Biomechanical Measurements 

 

 

Introduction

(Adapted from Instron Technical Literature)

 

Biomechanics

Biomechanics research explores the complex relationship between human biology and

mechanics. This interdiciplinary research plays a central role in advancing researchers toward a more complete understanding of human anatomical function. Among other topics, biomechanical research endeavors to increase our understanding of muscle and joint loads, properties of cortical and trabecular bone, properties of the spine, structure and function of articular cartilage, tendons and ligaments as well as friction and wear properties of articulating joints. In addition to characterizing the individual components of a biomechanical system, it is critical to study the system as a whole. Cadaveric studies continue to provide new and exciting revelations in the function and properties of all the major load-bearing and articulation systems of the human anatomy. Because they involve complete systems as well as individual components, biomechanical studies typically involve two or more axes of control and may incorporate several degrees of freedom. For

these tests, it is often desirable to measure many fedback parameters, including forces, torques and displacements on controlled and uncontrolled axes. Typical test configurations are based on standard axial or biaxial test systems, with the addition of one or more side-force actuators to provide external forces for simulating specific conditions. On occasion, completely custom systems are required to provide the desired test conditions. In nearly all cases involving control of two or more axes simultaneously, cross-axis compensation is required to ensure that the desired command and feedback parameters are achieved.

 

Biomaterials

Mechanical properties of materials are a major determinant of  their medical usage and biocompatibility with the human body.   For example: is the material strong enough to withstand repeated loading ?  Which materials are matched to the natural tissues? What resorbable materials will provide the necessary stability while the body is healing itself?

Mechanical testing of human tissues (both hard and soft)  are also needed to characterize their structure and mechanical properties.  To   develop replacement biomaterials, BMEs must understand the natural tissues they are replacing.

 

Testing soft  versus hard tissues

Despite  our best efforts replicate biological tissues with artificial materials,  tissue grafting remains the most desireable approach to restoring function of both hard and soft tissues. Surgeons have been using autografts for years for trauma reconstruction, heart bypass surgery and other cases where a patient's own tissue can be harvested from one part of the body for use elsewhere. As researchers look for more ingenious ways to repair tissue defects in an ever- widening array of medical conditions, the need to better characterize the various tissues of the body and improve tissue preservation techniques has intensified. To understand the properties of normal tissue, mechanical testing is typically performed on harvested tissue. Since tissue properties degrade quickly outside normal body conditions, testing is usually performed in a saline environment at body temperature. For tensile tests, special attention must be paid to the method of gripping the tissue to ensure a break in the gage section of

the specimen. Soft tissues pose the additional challenge of being susceptible to damage by contacting extensometers. Video extensometry provides an alternative by using a non-

contacting method of strain measurement that does not affect the properties of the soft tissue.

 

Despite researchers best efforts to imitate the function of biological tissues with man-made materials, for many medical cases, tissue grafting is the still the best approach to restoring function of hard and soft tissues. Surgeons have been using autografts for years for trauma reconstruction, heart bypass surgery and other cases where a patient's own tissue can be harvested from one part of the body for use elsewhere. As researchers look for more ingenious ways to repair tissue defects in an ever- widening array of medical conditions, the need to better characterize the various tissues of the body and improve tissue preservation techniques has intensified. To understand the properties of normal tissue, mechanical testing is typically performed on harvested tissue. Since tissue properties degrade quickly outside normal body conditions, testing is usually performed in a saline environment at body temperature. For tensile tests, special attention must be paid to the method of gripping the tissue to ensure a break in the gage section of

the specimen. Soft tissues pose the additional challenge of being susceptible to damage by contacting extensometers. Video extensometry provides an alternative by using a non-

contacting method of strain measurement that does not affect the properties of the soft tissue.

 

 

Bone TestingFemur bone

Bone is a complex material, composed of two types of tissue: cortical bone, the hard outer shell, and trabecular bone, the porous inner core. Bone testing is typically approached at two levels: the tissue level and the structure level of the whole bone (to see how bone geometry affects its mechanical properties).

There are several factors that influence the material properties of bone:

·         Mechanical properties of dry bone are very different from wet bone.

 

 

 

Medical Glove and Tubing Tensile  Testing

Medical gloves must be tested for tensile strength and percent ultimate elongation at break, before and after aging, as specified in ASTM D 412 and comparable ISO standards. This section discusses a test setup that not only accomplishes this, but also suggests a solution for measurement failures caused by traditional strain-measurement devices. Medical gloves can be made of a variety of materials, such as latex, nitrile, and vinyl. Whatever the material, certain minimum performance levels need to be determined as specified by the FDA.  Specifically, tensile strength and percent ultimate elongation at break, before and after aging, must be assessed.

Specifically, tensile strength and percent ultimate elongation at break, before and after aging, must be assessed. A test setup is needed that will:

·        Simulate aging

 

Plastic surgical tubing comes in many different sizes and shapes, with dozens of possible interconnections and fittings. Failure of any of these parts could seriously endanger the health of patients. This section discusses a test setup that measures tensile strength and elongation at break. It also offers suggestions on how to deal with gripping and measurement issues.

