Abstract:
An apparatus and method for determining dynamic indentation hardness values of a material using a propagating stress wave to make an indentation in the material. The invention provides such values without any prior knowledge of the material properties and enables the dynamic indentation hardness values to be directly compared to static indentation hardness values for the material.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 08/732,644 filed Oct. 7, 1998, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to an apparatus and method for determining the indentation hardness of a material, and more particularly, to an apparatus and method for determining the dynamic indentation hardness of a material. 
     BACKGROUND OF THE INVENTION 
     Indentation hardness measurements have been used for several decades by machinists and researchers in various fields. Indentation hardness is a measure of a material&#39;s resistance to penetration or permanent deformation. Indentation hardness measurements have been used to successfully monitor the effectiveness of processes such as heat treating, casting, forming, and welding. For the machinist, indentation hardness may indicate a material&#39;s resistance to cutting. 
     Indentation hardness measurements are typically obtained by subjecting a material to a static indentation force with an indentor for typically 10-15 seconds, measuring the area of the indentation and then calculating the static indentation hardness using a test standard such as, for example, ASTM Test Method E92-82 for Vickers indenters which is set forth below: 
     
       
         Vickers Hardness (HV)=2Psin(α/2)/d 2 =1.8544P/d 2  wherein 
       
     
     p=load (kgf) 
     d=mean indentation diagonal (mm) 
     α=face angle of diamond=136 degrees. 
     Although traditional static indentation hardness measurements provide insight into a material&#39;s behavior during a specific process, these measurements do not accurately and sufficiently represent a material&#39;s response during a dynamic event such as metal cutting or material removal during high speed machining. This is because deformation mechanisms of most materials are known to be rate dependent. For example, increased yield and flow stresses at higher strain rates in the case of metals, and increased fracture stress in the case of ceramics are rate dependent effects. Therefore, static indentation measurements do not completely represent a material&#39;s behavior under dynamic events. 
     Many engineering applications involve dynamic loading conditions, e.g., machining, crashworthiness of automotive structures, impacts of space debris on space structures, and penetration resistance of armor materials. Existing hardness testers either measure only static indentation hardness or use a rebound technique to measure dynamic indentation hardness. The rebound technique to measure dynamic indentation hardness assumes constant yield pressure, neglects thermal effects and neglects sensitivity to indenter velocity therefore making the measurement of dynamic indentation hardness indirect and inaccurate. Moreover, the dynamic indentation hardness determined by the rebound technique cannot be compared to existing static indentation hardness values. 
     Accordingly, there is a need for an apparatus and method that can measure indentation hardness at strain rates similar to those encountered in actual dynamic processes. With the increasing use of composites in every realm of technology, there is a demand for quick and accurate determination of effective properties of these new materials. Dynamic indentation hardness measurements plays a key role in these activities. 
     SUMMARY OF THE INVENTION 
     The invention provides an apparatus and method for determining the dynamic indentation hardness of a material. The invention requires no prior knowledge of material characteristics which simplifies the measurement of dynamic indentation hardness. The apparatus and method utilizes a propagating stress wave to make a single indentation in the material. By suitably positioning a load transducer, the dynamic indentation hardness of a material can be determined accurately. The apparatus and method is useful in evaluating dynamic material response under high rate loading processes such as material removal during machining, dynamic wear, impact loading, and dynamic fragmentation. The present invention is of significant value in assessing a material&#39;s ability to resist a specified dynamic load. Further, the invention can be used as a tool to quickly screen potential new materials to assess their suitability for dynamic applications. The obtained dynamic hardness can be directly compared to its static counterpart. 
     The invention provides for an apparatus and method for determining dynamic indentation hardness of a material using a propagating stress wave in a long slender bar. By mounting an indenter at one end and introducing a stress wave of suitable amplitude and duration at the other end, the invention delivers a single compressive stress pulse of required characteristics into the specimen material. 
     It is an object of the present invention to provide an improved apparatus and method for determining the indentation hardness of a material. 
     It is another object of the present invention to provide an apparatus and method for determining the dynamic indentation hardness of a material. 
     It is another object of the present invention to provide an apparatus and method for determining the dynamic indentation hardness of a material using loading pulses of less than one second. 
     It is another object of the present invention to provide an apparatus and method for determining indentation hardness having the load cell on the incident bar. 
     Other features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following detailed description, claims, and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a first embodiment of the invention; 
     FIG. 2 is a partial side view of a second embodiment of the invention; 
     FIGS.  3 ( a )-( e ) are charts of load in Newtons v. mean indentation diagonals in mm. squared for tool steels; 
     FIGS.  4 ( a )-( d ) are charts of load in Newtons v. mean indentation diagonals in mm. squared for stainless steels; 
     FIGS.  5 ( a )-( b ) are charts of load in Newtons v. mean indentation diagonals in mm. squared for FCC materials; 
     FIGS.  6 ( a )-( b ) are charts of load in Newtons v. mean indentation diagonals in mm. squared for HCP materials; 
     FIG. 7 is a chart of load in Newtons v. mean indentation diagonals in mm. squared for D- 2  Tool Steel for both placements of a load cell, front and back; 
     FIG. 8 is a chart entitled Change of Vickers Hardness under Dynamic Loading; and 
     FIG. 9 is a chart entitled Static and Dynamic Vickers Hardness. 
    
