A METHOD OF TESTING MATERIAL HARDNESS AND AN INDENTOR FOR CARRYING OUT THE METHOD

Abstract: Method of testing material hardness. An indentor is caused to penetrate into the material by means of a force (P) acting on the indentor, measurement characterizing the hardness being made for at least two separate magnitudes of this force. The penetration depth (L) caused by the forces is measured for the separate magnitudes to determine that the penetration depth variation in relation to the force increase is constant. The invention also relates to an indentor for carrying out the abovementioned material test.

Description: A METHOD DF TESTING MATERIAL HARDNESS AND AN INDENTOR FOR CARRYING OUT THE METHOD. TECHNICAL FIELD The present invention relates to a method of testing material hardness. In this respect, the invention relates to both exterior testing of material and testing inside material in a substantially cylindrical cavity. There is a need for the latter material testing, especially in building structures, and particularly concrete structures, but the invention is not limited to this particular field. The invention also relates to a testing body or indentor for carrying our the above-mentioned method of hardness testing in accordance with the invention. Finally, the invention relates to measuring equipment for hardness testing, in accordance with the invention, inside a cavity in material. BACKGROUND ART In material hardness testing an indentor is forced into the material. This is done by means of a predetermined loading force or imposed load, and deformations caused by the indentor in the tested material are measured. The hardness number corresponding to the loading force and deformations caused by it are given in tables. The indentors used can have varying shapes, such as of a shpere, cone or pyramid. A plurality of different methods are also known especially for testing finished concrete structures. In one known method of testing the pro¬ perties of a concrete surface, a testing body is allowed to bounce against the concrete surface, and certain conclusions can be drawn concering the properties of the concrete by means of sonic velocity measurements. In a known method for measuring inside the concrete material, a cast-in bolt is pulled out to determine the tensile strength. In another known method, an expansion bolt placed in a hole is pulled out to judge the deformation properties of These and other known testing methods often give test values which are difficult to interpret, and often require heavy and expensive testing equ pment. DISCLOSURE OF THE INVENTION The contour of the work diagram obtained by the penetration of an in- dentor into material for testing (hereinafter referred to as test ma¬ terial) is determined by the shape of the indentor. From the diagram can be read the point at which plastic deformation in the test material occurs, and the increase in plastic deformation corresponding to the increase in the imposed load, and this is considered, in accordance with the invention, to reflect the material hardness. The work dia¬ gram can thus be directly used as an indication ' of material hardness. Force path measurement data can also be processed to provide calculated values in the form of number or graphs. The testing method facilitates automation of the measurement and eliminates the need of subjective judgement. Testing can furthermore be carried out with minimized de¬ formation in the test material. Moreover, the testing method can show the alteration in hardness of the material due to cold working during the test. The interpretation of the force-penetration depth test values is simpler for certain given indentor embodiments, and the invention gives directions in respect of a specially suitable indentor for carrying out the above-mentioned testing method in accordance with the invention. With penetration into the test material by such an inden¬ tor in accordance with the invention, the test material surface acted on by the indentor increases linearly with penetration depth. The ma¬ terial zone, which is loaded from zero to the yield point of the ma¬ terial, increases in size from zero to a value obtained on passing the yield point. In continued penetration of the indentor into the test- material a zone is created where the material is loaded to the yield point. The increase in size of this zone is linearly proportional to the increase in penetration depth. The test comprises measuring the increase in size of the plastically deformed material zone caused by the increase in imposed load after passing the material yield point. C ' . It is here advantageous that the size of the elastically deformed ma¬ terial zone is constant after reaching the yield point, as well as that the size of the plastically deformed material zone is linearly propor¬ tional to the increased penetration depth. These conditions are ob- tained by using the special indentor in accordance with the invention. As stated by way of introduction, the method in accordance with the in¬ vention is also applicable to testing the material in a cavity made therein, especially in building structures and particularly concrete structures. Here it is essential to measure a well-defined concrete property, and such a property is the micro-hardness of the cement in the concrete. In the test, an indentor is inserted in the hole, the inner end of the indentor being