Mechanical property specimen with pre-prepared delayed crack and preparation method thereof

Abstract

The present application belongs to the technical field of metal welding performance sample preparation, and discloses a mechanical property sample with a prefabricated delayed crack and a preparation method thereof, which comprises the following steps: (1) first, weld the restraint weld seams on both sides, then directly weld the first test weld seam without preheating after the test piece cools to room temperature, place the test piece after it cools to room temperature after the first welding is completed, and then weld the second test weld seam, and repeat the process until the welding of the multilayer and multi-pass weld seam is completed; (2) after the welded piece cools completely, cut off the restraint weld seams at both ends, and cut off the non-penetration part at the bottom of the welded piece until the bottom crack is exposed, so that the weld root crack is converted into a surface crack after cutting, and a sampling test plate is prepared, and the crack position and shape are determined; (3) according to the crack position, the sampling test plate is cut to obtain a rough sample, and the rough sample is cut into a mechanical property sample under the condition that the delayed crack part is reserved. The present application has high accuracy.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of the preparation of metal welding performance test samples, and more particularly relates to a mechanical property test sample with a prefabricated delayed crack and a preparation method thereof. BACKGROUND

[0002] The delayed crack is a kind of welding crack that is common in welded steel structures and has great harm. If it is not found or not properly handled after being found, it will affect the stress performance of the structure, and in severe cases, it will even threaten the safety of the structure and cause major accidents. In recent years, many delayed cracks have occurred in some bridge steel structures during manufacturing or at the initial stage of bridge construction, causing a large amount of scrap steel and serious delays in construction schedule, and posing a great threat to the safety of the structure. At present, the research on delayed cracks mainly focuses on the welding structures of ships and pressure vessels. The amount of welds in bridge steel structures is large and is likely to produce delayed cracks, but the research on the influence of delayed cracks on the stress performance of bridge weld structures and the whole structure is still immature.

[0003] In order to reveal the failure mechanism of the structure with cracks and accurately evaluate the safety of the welded structure, the method of mechanical property test for the structure with cracks is often applied to solve such problems. However, how to prefabricate an initial crack that is consistent with the engineering welding defect is an important prerequisite for determining whether the test results are accurate and effective. At present, the prefabrication of cracks mainly includes manual cutting method, preloading method and pre-embedding special welding material method. The manual cutting method usually uses wire cutting to prefabricate cracks, but the width of the crack thus prefabricated is larger than that of the actual crack defect, especially it cannot accurately reflect the situation at the crack tip. The preloading method is to make a small notch by wire cutting and then perform fatigue loading to make the crack expand and become sharper. The pre-embedding special welding material method is to produce a crack by doping dissimilar metals. In summary, although the existing methods for prefabricating crack defects can produce cracks, they do not conform to the mechanism of the generation of delayed cracks in actual bridge steel structures, and cannot accurately reflect the generation of delayed cracks in actual bridge steel structures. After the delayed crack is prefabricated, how to reasonably utilize the crack and further process it into a mechanical property test sample that can be used for testing is a key problem for successfully performing the mechanical property test. SUMMARY

[0004] In order to overcome the above defects or improvement needs of the prior art, the present application provides a mechanical property test sample with a prefabricated delayed crack and a preparation method thereof, which provides an efficient and convenient prefabrication method of a delayed crack of a bridge steel inclined Y-shaped groove welding, so as to realize the simulation of the actual delayed crack, and then provides a mechanical property test sample processing idea, which reasonably and effectively utilizes the prefabricated delayed crack, and further processes the inclined Y-shaped groove test sample into a mechanical property test sample containing a surface crack, thereby providing a guarantee for the reliability and accuracy of the test research on the delayed crack of the bridge weld structure and the overall stress performance, and overcoming the defect that the prefabricated crack in the prior art cannot accurately simulate the generation of the delayed crack in the actual bridge steel structure.

