test are obtained, and the long-term mechanical test is performed on the other n tensile test pieces of step 1; Step 32: Calculate the strength ratio q of the remaining undamaged tensile specimens except the tensile specimens damaged during the first step stress S loading process in step 31 i ; Step 33: Use the damage accumulation-based failure time equivalent method to convert the failure time under multi-level loads into the failure time T under stress S i ; Wherein, the step 31 comprises: The first step stress S was applied to all specimens. After loading was completed, the load was maintained for a time of D. The number of specimens destroyed during the loading process g and the destruction time of the destroyed specimens during the loading and holding stage of the first step stress S were recorded; the undestroyed specimens were loaded with the second step stress 2S. After loading was completed, the load was maintained for a time of D. The destruction time of the destroyed specimens during the loading and holding stage was recorded; then the third step stress 3S was applied to the undestroyed specimens. After loading was completed, the load was maintained for a time of D. The destruction time of the destroyed specimens during the loading and holding stage was recorded. The above process was repeated until all specimens were destroyed. The destruction time of the specimens under the first step stress was recorded as T 1i , the failure time of the specimens under the second level step stress is T 2i , the failure time of the specimens under the third level step stress is T 3i , and so on; Wherein, the step 32 comprises: For the specimens that failed during the first step stress maintenance period, q i =S / f i , for the specimens that failed during the second step stress maintenance period, q i =2S / f i , for the specimens that failed during the third step stress maintenance period, q i =3S / f i , and so on; Wherein, the step 33 comprises: For the first step load, the failure time T i =T 1i , for the second level step load, the failure time Similarly, for the pth step load, the failure time And so on; Wherein, the step 33 comprises: The value of s is based on the strength ratio q of the specimen under the first step stress. i and the destruction time T 1i ,according to Regression analysis was performed to determine.

2. The method for accelerating the determination of the long-term tensile strength of reconstructed composite structural timber according to claim 1, characterized in that: In step 1, n is an even number not less than 40; And / or, in step 1, each tensile test piece is in the shape of a dumbbell, and the cross-sectional dimensions of two ends of the dumbbell-shaped tensile test piece are larger than the central cross-sectional dimension of the tensile test piece.

3. The method for accelerating the determination of the long-term tensile strength of reconstructed composite structural timber according to claim 1, characterized in that: The step 5 comprises: Set a design service life L of the reconstructed composite structural wood, use the fitting parameter values ​​A and B to calculate the long-term tensile strength ratio Q of the reconstructed composite structural wood corresponding to the design service life L: Q = A + Blog (L), and then according to the average value m of the short-term strength of all tensile specimens obtained in step 21, the long-term tensile strength f of the reconstructed composite structural wood is obtained. L , f L =m×Q.

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