Surgical tubing testing

Tensile test on surgical tubing with mini grips

·        Adhere to the relevant ASTM and ISO standards

 

 

 

Glossary of Materials Testing Terms

 

Adherence

The extent to which a coating bonds to a substrate.

Adherence Index

Measure of the adherence of porcelain enamel and ceramic coatings to sheet metal. (ASTM C-313)

Alpha Rockwell Hardness

Index of the resistance of a plastic to surface penetration by a specified indentor under specified load applied with a Rockwell hardness tester. Higher values indicate higher indentation hardness. (ASTM D-785)

Bend Test

Method for measuring ductility of certain materials. There are no standardized terms for reporting bend test results for broad classes of materials; rather, terms associated with bend tests apply to specific forms or types of materials. For example, materials specifications sometimes require that a specimen be bent to a specified inside diameter (ASTM A-360, steel products). A bend test for ductility of welds is given in ASTM E-190. Results of tests of fiberboard are reported by a description of the failure or photographs. (ASTM D-1037)

Bending Strength

Alternate term for flexural strength. It is most commonly used to describe flexure properties of cast iron and wood products.

Bond Strength

Stress (tensile load divided by area of bond) required to rupture a bond formed by an adhesive between two metal blocks. (ASTM D-952)

Break Detector

Feature in many Instron materials testing systems that detects the fracture of the test specimen. You can set up some systems to perform a user-selected action when specimen break is sensed.

Breaking Load

Load which causes fracture in a tension, compression, flexure or torsion test. In tension tests of textiles and yarns, breaking load also is called breaking strength. In tensile tests of thin sheet materials or materials in form of small diameter wire it is difficult to distinguish between breaking load and the maximum load developed, so the latter is considered the breaking load.

Breaking Strength

Tensile load or force required to rupture textiles (e.g., fibers, yarn) or leather. It is analogous to breaking load in a tension test. Ordinarily, breaking strength is reported as lb. or lb/in of width for sheet specimens.

Bulk Modulus of Elasticity

Ratio of stress to change in volume of a material subjected to axial loading. Related to Modulus of Elasticity (E) and Poisson’s Ratio (r) by the following equation: Bending Strength K = Er 3(1-2r).

 

 

Cleavage Strength

Tensile load (lb/in of width) required to cause separation of a 1-in. long metal-to-metal adhesive bond under the conditions set in ASTM D-1062.

Climbing Drum Peel Test

Method for determining peel resistance of adhesive bond between a relatively flexible and a rigid material. (ASTM D-1781).

Coefficient of Elasticity

An alternate term for modulus of elasticity.

Cohesive Strength

Theoretical stress that causes fracture in tension test if material exhibits no plastic deformation.

Complex Modulus

Measure of dynamic mechanical properties of a material, taking into account energy dissipated as heat during deformation and recovery. It is equal to the sum of static modulus of a material and its loss modulus. In the case of shear loading, it is called dynamic modulus.

Compressibility

Extent to which a material is compressed in test for compressibility and recovery of gasket materials (ASTM F-36). It is usually reported with recovery.

Compressibility and Recovery Test

Method for measuring behavior of gasket materials under short time compressive loading at room temperature. ASTM F-36 outlines a standard procedure. This test is not designed to indicate long term (creep) behavior and should not be confused with the plastometer test.

Compression-Deflection Test

Nondestructive method for determining relationship between compressive load and deflection under load for vulcanized rubber. (ASTM D-575)

Compression Fatigue

Ability of rubber to sustain repeated fluctuating compressive loads. (ASTM D-623)

Compression Set

The extent to which rubber is permanently deformed by a prolonged compressive load (ASTM D-395). Should not be confused with low temperature compression set.

Compression Test

Method for determining behavior of materials under crushing loads. Specimen is compressed, and deformation at various loads is recorded. Compressive stress and strain are calculated and plotted as a stress-strain diagram which is used to determine elastic limit, proportional limit, yield point, yield strength and (for some materials) compressive strength. Standard compression tests are given in ASTM C-773 (high strength ceramics), ASTM E-9 (metals), ASTM E-209 (metals at elevated temperatures) and ASTM D-695 (plastics).

Compressive Deformation

Extent to which a material deforms under a crushing load.

Compressive Strength

Maximum stress a material can sustain under crush loading. The compressive strength of a material that fails by shattering fracture can be defined within fairly narrow limits as an independent property. However, the compressive strength of materials that do not shatter in compression must be defined as the amount of stress required to distort the material an arbitrary amount. Compressive strength is calculated by dividing the maximum load by the original cross-sectional area of a specimen in a compression test.

Compressive Yield Strength

Stress which causes a material to exhibit a specified deformation. Usually determined from the stress-strain diagram obtained in a compression test. See also Yield Strength.