[0005] In order to achieve the above-mentioned purpose, according to one aspect of the present application, a preparation method of a mechanical property test sample with a prefabricated delayed crack is provided, characterized in that the method comprises the following steps:

[0006] (1) The opposite wedge surfaces of the two bridge steel plates and the restraint welds on both sides formed between the wedge surfaces and the slope surfaces form the test welds in the middle; wherein the thickness of the bridge steel plate is greater than or equal to 20mm;

[0007] (2) The restraint welds on both sides are welded first, and after the test sample is cooled to room temperature, the first test weld is directly welded without preheating, and after the first welding is completed, the test sample is cooled to room temperature and placed for more than 48 hours, and then the second test weld is welded, and the above process is repeated until the welding of the multiple layers of test welds is completed;

[0008] (3) After the welded part is completely cooled, the restraint welds at both ends are cut off, and the unpenetrated part at the bottom of the welded part is cut off until the bottom crack is exposed, so that the root crack of the weld is converted into a surface crack after cutting, and a sampling test plate is also made to determine the crack position and shape;

[0009] (4) The sampling test plate is cut to obtain a rough test sample, and the rough test sample is cut into a mechanical property test sample while the delayed crack part is reserved.

[0010] Further, when the test weld is welded, the welding rod does not need to be dried before welding.

[0011] Further, the welding material of the restraint welds should have the same or higher yield strength as the welding material of the test welds, and the welding material is dried before welding when the restraint welds are welded.

[0012] Further, in step (2), the weld is fully welded on the basis of the one-welded weld with a crack.

[0013] Further, on the basis that the restraint welds at both ends are cut off, the length of the test welds at both ends is also cut off by 10-20mm.

[0014] Further, the half of the thickness of the test piece is directly cut off in the non-welded part of the bottom of the cut-off plate.

[0015] Further, the same welding material is used for the second welding.

[0016] The application further provides a mechanical property test sample with a prefabricated delayed crack, which is prepared by the method for preparing a mechanical property test sample with a prefabricated delayed crack.

[0017] Overall, compared with the prior art, the mechanical property test sample with a prefabricated delayed crack and the preparation method thereof provided by the application mainly have the following beneficial effects:

[0018] 1. The application increases the restraint degree by increasing the size of the test piece, and reduces the hydrogen content loss of the test weld by combining measures such as not drying the welding rod and not preheating, to prepare a delayed crack of the weld, which is consistent with the situation that the delayed crack is caused by excessive restraint degree of the welded joint or non-standard processing, the welding rod is not dried before welding, the test piece cannot be preheated due to site restrictions, and the hydrogen content is relatively high, so the delayed crack prefabricated by the application is similar to the delayed crack of the bridge steel structure, and can reflect the actual engineering situation to a certain extent.

[0019] 2. On the basis of prefabricating the delayed crack, the application proposes a welding method of continuing to weld the Y groove, and then further proposes a cutting method of cutting off the non-welded part until the crack is exposed, to achieve the purpose of converting the delayed crack at the root into a surface crack, effectively utilize the prefabricated delayed crack, and lay a foundation for further processing the test piece into a mechanical property test sample for testing.

[0020] 3. The application makes a sampling test plate of a mechanical property test sample with a delayed crack, and different mechanical property test pieces with a prefabricated delayed crack such as a tensile test piece, a three-point bending test piece, and a fatigue test piece can be made on the sampling test plate according to requirements, which has a wide range of applications and high utilization value.

[0021] 4. The application proposes a new preparation method of a mechanical property test sample with a crack, compared with the traditional method of first making a mechanical property test sample and then manually prefabricating a crack, the application changes the processing sequence by first prefabricating a crack and then making a mechanical property test sample according to the crack state, so that the crack defect can be effectively utilized, the prefabricated delayed crack can more accurately simulate the delayed crack defect of the weld in engineering, and the reliability of the test result is ensured, which creates conditions for the bridge steel mechanical property test with a delayed crack. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1is the assembly schematic diagram of the welding member according to the present application;

[0023] Figure 2 is the A-A cross-sectional structure schematic diagram of the welding member assembly according to the present application; Figure 1

[0024] Figure 3 is the B-B cross-sectional structure schematic diagram of the welding member assembly according to the present application; Figure 1

[0025] Figure 4 is the sampling schematic diagram of each sample for judging the crack type after the first welding seam generates the crack;