expandable and acted on by an expansion body, axially movable by means of an axially directed tensile force, to provide a radially directed actuating force on the interior of the in- dentor, whereby the "inner end of the indentor expands and its end edge penetrates radially into the wall of the hole. The indentor is simul¬ taneously prevented from altering its axial position in the hole. For accurate and certain evaluation of the micro-hardness of said cement, good knowledge of the whole expansion process is required. This is achieved by continuous measurement of said actuating force and the movement of the expansion body in relation to the expandable indentor. These measurements can be presented graphically by an x-y tracer. The micro-hardness can be read from such a graph. The measured values can also be processed mathematically. The force variation is calculated in relation to the variation in ' said depth penetration. Values calculated thus can also be presented graphitacally by an x-y tracer and the micro-hardness can be read from the graph. The test result can also be presented as an obtained stable level on a lamp panel or display. This lamp panel or display can preferably be associated directly with the test equipment used in the test and which is situated outside the hole. The test method in accordance with the invention for testing material inside the material gives easily interpreted values and enables simple and easily handled equipment, which is stable and gives test values with very little spread. O PΪ What is characteristic for the invention to achieve the above-mentioned advantages in respect of both exterior and interior material testing is apparent from the following patent claims. In the following, the invention is described in deta l while referring to the appended drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a pyramidal ' or conical indentor's penetration into a test material. Figure 2 illustrates a work diagram for the penetration of an arbitrary indentor into a test material. Figure 3 illustrates the diagram in Figure 2, made into a diagram over the force-affected surface. Figure 4 illustrates the penetration of a spherical indentor into a test material. Figure 5 illustrates the work diagram for the penetration of a spherica indentor into a test material. Figure 6 illustrates the penetration by an annular indentor into a test material. Figure 7 illustrates the work diagram for the penetration of the annu¬ lar indentor into the test material. Figure 8 illustrates the diagram in Figure 7 made into a diagram over the force increase per penetration depth increase for the penetration of the annular indentor into the test material. Figure 9 illustrates the effect of cold working in the diagram of Figure 8. Figure 10 illustrates an embodiment of an apparatus with an indentor for material testing inside a material in a hole made therein. Figure 11 i Ilustrates the indentor to a larger scale and its function in testing in a hole in the material. Figure 12 is a detailed enlargement of Figure 11. Figure 13 is an embodiment of a sleeve portion of the indentor. Figure 14 illustrates another embodiment of the indentor sleeve portion. Figure 15, finally, illustrates a method of presenting measurements obtained in a test with an indentor according to Figures 10-14. BEST MODE FOR CARRYING OUT THE INVENTION In Figure 1 there is shown a pyramidal or conical indentor 2 which has attained a penetration depth L into a test material 1 under the action of an imposed load P. Figure 2 shows a diagram with a graph "a" showing the penetration depth in relation to the imposed load P for the indentor illustrated in Fig. 1. Within the area O-A the test material is only subjected to elastic de¬ formation. A portion of the test material has been loaded to the yield point at A. After A the size of both the material zone from uneffected up to the yield point, and the material zone loaded to the yield point is increased. If the hardness increase due to cold working is neglected, the yield point can be regarded as substantially constant. Thus the size of the material zone loaded to the yield point is linearly depen¬ dent on the imposed load acting within this zone. The size of the ma¬ terial zone from uneffected up to the yield point is also dependent on the imposed load acting within this zone. With good knowledge of the indentor's geometry, the graph "a" shown in Figure 2 can be made into the graph "b" of Figure 3, which shows the size Y of the material area acted on by the force P in relation to the O-.ϊH imposed load P. The test material is subjected to elastic deformation within the area 0-B. Within this area, the graph "b" has a curved shape At B the material begins to yield and thereafter the graph "b" has a substantially linear relationship between P and Y. The increase of the affected material area Y contains an increase of both elastically and plastically affected zones. The angle shown can be regarded as indicating material hardness. For calculating the angle α , at least two measured values of the imposed load P and the force corresponding to the value Y are needed. In order to be able to judge whether the measured values are taken within the area after B, at least one further measured value for each of P and Y is required. If at Least two values calculated from these measured values have the same size, the calculated value