Constant Amplitude

Digital function in the Model 8800 that maintains the amplitude of the command signal during changes in frequency of the signal.

Creep

Deformation that occurs over a period of time when a material is subjected to constant stress at constant temperature. In metals, creep usually occurs only at elevated temperatures. Creep at room temperature is more common in plastic materials and is called cold flow or deformation under load. Data obtained in a creep test usually is presented as a plot of creep vs. time with stress and temperature constant. Slope of the curve is creep rate and end point of the curve is time for rupture. As indicated in the accompanying diagram, the creep of a material can be divided into three stages. First stage, or primary creep, starts at a rapid rate and slows with time. Second stage (secondary) creep has a relatively uniform rate. Third stage (tertiary) creep has an accelerating creep rate and terminates by failure of material at time for rupture. See also Stress-Relaxation.

Creep rate graph

Creep Limit

Alternate term for creep strength.

Creep Rate

Time rate of deformation of a material subject to stress at a constant temperature. It is the slope of the creep vs. time diagram obtained in a creep test. Units usually are in/in/hr or % of elongation/hr. Minimum creep rate is the slope of the portion of the creep vs. time diagram corresponding to secondary creep.

Creep Recovery

Rate of decrease in deformation that occurs when load is removed after prolonged application in a creep test. Constant temperature is maintained to eliminate effects of thermal expansion, and measurements are taken from time load is zero to eliminate elastic effects. Creep Limit.

Creep Rupture Strength

Stress required to cause fracture in a creep test within a specified time. Alternate term is stress rupture strength.

Creep Strength

Maximum stress required to cause a specified amount of creep in a specified time. Also used to describe maximum stress that can be generated in a material at constant temperature under which creep rate decreases with time. An alternate term is creep limit.

Creep Test

Method for determining creep or stress relaxation behavior. To determine creep properties, material is subjected to prolonged constant tension or compression loading at constant temperature. Deformation is recorded at specified time intervals and a creep vs. time diagram is plotted. Slope of curve at any point is creep rate. If failure occurs, it terminates test and time for rupture is recorded. If specimen does not fracture within test period, creep recovery may be measured. To determine stress relaxation of material, specimen is deformed a given amount and decrease in stress over prolonged period of exposure at constant temperature is recorded. Standard creep testing procedures are detailed in ASTM E-139, ASTM D-2990 and D-2991 (plastics) and ASTM D-2294 (adhesives).

Crush Resistance

Load required to produce fracture in a glass sphere subjected to crush loading. (ASTM D-1213).

Crushing Load

Maximum compressive force applied during a compression or crushing test. For materials that do not shatter, crushing load is defined as the force required to produce a specified type of failure.

Crushing Strength

Compressive load required to cause a crack to form in a sintered metal powder bearing (ASTM B-438 and B-439). Cold crushing strength of refractory bricks and shapes is the gross compressive stress required to cause fracture. (ASTM C-133).

 

Deformation Energy

Energy required to deform a material a specified amount. It is the area under the stress-strain diagram up to a specified strain, as shown in the following diagram.

Deformation energy at yield stress

 

Deformation Under Load

Measure of the ability of rigid plastics to withstand permanent deformation and the ability of nonrigid plastics to return to original shape after deformation. Standard test methods for determining both types of deformation under load are given in ASTM D-621. For rigid plastics, deformation (which can be flow or flow and shrinkage) is re-ported as % change in height of specimen after 24 hours under a specified load. For nonrigid plastics, results are reported as % change in height after 3 hours under load and recovery in the 1-1/2 hour period following removal of the load. Recovery is % increase in height calculated on basis of original height. Crushing Strength.

Delamination Strength

Measure of the node-to-node bond strength of honeycomb core materials. It is equal to the tensile load applied to a honeycomb panel at fracture divided by its width times its thickness. (ASTM C-363)

Denier

The unit of linear density equal to the mass in grams per 9000 m of fiber, yarn, or other textile strand.

Dry Strength

Strength of an adhesive joint determined immediately after drying or after a period of conditioning in a specified atmosphere. (ASTM D-2475)

Ductility

Extent to which a material can sustain plastic deformation without rupture. Elongation and reduction of area are common indices of ductility.

Dynamic Creep

Creep that occurs under fluctuating load or temperature.

EASL

Elongation at a specified load.

Eccentricity of Loading

Distance between the actual line of action of compressive or tensile loads and the line of action that would produce a uniform stress over the cross section of the specimen.

Edge Tearing Strength

Measure of the resistance of paper to tearing when folded over a V-notch beam and loaded in a tensile testing machine. Results are re-ported in lb or kg. (See Tear Resistance.) Edge Tearing Strength.

Elastic Hysteresis

Difference between strain energy required to generate a given stress in a material and elastic energy at that stress. It is the energy dissipated as heat in a material in one cycle of dynamic testing. Elastic hysteresis divided by elastic deformation energy is equal to damping capacity.