[0026] Figure 5 , Figure 6 and Figure 7 are respectively the microstructure metallographic diagram of the welding seam sample with the crack;

[0027] Figure 8 , Figure 9 and Figure 10 are respectively the scanning electron microscope diagram of the welding seam sample with the crack;

[0028] Figure 11 is the schematic diagram of the restraint welding seam cutting of the welding seam sample with the crack;

[0029] Figure 12 is the schematic diagram of the bottom gap cutting of the welding seam;

[0030] Figure 13 is the sampling test plate diagram;

[0031] Figure 14 a), b) and c) in are respectively the sampling test plate rough sample schematic diagram, the tensile mechanical property sample thickness cutting schematic diagram and the tensile mechanical property sample finishing schematic diagram on the sampling test plate.

[0032] Figure 15 a), b) and c) in are respectively the sampling test plate three-point bending rough sample schematic diagram, the three-point bending sample thickness cutting diagram and the three-point bending property sample finishing schematic diagram.

[0033] ​​In all the drawings, the same reference signs are used to denote the same elements or structures, wherein: 1 - No. 1 steel plate, 2 - No. 2 steel plate, 3 - restraint weld, 4 - test weld bevel Y groove, 5 - No. 1 steel plate two side wedge surface, 6 - No. 2 steel plate wedge surface, 7 - No. 1 steel plate middle wedge surface, 8 - No. 2 steel plate slope surface, 9 - test weld groove root gap, 10 - test weld welded first pass, 11 - first pass weld, 12 - first pass weld crack, 13 - tensile mechanical property specimen with cracked weld, 14 - metallographic specimen with cracked weld, 15 - scanning electron microscope specimen with cracked weld, 16 - test weld after full welding, 17 - restraint weld cutting line, 18 - weld root crack, 19 - weld root gap, 20 - weld bottom porosity cutting line, 21 - sampling test plate, 22 - test weld on sampling test plate, 23 - weld crack on sampling test plate, 24 - tensile mechanical property coarse specimen, 25 - test weld on tensile mechanical property coarse specimen, 26 - weld crack on tensile mechanical property coarse specimen, 27 - weld crack depth control line on tensile mechanical property coarse specimen, 28 - thickness control line on tensile mechanical property coarse specimen, 29 - tensile mechanical property finished specimen, 30 - three-point bending coarse specimen, 31 - weld on three-point bending coarse specimen, 32 - weld crack on three-point bending coarse specimen, 33 - weld crack cutting line control on three-point bending coarse specimen, 34 - thickness control line on three-point bending coarse specimen, 35 - three-point bending finished specimen, 36 - notch on three-point bending finished specimen. DETAILED DESCRIPTION

[0034] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely intended to explain the present application and should not be used to limit the present application. In addition, the technical features involved in the various embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0035] The present application provides a method for preparing a mechanical property specimen with a prefabricated delayed crack, which mainly comprises the following steps:

[0036] Step one, two bridge steel plates are oppositely formed into two side restraint welds between the wedge surfaces and the slope surfaces, and are oppositely formed into a middle test weld between the wedge surfaces and the slope surfaces; wherein the thickness of the bridge steel plate is greater than or equal to 20 mm.

[0037] The thicker the thickness of the bridge steel plate used, the greater the constraint of the weld in the length direction by the two sides of the base material, the greater the longitudinal stress and strain of the weld, and at the same time, the constraint in the transverse direction will also increase due to the welding of the restraint weld on both sides, and the greater the constraint, the more likely it is to produce a delayed crack.

[0038] The bridge steel plate is beveled and assembled, and the weld bevel is preferably prepared by mechanical processing. During the preparation of the weld bevel, the bevel processing size and the assembly root gap should meet the requirements of GB / T 32260.2-2015 Metal Materials Welding Fracture Test Welding Cold Cracking Test Arc Welding Method Part 2: Self-restraint Experiment.

[0039] Step two, first weld the restraint weld on both sides, then weld the first test weld without preheating after the test piece cools to room temperature. After the first weld is completed, the test piece is cooled to room temperature and placed for more than 48 hours before the second test weld is welded. This process is repeated until the multi-layer and multi-pass weld is completed.