    
     Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown an apparatus  10  for measuring the dynamic indentation hardness of a material. Materials to be measured may include, for example, metals, ceramics, plastics and composites. The apparatus  10  includes a long slender incident bar  12  and a striker bar  14  both preferably fabricated of maraging steel. Preferably, the incident bar  12  is a modified Hopkinson pressure bar having a first end  16 , a second end  18  and a flange  20  adjacent end  18 . Optionally, the second end  18  can include a pulse shaper  22  that is mounted on flange  20  and is preferably copper. It should be noted however that a pulse shaper is not required for operation of the apparatus  10 . The striker bar  14  is launched such as from a gas gun at a predetermined velocity toward the incident bar  12 . The impact at the end  18  of the incident bar  12  generates a compressive stress pulse in the incident bar  12  which travels toward the end  16 . The amplitude of the stress pulse depends upon the velocity of the striker bar  14 . Stress pulse durations of 50-200 μs can be achieved by varying the length of the striker bar  14 . 
     Preferably, the end  18  of the incident bar  12  is modified with a momentum trap  24  which includes a collar  26 , or incident tube, surrounding the incident bar  12  and a reaction mass  28  on the incident bar  12 . The collar is positioned between flange  20  and the reaction mass. The momentum trap  24  is adapted to the configuration of the incident bar  12  and functions as described in Nemat-Nasser et al., Proc. R. Soc. Lond. A 435:371-391 (1991) which is incorporated herein by reference. More particularly and in operation, when the striker bar impacts the flange  20  at a given velocity it imparts a common axial strain to the collar  26  (incident tube) and the incident bar  16 . The compression pulse in the incident bar travels along this bar toward the specimen  38 . The compression pulse in the collar reflects from the reaction mass as compression, and reaches the transfer flange at the same instant that the tension release pulse which is reflected from the free end of the striker, reaches the end in contact with the transfer flange. The striker bar begins to bounce back, away from the transfer flange, as the transfer flange is loaded by the compression pulse traveling along the collar. This compression pulse then imparts a tensile pulse to the incident bar. It is instructive to note and with reference to the structure in FIG. 1, that when the incident tube in contact with the transfer flange at its one end, is free at the other end, then it serves as a “momentum trap” for tensile pulses moving in the incident bar toward the transfer flange. The tensile pulse reflects off the free end of the transfer flange as compression. This compressive pulse is then fully transferred into the incident tube in contact with the flange, when the impedances are matched. The compression then reflects off the free end of the tube as tension, and is trapped in the tube which begins to move away from the transfer flange once the reflected tension reaches the tube&#39;s end in contact with the flange. Hence, once the sample is loaded in compression by the initial compressive segment of the stress pulse, it will remain intact to be recovered, since all subsequent pulses which move toward the specimen are then tensile. Therefore, the initial strike impacts a compression pulse followed by a tension pulse and as described above all subsequent resulting pulses will be in tension relative to the indenter  36 . Accordingly, only a single compression stress pulse, the first stress pulse, reaches the end  16  of the incident bar  12 . Through use of the pulse shaper  22  on the end  18  of the incident bar  12 , loading and unloading rates can be customized. 
     Optionally, a strain gage  30  can be mounted on the incident bar  12  to confirm constant loading rates over a desired time duration. The strain gage  30  can be of various types such as, for example, model WK- 06 - 250 BF 10 C manufactured by M&amp;M of Raleigh, North Carolina. The strain gage  30  captures the complete history of the stress pulse which is then recorded on a high speed multi-channel digital oscilloscope  32 . A signal conditioner  34 , such as a wheatstone bridge, is positioned between the strain gage  30  and the oscilloscope  32 . 
     An indenter  36  is mounted on the end  16  of the incident bar  12 . Preferably, the indenter  36  is a diamond indenter of the Vickers type such as a Vickers diamond indentor available from Leco Corp. of St. Joseph, Michigan. However, it should be noted that other types of indenters can be used with the present invention such as ball or cone indenters. In the first embodiment as shown in FIG. 1, the specimen  38  of material to be tested is mounted to a load transducer such load cell  40 . The load cell  40  is mounted to a stationary surface  42 . Preferably, the load cell  40  is a high natural frequency load cell of, for example, 200 kHz such as model 9213 from Kistler of Amherst, N.Y. 
     In another embodiment as shown in FIG. 2, the load cell  40  can be positioned on the incident bar  12  adjacent the indenter  36 . The positioning of the load cell  40  is often dependent upon space limitations where the apparatus  10  is to function. Although the positioning of the load cell  40  between the specimen  38  and the surface  42  yields a cleaner electronic signal, the indentation hardness results for a particular specimen  38  for both load cell positions are the same as demonstrated in FIG. 7 for D- 2  Tool Steel. 
     The striker bar  14  impacting the incident bar  12  propagates a stress pulse along the incident bar  12  causes the indenter  36  to make a single indentation in the specimen  38 . Upon indentation, the complete loading history of the specimen  38  is captured by load cell  40  in conjunction with an amplifier  44 , such as a charge amplifier and recorded on the oscilloscope  32 . The mean indentation diagonal of the specimen  38  is then determined either manually such as with the aid of a microscope or automatically measured such as with the aid of a laser measurement system. 
     With the load value from the recorded signal on the oscilloscope  32  and the mean indentation diagonal value, the dynamic hardness can be calculated manually or with the aid of a computer. For example, with the use of a Vickers indenter, the present invention conforms to the requirements of ASTM Test Method E92-82 for static Vickers hardness measurements and extends the concepts to the dynamic range. Accordingly, the dynamic indentation hardness value is calculated with the following formula: 
     