Elastic Limit

Greatest stress that can be applied to a material without causing permanent deformation. For metals and other materials that have a significant straight line portion in their stress/strain diagram, elastic limit is approximately equal to proportional limit. For materials that do not exhibit a significant proportional limit, elastic limit is an arbitrary approximation (the apparent elastic limit).

Elastic Limit, Apparent

Arbitrary approximation of the elastic limit of materials that do not have a significant straight line portion on a stress/strain diagram. It is equal to the stress at which the rate of strain is 50% greater than at zero stress. It is the stress at the point of tangency between the stress- Elastic Hysteresis strain curve and the line having a slope, with respect to the stress axis, 50% greater than the slope of the curve at the origin.

 

Apparent Elastic Limit,

Elasticity

Ability of a material to return to its original shape when load causing deformation is removed.

Elongation

Measure of the ductility of a material determined in a tension test. It is the increase in gage length (measured after rupture) divided by original gage length. Higher elongation indicates higher ductility. Elongation cannot be used to predict behavior of materials subjected to sudden or repeated loading.

Embrittlement

Reduction in ductility due to physical or chemical changes.

Endurance

Alternate term for fatigue limit. Endurance.

Engineering Stress

Load applied to a specimen in a tension or compression test divided by the cross-sectional area of the specimen. The change in cross-sectional area that occurs with increases and decreases in applied load, is disregarded in computing engineering stress. It is also called conventional stress.

Event Detector

Digital function in the Model 8800 that looks for and trips on certain events, such as maximum peak, minimum peak, underpeak, over-peak, and specimen break. Can perform a number of actions, such as stop, hold, unload, transfer control mode, etc. upon trip. It is not used as a safety limit.

Extensometer

Instrument for measuring changes in linear dimensions. Also called a strain gauge. Frequently based on strain gauge technology.

 

LASE

Load At Specified Elongation.

Limits (Operational)

Feature in Instron materials and structural testing systems that suspends motion or shuts off the system when upper and/or lower bounds of actuator or crosshead travel, or force or strain, are reached during testing. Correct setting of operational limits by the operator, prior to testing, will reduce the risk of damage to test article and system and associated hazard to the operator.

Linear Density

Mass per unit length.

Load-Deflection Diagram

Plot of load versus corresponding deflection.

Load Protect

See Specimen Protect.

Maximum Fiber Stress

Maximum tensile or compressive stress in a homogeneous flexure or torsion test specimen. For a specimen loaded as a simple beam at its midpoint, maximum fiber stress occurs at mid-span and may be calculated by the formula (for rectangular specimens):

Maximum Fiber Stress Formula for Rectangular Specimens

where S is maximum fiber stress; P, load; L, span; b, width of the beam; and d, depth of the beam. For a circular cross section member loaded in torsion, maximum fiber stress may be calculated by the following formula:

 

Maximum Fiber Stress Formula for a Circular Cross Section Member

where T is twisting moment; r, original outer radius and J, polar moment of inertia of original cross section.

Mean Stress

Algebraic difference between maximum and minimum stress in one cycle of fluctuating loading, as in a fatigue test. Tensile stress is considered positive and compressive stress negative.

Minimum Bend Radius

Minimum radius to which a sheet or wire can be bent to a specified angle without failure.

Modulus

Alternate term for modulus of elasticity, often used in connection with rubber.

Modulus in Bending

Ratio of maximum fiber stress to maximum strain, within elastic limit of stress-strain diagram obtained in flexure test. Alternate term is flexural modulus of elasticity.

Modulus of Elasticity

Rate of change of strain as a function of stress. The slope of the straight line portion of a stress-strain diagram. Tangent modulus of elasticity is the slope of the stress-strain diagram at any point. Secant modulus of elasticity is stress divided by strainat any given value of stress or strain. It also is called stress-strain ratio.

 

Tangent Modulus of Elasticity

Tangent and secant modulus of elasticity are equal, up to the proportional limit of a material. Depending on the type of loading represented by the stress-strain diagram, modulus of elasticity may be reported as: compressive modulus of elasticity (or modulus of elasticity in compression); flexural modulus of elasticity (or modulus of elasticity in flexure); shear modulus of elasticity (or modulus of elasticity in shear); tensile modulus of elasticity (or modulus of elasticity in tension); or torsional modulus of elasticity (or modulus of elasticity in torsion). Modulus of elasticity may be determined by dynamic testing, where it can be derived from complex modulus. Modulus used alone generally refers to tensile modulus of elasticity. Shear modulus is almost always equal to torsional modulus and both are called modulus of rigidity. Moduli of elasticity in tension and compression are approximately equal and are known as Young’s modulus. Modulus of rigidity is related to Young’s modulus by the equation:

Modulus of Rigidity Formula

where E is Young’s modulus (psi), G is modulus of rigidity (psi) and r is Poisson’s ratio. Modulus of elasticity also is called elastic modulus and coefficient of elasticity.