[0040] The test weld material is selected according to the principle of "equal strength matching", and the electrode does not need to be dried before welding. The restraint weld material can have the same or higher yield strength as the test weld material, and should be dried before welding. During the second welding, the welding current should not be too large to prevent excessive melting of the first weld, which can cause loss of crack depth. The subsequent multi-layer and multi-pass weld is then completed.

[0041] The purpose of not drying the electrode is to reduce the loss of hydrogen content in the electrode, thereby more conducive to the formation of delayed cracks. At the same time, it is consistent with the situation in engineering practice that non-standard welding operation and not drying the electrode before welding leads to the formation of delayed cracks, and the crack formation mechanism is consistent. The test of the subsequent mechanical property specimen with delayed cracks is more close to the engineering requirements, and the results are more reliable. However, the electrode should not be soaked before welding, otherwise it may produce porosity defects rather than delayed crack defects.

[0042] The restraint weld material should have the same or higher yield strength as the test weld material, and the restraint weld should not produce cracks. Therefore, the material should be dried according to the material specification before welding to reduce the hydrogen content as much as possible and avoid the formation of hydrogen-induced cracks. If necessary, preheating, interpass temperature control, and post-heating measures can be taken.

[0043] The test weld does not need to be preheated before welding to prevent the escape of hydrogen in the weld, accelerate the cooling speed of the weld, and make it easier for the weld and heat-affected zone to produce hardened structures, increasing the probability of crack formation. The position of the first weld changes significantly, and the stress concentration is prominent, so the first weld is most likely to produce cracks, and subsequent welding will not produce cracks.

[0044] Since the delay crack has the characteristic of delayed generation, it can be generated after welding, for several days, months or even years, and the probability of generating the delay crack within 48 hours is large, so it is appropriate to observe the generation of the delay crack at 48 hours. The generation of the crack and the crack morphology can be observed by naked eye observation or methods such as penetrant testing, magnetic particle testing, etc. If no delay crack is generated or the required delay crack is short, the placement time can be extended to increase the probability of generating the delay crack and obtain a longer delay crack. If the crack property needs to be judged, a simple tensile specimen can be prepared by cutting off the part of the weld specimen with the crack at the crack position, and the crack section is observed after being pulled off. A metallographic specimen is prepared to observe the metallographic structure near the cross-section crack. A scanning electron microscope specimen is prepared to observe the cross-section crack morphology and cracking and expansion mode.

[0045] In the formula, the welded surface should be smooth, without scratches, dust, oil stains, rust and other contaminants, and the coating should be knocked off after each weld is completed, and the slag should be cleaned in time. The welding adopts CO2 gas shielded welding. The welding parameters of each layer and each pass can be executed by referring to Table 1.

[0046] The final purpose of the present application is to obtain a mechanical property specimen with a delay crack instead of only obtaining a delay crack. The purpose of the present application is different from that of the specification “GB / T 32260.2-2015 Metal Material Weld Joint Destructive Test Arc Welding Method Second Part: Self-restraint Experiment” which only welds one pass in the inclined Y groove to judge the cold crack sensitivity of the welded material. Therefore, the present embodiment welds the weld to be full on the basis of the obtained one pass with a crack, which is consistent with the process in the steel structure weld processing, and also provides conditions for subsequent mechanical property specimen preparation.

[0047] Table 1 Welding parameter table

[0048]

[0049] Step three, after the welded part is completely cooled, the restraint welds at both ends are cut off, and the non-penetration part at the bottom of the welded part is cut off until the bottom crack is exposed, so that the root crack of the weld is converted into a surface crack after cutting, and a sampling test plate is made to determine the crack position and morphology.

[0050] The cutting sequence is to cut the two sides in the width direction first and then cut in the thickness direction. Since it is difficult to cut along the thickness direction and it takes a long time, the restraint welds at both sides are cut off first, and then the cutting is performed after reducing the area in the thickness direction, so as to reduce the processing difficulty, shorten the processing time and save the cost.

[0051] On the basis of cutting off the restraint welds at both ends, the area of 10-20mm at both ends of the test weld is also cut off. Since the stress concentration exists in the connection part between the test weld and the restraint weld, the test weld will be affected, so the affected part is cut off.