can be considered as reflecting the material hardness. In Figure 4 there is shown a spherical indentor 3, which has penetrated into a test material 4. The affected area between the indentor 3 and test material 4 is linearly 'dependent on the depth of penetration L. Thus in the work diagram shown in Figure 5 the graph "c", for the spherical penetrator, has the same appearance as the curve "b" in the force-affected area diagram illustrated in Figure 3, both before and after the Line C (corresponding to the Line B). The penetration depth L thus represents the affected area Y. The angle β indicates the materi hardness. Figure 6 illustrates a specially formed indentor 5, which has been caused by the imposed load P to penetrate a depth L into a test mate¬ rial 6. The indentor is formed as an annular tip 7 with a profile, as shown, such that a line 8 at right angles to a plane through the tip divides the profile into two substantially symmetric profile halves 9 and 10 with substantially straight sides 11 and 12. The indentor can be provided with a hole 13 for the passage of a sensing means 14 for mea¬ suring the penetration depth L. With such an indentor, the elastically deformed zone size does not in¬ crease after attaining the yield point. The size of the plastically deformable zone is linearly dependent on the increase of the penetra¬ tion depth L. In Figure 7 there is shown the work diagram with graph "d" for the indentor 5 illustrated in Figure 6. The elastically deformed zone in- creases under the distance 0-D, subsequently to become constant. The plastically deformed zone increases after D. Thus, the increase of L after D directly represents the size of the plastically deformed zone, and the angle α the material hardness. Testing data according to the diagrams in Figures 3, 5 and 7 can be mathematically processed for calculating the increase in size of the plastically deformed zone in relation to the increase in the imposed load P acting on the zone. The value thus calculated can be regarded as giving the material hardness. A graphic depiction of the value "e" thus calculated is illustrated in Figure 8. The line E corresponds to the Lines B, C and D in Figures 3, 5 and 7. The graphical depiction has the same appearance as the noi— mal method of presentation for the tensile and compressive strengths of the material. The increase in hardness of the material due to cold working during the test is illustrated in Figure 9 by means of a graph "f" in a diagram corresponding to Figure 8 . In Figures 10-15 there is illustrated a preferred embodiment of a testing device and preferred indentors for material testing in a hole made in the material, and a method of presenting the test values. The test apparatus illustrated in Figure 10, generally denoted by the numeral 20, is illustrated in its position of use in the hole 21 in the test material 22. The indentor 23 comprises an expansion sleeve 24 and an expansion bolt 25. The expansion bolt 25 comprises a cone part 26 and a shaft part 27 partially provided with a thread. The test apparatus 20 comprises of a shield 28, support means 29, positional indicator 30, support for the indicator 31, nut 32 and handle 33. The support means 29 is provided with a strain gauge 34 and the posi- tional indicator 30 is provided with a strain gauge 35. The expansion sleeve 24 bears against the support means 29 so that there is a gap 36 between the test material 22 and the support means 29. The testing method is shown in Figure 11. The indentor 23 is arranged in the hole 21. The indentor sleeve part 24 is caused to expand by the cone part 26 of the indentor being drawn into the sleeve part 24 by the axial tensile force illustrated by P. The sleeve part 24 thereby maintains its axial position in the hole and only the cone part 26 makes an axial movement. On expansion, the edge 37 of the sleeve part 24 penetrates into the wall 38 of the hole 21. An annular pene- tration by the indentor in the test material is obtained. An enlarged depiction of the penetration location is shown in Figure 12 The sleeve edge 32 has penetrated into the hole wall 38. A groove with a triangular cross section is formed in the hole wall as a result of penetration. The penetration depth is denoted by "h" and the triangle legs s,. and s 2 - The bearing surface between the sleeve edge 37 and the wall 38 is (s. + s ? ) times the length of the sleeve edge. The size of this surface has a linear relationship to the penetration depth "h". The penetration depth "h" is dependent on the hardness of the wall material and the size of the radial imposed load p. For assessing a- terial hardness, it is enough to know the size variation of the bearing surface caused by the variation in a given imposed load. The change in the bearing surface is measured by measuring the relative movement between the sleeve 24 and cone 26. The alteration in the radially im¬ posed load "p" is measured by measuring the axial tensile force P acting on the cone 26 relative to the sleeve 24. The indentor sleeve 24 may have a form illustrated in Figure 13, and comprise two sleeve halves 39 and 40. The indentor 24 can also be formed as illustrated in Figure 14 where it is provided with three 0 ~ .'