Modulus of Rigidity

Rate of change of strain as a function of stress in a specimen subjected to shear or torsion loading. It is the modulus of elasticity deter-mined in a torsion test. Alternate terms are modulus of elasticity in torsion and modulus of elasticity in shear. Apparent modulus of rigidity is a measure of the stiffness of plastics measured in a torsion test (ASTM D-1043). It is "apparent" because the specimen may be deflected past its proportional limit and the value calculated may not represent the true modulus of elasticity within the elastic limit of the material.

Modulus of Rupture

Ultimate strength determined in a flexure or torsion test. In a flexure test, modulus of rupture in bending is the maximum fiber stress at failure. In a torsion test, modulus of rupture in torsion is the maxi-mum shear stress in the extreme fiber of a circular member at failure. Alternate terms are flexural strength and torsional strength.

Modulus of Strain Hardening

Alternate term for rate of strain hardening.

Modulus of Toughness

The work done on a unit volume of material as a simple tensile force is gradually increased from zero to the value causing rupture is de fined as the Modulus of Toughness. This may be calculated as the en-tire area under the stress-strain curve from the origin to rupture. Toughness of a material is its ability to absorb energy in the plastic range of the material.

Necking

Localized reduction of cross-sectional area of a specimen under tensile load. It is disregarded in calculating engineering stress but is taken into account in determining true stress.

Nominal Stress

Stress calculated on the basis of the net cross section of a specimen without taking into account the effect of geometric discontinuities such as holes, grooves, fillets, etc.

Offset Yield Strength

Arbitrary approximation of elastic limit. It is the stress that corresponds to the point of intersection of a stress-strain diagram and a line parallel to the straight line portion of the diagram. Offset refers to the distance between the origin of the stress-strain diagram, and the point of intersection of the parallel line and the 0 stress axis. Offset is expressed in terms of strain (often 0.2%).

Operating Stress

Stress imposed on a part in service.

Overstressing

Application of high fluctuating loads at the beginning of a fatigue test and lower loads toward the end. It is a means for speeding up a fatigue test.

 

Peel Resistance

Torque required to separate an adhesive and adherend in the climbing drum peel test (ASTM D-1781). It is a measure of bond strength.

Peel Strength

Measure of the strength of an adhesive bond. It is the average load per unit width of bond line required to part bonded materials where the angle of separation is 180 degrees and separation rate is 6 in/min. (ASTM D-903)

Plastic Deformation

Deformation that remains after the load causing it is removed. It is the permanent part of the deformation beyond the elastic limit of a material. It also is called plastic strain and plastic flow.

Plasticity

Tendency of a material to remain deformed, after reduction of the deforming stress, to a value equal to or less than its yield strength.

Plasticity Number

Index of the compressibility of rubber at elevated temperatures. Equal to 100 times the height of a standard specimen, after a 3 to 10 minute compression by a 5 kg load. (ASTM D-926)

Poisson’s Ratio

Ratio of lateral strain to axial strain in an axial loaded specimen. It is the constant that relates modulus of rigidity to Young’s modulus in the equation:

Formula

where E is Young’s modulus; G, modulus of rigidity; and r, Poisson’s ratio. The formula is valid only within the elastic limit of a material. A method for determining Poisson’s ratio is given in ASTM E-132.

Proof Stress

Stress that will cause a specified permanent deformation.

Proportional Limit

Highest stress at which stress is directly proportional to strain. It is the highest stress at which the curve in a stress-strain diagram is a straight line. Proportional limit is equal to elastic limit for many metals.

Proportional Limit

 

Rate of Strain Hardening

Rate of change of true stress as a function of true strain in a material undergoing plastic deformation. An alternate term is modulus of strain hardening.

Rationalization

The method of adding a self-ID and auto-calibration feature to the transducer.

Recovery

Index of a material’s ability to recover from deformation in the compressibility and recovery test (ASTM F-36), the deformation under load test (ASTM D-621) and the plastometer test (ASTM D-926). In the compressibility and recovery test, it usually is reported with compressibility and given as %. It is calculated by dividing the difference between recovered thickness and thickness under load, by the difference between original thickness and thickness under load. In the de-formation under load test, it indicates the extent to which a nonrigid plastic recovers from prolonged compressive deformation at an elevated temperature. It is given as %, and is calculated by dividing the difference between height recovered 1-1/2 hours after load is re-moved and height after three hours of loading, by the change in height under load. In the plastometer test, it indicates the extent to which an elastomer recovers from compressive loading at an elevated temperature. It is equal to plasticity number minus recovered height.

Recovery Test

Method for measuring compressibility and recovery of gasket and seal materials. (ASTM F-36)

Reduction of Area

Measure of the ductility of metals obtained in a tension test. It is the difference between original cross sectional area of a specimen and the area of its smallest cross section after testing. It is usually ex-pressed as % decrease in original cross section. The smallest cross section can be measured at or after fracture. For metals, it usually is measured after fracture and for plastics and elastomers, it is measured at fracture.