[0052] The half of the thickness of the specimen can be directly cut off in the non-welded part of the bottom of the plate, and the uneven welding thickness cannot be avoided, the non-welded part is not flat, and it is difficult to accurately estimate the cut-off part and it is very likely to need further processing, and the center line of the first weld is mainly located at the thickness center line position, so the half of the thickness can be cut off to ensure that the crack is exposed and flat.

[0053] Step four, cutting the sampling test plate according to the crack position to obtain a rough sample, and cutting the rough sample into a mechanical property sample under the condition of retaining the delayed crack part.

[0054] Among them, different mechanical property samples with pre-made delayed cracks such as tensile samples, three-point bending samples and fatigue samples can be made on the sampling test plate according to requirements, the application range is wide, and the utilization value is high.

[0055] The application also provides a mechanical property sample with a pre-made delayed crack, which is prepared by the preparation method of the mechanical property sample with a pre-made delayed crack.

[0056] The application will be further described in detail in the following specific embodiments.

[0057] Please refer to Figures 1-4 The application provides a bridge steel inclined Y groove welding delayed crack pre-preparation method, please refer to Figures 12-15 The application provides a bridge steel mechanical property sample with a pre-made delayed crack, and the preparation method comprises the following steps:

[0058] S1, select two Q345qd steel plates with a thickness of more than 20mm, use a planer and a milling machine or a numerical control machine tool to process a groove, the steel plate after processing includes a No. 1 steel plate 1 and a No. 2 steel plate 2, the No. 1 and No. 2 steel plates are assembled, the two side wedge surfaces 5 of the No. 1 steel plate and the opposite wedge surfaces 6 of the No. 2 steel plate form two sides of the to-be-welded restraint weld 3, the middle wedge surface 7 of the No. 1 steel plate and the slope surface 8 of the No. 2 steel plate form the middle test weld inclined Y groove 4, see Figures 1-3 .

[0059] In step 1, the thickness of the test plate is suggested to be at least 20mm thick. The thicker the specimen, the greater the restraint of the weld in the length direction by the base metal on both sides, the greater the longitudinal stress and strain of the weld, and at the same time, due to the two sides of the welded restraint weld, the transverse restraint will also increase with the increase of the thickness, and the greater the restraint, the easier the delayed crack is generated.

[0060] In this embodiment, the weld bevel is preferably prepared by mechanical processing. During the preparation of the weld bevel, the bevel processing dimensions and the assembly root gap should meet the requirements of "GB / T 32260.2-2015 Destructive testing of welds of metallic materials - Cold cracking test of welded parts - Arc welding method - Part 2: Self-restraint test".

[0061] S2. The welding material for the test weld shall be selected according to the principle of "equal strength matching" and there is no need to dry the welding rod before welding. The welding material for the restraint weld may have the same or higher yield strength as the welding material used for the test weld and shall be dried before welding.

[0062] The purpose of not drying the welding rod is to reduce the loss of hydrogen content, which is more conducive to the formation of delayed cracks. This also aligns with the situation in engineering practice where improper welding operations, such as not drying the welding rod before welding, lead to delayed cracks. The crack formation mechanism is consistent, making subsequent tests on mechanical property specimens with delayed cracks more closely reflect engineering requirements and yield more reliable results. However, the welding rod should not be soaked before welding; otherwise, it is likely to produce porosity defects rather than delayed crack defects.

[0063] The welding material selected for restraint weld 3 should have the same or higher yield strength as the welding material used for the test weld. Before welding, the welding material for restraint weld 3 should be dried according to the material specification to minimize the hydrogen content. The restraint weld 3 should avoid hydrogen-induced cracking. If necessary, preheating, interpass temperature control and post-heating measures can be taken.

[0064] S3, weld to form restraint welds 3 on both sides. After the specimen cools to room temperature, no preheating is required. Directly weld the first weld 11 of the test weld oblique Y groove 4. After the first weld is completed, cool the specimen to room temperature for at least 48 hours. The welding parameters are shown in Table 1.