       
         Dynamic Vickers Hardness (DHV)=2Psin(α/2)/d 2 =1.8544P/d 2   
       
     
     wherein 
     p=load (kgf) 
     d=mean indentation diagonal (mm) 
     α=face angle of diamond=136 degrees. 
     It should be noted that with the use of other indenters, corresponding formulas, such as the ASTM Test Methods, are to be employed. 
     Optionally, a integrated circuit could be conventionally constructed and made part of a dynamic indentation hardness measurement device to automatically make the above calculation. 
     Using the apparatus  10  and method described above, the dynamic indentation values can be compared to the static indentation values of a particular material to determine the rate sensitive nature of the material. Static indentation hardness values can be obtained from conventional static indentation hardness testers such as model V-100-C2 available from Leco Corp. For some materials such as metals, dynamic indentation hardness can be up to 30% greater than the static indentation hardness. See for example FIGS.  3 ( a )-( e ) pertaining to tool steels, FIGS.  4 ( a )-( d ) pertaining to stainless steels, FIGS.  5 ( a )-( b ) pertaining to FCC materials, FIGS.  6 ( a )-( b ) pertaining to HCP materials and FIGS. 8 and 9. 
     The apparatus and method described above for measuring dynamic indentation hardness enables the following advantages. 
     Unlike present dynamic indentation testers, the present invention uses the same scale as conventional static indentation testers therefore making comparisons possible and accurate. 
     The present invention determines dynamic indentation hardness values many scales of magnitude quicker than the prior art wherein the loading period is typically 10-15 seconds. In contrast, the loading times for the present invention are on the order of 50-200 μs. 
     Unlike other dynamic indentation testers, the present invention does not require any prior knowledge of the material to be tested. 
     Unlike other dynamic indentation testers, the present invention calculates a dynamic indentation hardness values that is load independent, which is a requirement of a material property. 
     The present invention enables all of the above advantages while maintaining simplicity of use.