Relative Modulus

Ratio of the modulus of a rubber at a given temperature to its modulus at 73° F. It is determined in the Gehman torsional test.

Relaxation

Rate of reduction of stress in a material due to creep. An alternate term is stress relaxation.

Residual Elongation

Measure of ductility of plastics. It is the elongation of a plastic specimen measured 1 minute after rupture in a tension test.

Rupture Resistance

Indication of ability of rubber to withstand tensile loading. It is the load required to rupture a rubber specimen under conditions set out in ASTM D-530.

Rupture Strength

Nominal stress developed in a material at rupture. It is not necessarily equal to ultimate strength. And, since necking is not taken into ac-count in determining rupture stre

S-N Diagram

Plot of stress (S) against the number of cycles (N) required to cause failure of similar specimens in a fatigue test. Data for each curve on an S-N diagram are obtained by determining fatigue life of a number of specimens subjected to various amounts of fluctuating stress. The stress axis can represent stress amplitude, maximum stress or minimum stress. A log scale is almost always used for the N scale and sometimes for the S scale. Relative Modulus.

Secant Modulus of Elasticity

Ratio of stress to strain at any point on curve in a stress-strain diagram. It is the slope of a line from the origin to any point on a stress-strain curve.

Set Point

Arithmetic mean of the excursions of the controlling waveform in the Model 8800; i.e., the algebraic sum of the positive and negative amplitudes of the waveform. It is roughly equivalent to "mean level" on earlier Instron Servohydraulic Testing Systems.

Shear Modulus of Elasticity

Tangent or secant modulus of elasticity of a material subjected to shear loading. Alternate terms are modulus of rigidity and modulus of elasticity in shear. Also, shear modulus of elasticity usually is equal to torsional modulus of elasticity. A method for determining shear modulus of elasticity of structural materials by means of a twisting test is given in ASTM E-143. A method for determining shear modulus of structural adhesives is given in ASTM E-229.

Shear Strength

Maximum shear stress that can be sustained by a material before rupture. It is the ultimate strength of a material subjected to shear loading. It can be determined in a torsion test where it is equal to torsional strength. The shear strength of a plastic is the maximum load required to shear a specimen in such a manner that the resulting pieces are completely clear of each other. It is reported in psi based on the area of the sheared edge (ASTM D-732). The shear strength of a structural adhesive is the maximum shear stress in the adhesive prior to failure under torsional loading (ASTM E-229). Methods for determining shear strength of timber are given in ASTM D-143 and ASTM D-198.

Specimen Protect

Feature in many Instron materials testing systems that limits the maximum force applied to the test article. When Specimen Protect is enabled, the actuator or crosshead moves automatically to ensure the force on the test article remains within the pre-set bounds. It is often used to protect specimens or components during set-up, prior to the setting of operational limits. Specimen Protect only functions in position control mode.

Splitting Resistance

Measure of the ability of felt to withstand tearing. It is the load required to rupture a slit felt specimen by gripping lips of the cut in jaws and pulling them apart (ASTM D-461). An alternate term is tear resistance.

Springback

Degree to which a material returns to its original shape after deformation. In plastics and elastomers, it is also called recovery.

Stiffness

Measure of resistance of plastics to bending. It includes both plastic and elastic behavior, so it is an apparent value of elastic modulus rather than a true value. (ASTM D-747)

Strain

Change per unit length in a linear dimension of a part or specimen, usually expressed in % Strain, as used with most mechanical tests, is based on original length of the specimen. True or natural strain is based on instantaneous length, and is equal to: ln × l lo , where l is instantaneous length and lo is original length of the specimen. Shear strain is the change in angle between two lines originally at right angles.

Strain Energy

Measure of energy absorption characteristics of a material under load up to fracture. It is equal to the area under the stress-strain curve, and is a measure of the toughness of a material. Splitting Resistance.

Strain Hardening Exponent

Measure of increase in hardness and strength caused by plastic deformation. It is related to true stress and true strain by the equation:
s = s0d h where s is true stress, s0 is true stress at unit strain, d is true strain and h is strain hardening exponent.

Strain Point

Temperature at which internal stress in glass is substantially relieved in about 1 hour. (ASTM C-336)

Strain Rate

Time rate of elongation.

Strain Relaxation

Alternate term for creep of rubber.

Strength Reduction Ratio

Alternate term for fatigue notch factor.

Stress

Load on a specimen divided by the area through which it acts. As used with most mechanical tests, stress is based on original cross-sectional area without taking into account changes in area due to applied load. This sometimes is called conventional or engineering stress. True stress is equal to the load divided by the instantaneous cross-sectional area through which it acts.

Stress Amplitude

One-half the range of fluctuating stress developed in a specimen in a fatigue test. Stress amplitude often is used to construct an S-N diagram.