[0065] Preheating is not required before welding the Y-groove 4 test weld to prevent hydrogen from escaping from the weld. The weld cools faster, and the weld and heat-affected zone are more likely to develop hardened structures, increasing the probability of crack formation.

[0066] The first weld 11 has a significant change in cross-section, with prominent stress concentration and high restraint. Therefore, the first weld 11 is most likely to develop cracks, and subsequent welding will basically not produce cracks.

[0067] S4. After standing for at least 48 hours, observe the crack formation. If it is necessary to determine the nature of the first weld crack 12, a portion of the cracked weld sample can be cut off at the location of the first weld crack 12 to prepare a simple cracked weld tensile test sample 13. After breaking, observe the crack cross-section; prepare a cracked weld metallographic sample 14 to observe the metallographic structure near the crack in the cross-section; prepare a cracked weld scanning electron microscope sample 15 to observe the crack morphology and cracking and propagation mode in the cross-section.

[0068] Wherein, whether the crack is generated and the crack morphology can be observed by naked eye observation or methods such as penetrant testing, magnetic particle testing and the like.

[0069] Since the delayed crack has the characteristic of delayed generation, it can be generated after welding, for several days, several months or even several years, and the probability of generating the delayed crack within 48 hours is high, so it is appropriate to observe the generation of the delayed crack at 48 hours, and if no delayed crack is generated or the required delayed crack is short, the placement time can be extended to increase the probability of generating the delayed crack and obtain a longer delayed crack.

[0070] The part of the crack-containing weld sample is intercepted for testing, as shown in Figure 4 : firstly, a crack-containing weld tensile judgment sample 13 is pulled off, whether the crack section has oxidation color is observed, if there is no oxidation color, the heat crack can be roughly excluded, and then the crack type is further judged by combining the microstructure analysis; then, the metallographic analysis is performed on the crack-containing weld metallographic sample 14, the crack generation position and the metallographic structure are observed, whether the martensite structure exists is judged, if it exists, the delayed crack is highly probable; finally, the scanning electron microscope analysis is performed on the crack-containing weld scanning electron microscope sample 15, the crack generation and expansion form is observed, if the transgranular expansion exists and the foregoing characteristics are combined, the crack type can be determined as the delayed crack.

[0071] S5, then the same welding material is used to perform the second welding, the welding current is not suitable to be too large, so as to avoid melting too much of the first welding, causing the loss of the crack depth, and then the welding of the subsequent multi-layer and multi-pass weld is completed, and the welding parameters are shown in Table 1.

[0072] The metallographic structure diagram in the embodiment is shown in Figures 5-7 , the scanning electron microscope diagram is shown in Figures 8-10 . It can be seen from Figures 5-7 that the crack is mainly generated in the heat affected zone, and the martensite structure exists around, which is preliminarily judged as the delayed crack; finally, the scanning electron microscope Figures 8-10 is combined, it is found that the crack expansion has the transgranular phenomenon, so the crack type is judged as the delayed crack.

[0073] Table 2 Chemical composition of the steel plate and the deposited metal of the welding wire

[0074]

[0075] Table 3 Basic mechanical properties of the steel plate and the deposited metal of the welding wire

[0076]

[0077] In the embodiment, the welded surface in steps 3 and 5 should be smooth, without scratches, dust, oil stains, rust and other pollutants. After each pass of the weld is welded, the flux is knocked off and the slag is cleaned in time.

[0078] In this embodiment, the welding in step 3 and step 5 adopts CO2 gas shielded welding.

[0079] In this embodiment, the welding parameters of each layer and each pass in step 3 and step 5 can be controlled according to Table 1.

[0080] In this embodiment, the test welds in step 3 and step 5 are welded, and a welding wire with a specification of Ф1.2 mm and a model of YCJ501-1 (GB / T 10045T492T1-1C1A-U) produced by Iron Anchor Company is selected. A welding machine with a model of Panasonic YD-350FR2 is used for welding. The welding power source is an inverter, the welding polarity is direct current reverse connection, the welding type is carbon dioxide gas shielded welding, the carbon dioxide flow is 15 L / min, the welding parameter control condition is shown in Table 1, the chemical composition of the steel plate and the welding wire is shown in Table 2, and the mechanical properties are shown in Table 3.