. ~ ϊ WI? ° fingei—like sleeve portions 41 kept together by a common sleeve portion 42. Testing in the apparatus illustrated in Figure 10 is carried out in the following manner. The hole 21 is drilled into the test material 22. The indentor 23 is mounted in the test apparatus 20 so that the sleeve 24 bears against the support means 29. The expansion bolt 25 is taken through the holes 43 and 44 in the shield 28 and support means 29, respectively, and is kept in its position by the nut 32. The indentor 23 is inserted into the hole 21 in the test material 22 so that the expansion sleeve 24 has en extension corresponding to the gap 36 outside the test material. By turning the nut 32, the expansion sleeve is caused to expand and penetrate into the test material 22. Axial forces thereby occurring are measured by the support means 29 provided with the strain gauges 34. The axial movement o.f the expander bolt 25 is measured by it actu¬ ating the position indicator 13 provided with the strain gauges 35. On termination of the test, the nut 32 is threaded off the expansion bolt 25 and the test apparatus 20 is removed. With force on the ex¬ pansion bolt 25, the latter is taken further into the hole 21 so that the grip between the cone 26 of the expansion bolt and the sleeve 24 ceases. After removing the sleeve 24 from the hole 21, the bolt 25 can also be removed. Figure 15 illustrates examples of the presentation of test values. The measurements made by the strain gauge 34 and 35 are presented by an x-y tracer. The curve k. illustrates the relative axial movement L for the expansion cone 26 in relation to the expansion sleeve 24 in response to axial forces P. In the initial area L1, the relationship between P and L is variable. The relationship subsequently becomes constant, the slope ": " indicating the material hardness. After mathematical processing of the measurement data, the size of the slope can be presented, possibly in the shape of a curve V.- as is also shown in Figure 15.

Claims: CLAIMS 1 A method of testing material hardness by means of a testing body or indentor penetrating into the material under the action of a force acting on the indentor, measurement characterizing the hardness taking place for at Least two separate magnitudes of the force, charac- terized in that for the separate magnitudes of the imposed force ( P ) the penetration depth CD caused by the forces is measured for deter¬ mining that the penetration depth variation is constant in relation to the increase of force. 2 Method as claimed in claim 1, characterized in that the measure¬ ment is carried out continuously during the test. 3 Method as claimed in claim 1 or 2, characterized in that the test is carried out with an indentor which, after attained yield point does not increase the size of the elastically deformed zone, but only the size of the plastically deformed zone, the latter being linearly dependent on the increase of the penetration depth. 4 Method as claimed in any of the preceding claims, the test being carried out by means of an indentor, the inner end of which is expandable, said indentor being inserted in a hole in the test material, characterized in that a radial force provided by an axially imposed force in the hole expands the inner end of the indentor for penetra¬ tion into the hole wall, the axial position of the indentor end pene¬ trating into the hole wall being maintained during the test. 5 An indentor for carrying out the method in accordance with any of the preceding claims under the action of a force acting thereon, measurement characterizing the hardness taking place for at least two separate magnitudes of the force, and whereby the penetration depths caused by the separate magnitudes of the imposed force are measured to determine that the variation in penetration depths in relation to the force increase is constant, characterized in that the part (7; 37) of the indentor ( 5; 23) disposed in the test material (6, 22) during penetration is formed as an annular knife-like edge with a profile G...PI which in cross section has substantially straight sides ( 11, 12 ) . 6 An indentor as claimed in claim 5, characterized in that said profile is substantially symmetr cal " in cross section relative a Line (8) through the edge (7) at right angles to the plane common to the edges. _ 7 An indentor as claimed in claim 6, characterized in that, in th space between the annular edges there is ovably disposed a sending means (14) for measuring relative movement between the indentor (5) and test material (6). 8 An indentor as claimed in claim 5 for test measurement in a hole in the test material, characterized in that the indentor (23) comprises a sleeve (24) insertable in the hole (21), the inner end of said sleeve inserted in the hole being expandable, and a shank ( 25) disposed through the sleeve and provided with a conical end portion (26) to expand the inner sleeve end for the penetration thereof into the test material on displacement of the conical end portion towards the sleeve end, the opposite end of the sleeve (24) being supported against a support part (29) arranged outside the hole (21). 9 An indentor as claimed in claim 8, characterized by means (34) for measuring axial force (P) for displacing the conical end portion ( 26 ) towards the sleeve end for said expansion, and means (35) for measuring the relative movement between the sleeve (24) and the coni- cal end portion for said expansion.

Inventors: TOOMINGAS I
Applicants: ERICSSON TELEFON AB L M, TOOMINGAS I
Application year: 1980
Main classification: G01N3
IPC codes: G01N3