Stress Concentration Factor

Ratio of the greatest stress in the area of a notch or other stress raiser to the corresponding nominal stress. It is a theoretical indication of the effect of stress concentrators on mechanical behavior. Stress concentration factor usually is higher than the empirical fatigue notch factor or strength reduction ratio, because it does not take into account stress relief due to local plastic deformation.

Stress Ratio

Ratio of minimum stress to maximum stress in one cycle of loading in a fatigue test. Tensile stresses are considered positive and compressive stresses negative.

Stress Relaxation

Decrease in stress in a material subjected to prolonged constant strain at a constant temperature. Stress relaxation behavior is determined in a creep test. Data often is presented in the form of a stress vs. time plot. Stress relaxation rate is the slope of the curve at any point.

Stress Rupture Strength

Alternate term for creep strength.

Stress-Strain Diagram

Graph of stress as a function of strain. It can be constructed from data obtained in any mechanical test where load is applied to a material, and continuous measurements of stress and strain are made simultaneously. It is constructed for compression, tension and torsion tests. An example is shown below.

Stress-Strain Ratio

Stress divided by strain at any load or deflection. Below the elastic limit of a material, it is equal to tangent modulus of elasticity. An alternate term is the secant modulus of elasticity.

Stress-Strain Ratio

Stripping Strength

Alternate term for peel strength.

Tangent Modulus of Elasticity

The instantaneous rate of change of stress as a function of strain. It is the slope at any point on a stress-strain diagram.

Tear Length

Measure of the drawability of sheet metal. Two small parallel slots are cut in the edge of the sheet to form a tab which is gripped and torn from the sheet. The variation in length of tabs torn in different directions is an indication of crystal orientation in the sheet (tabs torn in the direction of orientation are longer). The degree of orientation is an indication of difficulty to be expected in drawing the sheet to uniform shapes.

Tear Resistance

Measure of the ability of sheet or film materials to resist tearing. For paper, it is the force required to tear a single ply of paper after the tear has been started. Three standard methods are available for determining tear resistance of plastic films: ASTM D-1004 details a method for determining tear resistance at low rates of loading; a test in ASTM D-1922 measures the force required to propagate a precut slit across a sheet specimen; and ASTM D-1038 gives a method for determining tear propagation resistance that is recommended for specification acceptance testing only. Tear resistance of rubber is the force required to tear a 1 inch thick specimen under the conditions outlined in ASTM D-624. Tear resistance of textiles is the force required to propagate a single-rip tongue-type tear (starting from a cut) by means of a falling pendulum apparatus. (ASTM D-1424)

Tearing Strength

Tensile force required to rupture a pre-slit woven fabric specimen under the conditions outlined in ASTM D-2261 and ASTM D-2262. Edge tearing strength of paper is the force required to tear a specimen folded over a V-notch and loaded in a tensile test machine.

Tenacity

The tensile stress expressed as force per unit linear density of an unstrained specimen.

Tensile Modulus of Elasticity

Tangent or secant modulus of elasticity of a material subjected to tensile loading. Alternate terms are Young’s modulus and modulus of elasticity in tension. It can be measured in a tension test or in a dynamic test where it is related to resonant frequency on a cylindrical rod by the equation:
Tensile Modulus of Elasticity Equation
where E is modulus of elasticity; 1, length of the rod; p, density; f, resonant frequency; k, radius of gyration of the rod about an axis normal to the rod axis and plane of motion (d/4 for cylindrical rods) and j, a constant dependent on the mode of vibration. Tensile modulus of elasticity is approximately equal to compressive modulus of elasticity within the proportional limit.

Tensile Strength

Ultimate strength of a material subjected to tensile loading. It is the maximum stress developed in a material in a tension test.

Tension Impact Test

Method for determining energy required to fracture a specimen under shock tensile loading (ASTM D-1822).

Tension Set

Extent to which vulcanized rubber is permanently deformed after being stretched a specified amount for a short time. It is expressed as a % of the original length or distance between gage marks (ASTM D-412).

Tension Test

Method for determining behavior of materials under axial stretch loading. Data from test are used to determine elastic limit, elongation, modulus of elasticity, proportional limit, reduction in area, tensile strength, yield point, yield strength and other tensile properties. Tension tests at elevated temperatures provide creep data. Procedures for tension tests of metals are given in ASTM E-8. Methods for tension tests of plastics are outlined in ASTM D-638, ASTM D-2289 (high strain rates), and ASTM D-882 (thin sheets). ASTM D-2343 outlines a method for tension testing of glass fibers; ASTM D-897, adhesives; ASTM D-412, vulcanized rubber.

Tex

The unit of linear density equal to the mass in grams per 1000 m of fiber, yarn, or other textile strand.

Time for Rupture

Time required to rupture specimen under constant stress and temperature in a creep test.