[0081] In this embodiment, the purpose of step 5 is to obtain a mechanical property sample with a delayed crack, not only to obtain a delayed crack. The purpose of judging the cold crack sensitivity of the welding material is different from that of only generating a crack in the test weld 10 of the first pass in the specification “GB / T 32260.2-2015 Metal Materials Welding Damage Test Arc Welding Method for Welding Cold Crack Test Part 2: Self-restraint experiment”. Therefore, the weld is fully welded on the basis of the test weld 10 of the first pass, which is consistent with the process in the welding of steel structure welds, and also provides conditions for the subsequent mechanical property sample preparation.

[0082] S6, after the to-be-welded plate is completely cooled, the restraint welds 3 at both ends are cut off according to the restraint weld cutting line 17, the weld root gap 19 is cut off according to the weld bottom gap cutting line 20, and the weld root crack 18 will be cut off in part until the weld root crack is exposed and the surface is smooth, thereby a sampling test plate 21 for taking a mechanical property sample is prepared. The test weld 22 on the sampling test plate is smooth, the weld root crack is converted into a weld crack 23 on the surface of the sampling test plate after cutting, and the crack generation is observed.

[0083] The cutting sequence is to cut the two sides in the width direction first and then cut the thickness direction according to the weld bottom gap cutting line 20. Because it is difficult to cut along the thickness direction and it takes a long time, the restraint welds at both ends are cut off first, and then the cutting is performed after reducing the area in the thickness direction, thereby reducing the processing difficulty, shortening the processing time, and saving the cost.

[0084] In this embodiment, the two ends of the test weld 10-20 mm area are also cut off on the basis of cutting off the restraint welds at both ends according to the restraint weld cutting line 17. Because the affected part exists in the connection part between the fully welded test weld 16 and the restraint weld 3, the affected part is cut off, as shown inFigure 11 .

[0085] In this embodiment, the half of the thickness of the test piece can be directly cut off in the non-welded part of the bottom of the cut plate. Since the welding thickness is uneven, it is inevitable that the non-welded part is not flat, and it is difficult to accurately estimate the gap part to be cut off and it is very likely to need further processing. The first weld center line is mainly located at the thickness center line position, so the weld gap at the bottom can be cut off from the thickness center position to ensure that the crack is exposed and flat. The processing steps are shown in Figure 12 .

[0086] In this embodiment, whether the crack is generated and the crack morphology can be observed by visual observation or penetration testing, magnetic powder testing and the like.

[0087] S7, according to the crack position, the approximate position of the mechanical property sample is designed on the sampling test plate and cut into a rough sample. After cutting, the delay crack part is reserved for further processing of the test piece according to the requirements until the mechanical property test requirements are met.

[0088] It is recommended to process in sequence. If the thickness of the other end is processed first and then the crack depth end is cut, it is very likely that the thickness end will not be accurately cut due to processing errors, and in order to ensure the thickness of the final test piece, the crack depth of the crack depth end is sacrificed, resulting in uncontrollable crack depth. Therefore, cutting the crack position depth first is to accurately control the crack depth on the basis of ensuring the thickness of the final test piece and to reduce the influence of processing errors on the crack.

[0089] In this embodiment, if a tensile test piece with a crack is to be made to study the influence of the crack on the static performance of the tensile test piece, the tensile mechanical property rough sample 24 can be cut according to a) in Figure 14 , and then processed in a certain order: first, the weld crack 26 depth on the tensile mechanical property rough sample is determined by using penetration testing, magnetic powder testing and the like; then, it is selected whether to cut off part of the crack depth, and after the tensile mechanical property rough sample weld crack depth control line 27 reaches the required crack depth, the thickness of the other end is cut off according to the tensile mechanical property rough sample thickness control line 28 until the expected test piece thickness is reached, as shown in b) in Figure 14 ; finally, the test piece is made into a tensile mechanical property precision sample 29 with a delay crack by using a numerical control machine tool to meet the requirements of the test piece and size in the specification GB / T228.1-2010, as shown in c) in Figure 14 .

[0090] In this embodiment, if the quasi-static fracture toughness at the weld is to be studied, a three-point bending test piece can be made, and the test piece with a straight crack and meeting the size requirements of the specification is processed according to the requirements of GB / T21143-2014 Unified Test Method for Quasi-static Fracture Toughness of Metallic Materials. First, the test piece is cut according toFigure 15 a) cutting out the three-point bending rough sample 30, and then selecting whether to cut part of the three-point bending rough sample weld crack 32 according to the three-point bending rough sample weld crack cutting control line 33 of the three-point bending rough sample weld 31, and after reaching the required crack depth, cutting the thickness of the other side according to the three-point bending rough sample thickness control line 34 until reaching the expected specimen thickness, see Figure 15 b) in FIG. 3; finally, finishing the three-point bending finishing sample 35 to meet the requirements of the standard GB / T21143-2014 Unified Test Method for Metallic Materials Quasi-static Fracture Toughness, and cutting the three-point bending finishing sample notch 36 that can be clamped by the displacement gauge by wire cutting, see Figure 15 c) in FIG. 4. This method avoids the cumbersome process of opening a notch and pre-preparing a fatigue crack, and the pre-prepared delay crack is closer to the actual engineering defect.

[0091] In this embodiment, if a fatigue crack propagation rate test is to be made, a similar three-point bending manufacturing process is required, and attention should be paid to the requirements of the specimen and size in the standard GB / T6398-2017 Metal Materials Fatigue Test Fatigue Crack Propagation Method.

[0092] The pre-preparation of the delay crack is achieved by the foregoing embodiments, thereby obtaining a delay crack defect with the same engineering actual generation mechanism, and different mechanical performance test specimens with delay cracks can be made according to the requirements, thereby providing strong practical support for further mechanical tests, and providing a test basis for revealing the damage mechanism of the structure with cracks and accurately evaluating the safety of the welded structure.

[0093] Those skilled in the art will readily understand that the above description is only the preferred embodiment of the present application, and is not intended to limit the present application, and any modifications, equivalent replacements and improvements made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a mechanical property specimen with a pre-existing delay crack, characterized by, The method comprises the following steps: (1) forming two restraint welds on opposite wedge surfaces of two bridge steel plates and forming a test weld between the opposite wedge surfaces and slope surfaces; wherein the thickness of the bridge steel plate is greater than or equal to 20 mm; (2) welding the two restraint welds first, then welding the first test weld without preheating after the test piece cools to room temperature, and then placing the test piece to cool to room temperature for more than 48 hours after the first welding is completed, and then welding the second test weld, and repeating the above steps until the welding of the multiple-layer and multiple-pass weld is completed; (3) cutting off the restraint welds at both ends of the welded piece after the welded piece is completely cooled, and cutting off the non-penetration part at the bottom of the welded piece until the bottom crack is exposed, so that the root crack of the weld is converted into a surface crack after cutting, and a sampling test plate is prepared to determine the crack position and morphology; (4) cutting the sampling test plate according to the crack position to obtain a rough sample, and cutting the rough sample into a mechanical property sample while retaining the delayed crack part.

2. The method of claim 1, wherein the pre-existing delay crack is formed by: The welding rod does not need to be dried before welding the test weld.

3. The method of claim 2, wherein the pre-existing delay crack is formed by: The welding material of the restraint welds should have the same or higher yield strength as the welding material of the test weld, and the welding material is dried before welding the restraint welds.

4. The method of claim 1, wherein the pre-existing delay crack is formed by: In step (2), the weld is welded full based on the one weld with a crack.

5. The method of claim 1-4, wherein: The 10-20 mm length area at both ends of the test weld is also cut off based on the cutting off of the restraint welds at both ends.

6. The method of claim 1-4, wherein the method is characterized by: The non-penetration part at the bottom of the plate is directly cut off to half the thickness of the test piece.

7. The method of claim 1-4, wherein the method further comprises: The same welding material is used for the second welding.

8. A mechanical property test specimen with a pre-existing delayed crack, characterized by: The mechanical property sample is prepared by the method for preparing a mechanical property sample with a pre-prepared delayed crack according to any one of claims 1-7.

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