Torsion Test

Method for determining behavior of materials subjected to twisting loads. Data from torsion test is used to construct a stress-strain diagram and to determine elastic limit torsional modulus of elasticity, modulus of rupture in torsion, and torsional strength. Shear properties are often determined in a torsion test. (ASTM E-143)

Torsional Deformation

Angular displacement of specimen caused by a specified torque in torsion test. It is equal to the angular twist (radians) divided by the gage length (in.).

Torsional Modulus of Elasticity

Modulus of elasticity of material subjected to twist loading. It is approximately equal to shear modulus and also is called modulus of rigidity.

Torsional Strain

Strain corresponding to a specified torque in the torsion test. It is equal to torsional deformation multiplied by the radius of the specimen.

Torsional Strength

Measure of the ability of a material to withstand a twisting load. It is the ultimate strength of a material subjected to torsional loading, and is the maximum torsional stress that a material sustains before rupture. Alternate terms are modulus of rupture and shear strength.

Torsional Stress

Shear stress developed in a material subjected to a specified torque in torsion test. It is calculated by the equation:

Torsional Stress Equation

where T is torque, r is the distance from the axis of twist to the outermost fiber of the specimen, and J is the polar moment of inertia.

Toughness

Toughness is the resistance of a material to fracture or break. It is usually measured in units of energy.

True Strain

Instantaneous % of change in length of specimen in mechanical test. It is equal to the natural logarithm of the ratio of length at any instant to original length.

True Stress

Applied load divided by actual area of the cross section through which load operates. It takes into account the change in cross section that occurs with changing load.

 

 

 

Ultimate Elongation

Alternate term for elongation of material at rupture under tensile loading.

Ultimate Strength

Highest engineering stress developed in material before rupture. Normally, changes in area due to changing load and necking are disregarded in determining ultimate strength.

Wet Strength

Breaking strength of paper saturated with water. Also, the strength of an adhesive bond after immersion in water.

Yield Point

Stress at which strain increases without accompanying increase in stress. Only a few materials (notably steel) have a yield point, and generally only under tension loading.

Yield Point Elongation

Strain at yield point of a material. It is an indication of ductility.

Yield Strength

Indication of maximum stress that can be developed in a material without causing plastic deformation. It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of elastic limit. Offset yield strength is determined from a stress-strain diagram. It is the stress corresponding to the intersection of the stress-strain curve, and a line parallel to its straight line portion offset by a specified strain. Offset for metals is usually specified as 0.2%, i.e., the intersection of the offset line and the 0-stress axis is at 0.2% strain. Offset for plastics is usually 2%.

Yield Strength Elongation

Strain corresponding to yield strength of material. It is an indication of ductility.

Yield Value

Stress in an adhesive joint at which a marked increase in deformation occurs without an increase in load.

Young’s Modulus

Alternate term for modulus of elasticity in tension or compression.

Zero Suppression

Instron servohydraulic testing systems have a zero suppression feature that shifts the absolute zero of the command waveform to an offset or "apparent" zero. It is used to improve resolution when using a waveform that is small in relation to the full scale range in use. It is also used when the actuator is offset from its normal or absolute zero to accommodate large grips or long specimens.

 

 

 

 

 

 

Soft Tissue - Planar Biaxial Testing

The evaluation of the body's natural tissues, such as pericardium and collagen, is a crucial step in the development of new replacement tissues.

Soft tissue

 

Tibial insert

 

 

UHMWPE

UHMWPE (ultra high molecular weight polyethylene) is commonly used as an insert for one of the load-bearing, articulating surfaces in orthopaedic implants, such as the acetabular cup in the hip or tibial tray insert in the knee. This particular type of polyethylene is used because it has excellent biocompatibility and has a lower wear rate and coefficient of friction than other polymers.

 

Whenever a device is designed for use with humans, strict external and internal guidelines must be adhered to in order to assure its safety and efficacy. This requires in-depth knowledge of all aspects of the device, from raw materials properties to long-term performance under complex service conditions.

 

Medical Glove Tensile Testing

Medical gloves need to be tested for tensile strength and percent ultimate elongation at break, before and after aging, as specified in ASTM D 412 and comparable ISO standards. This section discusses a test setup that not only accomplishes this, but also suggests a solution for measurement failures caused by traditional strain-measurement devices. Medical gloves can be made of a variety of materials, such as latex, nitrile, and vinyl. Whatever the material, certain minimum performance levels need to be determined as specified by the FDA. Medical gloves can be made of a variety of materials, such as latex, nitrile, and vinyl. Whatever the material, certain minimum performance levels need to be determined as specified by the FDA. Specifically, tensile strength and percent ultimate elongation at break, before and after aging, must be assessed.

Specifically, tensile strength and percent ultimate elongation at break, before and after aging, must be assessed. A test setup is needed that will:

Simulate aging

 

Plastic surgical tubing comes in many different sizes and shapes, with dozens of possible interconnections and fittings. Failure of any of these parts could seriously endanger the health of patients. This section discusses a test setup that measures tensile strength and elongation at break. It also offers suggestions on how to deal with gripping and measurement issues.